wireless sensor networks introduction and its applications .ppt

Wireless Sensor Networks (WSNs) are networks made up of distributed autonomous sensors that monitor physical or environmental conditions, such as temperature, sound, pressure, and more, and transmit the data wirelessly to a central location. These networks are often used in various applications, including environmental monitoring, military applications, health monitoring, industrial automation, and smart homes.Key Components of a Wireless Sensor Network:
Sensor Nodes: These are devices that have sensors, a processor, memory, and a wireless transceiver. They collect data and communicate wirelessly.
Gateway (Sink): This node collects data from sensor nodes and forwards it to a base station or an external system, often connected to the internet.
Communication Protocols: Wireless communication protocols like Zigbee, Bluetooth, Wi-Fi, or proprietary protocols are used for data transmission between nodes.
Base Station: A central system that processes the collected data, which could be part of a larger information system or cloud-based platform.
Features of WSNs:
Scalability: Can support a large number of sensor nodes.
Low Power Consumption: Sensors are often designed to be energy-efficient due to limited battery life.
Self-Organization: Nodes often dynamically configure themselves to communicate with one another, especially in case of node failure or movement.
Low Bandwidth: Data collected is often low-volume but crucial for monitoring applications.
Common Applications:
Environmental Monitoring: Used to monitor parameters such as air or water quality, weather conditions, forest fires, etc.
Health Monitoring: Deployed in hospitals or homes for real-time health tracking of patients, such as heart rate and blood pressure.
Industrial Automation: Monitors machinery health, structural integrity, and environmental conditions in factories.
Smart Cities: Helps in traffic management, waste management, and monitoring public infrastructure.
Agriculture: Tracks soil moisture, weather conditions, and crop health to improve agricultural practices.
Challenges in WSNs:
Energy Efficiency: Nodes are typically battery-powered, and extending battery life is crucial.
Network Security: Protecting data and network integrity in environments where nodes might be physically accessible.
Data Integrity: Ensuring data collected by sensor nodes is accurate and reliable.
Latency: In some applications, real-time data transmission
with low latency is critical.
Wireless Sensor Networks (WSNs) are networks consisting of spatially distributed, autonomous sensors that monitor physical or environmental conditions and communicate the collected data wirelessly. These networks are essential for a wide range of applications, especially where real-time monitoring is required in remote or hazardous areas.Key Features of WSNs:
Wireless Communication: Sensor nodes communicate through wireless channels (e.g., ZigBee, Wi-Fi, Bluetooth, or proprietary protocols).
Energy Efficiency: Sensors are usual

1. Wireless Networks

2. A wireless network keeps devices connected to a network while still allowing them
the freedom to move about, unencumbered by wires. A wired network, on the other
hand, makes use of cables that connect devices to the network. These devices are
often desktop or laptop computers but can also include scanners and point-of-sale
machines.
In a Wi-Fi network, the medium (the radio frequency being used for the network) is a
shared resource, not just for the users of the network, but often for other
technologies as well (Wi-Fi operates in what are called ‘shared’ bands, where many
different electronic devices are approved to operate). This has several implications:
1) Unlike a wireless network, wired can’t both talk and listen at the same time, it is
“half duplex” .(Half-duplex mode :Half-duplex mode is when the sender can send the
data and also can receive the data one at a time. It is two-way directional i.e. bi-
directional communication but one at a time. - Full-duplex mode :Full duplex mode is
when the sender can send the data and also can receive the data simultaneously. It is
two-way directional i.e. bi-directional communication simultaneously.)
2) All users are sharing the same space.
3) Everyone can ‘hear’ all traffic going on. This has forced Wi-Fi networks to
implement various security measures over the years to protect the confidentiality of
information passed wirelessly.

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Wireless networks
Access computing/communication services, on the move
Wireless WANs
–Cellular Networks: GSM, GPRS, CDMA
–Satellite Networks: Iridium
Wireless LANs
–WiFi Networks: 802.11
–Personal Area Networks: Bluetooth
Wireless MANs
–WiMaX Networks: 802.16
–Mesh Networks: Multi-hop WiFi
–Adhoc Networks: useful when infrastructure not available

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GSM, GPRS, CDMA
GSM, GPRS, CDMA
Global System for Mobile communication, CDMA stands for Code
Division Multiple Access.
GSM supports transmitting data and voice both at once, but CDMA
does have not this feature. The main distinction between GSM and
CDMA is that in GSM, the customer information is put on a SIM
card which can be moved to a new mobile phone. Whereas only
mobile phones from a set of whitelisted companies can be used
with a CDMA network.

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GPRS
GPRS is an expansion Global System for Mobile Communication.
It is basically a packet-oriented mobile data standard on the 2G
and 3G cellular communication network’s global system for mobile
communication. GPRS was built up by European
Telecommunications Standards Institute (ETSI) because of the
prior CDPD, and I-mode packet switched cell advances.
GPRS overrides the wired associations, as this framework has
streamlined access to the packet information’s network like the
web.
Iridium Satellites are a system of 66 satellites orbiting the Earth,
working together to provide global voice and data communication
coverage. This Low Earth Orbit (LEO) satellite constellation
guarantees seamless connectivity, even in the most remote and
inaccessible areas of the world.

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IEEE 802.11
IEEE 802.11 is part of the IEEE 802 set of local area network
(LAN) technical standards, and specifies the set of medium
access control (MAC) and physical layer (PHY) protocols for
implementing wireless local area network (WLAN) computer
communication.
 IEEE 802.11 is used in most home and office networks to
allow laptops, printers, smartphones, and other devices to
communicate with each other and access the Internet without
connecting wires. IEEE 802.11 is also a basis for vehicle-
based communication networks with IEEE 802.11p.
The standards are created and maintained by the Institute of
Electrical and Electronics Engineers (IEEE) LAN/MAN
Standards Committee (IEEE 802).

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Limitations of the mobile environment
Limitations of the Wireless Network
limited communication bandwidth
frequent disconnections
heterogeneity of fragmented networks
Limitations Imposed by Mobility
route breakages
lack of mobility awareness by system/applications
Limitations of the Mobile Device
short battery lifetime
limited capacities

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Wireless v/s Wired networks
Regulations of frequencies
–Limited availability, coordination is required
–useful frequencies are almost all occupied
Bandwidth and delays
–Low transmission rates
•few Kbits/s to some Mbit/s.
–Higher delays
•several hundred milliseconds
–Higher loss rates
•susceptible to interference, e.g., engines, lightning
Always shared medium
–Lower security, simpler active attacking
–radio interface accessible for everyone
–secure access mechanisms important

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Wireless Network

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Wireless Network
Types of Wireless Network Connections
In addition to a LAN, there are a few other types of common wireless networks: personal-area network (PAN),
metropolitan-area network (MAN), and wide-area network (WAN).
LAN
A local-area network is a computer network that exists at a single site, such as an office building. It can be
used to connect a variety of components, such as computers, printers, and data storage devices. LANs consist
of components like switches, access points, routers, firewalls, and Ethernet cables to tie it all together. Wi-Fi is
the most commonly known wireless LAN.
PAN
A personal-area network consists of a network centralized around the devices of a single person in a single
location. A PAN could have computers, phones, video game consoles, or other peripheral devices. They are
common inside homes and small office buildings. Bluetooth is the most commonly known wireless PAN.
MAN
A metropolitan-area network is a computer network that spans across a city, small geographical area, or
business or college campus. One feature that differentiates a MAN from a LAN is its size. A LAN usually
consists of a solitary building or area. A MAN can cover several square miles, depending on the needs of the
organization. Large companies, for example, may use a MAN if they have a spacious campus and need to
manage key components, such as HVAC and electrical systems.
WAN
A wide-area network covers a very large area, like an entire city, state, or country. In fact, the internet is a
WAN. Like the internet, a WAN can contain smaller networks, including LANs or MANs. Cellular services are
the most commonly known wireless WANs.

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WIRED LAN

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WIRELESS LAN

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Reference model
Application
Transport
Network
Data Link
Physical
Medium
Data Link
Physical
Application
Transport
Network
Data Link
Physical
Data Link
Physical
Network Network
Radio

14. RF Basics

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Factors affecting wireless system design
Frequency allocations
–What range to operate? May need licenses.
Multiple access mechanism
–How do users share the medium without interfering?
Antennas and propagation
–What distances? Possible channel errors introduced.
Signals encoding
–How to improve the data rate?
Error correction
–How to ensure that bandwidth is not wasted?

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Frequencies for communication
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
Frequency and wave length: = c/f
wave length , speed of light c  3x10
8
m/s, frequency f
1 Mm
300 Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100 m
3 THz
1 m
300 THz
visible lightVLF LF MFHF VHFUHFSHFEHF infrared UV
optical transmission
coax cabletwisted
pair

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Wireless frequency allocation
Radio frequencies range from 9KHz to 400GHZ (ITU)
Microwave frequency range
–1 GHz to 40 GHz
–Directional beams possible
–Suitable for point-to-point transmission
–Used for satellite communications
Radio frequency range
–30 MHz to 1 GHz
–Suitable for omnidirectional applications
Infrared frequency range
–Roughly, 3x10
11
to 2x10
14
Hz
–Useful in local point-to-point multipoint applications within confined areas

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Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio
–simple, small antenna for cars
–deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellite communication
–small antenna, focusing
–large bandwidth available
Wireless LANs use frequencies in UHF to SHF spectrum
–some systems planned up to EHF
–limitations due to absorption by water and oxygen molecules
(resonance frequencies)
•weather dependent fading, signal loss caused by heavy rainfall etc.

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TRANSMITTER, RECEIVER
.

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Wireless transmission
Wireless communication systems consist of:
–Transmitters
–Antennas: radiates electromagnetic energy into air
–Receivers
In some cases, transmitters and receivers are
on same device, called transceivers.
Transmitter
Receiver
Antenna
Antenna

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Transmitters
Amplifier
Oscillator
Mixer FilterAmplifier
Antenna
Transmitter
Suppose you want to generate a signal that is sent at 900 MHz and
the original source generates a signal at 300 MHz.
•Amplifier - strengthens the initial signal
•Oscillator - creates a carrier wave of 600 MHz
•Mixer - combines signal with oscillator and produces 900 MHz
(also does modulation, etc)
•Filter - selects correct frequency
•Amplifier - Strengthens the signal before sending it
Source

22. Antennas

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Antennas
An antenna is an electrical conductor or system of
conductors to send/receive RF signals
–Transmission - radiates electromagnetic energy into space
–Reception - collects electromagnetic energy from space
In two-way communication, the same antenna can be
used for transmission and reception
Omnidirectional Antenna
(lower frequency)
Directional Antenna
(higher frequency)

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Radiation and reception of electromagnetic waves,
coupling of wires to space for radio transmission
Isotropic radiator: equal radiation in all directions
(three dimensional) - only a theoretical reference
antenna
Real antennas always have directive effects (vertically
and/or horizontally)
Radiation pattern: measurement of radiation around
an antenna
Antennas: isotropic radiator
zy
x
z
y x ideal
isotropic
radiator

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Antennas: simple dipoles
Real antennas are not isotropic radiators
–dipoles with lengths /4 on car roofs or /2 (Hertzian dipole)
 shape of antenna proportional to wavelength
Gain: maximum power in the direction of the main lobe
compared to the power of an isotropic radiator (with the same
average power)
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
simple
dipole
/
4
/
2

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Antennas: directed and sectorized
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
top view, 3 sector
x
z
top view, 6 sector
x
z
Often used for microwave connections or base stations
for mobile phones (e.g., radio coverage of a valley)
directed
antenna
sectorized
antenna

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Antenna models
In Omni Mode:
Nodes receive signals with gain G
o
In Directional Mode:
Capable of beam forming in specified direction
Directional Gain G
d
(G
d
> G
o
)

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Comparison of omni and directional
Issues Omni Directional
Strength Low High
Connectivity Low High
Interference Omni Directional
Cost & Complexity Low High

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Antennas: diversity
Grouping of 2 or more antennas
–multi-element antenna arrays
Antenna diversity
–switched diversity, selection diversity
•receiver chooses antenna with largest output
–diversity combining
•combine output power to produce gain
•co-phasing needed to avoid cancellation
+
/4/
2
/
4
ground plane
/
2/
2
+
/
2

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WHAT WE HAVE DONE TILL
NOW?

31. Signal Propagation and Modulation

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Signals
physical representation of data
function of time and location
signal parameters: parameters representing the value of data
classification
–continuous time/discrete time
–continuous values/discrete values
–analog signal = continuous time and continuous values
–digital signal = discrete time and discrete values
signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift 
–sine wave as special periodic signal for a carrier:
s(t) = A
t sin(2  f
t t + 
t)

33. Signal Propagation
Wireless communications systems are composed of one or more
“Antenna Sites”, “Tower Sites”, or “Cell Sites”.
Antennas mounted on these structures pump out wireless
communications signals to devices in the field via electromagnetic
waves.
Wireless signals are electromagnetic waves travelling through the
air. These are formed when electric energy travels through a piece of
metal -- for example a wire or antenna -- and waves are formed
around that piece of metal. These waves can travel some distance
depending on the strength of that energy.

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Signal propagation ranges
distance
sender
transmission
detection
interference
Transmission range
–communication possible
–low error rate
Detection range
–detection of the signal
possible
–no communication
possible
Interference range
–signal may not be
detected
–signal adds to the
background noise

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Propagation modes
Earth
Earth
Earth
a) Ground Wave Propagation
b) Sky Wave Propagation
c) Line-of-Sight Propagation
Transmission
Antenna
Receiving
Antenna
Signal
Signal
Ionosphere
Signal

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Attenuation
Strength of signal falls off with distance over
transmission medium
Attenuation factors for unguided media:
–Received signal must have sufficient strength so that
circuitry in the receiver can interpret the signal
–Signal must maintain a level sufficiently higher than noise
to be received without error
–Attenuation is greater at higher frequencies, causing
distortion
Approach: amplifiers that strengthen higher
frequencies

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Transmission Limitations
1.Attenuation
The strength of signal falls with distance over transmission medium. The
extent of attenuation is a function of distance, transmission medium, as
well as the frequency of the underlying transmission.
1..Distortion
Since signals at different frequencies attenuate to different extents, a signal
comprising of components over a range of frequencies gets distorted, i.e.,
the shape of the received signal changes. A standard method of resolving
this problem (and recovering the original shape) is to amplify higher
frequencies and thus equalize attenuation over a band of frequencies.
1.. Dispersion
Dispersion is the phenomenon of spreading of a burst of electromagnetic
energy during propagation. Bursts of data sent in rapid succession tend to
merge due to dispersion.

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Transmission Limitations contd....
5. Noise
The most pervasive form of noise is thermal noise, which is often modeled
using an additive Gaussian model. Thermal noise is due to thermal agitation
of electrons and is uniformly distributed across the frequency spectrum.
Other forms of noise include −
(a). .Inter modulation noise (caused by signals produced at frequencies that
are sums or differences of carrier frequencies)
(b). Crosstalk (interference between two signals)I
(c). Impulse noise (irregular pulses of high energy caused by external
electromagnetic disturbances).
Burst errors:- While an impulse noise may not have a significant impact on
analog data, it has a noticeable effect on digital data, causing burst errors.

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The above figure clearly illustrates how the noise signal
overlaps the original signal and tries to change its characteristics.

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Transmission Limitations contd....
Fading
 Fading refers to the variation of the signal strength with respect to time/distance
 Widely prevalent in wireless transmissions.
 Most common causes of fading in the wireless environment Multipath propagation and
mobility
Multipath propagation
In wireless media, signals propagate using three principles, which are reflection, scattering,
and Diffraction.
I.. Reflection occurs when the signal encounters a large solid surface, whose size is much
larger than the wavelength of the signal, e.g., a solid wall.
II.. Diffraction occurs when the signal encounters an edge or a corner, whose size is larger
than the wavelength of the signal, e.g., an edge of a wall.
III.. Scattering occurs when the signal encounters small objects of size smaller than the
wavelength of the signal.

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Attenuation: Propagation & Range

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Signal propagation
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d²
(d = distance between sender and receiver)
Receiving power additionally influenced by
–fading (frequency dependent)
–shadowing
–reflection at large obstacles
–refraction depending on the density of a medium
–scattering at small obstacles
–diffraction at edges
reflection scattering diffractionshadowing refraction

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Signal can take many different paths between sender and receiver due to reflection, scattering,
diffraction
Time dispersion: signal is dispersed over time
  interference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
  distorted signal depending on the phases of the different parts
Multi path propagation
signal at sender
signal at receiver
LOS pulses
multipath
pulses

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Effects of mobility
Channel characteristics change over time and location
–signal paths change
–different delay variations of different signal parts
–different phases of signal parts
 quick changes in the power received
(short term fading)
Additional changes in
–distance to sender
–obstacles further away
 slow changes in the average power
received (long term fading)
short term fading
long term
fading
t
power

45. Multiplexing Mechanisms

46. What is Multiplexing?
Multiplexing(sometimes contracted to muxing) is
the sharing of a medium or bandwidth. It is the
process in which multiple signals coming from
multiple sources are combined and transmitted
over a single communication/physical line.

47. What is Multiplexing?......
The multiplexed signal is transmitted over a
communication channel such as a cable. The multiplexing
divides the capacity of the communication channel into
several logical channels, one for each message signal or
data stream to be transferred.
A device that performs the multiplexing is called a
multiplexer (MUX), and a device that performs the reverse
process is called a demultiplexer (DEMUX or DMX).
Inverse multiplexing (IMUX) has the opposite aim as
multiplexing, namely to break one data stream into
several streams, transfer them simultaneously over
several communication channels, and recreate the
original data stream.

48. Types of Multiplexing
There are Five types of Multiplexing :
Frequency Division Multiplexing (FDM)
Time-Division Multiplexing (TDM)
Wavelength Division Multiplexing (WDM)
Code-division multiplexing (CDM)
Space-division multiplexing (SDM):

49. 1. Frequency Division Multiplexing :
Frequency division multiplexing is defined as a
type of multiplexing where the bandwidth of a
single physical medium is divided into a number
of smaller, independent frequency channels.

50. FDM CONTD.....
This allows a single transmission medium such as a
microwave radio link, cable or optical fiber to be shared by
multiple independent signals. Another use is to carry separate
serial bits or segments of a higher rate signal in parallel.
The most common example of frequency-division multiplexing
is radio and television broadcasting, in which multiple radio
signals at different frequencies pass through the air at the
same time. Another example is cable television, in which many
television channels are carried simultaneously on a single
cable. FDM is also used by telephone systems to transmit
multiple telephone calls through high capacity trunklines.
 Receivers must tune to the appropriate frequency (channel)
to access the desired signal.

51. FDM CONTD.....
In FDM, we can observe a lot of inter-channel
cross-talk, due to the bandwidth is divided into
frequency channels. In order to prevent the
inter-channel cross talk, unused strips of
bandwidth must be placed between each
channel. These unused strips between each
channel are known as guard bands.

52. 2. Time Division Multiplexing :
Time-division multiplexing is defined as a type of
multiplexing wherein FDM, instead of sharing a
portion of the bandwidth in the form of channels, in
TDM, time is shared.
Each connection occupies a portion of time in the link.
In Time Division Multiplexing, all signals operate with
the same frequency (bandwidth) at different times.
Time-division multiplexing (TDM) is a digital (or in
rare cases, analog) technology which uses time,
instead of space or frequency, to separate the
different data streams.

53. 2. Time Division Multiplexing :

Consider an application requiring four
terminals at an airport to reach a central
computer. Each terminal communicated at
2400 baud, so rather than acquire four
individual circuits to carry such a low-speed
transmission, the airline has installed a pair
of multiplexers.

54. 2. Time Division Multiplexing :

55. There are two types of Time Division
Multiplexing :
1) Synchronous Time Division Multiplexing
2) Statistical (or Asynchronous) Time Division Multiplexing
Synchronous TDM
Synchronous TDM is a type of Time Division Multiplexing where
the input frame already has a slot in the output frame. Time slots
are grouped into frames. One frame consists of one cycle of time
slots.
Synchronous TDM is not efficient because if the input frame has
no data to send, a slot remains empty in the output frame.
In synchronous TDM, we need to mention the synchronous bit at
the beginning of each frame.

56. Synchronous TDM :
.

57. Statistical TDM :
Statistical TDM is a type of Time Division Multiplexing where
the output frame collects data from the input frame till it is
full, not leaving an empty slot like in Synchronous TDM.
In statistical TDM, we need to include the address of each
particular data in the slot that is being sent to the output
frame.

58. 3. Wavelength Division Multiplexing :
Wavelength Division Multiplexing (WDM) is a
multiplexing technology used to increase the capacity
of optical fiber by transmitting multiple optical signals
simultaneously over a single optical fiber, each with a
different wavelength.
Each signal is carried on a different wavelength of
light, and the resulting signals are combined onto a
single optical fiber for transmission.
At the receiving end, the signals are separated by
their wavelengths, demultiplexed and routed to their
respective destinations.

59. 3. Wavelength Division Multiplexing :
.

60. 3. Wavelength Division Multiplexing :
WDM can be divided into two categories:
1) Dense Wavelength Division Multiplexing (DWDM) and
2) Coarse Wavelength Division Multiplexing (CWDM).
 DWDM is used to multiplex a large number of optical
signals onto a single fiber, typically up to 80 channels with a
spacing of 0.8 nm or less between the channels.
CWDM is used for lower-capacity applications, typically up
to 18 channels with a spacing of 20 nm between the
channels.

61. 3. Wavelength Division Multiplexing :

WDM has several advantages over other
multiplexing technologies such as Time Division
Multiplexing (TDM). WDM allows for higher data
rates and capacity, lower power consumption,
and reduced equipment complexity.
WDM is also flexible, allowing for easy
upgrades and expansions to existing networks.
It enables the transmission of large amounts of
data over long distances with high speed and
efficiency.

62. 3. Wavelength Division Multiplexing :

WDM is used in a wide range of applications,
including telecommunications, cable TV, internet
service providers, and data centers.
It enables the transmission of large amounts of
data over long distances with high speed and
efficiency.
Wavelength Division Multiplexing is used on fiber
optics to increase the capacity of a single fiber.

63. 4. Space-division multiplexing (SDM) :
Space Division Multiplexing (SDM) is a technique used in
wireless communication systems to increase the capacity of
the system by exploiting the physical separation of users.
In SDM, multiple antennas are used at both the transmitter
and receiver ends to create parallel communication
channels.
These channels are independent of each other, which
allows for multiple users to transmit data simultaneously in
the same frequency band without interference.
The capacity of the system can be increased by adding
more antennas, which creates more independent channels.

64. 4. Space-division multiplexing (SDM) :
.

65. 4. Space-division multiplexing (SDM) :
Examples are multiple-input and multiple-output
(MIMO), single-input and multiple-output (SIMO)
and multiple-input and single-output (MISO)
multiplexing.
Examples include an analogue stereo audio cable,
with one pair of wires for the left channel and
another for the right channel, and a multi-pair
telephone cable, a switched star network such as a
telephone access network, a switched Ethernet
network, and a mesh network.

66. 4. Space-division multiplexing (SDM) :
 SDM is commonly used in wireless
communication systems such as cellular
networks, Wi-Fi, and satellite communication
systems.
 In cellular networks, SDM is used in the form of
Multiple Input Multiple Output (MIMO)
technology, which uses multiple antennas at both
the transmitter and receiver ends to improve the
quality and capacity of the communication link.

67. 5. Code-division multiplexing (CDM) :
Code division multiplexing (CDM) is a technique used in
telecommunications to allow multiple users to transmit
data simultaneously over a single communication
channel.
In CDM, each user is assigned a unique code that is used
to modulate their signal. The modulated signals are then
combined and transmitted over the same channel.
At the receiving end, each user’s signal is demodulated
using their unique code to retrieve their original data.
In CDM, each user is assigned a unique spreading code
that is used to spread the data signal.
This spreading code is typically a binary sequence that is
much longer than the original data signal.

68. 5. Code-division multiplexing (CDM) :
.

69. 5. Code-division multiplexing (CDM) :
The spreading code is multiplied with the data signal
to generate a spread spectrum signal that has a much
wider bandwidth than the original data signal.
CDM is commonly used in wireless communication
systems such as cellular networks and satellite
communication systems. It allows multiple users to
share the same frequency band and increases the
capacity of the communication channel.
CDM also provides some level of security as the
signals of different users are difficult to intercept or
jam.

70. Spread Spectrum Technology

71. Sridhar Iyer IIT Bombay 71
Spread spectrum technology
Spread Spectrum refers to a system originally developed for military
applications, to provide secure communications by spreading the signal
over a large frequency band.
Figure 1 represents a narrow band signal in the frequency domain. These
narrowband signals are easily jammed by any other signal in the same
band. Likewise, the signal can also be intercepted since the frequency
band is fixed and narrow (i.e. easy to detect).
The idea behind spread spectrum is to use more bandwidth than the
original message while maintaining the same signal power.
A spread spectrum signal does not have a clearly distinguishable peak in
the spectrum. This makes the signal more difficult to distinguish from
noise and therefore more difficult to jam or intercept.
detection at
receiver
interference spread
signal
signal
spread
interference
f f
power power

72. Sridhar Iyer IIT Bombay 72
General Block Diagram
There are two predominant techniques to spread the spectrum:
1) Frequency hoping (FH), which makes the narrow band signal
jump in random narrow bands within a larger bandwidth.
2) Direct sequence (DS) which introduces rapid phase transition
to the data to make it larger in bandwidth.

73. Sridhar Iyer IIT Bombay 73
DSSS
In DSSS, each data bit is replaced with n bits
using a spreading code called chips, and the bit
rate of the chip is called as chip-rate.
The chip rate is n times the bit rate of the
original signal.
DSSS introduces rapid phase transition to the
data making it larger in bandwidth.
As the period T of a signal gets shorter in time
(or rate R increases), the bandwidth B of the
signal increases: R = 1/T = 2B (Nyquist Rate)

74. Sridhar Iyer IIT Bombay 74
DSSS

75. Sridhar Iyer IIT Bombay 75
DSSS
In DSSS technique, the data that needs to be transmitted
is split into smaller blocks.
After that, each data block is attached with a high data rate
bit sequence and is transmitted from the sender end to the
receiver end.
Data blocks are recombined again to generate the original
data at the receiver's end, which was sent by the sender,
with the help of the data rate bit sequence.
DSSS can also be classified into two types:
Wide Band Spread Spectrum
Narrow Band Spread Spectrum

76. Sridhar Iyer IIT Bombay 76
Advantages of Direct Sequence
Spread Spectrum
DSSS is less reluctant to noise; that's why the
DSSS system's performance in the presence of
noise is better than the FHSS system.
In Direct Sequence Spread Spectrum , signals
are challenging to detect.
It provides the best discrimination against
multipath signals.
In DSSS, there are very few chances of jamming
because it avoids intentional interference such as
jamming effectively.

77. Sridhar Iyer IIT Bombay 77
Disadvantages of Direct Sequence
Spread Spectrum (DSSS)
DSSS system takes longer time; that's why its
performance is slow.
It requires wide-band channels with small phase
distortion.
In DSSS, the pseudo-noise generator generates
a sequence at high rates.
Processing Gain is lower than DSSS.
Channel Bandwidth is less than FHSS.

78. Sridhar Iyer IIT Bombay 78
FSSS
The Frequency Hopping Spread Spectrum or FHSS allows
us to utilize bandwidth properly and maximum.
In this technique, the whole available bandwidth is divided
into many channels and spread between channels, arranged
continuously.
The frequency slots are selected randomly, and frequency
signals are transmitted according to their occupancy.
The transmitters and receivers keep on hopping on channels
available for a particular amount of time in milliseconds.
So, you can see that it implements the frequency division
multiplexing and time-division multiplexing simultaneously in
FHSS.

79. Sridhar Iyer IIT Bombay 79
Frequency Hopping Spread
Spectrum (FHSS)
In Frequency Hopping Spread Spectrum (FHSS), different
carrier frequencies are modulated by the source signal i.e. M
carrier frequencies are modulated by the signal.
At one moment signal modulates one carrier frequency and
at the subsequent moments, it modulates other carrier
frequencies.
A pseudorandom code generator generates Pseudo-random
Noise of some pattern for each hopping period Th.
The frequency corresponding to the pattern is used for the
hopping period and is passed to the frequency synthesizer.
The synthesizer generates a carrier signal of that frequency.

80. Sridhar Iyer IIT Bombay 80
FSSS

81. Sridhar Iyer IIT Bombay 81
FSSS

82. Sridhar Iyer IIT Bombay 82
FHSS can also be classified into two
types:
Slow Hopping: In slow hopping, multiple bits are
transmitted on a specific frequency or same
frequency.
Fast Hopping: In fast hopping, individual bits are
split and then transmitted on different
frequencies.

83. Sridhar Iyer IIT Bombay 83
DSSS Transmit/Receive
X
user data
chipping
sequence
modulator
radio
carrier
spread
spectrum
signal
transmit
signal
transmitter
demodulator
received
signal
radio
carrier
X
chipping
sequence
lowpass
filtered
signal
receiver
integrator
products
decision
data
sampled
sums
correlator

84. Sridhar Iyer IIT Bombay 84
FHSS Transmit/Receive
modulator
user data
hopping
sequenc
e
modulator
narrowband
signal
spread
transmit
signal
transmitter
received
signal
receiver
demodulator
data
frequency
synthesizer
hopping
sequenc
e
demodulator
frequency
synthesizer
narrowband
signal

85. Sridhar Iyer IIT Bombay 85
CELLULAR SYSTEMS

86. Sridhar Iyer IIT Bombay 86
Do you know that the first ever smart phone was
released in November 1999 by NTT DoCoMo
in Japan?

87. Sridhar Iyer IIT Bombay 87
Cellular System Infrastructure
Early wireless systems had a high-power transmitter,
covering the entire service area.
This required a very huge amount of power and was not
suitable for many practical reasons.
Cellular network is an underlying technology for mobile
phones, personal communication systems, wireless
networking etc.
The technology is developed for mobile radio telephone
to replace high power transmitter/receiver systems.
Cellular networks use lower power, shorter range and
more transmitters for data transmission.

88. Sridhar Iyer IIT Bombay 88
Cellular System Infrastructure
 The cellular system
replaced a large zone with a
number of smaller
hexagonal cells with a
single BS (base station)
covering a fraction of the
area.
Evolution of such a cellular
system is shown in the
given figures, with all
wireless receivers located in
a cell being served by a BS.

89. Sridhar Iyer IIT Bombay 89
Infrastructure of cellular systems
In a cellular structure, a MS (mobile
station) needs to communicate with the
BS of the cell where the MS is currently
located and the BS acts as a gateway to
the rest of the world.
Therefore, to provide a link, the MS
needs to be in the area of one of the
cells (and hence a BS) so that mobility of
the MS can be supported.
Several mobile switching centers are
interconnected to a PSTN (public
switched telephone network) and the
ATM (asynchronous transfer mode)
backbone. To provide a better
perspective of wireless communication
technology, simplified system
infrastructure for cellular system is
shown in the figure:

90. Sridhar Iyer IIT Bombay 90
Infrastructure of cellular systems

91. Sridhar Iyer IIT Bombay 91
Infrastructure of cellular systems
A BS consists of a base transceiver system (BTS) and a
BSC. Both tower and antenna are a part of the BTS, while all
associated electronics are contained in the BSC.
The HLR (home location register) and VLR (visitor location
register) are two sets of pointers that support mobility and
enable the use of the same telephone numbers worldwide.
The AUC (authentication center) unit provides authentication
and encryption parameters that verify the user's identity and
ensure the confidentiality of each cell.
The EIR (equipment identity register) is a database that
information about identity of mobile equipment.
Both AUC and EIR can be implemented as individual stand-
alone units or as a combined AUC/EIR unit.

92. Sridhar Iyer IIT Bombay 92
Infrastructure of cellular systems
The HLR is located at the MSC where MS is initially
registered and is the initial home location for billing and
access information.
In simple words, any incoming call, based on the calling
number, is directed to the HLR of the home MS where
the MS is registered. The HLR then points to the VLR of
the MSC where the MS is currently located.
Bidirectional HLR-VLR pointers help in carrying out
various functionalities.

93. Sridhar Iyer IIT Bombay 93
Redirection of a call to MS

94. Sridhar Iyer IIT Bombay 94
Role of Cells in Wireless Coverage
Coverage Area: Each cell has a defined
coverage area, typically represented
by a hexagon in network diagrams,
within which it can effectively
communicate with mobile devices.
Base Stations: The cell tower or base
station at the center of each cell
facilitates this communication. It
houses antennas, transceivers, and
other necessary equipment to handle
the radio communications with devices
in its vicinity.
Scalability: As the demand for network
coverage and capacity increases,
cellular networks can scale by adding
more cells or by implementing smaller
cells in dense areas.
.

95. Sridhar Iyer IIT Bombay 95
Communication Handling
Radio Frequency (RF) Management: Base
stations use RF signals to communicate with
devices. Each station operates on specific
frequency bands and channels to manage this
communication effectively.
Load Balancing: To ensure smooth
communication, cells dynamically manage the
load. If a cell becomes overcrowded with too
many users, calls or data sessions might be
handed off to adjacent cells to maintain quality
service.

96. Sridhar Iyer IIT Bombay 96
Techniques for sharing mobile-to-
base station radio spectrum
There is a certain radio spectrum that is allocated to the
base station and to a particular region and that now needs
to be shared. There are two techniques for sharing mobile-
to-base station radio spectrum:
Combined FDMA/TDMA: It divides the spectrum into
frequency channels and divides each channel into time
slots.
Code Division Multiple Access (CDMA): It allows the
reuse of the same spectrum over all cells. Net capacity
improvement.

97. Sridhar Iyer IIT Bombay 97
Cell Splitting
Managing Network Traffic: As the number of
users increases, networks employ cell splitting to
handle the traffic efficiently. This process involves
subdividing a large cell into several smaller cells.
Improving Coverage: Cell splitting not only
manages network traffic but also enhances
coverage, especially in areas with high user
density. Each smaller cell uses a portion of the
frequency spectrum allocated to the original cell,
thereby increasing the overall capacity of the
network.

98. Sridhar Iyer IIT Bombay 98
Shape of Cells
The coverage area of cellular networks are
divided into cells, each cell having its own
antenna for transmitting the signals.
Each cell has its own frequencies. Data
communication in cellular networks is served by
its base station transmitter, receiver and its
control unit.
The shape of cells can be either square or
hexagon −

99. Sridhar Iyer IIT Bombay 99
Square
A square cell has four neighbors at
distance d and four at distance Root 2
d.
Simplifies choosing and switching to
new antenna.
Hexagon
A hexagon cell shape is highly
recommended for its easy coverage
and calculations. It offers the following
advantages:-
Provides equidistant antennas
Distance from center to vertex equals
length of side
Shape of Cells
.

100. Sridhar Iyer IIT Bombay 100
Frequency Reuse
Using the same radio frequencies within a given area, that are separated
considerable distance, with minimal interference, to establish
communication.
Frequency reuse offers the following benefits −
Allows communications within cell on a given frequency
Limits escaping power to adjacent cells
Allows re-use of frequencies in nearby cells
10 to 50 frequencies per cell
For example, when N cells are using the same number of frequencies and
K be the total number of frequencies used in systems. Then each cell
frequency is calculated by using the formulae K/N.
In Advanced Mobile Phone Services (AMPS) when K = 395 and N = 7,
then frequencies per cell on an average will be 395/7 = 56. Here, cell
frequency is 56.

101. Sridhar Iyer IIT Bombay 101
Cell Cluster:
Considering a cellular system that has a total of S duplex
radio channels. If each cell is allocated a group of k
channels (k < S) and if the S channels are divided
among N cellsinto unique and disjoint channel groups of
same number of channels, then,
S = kN. 6.1
The N cells that collectively use the complete set of
available frequencies is called a cluster. If a cluster is
replicated M times within the system, the total number
of duplex channels or capacity,
C = M kN = MS. 6.2

102. Sridhar Iyer IIT Bombay 102
Cell Cluster:
In this example, The cluster size N = 7 and
the frequency reuse factor is 1/7 since each
cell contains one-seventh of the total
number of available channels.
The capacity is directly proportional to M.
The factor N is called the cluster size and is
typically 4, 7 or 12. If the cluster size N is
reduced while the cell size is kept constant,
more clusters are required to cover a given
area and hence more capacity is achieved
from the design viewpoint, the smallest
possible value of N is desirable to
maximize capacity over a given coverage
area. The frequency reuse factor of a
cellular system is 1/N,since each cell within
a cluster is assigned 1/N of the total
available channels in the system.
.

103. Sridhar Iyer IIT Bombay 103
Channel assignment strategies in
cellular system
 To allocate the available channels to the cells in a cellular
system Whenever a user wants to make a call request then
by using channel assignment strategies their requests are
fulfilled.
In CAS, there is efficient use of frequencies, time slots and
bandwidth.
Allocation of channels to cells in a cellular network.
Once the channels are allocated, cells may then allow users
within the cell to communicate via the available channels.
Channels in a wireless communication system typically
consist of time slots, frequency bands and/or CDMA pseudo
noise sequences.

104. Sridhar Iyer IIT Bombay 104
Types of channel allocations
There are three main types of channel assignment
strategies:
Fixed channel assignment (FCA)
Dynamic channel assignment (DCA)
Hybrid channel assignment (HCA)

105. Sridhar Iyer IIT Bombay 105
Fixed channel assignment (FCA)
In fixed channel assignment, each cell is allocated a
fixed or predetermined set of channels (voice channels).
If all channels in a cell are occupied, the call from a
mobile user is blocked and the user won’t receive
service.
Once the channels are allocated to the specific cells
then they cannot be changed.
Cells in this strategy are allowed to borrow channels
from adjacent cells if their channels are fully occupied
while adjacent cells have free channels.
No interference occurs by moving the channel from one
cell to another.

106. Sridhar Iyer IIT Bombay 106
Dynamic channel assignment (DCA)
In DCA, channels are assigned to cells on demand,
based on the current traffic load. i.e., there is no allocation
of predetermined set of channels (voice channels).
This strategy is more efficient than FCA, but it is also
more complex to implement.
First, DCA methods typically have a degree of
randomness associated with them and this leads to the
fact that frequency reuse is often not maximized.
Secondly, DCA methods often involve complex algorithm
that can be very computationally intensive and may
require large computing resources in order to be real-
time.

107. Sridhar Iyer IIT Bombay 107
Dynamic channel assignment (DCA)
Dynamic Channel Assignment is a strategy in which
channels are not permanently allocated to the cells
MSC assigns a channel only if it is not used and if it
will not cause co channel interference with any cell
‐
in range.
In this MSC has to collect real time data on channel
occupancy, traffic distribution, radio signal strength
indication of all channels on continuous basis
Thus increasing the computational load on MSC.

108. Sridhar Iyer IIT Bombay 108
Hybrid channel assignment (HCA)
HCA is a combination of FCA and DCA.
In HCA, each cell is allocated a fixed set of channels, but
additional channels can be dynamically assigned to cells if
needed.
This strategy offers a good balance between simplicity and
efficiency.
Hybrid Channel Allocation is a combination of both Fixed Channel
Allocation (FCA) and Dynamic Channel Allocation (DCA).
In this total number of channels or voice channels are divided into
fixed and dynamic sets.
If a user make a call then first fixed set of channels are utilized but
if all the fixed sets are busy then dynamic sets are used.

109. Sridhar Iyer IIT Bombay 109
Channel Borrowing
Channel Borrowing is one of the most straightforward hybrid allocation
schemes.
Here, channels are assigned to cells just as in fixed allocation schemes.
If a cell needs a channel in excess of the channels previously assigned to it,
that cell may borrow a channel from one of its neighboring cells given that a
channel is available and use of this channel won't violate frequency reuse
requirements.
Note that since every channel has a predetermined relationship with a specific
cell, channel borrowing is often categorized as a subclass of fixed allocation
schemes.
The major problem with channel borrowing is
 When a cell borrows a channel from a neighboring cell, other nearby cells are
prohibited from using the borrowed channel because of co-channel
interference. This can lead to increased call blocking over time.
 To reduce this call blocking penalty, algorithms are necessary to ensure that
the channels are borrowed from the most available neighboring cells; i.e., the
neighboring cells with the most unassigned channels.

110. Sridhar Iyer IIT Bombay 110
Extensions of the channel borrowing
approach
Two extensions of the channel borrowing approach are
Borrowing with Channel Ordering (BCO)
Borrowing with Directional Channel Locking (BDCL).
Borrowing with Channel Locking
BCO systems have two distinctive characteristics [Elnoubi]:
The ratio of fixed to dynamic channels varies with traffic load.
Nominal channels are ordered such that the first nominal
channel of a cell has the highest priority of being applied to a
call within the cell.

111. Sridhar Iyer IIT Bombay 111
Borrowing with Directional Channel
Locking(BDCL)
Borrowing with Directional Channel Locking
In BDCL, borrowed channels are only locked in nearby cells
that are affected by the borrowing.
This differs from the BCO scheme in which a borrowed
channel is locked in every cell within the reuse distance.
The benefit of BDCL is that more channels are available in the
presence of borrowing and subsequent call blocking is
reduced.
A disadvantage of BDCL is that the statement "borrowed
channels are only locked in nearby cells that are affected by
the borrowing" requires a clear understanding of the term
"affected."

112. Sridhar Iyer IIT Bombay 112
Advantages of Cellular Networks
Mobile and fixed users can connect using it. Voice and
data services also provided.
Has increased capacity & easy to maintain.
Easy to upgrade the equipment & has consumes less
power.
It is used in place where cables can not be laid out
because of its wireless existence.
To use the features & functions of mainly all private and
public networks.
Can be distributed to the larger coverage of areas.

113. Sridhar Iyer IIT Bombay 113
Disadvantages of Cellular Networks
It provides a lower data rate than wired networks like fiber
optics and DSL. The data rate changes depending on
wireless technologies like GSM, CDMA, LTE, etc.
Macrophage cells are impacted by multipath signal loss.
To service customers, there is a limited capacity that
depends on the channels and different access techniques.
Due to the wireless nature of the connection, security
issues exist.
For the construction of antennas for cellular networks, a
foundation tower and space are required. It takes a lot of
time and labor to do this.

114. Sridhar Iyer IIT Bombay 114
MULTIPLE ACCESS TECHNIQUES

115. Sridhar Iyer IIT Bombay 115
Multiple Access Techniques for
Wireless Communications
Multiple access schemes are used to allow
many mobile users to share simultaneously a
finite amount of radio spectrum.
The sharing of spectrum is required to achieve
high capacity by simultaneously allocating the
available bandwidth (or the available amount of
channels) to multiple users.

116. Sridhar Iyer IIT Bombay 116
MULTIPLE ACCESS TECHNIQUES
Multiple access techniques are used to allow a large
number of mobile users to share the
allocated spectrum in the most efficient manner.


As the spectrum is limited, so the sharing is required
to increase the capacity of cell or over a geographical
area by allowing the available bandwidth to be used
at the same time by different users.


And this must be done in a way such that the quality
of service doesn’t degrade within the existing users.

117. Sridhar Iyer IIT Bombay 117
DUPLEXING
In wireless communications systems, it is often
desirable to allow the subscriber to send
simultaneously information to the base station
while receiving information from the base
station, called duplexing.
Duplexing may be done using frequency or time
domain techniques:
Frequency division duplexing (FDD)
Time division duplexing (TDD)

118. Sridhar Iyer IIT Bombay 118
Frequency division duplexing (FDD)
To provides two distinct bands of frequencies for
every user.
In FDD, any duplex channel actually consists of
two simplex channels.
The forward band provides traffic from the base
station to the mobile, and the reverse band
provides traffic from the mobile to the base station.
Duplexer is used inside each subscriber unit and
base station, to allow simultaneous bidirectional
radio transmission and reception for both the
subscriber unit and the base station on the duplex
channel pair.

119. Sridhar Iyer IIT Bombay 119
Time division duplexing (TDD)
Uses time instead of frequency to provide both a forward
and reverse link.
In TDD, multiple users share a single radio channel by
taking turns in the time domain.
Individual users are allowed to access the channel in
assigned time slots, and each duplex channel has both a
forward time slot and a reverse time slot to facilitate
bidirectional Communication.
If the time separation between the forward and reverse
time slot is small, then the transmission and reception of
data appears simultaneous to the users at both the
subscriber unit and on the base station side.

120. Sridhar Iyer IIT Bombay 120
FDD AND TDD
Figure illustrates FDD and TDD techniques. TDD allows
communication on a single channel (as opposed to
requiring two separate simplex or dedicated channels)
and simplifies the subscriber equipment since a
duplexer is not required.

121. Sridhar Iyer IIT Bombay 121
Introduction to Multiple Access
The main aim in the cellular system design is to be
able to increase the capacity of the channel i.e. to
handle as many calls as possible in a given
bandwidth with a sufficient level of quality of service.


There are several different ways to allow access to
the channel. These includes mainly the following:
1. Frequency division multiple-access (FDMA)
2. Time division multiple-access (TDMA)
3. Code division multiple-access (CDMA)

122. Sridhar Iyer IIT Bombay 122
Frequency Division Multiple Access
(FDMA)
Frequency division multiple access (FDMA) assigns
individual channels to individual users.
In FDD systems, the users are assigned a channel as a pair
of frequencies; one frequency is used for the forward
channel, while the other frequency is used for the reverse
channel. The features of FDMA are as follows:


The FDMA channel carries only one phone circuit at a time.


If an FDMA channel is not in use, then it sits idle and
cannot be used by other users to increase or share
capacity.
It is essentially a wasted resource.

123. Sridhar Iyer IIT Bombay 123
It can be seen from Figure 2 that each user is allocated a unique frequency
band or channel. These channels are assigned on demand to users who
request service.
During the period of the call, no other user can share the same channel.

124. Sridhar Iyer IIT Bombay 124
FDMA
After the assignment of a voice channel, the base station and
the mobile transmit simultaneously and continuously.


The bandwidths of FDMA channels are relatively narrow (30
kHz in AMPS) as each channel supports only one circuit per
carrier. That is, FDMA is usually implemented in narrowband
systems.


The symbol time of a narrowband signal is large as compared
to the average delay spread. This implies that the amount of
intersymbol interference is low and, thus, little or no
equalization is required in FDMA narrowband systems.


The complexity of FDMA mobile systems is lower when
compared to TDMA systems,though this is changing as digital
signal processing methods improve for TDMA.

125. Sridhar Iyer IIT Bombay 125
FDMA
Since FDMA is a continuous transmission scheme, fewer
bits are needed for overhead purposes (such as
synchronization and framing bits) as compared to TDMA.
FDMA systems have higher cell site system costs as
compared to TDMA systems, because of the single channel
per carrier design, and the need to use costly bandpass
filters to eliminate spurious radiation at the base station.
The FDMA mobile unit uses duplexers since both the
transmitter and receiver operate at the same time. This
results in an increase in the cost of FDMA subscriber units
and base stations.
FDMA requires tight RF filtering to minimize adjacent
channel interference.

126. Sridhar Iyer IIT Bombay 126
Nonlinear Effects in FDMA
In a FDMA system, many channels share the same antenna at the
base station.
The power amplifiers or the power combiners, when operated at or
near saturation for maximum power efficiency, are nonlinear.
The nonlinearities cause signal spreading in the frequency domain
and generate intermodulation (IM) frequencies.
IM is undesired RF radiation which can interfere with other channels
in the FDMA systems.
Spreading of the spectrum results in adjacent-channel interference.
Intermodulation is the generation of undesirable harmonics.
Harmonics generated outside the mobile radio band cause
interference to adjacent services, while those present inside the
band cause interference to other users in the wireless system .

127. Sridhar Iyer IIT Bombay 127
Advanced Mobile Phone System
(AMPS)
The first US analog cellular system, the Advanced Mobile Phone
System (AMPS), is based on FDMA/FDD.
A single user occupies a single channel while the call is in
progress, and the single channel is actually two simplex channels
which are frequency duplexed with a 45 MHz split.
When a call is completed, or when a handoff occurs, the channel is
vacated so that another mobile subscriber may use it. Multiple or
simultaneous users are accommodated in AMPS by giving each
user a unique channel.
Voice signals are sent on the forward channel from the base
station to mobile unit, and on the reverse channel from the mobile
unit to the base station.
In AMPS, analog narrowband frequency modulation (NBFM) is
used to modulate the carrier.

128. Sridhar Iyer IIT Bombay 128
TIME DIVISION MULTIPLE ACCESS
In digital systems, continuous transmission
is not required because users do not use
the allotted bandwidth all the time.


In such cases, TDMA is a
complimentary access technique to
FDMA. Global Systems for Mobile
communications (GSM) uses the TDMA
technique.


In TDMA, the entire bandwidth is
available to the user but only for a finite
period of time. In most cases the
available bandwidth is divided into fewer
channels compared to FDMA.


The users are allotted time slots during
which they have the entire channel
bandwidth at their disposal, as shown in
Figure

129. Sridhar Iyer IIT Bombay 129
TIME DIVISION MULTIPLE ACCESS
TDMA shares a single carrier frequency with several users
where each users makes use of non-overlapping time
slots.
Data transmission in TDMA is not continuous, but occurs
in bursts. Hence handsoff process is simpler.
TDMA uses different time slots for transmission and
reception thus duplexers are not required.
TDMA has an advantage that is possible to allocate
different numbers of time slots per frame to different users.
Bandwidth can be supplied on demand to different users
by concatenating or reassigning time slot based on
priority.

130. Sridhar Iyer IIT Bombay 130
Buffer-and-Burst method
Systems transmit data in a buffer-and-burst method, thus
the transmission for any user is noncontinuous.
This implies that, unlike in FDMA systems which
accommodate analog FM, digital data and digital
modulation must be used with TDMA.
The transmission from various users is interlaced into a
repeating frame structure.
It can be seen that a frame consists of a number of slots.
Each frame is made up of a preamble, an information
message, and tail bits.
In TDMA/ TDD, half of the time slots in the frame
information message would be used for the forward link
channels and half would be used for reverse link channels.

131. Sridhar Iyer IIT Bombay 131

132. Sridhar Iyer IIT Bombay 132
Buffer-and-Burst method
In TDMA/FDD systems, an identical or similar frame
structure would be used solely for either forward or reverse
transmission, but the carrier frequencies would be different
for the forward and reverse links.
In general, TDMA/FDD systems intentionally induce
several time slots of delay between the forward and
reverse time slots for a particular user, so that duplexers
are not required in the subscriber unit.
In a TDMA frame, the preamble contains the address and
synchronization information that both the base station and
the subscribers use to identify each other.
Guard times are utilized to allow synchronization of the
receivers between different slots and frames.

133. Sridhar Iyer IIT Bombay 133
The features of TDMA
TDMA shares a single carrier frequency with several users
where each users makes use of non overlapping time slots.
The number of time slots per frame depends on several
factors such as modulation technique, available bandwidth
etc.
Data transmission in TDMA is not continuous but occurs in
bursts. This results in low battery consumption since the
subscriber transmitter can be turned OFF when not in use.
TDMA uses different time slots for transmission and
reception thus duplexers are not required.


TDMA has an advantage that is possible to allocate
different numbers of time slots per frame to different users.

134. Sridhar Iyer IIT Bombay 134
CODE DIVISION MULTIPLE
ACCESS
Code division multiple access technique is an example of multiple
access where several transmitters use a single channel to
send information simultaneously. Its features are as follows.
In CDMA every user uses the full available spectrum instead of
getting allotted by separate frequency.
CDMA is much recommended for voice and data
communications.
While multiple codes occupy the same channel in CDMA, the
users having same code can communicate with each other.
CDMA offers more air-space capacity than TDMA.
The hands-off between base stations is very well handled by
CDMA.

135. Sridhar Iyer IIT Bombay 135
CODE DIVISION MULTIPLE
ACCESS


In CDMA, the same bandwidth is occupied by all
the users, however they are all assigned separate
codes, which differentiates them from each other
shown in Figure


CDMA utilize a spread spectrum technique in which
a spreading signal (which is uncorrelated to the
signal and has a large bandwidth) is used to spread
the narrow band message signal.
In code division multiple access (CDMA) systems, the
narrowband message signal is multiplied by a very
large bandwidth signal called the spreading signal.

136. Sridhar Iyer IIT Bombay 136
CODE DIVISION MULTIPLE
ACCESS
All users in a CDMA
system, as seen from
Figure 5, use the same
carrier frequency and
may transmit
simultaneously.
Each user has its own
pseudorandom codeword
which is approximately
orthogonal to all other
codewords.
.

137. Sridhar Iyer IIT Bombay 137
CODE DIVISION MULTIPLE
ACCESS
In CDMA, the power of multiple users at a receiver
determines the noise floor after de-correlation.
If the power of each user within a cell is not
controlled such that they do not appear equal at
the base station receiver, then the near-far
problem occurs.
The near-far problem occurs when many mobile
users share the same channel. In general,the
strongest received mobile signal will capture the
demodulator at a base station.

138. Sridhar Iyer IIT Bombay 138
CODE DIVISION MULTIPLE
ACCESS
In CDMA,stronger received signal levels raise the noise
floor at the base station demodulators for the weaker
signals, thereby decreasing the probability that weaker
signals will be received.
To combat the near-far problem, power control is used in
most CDMA implementations. Power control is provided
by each base station in a cellular system and assures
that each mobile within the base station coverage area
provides the same signal level to the base station
receiver.
This solves the problem of a nearby subscriber
overpowering the base station receiver and drowning out
the signals of far away subscribers.

139. Sridhar Iyer IIT Bombay 139
The features of CDMA
Many users of a CDMA system share the same
frequency. Either TDD or FDD may be used.
Unlike TDMA or FDMA, CDMA has a soft capacity limit.
Multipath fading may be substantially reduced because
the signal is spread over a large spectrum.
Channel data rates are very high in CDMA systems.
Self-jamming is a problem in CDMA system. Self-
jamming arises from the fact that the spreading
sequences of different users are not exactly orthogonal.

140. Sridhar Iyer IIT Bombay 140

141. Sridhar Iyer IIT Bombay 141
Direct Sequence Spread Spectrum
(DS-SS)
This is the most commonly used technology for CDMA. In
DS-SS, the message signal is multiplied by a Pseudo
Random Noise Code.
Each user is given his own codeword which is orthogonal to
the codes of other users and in order to detect the user,
the receiver must know the codeword used by the
transmitter.
There are, however, two problems in such systems which
are discussed in the sequel.
CDMA and Self-interference Problem
CDMA and Near-Far Problem

142. Sridhar Iyer IIT Bombay 142
CDMA and Self-interference Problem


In CDMA, self-interference arises from the presence
of delayed replicas of signal due to multipath.


The delays cause the spreading sequences of the
different users to lose their orthogonality, as by
design they are orthogonal only at zero phase offset.


Hence in despreading a given user’s waveform,
nonzero contributions to that user’s signal arise from
the transmissions of the other users in the network.

143. Sridhar Iyer IIT Bombay 143
CDMA and Near-Far Problem


The near-far problem is a serious one in CDMA. This
problem arises from the fact that signals closer to the
receiver of interest are received with smaller attenuation
than are signals located further away.


Therefore the strong signal from the nearby transmitter will
mask the weak signal from the remote transmitter.


In TDMA and FDMA, this is not a problem since mutual
interference can be filtered.In CDMA, however, the near-far
effect combined with imperfect orthogonality between
codes (e.g. due to different time sifts), leads to substantial
interference.


Accurate and fast power control appears essential to
ensure reliable operation of multiuser DS-CDMA systems.

144. Sridhar Iyer IIT Bombay 144
Basic idea of these approaches can be explained in simple terms using
the cocktail party theory. In a cocktail party people talk to each other
using one of the following modes:
FDMA: When all the people group in widely separated areas and talk
within each group.
TDMA: When all the people are in the middle of the room, but they
take turn in speaking.
CDMA: When all the people are in the middle of the room, but
different pairs speak in different languages.

145. Sridhar Iyer IIT Bombay 145
Space Division Multiple Access (SDMA)
Space division multiple access or spatial division multiple
access is a technique which is MIMO (multiple-input multiple-
output) architecture and used mostly in wireless and satellite
communication. It has the following features:
All users can communicate at the same time using the same
channel.
SDMA is completely free from interference.
A single satellite can communicate with more satellites
receivers of the same frequency.
The directional spot-beam antennas are used and hence the
base station in SDMA, can track a moving user.
Controls the radiated energy for each user in space.

146. Sridhar Iyer IIT Bombay 146
Space Division Multiple Access
(SDMA)
Space division multiple access (SDMA) controls the radiated
energy for each user in space.
It can be seen from Figure that SDMA serves different users by
using spot beam antennas.
These different areas covered by the antenna beam may be
served by the same frequency (in a TDMA or CDMA system) or
different frequencies (in an FDMA system).
Sectorized antennas may be thought of as a primitive
application of SDMA.
In the future, adaptive antennas will likely be used to
simultaneously steer energy in the direction of many users at
once and appear to be best suited for TDMA and CDMA base
station architectures.

147. Sridhar Iyer IIT Bombay 147

148. Sridhar Iyer IIT Bombay 148
Space Division Multiple Access
(SDMA)
Adaptive antennas used at the base station (and eventually at the
subscriber units) promise to mitigate some of the problems on the
reverse link.
In the limiting case of infinitesimal beamwidth and infinitely fast
tracking ability, adaptive antennas implement optimal SDMA, thereby
providing a unique channel that is free from the interference of all
other users in the cell.
With SDMA, all users within the system would be able to
communicate at the same time using the same channel.
In addition, a perfect adaptive antenna system would be able to track
individual multipath components for each user and combine them in
an optimal manner to collect all of the available signal energy from
each user.
The perfect adaptive antenna system is not feasible since it requires
infinitely large antennas.

149. Sridhar Iyer IIT Bombay 149
Space Division Multiple Access
(SDMA)
The reverse link presents the most difficulty in cellular systems for
several reasons.
First, the base station has complete control over the power of all the
transmitted signals on the forward link.
However, because of different radio propagation paths between each
user and the base station, the transmitted power from each
subscriber unit must be dynamically controlled to prevent any single
user from driving up the interference level for all other users.
Second, transmit power is limited by battery consumption at the
subscriber unit, therefore there are limits on the degree to which
power may be controlled on the reverse link.
If the base station antenna is made to spatially filter each desired user
so that more energy is detected from each subscriber, then the
reverse link for each user is improved and less power is required.

150. Sridhar Iyer IIT Bombay 150
ALOHA

151. Sridhar Iyer IIT Bombay 151
ALOHA
ALOHAnet, also known as the ALOHA System,
or simply ALOHA, was a pioneering computer
networking system developed at the University
of Hawaii in the early 1970s for packet radio
networks.
An effective solution to provide for wireless
access to computer systems.
However, it can be used in any situation where
multiple devices share a common
communication channel.

152. Sridhar Iyer IIT Bombay 152
ALOHA
In its simplest form, later known as Pure ALOHA, remote units
communicated with a base station (Menehune) over two
separate radio frequencies (for inbound and outbound
respectively).
This is known as a random access technique, and it is
asynchronous because there is no coordination between
devices.
 When multiple devices attempt to transmit data at the same
time, it can result in a collision, where the data becomes garbled.
In this case, each device will simply wait a random amount of
time before attempting to transmit again.
The basic concept of the ALOHA protocol can be applied to any
system where uncoordinated users are competing for the use of
a shared channel.

153. Sridhar Iyer IIT Bombay 153
ALOHA
Nodes did not wait for the channel to be clear before
sending, but instead waited for acknowledgement of
successful receipt of a message, and re-sent it if this was
not received.
Nodes would also stop and re-transmit data if they detected
any other messages while transmitting. While simple to
implement, this results in an efficiency of only 18.4%.
The Aloha network introduced the mechanism of
randomized multiple access, which resolved device
transmission collisions by transmitting a package
immediately if no acknowledgement is present, and if no
acknowledgment was received, the transmission was
repeated after a random waiting time.

154. Sridhar Iyer IIT Bombay 154
NOTE:
The probability distribution of this random waiting
time for retransmission of a package that has not
been acknowledged as received is critically
important for the stability of Aloha-type
communication systems.
The average waiting time for retransmission is
typically shorter than the average time for
generation of a new package from the same client
node, but it should not be allowed to be so short
as to compromise the stability of the network,
causing a collapse in its overall throughput.

155. Sridhar Iyer IIT Bombay 155
Aloha Rules
Any station can transmit data to a channel at
any time.
It does not require any carrier sensing.
Collision and data frames may be lost during
the transmission of data through multiple
stations.
Acknowledgment of the frames exists in Aloha.
Hence, there is no collision detection.
It requires retransmission of data after some
random amount of time.

156. Sridhar Iyer IIT Bombay 156
Types of Aloha

157. Sridhar Iyer IIT Bombay 157
Pure ALOHA
Pure ALOHA refers to the original ALOHA protocol.
The idea is that each station sends a frame whenever one is
available.
Because there is only one channel to share, there is a chance
that frames from different stations will collide.
The pure ALOHA protocol utilizes acknowledgments from the
receiver to ensure successful transmission.
When a user sends a frame, it expects confirmation from the
receiver.
If no acknowledgment is received within a designated time
period, the sender assumes that the frame was not received
and retransmits the frame.

158. Sridhar Iyer IIT Bombay 158
Pure ALOHA(IMPLEMENTATION)
The original version of the protocol (now called Pure
ALOHA, and the one implemented in ALOHAnet)
was quite simple:
If you have data to send, send the data
If, while you are transmitting data, you receive any
data from another station, there has been a
message collision.
All transmitting stations will need to try resending
later.

159. Sridhar Iyer IIT Bombay 159
Pure ALOHA(ASSUMPTIONS)
To assess Pure ALOHA, there is a need to predict its
throughput, the rate of (successful) transmission of frames.
First make a few simplifying assumptions:
All frames have the same length.
Stations cannot generate a frame while transmitting or trying to
transmit. That is, while a station is sending or trying to resend a
frame, it cannot be allowed to generate more frames to send.
The population of stations attempting to transmit (both new
transmission and retransmissions) follows a Poisson
distribution.

160. Sridhar Iyer IIT Bombay 160

161. Sridhar Iyer IIT Bombay 161
IN THE FIGURE
As we can see in the figure above, there are four stations for
accessing a shared channel and transmitting data frames.
Some frames collide because most stations send their frames
at the same time.
Only two frames, frame 1.1 and frame 2.2, are successfully
transmitted to the receiver end.
At the same time, other frames are lost or destroyed.
Whenever two frames fall on a shared channel simultaneously,
collisions can occur, and both will suffer damage.
If the new frame's first bit enters the channel before finishing
the last bit of the second frame.
Both frames are completely finished, and both stations must
retransmit the data frame.

162. Sridhar Iyer IIT Bombay 162
Pure ALOHA(COLLISION)
When two frames attempt to occupy the channel
simultaneously, a collision occurs and both frames
become garbled.
If the first bit of a new frame overlaps with the last bit of a
frame that is almost finished, both frames will be
completely destroyed and will need to be retransmitted.
If all users retransmit their frames at the same time after
a time-out, the frames will collide again.
To prevent this, the pure ALOHA protocol dictates that
each user waits a random amount of time, known as the
back-off time, before retransmitting the frame.
This randomness helps to avoid further collisions.

163. Sridhar Iyer IIT Bombay 163
Pure ALOHA

164. Sridhar Iyer IIT Bombay 164
SLOTTED ALOHA

165. Sridhar Iyer IIT Bombay 165
Slotted ALOHA
Slotted ALOHA was
introduced in 1972 by
Robert as an
improvement over
pure ALOHA.
Here, time is divided
into discrete intervals
called slots,
corresponding to a
frame.

166. Sridhar Iyer IIT Bombay 166
SLOTTED ALOHA
Pure Aloha has a very high chance of hitting a frame, hence
the slotted Aloha is intended to outperform its efficiency.
In slotted Aloha, the shared channel is split into fixed time
intervals called slots.
As a result, if a station wants to send a frame to a shared
channel, it can only do so at the start of the slot, and only
one frame can be sent to each slot.
Additionally, the station must wait until the beginning of the
slot for the subsequent transmission if it is unable to
transfer data at the beginning of the slot.
However, sending a frame at the start of two or more station
time slots still carries the risk of a collision.

167. Sridhar Iyer IIT Bombay 167

168. Sridhar Iyer IIT Bombay 168
FIGURE:
The figure shows how the channel is divided
into slots.
A station can start its transmission only at the
start of the slot.
So the only possible condition for collision is if
two or more stations start transmission in the
same slot.
This condition is shown in Frame 1.2 of Station
1 and Frame 4.1 of Station 4.

169. Sridhar Iyer IIT Bombay 169
Vulnerable Time for Slotted Aloha
The Vulnerable Time
in the case of Slotted
Aloha is equal to the
transmission time of
the station.
The figure given
below shows the
Vulnerable time for
Slotted Aloha.
.
It is because we bound the transmission of
stations with the slots.

170. Sridhar Iyer IIT Bombay 170
Protocol Flow Chart for ALOHA

171. Sridhar Iyer IIT Bombay 171
Throughput of Slotted ALOHA:
The quantity of successful transmissions at each time slot
determines the throughput of the Slotted Aloha protocol.
The Slotted Aloha protocol has a maximum throughput of
about 18.4%.
This is because there is a significant risk of collisions when
numerous nodes try to transmit at the same time, which causes
missed packets and a decreased overall throughput.
When less than or equal to 37% of the network’s total nodes
are actively transmitting data, the maximum throughput is
reached.
Due to the high frequency of collisions, the throughput of
Slotted Aloha is typically substantially lower than 18.4% in
practice

172. Sridhar Iyer IIT Bombay 172
Throughput of Slotted ALOHA:
The maximum throughput of a slotted ALOHA channel is
given by the formula:
Throughput (S) = G x e-
G
The maximum Throughput occurs at G = 1,
i.e. S = 1/e = 0.368
Where:
G = the offered load (or the number of packets being
transmitted per time slot). The offered load is a measure
of the number of nodes attempting to transmit in a given
time slot.

173. Sridhar Iyer IIT Bombay 173
Comparison of the throughput as a function of offered load
for Pure and Slotted ALOHA

174. Sridhar Iyer IIT Bombay 174
The throughput is a function of the offered load and it ranges
from 0 to 1.
As the offered load increases, the throughput decreases as
more collisions occur, resulting in less successful
transmissions.
The maximum throughput is achieved when the offered load is
equal to 0.37 and it is approximately 0.184.
It is important to note that the above equation assumes that all
the packets are of the same length and that the channel is
error-free.
In practice, the throughput is usually much lower than this due
to a number of factors such as packet errors, channel noise,
and the overhead of retransmissions.
Throughtput of slotted aloha

175. Sridhar Iyer IIT Bombay 175
Assumption of Slotted ALOHA:
All frames are of the same size.
Time is divided into equal-sized slots, a slot
equals the time to transmit one frame
Nodes start to transmit frames only at beginning
of slots.
Nodes are synchronized.
If two or more nodes transmit in a slot, all nodes
detect collision before the slot ends.

176. Sridhar Iyer IIT Bombay 176
Advantages of Slotted ALOHA:
Simplicity: The Slotted Aloha protocol is relatively
simple to implement and understand, making it an
easy option for low-complexity networks.
Flexibility: Slotted Aloha can be used in a wide
range of network environments, including those
with varying numbers of nodes and varying traffic
loads
Low overhead: Slotted Aloha does not require
complex management or control mechanisms,
which can help to reduce the overhead and
complexity of the network.

177. Sridhar Iyer IIT Bombay 177
Disadvantages of Slotted ALOHA:
Low throughput: The maximum throughput of the
Slotted Aloha protocol is relatively low at around
18.4%, which can be limiting for high-bandwidth
applications.
High collision rate: The high collision rate in slotted
ALOHA can result in a high packet loss rate, which
can negatively impact the overall performance of
the network.
Inefficiency: The protocol is inefficient at high loads,
as the efficiency decreases as the number of
nodes attempting to transmit increases.

178. Sridhar Iyer IIT Bombay 178
Pure aloha v/s slotted aloha

179. Sridhar Iyer IIT Bombay 179
CDMA
 It is a method of channel access and is an example of multiple
access as well.
Multiple access states that data can be transmitted simultaneously
to a single communication channel through several transmitters.
CDMA (Code-Division Multiple Access) refers to any of several
protocols used in second-generation (2G) and third-generation
(3G) wireless communications.
As the term implies, CDMA is a form of multiplexing, which allows
numerous signals to occupy a single transmission channel,
optimizing the use of available bandwidth.
The technology is used in ultra-high-frequency (UHF) cellular
phone systems in the 800 megahertz (MHz) and 1.9 gigahertz
(GHz) bands.

180. Sridhar Iyer IIT Bombay 180
CDMA

181. Sridhar Iyer IIT Bombay 181
CDMA
CDMA employs analog-to-digital conversion (ADC) in
combination with spread spectrum technology.
Audio input is first digitized into binary elements. The
frequency of the transmitted signal is then made to vary
according to a defined pattern code.
This enables the signal to be intercepted only by a
receiver whose frequency response is programmed with
the same code, following along with the transmitter
frequency.
There are trillions of possible frequency sequencing
codes, which enhances privacy and makes cloning
difficult.

182. Sridhar Iyer IIT Bombay 182
Features of CDMA
At a defined time, it enables more users to communicate and thus
offers enhanced capacity for voice and data communication.
Many of the channels in CDMA use a complete spectrum.
To reduce interference & noise and thereby increase the
efficiency of the network, CDMA systems make use of power
control.
To protect its signals, CDMA encodes user transmissions into
separate and special codes.
The same frequency can also be used by all cells in CDMA
systems.
There is also no fixed limit to the number of participants in a
CDMA system, but performance degrades with an increment in
the number of participants.

183. Sridhar Iyer IIT Bombay 183
How does CDMA work?
Cell clusters form the cellular structure of wireless
CDMA networks.
Each cell in a cell cluster has a transceiver with the
necessary transmitting power and mobile units
distributed around the cell's coverage area.
Every mobile unit runs a transceiver, which consists
of a low-power transmitter and a sensitive receiver
operating with a wireless cellular environment.
The characteristics of the cellular environment
include multipath propagation, access interference
and fading.

184. Sridhar Iyer IIT Bombay 184
How does CDMA work?
The near-far (N-F) effect plays a significant role in
the quality of service (QoS) for CDMA systems.
It refers to a phenomenon that occurs when a
user near the base station sends out a
transmission that interferes with and overpowers
a weaker transmission signal coming from a user
further away.
To this end, CDMA network providers use
receivers that are resistant to the N-F effect; they
also use tight power control schemes.

185. Sridhar Iyer IIT Bombay 185
How does CDMA work?
The CDMA channel is nominally 1.23 MHz wide.
CDMA networks use a scheme called soft handoff, which minimizes
signal breakup as a handset passes from one cell to another.
The combination of digital and spread spectrum modes supports
several times as many signals per unit of bandwidth as analog
modes.
CDMA is compatible with other cellular technologies; this enables
nationwide roaming.
The original CDMA standard, also known as CDMA One, offers a
transmission speed of only up to 14.4 kilobits per second in its
single channel form and up to 115 Kbps in an eight-channel form.
CDMA2000 and Wideband CDMA (W-CDMA) deliver data many
times faster.

186. Sridhar Iyer IIT Bombay 186
CDMA2000 standards:
The CDMA2000 family of standards include
single-carrier Radio Transmission
Technology (1xRTT),
Evolution-Data Optimized Release 0,
EVDO Revision A and EVDO Rev. B.
People often confuse CDMA2000, which is a
family of standards supported by Verizon and
Sprint, with CDMA, which is the physical layer
multiplexing scheme.

187. Sridhar Iyer IIT Bombay 187

188. Sridhar Iyer IIT Bombay 188
Difference between GSM and CDMA?

189. Sridhar Iyer IIT Bombay 189
Which is better: CDMA or GSM?
With GSM networks, users can transmit data and make voice calls at the
same time, an impossible feat for CDMA networks.
But this is hardly the reason behind GSM's popularity.
A big driver was Europe's 1987 law that required the use of GSM.
Another reason was that GSM resulted from an industry consortium,
while CDMA was, for the most part, owned by Qualcomm, making
GSM-powered devices cheaper to make and use.
CDMA and GSM standards apply only to 2G and 3G connectivity.
As the switch to fourth-generation wireless began in earnest in 2010,
carriers adopted Long-Term Evolution (LTE), the global standard for 4G
networks. Consequently, the distinction between CDMA and GSM is
becoming less important as CDMA phones and devices powered by
GSM networks vanish into history.
But, for now, 2G and 3G networks still serve as backups for areas with
weak 4G LTE signals.

190. Sridhar Iyer IIT Bombay 190
CSMA
CSMA/CD
CSMA/CA

191. Sridhar Iyer IIT Bombay 191

192. Sridhar Iyer IIT Bombay 192
CSMA (Carrier Sense Multiple
Access)
It is a carrier sense multiple access based on media access protocol to
sense the traffic on a channel (idle or busy) before transmitting the data.
It means that if the channel is idle, the station can send data to the
channel.
Otherwise, it must wait until the channel becomes idle. Hence, it
reduces the chances of a collision on a transmission medium.
Carrier Sense Multiple Access (CSMA) is a network protocol for carrier
transmission that operates in the Medium Access Control (MAC) layer.
 It senses or listens whether the shared channel for transmission is busy
or not, and transmits if the channel is not busy.
Using CMSA protocols, more than one users or nodes send and receive
data through a shared medium that may be a single cable or optical
fiber connecting multiple nodes, or a portion of the wireless spectrum.

193. Sridhar Iyer IIT Bombay 193
Working Principle
When a station has frames to transmit, it attempts to detect
presence of the carrier signal from the other nodes connected
to the shared channel.
If a carrier signal is detected, it implies that a transmission is in
progress.
The station waits till the ongoing transmission executes to
completion, and then initiates its own transmission. Generally,
transmissions by the node are received by all other nodes
connected to the channel.
Since, the nodes detect for a transmission before sending their
own frames,collision of frames is reduced. However, if two
nodes detect an idle channel at the same time, they may
simultaneously initiate transmission. This would cause the
frames to garble resulting in a collision.

194. Sridhar Iyer IIT Bombay 194
CSMA Access Modes
1-Persistent
Non-Persistent
P-Persistent
O- Persistent

195. Sridhar Iyer IIT Bombay 195
CSMA Access Modes
1-Persistent: In the 1-Persistent mode of CSMA that
defines each node, first sense the shared channel and if
the channel is idle, it immediately sends the data. Else it
must wait and keep track of the status of the channel to
be idle and broadcast the frame unconditionally as soon
as the channel is idle.
Non-Persistent: It is the access mode of CSMA that
defines before transmitting the data, each node must
sense the channel, and if the channel is inactive, it
immediately sends the data. Otherwise, the station must
wait for a random time (not continuously), and when the
channel is found to be idle, it transmits the frames.

196. Sridhar Iyer IIT Bombay 196
CSMA Access Modes
P-Persistent: It is the combination of 1-Persistent and
Non-persistent modes. The P-Persistent mode
defines that each node senses the channel, and if the
channel is inactive, it sends a frame with a P
probability. If the data is not transmitted, it waits for a
(q = 1-p probability) random time and resumes the
frame with the next time slot.
O- Persistent: It is an O-persistent method that defines
the superiority of the station before the transmission
of the frame on the shared channel. If it is found that
the channel is inactive, each station waits for its turn
to retransmit the data.

197. Sridhar Iyer IIT Bombay 197

198. Sridhar Iyer IIT Bombay 198
What are the main types of CSMA
protocols?
There are three main types of CSMA protocols:
1) CSMA-CD (Collision Detection), which is
primarily used in Ethernet networks;
2) CSMA-CA (Collision Avoidance), which is used
in wireless networks; and
3) CSMA with implicit acknowledgment, which is
used in slotted networks like Token Ring.

199. Sridhar Iyer IIT Bombay 199
CSMA/ CD
It is a carrier sense multiple access/ collision detection
network protocol to transmit data frames.
The CSMA/CD protocol works with a medium access control
layer. Therefore, it first senses the shared channel before
broadcasting the frames, and if the channel is idle, it transmits
a frame to check whether the transmission was successful.
If the frame is successfully received, the station sends
another frame.
If any collision is detected in the CSMA/CD, the station sends
a jam/ stop signal to the shared channel to terminate data
transmission.
After that, it waits for a random time before sending a frame
to a channel.

200. Sridhar Iyer IIT Bombay 200
In the diagram, A starts sending the first bit of its frame at t1 and since C sees
the channel idle at t2, starts sending its frame at t2. C detects A’s frame at t3 and
aborts transmission. A detects C’s frame at t4 and aborts its transmission.
Transmission time for C’s frame is, therefore, t3-t2 and for A’s frame is t4-t1
So, the frame transmission time (Tfr) should be at least twice the maximum
propagation time (Tp). This can be deduced when the two stations involved in a
collision are a maximum distance apart.

201. Sridhar Iyer IIT Bombay 201
Process: The entire process of collision
detection can be explained as follows:
Throughput
and
Efficiency:
The
throughput of
CSMA/CD is
much greater
than pure or
slotted
ALOHA.
For the 1-
persistent
method,
throughput is
50% when
G=1.
For the non-
persistent
method,
throughput
can go up to
90%.

202. Sridhar Iyer IIT Bombay 202
Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA)
The basic idea behind CSMA/CA is that the station should be
able to receive while transmitting to detect a collision from
different stations.
In wired networks, if a collision has occurred then the energy
of the received signal almost doubles, and the station can
sense the possibility of collision.
In the case of wireless networks, most of the energy is used
for transmission, and the energy of the received signal
increases by only 5-10% if a collision occurs.
It can’t be used by the station to sense collision. Therefore
CSMA/CA has been specially designed for wireless networks.

203. Sridhar Iyer IIT Bombay 203
CSMA/ CA
It is a carrier sense multiple access/collision avoidance
network protocol for carrier transmission of data frames.
It is a protocol that works with a medium access control
layer. When a data frame is sent to a channel, it receives
an acknowledgment to check whether the channel is clear.
If the station receives only a single (own)
acknowledgments, that means the data frame has been
successfully transmitted to the receiver.
But if it gets two signals (its own and one more in which
the collision of frames), a collision of the frame occurs in
the shared channel. Detects the collision of the frame
when a sender receives an acknowledgment signal.

204. Sridhar Iyer IIT Bombay 204
These are three types of strategies
InterFrame Space (IFS): When a station finds the
channel busy it senses the channel again, when the
station finds a channel to be idle it waits for a period
of time called IFS time. IFS can also be used to
define the priority of a station or a frame. Higher the
IFS lower is the priority.
Contention Window: It is the amount of time divided
into slots. A station that is ready to send frames
chooses a random number of slots as wait time.
Acknowledgments: The positive acknowledgments
and time-out timer can help guarantee a successful
transmission of the frame.

205. Sridhar Iyer IIT Bombay 205
Process: The entire process of collision avoidance
can be explained as follows:

206. Sridhar Iyer IIT Bombay 206
Characteristics of CSMA/CA
Carrier Sense
Multiple Access
Collision Avoidance
Acknowledgment (ACK)
Fairness
Binary Exponential Backoff
Interframe Spacing
Adaptive Behavior

207. Sridhar Iyer IIT Bombay 207
Advantages of CSMA
Increased efficiency
Simplicity
Flexibility
Low cost
Disadvantages of CSMA
Limited scalability
Delay
Limited reliability
Vulnerability to attacks

208. Sridhar Iyer IIT Bombay 208
Comparisonof various protocols:

209. Sridhar Iyer IIT Bombay 209
BTMA/DBTMA

210. Sridhar Iyer IIT Bombay 210
Busy Tone Multiple Access
Busy Tone Multiple Access (BTMA) is a
communication method used in wireless networks
to enable efficient sharing of communication
channels.
In contrast to other access methods, BTMA relies
on the use of special tones, the so-called "busy
tones", to signal the occupancy of a channel.
This innovative technology minimizes collisions
and interference, resulting in improved channel
utilization and increased overall performance in
wireless communication systems.

211. Sridhar Iyer IIT Bombay 211
How it works
Here is a basic explanation of how it works:
Generate busy tones: Each user who intends to use a communication
channel emits a special "busy tone". This tone serves as a signal to other
users to indicate that a device is currently using this channel.
Channel Monitoring: Other users in the area continuously monitor the
available channels for the presence of busy tones. This is done to find out
which channels are currently occupied by other devices and which are free.
Channel Access: When a user needs a free channel, it searches for
channels without an active busy tone. Once such a channel is found, the
user can start communicating without any interference.
Collision avoidance: Using busy tones minimizes the likelihood of
collisions. If multiple users try to use the same channel at the same time,
they can detect each other's busy tones and switch to another free channel
to prevent collisions.
Dynamic Adjustment: BTMA allows dynamic channel allocation. Users
can update their busy tones to reflect changes in their communication
needs, allowing for flexible channel usage.

212. Sridhar Iyer IIT Bombay 212
Effieciency of BTMA
The efficiency of BTMA lies in the early detection
of channel occupancy through busy tones. This not
only reduces collisions, but also minimizes channel
allocation delays, which is particularly beneficial in
time-critical applications such as cellular networks
and wireless sensor networks.
Overall, the way BTMA works enables efficient and
fair use of radio channels in wireless networks and
helps improve the overall performance and
reliability of wireless communication systems.

213. Sridhar Iyer IIT Bombay 213
Areas of application
Some of the main areas where BTMA can be used are:
Mobile Networks: In mobile networks, especially in high traffic
areas such as urban environments, BTMA can help maximize
the capacity and efficiency of communication channels.
Wireless Sensor Networks: BTMA is useful in wireless sensor
networks where sensors collect and transmit data in real time.
Industrial automation: In industrial environments, BTMA can
help ensure reliable and interference-free communication.
Ad hoc networks: BTMA can also be used in wireless ad hoc
networks where devices temporarily communicate with each
other without relying on prior infrastructure.
IoT Applications: In the Internet of Things (IoT), BTMA
techniques can help improve the efficiency of wireless
communication between IoT devices.

214. Sridhar Iyer IIT Bombay 214
Dual Busy Tone Multiple Access
(DBTMA)
Dual Busy Tone Multiple Access (DBTMA) is an
evolution of Busy Tone Multiple Access (BTMA)
technology.
DBTMA is designed to further increase the
efficiency of using communication channels in
wireless networks and optimize performance.
Essentially, DBTMA combines two different
types of busy tones to further coordinate
channel allocation and usage.

215. Sridhar Iyer IIT Bombay 215
Dual Busy Tone Multiple Access
working?
How Dual Busy Tone Multiple Access works typically includes the following
aspects:
Busy Tones for Sending and Receiving: In DBTMA, users use not only one
busy tone to indicate that they want to send, but also another busy tone to
indicate that they are ready to receive. This enables more sophisticated
signaling and coordination between sending and receiving.
Channel Assignment: When a user is ready to transmit, they send out the
appropriate busy tone to indicate their desire to communicate. Other users can
recognize this sound and assign the channel accordingly.
Collision avoidance: DBTMA further reduces collisions because there are
separate busy tones for sending and receiving. This means that users who
want to send will not conflict with users who only want to receive, and vice
versa.
Dynamic Adjustment: Like BTMA, DBTMA allows busy tones to be
dynamically adjusted to reflect changes in users' communication needs. This
further increases the flexibility of the system.

216. Sridhar Iyer IIT Bombay 216
THE DBTMA PROTOCOL
In the DBTMA protocol, two narrow-bandwidth tones are
implemented with enough spectral separation on the single shared
channel. BTt (the transmit busy tone) and BTr (the receive busy
tone), indicate whether the node is transmitting RTS packets or
receiving data packets, respectively.
The transmit busy tone (BTt) provides protection for the RTS
packets to increase the probability of successful RTS reception at
the intended receiver.
We use the receive busy tone (BTr) to acknowledge the RTS packet
and provide continuous protection for the transmitted data packets.
All nodes sensing any busy tone are not allowed to send RTS
requests. When the start of the signal is sensed, a node sending the
RTS packet is required to abort such transmission immediately.
Indeed, the RTS packets and the receive busy tone solve the
hidden- and the exposed-terminal problems.

217. Sridhar Iyer IIT Bombay 217
The operation of the DBTMA protocol will be explained by the way
of a network example, shown in Fig. 2.
In this figure, a solid line between any two nodes indicates that the
nodes can hear each other.
Hence, node C is a hidden terminal to the transmission from node A to
node B, and node E is an exposed terminal, if it wants, for example, to
communicate with node F (but not with node A).

218. Sridhar Iyer IIT Bombay 218
Wireless Local Loop

219. Sridhar Iyer IIT Bombay 219
Wireless Local Loop
This system, also known as fixed wireless access or fixed
radio
Local loop is a circuit line from a subscriber’s phone to the
local central office (LCO). But the implementation of local
loop of wires is risky for the operators, especially in rural and
remote areas due to less number of users and increased
cost of installation.
Hence, the solution for it is the usage of wireless local loop
(WLL) which uses wireless links rather than copper wires to
connect subscribers to the local central office.
In developing economies, WLL is expected to help unlock
competition in the local loop, enabling new operators to
bypass existing wire line networks to deliver POTS and data
access.

220. Sridhar Iyer IIT Bombay 220
Wireless Local Loop
WLL systems are easy to integrate with a modified public telephone
network (PSTN), and they can usually be installed within a month of
acquiring equipment, much faster than traditional wiring, which can
take months to set up and years to increase the capacity to meet the
growing demand for communication services.
There are WLL systems based on Code Division Multiple Access for
high-density, high-growth urban and suburban settings (CDMA).
Telecommunications systems such as TDMA (Time Division Multiple
Access) and GSM (Global System for Mobile) are also available.
Digital WLL systems can offer higher-speed fax and data services in
addition to providing better speech quality than analog systems.
Existing operations support systems (OSS) and transmission and
distribution systems are also compatible with WLL technology.

221. Sridhar Iyer IIT Bombay 221
WLL Architecture:
The Wire
­less Local Loop (WLL) architecture re­places traditional
copper wires with wire
­less links, connecting subscribers to the local
central office.
It consists of several components, including the PSTN (Public
Switched Te
­lephone Network), Switch Function, WANU (Wire­less
Access Network Unit), and WASU (Wire
­less Access Subscriber Unit).
The PSTN serves as a circuit-switched ne
­twork, while the Switch
Function manages conne
­ctions between WANUs. The WANU takes
care of authentication, ope
­ration, routing, and data transmission,
whereas the WASU is installe
­d at the subscriber’s location.
With its cost-effe
­ctiveness, enhanced security through digital
encryption, scalability options, and various features like internet
access, voice services, data transfe
­r capabilities, and fax services –
WLL prove
­s to be a dependable­ solution for telecommunication
require
­ments specifically in remote­ or rural areas.

222. Sridhar Iyer IIT Bombay 222

223. Sridhar Iyer IIT Bombay 223
WLL components:
1.PSTN: It is Public Switched Telephone Network
which is a circuit switched network. It is a
collection of world’s interconnected circuit
switched telephone networks.
2.Switch Function: Switch Function switches the
PSTN among various WANUs.

224. Sridhar Iyer IIT Bombay 224
WLL components:
3.WANU: It is short for Wireless Access Network Unit. It is present at
the local exchange office. All local WASUs are connected to it. Its
functions includes: Authentication, Operation & maintenance,
Routing, Transceiving voice and data. It consists of following sub-
components:
Tansceiver: It transmits/receives data.
LL Controller: It controls the wireless local loop component with
WASU.
AM: It is short for Access Manager. It is responsible for authentication.
HLR: It is short for Home Location Register. It stores the details of all
local WASUs.
4.WASU: It is short for Wireless Access Subscriber Units. It is present
at the house of the subscriber. It connects the subscriber to WANU
and the power supply for it is provided locally.

225. Sridhar Iyer IIT Bombay 225
Advantages of WLL:
It eliminates the first mile or last mile
construction of the network connection.
Low cost due to no use of conventional copper
wires.
Much more secure due to digital encryption
techniques used in wireless communication.
Highly scalable as it doesn’t require the
installation of more wires for scaling it.

226. Sridhar Iyer IIT Bombay 226
Features of WLL:
Internet connection via modem
Data service
Voice service
Fax service

227. Sridhar Iyer IIT Bombay 227
What is GSM?

228. Sridhar Iyer IIT Bombay 228
What is GSM
GSM (or Global System for Mobile
Communications) is defined as a set of mobile
communications standards and protocols
governing second-generation or 2G networks,
first developed and deployed in Europe.
It uses 4 different frequency bands of 850 MHz,
900 MHz, 1800 MHz and 1900 MHz . It uses
the combination of FDMA and TDMA.

229. Sridhar Iyer IIT Bombay 229
GSM is having 4 different sizes of
cells are used in GSM :
Macro : In this size of cell, Base Station
antenna is installed.
Micro : In this size of cell, antenna height is less
than the average roof level.
Pico : Small cells’ diameter of few meters.
Umbrella : It covers the shadowed (Fill the gaps
between cells) regions.

230. Sridhar Iyer IIT Bombay 230

231. Sridhar Iyer IIT Bombay 231
The Working of a GSM Network
GSM is a digital cellular communication standard that is
universally accepted.
The European Telecommunications Standards Institute created
the GSM standard to define the procedures for second-
generation digital mobile networks.
It is a wide-area communications technology program that
utilizes digital radio channeling to bring forth audio, information,
and multimedia communication systems.
GSM is a mobile network. This implies that devices interact with
it by looking for nearby cells.
GSM, including other technological advances, has influenced
the evolution of mobile wireless telecommunication services.
A GSM system manages communication between mobile
stations, base stations, and switching systems.

232. Sridhar Iyer IIT Bombay 232
The Working of a GSM Network

Every GSM radio channel is 200 kHz wide and is additionally
divided into frames of 8-time slots.

The global system for mobile communication (GSM) was first
known as Groupe Special Mobile, which is the reason for the
acronym.

The GSM system comprises mobile stations, base stations,
and intertwining switching systems.

The GSM program enables 8 to 16 audio users to share every
radio channel, and every radio transmission location may have
multiple radio channels.

Because of its simplicity, affordability, and accessibility, GSM
is presently the most commonly used network technology in
the Internet of Things (IoT)Opens a new window applications.

233. Sridhar Iyer IIT Bombay 233

234. Sridhar Iyer IIT Bombay 234
Home Location Register
(HLR)
Visitor Location Register
(VLR)
Equipment Identity Register
(EIR)
Authentication Center
(AuC)
The additional components of the GSM architecture comprise of
databases and messaging systems functions
SMS Serving Center
(SMS SC)
Gateway MSC (GMSC)
Chargeback Center
(CBC)
Transcoder and
Adaptation Unit (TRAU)

235. Sridhar Iyer IIT Bombay 235
The following diagram shows the GSM network along with the
added elements −

236. Sridhar Iyer IIT Bombay 236
The Architecture of GSM
The GSM architecture is made up of three central
systems. The following are the primary
components of the GSM architecture:
The network switching system (NSS)
The mobile station (MS)
The base station system (BSS)
The operations and support system (OSS)

237. Sridhar Iyer IIT Bombay 237
The network switching system
(NSS)
NSS is a GSM element that provides flow
management and call processing for mobile
devices moving between base stations. The
switching system consists of the functional units
listed below.Mobile Services Switching
Center (MSC): Mobile Switching Center is
integral to the GSM network architecture’s
central network space. The MSC supports call
switching across cellular phones and other fixed
or mobile network users. It also monitors
cellular services, including registration, location
updates, and call forwarding to a roaming user.

238. Sridhar Iyer IIT Bombay 238

239. Sridhar Iyer IIT Bombay 239
The network switching system
(NSS)
Home Location Register (HLR): It is a set of data items used for
storing and managing subscriptions. It provides data for each
consumer as well as their last known position.
The HLR is regarded as the most significant database because it
preserves enduring records about users. When a person
purchases a membership from one of the operators, they are
enlisted in that operator’s HLR.
Visitor Location Register (VLR): VLR is a database that provides
subscriber information necessary for the MSC to service
passengers. This includes a short-term version of most of the data
stored in the HLR.
The visitor location register can also be run as a standalone
program, but it is usually implemented as a component of the
MSC.

240. Sridhar Iyer IIT Bombay 240
The network switching system
(NSS)
Equipment Identity Register (EIR): It is the component
that determines if one can use particular mobile equipment
on the system. This consists of a list of every functioning
mobile device on the system, with each mobile device
recognized by its own International Mobile Equipment
Identity (IMEI) number.
Authentication Center (AuC): The AUC is a unit that
offers verification and encryption factors to ensure the
user’s identity and the privacy of every call. The verification
center is a secure file that contains the user’s private key in
the SIM card. The AUC shields network operators from
various types of fraud prevalent in the modern-day cellular
world.

241. Sridhar Iyer IIT Bombay 241
The mobile station (MS)
The mobile station is a cell phone with a display, digital
signal processor, and radio transceiver regulated by a
SIM card that functions on a system.
Hardware and the SIM card are the two most essential
elements of the MS. The MS (Mobile stations) is most
widely recognized by cell phones, which are
components of a GSM mobile communications network
that the operator monitors and works.
Currently, their size has shrunk dramatically while their
capabilities have skyrocketed. Additionally, the time
between charges has been significantly improved.

242. Sridhar Iyer IIT Bombay 242
The base station system (BSS)
It serves as a connection between the network subsystem and the mobile
station. It consists of two parts:
The Base Transceiver Station (BTS): The BTS is responsible for radio
connection protocols with the MS and contains the cell’s radio
transceivers. Companies may implement a significant number of BTSs
in a big metropolitan area. Each network cell has transceivers and
antennas that make up the BTS. Based on the cell’s consumer density,
every BTS includes anywhere from one to sixteen transceivers.
The Base Station Controller (BSC): The BSC is responsible for
managing the radio resources of one or more BTS(s). This manages
radio channel configuration and handovers. The BSC serves as the link
seen between mobile and MSC. It allocates and emits MS frequency
bands and time slots. Additionally, the BSC is responsible for intercell
handover and transmits the BSS and MS power within its jurisdiction.

243. Sridhar Iyer IIT Bombay 243
The operations and support system
(OSS)

The operation support system (OSS) is a part of the
overall GSM network design. This is linked to the NSS
and BSC components. The OSS primarily manages the
GSM network and BSS traffic load.

As the number of BS increases due to customer
population scaling, a few maintenance duties are shifted
to the base transceiver stations, lowering the system’s
financial responsibility.

The essential purpose of OSS is to have a network
synopsis and assist various services and maintenance
organizations with their routine maintenance
arrangements.

244. Sridhar Iyer IIT Bombay 244
The operations and support system (OSS)

245. Sridhar Iyer IIT Bombay 245
GSM Architecture-sub system
GSM is a network with hierarchical structure with
architecture consisting of basic 3 sub – systems
namely:
Radio Sub System
Network Sub System
Operation Sub System

246. Sridhar Iyer IIT Bombay 246
GSM Architecture-sub system
It comprises of all radio specific entities. It is connected to other sub-
systems like NSS and OSS by A – interface and O – interface connect
MS to the network. The entities belonging to radio sub system are
Mobile station (MS)
Base Transceiver System (BTS)
BSC Base Station Controller

The three entities form a hierarchical structure as shown in figure. There
are number of MS, which connect to a BTS, Number of BTS are
connected to a BSC. Hence,
No. of MS > No of BTS > No. of BSC

247. Sridhar Iyer IIT Bombay 247
BSC with BTS’s and MS’s form Base station subsystem .BSS are
connected to MSC in NSS and OMC in OSS layer.
FIGURE: ENTITIES OF RADIO SUB SYSTEM

248. Sridhar Iyer IIT Bombay 248
Mobile Station
It can be any hand-held devices like phone, tablet,
PDU, laptop which connects to GSM network. It
consists of necessary network and software to
transmit and receive GSM data.
It consists of a user terminal to access the device
and SIM card through which user can access its
account in GSM, connects to network and uses
services of network.
The MS for GSM 900 have transmit power of 2W
whereas for GSM 1800 1W is enough because the
cell sizes are smaller.

249. Sridhar Iyer IIT Bombay 249
Mobile Station
IMEI or International mobile equipment identity is a 15 digit number identifier
associated with the mobile station. It is a useful tool to prevent stolen handset from
accessing a network. IMEI numbers are stored on devices beneath the battery
printed on a small while label. It can also be retrieved by dialing *#06# from the MS
Subscriber Identity Module
Through SIM user can access its account in GSM, connects to network and uses
services of network. Following information is stored in SIM
Card serial number, type and list of subscribed services
IMSI number (International mobile Subscriber Identity): A 15-digit number allocated
to uniquely identify the subscribers. It does not change when user moves from one
network to another. It is used to generate cipher key, TMSI and LAI from service
provider. It constitutes of 3 parts
3 digit mobile country code
2 digit mobile network code
10 digit MSIN number (Mobile Subscriber Identity Number) The same format is used
by service provider all over the world.

250. Sridhar Iyer IIT Bombay 250
Mobile Station
For eg. If IMSI is 310 15 0123456789
310 – Country Code for USA; 15 – AT & T Mobility; MSIN –
0123456789
PIN (Personal Identification Number) used to lock and unlock the
MS.
PUK – Pin Unblocking Key enables subscribers to unlock the
SIM if it is accidentally or deliberately lock
Ki – 128 bit authentication key used for authentication and cipher
key generation during encryption
Kc : Cipher key

251. Sridhar Iyer IIT Bombay 251
GSM - The Base Station Subsystem
(BSS)
The BSS is composed of two parts −
The Base Transceiver Station (BTS)
The Base Station Controller (BSC)
The BTS and the BSC communicate across the specified Abis
interface, enabling operations between components that are
made by different suppliers. The radio components of a BSS
may consist of four to seven or nine cells. A BSS may have
one or more base stations. The BSS uses the Abis interface
between the BTS and the BSC. A separate high-speed line
(T1 or E1) is then connected from the BSS to the Mobile
MSC.

252. Sridhar Iyer IIT Bombay 252
The Base Station Subsystem (BSS)

253. Sridhar Iyer IIT Bombay 253
The Base Transceiver Station (BTS)
The BTS houses the radio transceivers that define a cell and handles
the radio link protocols with the MS. In a large urban area, a large
number of BTSs may be deployed.
The BTS corresponds to the transceivers and antennas used in each
cell of the network. A BTS is usually placed in the center of a cell. Its
transmitting power defines the size of a cell. Each BTS has between 1
and 16 transceivers, depending on the density of users in the cell.
Each BTS serves as a single cell.

254. Sridhar Iyer IIT Bombay 254
The Base Station Controller (BSC)
The BSC manages the radio resources for one or more
BTSs. It handles radio channel setup, frequency hopping,
and handovers.
 The BSC is the connection between the mobile and the
MSC. The BSC also translates the 13 Kbps voice channel
used over the radio link to the standard 64 Kbps channel
used by the Public Switched Telephone Network (PSDN)
or ISDN.
It assigns and releases frequencies and time slots for the
MS. The BSC also handles intercell handover. It controls
the power transmission of the BSS and MS in its area. The
function of the BSC is to allocate the necessary time slots
between the BTS and the MSC.

255. Sridhar Iyer IIT Bombay 255
The additional functions include−
Control of frequency hopping
Performing traffic concentration to reduce the number of lines from the MSC
Providing an interface to the Operations and Maintenance Center for the BSS
Reallocation of frequencies among BTSs
Time and frequency synchronization
Power management
Time-delay measurements of received signals from the MS

256. Sridhar Iyer IIT Bombay 256
GSM network areas
In a GSM network, the following areas are defined −
Cell − Cell is the basic service area; one BTS covers one cell. Each cell is
given a Cell Global Identity (CGI), a number that uniquely identifies the cell.
Location Area − A group of cells form a Location Area (LA). This is the area
that is paged when a subscriber gets an incoming call. Each LA is assigned
a Location Area Identity (LAI). Each LA is served by one or more BSCs.
MSC/VLR Service Area − The area covered by one MSC is called the
MSC/VLR service area.
PLMN − The area covered by one network operator is called the Public
Land Mobile Network (PLMN). A PLMN can contain one or more MSCs.

257. Sridhar Iyer IIT Bombay 257
Top 4 Applications of GSM
1. Sending and receiving short messages
2. GSM and data security
3. GSM for mobile system handover
4. GSM in medical services

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GPRS (General Packet Radio Service)

259. Sridhar Iyer IIT Bombay 259
GPRS (General Packet Radio
Service)

GPRS is a mobile communications standard that runs on 2G and
3G networks to enable moderately high-speed data transfers.

GPRS stands for General Packet Radio Service. It is the modified
version of GSM architecture. GPRS is a packet-oriented mobile
data mechanism, that can carry data packets as well.

In GSM architecture, only voice signals can be transported, so
being an enhanced version GPRS is able to transmit voice as well
as data packets. It uses the same physical radio channel as GSM
does, the only difference is it has a new logic defined for the radio
channel.

General packet radio service (GPRS) is a mobile communications
standard that operates on 2G and 3G cellular networks to enable
moderately high-speed data transfers using packet-based
technologies.

260. Sridhar Iyer IIT Bombay 260
GPRS Network Architecture
GPRS tries to make maximum use of the existing
physical structure of GSM.
It has introduced a new entity named GPRS
support nodes(GSN) whose responsibility is to
route and deliver a data packet. GSN is of two
types:
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)

261. Sridhar Iyer IIT Bombay 261
Network Architecture Of GPRS In Wireless Communication

262. Sridhar Iyer IIT Bombay 262
Network Architecture Of GPRS In
Wireless Communication
It is a packet switched wireless data communication technology.
GPRS enables mobile devices to access the internet and other data
services over a cellular mobile network.
The main elements of a GPRS architecture are
Mobile Station (MS),
Base Station Subsystem (BSS),
Serving GPRS Support Node (SGSN),
Gateway GPRS Support Node (GGSN),
Home Location Register (HLR),
Authentication Center (AuC),
Operation and Maintenance Center (OMC),
Charging Gateway (CG) and so on.

263. Sridhar Iyer IIT Bombay 263
Network Architecture Of GPRS In
Wireless Communication
Entire GPRS network can be divided for understanding
into following basic elements.
Packet Control Unit (PCU) : This PCU is the core unit
to segregate between GSM and GPRS traffic. It
separates the circuit switched and packet switched
traffic from the user and sends them to the GSM and
GPRS networks respectively which is shown in the
figure above. In GPRS PCU has following two paths.
1. PCU-MSC-GMSC-PSTN
2. PCU-SGSN-GGSN-Internet (packet data network)

264. Sridhar Iyer IIT Bombay 264
Network Architecture Of GPRS In
Wireless Communication
Serving GPRS Support Node(SGSN):
It is similar to MSC of GSM network. SGSN functions are
outlined below.
• Data compression which helps minimise the size of
transmitted data units.
• Authentication of GPRS subscribers.
• Routing of data to the corresponding GGSN when a
connection to an external network is needed.
• Mobility management as the subscriber moves from one
PLMN area to the another PLMN, and possibly one SGSN to
another SGSN.
• Traffic statistics collections.

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Network Architecture Of GPRS In
Wireless Communication
Gateway GPRS Support Node(GGSN) : GGSN is the gateway
to external networks such as PDN (packet data network) or IP
network. It does two main functions. It is similar to GMSC of
GSM network
• Routes mobile destined packet coming from external IP
networks to the relevant SGSN within the GPRS network
• Routes packets originated from a user to the respective
external IP network
Border Gateway (BG) : It is a kind of router which interfaces
different operators GPRS networks. The connection between
two border gateways is called GPRS tunnel. It is more secure
to transfer data between two operators using their own PLMN
networks through a direct connection rather than via the
public Internet which is less secure.

266. Sridhar Iyer IIT Bombay 266
Network Architecture Of GPRS In
Wireless Communication
Charging Gateway (CG) : GPRS users have to be charged
for the use of the network, this is taken care by Charging
gateway. Charging is done based on Quality of Service or
plan user has opted either prepaid or post paid.
DNS server : Connected at ISP location or at IP network. It
converts domain name to IP addresses required to
establish internet connection and to deliver web pages on
user's terminal screen.
Intra PLMN : An IP based network inter-connecting all the
above mentioned GPRS network elements in one PLMN
area.
Inter PLMN : Connection between two different PLMN areas.

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268. Sridhar Iyer IIT Bombay 268
How Does GPRS Work?
The global system for mobile communications (GSM) is the primary
standard for the second generation (2G) cellular network, while GPRS
is an improved version.
GPRS is not like GSM’s short messaging service (GSM-SMS), which
has a message length limit of 160 bytes. GPRS has a theoretical
maximum speed of 115 kbps, although most networks operate at
roughly 35 kbps. GPRS is sometimes known as 2.5G unofficially. It’s a
third-generation route to gain availability on the internet.
GPRS can operate from either symmetric or asymmetric configuration,
whereas frequency for either direction is determined by which one of the
12 multislot provider classes are chosen.
The number of time slots for every path is determined by the multislot
service class, for every time slot propping up a theoretical connection
speed of 21.4 kbps. One of the most basic is service class 1, which
allows a one-time slot for each path. Service class 12 is by far the most
proficient, with four-time slots in every direction.

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270. Sridhar Iyer IIT Bombay 270
Features of GPRS
GPRS is a wireless communication service that allows data to be
transmitted over a cellular network.
GPRS uses packet-switching technology to transmit data, which
means that data is divided into small packets and sent over the
network in a more efficient way.
GPRS offers always-on connectivity, which means that a user can
stay connected to the network at all times, without having to establish
a connection every time they want to send or receive data.
GPRS provides faster data transfer rates compared to the earlier
generation of cellular networks, such as GSM.
GPRS enables new applications and services to be developed, such
as mobile internet browsing and email.
GPRS is a precursor to modern cellular data technologies, such as
3G and 4G.x

271. Sridhar Iyer IIT Bombay 271
Advantages of GPRS
A high-speed data transfer cost is offered to mobile devices
through General Packet Radio Service or GPRS.
Web browsing, email sending and receiving, and online
shopping are just a few of the online services that GPRS users
can access while they are on the move.
Because GPRS is always operational, customers can access
the internet quickly and without any problems without utilizing
dial-up.
GPRS offers a cost-effective approach to transmitting statistics
because it only charges for the volume of data transferred, not
for the amount of time spent online.
GPRS offers users a flexible option because it functions well
with a variety of mobile devices.

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EDGE(Enhanced Data Rate for GSM Evolution)

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EDGE(Enhanced Data Rate for GSM
Evolution)
EDGE (Enhanced Data Rate For GSM Evolution) provides
a higher rate of data transmission than normal GSM.
It uses a backward-compatible extension of GSM of digital
mobile technology.EDGE has a pre-3G radio technology
and uses part of ITU’s 3G definition.
 It can work on any network deployed with GPRS (with
necessary upgrades).Enhanced Data rates for Global
Evolution (EDGE) are a radio based high-speed mobile
data standards.
A technology that gives Global System for Mobile
Communications (GSM) the capacity to handle services
for the third generation of mobile network.

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Evolution

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EDGE (Enhanced Data Rate for
GSM Evolution)
EDGE allows for a faster transmission rate than standard
GSM. It makes use of a backward-compatible GSM digital
mobile technology extension.
With the proper modifications, it can work on any GPRS
network.
As part of the GSM family, 3GPP has standardized EDGE.
Compact-EDGE, a variation, was created for usage in a
section of the Digital AMPS network frequency.
EDGE provides higher bit-rates per radio channel due to the
introduction of sophisticated data coding and transmission
technologies, resulting in a threefold improvement in capacity
and performance over a standard GSM/GPRS connection.

276. Sridhar Iyer IIT Bombay 276
Working
It uses 8PSK modulation in order to achieve a
higher data transmission rate.
The modulation format is changed to 8PSK
from GMSK(Gaussian minimum-shift keying).
This provides an advantage as it is able to
convey 3 bits per symbol, and increases the
maximum data rate.
However, this upgrade required a change in the
base station.

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278. Sridhar Iyer IIT Bombay 278
Features
It provides an evolutionary migration path from
GPRS to UMTS.
It is standardized by 3GPP.
EDGE is used for any packet switched
application,like an Internet connection.
EDGE delivers higher bit-rates per radio
channel and it increase the capacity and
performance.

279. Sridhar Iyer IIT Bombay 279
The Technology behind EDGE:
 EDGE is primarily a radio interface improvement, but in
a more general context it can also be viewed as a
system concept that allows the GSM and TDMA/136
networks to offer a set of new services.
The first stepping stone in migration path to third
generation wireless mobile services (3G) is the General
Packet Radio Services, GPRS, a packet-switched
technology that delivers speeds of up to 115kbps.
If GPRS is already in place, Enhanced Data rates for
Global Evolution (EDGE) technology is most effective
as the second stepping stone that gives a low impact
migration.

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Advantage Disadvantage
It has higher
speed.
It is an “always-on”
connection
It is more reliable
and efficient
It is cost efficient
It consumes more
battery.
hardware needs
upgradation.

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What Is UMTS
Universal mobile telecommunication system (UMTS)
refers to the third generation (3G) mobile network
built on the global GSM standard, compatible with
data transfer up to 2 megabits per second.
The Universal Mobile Telecommunications System
(UMTS) is a broadband, packet-based, 3G mobile
cellular system based upon GSM standards. The
specifications of UMTS covers the entire network
system, including the radio access network, the core
network and user authentication.

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What Is UMTS
It is a pioneering wireless radio technology associated with
third-generation (3G) cellular networks. AT&T first deployed it in
North America in the early 2000s, and its use spread globally
over the next few years.
Today, UMTS is used interchangeably with 3G. Unlike global
system for mobile communications (GSM) – which was widely
used before the deployment of UMTS – UMTS offers faster data
transfer, improved cellular capabilities, greater range/bandwidth,
and better radio spectrum efficiency.
This ensures a better method of transferring data and a better
customer experience. Although UMTS uses code division
multiple access (CDMA) technology, it has a broader bandwidth
than other CDMA systems, e.g., CDMA2000. So, it is
sometimes referred to as wideband CDMA or WCDMA.

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Features
UMTS is a component of IMT-2000 standard of the
International Telecommunications Union (ITU),
developed by 3GPP.
It uses wideband code division multiple access (W-
CDMA) air interface.
It provides transmission of text, digitized voice, video and
multimedia.
It provides high bandwidth to mobile operators.
It gives a high data rate of 2Mbps. For High-Speed
Downlink Packet Access (HSDPA) handsets, the data-
rate is as high as 7.2 Mbps in the downlink connection.
It is also known as Freedom of Mobile Multimedia
Access (FOMA).

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285. Sridhar Iyer IIT Bombay 285
UMTS network architecture
The UMTS network is divided into three main
components:
User equipment (UE) refers to UMTS-compatible
devices such as smartphones and tablets.
The UMTS Terrestrial Radio Access Network
(UTRAN) handles the radio communications
between mobile devices (UE) and the network.
The core network (CN) manages the routing of
calls and data, along with other administrative
functions.

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287. Sridhar Iyer IIT Bombay 287
User Equipment, UE
The USER Equipment or UE was a major element of the
overall 3G UMTS network architecture. It formed the final
interface with the user.
In view of the far greater number of applications and
facilities that it could perform, the decision was made to
call it a user equipment rather than a mobile.
However it was essentially the handset (in the broadest
terminology), although having access to much higher
speed data communications, it could be much more
versatile, containing many more applications.
It consists of a variety of different elements including RF
circuitry, processing, antenna, battery, etc.

288. Sridhar Iyer IIT Bombay 288
There were a number of elements
within the UE:
UE RF circuitry: The RF areas handled all elements of the signal,
both for the receiver and for the transmitter. One of the major
challenges for the RF power amplifier was to reduce the power
consumption.The form of modulation used for W-CDMA
required the use of an RF linear amplifier. These inherently take
more current than non linear amplifiers which could be used for
the form of modulation used on GSM. Accordingly to maintain
battery life, measures were introduced into many of the designs
to ensure the optimum efficiency.
Baseband processing: The base-band signal processing
consisted mainly of digital circuitry. This was considerably more
complicated than that used in phones for previous generations.
Again this had been optimised to reduce the current
consumption as far as possible.

289. Sridhar Iyer IIT Bombay 289
There were a number of elements
within the UE
Battery: While current consumption has been minimised as far as possible
within the circuitry of the phone, there had been an increase in current
drain on the battery. With users expecting the same lifetime between
charging batteries as experienced on the previous generation phones,
this had necessitated the use of new and improved battery technology.
Lithium Ion (Li-ion) batteries started to be more widely used to address
this issue.These phones needed to remain small and relatively light while
still retaining or even improving the overall life between charges.
Universal Subscriber Identity Module, USIM: The UE also contained a
SIM card, although in the case of UMTS it was termed a USIM (Universal
Subscriber Identity Module). This was a more advanced version of the
SIM card used in GSM and other systems, but embodied the same types
of information. It contained the International Mobile Subscriber Identity
number (IMSI) as well as the Mobile Station International ISDN Number
(MSISDN).

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3G UMTS Radio Network Subsystem
This was the section of the 3G UMTS / WCDMA
network that interfaced to both the UE and the
core network
- it handled the wireless communications
elements of the network.
The overall radio access network, i.e.
collectively all the Radio Network Subsystem
was known as the UTRAN or UMTS Radio
Access Network.

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3G UMTS Core Network
The 3G UMTS core network architecture was a migration of that used
for GSM with further elements overlaid to enable the additional
functionality demanded by UMTS. In view of the different ways in
which data could be carried, the UMTS core network was split into
two different areas:
Circuit switched elements: These elements were primarily based
on the GSM network entities and carry data in a circuit switched
manner, i.e. a permanent channel for the duration of the call.
Packet switched elements: These network entities were designed
to carry packet data. This enabled much higher network usage as the
capacity could be shared and data was carried as packets which
were routed according to their destination.
Some network elements, particularly those that were associated with
registration were shared by both domains and operated in the same
way that they did with GSM.

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293. Sridhar Iyer IIT Bombay 293
Circuit switched elements
The circuit switched elements of the UMTS core
network architecture included the following
network entities:
Mobile switching centre (MSC): This was
essentially the same as that within GSM, and it
managed the circuit switched calls under way.
Gateway MSC (GMSC): This was effectively the
interface to the external networks.

294. Sridhar Iyer IIT Bombay 294
Packet switched elements
The packet switched elements of the 3G UMTS core network architecture included
the following network entities:
Serving GPRS Support Node (SGSN): As the name implies, this entity was first
developed when GPRS was introduced, and its use has been carried over into the
UMTS network architecture. The SGSN provided a number of functions within the
UMTS network architecture.
Session management: The SGSN managed the data sessions providing the
required quality of service and it also managed what were termed the PDP (Packet
data Protocol) contexts, i.e. the pipes over which the data was sent.
Interaction with other areas of the network: The SGSN was able to manage its
elements within the network only by communicating with other areas of the network,
e.g. MSC and other circuit switched areas.
Billing: The SGSN was also responsible billing. It achieved this by monitoring the
flow of user data across the GPRS network. CDRs (Call Detail Records) were
generated by the SGSN before being transferred to the charging entities (Charging
Gateway Function, CGF).

295. Sridhar Iyer IIT Bombay 295
Packet switched elements
Gateway GPRS Support Node (GGSN): Like the
SGSN, this entity was also first introduced into the
GPRS network. The Gateway GPRS Support Node
(GGSN) was the central element within the UMTS
packet switched network.
It handled inter-working between the UMTS packet
switched network and external packet switched
networks, and could be considered as a very
sophisticated router. In operation, when the GGSN
received data addressed to a specific user, it checked
if the user was active and then forwarded the data to
the SGSN serving the particular UE.

296. Sridhar Iyer IIT Bombay 296
Key benefits of UMTS
High-speed data transfer:With speeds up to two Mbps, UMTS enables faster data
transfer, making it ideal for video streaming and large downloads. With the ability
to transfer more data more quickly, high-speed data transfer enables faster
communication, efficient remote work, and quick access to cloud services.
Improved bandwidth and capacity:UMTS networks can handle more
simultaneous users per cell, reducing the likelihood of network congestion. Higher
bandwidth is especially crucial for businesses dealing with large volumes of data,
supporting applications like video conferencing, cloud computing, and real-time
analytics.
Global roaming:UMTS operates on internationally recognized frequencies,
allowing for seamless connectivity across different geographical locations. This
feature is essential for businesses with a global footprint. It supports international
communication, travel, and operations without the need for multiple devices or
services.
Enhanced security:UMTS networks offer advanced security features, including
improved encryption for voice and data transmission. Enhanced security
safeguards sensitive corporate data and communications. It also reduces the risk
of data breaches and cyber threats, which is crucial in an era where digital
information is a valuable asset.

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Challenges and limitations
High infrastructure costs:If you’re building a UMTS network
yourself, setting it up and maintaining it requires a significant
investment. These costs include new hardware and software, as
well as the deployment of additional base stations to ensure wide
coverage and high-quality service.
Compatibility issues:UMTS isn’t backward compatible with some
older 2G devices, requiring users to upgrade their devices. In
addition to the cost of upgrading existing infrastructure and
devices to be UMTS-compatible, integrating UMTS with legacy
systems can lead to increased costs and complexity.
Spectrum licensing:Obtaining the necessary licenses to operate
on relevant frequency bands can be both costly and
complex.Spectrum is a limited resource, and acquiring the rights
to use specific frequency bands often involves participating in
competitive bidding or auctions for those looking to set up their
own UMTS networks.

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CDMA2000
CDMA2000 is a code division multiple access
(CDMA) version of IMT-2000 specifications
developed by International Telecommunication
Union (ITU).
It includes a group of standards for voice and
data services −
Voice − CDMA2000 1xRTT, 1X Advanced
Data − CDMA2000 1xEV-DO (Evolution-Data
Optimized)

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CDMA2000
The nets CDMA2000 are compatible with the
nets cdmaOne, that which protects the
investments of the operators cdmaOne and it
provides a simple and economic migration to
the following generation.
Also, the nets CDMA2000 offers improvements
in the voice quality and support for data
multimedia services.

300. Sridhar Iyer IIT Bombay 300
Evolution of Cdma2000
CDMA2000:
- Common denomination for IMT-2000 CDMA Multi-Carrier.
CDMA2000 1X (October 2000):
- 3G Technology that it duplicates the voice capacity.
- It provides data transmission speeds up to 307 kbps in a single
carrier (1.25 MHz, or 1X).
CDMA2000 1xEV :
- Evolution of CDMA2000 1X that it offers bigger data
transmission speed can offer up to 2.4 Mbps in a single carrier
the same as the previous one (1.25 Mhz).
CDMA2000 1xEV-DO (firsts of 2002):
- 3G Technology that only uses a carrier of 1.25MHz for data.
- It reaches transmission speeds of up to 2.4 Mbps.

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Architecture Diagram CDMA 2000

302. Sridhar Iyer IIT Bombay 302
Architecture:
Logical channels Carry data over the air and are mapped directly to
the physical channels( logical channels)
o Dedicated Traffic Channel (f/r-dtch): A point to point logical
channel that carries data or voice traffic over a dedicated physical
channel.
o Common Control channels (f/r-cmch control) : These are used to
carry MAC messages with shared access for several Terminals.
Dedicated signalling Cannel (f/r-dsch): A point to point logical
channel that carries upper layer signalling traffic over a dedicated
physical channel, for a single terminal.
o Common Signalling Channel (f/r-csch): A point to multipoint
logicalchannel that carries upper layer signalling traffic over a
common physical channel, with shared access for several terminals.

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Evolution - 2G to 3G

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Interim Standard 95 (IS-95)
Also known as cdmaOne
• 64 users in a 1.25 MHz channel.
• Can be used in 800 MHz and 1900 MHz bands.
• Sprint and Verizon in the U.S.
• Spectrum bandwidth:
• 1850 to 1910 MHz Mobile to Base
• 1930 to 1990 MHz Base to Mobile
• Channels are 1.25 MHz
• 3.75 MHz in CDMA 2000, 5 MHZ in UMTS
• Results in approximately only 48 forward/reverse channel pairs in IS-95.
• Adjacent cell phone towers use the exact same channels as all other towers.
• This is a major difference.
• Allows for much better frequency reuse and makes setting up a cellular
network much easier.

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Upgrade path from IS-95A to IS-95B
for 2.5G CDMA
Only one upgrade path for IS-95
• Users can use up to 8 CDMA codes
simultaneously.
• 14.4 kpbs * 8 = 115.2 kbps
• Practical throughput is 64 kbps that can actually
be achieved.
• Also changes the method of handoff between
base stations.

306. Sridhar Iyer IIT Bombay 306
cdma2000
• From IS-95/IS-95B
• Works within original 2G CDMA channel
bandwidth of 1.25 MHz.
• Allows wireless carriers to introduce 3G in a
gradual manner.
• Can introduce 3G capabilities at each cell
• Do not have to change out entire base stations
• Do not have to use different spectrum.

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cdma2000 1xRTT
1X = one times the original IS-95 (cdmaOne) channel
bandwidth.
• RTT = Radio Transmission Technology
• Commonly just referred to as cdma2000 1X.
• Instantaneous data rate of 307 kbps.
• Typical rates up to 144 kbps
• Uses rapidly adjusting rates.
• No additional RF equipment is needed.
• All changes made in software or with additional hardware.

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cdma2000 1xEV
• EV = Evolutionary enhancement
• High data rate packet standard overlaid on existing IS-95, IS-95B, and
cdma2000 networks.
• 1xEV-DO
• Data only channel
• Restricts a shared 1.25 MHz channel strictly to data users.
• Supports greater than 2.4 Mbps throughput per user.
• Actual data rates usually much lower.
• Typical: Several hundred kbps.
• Highly dependent on number of users, propagation conditions, and
velocity of mobile.

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Cdma One Network

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MSC BASED CORE NETWORK
FOR CDMA 2000 1x SYSTEM

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Interworking Function(IWF)
IWF acts as a gateway between the wireless
CDMA networks and the wireline Public
Switched Telephone Network (PSTN/packet
data networks).
• IWF provides the interworking and protocol
conversion required for offering data services to
cdma One Mobile subscribers.
• The IWF functionality may be implemented in
BSC or MSC.

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Features
CDMA2000 is a family of technology for 3G mobile
cellular communications for transmission of voice, data
and signals.
It supports mobile communications at speeds between
144Kbps and 2Mbps.
It has packet core network (PCN) for high speed
secured delivery of data packets.
It applies multicarrier modulation techniques to 3G
networks. This gives higher data rate, greater bandwidth
and better voice quality. It is also backward compatible
with older CDMA versions.
It has multi-mode, multi-band roaming features.x

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3G vs 4G vs 5G Comparison Table