Study of Space Communications Spread-Spectrum Systems, Phase 5 and 6. Part 2:Coding and Modulation

摘要

Trellis codes are obtained for use with noncoherent M-ary frequency shift keying(FSK) transmission as would be used in a hopped spread spectrum system. These codes give improved performance in additive noise but do not require an increase bandwidth. In the trellis coding, the signal set of uniformly spaced tones is expanded from noncoherently orthogonal M-ary FSK to 2M-ary noncoherent FSK. To avoid significant bandwidth expansion, nonorthogonal tone spacing is permitted. The effects on error performance of tone spacing and of the number of states in the code is investigated. Performance in the presence of noise is analyzed and it is concluded that a coding gain of from 2 to 4 dB can be achieved with a simple 4-state trellis code. The exact gain depends on the choice of signal spacing. In a jamming environment, the coding gains are much more dramatic. New results on the error performance of trellis codes in noise and in partial-band jamming, with noncoherent detection and signal sets that are not orthogonal, are presented. Simulations were carried out which yield performance within a fraction of a dB of that predicted by the theory. A practical self-normalizing decoding metric is shown to give performance very close to that predicted by the idealized jamming bound considerations. There appears to be significant improvement in the 4-state over the 2-state codes, but little improvement is seen in going from 4-state to 8-state codes.