Adaptive data rate communications for high frequency radio channels

An alternative approach to the problem of multipath fading on HF ionospheric scatter channels is considered: that of adapting the data rate to the channel conditions. The performance of a simple on/off data rate system is only 4.3 dB shy of non-fading performance. A more sophisticated four-rate (three non-zero rates) trellis-coded system gains 6.7 dB more without interleaving and without bandwidth expansion. At a bit error rate of 10^-5, this is equivalent to 39 dB of coding gain. Implementation problems for adaptive data rate systems include noisy feedback channels, increased overhead, synchronization, sensitivity to propagation delay, and buffer control. It is noted that these problems are challenging, but not insurmountable