Research On Demodulation Method Of OFDM/CDMA Superimposed Signal

With the development of machine-type communication,the number of terminals connected to the communication system have increased dramatically.In order to deal with the problem of massive connections,we need to further enhance the system capacity.Based on the existing communication system,this paper proposes a scheme to superimpose OFDM signals and CDMA signals in the same time-frequency domain.The superimposed technology achieved bearing more terminals on the same timing domain.Therefore,OFDM/CDMA superimposed signals have stronger information transmission capability and it can effectively increase system capacity.This paper mainly research the demodulation method of the superimpose signals.In order to improve the demodulation performance of the OFDM/CDMA superimposed signals,we have studied and determined the serial interference cancellation demodulation scheme for OFDM/CDMA superimposed signals firstly.Through the simulation results,we analyze the shortcomings of the existing serial interference cancellation scheme in this paper.After that we proposed an iterative based serial interference cancellation improved scheme.In the new scheme,we use the interference cancellation algorithm to solve the CDMA signal,then use the interference cancellation algorithm to solve the OFDM signal again.This process can be iterated many times to improve the demodulation performance of OFDM signals.Simulation results show that the improved scheme has better performance and it is more suitable for OFDM/CDMA superimposed signals.Secondly,in order to eliminate the influence of frequency offset on OFDM/CDMA superimposed signals,we have improved the frequency offset estimation algorithm for OFDM/CDMA superimposed signals based on the Moose algorithm.The Moose algorithm calculates the phase difference between the two repeated sequences to obtain the frequency offset estimation on the receiving end.However,these two repeated sequences may be interfered by CDMA signals and the accuracy of the estimate will be affected.To solve the problem,two identical sets of pilot sequences were placed before the OFDM/CDMA superimposed signal.In this way,the CDMA signal will not interfere the pilot sequences.The Moose algorithm uses these two pilot sequences to obtain the value of frequency offset estimation.The simulation results show that the MSE of the frequency offset valueestimated by the improved frequency offset estimation algorithm has an order of magnitude improvement over the MSE estimated by the original algorithm when the SNR of OFDM/CDMA superimposed signals is 10 d B.As the number of users increases,the estimated performance of the improved algorithm is more accurate.Based on the above results,the general demodulation method is given.Specifically,we perform frequency offset estimation and compensation at the receiving end of the OFDM/CDMA superimposed signals.Then we use the channel estimation algorithm and iterative serial interference cancellation scheme to perform demodulation of superimposed signals.Simulation experiments can show that the OFDM signal and CDMA signal obtained from the solution have good BER performance under AWGN and multipath fading channel.The experimental results further prove that OFDM/CDMA superimposed signal transmission method has practical feasibility.