Performance of Kalman decision-feedback equalization in HF radio modems

The performance of fast-adapting Kalman algorithms in adjusting the tap weights of digital equalizers for high-speed, serial HF radio modems is investigated. Experiments on a large realistic set of simulated HF channels, and on some actual channels, have been conducted using 2400 bps QPSK. Results are presented for linear and decision-feedback equalizers with symbol and half-symbol (fractionally) spaced taps. The bit error rate and the optimum values of several algorithmic parameters in reference-directed mode, and the relative resistance of the various configurations to "crashing" in decision-directed mode, have been determined as a function of the channel characteristics. The results show the superiority of the Kalman, fractionally-tapped decision-feedback equalizer and point out the uniqueness and relative difficulty of equalizing the HF radio channel. At slow fade rates, reference-directed performance within 3-5 dB of optimum is possible. With fade rates above 0.5 Hz though, the technique becomes fade-rate limited. A theoretical argument leads to a new definition of a "slow fading channel" for exponentially-aged Kalman equalizers. Performance in decision-directed mode, necessary for actual communication, was much poorer than expected. It was concluded that the continously-high Kalman Gain necessary on the HF channel results in such adaptation at each iteration that even isolated decision errors can seriously disrupt the adaptation process. In spite of the decision-directed degradation, the serial equalizer technique is shown superior to currently-used parallel modems if the fade rate is less than 0.5 Hz and the transmissions are peak power limited.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat