A high-speed HF parallel-tone modem

There has been a revival of interest in 2400 bps parallel-tone HF modems following the successful application of trellis-coded modulation (TCM) to the MIL-STD-188-110A 16-tone modem format. To date, codes optimised for additive white Gaussian noise (AWGN) have been utilised. This paper discusses why these codes are sub-optimal for the HF channel and describes a number of enhancements that provide useful improvements for multipath Rayleigh-fading channels without incurring additional computation. Results for CCIR Good and Poor channels are given which contrast the performance the 256-state Ungerboeck code used in the earlier work against coded modulation schemes described in this paper which have been specifically designed for fading channels. A discussion on methods of applying interleaving to such a modem follows, which leads to the description of a novel interleaving approach which maximises time diversity for a given delay. Results are given for interleaving delays of 1.2, 5.0 and 10.0 seconds. The paper concludes with a discussion of theoretical bounds for coded modulation schemes which suggests additional improvements which can be made to increase throughput and further reduce error-rates. These are illustrated with preliminary results from our newest 4000 bps HF modem