Chap# 2-Multiple Access Techniques explained

Multiple access techniques are essential for allowing multiple users to share the same communication resources effectively and efficiently. Here are the primary types of multiple access techniques used in wireless communication:

Time Division Multiple Access (TDMA):

Description: In TDMA, the available bandwidth is divided into time slots. Each user is assigned a specific time slot during which they can transmit or receive data. This ensures that multiple users can share the same frequency channel without interference by transmitting in different time intervals.
Applications: TDMA is used in various systems, including 2G GSM cellular networks and certain satellite communications.
Frequency Division Multiple Access (FDMA):

Description: FDMA allocates separate frequency bands to each user. Each user operates on a distinct frequency channel, minimizing interference between users. The total available bandwidth is divided into multiple frequency bands, each assigned to a different user.
Applications: FDMA is commonly used in analog cellular systems, traditional broadcast radio, and television.
Code Division Multiple Access (CDMA):

Description: In CDMA, each user is assigned a unique code that is used to modulate their signal. Multiple users can transmit simultaneously over the same frequency band, and the receiver uses the unique code to distinguish between different signals. This technique allows efficient use of the available bandwidth and provides resistance to interference and eavesdropping.
Applications: CDMA is widely used in 3G cellular networks (such as CDMA2000 and WCDMA) and in GPS systems.
Orthogonal Frequency Division Multiple Access (OFDMA):

Description: OFDMA is an extension of Orthogonal Frequency Division Multiplexing (OFDM). In OFDMA, the available frequency spectrum is divided into orthogonal subcarriers, and users are assigned different subcarriers. This technique allows simultaneous transmission by multiple users with minimal interference.
Applications: OFDMA is used in 4G LTE, WiMAX, and some modern Wi-Fi standards (such as Wi-Fi 6).
Space Division Multiple Access (SDMA):

Description: SDMA uses spatial separation to allocate resources to users. It leverages multiple antennas (MIMO technology) to direct signals towards specific users, allowing multiple users to share the same frequency channel simultaneously. By focusing signals in different spatial directions, SDMA minimizes interference.
Applications: SDMA is utilized in advanced MIMO systems, 5G networks, and satellite communications.
Non-Orthogonal Multiple Access (NOMA):

Description: NOMA allows multiple users to share the same time and frequency resources by assigning different power levels to each user. Signals are superimposed, and advanced signal processing techniques are used to separate the signals at the receiver. This technique improves spectral efficiency and user fairness.Multiple access techniques are essential for allowing multiple users to share the same

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https://www.electronics-notes.com/articles/connectivity/2g-gsm/rf-
air-interface-slot-burst.php

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24. Spread Spectrum
FHSS
Frequency Hopping Spread Spectrum
DSSS
Direct Sequence Spread Spectrum
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39. For each channel the base station generates a unique code
that changes for every connection.
The base station adds together all the coded transmissions
for every subscriber.
The subscriber unit correctly generates its own matching
code and uses it to extract the appropriate signals.
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40. It must be deterministic. The subscriber station must be able to
independently generate the code that matches the base station
code.
It must appear random to a listener without prior knowledge of
the code (i.e. it has the statistical properties of sampled white
noise).
The cross-correlation between any two codes must be small
The code must have a long period (i.e. a long time before the
code repeats itself).
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41. The FEC coded Information data modulates the pseudo-random code.
Chipping Frequency (fc):the bit rate of the PN code.
Information rate (fi):the bit rate of the digital data.
Chip:One bit of the PN code.
Epoch:The length of time before the code starts repeating itself (the period of the
code). The epoch/duration must be longer than the round trip propagation delay
(The epoch is on the order of several seconds).
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42. the bandwidth of a digital signal is twice its bit rate. The bandwidths of the information
data (fi) and the PN code are shown together. The bandwidth of the combination of the
two, for fc>fi, can be approximated by the bandwidth of the PN code.
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43. This is a theoretical system gain that reflects the relative
advantage that frequency spreading provides. The processing
gain is equal to the ratio of the chipping frequency to the data
frequency:
There are two major benefits from high processing gain:
Interference rejection: the ability of the system to reject interference is directly
proportional to Gp.
System capacity: the capacity of the system is directly proportional to Gp.
So the higher the PN code bit rate (the wider the CDMA bandwidth), the better the
system performance.
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A PN generator is typically made of N cascaded flip-flop circuits and a
specially selected feedback arrangement as shown in figure below:
The flip-flopcircuits when used in this way are called a shift register,
since each clock pulse applied to the flip-flops causes the contents of each
flip-flop to be shifted to the right.
The period of the PN sequence is:
PN Sequence Generation
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Example:
Startingwiththeregisterinstate001
the next 7 states are :
100, 010, 101, 110, 111, 011
and then 001 again and the states
repeats
The output taken from the right-most flip-flop is 1001011 and then
continue to repeat.
The three-stage shift register shown, the period is
PN Sequence Generation
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The tap connections are based on primitive polynomials on the order of the
number of registrars.
The polynomial should be irreducible for the sequence to be an m-sequence
and have the desired properties.
PN Sequence Generation
For example, IS-95 specifies the in-phase PN generator shall be built
based on the characteristic polynomial:
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IS-95 and IS-2000 use two types of m-sequences but have special names
and uses and are called:
Long codes and Short codes
Long codes andShort codes
Long code
The long PN code is generated by a 42-stage linear shift register.
The length of the Long code is
This code runs at the chip frequency of 1.2288 Mc/s
The time it takes to recycle this length of code at this speed is 41.2 days
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Itisusedtobothspreadthesignalandtoencryptit.
Acyclicallyshiftedversionofthelongcodeisgeneratedbythecellphone
duringcallsetup.
TheshiftiscalledtheLongCodeMaskandisuniquetoeachphonecall
Long codes andShort codes
Long Codes
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49. Short code
The short code used in CDMA system is based on a m-sequence created
from a LFSR of 15 registers.
The code Length is L =
Long codes andShort codes
The short code repeats every 26.666 milliseconds. The sequences repeat
exactly 75 times in every 2 seconds.
These codes are used for synchronization in the forward and reverse
links and for cell/basestation identification in the forward link.
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During call setup, the mobile looks for a short code and needs to be able
find it fairly quickly as two seconds is the maximum time that a mobile will
need to find a base station.
If one is present because in 2 seconds the mobile has checked each of
the allowed base stations in its database against the network signal it is
receiving.
Each base-station is assigned one of these codes.
Since short code is only one sequence, each station gets the same
sequence but cyclically shifted.
Long codes andShort codes
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PN Offset and PN Roll
Different cells and cell sectors all use the same short code, but use
different phases or shifts, which is how the mobile differentiates one base
station from another.
The phase shift is known as the PN Offset
For short code there can be 32,768 PN offsets.
The moment when the Short code wraps around and begins again is called
a PN Roll
There are actually two short codes per base station. One for each I and Q
channels to be used in the quadrature spreading and despreading of
CDMA signals.
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From properties of the m-sequences, the shifted version of a m-sequences
has a very small cross correlation and so each shifted code is an
independent code.
if two adjacent offsets are used, a multi-path of the leading sequence
(delayed by exactly one chip) would look identical to the lagging sequence.
In IS-95, a 64 chip separation is recommended for each adjacent station.
This gives 512 different short PN offsets used for different cells and cell
sectors, that is how the mobile differentiates one base station from another.
PN Offset and PN Roll
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Simplified model for code usage in CDMA
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End Of Chapter # 2