Chap# 2-Multiple Access Techniques explained

EE-494 Wireless communication
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https://www.electronics-notes.com/articles/connectivity/2g-gsm/rf-
air-interface-slot-burst.php

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Spread Spectrum
FHSS
Frequency Hopping Spread Spectrum
DSSS
Direct Sequence Spread Spectrum
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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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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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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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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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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-flop circuits 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:
Starting with the register in state 001
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
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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.
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 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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It is used to both spread the signal and to encrypt it.
A cyclically shifted version of the long code is generated by the cell phone
during call setup.
The shift is called the Long Code Mask and is unique to each phone call
Long Codes
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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 =
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/base station 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.
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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

Chap# 2-Multiple Access Techniques explained

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Chap# 2-Multiple Access Techniques explained