Constant Envelope OFDM with Channel Coding

Abstract

This paper studies the application of convolutional coding to constant envelope orthogonal frequency division multiplexing (CE-OFDM). The noise at the output of the phase demodulator (in the CE-OFDM receiver) is shown to be approximately Gaussian at high SNR and relatively low modulation indices. This allows the use of the conventional metric in the Viterbi decoding algorithm. A bound on the performance of convolutionally coded CE-OFDM is derived and is plotted along side the performance curves obtained through simulation. Both the cases with and without a phase unwrapper are studied. The phase unwrapper results in large bursts of errors at moderate to high modulation indices due to cycle slips. Such burst errors overwhelm the convolutional code and render it useless. Fewer burst errors are generated in the absence of a phase unwrapper. Further adding a simple interleaver is beneficial to counter the burst errors. It is shown that channel coding with interleaving provides good performance for CE-OFDM for low to moderate modulation indices while correcting burst errors due to phase wrapping