Page 150 - Proceedings of the 2017 ITU Kaleidoscope

2017 ITU Kaleidoscope Academic Conference

means of digital signal processing, every LED channel on maintains independence with each other.
the array will be recognized and analyzed individually. After
the demodulation, a frame of original digital data stream is P 1(t) I 1(t)
obtained from the image [6].
However, there exists spatial inter-symbol interference (SISI)
P i(t) Free Space Channel O\E I j(t) Imaging ĞŵŽĚƵůĂƟŽŶ Parallel to Serial Output Data
in MIMO-IS-VLC system which will degrade the system
Input Data
Modulation
Serial to parallel
performance. In ideal imaging system, the light reaching the
P n×n(t) I n×n(t)
imaging plane is the target light of imaging objects. Nev-
ertheless there will be non-target stray light in a practical
LED Array Lens Noise Image Sensors
imaging system leading to the interference between different
LED channels. Fig. 2: The block diagram of MIMO-IS-VLC system.
The stray light is defined as the irregular incident light of the
imaging object. It is the major factor contributed to the SISI. Assuming that P LED is the rated optical power of LEDs and
As is shown in Fig.1(c), the LED of on status has similar s i (t) is the input digital signal of the i channel, there is:
light intensity contour distribution which can be equivalent
to concentric circles. However, in the region that LEDs of P i (t) = P LED , s i (t) = 1 (1)
on status are relatively concentrating, the outermost of each 0, s i (t) = 0
concentric circles is going to connect together and generates
Where 1 ≤ i ≤ n×n, P i (t) is the i channel’s LED transmit-
a larger area. It reveals the fact that in MIMO-IS-VLC sys-
ting optical power.
tem, different LED channels will influence each other and
Secondly a high speed CCD or CMOS device with lens cap-
produces the SISI.
tures the picture of the LED array and transfers it to be pro-
A, B and C in Fig.1(c) is the position of LEDs of off status.
cessed. The image processing algorithm segments the picture
According to the light intensity contour, the light intensity
and every single LED channel will be analyzed individually.
value of LED on position A is higher than that of the other
Then the LED optical signal is demodulated to digital signal
two position and the light intensity value of LED on C is the
and a n × n data matrix is generated.
lower. B is on the boundary of light intensity contour among
At last, after the parallel-to-serial conversion, a frame of the
A and C. The interference light intensity from other LED
transmitted serial data is obtained.
channels makes it easier for A LED to be recognized as a on
status LED when applying a constant judgment threshold to
demodulate the signal. From point of view of optical wire- 2.2. Distribution model of the stray light
less communication system, the spread distribution of LEDs’
light intensity on the image plane gives rise to the SISI and The spatial inter-symbol interference of the MIMO-IS-VLC
degrades the system performance. The paper [7] studies the system is mainly caused by the stray light. The stray light
impacts of SISI on the visual MIMO system’s capacity from can be considered as a additively background noise compo-
the prospective of the imaging process. However, the gen- nent to the system. There are two main sources for the stray
eration mechanism of SISI is not discussed and the further light: One is the external light such as the sunlight, the in-
analysis such as the estimation of the SISI noise intensity is door illumination light and the atmospheric diffusion light.
not provided. It is considered as a constant background light and the light
intensity value can be expressed as K. The other one is the
For the purpose of modeling and analyzing the SISI in
irregular incident light of the imaging object. In the ideal
MIMO-IS-VLC system, a more specific system description
condition, the incident light of the imaging object follows a
will be given in Section 2. Then the generation mechanism of
certain angle. However, a part of imaging light reach the im-
the stray light and the mathematical description is discussed.
age sensor at an uncertain angle and lead to the disorder of
In Section 3, the mathematical expression of the signal-to-
imaging process. In particular, there are three reasons ac-
noise (SNR) and the bit error rate (BER) of MIMO-IS-VLC
count for the irregular incidence.
system with SISI is deduced and the corresponding simula-
tion result is demonstrated and analyzed. The conclusion is
• The diffraction caused by the grating edge and the grat-
obtained in Section 4.
ing slit takes place.
• The refraction takes place because the camera lens is
2. THE SYSTEM MODEL
contaminated and the surface of the optical compo-
nents such as gratings, prisms, lenses, filters and other
2.1. System Description
is not flat.
The block diagram of MIMO-IS-VLC system is shown in • The reflection takes place on the inner wall of the tube.
Fig.2. Firstly, a frame of serial input data is converted to the
parallel data and drives a n × n LED array by OOK mod- For the image sensor, the incident angle of the stray light is
ulation scheme. According to the given protocol, each dig- stochastic but its spatial distribution has characteristic. Ac-
ital signal channel controls a specific LED on the array and cording to [8], the spatial distribution of stray light can be

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