Research On 5G Air Interface F-OFDM

The development of ICT(Information Communications Technology)and the global digital wave indicate that the next generation mobile communication network 5G is about to be born.5G supports rich business scenarios,different business scenarios require different parameters.The traditional OFDM(Orthogonal Frequency Division Multiplexing)has become the core wave of 4G,because of its simplicity and high transmission efficiency.However,the parameters of the whole system bandwidth of OFDM are fixed,the band spectrum leakage high,the shortcomings show that OFDM has been difficult to meet the needs of 5G.Filter based OFDM(Filtered-OFDM)is a new type of air interface that can solve the traditional OFDM defects.It divides the system bandwidth into multiple subbands,and each sub-band parameter can be configured according to the actual business scenario.Subband filter of F-OFDM can restrain subband band spectrum leakage.Moreover,the core wave of the F-OFDM sub-band is still OFDM,that is to say,F-OFDM achieved forward compatibility.In other words,the new module of F-OFDM is main the filter of sub-band.Sub-band filter mainly involves two key issues: first,The generation of the subband filter should not be too complicated,in principle,it can act as a F-OFDM filter as long as the filtering performance is good enough,but considering the F-OFDM with the parameters according to the actual service configuration,subband filter parameters will change,so the generation of subband filter should be easy to achieve;second,the filtering operation has high computational complexity,Filtering operations are usually performed by linear convolution,to avoid interference between subbands,subband filters are often of great length,This results in very high computational complexity of filtering operations,while filtering operations with high computational complexity are difficult to be accepted by practical applications of 5G communications.In this paper,based on the background of the mobile communication system,the basic content related to the air interface technology is sorted out.Combining F-OFDM subband symbol,the reasons of the high spectrum leakage of F-OFDM subband are obtained from time domain to frequency domain: the subband symbol spectrum consists of sinc functions,and the sidelobe of the sinc function causes the subband out of spectrum leakage.Then,subband filters are generated based on soft cut of window function.Using this method to generate the filter is relatively simple,the configuration of subband parameters according to the actual business won't be affected.Then the computational complexity of the filtering operation is analyzed according to the equivalent relation between time domain cyclic convolution and frequency domain multiplication.Low complexity filtering operation is realized in frequency domain by overlap save method(OLS),the computational complexity of the frequency domain filtering operation is compared with the computational complexity of the time-domain convolution filtering operation.According to the formula: the computational complexity of the frequency domain multiplicative filtering operation is far less than the computational complexity of the time-domain convolution filtering operation,the former can improve the time efficiency of the F-OFDM system.In the end of this paper,with the MATLAB platform,the simulation of F-OFDM downlink is realized.In FDATool,it is simple and easy to implement when the window function is used to generate subband filters.From the simulation results: the power spectral density of the OFDM began to appear sidelobe about at-30 dB F-OFDM subband filter suppresses the subband band spectrum leakage,the power spectral density of about OFDM of sub-bands began to appear about at-50 d B;on computational complexity in subband filtering operations,this paper deduces a calculation,which is the low complexity frequency-domain filtering operation complexity and the linear convolution filtering operation complexity,comparing with this formula,the ratio of the actual running is about half of the theoretical value of the formula;besides,depending on the length of the filter and the length of the entire data segment,After determining the subband filter length,the sub data segments should be as long as possible in combination with the values that facilitate the IFFT/FFT transform.