Asymmetric Physical-layer Network Coding And Its Application In Underwater Acoustic Communication

The physical-layer network coding(PNC)can be used to improve the throughput of the two-way relay transmission system in wireless communication networks.In the two-way relay(TWRC),the communication situation we often consider is the system with two symmetric links.But in the reality,it is common that two channel links are asymmetric.Then how to reduce the impact of poor links on communication performance is a major problem in practical communication systems.The asymmetric two-way relay transmission system is very common in both terrestrial and underwater channels.But the underwater acoustic communication system is very complex,and the multipath characteristics,time delay and limited bandwidth make it difficult to guarantee the performance of communication.In the underwater acoustic communication system,the asymmetry of the channel has to be considered.In this thesis,physical-layer network coding is applied to asymmetric two-way relay communication.And the application of PNC in underwater acoustic communication is also studied.In order to ensure the effectiveness and reliability of the underwater acoustic communication system,the physical-layer network coding and the traditional communication technology are combined to improve the throughput of the shallow sea acoustic communication.The main contents of this thesis are as follows:First,an asymmetric two-way relay transmission system model is established to analyze the impact of asymmetric channel on the performance of physical-layer network coding,and the asymmetric channel physical-layer network coding scheme is proposed.The feasibility and effectiveness of the scheme are verified by numerical simulation.The simulation result shows that physical-layer network coding can improve system throughput of the system with a little reliability degradation.Secondly,based on the analysis of underwater acoustic channel characteristics a symmetric two-way relay communication system of underwater acoustic channel is established using the ray model.This model can well describe the multipath and time delay of underwater acoustic communication environment.In the underwateracoustic communications,the multipath and signal fading are unavoidable.We combine Direct Sequence-Code Division Multiple Access(DS-CDMA)with channel equalizer and RAKE receiver to overcome the influence of the underwater signal multipath.In order to improve the effectiveness of underwater acoustic communication,we combine analog network coding(ANC)with the DS-CDMA.Finally,considering the asymmetry of communication links,an asymmetric two-way relay underwater acoustic communication system model based on ray model is established.In order to solve the problem of asymmetric links in the system,a physical-layer network coding scheme with asymmetric modulation is proposed.The idea of asymmetric modulation is to make full use of the good links and minimize the error of poor links.For physical-layer network coding under asymmetric modulation,the superposition of asymmetric modulation is obtained by channel equalization preprocessing.The superimposed signals are coded and mapped to get the network coding information,which is the relay mapping scheme.The receivers can get the information of the source nodes and complete the data process.The performance of the scheme is verified by simulations.