Abstract
Visible light communications (VLC) have received significant attention as a way of moving part of the saturated indoor wireless traffic to the wide and unregulated visible optical spectrum. Nowadays, VLC are considered as a suitable technology, for several applications such as high-rate data transmission, supporting internet of things communications or positioning. The signal processing originally derived from radio-frequency (RF) systems such as cooperative or precoding schemes can be applied to VLC. However, its implementation is not straightforward. Furthermore, unlike RF transmission, VLC present a predominant line-of-sight link, although a weak non-LoS component may appear due to the reflection of the light on walls, floor, ceiling and nearby objects. Blocking effects may compromise the performance of the aforementioned transmission schemes. There exist several surveys in the literature focused on VLC and its applications, but the management of the shadowing and interference in VLC requires a comprehensive study. To fill this gap, this work introduces the implementation of cooperative and precoding schemes to VLC, while remarking their benefits and drawbacks for overcoming the shadowing effects. After that, the combination of both cooperative and precoding schemes is analyzed as a way of providing resilient VLC networks. Finally, we propose several open issues that the cooperative and precoding schemes must face in order to provide satisfactory VLC performance in indoor scenarios.
Highlights
Visible light communications (VLC) are envisaged as a mature and promising technology for future wireless communications [1,2,3]
This survey focuses on indoor scenarios because; (i) LoS-link blockage is more probable in indoors than outdoors; (ii) more complex network topologies are used indoors; (iii) outdoor scenarios present a reduced and spaced number of access point (AP) with a higher transmit power, which significantly reduces the cooperation possibilities
As VLC-enabled devices may not have the chance to feed their channel condition back to the optical APs frequently, when the number of devices to be served per small cell is too large, it is desirable that the resource allocation scheme of the multi-cell VLC systems corresponds to a pre-defined strategy that provides as good coverage as possible
Summary
Visible light communications (VLC) are envisaged as a mature and promising technology for future wireless communications [1,2,3]. Using wider beams, which recalls that it is required for providing a uniform illumination (see Figure 2) and leads to a poor spectral efficiency as it can be seen, because most of the locations of the scenario are subject to overlapping from several optical transmitters, i.e., to inter-cell interference At this point, the two main requirements for VLC are presented: providing satisfactory illumination and data transmission simultaneously, which involves a trade-off between overlapping and interference. Our goal is to present a clear and comprehensive picture of the cooperative and precoding schemes for VLC and how they are useful for overcoming the harmful blocking and shadowing effects They invoke techniques that exploit multiple rays, arriving at the destination, either by reflection [45] or by using different transmission points [31], the cooperation among APs [46,47], and signal processing techniques associated to reduce or manage interference [48]
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