 modulation and non-coherent demodulation are normally used Dixon, R.C. (1976) Spread Spectrum Systems, Wiley, New York. in FHSS, rather than PSK and coherent demodulation. Forney, G.D. (1970) Convolutional codes 1: Algebraic structure, In block hopping FHSS, the input signal is first modulated using IEEE Trans. Info. Theory, (IT-16) pp. 720-738. a conventional binary or M-ary FSK modulator, whose output is one Lin, S. and Costello, D.J. (1983) Error control coding: fundamen- of M frequencies within a bandwidth W. This block of frequencies tals and applications, Prentice-Hall, Englewood Cliffs, NJ. is then shifted to somewhere in the full bandwidth LW by mixing it with a local oscillator signal derived from a frequency synthesiser Michelson, A.M. and Levesque, A.H. (1985) Error-control tech- controlled by a PN generator. The receiver has a matching PN niques for digital communication, Wiley-Interscience, New generator, frequency synthesiser and mixer, which shift the fre- York. quency block down to baseband again, where it is demodulated by NBS (1977) National Bureau of Standards. Data encryption stand- a conventional FSK demodulator. An alternative approach, which ard, Federal Information Processing Standard (FIPS) Publica- has better resistance to some kinds of jamming, but requires a more tion No. 46. complex demodulator, is called independent tone hopping. In this, Peterson, W.W. and Weldon, E.J. (1972) Error-correcting codes, each digit value is combined with the output of the PN generator, to 2nd ed., MIT Press, Cambridge, MA. control the frequency synthesiser directly. The resulting frequency separation of the tones corresponding to different digit values at any Proakis, J.G. (1989) Digital communications, 2nd ed., McGraw- particular time can be up to the full channel bandwidth. Hill, New York. The frequency hopping rate is usually chosen to be equal to or Rivest, R.L., Shamir, A. and Adelman, L. (1978) A method for faster than the symbol rate. If they are equal, it is referred to as obtaining signatures a