hmetic operation particular PN pattern, the interference from the other PN sequences involving E. The recipient recovers the plaintext using further is therefore low. The chosen sequences must also have noise-like simple modulo-N operations which rely on knowledge of the factors autocorrelation functions, to help the receiver to synchronise cor- References 14/13 rectly to the partially unknown timing of the transmitter. Some hopping. Fast hopping is used to combat a follower jammer, which often-used sequences with these properties are called Gold and attempts to detect the tone(s) and immediately broadcasts other Kasami sequences (Proakis, 1989). tones with adjacent frequencies. However, when fast hopping is Direct sequence spread spectrum requires the overall channel used, the fact that phase coherence cannot be maintained across (including, where relevant, equalisation in the receiver) to have hops means that the energy from the successive hops in one symbol approximately unity gain, pure delay characteristics over the whole must be combined incoherently; this causes a non-coherent combin- signal bandwidth. This is achievable for local radio systems and ing loss. transmission lines, but can be much harder to achieve over a wide FHSS signals are primarily used in AJ and CDMA systems. bandwidth in long distance radio links. Usually, FHSS is preferred over DSSS because it has less severe timing requirements and is less sensitive to channel gain and phase 14.8.3 fluctuations. Frequency hopping spread spectrum In Frequency Hopping Spread Spectrum (FHSS), the available channel bandwidth LW is divided into L slots of bandwidth W. In 14.9 References any signalling interval the signal occupies only one slot or a few (<<L) slots. The spreading of the spectrum arises because the active Diffie, W. and Hellman, M.E. (1976) New direction in crypto- slot frequency 'hops' around pseudo-randomly. Because of the graphy, IEEE Trans. Inform. Theory, pp. 644-654. difficulty of maintaining phase references as the frequency hops, FSK