Performance Improvement for Coded OFDM Systems with Adaptive Interleaving in Frequency Selective Fading Channel

摘要

In an attempt to improve the performance under frequency selective fading environment, we develop in this paper an orthogonal frequency division multiplex (OFDM) system in which adaptive interleaving is applied. The adaptive interleaving is a method that assigns symbols adaptively to the subcarriers in order to cope with frequency selective fading based on a channel state information (CSI) sent back from the reception end. The concept of adaptive interleaving is to maximize a free Euclidean distance in the limited interleave size. In this paper, we extend the method by an introduction of bit interleaving and multiple trellis coded modulation (MTCM). MTCM assigns two or more symbols to one trellis branch and shows good performance in frequency selective fading. If we could assign those set of symbols with an aid of the adaptive interleaving, the performance improvement can be expected. Another improvement method considered in this paper is the use of bit interleaving. The bit interleaving techniques randomize the effect of channel more efficiently compared to the case of symbols interleaving. Thus the further performance improvement is expected. One draw back is that since the interleaving process is done in bit level, bit interleaving can not be applied to TCM nor MTCM. In this paper, we mainly focus on adaptive bit and symbol interleaving and discuss the performance from the point of interleaving effect, and the error correcting code (convolutional code and MTCM).