Research On HARQ Schemes For OFDM Systems

With the rapid development of information industry and the continuous emergence of new types of business, people's demand of data transfer rate and the quality of service of wireless communication becomes higher and higher. How to improve the systems reliability without increasing the system bandwidth and reducing data transfer rate, this becomes a concerned problem for the researchers of wireless communication.Orthogonal Frequency Division Multiplexing (OFDM) is a technology which disperses a high rate data stream to several orthogonal sub-carriers for transmission. Due to its effective on overcoming the multiple fading effect, high spectrum efficiency as well as the feasibility of low-cost transceiver implementation, has become one of the necessary technology for the next generation of mobile communication.Hybrid Automatic Repeat Request (HARQ) has been proved to be an error control scheme with high efficiency, which utilizes the advantages of Forward Error Control (FEC) and Automatic Repeat Request (ARQ). This scheme not only has a small time delay, but also can effectively improve the system throughput, so it receives more and more attention. The goal of this paper is to design new HARQ scheme that suite to OFDM system, in order to realize the compromise of communication system's reliability and validity.This paper firstly deeply studied the check matrix of rate compatible low density parity check codes'puncturing algorithm and extending algorithm, the adaptive interleaving technology which based on the OFDM systems' channel estimation and do some improvement to it, the technology of constellation rearrangement which can average the bit reliability of the same symbol, etc. This paper proposed a new PIR-HARQ scheme which based on the constellation rearrangement under the OFDM system. The retransmission information of this scheme can get frequency diversity gain through the adaptive interleaving technology, also because retransmission codes contain new redundant information, so in the receiver after merger with the previous codes, the codes rate can be reduced, therefore the scheme can get decoding gain. Simulation shows that the scheme has a significant gain in system BER, time delay and throughput than other HARQ scheme.Then this paper studied again the decoding process of the LDPC code and the unequal error protection of irregular LDPC code, finding that even in decoding failed, the variable node of irregular LDPC code which has the large degree still has high correct decoding probability. So if these bits which corresponded to the large variable node were retransmitted, it cannot provide anymore new information to other bits in decoding. So this paper proposed an improve HARQ scheme based on the degree distribution of the LDPC code, which also used the OFDM systems' sub-carrier mapping. This scheme chooses the middle degree variable nodes for retransmission. Simulation shows that this scheme has a significant gain in system BER, time delay and throughput than the scheme which choose the large degree variable nodes for retransmission.