Doppler Frequency Shift Suppression For GFDM System In High Movable Communication Scenarios

In 5G many new scenarios are proposed,such as Internet of Things(IoT),Internet of Vehicle(IoV),Virtual Reality(VR),and so on.This requires the air interface waveform in the 5G can simultaneously meet the different requirements of different scenarios,such as shorter communication latency,larger communication bandwidth and system capacity,and so on.In 4G the air waveform technology is Orthogonal Frequency Division Multiplexing(OFDM),which can't simultaneously meet the different requirements of different scenarios.Moreover,the OFDM signal has large out of band leakage,sensitive to the frequency offset and large peak to average power ratio(PAPR),all of this make OFDM doesn't suit for the 5G,and make it necessary to have a new kind air waveform to meet the requires in 5G.In this paper,5G and High Dynamic Communication Scenarios(HDCS)are the main research background,the purpose is to improve the new air interface waveform--Generalized Frequency Division Multiplexing(GFDM)Bit Error Ratio(BER)performance in HDCS and reduce the system complexity,making this technique have a better adaptability and application in 5G.The improvement of GFDM communication system is mainly divided into the following two steps:As is known,in the original GFDM system,there have several standard schemes,such as the Zero Forcing(ZF)receiver,Matching Filter(MF)receiver and Minimum Mean Square Error(MMSE)receiver,and those schemes all adopt the matrix way in the receiver with the purpose of have an intuitive and simple signal expression.However,when the subcarrier number and the symbol number are large(such as in 4G LTE,subcarrier number is 2048,symbol number is 15),the computation of the system will be very large,is not conducive to the realization of the actual system.So the first step is to reduce the complexity of the system.In the research process,founding that the GFDM signal expression is equal to the Gabor expansion of a signal in the critical sampling condition,so it can use the critical sampling Gabor expansion to represent the GFDM modulation signal and the Gabor transform represent the GFDM demodulation signal.In the Gabor transform,the analysis window function and the synthesis window function satisfy the double orthogonality condition(i.e.,the Wexler-Raz dual identity).So the analysis window function of the receiver can be obtained via the double orthogonality condition when the synthesis window function is known,in this process this identity can be solved in the Zak domain,which can be greatly reduce the difficulty and complexity.Then the Gabor expansion can be obtained with the analysis window function.Due to avoid the matrix inversion operation,this way greatly reduces the complexity of the system.This is the proposed GFDM method based on Gabor transform and expand in this paper.In the GFDM system,the neutron carrier is no longer strictly orthogonal,and then the sensitivity of the frequency offset is lower than the OFDM system,which let GFDM system have a better Anti-Time Selective Fading Performance(ATSFP)compared to the OFDM system in the presence of Doppler Frequency Shift(DFS)channel.But the DFS will also have a great impact on the communication performance of GFDM system when the relative speed is very large between the transceiver.In the other hand,high mobility is also an important application scenario as required in the fifth generation mobile communication.So the second step is to improve the ATSFP of the GFDM systemIn this paper,the chirp basis function is used to replace the original sine basis function in the GFDM signal.In this way the subcarrier frequency is not a certain value,but time-vary,let the GFDM system obtain an improved ATSFP.The concept of fractional Fourier transform can also be used for reference in the actual signal processing,i.e.,via selecting the optimal transform order,let the system has the best performance in the corresponding transform domain.So as to achieve the purpose of improving the system's performance to resist the channel time selective fading.As the simulation results shows,the implementation of GFDM based on Gabor transform and expansion can effectively reduce the complexity of GFDM system,generalized fractional division multiplexing method can effectively improve the ATSFP of the system.Therefore the following conclusions can be made: the two steps operation can let the GFDM system achieve a reduction of system complexity and an enhanced ATSFP.