| With the widespread commercial use of the 5th generation mobile network(5G),5G+ applications continue to emerge,along with the transformation of smart mobile devices,cloud applications,5G telemedicine,remote office,short video,live broadcast and other ways of life and social interaction,as well as the emergence of applications such as the Internet of Things and autonomous driving,the number of network connections and data traffic have increased dramatically.The 6th generation mobile network(6G)will request ubiquitous connection,ultra-dense connection,ultra-high speed,and efficient energy consumption.Due to the shortage of spectrum resources,the wireless communication based on radio frequency(RF)cannot meet these requirements mentioned above and needs to find new frequency band resources.At the same time,with the release of projects of "Carbon peak"and "Carbon neutrality" in China,major communication operators and communication research institutions have begun to study new green communication technologies.Optical wireless communication(OWC)is a cutting-edge communication technology that combines the characteristics of optical communication and wireless communication.With the original characteristics of rich spectrum resources,THz-level communication bandwidth,strong security,high confidentiality,anti-electromagnetic interference,green low carbon,low cost and flexible deployment,OWC is the most potential supplement/substitute communication technology for RF wireless communication.Hence OWC has a wide range of application prospects.Visible light communication(VLC),which is still in the development stage,is the most widely studied technology in the field of OWC.The multiple access technology of online bidirectional VLC,the online high-speed VLC technology and the network architecture of dense VLC,which are the research hotspots,are the bottlenecks of the practical development of VLC at present.Focusing on the needs and problems of the practical development of online bidirectional VLC,the multiple access technology of online bidirectional VLC,online high-speed VLC technology and network architecture of dense VLC are studied and some technical solutions are proposed in this paper.The innovations obtained are as follows:Firstly,a slot-based central-coordinated resource-reserved multiple access scheme.Aiming at indoor VLC channel characteristics,a low-complexity,easy-to-implement and low-cost central-coordinated resource-reserved multiple access scheme for online bidirectional VLC network is proposed,and the corresponding online bidirectional VLC system is designed.The terminal access contention avoidance,centralized control mechanism and resource reservation mechanism are designed.The feasibility of the proposed scheme is verified by the theoretical analysis,numerical simulation and experimental verification of system throughput and terminal access delay.The experimental results of the online bidirectional VLC system based on the proposed multiple access scheme show that the system normalized throughput of is 0.91,and the end-to-end average access delay of the terminal is 1338.6 ms.Secondary,an online high-speed VLC technical scheme combined with wavelength division multiplexing(WDM)and feed-forward equalizer(FFE).Aiming at the problem of high frequency attenuation in VLC channels,a red green blue light emitting diode(RGB-LED)based high-speed VLC solution,which integrates WDM and FFE,is proposed.And an online high-speed VLC system with the proposed scheme is designed and is verified through theoretical analysis and experiments.Under the non-return-to-zero On-off Keying(NRZ-OOK)modulation mode,the experimentally measured 3 dB bandwidths of the three channels of red,green and blue wavelengths are 167.8 MHz,148.6 MHz and 134.8 MHz,the theoretical data rates can reach 335.6 Mbps,297.2 Mbps and 269.6 Mbps,the data rates with zero-bit error rate(BER)are 240 Mbps,260 Mbps and 250 Mbps,respectively.The RGB-LED based integrated WDM and FFE online high-speed VLC systems is 750 Mbps with BER equal to 0.Finally,a dense VLC user-centric cell-free network architecture and a space-wave-time division multiple access scheme.Aiming at the problems of massive VLC access point(AP)management,user resource allocation,and user switching between APs in dense VLC deployment,an RGB-LED based user-centric cell-free network architecture for dense VLC is proposed.The AP structure based on RGB-LED is presented,the principles of user-centric and cell-free design,and the update strategy of access point group(APG)are described.The signal to interference plus noise ratio(SINR)is analyzed through theoretical simulation with scenarios of user equipment(UE)located on the centric regions of AP and the overlapping regions of APs respectively.The simulation results show that the SINR of the UE’s receiver is more than 10 dB.Based on the proposed network architecture,combined with the slot-based central-coordinated resource-reserved multiple access scheme,WDM and FFE,a space-wave-time division multiple access scheme is proposed,and an online VLC system based on space-wave-time division multiple access is designed.The theoretical and simulated results show that the system normalized throughput reaches 0.71 and the end-to-end access delay is less than 35 ms with NRZ-OOK modulation and system data rate of 750 Mbps. |
