无线通信与网络chap7.ppt

Spread Spectrum Chapter 7 Spread Spectrum Input is fed into a channel encoder Produces analog signal with narrow bandwidth Signal is further modulated using sequence of digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number generator Effect of modulation is to increase bandwidth of signal to be transmitted Spread Spectrum On receiving end, digit sequence is used to demodulate the spread spectrum signal Signal is fed into a channel decoder to recover data Spread Spectrum Spread Spectrum What can be gained from apparent waste of spectrum? Immunity from various kinds of noise and multipath distortion Can be used for hiding and encrypting signals Several users can independently use the same higher bandwidth with very little interference Frequency Hoping Spread Spectrum (FHSS) Signal is broadcast over seemingly random series of radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input signal Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is selected Frequency Hoping Spread Spectrum Channel sequence dictated by spreading code Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only at knocking out a few bits Frequency Hoping Spread Spectrum FHSS Using MFSK MFSK signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signal For data rate of R: duration of a bit: T = 1/R seconds duration of signal element: Ts = LT seconds Tc ? Ts - slow-frequency-hop spread spectrum Tc Ts - fast-frequency-hop spread spectrum FHSS Performance Considerations Large number of frequencies used Results in a system that is quite resistant to