Research On Adaptive Modulation For Underwater Acoustic Communication

With the exploration and use of marine resources by humans,the development of underwater acoustic communication technology is paid more and more attention to scholars.However,the underwater acoustic channel environment is abominable,which has the characteristics of fast change,multi-path,large delay,narrow bandwidth,etc.,which makes the signal have serious Inter-Symbol Interference(ISI)and limited throughput in the transmission process.In response to the above problems,the following studies are carried out in this paper:First,the BELLHOP model was used to simulate the actual underwater acoustic channel and analyze the characteristics of the underwater acoustic channel.On this basis,the main research content of this thesis is made up of basic knowledge,including the commonly used blind equalization algorithm and modulation algorithm,and the reason why the system adopts Multi-Modulus Algorithm(MMA)is given.Secondly,for the serious ISI phenomenon caused by the multipath propagation of the underwater acoustic channel,the blind equalization detection technology can compensate for the channel distortion caused by the underwater acoustic channel characteristics and suppress the ISI.Considering that the existing blind equalization technique is based on the fixed tap length and the tap length has an effect on the system performance,variable tap length equalization is proposed.It uses a tap update algorithm to adaptively select the optimum tap length when the channel environment changes.By comparing and analyzing the two algorithms,the effectiveness of the variable-tap blind equalization detection algorithm is verified and the performance of blind equalization is improved.In addition,considering the sparse characteristics of the underwater acoustic channel affects the performance of blind equalization.For this reason,this paper adopts the blind equalization algorithm of fractional order norm to improve the cost function,and verify that it improves the performance of the system.Finally,for the problem that the narrow bandwidth of the underwater acoustic channel causes the limited throughput of the underwater acoustic communication system,the adaptive modulation technique is applied to the underwater acoustic communication system to improve the system throughput.The basic principle of adaptive modulation technology is to ensure the transmission efficiency of the communication system when applying high-order modulation to the higher quality of the underwater acoustic channel;to ensure that the transmission reliability of the communication system when applying the low-order modulation to the poor quality of the underwater acoustic channel.Adaptive modulation technology is an effective measure to improve throughput.This dissertation compares traditional Signal-to-Noise Ratio(SNR)based adaptive modulation algorithm and proposes the use of Steady-state Mean Square Error(SMSE)for adaptive modulation and compares the performance of the two.