Investigating the benefits of iterative equalization and decoding of STANAG 4539 HF waveforms

The area of digital communications has undergone a significant transformation in the last twenty years due mostly to the discovery of iterative forward error correction (FEC) codes (such as turbo codes) and iterative demodulation techniques (such as turbo equalization). Waveforms developed for the High Frequency (HF) band typically place an interleaver between the FEC scheme and the transmitted symbols. The purpose of this interleaver is to decorrelate the errors caused by slow fading multipath channels encountered on HF. If no interleaver is used, most standard EEC schemes become ineffective (e.g. convolutional codes). Although HF waveforms were not designed with the notion of iterative demodulation techniques in mind, it turns out that these techniques can be applied to single-carrier HF waveforms which utilize FEC and interleaving by iteratively exchanging soft information between the equalizer and the FEC decoder. This paper will investigate the benefits of applying iterative demodulation techniques to the 3200 and 9600 bit per second waveforms defined in STANAG 4539 as a function of the Doppler spread and multipath spread encountered on HF channels.