1. 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.

2. Spread spectrum

3. 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

4. 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

5. Spread Spectrum

6. 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

7. 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

8. 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

9. 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)

10. Frequency Hoping Spread Spectrum

11. Figure 6.28 Frequency hopping spread spectrum (FHSS)

12. Figure 6.29 Frequency selection in FHSS

13. Figure FHSS cycles

14. Figure 6.31 Bandwidth sharing

15. Frequency Hopping Spread Spectrum

16. Slow Frequency-Hop SS

17. Fast Frequency-Hop SS

18. 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)

19. Approximate Spectrum of DSSS Signal

20. 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

21. Figure DSSS

22. Direct Sequence Spread Spectrum (DSSS)

23. DSSS Using BPSK