Research On The Optimization Method Of Downlink For Indoor Visible Light Communication System

The ever increasing number of mobile communication users and various mobile serves poses great challenge to the limited radio frequency(RF) spectrum.Visible light communication(VLC) use commercially available light emitting diodes(LED) to transmit signals by flickering at high speed,and can realize lighting and communication functions at the same time.Compared with traditional RF communication,VLC is characterized by numerous advantages,such as high security,low power consumption,no electromagnetic interference,and license-free band usage,etc.So VLC can used as an alternative resolution for next-generation wireless optical communications and thus researchers from all over the world have been paid extensive attention and support on VLC.But there are still many issues needed to be solved in actual application scenarios.On the one hand,downlink coverage is directly related to the quality of illuminance and communication since the light of LED is used as carrier in VLC.But inherent Lambertian emission pattern of single LED and multipath diffuse channel characteristics would cause nonuniform distribution of optical power and illuminance intensity on the receiving plane.On the other hand,there are multiple LEDs installed on the ceiling to guarantee seamless coverage,so inter-cell interference between adjacent cells can affect the performance of downlink.Therefore,we are mainly focus on the optimization method of coverage uniformity and interference mitigation for indoor VLC downlink in this thesis.In order to solve the problem of nonuniform distribution of optical power and illuminance intensity on the receiving plane.An improved genetic simulated annealing algorithm(IGSAA) is proposed in chapter three to adjust the transmitted optical power for LED arrays.In our scheme,LED arrays are equipped with power factors(PFs) and we construct chromosomes in algorithm with PFs.Fitness function is designed in the view of the differences of received power among different positions on receiving plane.Adaptive mutation probability based on the information of optimal chromosome and evolutional generation is also designed.And then an optimal set of PFs can be obtained to adjust the transmitted optical power.Numerical results show that the proposed algorithm can effectively decrease the fluctuation of received optical power and illumination intensity.So it can improve the downlink coverage performance of indoor VLC system.In order to solve the problem that inter-cell interference may deteriorate the signal-to-interference-noise ratio(SINR) of user and then degrade the system average spectral efficiency(ASE) and user data rate.Two interference graph-based dynamic frequency reuse algorithms called Dynamic Frequency Reuse(DFR) and Weighted Dynamic Frequency Reuse(W-DFR) are proposed in chapter four.The interference graph is constructed based on the coverage area of LED access point(AP) and user requirements.The interference graph makes LEDs can transmit data without interference and select the minimum sub-bands needed for frequency reuse.DFR algorithm reuses the system frequency equally across service-providing cells to mitigate spectrum interference.While W-DFR algorithm can reuse the system frequency by using the bandwidth weight(BW),which is defined based on the number of service users.Numerical results show that both of the proposed schemes can effectively improve the ASE of the system and data rate of users.Besides,W-DFR can maintain the transmission performance when users are unevenly distributed within the room.