Research On Link Adaptation Based On HARQ In Underwater Acoustic Communication

Underwater acoustic channel experiences severe Multi-path extension, high ambient noise, limited bandwidth, large transmission and propagation delay, fluctuation effect and great doppler frequency shift. These features greatly reduce the reliability and effectiveness of underwater acoustic communication system. FEC corrects errors through adding redundant bits into data packets; ARQ performs error control through retransmission technology; HARQ technology combines FEC and ARQ to improve the reliability of the communication system. Adaptive modulation and coding technology is one kind of link adaptation. It changes the modulation and coding scheme according to the channel state, optimizing the link performance. This thesis combines HARQ and link adaptation in underwater acoustic systems. It can improve the throughput performance and reliability of underwater acoustic channel.The principle of three types of HARQ is firstly introduced and the performance of HARQ based on Turbo is then studied and simulated in this thesis. Results show that frame error rates of different MCS have significant differences. The lower Coding rate, the smaller frame error rate of MCS, while the higher the modulation order, the greater frame error rate.Subsequently, the expected throughput of three different types of HARQ is analyzed both in time-invariant channel model and time-varying Nakagami-m fading channel model. This thesis analyzes the state transition probability of selective ARQ. Based on this kind of expected throughput, link adaptation is implemented with the throughput of underwater acoustic channel to be optimized. Simulation results show that, for underwater acoustic channel with limited bandwidth and large delay, throuput performance through adaptive modulation and coding technology is better than that with a fixed modulation and coding scheme. Meanwhile, throuput of Incremental Redundancy is better than that of Chase Combining. According to different channel states, adaptive selection of ARQ types achieves higher throughput performance than only one single type of ARQ. Besides, this thesis analyzes the delay performance of the channel when it achieves the optimal throuput. Results show that delay of underwater acoustic channel is large and delay of Incremental Redundancy is smaller than that of Chase Combining.Finally, the thesis extends point to point link model to underwater acoustic relay network. This thesis analyzes HARQ and adaptive modulation and coding technology in the network with one single relay. This thesis makes use of Markov model to analyze the transmission process. State transition probability and expected throuput is calculated based Markov. Simulation results show that in this single relay network, adaptive modulation and coding technology can make up the lack of a single modulation and coding scheme to enhance link throughput. Relay selection with the optimal location is also discussed. Results show that relay in the middle between source and destination nodes has better throughput performance.