On the Effect of Multipath Reflections in Indoor Visible Light Communication Links: Channel Characterization and BER Analysis

Abstract

Owing to its several advantages over other wireless schemes, visible light communication (VLC) shall be at the forefront of optical wireless communication technology. However, due to multipath reflections and spatial distribution of light-emitting diode (LED) transmitters, there is an inherent delay spread in the VLC channels. We perform a comprehensive quantitative study on the effect of several practical factors like LED semi-angle, wall reflectivity, number of reflections, number of LED panels, room size, and user locations on the channel delay parameters, namely RMS delay spread and coherence bandwidth of the channel. We present the detailed derivation of the multipath VLC channel model and incorporate the effect of inter-symbol interference in bit error rate (BER) performance of the multipath VLC system. We analyze the average BER of the system under different practical scenarios and determine the penalty in signal to noise ratio entailed by a change in the system parameters mentioned above. We conclude that it is sufficient to model up to three reflections in the VLC channel to emulate the effect of multipath propagation on the channel characterization and BER analysis of the system. The results and analyses presented herein provide critical insights into the effect of multipath reflections in indoor VLC links, particularly the BER performance and channel delay characteristics. We also provide some key recommendations for the design of practical VLC systems by outlining the data rates that can be served under different system configurations. INDEX TERMS multipath channel, delay spread, coherence bandwidth, inter-symbol interference, bit error rate, visible light communication. I. INTRODUCTION Visible light communication (VLC) is an upcoming optical wireless communication technology that integrates communication and illumination by utilizing the illumination infrastructure of white light-emitting diodes (LEDs) for data communication [1-4]. Due to rapid advancements in the field of solid-state lighting devices [5] and a simultaneous boom in Internet traffic over the last few decades, the research on VLC has garnered much interest in the area of wireless communication systems. VLC opens up the untapped visible range (~ 400-700 THz) of the electromagnetic spectrum that is unlicensed and free. Thus, VLC promises to support very high data rates and enables inexpensive indoor wireless communication. Besides this, VLC also benefits from advantages like radiation safety and less electromagnetic interference as compared to other wireless communication technologies. Moreover, since visible light signals cannot penetrate through walls, so the transmission of user data over such signals is confined to a room, which makes it difficult for eavesdroppers to intercept data [3]. However, the non-penetrability of visible light signals means that they suffer reflections at the walls. These reflections create multiple paths from the LED source to the receiver, giving rise to the problem of delay spread because the light signals...

Dimming-Based Modulation Schemes for Visible Light Communication: Spectral Analysis and ISI Mitigation

“…11) due to reflections occurring at the walls of the indoor environment. The reflections are mostly diffused in nature, and hence, can be modeled using the Lambertian radiation model [55]. The multipath propagation occurs via two types of paths.…”

“…Due to the multipath propagation of visible light signals, the received optical signal in the indoor VLC link experiences time dispersion leading to ISI. The multipath channel model for indoor VLC systems is explained in great detail in [55], where the channel impulse response between the i th source S i and the receiver R is derived to be,…”

“…where K is the reflection order defined as the total number of reflections modeled in the system, N P is the total number of LED panels, and h (0) i (t; S i , R) and h (k) i (t; S i , R) are the impulse responses [55] of the LoS path and NLoS paths, respectively, as shown in ( 33) and ( 34) at the bottom of the page. The various physical parameters are depicted in Fig.…”

“…The user with the photodiode receiver (R) is located near the corner of the room, i.e., at (2, 2, −1.5). The user is assumed to be fixed and its location is chosen in order to study the worst-case BER performance [55]. The relative positions of the LED sources and the user is depicted in Fig.…”

“…Visible light signals emanating from the LED sources undergo reflections (quantified by the reflection order, K) at the walls which are divided into reflective elements with the same reflectivity (ρ). It has been shown in [55], [56] that the multipath nature of the VLC channel can be adequately modeled with K = 3 and ρ = 0.8. After traversing the multipath VLC channel, the signals reach the photodiode receiver where noise is added to the received signal.…”