Research On The Key Technologies Of Variable Rate Low Density Signature Communication

In the mobile communication system,due to the openness of the wireless channel,the electromagnetic wave signal propagating in space is easily intercepted by a third party,threatening the security of the information transmitted therein as a result.With the rapid development of the fifth generation of mobile communications,people are increasingly transmitting important private information through wireless networks,including personal identification information,financial information,etc.How to ensure the security of the transmitted information will become the focus of research.In recent years,a theory called low probability of interception communication has received extensive attention because of some breakthrough research in communication security.Unlike traditional encryption technology that focuses on protecting the confidentiality of transmitted content,low probability of interception communication pay more attention to the protection of the transmission process.From the perspective of physical layer security,variable rate can achieve low probability of interception communication by eliminating the typical characteristics of communication waveforms.This paper studies the variable rate low density signature communication system,applies LDS technology to the variable rate communication system,and proposes two design schemes of variable rate low density signature communication system.In scheme one,each user transmits in a different rate change mode,and in scheme two,each user transmits in the same rate change mode.Considering the different complexity of the two schemes,the complexity of scheme one is too high.In this paper,simulation analysis is carried out for scheme two.Different LDS codebooks are selected to increase the diversity of codebook combinations in the variable rate system and improve the safety.Through simulation,it is found that in variable rate communication systems,Inter Symbol Interference(ISI)has a greater impact on user performance at high signal-to-noise ratios.Finally,this paper also studies the ISI elimination algorithm in variable rate low density signature communication system,including equalization algorithm based on minimum mean square error criterion,ISI pre-elimination algorithm and ISI elimination algorithm based on guard interval.The equalization algorithm is used to eliminate ISI at the receiving end,and uses the MMSE criterion to calculate the coefficients of the equalizing filter at the receiving end.Since ISI is artificially introduced in the variable rate communication system,and the transmission sequence and the filter coefficients at the transmission end are known,the ISI corresponding to each chip can be calculated.The ISI pre-cancellation scheme is to subtract the calculated ISI after the spread spectrum signal is generated,which can effectively reduce the generation of ISI at the receiving end.The ISI elimination scheme based on guard interval is to add several zero elements before and after the chip,which can prevent the time-domain tailing of the chip from affecting the front and back chips.The simulation results show that the three methods can effectively reduce the impact of ISI on the variable rate LDS communication system and improve the bit error performance of the variable rate LDS communication system.