Constant envelope waveforms for use on HF multipath fading channels

Abstract

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 large variation in the peak-power of the waveform relative to the average-power (about 3-6 dB depending on the symbol modulation). The effect of this peak power to average power ratio (PAPR) is that an average power 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. Similarly, orthogonal frequency division multiplexing (OFDM) waveforms suffer from an even larger PAPR, typically 6-14 dB. An alternative to these waveforms is to use constant-amplitude (or envelope) waveforms such as continuous phase modulation (CPM) or constant envelope OFDM (CE-OFDM). This paper will investigate the potential of modifying the low data rate waveforms in US MIL-STD-188-110B (110B) with constant-amplitude variants in an effort to improve on-air performance by maximizing the average transmit power. These new constant envelope waveforms will be compared to the standardized 110B single-carrier HF waveforms and to new candidate OFDM waveforms on several HF channels in order to understand the performance trade-offs offered by the new waveforms.