Research On Muliple Access Technique Of Visible Light Communication

Recently,visible light communication(VLC)has been a hot research in wireless communication area,due to a large number of advantages over traditional communication,such as the free broadband spectrum resources independent of radio frequency(RF)systems,fast transmission rate,free license,higher privacy,better Compliance,green,and so on.The signal transmission device in VLC system is based on light-emitting diodes(LEDs),which will replace the conventional lighting source and become the preferred source of new-generation lighting,thanks to their long operating life,small volume,low power dissipation,small heat and high-humidity resistance.Therefore,VLC is of great practical significance.This paper is focused on the multiple access technique of visible light communication.Firstly,the basic VLC system consists of transmitter,receiver and communication channel.At the transmitter,the original information is carried by the electrical signal.Then,the electrical signal will be processed by rectification and filter circuits.After that,it will be changed into optical signal by LED,propagating in free space.At the receiver,the optical signal will be converted into induced current by photodiode(PD)when the light shines on PD.Eventually,the original information can be detected or reconstructed by the receiver circuits amplifying,shaping,filtering and regenerating.Generally speaking,the communication channel is regraded as the path for the original signal from LED to PD in terms of theoretical analysis.However,in practical calculation,it should be considered as the path for the electrical signal between the transmitter and receiver circuits.In this paper,the principles of LED-lighting and light-mixing based on colorimetry are analyzed emphatically.Also,the method to calculate the color temperature(CT)and color index(CRI)of LED light is described in particular.Furthermore,the mathematical formula of the channel response is listed,which is assumed to be known before communication.Secondly,the wavelength devision multiple access is unique to optical communication.The emiting light source is composed of multiple monochromatic LEDs,which can emit different wavelengh of lights.The lights with different wavelength can be used as a subchannel.Therefore,signals can be transmitted in relevant subchannel by LEDs with different direct-current(DC)bias.In order to lighting,all lights emitted by LEDs should be mixed into white with CT and CRI constraint.In this paper,a new joint precoer and offset design algorithm for red/green/blue/amber(RGBA)LED is proposed to minimize the system mean squared error.We resort to approximation to relax the original problem into a semidefinite programming(SDP)problem,which is called semidefinite relaxation(SDR)programming.Simulation results are provided to illustrate the performance of the proposed SDR algorithm,which can not only solve multiple access for various user number,but also can obtain better results than SDP algorithm proposed by others.Thirdly,the orthogonal frequency division multiple access is much mature in RF communication,which has been proved to improve the spectrum efficiency.In this paper,a multiple-input single-output(MISO)VLC system is considered,whose transmitter consists of multiple white LED access point.Users are assumed to be equipped with a PD,and be orthogonal subcarrier divided.For this system,this paper aims to find the optimal power allocation for each user to the minimize the symbol error rate(SER)or to maximize the channel mutual information(MI).Simulation results show that the precoder obtained by this paper is superior to that obtained by the equal distribution algorithm in the error rate,mutual information and PAPR performance.Finally,the main feature of non-orthogonal multiple access(NOMA)is that all users'signals are multiplexed in the power domain at the transmitter,and they are detected by successive interference cancellation(SIC)at the receiver.NOMA has been recognized as a powerful technique for improving the spectral efficiency by using the difference between the path losses of multiple users.In this work,concerning one-LED two-user system,one-LED multiple-user system and multiple-LED multiple-user system,two types of optimal transmitter design algorithms under sum-rate maximization and max-min fairness criteria are proposed,where quality of service(QoS)constraints are included.Simulation results demonstrate that NOMA can provide remarkable spectral efficiency gains over traditional orthogonal multiple access(OMA)in the context of VLC downlinks when the channels of different users hold some distance with each other.