A high-speed modem with built-in HF noise tolerance

Recent advancements in high frequency (HF) modem design have shown that superior performance an a fading channel can be achieved using trellis-coded modulation (TCM) overlaid on a parallel-tone modem format. In practice an HF modem has to operate not only under channel fading conditions but also in the presence of additive noise which is likely to be non-Gaussian and coloured. This paper describes a parallel-tone TCM HF modem designed to be tolerant to noise generated by other HF users, the largest component of HF noise. The design of a noise-resistant modem was guided by earlier studies an the HF noise environment. Results show the HF power spectrum to be impulsive in frequency and reasonably stable over time. Using a maximum-likelihood formulation it is shown that the TCM parallel-tone modem can be easily modified so that it's performance is greatly improved in HF noise. A software system capable of simulating additive white Gaussian noise (AWGN) and swept carrier wave (CW) interference was constructed using a complete TCM parallel-tone modem. The advantages of the use of new error-control decoder metrics, Fourier transform windowing and interleaving are demonstrated. In AWGN both the original and modified modems displayed near. Identical performance while the new modem showed a substantial improvement when subject to swept CW interference. Only a modest computational overhead is required to achieve these considerable improvements.<
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