Research On Hybrid ARQ Retransmission Schemes Based On Protograph LDPC Codes In Underwater Acoustic Channels

Underwater acoustic communication technology is the key technology for the development of marine resources.However,channel characteristics of underwater acoustic channel,such as fast time-variant signal,limited transmitting bandwidth,multipath effect and severe frequency-dependent attenuation,bring great challenges to the design of the underwater acoustic communication system.Low density parity check(LDPC)codes are error correction codes with Shannon-limit approaching performance and protograph LDPC codes are a class of LDPC codes with excellent performance.Protograph LDPC codes not only has excellent error correction performance,but also has a simple structure easy to optimize.Thus,protograph LDPC codes can be used as an error correction code for underwater acoustic communication.At present,the researches focus on optimizing the decoding threshold of LDPC codes about the construction of LDPC codes in underwater acoustic channel.However,the LDPC codes constructed in this way have the error floor problem.In addition,the bandwidth of the underwater acoustic channel is limited,so the LDPC code length is generally short and often cannot be correctly decoded when the channel conditions are poor.Therefore,hybrid automatic repeat request(HARQ)technology is often used to ensure reliable communication.Reliability-based HARQ(RB-HARQ)scheme regards the soft information of iterative decoder as the reliability and retransmits the bits with lower reliability,which is suitable for underwater acoustic channel.However,the RBHARQ scheme has the problems that the amount of data in the backward transmission channel is too large.In order to solve above problems,this paper studies the HARQ scheme of protograph LDPC codes in underwater acoustic channel.To solve the error floor problem in underwater acoustic channel,we propose an optimization method of the protograph LDPC code.For the optimization of protograph,a good protograph has the characteristics that the degrees of check nodes are almost equal and includes variable nodes with degree 1.In addition,the number of variable nodes with degree 2 should be adjusted to tradeoff between waterfall areas and error floor areas.For lifting methods,the concept of virtual check node is proposed to represent multipath effect in the underwater acoustic channel and the traditional progressive edge growth(PEG)/approximate cycle extrinsic message degree(ACE)hybrid construction method is improved for protograph LDPC codes.Finally,decoding threshold detection and ACE detection of the optimized protograph LDPC codes ensure that the constructed protograph LDPC code has good performance in both waterfall area and error floor area.Simulation results show that the optimized protograph LDPC codes by proposed method has better error correction performance.To solve the problem of the large amount of data in the backward channel of the RB-HARQ scheme in underwater acoustic channel,we propose two modified RBHARQ schemes in terms of reducing the number of feedback indices and the search space of retransmitted bits.Firstly,to reduce the number of feedback indices,the concept of neighbor variable node set is proposed and the neighbor variable node set with the smallest average reliability will be retransmitted.Secondly,to reduce the search space of retransmitted bits,since variable nodes with larger degree play a greater role in the decoding process,the variable nodes with large degrees and low reliability have high retransmission priorities.Simulation results show that the throughput of the two modified schemes are better than the original RB-HARQ scheme.