Research Of HARQ Combining Techniques In OFDM And MIMO-OFDM Systems

Orthogonal Frequency Division Multiplexing technology is robust to the frequency selective fading in the broadband wireless multipath channel. Multiple-Input Multiple-Output technology can increase the channel capacity of wireless in flat channels. While Hybrid Automatic Repeat reQuest technique can guarantee data reliability. The systems which combine the OFDM technique or MIMO technique and HARQ protocol can trade off validity and reliability of wireless communication effectively. Under this background, this paper focuses on the study of HARQ technology in OFDM and MIMO-OFDM systems, the contributions of this thesis are as follows.Firstly, the principles of OFDM , MIMO and HARQ techniques are expatiated on in this thesis. Moreover, this thesis studies the implementation schemes and retransmission mechanism of HARQ technique, and analyses Chase Combination and incremental redundance combination.Secondly, this thesis studies conventional HARQ algorithms which are oriented to hard decision, builds a complete OFDM-HARQ system model, proposes a novel combining HARQ scheme based on majority logic decision principle and channel impulse response power. The novel algorithm integrates majority logical decision principle with optimality decision principle to combine the transmitted packets before decoding at the receiver. By contrast with the conventional MRC algorithm, the proposed algorithm requires less mount of memory and has lower complexity when reserving good performance.Thirdly, this thesis studies conventional HARQ algorithms which are oriented to soft decision, builds a complete OFDM-HARQ system model, proposes a novel HARQ algorithm based on Log-Likelyhood-Ratio and Signal Constellation Rearrangement. In the novel algorithm, Signal Constellation Rearrangement is employed to average out LLRs between bits in a mapping symbol, thereby reducing the variations in bit reliabilities. Compared with conventional LLR combining algorithms, the proposed algorithm can improve the system performances at the expense of adding a small amount of memory and complexity.At last, this thesis studies the HARQ iterative combining technique in the MIMO-OFDM systems. HARQ technique is combined with iterative signal processing technique in which a soft-information combination module is embeded after MMSE-IC equalization. In this scheme, the conventional external information algorithm is replaced by a HARQ algorithm based on soft-information combining. In contrast with conventional iterative receiving technology, the proposed scheme can improve system performances effectively at the expense of adding a small amount of complexity.