| In recent years,activities in shallow water are increasing and underwater acoustic communications attract more and more attention of researchers.In many application cases,underwater acoustic communications with high performance and high rate are needed.The Multiple-input multiple-output(MIMO)approach can achieve richly spatial degrees of freedom based on the low spatial correlation of acoustic channels from different transmit elements to different receive elements.Therefore,the MIMO approach can improve the performance and rate of underwater acoustic communications.The time reversal(TR)approach has low computational complexity and its spatial-temporal focusing can reduce the inter-symbol interference(ISI)caused by multipath propagation in shallow water channels and the co-channel interference(CCI)due to spatial correlation.Therefore,the TR approach is benefit to achieving good communication performance with relatively low computational complexity.TR approaches includes active TR(ATR)and passive TR(PTR).In time-varying underwater environment,estimated channel responses in PTR process are more identical to the actual channel response than that of ATR process,since the time interval between the probe signal and the information signal in PTR process only needs to be larger than the length of multipath propagation and to be shorter than that of ATR process.Therefore,PTR suits to underwater acoustic communications better than ATR.In order to meet the increasing demand of underwater acoustic communications with high rate and high performance,the study on MIMO-PTR underwater acoustic communications is important in theory and in practice.Based on the present status and development trend of MIMO-PTR underwater acoustic communications and the characteristics of PTR and MIMO,the suppression methods of residual ISI and CCI in the signals processed by PTR were researched exhaustedly in the thesis.Theoretical derivation,simulation and experiment were used to demonstrate the validity of the proposed method.The main research topics in the thesis are as follows.1.The interference suppression performance of PTR in acoustic communications was studied.The spatial-temporal focusing of PTR can suppress the multipath propagation of acoustic channels and reduce the spatial correlation of acoustic channels for different information signals,and therefore the ISI and CCI can be reduced and the communication performance can be improved through using PTR.The interference suppression performance of PTR determines the residual CCI and ISI,and further determines the computational complexity of other approaches used as post-processors of PTR.The researches of PTR underwater acoustic communications mainly focus on two fields:single-input multiple-output(SIMO)PTR underwater acoustic communications and MIMO-PTR underwater acoustic communications exploiting spatial diversity and spatial multiplexing simultaneously.Therefore,based on the present status of PTR underwater acoustic communications,the interference suppression performance of PTR was analyzed.Firstly,the expression of the signal-to-interference-plus-noise ratio used as performance indicator was deduced in theory.Then,based on the shallow water channel model built on ray acoustics and channel geometry,the effects of the number of receive elements,the number of transmit elements,and the symbol signal-to-noise ratio on the interference suppression performance of PTR was analyzed in simulation.Results shows that after PTR process in SIMO-PTR underwater acoustic communications and MIMO-PTR underwater acoustic communications exploiting spatial diversity and spatial multiplexing simultaneously,there always is some residual interference.In order to further improve the communication performance,other approaches such as adaptive equalization and interference cancellation must be exploited in post-process.2.The MIMO-PTR underwater acoustic communications using TR space-time block coding(TR-STBC)were researched.In PTR underwater acoustic communications,the interference suppression performance of PTR is related to the spatial diversity gain.When the number of receiver elements is small,the existing SIMO-PTR underwater acoustic communication method has relatively low spatial diversity gain only using receiver diversity.In such a case,PTR has relatively low interference suppression performance and the adaptive equalizer used as post-processor is relatively complex.In order to improve the interference suppression performance of PTR,a MIMO-PTR underwater acoustic communication method using TR-STBC was proposed.In the proposed method,a 1/2 rate TR-STBC scheme combined with PTR was designed based on the principle of two-element TR-STBC scheme suitable for frequency-selective channels and 1/2 rate STBC scheme suitable for any number of transmit elements.The principle of the proposed method was analyzed in theory and the validity of the proposed method was verified by the simulation and experiment.The results show that the proposed method can improve the interference suppression performance of PTR using transmit diversity additionally,and therefore compared with the SIMO-PTR underwater acoustic communication method,the proposed method has better communication performance and lower complex equalizer used as post-processor.3.The MIMO-PTR underwater acoustic communication method using successive IS I and CCI cancellation was explored.Although the spatial-temporal focusing of PTR process in MIMO-PTR underwater acoustic communications exploiting spatial diversity and spatial multiplexing simultaneously can reduce the ISI and CCI,there are residual ISI and CCI.The residual interference has bad effect on the communication performance.In order to reduce the residual ISI and CCI in the signal after PTR process with lower computational complexity,the MIMO-PTR underwater acoustic communication method using successive ISI and CCI cancellation was proposed based on the principle of successive interference cancellation(SIC)and the characteristics of composite channel response after PTR process.The proposed method was analyzed in theory and verified by the data collected in the experiments conducted respectively in a pool and the Songhua Lake in Jilin province.The results show that since the residual interference can be further reduced using the proposed method,the proposed method has better communication performance in contrast with the MIMO-PTR underwater acoustic communication without any post-processor.Furthermore,when the communication performance of proposed method is similar to that of the existing MIMO-PTR underwater acoustic communication method using SIC,the proposed method has lower computational complexity.4.The MIMO-PTR underwater acoustic communication method using filtered multitone(FMT)modulation was investigated.When the frequency band is wide and single-carrier(SC)modulation is used,there are large expand of residual ISI and CCI in the signal after PTR process of the MIMO-PTR underwater acoustic communication exploiting spatial diversity and spatial multiplexing simultaneously,and therefore the performance after PTR process is relatively low and the approach used as post-process is relatively complex.To solve this problem,the MIMO-PTR underwater acoustic communication method using FMT modulation was proposed based on the characteristics of FMT approach.In the proposed method,the communication frequency band was divided into several separate subcarriers without guard bands using FMT approach and the information symbols processed by the serial-to-parallel converter was modulated onto different subcarriers.The proposed method was analyzed in theory and verified by the designed experiment.The results show that the symbol interval is expanded and the expand of residual interference in the signal after PTR process are reduced using FMT modulation,and therefore compared with the MIMO-PTR underwater acoustic communication using SC modulation,the proposed method has better communication performance and lower computational complexity in channel estimation,PTR and interference cancellation. |
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