Satellite Notes

Spread Spectrum

Spread Spectrum

 Input is fed into a channel encoder

o Produces analog signal with narrow bandwidth

 Signal is further modulated using sequence of digits

o Spreading code or spreading sequence o Generated by pseudonoise, or pseudo-random number generator

 Effect of modulation is to increase bandwidth of signal to be transmitted  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?

o Immunity from various kinds of noise and multipath distortion o Can be used for hiding and encrypting signals o 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

o A number of channels allocated for the FH signal o Width of each channel corresponds to bandwidth of input signal

 Signal hops from frequency to frequency at fixed intervals

o Transmitter operates in one channel at a time o Bits are transmitted using some encoding scheme o 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

o Eavesdroppers hear only unintelligible blips o Attempts to jam signal on one frequency succeed only at knocking out a few bits

Frequency Hopping 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:

o duration of a bit: T = 1/R seconds o duration of signal element: Ts = LT seconds

 Tc ≥ Ts - slow-frequency-hop spread spectrum

o Cheaper to implement but less protection against noise/jamming o...