Research On Key Technologies Of Analog Mobile Fronthaul Broadband Wireless Signal Based On Radio Over Fiber

In the face of the development of 5G wireless communication with ultra-high speed and ultra wideband,the MFH system based on CPRI needs to digitize the analog signal for transmission,which greatly increases the transmission capacity of the system;the MFH system based on RoF can not only meet the requirement of high speed and large capacity,but also has the advantages of simple structure and convenient deployment,which has become a research hotspot in recent years.Among them,how to maintain high fidelity and high frequency spectrum utilization in the transmission of broadband wireless signal has become an important problem to be solved in the future.In this paper,we first analyze the nonlinear interference based on RoF in analog optical links and the same frequency self-interference in simultaneity and the same frequency full duplex communication.Then,we summarize the research status of routinization technology and the same frequency self-interference cancellation technology in analog optical links at home and abroad,as well as the problems of existing schemes,three schemes are proposed to improve the performance of 5G analog MFH system:(1)Based on the research of traditional APL linearization scheme,this paper proposes a scheme to linearize the direct modulation link by using the principle that the even order harmonics of the photocurrent generated by the balanced detection of two inverted optical signals generated by push-pull modulation can cancel each other.The simulation results show that all even order nonlinear interferences in APL can be eliminated and the linearization performance of optical link can be improved.When the data rate is 6.4Gb/s,8 channels of 200MHz frequency division multiplexing 16-QAM-OFDM signals can transmit at 0.4km when EVM<12.5%.(2)Based on the research of existing co-time co-frequency self-interference cancellation schemes,we propose a scheme using cascaded MZMs for self-interference cancellation in co-time co-frequency full duplex communication.Firstly,the two cascaded MZMs are biased at opposite orthogonal bias points to achieve the purpose of reverse phase between interference and the reference signal.Then,the amplitude and time of the input reference RF signal and the optical delay line between the cascaded MZMs are matched to achieve the purpose of eliminating the same frequency self-interference.Under the condition that the spectrum of wideband interference and useful signal will not overlap,the simulation link of the scheme is built to prove its validity and feasibility.The final results show that the proposed scheme can eliminate narrowband single frequency interference by 32dB at 40GHz and 16-QAM-OFDM interference with 14GHz bandwidth by 46dB at 40GHz.At the same time,the signal-to-noise ratio of 16QAM signal with bandwidth of 2.8GHz can reach 29.5dB and EVM can reach 7.7%after eliminating self-interference for the same frequency.(3)On the basis of studying the principle of multi-path self-interference cancellation,this paper proposes a scheme to eliminate multi-path self-interference with the same frequency by using two-stage MZMs which are biased at the opposite orthogonal offset points to realize inverse modulation of self-interference with the same frequency between the reference signal and uplink signal.The simulation results show that the ratio of the maximum interference to the uplink signal is 25.7dB,the signal-to-noise ratio of the recovered uplink signal is 29dB and the EVM is 8.2%,which meets the requirements of 3GPP standard for 16QAM signal transmission in 5G system.