Error-control options for parallel-tone HF modems

DSTO is pursuing a program to adapt the configuration and operational parameters of military radio systems to optimize channel utility. This led the authors to compare the relative performance of 2400 bps MIL-STD-188-110A HF modems in a variety of channel environments. They show that the performance differences between the three waveforms are primarily a function of the error-control coding employed which in turn prompted them to investigate options to improve the error control on the earlier parallel-tone modems. This paper concentrates on the provision of digital voice transmission over systems employing a 16-tone modem structure where feedback adaptation is not available. This mandated a processing delay of no more than a few tenths of a second and a minimum throughput of 2400 bps. The approach adopted was to extend the signalling constellation of each of the tones from QPSK to 8 PSK and to apply rate 2/3 error-correcting codes to the bit stream before transmission. Results are given for two BCH codes, two Reed-Solomon codes, a convolutional code and a TCM scheme for both simulated and real HF channels. The results presented indicate that the parallel-tone format coupled to a TCM scheme showed clear superiority to the single-tone modem. The paper concludes with a discussion of the issues to be addressed in the design of an optimal parallel-tone error-control strategy