Broadband Visible Light Communication System:Design And Analysis

The evolving explosion in high speed wireless applications is requiring the use of abundant spectrum of the visible light for communications to mitigate the immediate demand for spectrum and meet the demands of the next generation communication technologies.Visible light communication(VLC),as a potential and promising technology for high rate services,boasts many attractive characteristics such as,low cost,safety,high spatial reuse,etc.Generally,VLC systems exploit intensity modulation and direct detection(IM/DD),for which an electrical signal has to be transformed into a real and nonnegative waveform for driving light-emitting diodes(LED).Direct-current-biased optical orthogonal frequency division multiplexing(DCO-OFDM)is widely used in broadband visible light communications(VLC)for high rate transmissions.Because of its high spectral efficiency and simple implementation,DCO-OFDM is attracting huge research interest from both academia and industry.In this thesis,we will focus on DCO-OFDM in broadband VLC.The major results and contributions of this thesis are concluded as follows:Due to the unipolarity of optical signals,a direct current(DC)bias along with proper clipping is adopted in DCO-OFDM.A high DC bias causes a waste of power and a low DC bias causes a severe clipping distortion.Thus,the DC bias and information-carrying power shall be jointly optimized to balance clipping distortion and power consumption.We consider the bias and power optimization problem under the optical and/or electrical power constraints over flat and dispersive channels.In the flat case,we analytically characterize the optimal solution,based on which several important insights are provided.We also investigate the relation and impact of the two constraints,and propose efficient algorithms to obtain the optimal solution.In the dispersive case,we identify the scope of the optimal solution,and provide efficient algorithms to jointly optimize the bias and power.The optimality and superiority of the proposed methods are validated by numerical results.VLC equipped with multiple LEDs can provide near ubiquitous indoor coverage for both communication and illumination.We introduce the novel concept of biased beamforming to explore the full potential of multicarrier multi-LED VLC systems.The biased beamforming including both bias and beamforming,can cooperate multiple LEDs based on multicarrier schemes.Biased beamforming includes two components to be jointly designed: a direct current(DC)bias on each LED and a beamforming vector on each subcarrier.We first analyze the impact on clipping in multi-LED VLC systems.Next,we consider the joint optimization of beamforming and biasing to maximize data rate.We find the optimal beamformer and analytically characterize its structure.We further optimize the bias on each LED and provide the globally optimal solution in closed form for flat channels,which leads to several critical insights that are helpful for practical systems.We also derive a simplified near-optimal solution for dispersive channels and develop an efficient method for biased beamforming.The performance of the proposed biased beamforming design outperforms existing solutions.VLC offers a convenient way to serve a small-scale cell and provides multiuser access with high spatial multiplexing.For higher throughput,we study the resource allocation problem for an orthogonal frequencydivision multiplexing access(OFDMA)scheme for downlink transmissions and present a joint design of the bias level,power and subcarrier allocation.Decomposing the original optimization into subproblems,we first develop an efficient algorithm for the bias optimization,along with its performance analysis in terms of convergence properties and the global optimality.We then propose several power and subcarrier allocation strategies,including an optimal algorithm and a near-optimal simplified method,to satisfy different complexity and accuracy requirements.Finally,we present two algorithms to jointly optimize the bias,power and subcarrier,and analyze the convergence properties in terms of both the objective and the solution.Comprehensive simulation results under practical system setups illustrate the effectiveness of the proposed algorithms.