Avoiding PAPR degradation in Convolutional Coded OFDM Signals

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is a promising technique for high bit rate transmission in wireless communications systems. Convolutional coding is often used in conjunction with OFDM to improve the reliability of transmission. However, in this paper, we show that the peak to average power ratio (PAPR) statistics of convolutional coded OFDM (C-COFDM) signals can be significantly degraded when compared with uncoded-OFDM. We have found that this degradation can occur for code rates R < 1/2 and relatively low constraint lengths K=3 through K=6. For these codes, it is especially important to use PAPR reduction techniques to counteract this degradation. We further demonstrate that the use of Guided Scrambling (GS) as a PAPR reduction technique does not help in all of the cases, and therefore that reduction techniques applied after convolutional encoding, such as Selected Mapping (SLM), should be used instead.