Selectrive Fading Limitations of the Kathryn Modem and Some System Design Considerations

Digital data communication in the HF band is influenced adversely by multipath and Doppler smear which cause frequency-selective and time-selective fading, respectively. As a result of the selective fading, an increase in signal-to-noise ratio brings in diminishing returns as far as reduction in error probability is concerned. Careful attention to system design is necessary in an HF modem if its performance is not to be limited by selective fading effects. This paper determines the major error probability limitations caused by multipath and Doppler smear in the Kathryn modem, a new HF modem of considerable current interest. An analysis is also made of the effectiveness of two techniques for reducing these error probability limitations. Specifically, the effectiveness of time-guard bands in reducing the error probability caused by multipath smear and the effectiveness of delay compensation in reducing the decorrelation of the pilot tone caused by the combined effects of Doppler smear and a nonzero bandwidth pilot-tone filter are studied in detail. In addition, the error probability caused by synchronization error is evaluated. The effects of diversity combining are studied in all cases.