Constant-amplitude waveform variations of US MIL-STD-188-110B and STANAG 4539

Modern single-carrier HF waveforms require tight digital and analog filters in order to meet current HF bandwidth allocations. Unfortunately, this filtering process produces a variation in the peak-power of the waveform relative to the average-power. The effect of this variation is that a back-off is required at the input to a power-amplifier (PA) (when using a peak-power limited PA) to avoid operating in the non-linear region of the PA. Operating in the non-linear region can result in significant spectral regrowth and distortion of the waveform. The recommended PA back-off is typically between 3–6 dB (depending on the amount of filtering and the modulation type used by the HF waveform). The net effect of this back-off is to reduce the on-air average transmit power by the back-off amount or to require a much larger PA in order to achieve a desired on-air average transmit power. An alternative to current low data rate single-carrier HF waveforms is to use constant-amplitude waveforms such as continuous phase modulation (CPM) where the bandwidth of the waveform is controlled in the carrier phase-domain rather than in the time-domain (via filtering) eliminating the need for PA back-off. This paper will investigate the potential of modifying the low data rate waveforms in US MIL-STD-188-110B and STANAG 4539 with constant-amplitude variants in an effort to improve performance by maximizing the average TX power. Coded and uncoded waveforms will be studied with an emphasis on developing some waveforms with low latency for digital voice applications. These new waveforms will be compared to the standardized HF waveforms on several HF channels in order to understand the performance trade-offs offered by these new waveforms.