An investigation of constant-envelope variations of OFDM waveforms for use on HF multipath fading channels

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) has become a very popular technique for digital data transmission on multipath fading channels due to its low computational complexity and simple equalization process. However, the multipath component of these types of channels causes a phenomenon known as frequency selective fading. This type of fading can severely degrade or completely eliminate the signal energy of many of the OFDM tones producing an irreducible error rate, even when no noise is present. Consequently, most OFDM systems operating in multipath fading environments utilize some form of forward error correction (FEC) and block interleaving. OFDM waveforms which utilize FEC are usually referred to as coded OFDM (COFDM). One of the main drawbacks of OFDM and COFDM waveforms is the very large peak power to average power ratio (PAR) which requires the use of very linear power amplifiers (PA) and a large power back-off into the PA. In recent years there has been much interest in creating constant-envelope variations of OFDM and COFDM waveforms in order to overcome the PAR drawback. This paper will investigate constant-envelope (CE) variants of OFDM and COFDM waveforms for use on HF multipath fading channels.