Demo: A Context Aware Algorithm for an Adaptive Visible Light Communication System
Résumé
Recently, Visible Light Communication systems
have gained a lot of attention. In this work we propose
a context-aware and adaptive Visible Light Communication
(VLC) system, able to dynamically react to the environmental
changes in order to keep a good communication quality.
In particular, we focus on a frame synchronization technique
which is implemented by appending a preamble (repetitive
insertion of sequences), to the transmitted data. At the receiver,
a clean copy of the appended message is correlated
with the received symbol stream for frame alignment. The
size N (number of bits) of the preamble impacts on the performance
of the communication system. Indeed, a short dimension
of the preamble is to be preferred to reduce the control
overhead (i.e. it is not carrying data information) but it
could be not sufficient to perform a good carrier recovery, especially
in the case of noisy environmental conditions. Since
different external environmental conditions need different
values of preamble length, the system must be able to gather
information about its environment at any given time and
adapt its behaviors accordingly (context awareness). Based
on these premises, we propose a dynamic computation of N
as ideal size of preamble for carrier recovery by modeling
it as a multi-arm bandit problem and apply Thompson sampling
to select in a fast and efficient way the best value of N
[3]. Specifically, an agent tries to achieve as much award as
possible by playing the most rewarding arm among J arms
(J in our case represents the possible choices of the size N,
that could be potentially unlimited but not all the sizes are
meaningful, so we consider a limited sub-set). Each arm rewards
randomly upon being played according to an unknown
distribution. Our goal is the minimization of the exploration
to find the most rewarding arm. The learning approach has
been implemented to the receiver side. This choice is motivated
by the fact that in the receiving path all data needed to implement the algorithm are known. We assumed that after
the receiver computes the ideal value of N, it communicates
this value to the transmitter that will consequently adapt the
next frame. The algorithm has been implemented on a couple
of low cost VLC prototypes consisting in an Arduino board,
a driving circuit and a led array in the transmitting stage, a
photo-diode, a trans-impedance amplifier and a second Arduino
board in the receiving path. Transmitted signal is generated
through software and the received one is processed
through a proper ”Virtual Instrument”, developed using the
commercial software LabView. Experimental results have
shown the impact played by a correct choice of the parameter
N on the reduction of the recovered carrier frequency
variance and Bit Error Ratio (BER) in different environmental
conditions.
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