Research On Multiple Access Technology Of Transform Domain Communication System

With the continuous development of wireless communication technologies and the increasing services supported by wireless communications,electromagnetic spectrum resources are running out.Cognitive radio(CR)technology monitors and senses the changing electromagnetic environment and adjusts system parameters according to the sensing results to realize dynamic access to unoccupied frequency bands.In this way CR enables intelligent,reliable and efficient as well as efficiently improves spectrum utilization.CR has attracted widespread attention from academic and industrial areas since it was put forward.As one of candidate technologies for CR transceivers,transform domain communication system(TDCS)designs signal waveforms in the transform domain(such as frequency domain,wavelet domain or other abstract domains)according to spectrum sensing and decision results and discovers unoccupied spectrum for information transmission.It has strong anti-interference ability and has low probability of intercept(LPI)and low probability of detect(LDI)characteristics.This thesis mainly studies the multiple access technology of TDCS.Firstly,the system model of single-user TDCS and the principle of TDCS multiple access are introduced,and the multiple access performance of TDCS is simulated.Simulation results show that TDCS with 1024 subcarriers and serial interference cancellation demodulation can accommodate about 6 users under additive Gaussian white noise channel.Aiming at the problem of limited user capacity of traditional TDCS multiple access algorithm,this thesis studies a non-orthogonal multiple access technology,interleave division multiple access(IDMA).Combining IDMA technology and TDCS,a TDCS-IDMA system based on BPSK and CCSK modulation is proposed,and the iterative detection algorithms under single-path real channel and quasi-static multipath channel is deduced,respecively.The simulation results show that TDCS-IDMA system can accommodate more than 16 users when the number of subcarriers is 1024 and the spread spectrum length is 16 while ensuring the anti-jamming performance of TDCS,which effectively improved user capacity of TDCS.Then the TDCS multiple access technology based on spectrum soft decision is studied.The traditional hard decision mode selects 30%~40% of the maximum noise power as the decision threshold.This method has poor performance when the interference signal occupies a wider frequency band.In this paper,by selecting a high threshold and a low threshold as the decision threshold,the traditional hard decision mode is changed to soft decision mode,which can effectively improve the system performance and covert communication ability.On this basis,the peak-to-average power ratio(PAPR)problem of multi-user TDCS is studied,and its mathematical model and the reasons for high PAPR are given.At the same time,the design algorithm of frequency domain sequence with good correlation under spectrum constrains is proposed and verified by simulation.Simulation results show that using this sequence to generate the fundamental modulation waveform of TDCS can effectively reduce the PAPR of soft decision TDCS.Finally,this thesis carries out prototype verification and performance tests of the multi-user TDCS system.The multi-user TDCS system hardware test bench was built by using Universal Software Radio Peripheral(USRP),and the link design and software and hardware implementation schemes were introduced in detail.The system functions are demonstrated in the presence of interference.