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Spread Spectrum modulation technique
In the concept of spread spectrum modulation technique, input is fed into a channel encoder that
produces an analog signal with a relatively narrow bandwidth around some centre frequency.
This signal is further modulated using a sequence of digit known as spreading code
or spreading sequences.Typically but not always the spreading code is generated by a
PseudoNoise generator.
The effect of this modulation is to increase the bandwidth (spred the spectrum) of the signal to be
transmitted. On the receiving end, the same digit sequence is used to demodulate the spread
spectrum signal. Finally the signal is fed into a channel decoder to recover the data.
Introduction:
Initially developed for military applications during II world war, that was less sensitive to intentional
interference or jamming by third parties.
Spread spectrum technology has blossomed into one of the fundamental building blocks
in current and next-generation wireless systems.
Problem of radio transmission
Narrow band can be wiped out due to interference
To disrupt the communication, the adversary needs to do two things,
(a) to detect that a transmission is taking place and
(b) to transmit a jamming signal which is designed to confuse the receiver.
.Solution
A spread spectrum system is therefore designed to make these tasks as difficult as
possible.
Firstly, the transmitted signal should be difficult to detect by an adversary/jammer, i.e.,
the signal should have a low probability of intercept (LPI).
Secondly, the signal should be difficult to disturb with a jamming signal, i.e., the
transmitted signal should possess an anti-jamming (AJ) property
Remedy spread the narrow band signal into a broad band to protect against interference
In a digital communication system the primary resources are Bandwidth and Power. The study of digital communication system deals with efficient utilization of
these two resources, but there are situations where it is necessary to sacrifice their
efficient utilization in order to meet certain other design objectives.
For example to provide a form of secure communication (i.e. the transmitted
signal is not easily detected or recognized by unwanted listeners) the bandwidth of the
transmitted signal is increased in excess of the minimum bandwidth necessary to transmit
it. This requirement is catered by a technique known as “Spread Spectrum
Modulation”.
The primary advantage of a Spread – Spectrum communication system is its
ability to reject ‘Interference’ whether it be the unintentional or the intentional
interference.
The definition of Spread – Spectrum modulation may be stated in two parts.
1. Spread Spectrum is a mean of transmission in which the data sequence occupies a
BW (Bandwidth) in excess of the minimum BW necessary to transmit it.
2. The Spectrum Spreading is accomplished before transmission through the use of
a code that is independent of the data sequence. The Same code is used in the
receiver to despread the received signal so that the original data sequence may be
recovered.
Frequency hopping multiple access (FHMA)
Frequency Hopping Multiple Access (FHMA) is a spread-spectrum transmission technology,
which permits simultaneous voice or data communications to share the exact same communication
medium.
This is done by allowing the receiving and transmitting stations to adjust the frequency swiftly in
a pseudorandom sequence between several discrete radio channels. Transceivers are synchronized with a
hopping sequence computed from a predefined algorithm. This hopping sequence can be modified
effectively to avoid various other interference and transmissions in the same frequency band.
There are 2 types of frequency hopping.
1. Fast FH fast fh change the carrier frequency several times during transmission of one
symbol i.e. transmission of each separate symbol is spread over a large bandwidth,so the effect
of fading or interference is occur.
CDMA – Code Division Multiple Access
Fully digital wireless data transmission system
• Not designed for voice at all
Uses special random numbers to encode bits of information.
Allows multiple access by assigning different users different random numbers on the
same channel.
• Users have control of a very wide channel bandwidth 1.5 to 5 MHz
The only limit to the system is the computing prowess of the base station and it’s ability
to separate noise from actual data.
• Shannon’s Theorem / SQR
CDMA is a transmission technique to pass information from the mobile to the base station and
from the base station back to the mobile.
CDMA Analogy
10 people in a room.
5 speak English, 2 speak Spanish, 2 speak Chinese, and 1 speaks Russian.
Everyone is talking at relatively the same time over the same medium – the air.
Who can listen to whom and why?
Who can’t you understand?
Who can’t speak to anyone else?
Spread Spectrum.
A signal takes up 6 – 10 times the bandwidth that it needs at a minimum.
This seems deliberately inefficient.
The military used spread spectrum communications because the signal is:
Difficult to block.
Difficult to listen in on.
Difficult to even identify from noise.
Much more difficult to tune into a certain frequency.
In CDMA, all users share the same 1.25 MHz bandwidth.
They all transmit a signal that’s the exact same size, 1.25 MHz
There’s actually .02 MHz of a guard band, meaning that the actual bandwidth is
1.23 MHz.
This would be like 100 AM radio stations all transmitting on the exact same frequency.
However, with CDMA, unique digital codes are used to separate each of the mobile phones.
Essentially, this makes each mobile phone speak a different language.
Also, it’s language is very unpredictable, it starts at a random language and
changes in random fashion with a given seed.
Also, the base station can speak every language as long as it is synchronized.
Also, the languages are special in that they will be able to mathematically never
interfere with each other.
Each bit of the conversation is encoded with this special code.
The base station and mobile phone have an algorithm for generating pseudo random numbers.
Uses something called Walsh Vectors.
This mathematical function has a way to generate 128 bit random numbers that
are orthogonal to every other random number that is has generated.
This random number generator has a very large period.
When they both start at the same seed (the time), both the mobile and base station should
generate the same random numbers.
The random number is actually only 32 bits.
This random number is convoluted with the data.
Also, a time stamp is added.
And error codes are added.
The result is 128 bits that represent only 1 bit of data.
This is a very computationally intense process.
• But, modern cell phones have fast processors.
So, the original 9600 bps of conversation has been multiplied to 1.23 Mbps.
These codes are designed to never interfere with any other codes to a very high probability.
Example (on board)
The base station, using the mobile’s known code, can convolute this code with everything that it
received.
This convolution results in only what the mobile sent.
Advantages of CDMA
Capacity is CDMA's biggest asset. It can accommodate more users per MHz of bandwidth than
any other technology.
3 to 5 times more than GSM
CDMA has no built-in limit to the number of concurrent users.
CDMA uses precise clocks that do not limit the distance a tower can cover.
CDMA consumes less power and covers large areas so cell size in CDMA is larger.
CDMA is able to produce a reasonable call with lower signal (cell phone reception) levels.
CDMA uses Soft Handoff, reducing the likelihood of dropped calls.
CDMA's variable rate voice coders reduce the rate being transmitted when speaker is not talking,
which allows the channel to be packed more efficiently.
Has a well-defined path to higher data rates.
Multiuser Detection(MUD)
A cellular system has a number of mobiles which communicate with one base station
(BS). The BS has to detect all the signals whereas each mobile is concerned with its own
signal. This implies that the BS must know all the chip sequence. In multiuser detection, one of
the main drawbacks is that of complexity. There is always a trade -off between complexity and
performance of the system. Due to above mentioned two points, the main use of the multiuser
detection system is for the BS, or in the reverse link (mobile to BS).
The Base Station records information only on the mobiles in its own cell. This limits
improvements to be expected in a MUD system
MUD Concepts and Techniques:
A baseband model of a CDMA uplink is shown below.
The signal received at the BS is the superposition of signals from all users, multipath
components for each user’s signal, and Additive White Gaussian Noise (AWGN). The figure
also includes channel encoders for each transmitter.