2.Cellular Networks_The final stage of connectivity is achieved by segmenting the comprehensive service area into several compact zones, each called a cell.

A cellular network, frequently referred to as a mobile network, is a type of communication system that enables wireless communication between mobile devices. The final stage of connectivity is achieved by segmenting the comprehensive service area into several compact zones, each called a cell.

1. Cellular Networks
T.JEYA.,ASSISTANT PROFESSOR.,
DEPARTMENT OF COMPUTER SCIENCE,
SAC WOMEN’S COLLEGE,CUMBUM.

2. Cellular networks: From 1G to 3G
•1G: First generation wireless cellular: Early 1980s
–Analog transmission, primarily speech: AMPS (Advanced Mobile
Phone Systems) and others
•2G: Second generation wireless cellular: Late 1980s
–Digital transmission
–Primarily speech and low bit-rate data (9.6 Kbps)
–High-tier: GSM, IS-95 (CDMA), etc
–Low-tier (PCS): Low-cost, low-power, low-mobility e.g. PACS
•2.5G: 2G evolved to medium rate (< 100kbps) data
•3G: future Broadband multimedia
–144 kbps -384 kbps for high-mobility, high coverage
–2 Mbps for low-mobility and low coverage
•Beyond 3G: research in 4G

3. •Frequency allocation
•Licensed
•Many providers
•Multiple Access
•Many users
•Wide area of coverage
•Traffic management
•Location management
•High mobility (in cars, trains)
•Multiple suppliers
•Handoff management, roaming
•General principles
•Handled differently by different generations
Issues Vital to cellular

4. Multiple Access Techniques: How to allocate users
Time
Frequency
Session1
Session2
Session3
Session4
Frequency Division
Multiple Access (FDMA)
1G Cellular (AMPS)
Time
Frequency
Time Division
Multiple Access (TDMA)
2G TDMA
3G TDMA
Session2Session3Session1Session4
Time
Frequency
Code Division
Multiple Access (CDMA)
All sessions
based on a
code
2G CDMA (IS-95)
3G CDMA

5. A Cellular Network
Public
Switched
Telephone
Network
(PSTN)
Mobile
Telephone
Switching
Center
(MTSC)
Base Transceiver Station (BTS)
Mobile User
Cell 1
Cell 2
Cordless connection
Wired connection
HLR VLR
HLR = Home Location Register
VLR = Visitor Location Register

6. Overview of Location Services
Cell-id based location.
assigned an id of the cell that you are in.
cell-id is stored in a database.
As you move from one cell to another, you are assigned a
different cell-id and the location database is updated.
most commonly used in cellular networks. (HLR, VLR)
Neighborhood polling: Connected mobile units only move to
adjacent cells
Angle of arrival (AOA). the angle at which radio waves from your
device "attack" an antenna is used to calculate the location of
the device.
Time taken. In this case, the time taken between the device and
the antenna is used to calculate the location of the device.
Network assisted Global Positioning System (GPS). a GPS chip
is installed inside a phone and thus the location of the user is
tracked.

7. Cellular System
Handoffs (typically 30 mseconds):
1. At any time, mobile station (MS) is in one cell and under the control of a BS
2. When a MS leaves a cell, BS notices weak signal
3. BS asks surrounding BSs if they are getting a stronger signal
4. BS transfers ownership to one with strongest signal
5. MTSO assigns new channel to the MS and notifies MS of new boss
Public
Switched
Telephone
Network
(PSTN)
Mobile
Telephone
Switching
Center
(MTSC)
Cell 1
Cell 2
HLRVLR

8. Frequency Reuse
The concept of frequency reuse is based on assigning to
each cell a group of radio channels used within a small
geographic area
Cells are assigned a group of channels that is completely
different from neighbouring cells
The coverage area of cells is called the footprintand is
limited by a boundary so that the same group of channels can
be used in cells that are far enough apart

9. Frequency Reuse
•Cells with the
same number have
the same set of
frequencies
Frequency Reuse

10. Frequency Reuse using 7
frequencies allocations
f4
f3
f2
f1
f6
f7
f5 f4
f3
f2
f1
f6
f7
f5
f4
f3
f2
f1
f6
f7
f5
f4
f3
f2
f1
f6
f7
f5
f4
f3
f2
f1
f6
f7
f5
Each cell is generally 4 to 8 miles in diameter with a lower limit
around 2 miles.

12. 0G Wireless
•Mobile radio telephones were used for military
communications in early 20th century
•Car-based telephones first introduced in mid 1940s
–Single large transmitter on top of a tall building
–Single channel used for sending and receiving
–To talk, user pushed a button, enabled transmission and disabled
reception
–Became known as “push-to-talk” in 1950s
–CB-radio, taxis, police cars use this technology
•IMTS (Improved Mobile Telephone System) introduced in
1960s
–Used two channels (one for sending, one for receiving)
–No need for push-to-talk
–Used 23 channels from 150 MHz to 450 MHz

13. First-Generation Cellular
•Advanced Mobile Phone Service (AMPS) invented at Bell Labs
and first installed in 1982
•Used in England (called TACS) and Japan (called MCS-L1)
•Key ideas:
–Exclusively analog
–Geographical area divided into cells (typically 10-25km)
–Cells are small: Frequency reuse exploited in nearby (not adjacent) cells
–As compared to IMTS, could use 5 to 10 times more users in same area by
using frequency re-use (divide area into cells)
–Smaller cells also required less powerful, cheaper,smaller devices

15. E
A
D
F
G
C
B
E
A
D
F
G
C
B
E
A
D
F
G
C
B
Cell Design
•Cells grouped into a cluster of seven
•Letters indicate frequency use
•For each frequency, a buffer of two cells is used before reuse
•To add more users, smaller cells (microcells) are used
•Frequencies may not need to be different in CDMA (soft handoff)

16. Cellular Network Organization
•Cell design (around 10 mile radius)
–Served by base station consisting of transmitter,
receiver, and control unit
–Base station (BS) antenna is placed in high
places (churches, high rise buildings) -
•Operators pay around $500 per month for BS
–10 to 50 frequencies assigned to each cell
–Cells set up such that antennas of all neighbors are
equidistant (hexagonal pattern)
•In North America, two 25-MHz bands allocated to
AMPS
–One for transmission from base to mobile unit
–One for transmission from mobile unit to base

17. Approaches to Increase Capacity
•Adding/reassigning channels -some channels
are not used
•Frequency borrowing –frequencies are taken
from adjacent cells by congested cells
•Cell splitting –cells in areas of high usage
can be split into smaller cells
•Microcells –antennas move to buildings,
hills, and lamp posts

18. Security Issues with 1G
•Analog cellular phones are insecure
•Anyone with an all band radio receiver can listen in
(many scandals)
•Theft of airtime:
–all band radio receiver connected to a computer
–can record 32 bit serial number and phone number
of subscribers when calling
–can collect a large database by driving around
–Thieves go into business -reprogram stolen
phones and resell them

19. Second Generation Cellular
•Based on digital transmission
•Different approaches in US and Europe
•US: divergence
–Only one player (AMPS) in 1G
–Became several players in 2G due to competition
–Survivors
•IS-54 and IS-135: backward compatible with AMPS frequency
allocation (dual mode -analog and digital)
•IS-95: uses spread spectrum
•Europe: Convergence
–5 incompatible 1G systems (no clear winner)
–European PTT development of GSM (uses new
frequency and completelydigital communication)

23. Advantages of Digital
Communications for Wireless
•Voice, data and fax can be integrated into a
single system
•Better compression can lead to better
channel utilization
•Error correction codes can be used for better
quality
•Sophisticated encryption can be used

24. Differences Between First and
Second Generation Systems
•Digital traffic channels –first-generation systems
are almost purely analog; second-generation
systems are digital
•Encryption –all second generation systems
provide encryption to prevent eavesdropping
•Error detection and correction –second-generation
digital traffic allows for detection and correction,
giving clear voice reception
•Channel access –second-generation systems allow
channels to be dynamically shared by a number of
users

25. Integrating Data Over Cellular
•Direct access to digital channel
•Voice and data using one handset
•PCS 1900 (GSM-1900)
–9.6 kbps circuit switched data
–14.4 kbps under definition
–Packet mode specified
–Short message service
•IS-95-based CDMA
–13 kbps circuit switched data
–Packet mode specified
–Short message service

26. GSM (Global System for Mobile Communications)
•Completely designed from scratch (no backward
compatability)
•Uses 124 channels per cell, each channel can
support 8 users through TDM (992 users max)
•Some channels used for control signals, etc
•Several flavors based on frequency:
–GSM (900 MHz)
–GSM 1800 (called DCS 1800)
–GSM 1900 (called DCS 1900) -used in North America
•GSM 1900 phone only works in North America.
•In Europe, you can transfer your SIM(Subscriber
Identity Module) card to a phone of the correct
frequency. This is called SIM-roaming.

31. GSM (2G-TDMA)
•Circuit mode data
–Transparent mode
–Non-transparent mode using radio link protocol
–Data rate up to 9.6kb/s
•Short message service
–Limited to 160 characters
•Packet mode data: Plans for GSM Phase 2+
•Architecture specification very detailed
(500 pages)
•Defines several interfaces for multiple
suppliers

33. Mobile Station and Base Station Subsystem (BSS)
Mobile station
•Mobile station communicates across Um interface (air
interface) with base station transceiver in same cell as
mobile unit
•Mobile equipment (ME) –physical terminal, such as a
telephone or PCS
–ME includes radio transceiver, digital signal processors and
subscriber identity module (SIM)
•GSM subscriber units are generic until SIM is inserted
–SIMs roam, not necessarily the subscriber devices
BSS
•BSS consists of base station controller and one or more
base transceiver stations (BTS)
•BSC reserves radio frequencies, manages handoff of
mobile unit from one cell to another within BSS, and
controls paging

34. Network Subsystem Center
Mobile Switching Center (MSC) is at core; consists
of several databases
•Home location register (HLR) database –stores
information about each subscriber that belongs to
it
•Visitor location register (VLR) database –
maintains information about subscribers currently
physically in the region
•Authentication center database (AuC) –used for
authentication activities, holds encryption keys
•Equipment identity register database (EIR) –
keeps track of the type of equipment that exists at
the mobile station

35. GSM Location Services
Public
Switched
Telephone
Network
(PSTN)
Gateway
MTSC
VLR
HLR
Terminating
MSC
1
1. Call made to mobile unit (cellular phone)
2. Telephone network recognizes number
and gives to gateway MSC
3. MSC can’t route further, interrogates
user’s HLR
4. Interrogates VLR currently serving user
(roaming number request)
5. Routing number returned to HLR and
then to gateway MSC
2
3
4
5
5
6
6. Call routed to terminating MSC
7. MSC asks VLR to correlate call to
the subscriber
8. VLR complies
9. Mobile unit is paged
10. Mobile unit responds, MSCs convey
information back to telephone
7 8
9
BTS
910
10
10 10
10
Legend: MTSC= Mobile Telephone Service Center, BTS = Base Transceiver Station
HLR=Home Location Register, VLR=Visiting Location Register

36. GSM Protocol Architecture
BSSMAP = BSS Mobile Application part
BTSM = BTS management
CM = Connection Management
LAPD = Link Access Protocol, D Channel
Base Transceiver
Station
Mobile
Station
Radio
LAPDm
RRM
Radio
LAPDm
RRM
MM
CM
64 Kbps
LAPD
BTSM
64 Kbps
MTP
SCCP
Base Station
Controller
64 Kbps
LAPD
BTSM
BSSMAP
64Kbps
MTP
SCCP
MM
CM
BSSMAP
Mobile Service
Switching Center
MM = Mobility Management
MTP = Message Transfer Part
RRM = Radio Resources Management
SCCP = Signal Connection Control Point

37. Functions Provided by Protocols
•Protocols above the link layer of the GSM
signaling protocol architecture provide
specific functions:
–Radio resource management: controls setup,
termination and handoffs of radio channels
–Mobility management: location and security
(MTSO)
–Connection management: connects end users
–Mobile application part (MAP): between
HLR,VLR
–BTS management: management base system

38. 2G CDMA Cellular
IS-95 is the best known example of 2G with
CDMA
Advantages of CDMA for Cellular
•Frequency diversity –frequency-dependent
transmission impairments have less effect on
signal
•Multipath resistance –chipping codes used for
CDMA exhibit low cross correlation and low
autocorrelation
•Privacy –privacy is inherent since spread
spectrum is obtained by use of noise-like signals
•Graceful degradation –system only gradually
degrades as more users access the system

39. Drawbacks of CDMA Cellular
•Self-jamming –arriving transmissions from
multiple users not aligned on chip boundaries
unless users are perfectly synchronized
•Near-far problem –signals closer to the receiver
are received with less attenuation than signals
farther away
•Soft handoff –requires that the mobile acquires
the new cell before it relinquishes the old; this is
more complex than hard handoff used in FDMA
and TDMA schemes

40. Types of Channels Supported by
Forward Link
•Pilot (channel 0) -allows the mobile unit to
acquire timing information, provides phase
reference and provides means for signal strength
comparison
•Synchronization (channel 32) -used by mobile
station to obtain identification information about
cellular system
•Paging (channels 1 to 7) -contain messages for
one or more mobile stations
•Traffic (channels 8 to 31 and 33 to 63) –the
forward channel supports 55 traffic channels

41. Forward Traffic Channel Processing Steps
•Speech is encoded at a rate of 8550 bps
•Additional bits added for error detection
•Data transmitted in 2-ms blocks with forward error
correction provided by a convolutional encoder
•Data interleaved in blocks to reduce effects of errors
•Data bits are scrambled, serving as a privacy mask
•Power control information inserted into traffic channel
•DS-SS function spreads the 19.2 kbps to a rate of 1.2288
Mbps using one row of 64 x 64 Walsh matrix
•Digital bit stream modulated onto the carrier using QPSK
modulation scheme

42. Wireless Network Evolution to 3
rd
Generation
Enabling Technologies
AMPS
GSM
IS-95
GPRS
CDMA-2000
1XRTT
EDGE
CDMA2000
3XRTT
(UMTS)
2.5G
3G
2G
2 Mbps
500 kbps
150 Kbps
100 Kbps
50 Kbps
10 Kbps
1999 2000 2001 2002 2003
TDMA Migration
1G-2G Migration
CDMA Migration
1980
1G
1 Kbps
W-CDMA
(UMTS)

43. •Fig 8-13

44. 2G Technologies
cdmaOne (IS-95)GSM, DCS-1900 IS-54/IS-136
PDC
Uplink Frequencies
(MHz)
824-849 (Cellular)
1850-1910 (US PCS)
890-915 MHz (Eurpe)
1850-1910 (US PCS)
800 MHz, 1500 Mhz
(Japan)
1850-1910 (US PCS)
Downlink Frequencies 869-894 MHz (US
Cellular)
1930-1990 MHz (US
PCS)
935-960 (Europa)
1930-1990 (US PCS)
869-894 MHz (Cellular)
1930-1990 (US PCS)
800 MHz, 1500 MHz
(Japan)
Deplexing FDD FDD FDD
Multiple Access CDMA TDMA TDMA
Modulation BPSK with Quadrature
Spreading
GMSK with BT=0.3 p/4 DQPSK
Carrier Seperation 1.25 MHz 200 KHz 30 KHz (IS-136)
(25 KHz PDC)
Channel Data Rate 1.2288 Mchips/sec 270.833 Kbps 48.6 Kbps (IS-136)
42 Kbps (PDC)
Voice Channels per
carrier
64 8 3
Speech Coding CELP at 13Kbps
EVRC at 8Kbps
RPE-LTP at 13 Kbps VSELP at 7.95 Kbps

45. Alternatives to 3G Cellular
•Majortechnicalundertakingwithmany
organizationalandmarketingovertones.
•Questionsabouttheneedfortheadditional
investmentfor3G(happywith2.5G)
•WirelessLANinpublicplacessuchasshopping
mallsandairportsofferoptions
•Otherhigh-speedwireless-datasolutionscompete
with3G
–Mobitexlowdatarates(nominally8Kbps),itusesanarrowband
(2.5KHz)ascomparedto30KHz(GSM)and5MHz(3G).
–Ricochet:40-128kbpsdatarates.Bankruptcy
–Flash-OFDM:1.5Mbps(upto3Mbps)

46. Major Mobile Radio Standards
USA
Standard Type Year
Intro
Multiple
Access
Frequency
Band
(MHz)
ModulationChanne
l
BW
(KHz)
AMPS Cellular 1983FDMA 824-894 FM 30
USDC Cellular 1991TDMA 824-894 DQPSK 30
CDPD Cellular 1993FH/Packet 824-894 GMSK 30
IS-95 Cellular/PCS1993CDMA 824-894
1800-2000
QPSK/BPSK 1250
FLEX Paging 1993Simplex Several 4-FSK 15
DCS-1900
(GSM)
PCS 1994TDMA 1850-1990 GMSK 200
PACS Cordless/PC
S
1994TDMA/FDMA 1850-1990 DQPSK 300

47. Major Mobile Radio Standards -
Europe
Standard Type Year
Intro
Multiple
Access
Frequency
Band
(MHz)
ModulationChanne
l
BW
(KHz)
ETACS Cellular 1985FDMA 900 FM 25
NMT-900Cellular 1986FDMA 890-960 FM 12.5
GSM Cellular/PCS1990TDMA 890-960 GMSK 200KHz
C-450 Cellular 1985FDMA 450-465 FM 20-10
ERMES Paging 1993FDMA4 Several 4-FSK 25
CT2 Cordless 1989FDMA 864-868 GFSK 100
DECT Cordless 1993TDMA 1880-1900 GFSK 1728
DCS-1800Cordless/PC
S
1993TDMA 1710-1880 GMSK 200

48. 4G Systems
•Wireless networks with cellular data rates of 20
Mbits/second and beyond.
•AT&T has began a two-phase upgrade of its wireless
network on the way to 4G Access.
•Nortel developing developing features for Internet
protocol-based 4G networks
•Alcatel, Ericsson, Nokia and Siemens found a new
Wireless World Research Forum (WWRF) for research on
wireless communications beyond 3G.
•Many new technologies and techniques (multiplexing,
intelligent antennas, digital signal processing)
•Industry response is mixed (some very critical)