Visible light communications: analysis of cooperative communication and amplification

Abstract

Wireless radio frequency (RF) communication has seen an enormous growth in the last decade, with users enjoying its high data rates and convenience. However, as the demand of mobile users and devices employed in the Internet of Things grows, the use of the radio frequency spectrum approaches saturation. In search for new solutions, visible light communication (VLC) emerges as a potential technology that exploit the visible spectrum to transmit information. However, despite its advantages, VLC still presents limitations that must be addressed before it is deployed in the real world. In this sense, this work contributes with the development of VLC systems by means of a versatile simulation model that enables the calculation of performance parameters, such as received power, signal-to-noise ratio (SNR), bit error rate (BER) and error vector magnitude (EVM) in customizable environments. The model is then employed to evaluate relay-assisted cooperative VLC links, in which, besides the source and the destination, a relay node is added to the path to support communication. This is accomplished by using the amplify-and-forward technique at the relay node, where the source signal is amplified and retransmitted to the destination, at which point both source and relay signals are combined and processed. Simulation results show that cooperation improves the link performance and even allow communication in cases where it wouldn’t originally be feasible. Besides simulations, the cooperative VLC links were tested experimentally. Using different path configurations it is demonstrated that SNR improves by an amount varying from 4 to 8 dB and BER results improves by up to one order of magnitude. As amplification is an important aspect of the employed amplify-and-forward technique, a simulation of optical and electrical amplification in VLC links is also accomplished. A comparison of both schemes is presented with results showing that the performance of an optical amplifier of 3.7 dB is comparable to a gain of 50 dB solely in the electrical domain. Finally, a specific application of VLC in the transmission of standard downlink mobile signals for LTE (4G) and 5G is evaluated. Experimental results show EVM values of 7 and 12 dB below the EVM limit for 4G and 5G, respectively, at 3 m distance.