Spectrum

Spread spectrum:-
As the name suggest ,spread spectrum techniques involve spreading the band width needed to tansmit the data - which does not make sense at the first sight.Spreading the bandwidth has a several advantages.The main advantage is the resistance to narrow band interferance .Idealized narrowband signal from a sender of user data .The sender now spreads the signal ..i.e converts the narrow band signal to broad band signal.The energy neededto transmit the signal is the same , but it is now spread over a large frequncy range.The power level of the spread signal without losing data .Depending on the generation and reception of the spread signal, the power level of the signal can even be low as background noise.This makes it difficult todistingiush the user signalfrom background noise and thus hard to detect
During transmission , narrowband and broad band interferance add to signal.The sum of interferance and user signal is received .The receiver now knows how to de-spread the signal,converting the spread signal into narrowband signal again,while spreading the narrowband interferance and leaving the broadband interferance.The receiver can reconstruct the original data because the power level of the user signal is high enough ,ie the signal is much stronger than the reamining interferance.The folowing sections show how spreading can be performed.
The seperation of channels ,CDM is now used insted of FDM .This application shows the tight coupling of CDM and spred spectrum .Spreading the narrowband signal is acheived using a special code.