Iterative Bi-directional Kalman-DFE equalizer for the high data rate HF waveforms in the HF channel

In this paper, we present the Kalman decision feedback equalizer (Kalman-DFE) as a solution for the high data rate HF waveforms. We show that the basic performance for the Kalman-DFE is far from the standard requirements of the HF and the performance breaks down for the higher constellation. As a result, we add two major enhancements for the Kalman-DFE: the bi-directional and iterative structures. The bi-directional structure successfully prevents the performance break down at the higher constellation and enhances the overall performance. The iterative structure then enhances the performance significantly (about 6dB gain at 1e-5 BER for 64QAM after three iterations). We show that the proposed equalizer in the final form achieves significant performance enhancements over the normal one. Moreover, the proposed iterative Kalman-DFE is much simpler than turbo equalizer, especially for the fractional-spaced model, for two reasons: first, the direct adaptation structure used in Kalman filter is much simpler than the indirect adaptation structure usually used with the turbo structure (it does not require matrix inversion). Secondly, the iterative structure (exchanges hard information) is simpler than turbo (exchanges soft information) as the later requires a lot of changes in both equalizer and decoder to support soft information. We perform the simulations on the MIL-STD-188-110 (Appendix C) waveforms, transmitted over an ITU-R poor channel (commonly used channel to test HF modem).