Error Distribution and Diversity Performance of a Frequency-Differential PSK HF Modem

A 4800-bit/s digital data modem is operated over a simulated HF channel to determine the bit-error rate and error distributions of the received serial binary data stream. The measured error distributions for multipath-limited conditions are compared with back-to-back operation and theoretically determined random distributions, and are also used to evaluate forward acting error correction assuming half-rate random error-correcting block codes. This improvement in bit-error rate is compared with that measured by operating the modem at 2400 bit/s with dual in-band frequency diversity. The error distributions for back-to-back operation approach random except for small block lengths where the systematic error resulting from the frequency-differential PSK coding appears. The error distributions for multipath-limited operation deviate from random especially for small multipath delay spreads (low biterror rates) where the correlation bandwidth is large. Forward error correction produces larger improvement factors as the code block length is increased. However, the improvement factor barely exceeds two orders of magnitude for the largest block considered. In fact, the burst nature of the errors is such that improvement factors actually decrease in most cases for multipath-limited operation as the raw bit-error rate decreases. On the other hand, inband diversity is more effective for small multipath delay spreads than for large ones. However, forward error correction appears to offer more potential especially if the bit errors can be spread over a greater number of bits.