Research On OFDM Underwater Acoustic Communication System Based On Polar Codes

The underwater acoustic(UWA)channel has the characteristics of time-frequency double-selection,which is mainly affected by multipath effect,doppler frequency shift and noise factor.And it is one of the most difficult wireless communication channels.Therefore,how to improve the reliability of data transmission in the underwater acoustic channel has become a hot issue.In order to solve this problem,this paper first attempts to apply the polar code to the OFDM underwater acoustic communication system to effectively improve the performance of OFDM underwater communication system.Polar code is a new channel coding research program,and it is current the only proved channel coding scheme which can achieve the Shannon capacity under binary discrete memoryless channel.And it has a clear encoding scheme,a lower encoding and decoding complexity.Therefore,the application of the polar code to the underwater acoustic communication system has important practical significance and value.The work of this paper is to study the application of polar code in underwater acoustic communication system.The main works are listed as follow:(1)According to the channel characteristics,three kinds of approximate underwater acoustic channel model-time invariant channel,quasi-stationary channel and time-varying channel are established.The characteristics of the channel and the realization method are given.(2)For the time-invariant channel and the quasi-stationary channel,this paper studies the bit error rate performance of the cyclic elimination check(CA_SCL)decoding algorithm with cyclic redundancy check of polar code under different code length,search width and multipath structure.Taking into account the multipath factor,this paper requires channel estimation before the decoding operation.Then,Vardy's proposed polarization code construction method is used to realize the selection of information position.The simulation results show that the CA_SCL decoding algorithm of the polar code can achieve the bit error rate performance required by underwater acoustic communication.Compared with the existing quasi-cyclic low-density parity check(QC_LDPC)code,the CA_SCL algorithm has the similar performance to the belief propagation(BP)decoding algorithm of quasi-cyclic low-density parity(QC_LDPC)code.At the specific signal-to-noise ratio,the CA_SCL decoding algorithm is superior to the QC_LDPC code BP decoding algorithm.Therefore,the polar codes can be considered a candidate code word for reliable UWA communication process of the time-invariant channel and the quasi-stationary channel.(3)For the time-varying channel,this paper studies the error rate performance of CA_SCL decoding algorithm under different code length,search width,subcarrier number and doppler frequency offset.Considering the existence of doppler shift,this paper needs channel estimation,equalization and subcarrier interference(ICI)suppression before decoding operation to make the bit error rate of the system reach the decoding requirement value.Then,take a new construction method-Monte Carlo structure to achieve the polar code of the information location selection process.The simulation results show that the CA_SCL decoding algorithm of the polar code can achieve the bit error rate performance required by the system under the specific signal to noise ratio.Therefore,the polar codes can be considered a candidate code word for reliable UWA communication process of the time-varying channel.