Research On Relaxation Of Timing And Frequency Synchronization For GFDM System

In recent years,the demand of data flow is increasing rapidly,multi-carrier modulation(MCM)which is efficient and flexible is becoming a hot spot of current research.Among kinds of MCM,orthogonal frequency division multiplexing(OFDM)is widely used in the wireless communication system,which has low complexity of transceiver,strong ability to resist multipath interference and high spectrum efficiency.But it also exhibits some intrinsic drawbacks which limit its application,such as large out-of-band radiation,high peak-to-average ratio(PAPR)and strict time and frequency synchronization is required.Generalized Frequency Division Multiplexing(GFDM)is an innovative multi carrier modulation technology.It has its unique advantages while keeping the same advantages as OFDM,such as the pulse shaping filter can be selected flexibly to obtain low out-of-band radiation and the time-frequency structure can be adjusted flexibly to meet the needs of different scenes.The purpose of this paper is to further enhance the robustness of timing offset(TO)and carrier frequency offset(CFO)for GFDM in multi-user asynchronous transmission scenario.The main content is as follows:Firstly,we study the basic principle of GFDM system,and introduce the GFDM's transmitter and receiver model in detail from the viewpoint of frequency domain.In order to further understand the communication performance,we compare the BER performance when using different receiver and different roll-off factor of raised cosine(RC)filter under AWGN and multipath fading channel.Moreover,the similarities and differences of GFDM,OFDM,SC-FDE and SC-FDM in frame structure are compared,it illustrates that the GFDM frame structure is flexible.Then,we take 3MHz and 20MHz bandwidth for example to design GFDM frame structure,compared with OFDM,GFDM has stronger ability to resist time-frequency offset and is more suitable for multi-user asynchronous transmission scenario.Secondly,we study the method for GFDM to resist timing offset.In presence of timing offset,the periodic extension of the signal is discontinuous,and thus spectral leakage occurs.The spectral leakage is the source of multi-user interference.To decrease the discontinuities at the signal boundaries and hence reduce the impact of spectral leakage,we multiply signal by window function in time domain at both the transmitter and the receiver to make the signal boundary gradually transit to zero.Then simulation is given to prove that when TO is greater than the cycle prefix length,MUI can be reduced 4dB when window is utilized,and meanwhile the out-of-band radiation can be reduced 40dB.Thirdly,we study the method for GFDM to resist carrier frequency offset.First of all,we compare the BER and out-of-band radiation performance when using different pulse shaping filter.Usually,GFDM system adopts the raised cosine(RC)filter as its pulse shaping filter.However,when using RC filter,adjacent subcarrier's frequency spectrum has some overlap,which makes it more sensitive to CFO.Thus,we propose use partial response coding technology in GFDM system to resist CFO,the simulation verifies the advantages.