Visible Light Communications (Theory and Applications) || Lighting and Communications: Devices and Systems

摘要： An important step in the design of a visible light communications (VLC) system is to comprehend the limitations arising from the optical wireless channel. Accurate channel characterization is an important prerequisite to set the system parameters appropriately in order to establish a high-quality link since it permits better exploitation of the available energy and spectral resources in view of optimizing the system design. An accurate channel model is also necessary to precisely predict the performance of VLC systems. In this chapter, we address channel modeling for VLC systems mainly focusing on indoor systems. We introduce different sources of impairment in VLC systems arising from beam propagation or transmitter (Tx)/receiver (Rx) devices. Indeed, the latter could be attributed to the “global channel” comprising the blocks between the signal Tx and signal detection at the Rx. After this introduction, we give an overview of different propagation modes in Section 3.2. In addition, we explain methods for numerical channel simulation. Analytical channel modeling for the cases of single- and multiple-source systems is presented in Section 3.3. Then, in Section 3.4, we outline the limitations arising from the aggregate channel while focusing on the problem of intersymbol interference (ISI) and how it affects the link performance particularly in the absence of a line of sight (LOS). This could be due to multipath-induced channel time dispersion, that is, multiple reflections from people and objects within an indoor environment, the lack of sufficient bandwidth of the transmitting device, most commonly a light-emitting diode (LED), the photodetector (PD), and the cabling used for lighting installations. Limitations due to the LED nonlinear characteristics could also be considered as an impairment of the global channel, which is the subject of Section 3.5 where channel distortion modeling is investigated. Finally, channel modeling for multiple-input multiple-output (MIMO) VLC systems will be presented next in Section 3.6.