Spread spectrum signal is different from normal signal in the many aspects. These is a invariably uses some kind of coding this signal occupies an large bandwidth than that of normal signal therefore the name is spread spectrum. Under spread spectrum spreading at the transmitter and despreading at the receiver is obtained with the help of this code word.
The code word associated with an spread spectrum signal is independent of the information carried by the signal only specially designed receiver can it to recover the information.
the most important thing about about the spread spectrum is pseudo random in nature, so so this makes it appear like random noise therefore the normal ratio cannot be modulated the spread spectrum signal.
Classification of spread spectrum modulation techniques
Spread spectrum modulation technique is divided into two types
1) the avoidance type system
2) averaging type system.
The averaging type system reduces the interference by averaging it over a long period. the direct sequence spread spectrum system is an averaging system.
Avoidance system reduces the interference by making the signal avoid the interference over a large fraction of time. the avoidance system are further classified depending on the type of modulation used. some of the avoidance type system are classified further using different modulation techniques are as follows.
1) frequency hopping system
2) time hoping system
3) chirp
4) Hybrid modulation system
The basic elements of spread spectrum digital communication system.
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| diagram of Spread specrum digital communication systeam |
Operation of transmitter:-
Operation of transmitter it can consists of two elements channel encoder and modulator. Information sequence is an input to the channel encoder it is and binary information sequence. The same signal is recovered at the output of system as output data signal.
In which additional block is used called psuedorandom Pattern Generator. In which one of them is connected to the moderator of translatory while other is connected to the d modulator at receiving side. Both these generator are identical to each other.
This generate generate Shuru random binary sequence it is impressed on the transmitted signal at the modulator. those the modulated signal along with the pseudo random sequence Travels over the communication channel.
this sequence spread the signal randomly over a wide frequency band.
Does the output of the modulated signal is spread spectrum signal.
Operation of the receiver:-
- The pseudo random sequence is removed from the signal by the other pseudo random generator operating at the receiver.
-Thus the pseudo random Pattern Generator operator in synchronisation with each other.
- the synchronisation between this Pattern Generator is achieved before the beginning of the signal transmission.
- this is done by transmitting a fixed pseudo-random which a receiver can recognise even in presence of interference. Once this synchronisation is established it is possible to begin the transmission.
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