Multi-site spatially diverse demodulation of HF propagated signals

This paper discusses a project to develop and evaluate multi-site combination algorithms to improve demodulation of high frequency (HF) signals. The approach to achieve this goal includes (1) implementing a single-site MS110B serial demodulator, (2) calculating quality metrics at various points in the demodulation process, and (3) collecting on-air skywave propagated signal samples with known content for thorough algorithm evaluation. Both simulated and experimental results show that combining the complex raw symbols prior to hard decision and combining the pre-FEC corrected bits yield the highest gain. The latter is especially attractive due to the lower amount of data necessarily communicated to a multi-site demodulator. In addition, using a measure of the bit disagreement between sites as a soft decision into the FEC decoder improves the performance of the error correction on the combined result.