2.4 kbit/s adaptive serial modems for HF skywave paths

It is only recently that the processing power required by adaptive modems of this type has been available from digital signal processors (DSPs) suitable for use in modems. This is especially true in the case of a channel estimating modem of the type described, where a near-maximum likelihood detector (NMLD) is employed. Prototype 2.4 kbit/s modems have been developed with the aim of achieving the best possible performance on HF skywave paths subject to multipath, fast fading and interference. Of particular interest are paths subject to high auroral absorption effects, where fade rates of 5 to 10 Hz are not uncommon. The channel estimator plus near-maximum likelihood detector system proposed and developed by Clark, Asghar and McVerry (1981) was chosen as the structure best able to cope with fast fading conditions.<>