Study On High Efficiency Radio-over-Fiber Mobile Fronthaul Systems

In recent years,C-RAN(centralized,coorative and clean cloud radio access network)with centralized baseband unit(BBU)pool and distributed remote radio heads(RRHs)has attracted more and more attentions with the explosion of data traffic.However,affected by the bandwidth of the fronthaul segment between BBU and RRH,it is difficult to satisfy the requirements of the new services and applications in the 5 th Generation Mobile Communications(5G).Currently,the interface between BBU and RRH is based on common public radio interface(CPRI),and the transmitted data over fronthaul links is sampled data.If CPRI continues to be used for fronthaul segment,the bandwidth between BBU and RRHs will be up to the level of 1 Tb/s,as massive multiple-input multiple-output(MIMO)is adopted in 5G.The high-bandwidth will limit the potential cost savings of C-RAN.As an analog signal transmission technology,radio-over-fiber(RoF)provides a new low-cost solution for fronthauling without increasing the bandwidth caused by data sampling and quantization.However,the new technologies adopted in 5G also pose new requirements and challenges to RoF systems.At present,a fronthaul link between a BBU and a RRH tends to use a single or a bundle of parallel single-mode fibers(SMFs).Due to the continuously enhanced MIMO,the increased radio frequency(RF)channels will consume significant fibers.Meanwhile,the new technologies,such as ultra dense deployment,multicarrier and higher order modulation,will make the fronthaul segment suffer from an increased interference and severe non-linear,thus lead to a decreased transmission reliability.In order to reduce the needed fiber counts and ensure the transmission reliability,this paper investigated two methods that transmitted MIMO signals over a single fiber and proposed a high-gain forward-error-correction(FEC)technology for RoF fronthaul systems.The details and novelty are as follows:1.High fiber utilization of MIMO wireless signals transmission over a single spatially multiplexed fibera)Taking advantage of the inherent nature of modal dispersion,multimode fiber(MMF)behaves similar to wireless multipath propagation.Based on this analogy,this paper proposes to transmit MIMO wireless signals over a single MMF.The channel condition,EVM and throughput performance for different fiber length links under a range of fiber bending conditions are investigated.The experimental results show that,after 1km MMF transmission,the EVM performance of 3 × 3 MIMO could be as low as 2.38%and 2.97%,and the average throughput performance could be 2.76 and 2.27 times larger than that of single-input single-ouput case at 2.4GHz and 5.8GHz bands,respectively.In addition,based on the observed time-varying feature of the MMF channel,the statistic characteristics of the RF MIMO channel enabled by SDM are experimentally investigated.The impacts of mode coupling on the MIMO channel coefficients,channel matrix and channel capacity have been analyzed over different fiber lengths.The results indicate that,MIMO transmission benefits from the greater fiber length with stronger mode coupling.Since MMFs have been widely deployed in short-reach radio access networks,the practical application of this system will bring enormous cost-savings by reusing the widely installed MMFs.b)Taking advantage of the inherent parallelism of weakly-coupled multicore fiber(MCF),this paper experimentally investigates the performance o f MIMO transmission over a single weakly-coupled MCF by comparing with th at of six parallel SMFs,in terms of channel condition and EVM performance.The experimental results show that,after 2 km MCF transmission,the EVM performance of 6 X 6 MIMO could be as low as 0.88%and 1.63%at 2.4 GHz and 5.8 GHz bands,respectively,which shows a nearly identical performance with that of six parallel SMFs.Meanwhile,the impact of the transmission direction among the cores and the compatibility of MCF-based SDM and polarization division multiplexing are also evaluated.All these will provide a useful reference for the practical application of MCF to make full use of the multiple cores.By using a single MCF to replace a bundle of SMFs,it will significantly reduce the required fiber counts and the space occupation,as well as the installation and maintaince difficulty.2.Improved decoding of high-gain staircase codes(SCCs)with marked bitsTo ensure the reliable transmission of information,SCCs with hard decision decoding have attracted much attentions in the optical communication community due to their outstanding performance and low decoding complexity.In this paper,a novel decoding algorithm for SCCs is proposed,which partially uses soft information from the channel.The proposed algorithm is based on marking certain number of highly reliable and highly unreliable bits.These marked bits are used to improve the miscorrection-detection capability and the error-correcting capability of the SCC decoder,respectively.For SCCs with 2-error-correcting BCH component codes,the proposed algorithm improves upon standard SCC decoding by up to 0.30 dB at a bit-error rate(BER)of 10-7.The proposed algorithm only requires slightly modifications to the decoding structure of standard SCC.Furthermore,the algorithm is based on marking bits only,and thus,no soft information need to be saved.Marked bits do not need to be tracked during the iterative process either.In addition,we also considered SCCs with geometric shaping to analyze the performance of constellations with 64 points.The simulation results indicate that,it could yield around 0.24 dB additional gain at a BER of 1 0-6,when compared to SCCs with regular 64-QAM Given a channel,high-gain FEC can result in a much lower BER performance,thus can keep the transmission reliable to better support the new technologies of 5G.