A new turbo-coded OFDM system using orthogonal code multiplexing

In this paper, we propose a new transmission scheme that can improve greatly the performance of turbo-coded orthogonal frequency division multiplexing (OFDM) systems by making all the turbo-coded symbols have the same reliability for OFDM transmission over a frequency selective fading channel. The same reliability, that is, the same fading can be accomplished through multiplexing of turbo-coded symbols using distinct orthogonal codes and spreading over the whole effective subcarriers (hereafter, called the orthogonal code multiplexing (OCM)). This idea of the same fading for all symbols is the key of the performance improvement using the proposed scheme. As for the orthogonal code selection, a set of Walsh codes could be the best selection in terms of the complexity. However, in the fast Fourier transform (FFT) implementation of OFDM, the number of the whole effective subcarriers may be slightly less than the number of FFT points used. Hence, we choose the set of the discrete Fourier transform (DFT) basis sequences as a good substitute for a set of orthogonal codes, since the code set can hold the orthogonality irrespective of the length. We perform computer simulations using the log-maximum-a-posteriori (log-MAP) algorithm for iterative decoding in order to assess the performance of the proposed system and to compare it with that of the conventional turbo-coded OFDM system.