Research On Peak-to-average Power Ratio Suppression In Multi-IFoF Based Mobile Fronthaul System

The multi-intermediate-frequency-over-fiber(Multi-IFoF)based analog mobile fronthaul has great potential in the fifth generation(5G)and future mobile communications.Multi-IFoF technology up-converts multiple Orthogonal Frequency Division Multiplexing(OFDM)signals to intermediate frequency,and then aggregates them to one channel through frequency division multiplexing.Finally,the aggregated signal is sent from the baseband unit(BBU)pool to the remote radio header(RRH)via the optical fiber link.OFDM signal inherently has the problem of high peak to average power ratio(PAPR).In addition,the aggregation of multiple Intermediate Frequency(IF)OFDM signals will result in peak regeneration and higher PAPR.Signal passing through power amplifier and photoelectric device will introduce nonlinear distortion,which will seriously affect the transmission performance of the Multi-IFoF system.The conventional PAPR suppression algorithms of OFDM signal cannot be applied due to the peak regeneration.Furthermore,the low latency characteristics of mobile fronthaul network have high requirements on the computational complexity of the PAPR reduction algorithm.Therefore,this paper proposes a novel low-complexity PAPR reduction scheme for the high PAPR problem of the analog mobile fronthaul system based on Multi-IFoF technology.Firstly,the paper introduce the Multi-IFoF based mobile fronthaul system.The PAPR problem of Multi-IFoF based mobile fronthaul system is mainly analyzed.We observe the PAPR characteristics of multi-IF OFDM signals by changing the number of IF carriers,and analyze the limitations of traditional PAPR reduction algorithms of OFDM system directly used in Multi-IFoF system.Taking the selected mapping(SLM)algorithm to reduce PAPR as an example,the simulation results show that the PAPR of multi-IF OFDM signal will increase as the number of IF carriers increases.But increasing to a certain degree will approach a saturation state.In addition,as the number of IF carriers increases,the PAPR reduction effect of SLM algorithm is getting worse and worse.It indicates that it is not feasible to apply PAPR reduction algorithm before IF carriers multiplexing.Then,according to the characteristics of the multi-IF OFDM signal,we adopt a peak shrinking and interpolating(PSI)PAPR reduction algorithm in the time domain for the Multi-IFoF based mobile fronthaul system.The main idea of PSI algorithm is to extract the high peaks of the signal and shrink them,and then interpolate them into the original signal to reduce the PAPR.We compare PSI algorithm with the previously proposed tone reservation(TR)algorithm and phase pre-distortion(PDP)algorithm.The simulation results show that the PSI algorithm can reduce the PAPR by 4.3d B while ensuring low computational complexity,which is significantly better than the 1.6d B of TR and 3.2d B of PDP.Finally,In view of the disadvantage of PSI algorithm that a large amount of sideband information is needed for distortionless recovery in the receiver.We propose a scheme that combines cyclic delay and peak shrinking and interpolating(CD-PSI).The CD-PSI algorithm performs once delay search combination in the process of generating multi-IF OFDM signal.It is equivalent to reducing the peak size by preprocessing the signal.Next,PSI algorithm only needs to process a small number of peaks to achieve the expected PAPR reduction effect.The simulation results show that the sideband information overhead of the CD-PSI algorithm is reduced by up to 8 times compared with the PSI algorithm under the same PAPR reduction performance,meanwhile,the computational complexity is similar.In order to confirm the feasibility of the CD-PSI algorithm in Multi-IFoF system,the experiment demonstrates that the EVM of the received signal is reduced by 0.005 after the transmission through 20 km single-mode fiber by applying the CD-PSI algorithm.In summary,this thesis adopts a low computational complexity PSI algorithm to reduce PAPR to address the high PAPR problem of Multi-IFoF based mobile fronthaul system.On this basis,a novel CD-PSI scheme is further proposed,which effectively reduces the sideband information overhead and improves the transmission performance of Multi-IFoF system.