Optimized Resource Allocation in Multi-user WDM VLC Systems

Abstract

In this paper, we address the optimization of wavelength resource allocation in multiuser WDM Visible Light Communication (VLC) systems. A Mixed Integer Linear Programming (MILP) model that maximizes the sum of Signal-to-Interference-plus-Noise-Ratio (SINR) for all users is utilized. The results show that optimizing the wavelength allocation in multiuser WDM VLC systems can reduce the impact of the interference and improve the system throughput in terms of the sum of data rates for up to 7 users.

Full text

Optimized Resource Allocation in Multi-user WDM VLC Systems
Sarah O. M. Saeed1, Sanaa Hamid Mohamed1, Osama Zwaid Alsulami1, Mohammed T. Alresheedi2, and
Jaafar M. H. Elmirghani1
1School of Electronic and Electrical Engineering, University of Leeds, LS2 9JT, U.K.
2Department of Electrical Engineering, King Saud University, Riyadh, Kingdom of Saudi Arabia
elsoms@leeds.ac.uk, elshm@leeds.ac.uk, ml15ozma@leeds.ac.uk, malresheedi@ksu.edu.sa,
j.m.h.elmirghani@leeds.ac.uk
ABSTRACT
In this paper, we address the optimization of wavelength resource allocation in multi-user WDM Visible Light
Communication (VLC) systems. A Mixed Integer Linear Programming (MILP) model that maximizes the sum of
Signal-to-Interference-plus-Noise-Ratio (SINR) for all users is utilized. The results show that optimizing the
wavelength allocation in multi-user WDM VLC systems can reduce the impact of the interference and improve
the system throughput in terms of the sum of data rates for up to 7 users.
Keywords: Visible Light Communication (VLC), resource allocation, Mixed Integer Linear Programming
(MILP), Signal-to-Interference-plus-Noise-Ratio (SINR), On-Off Keying (OOK), Wavelength Division
Multiplexing (WDM), Poisson Point Process (PPP).
1. INTRODUCTION
The demands for high data rates are expected to dramatically increase as Internet-connected devices are
expected to be more than half of the global population by 2022 generating an increased global IP traffic by three
folds [1]. Hence, enhanced and future-proof technologies are needed to continue fulfilling these future demands.
The radio spectrum has already become congested and to cope with the increasing bandwidth requirements, it
needs to be supplemented by frequencies that can support high data rates such as infrared, visible, and ultraviolet
spectrum technologies which are used in optical wireless communication (OWC) systems. While OWC has
attracted the attention of researchers for more than three decades [2], Visible Light Communication (VLC) has
gained more interest as the communication can be a by-product of existing illumination infrastructure. The use of
Solid-State Lighting (SSL) for indoor illumination has increased over the past decade and is expected to provide
75% of the aggregate global lighting by 2030 [3]. SSL devices are characterised by longer lifespan, and energy
efficiency in addition to their ability to switch between different light intensity levels at a very fast rate [4]. The
last property is a unique one compared to conventional lighting technologies, which can be exploited in
communication to encode data.
While VLC systems have the potential to provide high data rates, indoor VLC channels suffer from multipath
propagation which causes Inter-Symbol Interference (ISI) and Co-Channel Interference (CCI) in systems with
multiple users. Both, ISI and CCI result in decreasing the data rate of the VLC system. Changing the number of
transmitters and receivers, in addition to controlling their directionality creates several favourable configurations
for VLC systems that can minimize the dispersion resulting from multipath propagation and increase the SNR at
the receiver [5]–[9].
Many studies aimed at increasing the achievable data rate and previous work showed that multi-gigabit per
seconds could be obtained [10]–[16] . In [10]-[12] multi-beam power adaptation with imaging receivers was used,
while in [13], the beam angle and beam power were adapted with reception using an Angle Diversity Receiver
(ADR). Delay adaptation with imaging diversity receiver was used in [14], and also in [15], [16] with the addition
of relay nodes. Holograms to steer the whole beam power to the user’s location are used in [17]–[20]. The
adaptation techniques used in [10]–[16] enabled the achievement of up to 10 Gbps data rate at an increase in the
computational cost. To avoid this cost, a Fast Computer-Generated Hologram (FCGH) algorithm was used in [17]
and [21] and reported in [17] to achieve up to 25 Gb/s. Line strip multi-beam spot diffusing transmitters were
studied in [22], [23] as a means to achieve high data rates in the absence of line of sight components. The problem
of glare due to VLC uplink was tackled in [24] introducing an infrared uplink design in VLC.
The previous studies outlined above considered a single-user scenarios. Optimizing the allocation of a
transmitter to a user in a single user scenario was done in [25] using Sub-Carrier Multiplexing (SCM) while
considering the mobility of the single user. The use of SCM in [25] was extended to a multi-user scenario in [26]
using multi-branch transmitters and also in [27] with Wavelength Division Multiplexing (WDM). In multi-user
scenarios, the interference between users leads to degradation in the signal quality, hence, multiple access
optimization techniques that allow efficient use of different resources such as wavelength, space, time, and power,
have recently received increased attention [28].

In this paper, we optimize the allocation of wavelengths and multiple access points to users in WDM VLC
systems by using a Mixed Integer Linear Programming (MILP) model that maximizes the sum of Signal to
Interference-plus-Noise-Ratio (SINR) for all users. The rest of this paper is organized as follows: The system
model and the parameters used are introduced in Section 2. The results are provided in Section 3 while the
conclusions and future work are provided in Section 4.
2. SYSTEM MODEL
In this work, we consider an unfurnished room with no doors or windows and a (length × width × height)
dimensions of (8 m × 4 m × 3 m) as shown in Figure 1. Eight lighting units (also access points) are used to provide
adequate illumination in the room according to the EU and ISO standards [14], [15]. To achieve high modulation
bandwidth, Laser Diodes (LDs) are used and the white illumination is obtained using four-colours: Red, Yellow,
Green, and Blue (RYGB) LDs as in [14], [15], [17], [18], [29]. Lambertian radiation with 70° half power semi-
angle is assumed to ensure the room is well lit. Users are randomly distributed over the communication floor which
is a plane 1 m above the ground (i.e. the height of a typical office desk). We considered a wide Field Of View
(wFOV) receiver that uses On-Off Keying (OOK) modulation; and for simplicity, we only considered the Line-
Of-Sight (LOS) component. We assumed receivers with equal wavelength responsivities and with tuneable filters.
Therefore, when a user is assigned to a wavelength, all other wavelengths can be ignored. The key parameters
considered are summarized in Table 1.
Figure 1. Room configuration with 8 access points and 10 users distributed over the communication floor with
random allocation of wavelengths. A solid line indicates that an allocated wavelength is used for communication.
A dot-dashed line indicates that a wavelength is used for illumination only. A dotted line indicates interference
between communication channels which is illustrated for the blue wavelength only.
For such a system, the SINR for the ith user can be given by [21]:
 






  (1)
where  is the responsivity of the photodetector in (A/W),  is the transmitted optical power,  is the optical
channel gain for the ith user, between the user and the serving access point,  is the interference
received by the ith user from other access points with the same wavelength, and  is the receiver’s total noise
standard deviation given by:



















(2)
where  represent the background light shot noise, shot noise associated with the signal, and the
preamplifier noise, respectively calculated as in [30]. As stated in the previous section, a receiver can tune to a
specific wavelength, hence, the background light shot noise is considered only for that wavelength. We considered
the PIN-FET receiver proposed by Kimber et. el. [31] with noise spectral density of 10pA/√Hz and a bandwidth
of 7 GHz.

Table 1. Key parameters for the system model.
Parameter
Value
Room dimensions: width×length×height
4m×8m×3m
Communication floor height
1m
Number RYGB LDs lighting units
8
Location of lighting units
(1 m, 1 m, 3 m), (1 m, 3 m, 3 m), (1 m, 5 m, 3 m), (1 m, 7 m, 3 m),
(3 m, 1 m, 3 m), (3 m, 3 m, 3 m), (3 m, 5 m, 3 m), (3 m, 7 m, 3 m)
Transmitted powers for RYGB colour
800, 500, 300, 300 mW
Half-power semi-angle
70°
Photodetector area
1 cm2
Photodetector Responsivity
0.4 A/W
Receiver noise current density
10 pA/√Hz
Receiver electrical bandwidth
7 GHz
We utilized a MILP model to optimize the allocation of the wavelengths (R, Y, G, and B) and access points
(1 to 8) to users randomly distributed over the communication floor in a Poison Point Process (PPP). The model
maximizes the total SINR by maximizing the sum of the received optical signal power for all users and minimizing
the sum of the receiver’s noise for all users and the sum of the interference for all users. The model ensures that
all users are served and that each user is allocated to a unique combination of wavelength and access point. It is
assumed that the allocation is done by a centralized controller (see Figure 1) that has prior knowledge of the users’
locations and their received power from all access points.
3. RESULTS AND DISCUSSION
(a) (b)
Figure 2. (a)Throughput (sum of rates) in Gb/s vs number of users. (b) Average SINR per user in dB vs number
of users.
Figure 2 shows the results for the overall system throughput and average SINR per user. Each point in the
curves is obtained by averaging the results for five random locations of users over the communication floor
generated using PPP. Figure. 2 (a) shows the sum of the achievable data rates at a BER of 10-9, where users that
could not achieve this BER are not considered. It was noticed that for up to two users, each user can achieve 10
Gb/s. As the number of users increases beyond two users, the throughput increases at a lower rate as a result of the
increase in the interference. The maximum throughput is achieved when the number of users in the room is 7 for
the set of parameters considered, and depending on the random locations of users, mostly, three of the wavelengths
are used twice and one wavelength is used once (i.e., this wavelength has zero interference). As the number of
users increases and the distance between the users decreases, the impact of the interference becomes more
significant resulting in decrease in the achieved system throughput, where the throughput decreases at a faster rate.
This can be attributed to the fact that serving all users results in increase in the use of the same wavelengths. Figure
2 (b) shows the decrease in the average SINR per user as the number of users increases, which is also attributed to
the increase of the interference.
4. CONCLUSIONS AND FUTURE WORK
In this paper, we optimized the wavelength allocation in a multi-user WDM VLC system with wFOV receivers
using a MILP model that maximizes the sum of SINR for all users. The results show an overall increase in the
system throughput for up to seven users. However, increasing the number of users beyond that results in a reduction

in the achievable throughput. Future work includes the consideration of the first and the second order reflections
of the transmitted optical power. This will account for multipath dispersion and will also account for situations
where no LOS power component is available. Furthermore, to reduce the impact of the interference in WDM VLC
systems, cell size optimization, in addition to Time Division Multiplexing (TDM) and different transmitter and
receiver architectures can be considered.
ACKNOWLEDGEMENTS
The authors would like to acknowledge funding from the Engineering and Physical Sciences Research Council
(EPSRC) for the TOWS project (EP/S016570/1). The first author would like to acknowledge her PhD funding
from the School of Electronic and Electrical Engineering, University of Leeds, UK and from the Ministry of Higher
Education and Scientific Research, Sudan. All data are provided in full in the results section of this paper.
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