Research On Ultra-long Delay Spread Suppression Based On Time Reversal Of OFDM Communication Systems

In the orthogonal frequency division multiplexing(OFDM)communications,the cyclic prefix(CP)is added to ensure that the subcarriers are orthogonal to each other in the symbol period and the multipath-induced intersymbol interference(ISI)decreases.In the general communication environment,the maximum delay spread(MDS)of the channel is less than the CP length,so the ISI can be effectively reduced,and the communication performance can be guaranteed.However,in extremely complex multipath environment,such as spacecraft,underground tunnels,submarine cabin and other reverberation environment,MDS will be much longer than the conventional CP duration,which leads to the overlap and mutual interference of the transmission signal waveform.If the duration of the CP is not changed,the ISI reduction capability of CP will lose efficacy and the system performance will deteriorate significantly.If the duration of the CP is increased,the spectrum resource utilization rate of the OFDM communication will be greatly reduced and the performance will be degraded.Based on the temporal and spatial focusing mechanism of time reversal(TR),the thesis studies the ultra-long delay spread(ULDS)suppression method for OFDM communication systems.The ULDS channel and the effective equalization method are mainly studied.The main work of the thesis is as follows:Firstly,the thesis analyzes the suppression method of inter-symbol interference caused by multipath in OFDM communication,and explains the necessity of the research.This thesis describes a wide range of TR applications.TR has the space-time focusing characteristics and the ability to suppress the delay spread of channel,so it has great significance to study the performance of OFDM system in ULDS channel.Secondly,the multi-input multiple output(MIMO)channel correlation in complex environments and the communication performance of TR and MIMO technology are studied.In addition,the ratio of the main to maximum side-lobe ratio(MSR)of ULDS channel is studied,and the communication performance of the TR-OFDM under the channel is analyzed.The conclusion is that it has good communication performance for the case of a specific MSR.The simulation of the pre-TR and the post-TR in the complex communication environment and the simple communication environment results in the conclusion that the pre-TR is slightly better than the post-TR in the ULDS channel.Finally,the method of ordered successive interference cancellation(OSIC)is analyzed theoretically.On this basis,the thesis presents a combination of TR and OSIC detection methods,and compares the performance of different sort detection.In addition,the thesis proposes the equalization technique of combination of TR and recursive least squares(RLS).The simulation results show that TR-RLS equalization has a certain effect on the suppression of ULDS.The content of this thesis is summarized.The future research direction is pointed out.