Research On Subcarrier Allocation Of VLC-OFDM System Under Statistical Delay QoS Guarantee

With the rapidly increasing demand of wireless bandwidth,the shortage of RF spectrum becomes more and more severe.Visible light communication(VLC)has the advantages of ultra-bandwidth,being harmless to human body,secure information transmission,and so on.It has become one of the most important broadband wireless networks in the next generation communication.Orthogonal frequency division multiplexing(OFDM)modulation technology achieves parallel transmission,which has the characteristics of high data rate,is widely used in VLC system.Appropriate resource management schemes can effectively improve the performance of VLC-OFDM system.The next generation wireless networks appear to be heterogeneous,how to utilize the limited spectrum resources to guarantee the quality of service(Qo S)of different types of services is a challenging problem.In the future,everything will access to the internet,an access point(AP)manage a large number of terminals,the use of centralized resource management scheme will increase the burden of AP,and reduce the system reliability.At the same time,with the rapid development of hardware technology,the computing capability of terminals is greatly improved.In the next generation wireless network,the performance of distributed algorithms which need the participation of terminals will be better than the centralized algorithms'.Delay is the most basic communication quality of service(Qo S)indicator.Since wireless fading channels are random and time-varying,deterministic delay of business is difficult to guarantee.Effective capacity theory considers the statistical delay Qo S constraint from the perspective of link layer.It reflects the performance of network throughput under statistical delay Qo S guarantee,and ensures that the delay violation probability of service would not exceeding the specified delay boundary.Effective capacity theory provides the theoretical basis for the statistical delay guarantee of next generation wireless communication system.In this paper,we study the subcarrier allocation problem in indoor VLC-OFDM system under statistical Qo S guarantee.Considering the poor robustness of the optical line of sight(LOS)link,the effective capacity of the VLC-OFDM system is modeled by introducing the LOS blocking probability,and then establish the subcarrier allocation problem as an integer optimization problem to maximize the system effective capacity.The bandwidth allocation factor is used to transform the integer optimization problem into continuous optimization problem.The mathematical optimization shows that the continuous optimization problem after transformation is a convex optimization problem.Based on the convex optimization theory,a quasi-distributed algorithm which needs the terminals and AP to work collaboratively is proposed to solve the optimal allocation strategy of bandwidth allocation factor.In the implementation of the algorithm,the AP point and the terminal need to exchange information,slower algorithm convergence speed will lead to more bandwidth wasted on the implementation of the bandwidth allocation algorithm,which will reduce the bandwidth utilization.Therefore,in addition to the quasi-distributed algorithm,in this paper we also optimizes the algorithm to speed up its convergence.The number of subcarriers in the OFDM system is the discrete,however the number of subcarriers obtained by solving the continuous optimization problem is continuous.Therefore,the continuous subcarriers needs to be discretized.The general discretization method are downwards and upwards.Downwards lead to a waste of bandwidth,while upwards lead to the bandwidth allocated to terminals beyond the total bandwidth in the system.Regarding the issue above,in this paper we then studies the optimal subcarrier discretization strategy.We first define the overall effective capacity of the system,considering the fairness,we then introduces the bandwidth allocation factor and utility function to build the continuous optimization problem with fairness and solve it by the quasi-distributed algorithm stated above.After obtaining the bandwidth allocation factor of the all terminals,next we introduce the terminal discretization factor to model the subcarrier discretization problem as the 0-1 programming problem of maximization the overall effective capacity of the system,and obtain the optimal discretization strategy by the genetic algorithm.