Link Dsesign And Optimization For Visible Light Communication

Driven by the demand of broadband and high capacity wireless communication,radio frequency(RF)spectrum resources have become increasingly scarce.Visible light communication(VLC)is regarded as an effective supplement to traditional RF communication by virtue of its abundant available spectra and low cost.However,due to the current semiconductor manufacturing level,the modulation bandwidth of ordinary light-emitting diodes(LED)is very limited.Thus,the effective bandwidth of ordinary diodes is mostly narrower than ten MHz,which brings severe challenges to high-capacity VLC involved wireless applications.In addition,the frequency of visible light is typically high,and the path-loss fading brings considerable trouble.Besides,it is almost impossible for VLC to penetrate obstacles,leading to considerable coverage blind spots.Thereafter,many recent research efforts address the spectrum efficiency and coverage capability for VLC systems.As mentioned above,this thesis will propose a link design and optimization scheme for the VLC system.The contributions of this work are summarized as follows,1)In view of the low communication capacity caused by the narrow bandwidth of LEDs,we will propose a VLC capacity optimization method based on non-orthogonal multiple access(NOMA)technology.The critical parameters,such as user quality of service(QoS),transmit power,LED operating region(LOR)of LEDs and the number of users,are jointly considered in this work.Taking the sum-rate as the optimization goal,we design the power allocation algorithm to maximize the sum-rate.The analysis results show that if compared with random and optimal fixed power allocation algorithms,the proposed scheme in this thesis produces a significant gain.The proposed optimization algorithm is close to the global search algorithm.2)In view of the large path-loss of the visible light wireless channel and poor penetration capability,a visible light(VL)-RF hybrid transmission scheme was designed,which helps to realize the trade-off between transmission rate and coverage capability.In this thesis,we take the transmission rate and the LED Operating Region(LOR)of diodes as constraints and proposes a precoding algorithm to maximize the minimum rate for those users in weak channels.The results show that the proposed algorithm can improve the coverage for the VLC.In conclusion,the link enhancement scheme and the optimization algorithm are proposed in this thesis to address the challenges of VLC technology.The contributions presented in this thesis are expected to shed some light on the practical applications and academic research for the VLC technologies.