Research On Time-Offset GFDM Communication Under Doubly Selective Channels

Generalized frequency division multiplexing(GFDM)is a data-block-based multi-carrier modulation scheme with the advantage of flexible structure,low out-of-band radiation,and high spectrum utilization,and it is considered as one of the candidate waveforms for the Beyond Fifth Generation(B5G)mobile communication systems.However,due to the introduction of prototype filters,there exists intrinsic interference in the GFDM system.In B5G mobile communication systems,the high mobility of the devices will bring serious Doppler effect and makes the channel possess the doubly selective characteristics.As a result,the signal to interference ratio(SIR)performance of the GFDM system will significantly degrade,and the application of the GFDM system will be limited.This thesis first introduces the GFDM system model,and compares the GFDM system with other multi-carrier modulation schemes.The advantages of the GFDM system is presented,and a low-complexity GFDM system implementation algorithm is analyzed.Then,the effects of Symbol Timing Offset(STO),Carrier Frequency Offset(CFO)and doubly selective channel on the average signal-to-interference ratio of each subsymbol are analyzed theoretically and simulated.It is found out that the main reason for the conventional GFDM system's sensitivity to the Doppler frequency shift is the discontinuous energy distribution structure of the first sub-symbol in the conventional GFDM data block.According to this,a time-offset GFDM system is proposed by cyclic shifting the prototype filter of the conventional GFDM system.Theoretical analysis and simulation results show that the TO-GFDM system improves the average SIR under the influence of doubly selective channel and CFO without sacrificing robustness to STO.Focus on the TO-GFDM system under doubly selective channel,a pilot-aided joint iterative channel estimation and symbol detection algorithm is proposed in this thesis.Using the least mean square error estimation,the proposed method estimates the generalized channel matrix by the least squares estimation result of the pilots.With Parallel Interference Cancellation(PIC)or Serial Interference Cancellation(SIC),this algorithm can progressively decrease the interference signal and improve the performance of channel estimation and symbol detection by the information exchanging between the channel estimation unit and the symbol detection unit.The results of simulation demonstrate that in doubly selective channels,the iteration can effectively improve the channel estimation accuracy and Bit Error Rate(BER).And the SIC makes a better influence than PIC and shows a better BER.Furthermore,the BER performance of the conventional GFDM system and the TO-GFDM system under this algorithm is compared,and verifies the advantage of TO-GFDM system under doubly selective channel.