Spread Spectrum A signal that occupies a bandwidth

Spread Spectrum • A signal that occupies a bandwidth of B, is spread out to occupy a bandwidth of Bss • All signals are spread to occupy the same bandwidth Bss • Signals are spread with different codes so that they can be separated at the receivers. • Signals can be spread in the frequency domain or in the time domain. 6. 1

Spread spectrum 6. 2

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

Gains • Immunity from various noise and multipath distortion – Including jamming • Can hide/encrypt signals – Only receiver who knows spreading code can retrieve signal • Several users can share same higher bandwidth with little interference – Cellular telephones – Code division multiplexing (CDM) – Code division multiple access (CDMA)

Frequency Hoping Spread Spectrum

Figure 6. 28 Frequency hopping spread spectrum (FHSS) 6. 11

Figure 6. 29 Frequency selection in FHSS 6. 12

Figure 6. 30 FHSS cycles 6. 13

Figure 6. 31 Bandwidth sharing 6. 14

Frequency Hopping Spread Spectrum

Slow Frequency-Hop SS

Fast Frequency-Hop SS

Direct Sequence Spread Spectrum (DSSS) • Each bit in original signal is represented by multiple bits in the transmitted signal • Spreading code spreads signal across a wider frequency band – Spread is in direct proportion to number of bits used • One technique combines digital information stream with the spreading code bit stream using exclusive. OR (Figure 7. 6)

Approximate Spectrum of DSSS Signal

Direct Sequence Spread Spectrum (DSSS) • Each bit represented by multiple bits using spreading code • Spreading code spreads signal across wider frequency band – In proportion to number of bits used – 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code • One method: – – Combine input with spreading code using XOR Input bit 1 inverts spreading code bit Input zero bit doesn’t alter spreading code bit Data rate equal to original spreading code • Performance similar to FHSS

Figure 6. 32 DSSS 6. 21

Direct Sequence Spread Spectrum (DSSS)

DSSS Using BPSK