Praneeth Varma, G V S S and Sharma, G V V
(2018)
Design and Performance of Visible Light
Communication Systems.
PhD thesis, Indian Institute of Technology hyderabad.
Abstract
Light has traditionally been used for making objects visible to the naked eye. Lately,
there has been tremendous interest in using it for free space communication. This
has simultaneously been accompanied by significant interest in light emitting diodes
(LEDs) that have been replacing conventional light sources in almost all applications
like television, traffic lights, homes and offices etc. LEDs are better than existing
incandescent lamps in terms of long life expectancy, high tolerance to humidity, low
power consumption, and minimal heat generation. Fair amount of existing literature
has focused on achieving uniform irradiance over a planar surface [1–7]. This has
been addressed as the problem of finding the optimal LED geometry at the light
source to achieve uniform irradiance. Several computationally intensive optimization
routines like evolutionary, genetic algorithms were used for power allocation for the
LED sources to realise uniform irradiance on the incident surface.
The most practical scenario would be the case when the LEDs are placed randomly
at the source with uniform illumination being achieved through power allocation,
keeping the total power constant. This is addressed as the first problem in this thesis
by considering a binomial point process (BPP) based stochastic geometry. Further, a
simple meta-heuristic power allocation scheme is proposed for uniform irradiance on
the incident surface. Power allocation is done by maximizing a metric for uniformity
of the signal to noise ratio (SNR) at the output of the photo-detector.
The performance of a stochastic visible light communication (VLC) system based
on a BPP with a heuristic power allocation scheme to provide uniform irradiance is
analyzed in terms of bit error rate (BER). By intelligently using various approximations,
an analytical expression for the BER for the BPP based VLC is obtained . This
expression is then used to numerically obtain the optimum number of LEDs required
for a stochastic VLC system. Such results for VLC are rare and usually restricted to
light sources with a fixed geometry. The asymptotic BER for a circular BPP VLC is
obtained in closed form and found to be numerically close to that for a square BPP.
The BER expression is used to estimate the cost of the VLC system in terms of the
number of LEDs required for optimum system performance.
While a BPP based stochastic model is a powerful tool for resource allocation for
VLC, it is useful to consider power allocation for a VLC based on a single realization
of a stochastic point process. Here, a Matern type II hard-core point process (HCPP)
is desirable, since it accounts for minimum separation between any two LEDs for
better coverage. This is the focus of the next problem addressed in the thesis. The
variance of the power on the receiver plane is used as an objective function. Under
some conditions, this function is shown to be convex, allowing for optimum power
allocation. Through numerical results, it is shown that this approach is superior to
existing techniques for power allocation.
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