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...

Visible light communication (VLC) is being envisioned as an enabling technology to provide the much-needed spectral relief for the ever-increasing demand for Internet connectivity and data consumption. Since VLC uses illumination sources for lighting as well as communication, it is required to provide dimming control for proper lighting and enhanced error performance for reliable data communication. In this paper, we address both these issues holistically. We formulate and study the power spectral densities of dimming-based modulation schemes, namely variable on-off keying (VOOK) and variable pulse position modulation (VPPM), and hence, derive their bandwidth requirements and spectral efficiencies. Moreover, the capacity of VLC systems is severely limited by the inter-symbol interference (ISI) occurring as a result of the multipath propagation of light signals in VLC. We propose to ameliorate the error performance of VLC systems by using channel equalization for ISI mitigation, thereby enhancing the system capacity. We develop the analytical model of a dimmable VLC system employing channel equalization and use this model to study the effect of dimming and data rate on the error performance of VOOK and VPPM schemes. We present simulation and analytical results to show that the performance of dimming-based modulation schemes is significantly improved using channel equalization.INDEX TERMS Dimming, ISI mitigation, channel equalization, visible light communication.

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