Serial modems for frequency selective slow fading channels

The application of interest here is that where the relative fading rate is less than 10^-3. As an example of this, the paper considers the particular problem of transmitting digital data at 4800 bit/s, using 4-QAM, over voiceband HF radio links. Conventional digital modems designed for use over voiceband HF radio links are parallel modems that operate at 2400 bit/s, with the simultaneous transmission of several signals. These are simple and robust but have the characteristic that, at the higher fading rates, the bit error does not decay to zero as the signal-to-noise ratio rises, decaying instead to a value that becomes constant over the higher signal-to-noise ratios, and is typically in the range 10^-4 to 10^-2. Although the residual error rate is of no great disadvantage for the transmission of digitally coded speech, it can seriously degrade the performance of the system when used for the transmission of data, where such high error rates cannot be tolerated. Furthermore, at the higher transmission rate of 4800 bit/s, the effect becomes more serious and parallel modems become even less well suited to the transmission of data. Thus, serial modems would generally be preferred here. The channel here is a good illustration of a frequency selective, slow fading channel, and the technique presented in the sequel is an effective means of combating the distortion that arises due to multipath