Enhanced HARQ scheme based on rearrangement of signal constellations and frequency diversity for OFDM systems

In multi-carrier modulation systems such as OFDM, several subcarriers are used for transmission and their received signal quality is different from one subcarrier to another in frequency selective fading channels. In this paper, an enhanced hybrid ARQ scheme for OFDM systems is proposed. When data is requested for retransmission, we change the bit interleaving mode in such a manner that the data sequence is changed. Since the coded bit is assigned to different subcarriers and positions of modulation symbols in different retransmissions we can take advantage of this effect, and achieve both frequency- and modulation constellation diversity. Results from computer simulations confirm that the proposed scheme improves the throughput performance. After two transmissions, and with packet error rate (PER) of 10^-4, the proposed scheme has a gain of approximately 3 dB compared to a scheme employing Chase combining. The performance gain is even more for three transmissions (2 retransmissions).