Convolutional Codes for Wireless Packet Data Systems

Licentiatavhandling, 2000

Automatic Repeat reQuest (ARQ) schemes can guarantee an almost error free reception strongly required in data communications. Hybrid type-II ARQ schemes are supplied with channel coding to further improve the performance. In these schemes, error correction is performed adaptively which makes them favorable for applications in time varying channels. We have proposed a hybrid type-II ARQ scheme based on High Rate Optimized-Rate Compatible Convolutional (HRO-RCC) codes which provides the best performance compared to the other considered schemes, in all the channel conditions. Furthermore, the performance of the proposed scheme has been investigated for different packet sizes and constraint lengths of the convolutional encoder, in more detail. Tailbiting coding in hybrid type-II ARQ schemes has also been studied in this work. Implementing tailbiting coding, has the prospect of improving the throughput through the avoidance of using the tail bits. However, applying sub optimum decoding approaches due to the extensive complexity of the optimum decoding is inevitable. Our results show that employing very short packets in slow fading channels can accomplish slight improvement in throughput. For all other considered cases the loss is considerable. The throughput performance of the ARQ schemes has been evaluated analytically for slow and fast fading channels as well as AWGN channel. Analytical results have been compared with corresponding simulation results for fast fading channels. Finally, the proposed scheme has been combined with a blind Adaptive Modulation System (AMS) where the modulation is gradually changed from a high constellation to low constellations when the channel bit error probability is high. The adaptation of coding rate/modulation is only controlled by the ACK/NACK feedback signal and no channel state information is needed at the transmitter. The simulation results show that the proposed scheme, in spite of having a simple structure, can improve the performance considerably in comparison with the hybrid type-II ARQ scheme combined with fixed modulation systems.