| Optical wireless communications,including visible light communicaitons?VLC?and radio over fiber communications?RoF?,are the enabling technologies for the future wireless access network.In order to satisfy increasing demands of high-bandwidth data service for the user ends,advanced digital signal processing?DSP?technologies are indispensable.At prsesnt,one major technical challenge is to design coded modulation schemes tairloed to large-capacity communication system.It aims to jointly optimize the concatenation of forward error correction?FEC?and modulation format to provide transmitted signal with high spectral efficiency and robustness against noises.Most researches have beed done under the framework of bit-interleaved coded modulation?BICM?due to its simplicity,low latency,high parrellesim and independent design of FEC.Thus,BICM has become the de facto standard for a variety of communication systems.Particularly,polar code is emerging as one of the FEC winners for the latest5G standard,which has already been theoretically proved to be able to approach Shannon limit.Besides,constellation shaping is another type of DSP technology that enables communication systems to approach Shannon limit.Currently,probabilistic amplitude shaping?PAS?is recommended to be one of the best CS solutions,with advantages of rate adapility,low complexity and Shannon-limit-approaching ability.This thesis introduces our researches on advanced coded modulation schemes,including polar-coded bit-interleaved coded modulation?BIPCM?,PAS,in the scenario of optical wireless communications,i.e,VLC and RoF.The main works and novelties are as follows:1.Based on a VLC system adopting 128 quadrature-amplitude modulation?QAM?and orthogonal frequency division multiplexing?OFDM?,an encoder-subcarrier mapping method is proposed to overcome frequency fading for polar code.The experimental results show that,at sampling rate of AWG with 150 MSa/s,the system can realize error free transmission with high spectral efficiency?SE?up to 5.77 bit/s/Hz by using the proposed method.At sampling rate of 175 MSa/s,its measured Q-factor is 9.86 dB,which improves by 5.12 dB compared to the case without coding.2.A BIPCM scheme based on orthogonal circulant matrix transform?OCT?precoding is proposed and experimentally demonstrated in an OFDM VLC system.To overcome the fading effect on a mapped bit-level,the scheme employs the Monte Carlo method to jointly design a polar code concatenated with 256-QAM.In addition,OCT precoding is used to mitigate the frequency fading effect across OFDM subcarriers.It exhibits the advantages of high reliability,low latency and low computational complexity.The experimental results show that the system can achieve a net data rate of 343 Mb/s over 80-cm free-space transmission with a bit error rate?BER?below1×10-3.At the receiver,its number of complex multiplications is reduced by 33%in comparison with BIPCM using an optimized quadratic polynomial permutation?QPP?interleaver.3.Tailored to a W-band RoF system,a 64-QAM PAS scheme adapts to large number of OFDM subcarriers is proposed.Thanks to the OCT precoding method,one optimized probabilistic distribution is globally implemented across 240 subcarriers by merely one constant composition distribution matcher?CCDM?.The proposed PAS scheme for OFDM is experimentally demonstrated in the RoF system,with 20-km single mode fiber?SMF?and 0.5-m free-space transmission.Experimental results show that compared to the uniform 64-QAM,there is 1.07 dB shaping gains for system sensitivity at BER of 1×10-4.Meanwhile,it inherits the advantage of flexible capacity granularity from the conventional single-carrier PAS. |
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