Turbo equalisation based on data directed estimation for standard high frequency waveforms

Abstract

Conventional receivers for high-frequency (HF) waveforms using phase-shift keying (PSK) or quadrature amplitude modulation (QAM) symbol mapping contain an equaliser to equalise channel effects and a forward error correction (FEC) decoder to improve bit error rate (BER) performance at low signal-to-noise ratios (SNRs). The equaliser is an important component of an HF waveform receiver since the ionospheric channel used for skywave propagation commonly causes a signal to propagate along multiple paths which often results in inter-symbol interference (ISI) at the receiver. Turbo equalisation, which is the process of feeding back information from the decoder to the equaliser to improve symbol estimates, has been shown to improve the performance of a receiver when ISI is present in the channel. Turbo data directed estimation (TDDE) is a turbo equalisation method based on the data directed estimation (DDE) equaliser which is suited for waveforms that interleave known symbol sections, used for channel estimation, with the unknown data symbols.In this work, the TDDE algorithm is applied to standardised MIL-STD-188-110D Appendix D (MIL-STD-188-110D App. D) HF waveforms. This standard was selected since it is still relevant and actively being maintained and updated. The 3 kHz bandwidth variants of the low data rate waveforms in MIL-STD-188-110D App. D, referred to as waveform number 1 and waveform number 2, were studied in this work since it was expected that it would be more challenging to improve the performance of these waveforms than the less robust and higher data rate waveforms with denser constellations. The designed TDDE receiver was tested using the methods specified in MIL-STD-188-110D App. D and using equipment from a reliable supplier of HF communications equipment, Rapid Mobile (Pty) Ltd (RapidM). The tests showed promising results for the TDDE algorithm when applied to MIL-STD-188-110D App. D waveform number 1 and waveform number 2, with consistent performance improvements in the order of 2 dB when using a Poor channel as defined in MIL-STD-188-110D App. D. Permutations of the TDDE algorithm were also experimented with in this work, but none provided substantial changes in performance.