Research On Novel Systems And Several Key Techniques For Optical Wireless Communications

With the rapid developlemt of mobile internet,Internet of Things(Io T),augmented reality/vitural reality,autonomous driving,artificial intelligence and other new application technologies,we are approaching an era of Internet of Everything,which mainly relies on mobile communication technologies.In future,various kinds of new services and application scenarios with huge increase of mobile data traffic and massive device connectivity are forcing a revolution of next-generation communication networks.Optical wireless communications with higher spectrum such as millimeter-wave and visible light bands adopt advanced modulation formats,novel multiple access technology and advanced digital signal processing,forming novel optical wireless communications with higher band-width,higher spectrum efficiency,and more user access.In view of the background above,the dissertation maily focuses on researches on some key issues such as channel estimation and equalization,power allocation,error propagations,hardware-efficient algorithms,capacity enhancement and performance improvement in novel optical wireless communications.The contents and innovative results of the dissertation are summarized as follows.Firstly,researches about modulation formats with high-spectral efficiency.To enhance the system performance such as transmission rate and the robustness to interference,offset quadrature amplitude–modulation based orthogonal frequency division multiplexing(OQAM/OFDM)is investigated.As compared to traditional orthogonal frequency division multiplexing(OFDM)technique,OQAM/OFDM has many advantages such as low out-of-band leakage,higher spectral efficiency,stronger robustness to inter-symbol-interference(ISI).While these advantages exist in OQAM/OFDM at the cost of only real orthogonality among subcarriers,leading to intrinsic imaginary interference(IMI).IMI will make the channel estimation and equalization becomes more complicated.(1)To solve the issue above,we proposed improved channel estimation method for OQAM/OFDM systems.It converts the estimated frequency response into time domain for elimination of IMI and noise outside the channel state information.(2)Blind equalization is proposed and experimentally demonstrated for OFDM/OQAM-VLC system to improve the system performance.It does not need any pilot overhead.Moreover,compared with conventional interference approximation method(IAM),it exhibits 3.2-and 2.7-d B overall improvement in Q²-factor using blind equalization based on the constant modulus algorithm(CMA)and constant norm algorithm(CNA),respectively.Secondly,researches about power domain NOMA.The multiple access schemes for traditional optical wireless communication systems are mainly based on time domain,freuquency domain,code domain to realize the multiplexing with multiple dimensions.In this dissertation,we further investigate the power domain NOMA for achieving signal multiplexing with higher dimension to obtain larger access capacity,more user connectivity number.Owing to very limited bandwidth and severe power fading issues,we hence investigate the integration of NOMA and optical wireless communication systems to maximally release the potential of NOMA.(1)We propose a digital signal processing(DSP)-aided technique to optimize the power ratio among users for orthogonal frequency division multiplexing(OFDM)-based non-orthogonal multiple access(NOMA)in an integrated optical fiber and millimeter wave(mm Wave)wireless communication system.The proposed technique can mitigate the issue of high frequency fading and make all subcarriers have a uniform signal-to-noise ratio(SNR).Experimental results show that the optimal power ratios on all effective subcarriers are optimized at the same level and the users'performance is significantly improved.(2)We propose and experimentally demonstrate a subcarrier pairwise coding(PWC)scheme to mitigate the error propagation issue for NOMA-VLC.Without extra coding overhead,the proposed scheme will not decrease the spectral efficiency.Experimental results show up to 3.3-d B Q²-factor improvement can be achieved by the proposed scheme(3)We first propose and experimentally demonstrate an OQAM/OFDM based NOMA for multi-user MIMO-VLC system.Experiemtal results show that the proposed MIMO-NOMA scheme exhibits better BER performance than the conventional OQAM based MIMO-OFDM.With the optimized power ratio,an aggregate capacity of 3.2 Gbit/s could be achieved.(4)OQAM/OFDM-NOMA is proposed in the multi-user asynchronous multi-cell VLC to improve the system performance.OQAM/OFDM-NOMA modulation can effectively improve system capacity and flexibi lity.For the user fairness and robustness against asynchronous time-offset,the proposed OQAM/OFDM-NOMA modulation can significantly outperform the conventional OFDM-NOMA modulation in the multi-user asynchronous multi-cell VLC system.Thirdly,real-time field-programmable gate array(FPGA)based demonstrations for optical wireless communication systemsThe signal processing schemes are all performed offline in optical wireless communication systems as introduced before.They fail to consider some issues that exist in pratical systems such as feasibil ity of hardware,usage of resources,and latency.The validation of real-time systems as well as its signal processing algorithm is a key step for the conversion from science researches to industrial applications.Hence,the real-time investigation for optical wireless communication systems can be divided into following two aspects.(1)We have proposed a real-time software-reconfigurable dynamic power-and-subcarrier allocation scheme for OFDM-NOMA based VLC.All signal process algorithms are carried out by using one FPGA.The flexibility and reliability of the proposed scheme are validated in a FPGA based real-time multi-user VLC system.It can work very well to meet users'demands.Experimental results show that a 1.84-Gbit/s real-time OFDM-NOMA transmission can be realized with several hours stability.(2)A hybrid in-house optical wireless access is proposed using OFDM-NOMA to multiplex both wired and wireless signal.Such hybrid access can provide high-speed and stable wired access with fiber and suppor t flexible and secure seamless coverage with VLC as wireless access.By adjusting the power ratio between fiber user and VLC user in real-time,the system could flexiblily satify users'demands and enable better user fairness.Forthly,researches about low-cost optical wireless communication system.Optical camera communication(OCC)based optical wireless communication system is capable of supporting low-cost wireless connectivity,and thus could accomondate the demands of future Io T.CMOS cameras are used as receivers for OCC system.The trasmission rate is mainly limited by the frame rate of CMOS cameras.To improve the data rate and consider the feasibility in mobile scenarios,the researches include following three parts.(1)We propose and experimentally demonstrate a multilevel modulation scheme for VLC system utilizing the overlapping of two light sources for the first time,and the two lightsources are modulated by an OOK and a Manchester signal respective ly.At the receiver,a CMOS camera can demodulate the Manchester and the OOK signal simultaneously.The experimental results demonstrate that the proposed mu ltilevel modulation scheme can achieve a net data rate of 4.32 kbit/s.(2)We propose and experimentally demonstrate a simple and elegant modula tion format shifting(MFS)scheme by continuously changing the modulation formats between OOK and Manchester signal for optical camera communication(OCC)using a commercially available mobile phone.Leveraging the MFS scheme,extra data bits are delivered without reducing the pixel width of each bit,resulting in higher transmitted data rate.Meanwhile,multi-thresholding scheme is proposed for modulation format identification and signal decoding at the receiver.The experimental results demonstrate that t he proposed MFS scheme can achieve a data rate of 8.16 Kbps with the bit error rate(BER)of 2.9×10^-3 at the illuminance of 400lux.(3)Mobile OCC system using mobile phone is firstly proposed and experimentally demonstrated and its experimental character ization of user mobility by varying moving speed and lateral distance is also investigated.The dynamic CMS algorithm is proposed for the mobile OCC system to select an appropriate column matrix while avoiding the blooming effect.The experimental results show that the BER performance is proportional to the moving speed and lateral distance and it will get degraded with the increase of moving speed and the increase of lateral distance.Furthermore,the proposed mobile OCC system using mobile phone can achie ve a data rate of 4.08 kbps under the low illuminance of 275 lx.