Techniques for the synchronisation and demodulation of fast frequency hopped M-ary frequency shift keying

Gibbs, Jonathan Alastair (1991) Techniques for the synchronisation and demodulation of fast frequency hopped M-ary frequency shift keying. University of Southampton, Doctoral Thesis.

Abstract

In this thesis we consider the synchronisation and demodulation of fast frequency hopped M-ary FSK with convolutional coding in the presence of jamming, multiple access noise, frequency uncertainty and random frequency variations. We consider three modulation formats, 4-FSK, 8-FSK and 16-FSK and describe a family of 1/2 rate constraint length 7 symbol-based convolutional codes. We investigate the performance of the proposed systems with 2nd order diversity, in addition to the 1/2 rate encoding, and highlight the worst-case partial-band tone jammer waveform for such schemes. Consideration is given to a receiver based upon the offset FFT, where these transforms are then processed in order to achieve maximum a posteriori carier frequency synchronisation, demodulation and decoding of the received signal. Two hop-by-hop AGC techniques are described, the SNORM and the DMAX techniques; the former normalises the OFFT amplitudes with reference to the total power within the OFFT whilst the latter performs the normalisation based upon the maximum amplitude slot present within the OFFT. The worst-case partial-band tone jammer is generalised for the OFFT receiver when transform sizes exceed the modulation set size. A Viterbi frequency synchroniser is presented. This technique combines frequency synchronisation with the soft decision decoding of the convolutional code. By considering both the mean and the standard deviation of the signal slot amplitudes across the four OFFT slots which comprise each branch metric of the Viterbi decoding process, a novel modified metric is derived. This metric alters the worst-case partial-band jammer from a single tone to a more broad-band dammer waveform. An improvement in the worst-case partial-band tone jamming BER performance is experienced with all modulation formats for large OFFTs and for OFFTs of all sizes for modulation formats where M ≥ 8. The AWGN GER performance is degraded by 0.5-0.7 dB. The Viterbi frequency synchroniser is shown to degrade the BER peformance for 8-FSK by approximately 0.75 dB when compared to perfect carrier frequency synchronisation. The metric is also shown to increase the number of simultaneous users in an FFH 8-FSK multiple access environment, especially when the power control of the different accesses in the system is poor. Implementation of the synchroniser and metric calculation are investigated and the performance of 8-FSK with 4-bit quantisation of the soft decision information is shown to degrade the BER performance by 0.25 dB. Coarse code and carrier frequency synchronisation methods for spread spectrum communications in general are reviewed. The requirement for additional intermediate frequency tones in order to achieve frequency synchronisation is shown to be unnecessary when MFSK detection uses the OFFT. Two techniques for the synchronisation of fast frequency hopped MFSK are described and analysed in AWGN. These are the bandwidth identifcation algorithm and the tone folding detector. These are compared with a Viterbi based detector which exploits the structure which is inherent within the convolutionally encoded data sequence. The superiority of the Viterbi detector is shown and the technique is modified to utilise the modified metric calculation which was developed for demodulation. A jammer classifier is described, based upon mean output of the DMAX AGC in order to allow thresholding to be achieved in the presence of different jammer types.

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