Research On Multi-user Underwater Acoustic OFDM Downlink Communication Based On Superposition Coding

The severe limitation of communication bandwidth is one of the major bottlenecks in improving the data transmission rate of underwater networks.How to improve the spectrum utilization of the multi-user communication in the physical layer within a limited bandwidth is a key issue to be solved urgently.The power domain multiple access technology is one of non-orthogonal multiple access(NOMA)technologies.The technology is to superimpose and transmit downlink multi-user data in the power domain,implement each user pair and differentiate each user's own information,and it can also have greater performance improvements over orthogonal multiple access technologies.This paper first analyzes the performance of its overall channel capacity for non-orthogonal multiple access methods,and compares the orthogonal multiple access methods,and proves that non-orthogonal methods increase the channel capacity,thus introducing superposition coding.The algorithm and the principle of superposition coding are introduced in detail.The algorithm realizes the differentiation of downlink users by using power domain information.Since each user shares the time-frequency freedom of the channel,the superposition coding can bring greater gain to the communication rate of the downlink communication system.Finally,the difference in channel capacity between orthogonal and non-orthogonal modes is obtained through simulation.Based on this,the problem of allocation of power factors involved in superposition coding is preliminarily explored.Secondly,because the distribution of power factor will directly affect the demodulation effect of the downlink two users,a power factor allocation method based on the average signal to noise ratio of downlink users is proposed,and the effectiveness of the method is verified by the simulation of underwater acoustic sparse multipath channel.Since the power domain superposition is equivalent to artificially introducing information interference at the transmitting end,it is more important for each user in the downlink communication to correctly distinguish their own information.In this paper,symbol-level continuous interference cancellation and codeword-level continuous interference cancellation receiving algorithms are proposed.According to the communication scenario of underwater two-user underwater sparse multipath channels,the BERs of remote users and near-end users under different receiving algorithms are performed.Simulation analysis.On this basis,this paper considers the influence of channel coding selection on the capacity of superposition coding channel,introduces LDPC code,and compares it with convolutional code system,and verifies the superiority of LDPC code.Finally,based on the code-level receiving algorithm,this paper improves the detection method of the superimposed signal,adopts the MMSE detector using prior information,and uses the quasi-random code LDPC code to form the non-iterative receiving algorithm in this mode.The iterative receiving algorithm and the continuous interference cancellation receiving algorithm,through the same simulation conditions,the error performance of the analysis and comparison,and ultimately obtainable iterative detection of the receiving system for the receiver demodulation performance has greatly improved.Finally,receiving and demodulating the downlink communication system of the near-end user under the sparse multi-path underwater acoustic channel,the effectiveness of the proposed receiver algorithm is verified.