Article
Multi-User Precoder Designs for RGB Visible Light
Communication Systems
Roser Viñals
1,2
, Olga Muñoz
1,
* , Adrián Agustín
1
and Josep Vidal
1
1
The Department of Signal Theory and Comunications, Universitat Politècnica de Catalunya (UPC),
08034 Barcelona, Spain; roser.vinalsterres@epfl.ch (R.V.); adrian_agustin@telefonica.net (A.A.);
josep.vidal@upc.edu (J.V.)
2
Signal Processing Laboratory (LTS5), École Polytechnique Fédérale de Lausanne (EPFL),
1015 Lausanne, Switzerland
* Correspondence: olga.munoz@upc.edu
Received: 22 October 2020; Accepted: 25 November 2020; Published: 29 November 2020
Abstract:
In this paper, we design linear precoders for the downlink of a visible light communication
(VLC) system that simultaneously serves multiple users. Instead of using phosphor-coated white
light-emitting diodes (PWLEDs), we focus on Red-Green-Blue light-emitting diodes (RGB-LEDs)
that allow modulating three separate data streams on the three primary colors of the RGB-LEDs.
For this system, we design a zero-forcing (ZF) precoder that maximizes the weighted sum rate for
a multilevel pulse amplitude modulation (M-PAM). The precoding design in RGB-based systems
presents some challenges due to the system constraints, such as the limited power, the non-negative
amplitude constraints per light-emitting diode (LED), and the need to guarantee white light emission
while transmitting with RGB-LEDs. For comparison purposes, we also consider the ZF design for a
PWLED-based system and evaluate the performance of both a PWLED- and an RGB-based system.
Keywords:
visible light communications; multi-user; linear precoding; adaptive modulation; zero forcing;
weighted sum-rate maximization; Red-Green-Blue; RGB-LEDs
1. Introduction
Due to the limitation in the availability of spectrum and the millimeter wave (mmWave) signals
propagation problems, short-range Visible Light Communications (VLC) has received much attention
from the research community, experiencing significant advances in the last years. VLC technology can
operate in the non-regulated visible light spectrum of approximately 400 THz using light-emitting diodes
(LEDs) and photodiodes (PD) as transmitters and receivers, respectively, for downlink (DL) transmissions.
LEDs can switch at different light intensity levels at a high-speed rate but imperceptible to the human
eye. Because of that, off-the-shelf LEDs commonly used for illumination can also act as communication
transmitters, being this one of the most exciting features of VLC systems [1].
Figure 1 shows an illustrative scenario of application. In the figure, several passengers in a rail wagon
receive the information transmitted by a set of LEDs placed in the wagon ceiling. An indoor broadcast
system such as this requires the design of VLC techniques dealing with the multi-user (MU) interference
while meeting with the specificities of the VLC signals. Combating the DL interference has been an active
area of research of Radio Frequency (RF) systems for many years. However, applying the available RF
techniques directly to VLC systems is not possible because of the differences between RF and VLC systems
Sensors 2020, 20, 6836; doi:10.3390/s20236836 www.mdpi.com/journal/sensors
