| One of the major disadvantages of Orthogonal Frequency Division Multiplexing (OFDM) is the large peak to average power ratio (PAPR). Large power fluctuations present in OFDM are sensitive to nonlinearities in the system including the power amplifier. A power backoff at the amplifier is usually employed to reduce the OFDM signal distortion and out-of-band spectral leakage. Such a backoff not only reduces the radiated power, but also considerably decreases the efficiency of the amplifier.
In [1] and [2], a new constant envelope OFDM (CEOFDM) waveform was proposed. In CEOFDM, the OFDM signal is embedded within the phase of a constant envelope carrier signal. The resulting CEOFDM waveform alleviates the PAPR problem. The spectrum, signal space properties and performance in AWGN were presented in [2]. The use of low modulation indices was deemed necessary to avoid phase wrapping and the associated performance loss.
In this paper, we study the application of convolutional coding to CEOFDM. The noise at the phase demodulator 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 convolutionaly coded CEOFDM 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 addresses any burst errors due to the occasional phase wrapping and is shown to improve performance by 1dB for moderate modulation indices. It is shown that channel coding with interleaving provides good performance for CEOFDM with low to moderate modulation indices while correcting burst errors due to phase wrapping.
[1] S.C. Thompson, J.G. Proakis, and J.R. Zeidler, “Binary OFDM Phase Modulation,” in Proceedings of IEEE Milcom ‘03, Boston, MA, Oct 2003.
[2] S.C. Thompson, A.U. Ahmed, J.G. Proakis, and J.R. Zeidler, “Constant Envelope OFDM Phase Modulation: Spectral Containment, Signal Space Properties and Performance,” in Proceedings of IEEE Milcom ‘04, Monterey, CA, Nov 2004. |