Research On LEDs Selection And Modulation Technology In Indoor VLC

With the development of the smart terminals,the global data service is growing rapidly,which brings a huge challenge to the already intense radio frequency(RF)spectrum resources.In this case,visible light communication(VLC),a new environmentally-friendly and energy-saving communication method with rich frequency spectrum resource comes into being.With the advantages of ultra-high bandwidths,no electromagnetic interference,low power consumption and using spectrum without authorization,VLC gets a lot of attention since it arose.VLC has been a research hotspot in recent years.In actual system,in order to have a uniform indoor illumination,the transmitter always consists of LED array that has lots of LEDs.However,the optical path difference among the multiple LEDs triggers inter-symbol interference(ISI).Besides,the ISI caused by multiple LEDs exacerbates the nonuniform distribution of the Signal Noise Ratio(SNR).And with the dynamic change of the modulation order,the ISI brings a modulation order selection trouble.In this paper,we proved that the received SNR is not optimal if all LEDs are used for communication by comparing the received SNRs corresponding to different C-LEDs(Communicating LEDs)deployments,which arouses our interests to explore the C-LEDs selection algorithm.In this paper,the optimal C-LEDs selection problem was constructed as a SNR optimization problem.For a single user,we presented a valid ratio threshold-based C-LEDs selection scheme.In the scenario that multiple users participate in communication at the same time,a selecting scheme based on evolutionary algorithm was proposed.On the problem of modulation order selection,a modulation order selection based on step-by-step method was proposed.The main contributions can be summarized as follows:(1)We investigated the C-LEDs selection problem under the downlink single terminal communication situation.The LED signal contains both the positive component and the negative component in a new perspective.The positive component provides useful power while the negative component causes ISI,the ratio of the two components is defined as Valid Ratio(VR),and the threshold value of VR expressed as Th VR.For one user scenario,we proposed a VR threshold-based scheme to optimize SNR.An LED is chosen to be C-LED only when its VR is greater than VR threshold,otherwise it provides illumination only.Thus a proper LED deployment can be chosen by setting a proper Th VR,and high data rate can beachieved.Besides,the influence on SNR and C-LEDs from Field of View(FOV)and data rates was discussed in this paper.(2)In the downlink broadcast VLC system,there are always multiple users participating in communication at the same time.In this scenario,the VR threshold-based scheme is no longer suitable,because the multiple users locating in different positions always have different SNRs in one C-LEDs layout.Besides,their received SNRs change irregularly with different layouts of C-LEDs.A C-LED beneficial to one user may cause serious interference to other terminals nonetheless.Therefore,we constructed an optimization problem that maximizing the minimum SNR of all users with the purpose that the BER quality of all users can be guaranteed.In order to find the optimal C-LEDs,a selecting scheme based on evolutionary algorithm was proposed and was validated by simulations.(3)Due to the distances of the different receiver to LEDs are different,the distribution of SNR is nonuniform.And the ISI caused by multiple LEDs exacerbates the nonuniform distribution of the SNR.In order to take full advantage of the channel state,we presented an adaptive location Multiple Pulse Position Modulation(MPPM).Every locations have different SNRs,so the receiver in different location can use different MPPM modulation orders.It can use a high order MPPM where has high SNR,however a high modulation order mode can increase ISI and reduce the SNR.With the dynamic change of the modulation order,the ISI brings a huge challenge to choose modulation order.Therefore,we raised to use the step-by-step method to find out the best modulation order.Furthermore,we also analyzed the adaptive location modulation order selection problem in the scenario that partial LEDs were used for communication.In the paper,we investigated the C-LED selection problem and the MPPM adaptive-location order selection problem in indoor VLC system from a new perspective.Maybe the result of our research can provide some theoretical basis and reference value for the wide application of VLC.