Spread spectrum: FHSS and DSSS

Spread spectrum

Spread spectrum (SS) is a technique in which a signal of bandwidth B is deliberately spread over a much larger bandwidth Bss before transmission, using a code (spreading sequence) that is independent of the data. The receiver uses the same code to despread it. Two requirements: Bss must be much larger than B, and the spreading code must be independent of the message.

Advantages: resistance to jamming and interference, low probability of interception, privacy, and multipath resistance.

Frequency Hopping Spread Spectrum (FHSS)

The available bandwidth is divided into 2^k channels (hop frequencies). A pseudorandom code generator (PN) produces a k-bit pattern, and the frequency synthesizer moves the carrier to the corresponding frequency. The carrier stays on each frequency for a short hop period and then jumps to the next. Sender and receiver use the same PN sequence, so they hop together.

 Data --> [Modulator] --> [Freq synthesizer] --> Tx
 (FSK/PSK)                     ^
                        [PN code generator]
 Rx: Signal --> [Freq synthesizer] --> [Demod] --> Data
                       ^
              [PN code generator, same seed]

An eavesdropper without the PN sequence hears only short bursts, and narrow-band interference hurts only the hops that coincide with it. Example: Bluetooth hops 1600 times per second over 79 channels of 1 MHz.

Direct Sequence Spread Spectrum (DSSS)

Each data bit is replaced by n chips using a spreading code (chip sequence), which has a much higher rate. The data signal is XOR-ed (multiplied) with the PN chip sequence, so the bandwidth increases by factor n (processing gain).

 Data --> [XOR] --> [Modulator BPSK] --> Tx
            ^
   [PN chip generator]
 Rx: Signal --> [Demod BPSK] --> [XOR] --> Data
                                   ^
                          [same PN chips]

Example: Wi-Fi 802.11b uses an 11-chip Barker sequence 10110111000. A data bit 1 becomes 10110111000, and 0 becomes its complement. The spread signal looks like noise to receivers without the code.