System Performance Optimizations Of Indoor Visible Light Communications Using LEDs

Owning to the advantages of low cost,low power consumption,high modulation rate,high security,and unlicensed spectrum,light emitting diode(LED)are capable of providing lighting function as well as wireless data transmission.This is known as visible light communication(VLC),which has aroused emerging interests in the field of short distance high data rate wireless communications in recent years.Based on the in-depth study of semi-angle at half power(SAHP)parameters on the influence of light intensity distribution,our research includes the following aspects: 1)Since the calculation of indoor impulse response is complex and time consuming,we propose a simplified calculation method of indoor impulse response including multiple reflections,based on classical recursive algorithms.2)An optimization method for SAHP parameters is proposed to improve the uniformity of VLC system,it can enhance the signal-to-noise ratio(SNR)as well as its distribution.3)An optimization is proposed based on modulation parameters and non-identical SAHP parameters to reduce the VLC power consumption while providing brightness dimming control.The system power consumption can be minimized according to different user requirements.The detailed contents of the paper are as follows:Firstly,we introduce the basic knowledge of VLC,the model of light emitting and detecting,the impulse response of directed light and multiple reflection light.Based on the classical recursive method,we propose a simplified calculation method to calculate the impulse response of multiple reflection light.By simplifying the simulation process and reducing redundant calculation steps,the simplified calculation can reduce 40% of total computational amount with the same accuracy.Secondly,by studying the LED lambertian model of emitting light,we deduce the equation of LED illuminance with the lambertian order,simulate the single LED illuminance distribution in terms of different SAHP parameters and analyze the influence of SAHP on LED illuminance.Two methods of enhancing SNR unifomity based on SAHP parameter are proposed: individually optimization(IOP)and wholly optimization(WOP).They are implemented for four LED alignments and the results show that: compared to non-optimized situation,both WOP and IOP can improve the SNR and root mean square(RMS)delay spread for VLC system.For most situation,IOP is superior to WOP.Thirdly,on the basis of studying the sub-carrier 4-pulse position modulation(SC-4PPM)which is capable of providing brightness dimming control in VLC system,we propose an optimization to provide brightness dimming control and enhance SNR uniformity,in the condition of satisfying both communication and lighting requirements.For the Evenly alignment,the simulation results show that it can achieve uniformity improvement while supporting dimming control function.Tolerance ability of the proposed optimization is analyzed by introducing deviations on SAHP parameter,modulation parameters and room size parameters.Tolerance analysis shows that our method is robust against practical parameters deviations.Feasibility evaluations show that the optimization can be employed in various practical situations,such as different LED powers,receiver field of view(FOV),LED numbers and room dimensions.Fourthly,by analyzing the influence of SAHP parameter on LED illuminance,we propose the idea of power consumption based on SAHP parameter in VLC,then we define the power efficiency of the dimming VLC system using SC-4PPM modulation parameters.We propose four methods of optimization based on modulation parameter,SAHP parameters,FOV of detector.They are stable identical SAHP(SS),variable identical SAHP(VS),variable non-identical SAHPs(VNS),and variable identical SAHP and variable FOV(VISF).By implementing the four optimizations for four LED alignments,the results show that these optimizations can enhance VLC power efficiency.VS can achieve higher power efficiency than SS,VNS can obtain the minimum SNR variation,VISF can achieve the highest power efficiency among all the four optimizations.Among the four LED alignments,Evenly alignment obtains the best overall performance.Finally,our contributions and innovations are summarized.The future research for VLC in terms of the trend and existing problems are also mentioned.