A New Adaptive Modem for Long Haul HF Digital Communications at Data Rates Greater than 1 bps/Hz

Reliable digital communication over long-haul HF circuits, at rates above 1 bps/Hz, is possible if an adaptive equalizer technique is used which will successfully follow the fading characteristics of the time-dispersive channel. This paper summarizes an investigation of this tracking problem which led to the selection of a square-root Kalman algorithm to update the coefficients of a decision feedback equalizer (DFE). The design and implementation of a new HF modem utilizing the Kalman DFE in conjunction with a continuous GO BACK N ARQ strategy is described. Results of performance tests at data rates from 2400 to 9600 bps using a 3 kHz real-time HF channel simulator are presented. Both low error rate and high throughput are achieved using the combined adaptive channel equalization and ARQ techniques. Preliminary results from tests over a long-haul live link are also reported.