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DIRECT DETECTION IN OPTICAL
COMMUNICATION USING INTENSITY
MODULATION V.Vadivu#1, M.Sathya#2, S.Anjugam#3,R.Ramya #4,
#1, #2, #3 PG students [Communication system], Dept.of ECE,
As-salam engineering and technology, Aduthurai, Tamilnadu, India.
#4Assistant Professor [O.G], Dept.of ECE, As-salam engineering and technology, Aduthurai , Tamilnadu, India.
Abstract— Optical wireless communication (OWC) refers
to communication through an unguided medium using
modulated light. In this paper, the performance of OWC
systems employing intensity modulation and direct
detection (IM-DD), established by modulating light
intensity at the source and using an intensity detector at
the destination is proposed. The capacity of the intensity-
modulation direct-detection optical broadcast channel
(OBC) is investigated, under both average and peak
intensity constraints. An outer bound on the capacity
region is derived by adapting Bergmans’ approach to the
OBC. Inner bounds are derived by using superposition
coding with either truncated-Gaussian (TG) distributions
or discrete distributions. At high signal-to-noise ratio
(SNR), it is shown that the TG distribution is nearly
optimal.
Keywords— Intensity-modulation; optical broadcast;
capacity region; truncated Gaussian; discrete inputs.
I. INTRODUCTION
The Optical Wireless Communications is a type of
communications system that uses the atmosphere as a
communications channel. The OWC systems are attractive to
provide broadband services due to their inherent wide
bandwidth, easy deployment and no license requirement. The
idea to employ the atmosphere as transmission media arises
from the invention of the laser. However, the early
experiments on this field did not have any baggage of
technological development derived from the fiber optical
communications systems, because like this, the interest on
them decreased. At the beginning of the last century, the
OWC systems have attracted some interest due to the
advantages mentioned above. However, the interaction of the
electromagnetic waves with the atmosphere at optical
frequencies is stronger than that corresponding at microwave.
The intensity of a laser beam propagating through the
atmosphere is reduced due to phenomena such as scattering
and molecular absorption, among other. The changes in the
refractive index of the atmosphere due to optical turbulence
affect the quality of laser beam through distortion of its phase
front and random modulation of its optical power. Also the
presence of fog may completely prevent the passage of the
optical beam that leads to a no operational communications
link. The information signal analog or digital is applied to the
optical transmitter to be sent through the atmosphere using an
optical antenna. At the receiver end the optical beam is
concentrated using an optical antenna to the photo-detector
sensitive area which output is electrically processed in order
to receiver the information signal.
II. EXISTING SYSTEM
In the existing systems the direct detection is used in
optical communication. By using direct detection the signal
from input side is directly given to the receiver side with an
unwanted noise. Orthogonalizing users this way allows
serving multiple users in the OBC without interference.
Hence, the channel from the transmitter to each receiver
reduces to an IM-DD P2P, and capacity results on IM-DD
P2P channels can be applied. For a Gaussian broadcast
channel which is physically degraded by nature, superposition
coding (SC) is optimal and orthogonalizing users is not
efficient. The performance of SC in the OBC in terms of bit-
error rate and throughput
III. PROPOSED SYSTEM
The goal of this paper is to study the capacity of the
N-user OBC, which models the downlink in VLC. The main
focus is finding simple closed-form statements on the channel
capacity. This requires developing outer and inner bounds on
the capacity region of the channel. To this end, we modify
Bergmans’ outer bound to obtain outer bounds on the capacity
region of the OBC. Then, we develop inner bounds on the
capacity region based on SC, where the source sends the sum
of several symbols, each of which is desired by one user.
The main contributions of the paper can thus be
summarized as providing:
1) Outer and inner bounds on the OBC capacity region,
2) The high-SNR capacity within a small constant gap, and
3) The low-SNR capacity.
OPTICAL BROADCAST CHANNEL
Consider an N-user optical broadcast system, where
information needs to be conveyed from a light source to users.
Coherent receivers in which the received optical field is mixed
with the field generated by a local optical oscillator (laser)
through a beam combiner or coupler, and the resulting signal
is photo-detected.
Fig. 1 Optical Broadcast Channel
BLOCK DIAGRAM OF OPTICAL COMMUNICATION
Fig. 2 Optical Communication diagram
The input signal is generated as binary symbols. This
signal doesn’t have polarity at this stage. If the binary signals
are generated we need to process encoding. Encoding is a
method which converts the individual binary bit to another
format of binary data’s. In our implementation we are using
Convolutional encoding. Now we have to modulate the binary
signal using QPSK. Then we need to spit those polarized
signal into two sections. X- Real part and Y- imaginary part of
the symbol. Again we need to do the symbol mapping. IFFT
and FFT is the process of Frequency division multiplexing
and demultiplexing respectively.
DETAILED WITH OPTICAL CHANNEL
Fig. 3 Detailed with optical channel diagram
Then we have to convert those parallel signals into serial
for transmission of symbols into single channel or medium. So
the signals we are having is digital. To pass through the fiber
channel we have to change to analog. Then the signals are
transmitted to the channel as dual signal. In fiber optic
channel, the first and initial process is the IQ-modulator
(In-phase and Quadrature –phase).
PBC is the polarized beam combiner to pass into fiber.
Here we are using SSMF type of fiber. Then we have the
design of coherent receiver. Coherent receiver has number
steps to accurately recover the original signal of transmitter.
To reduce the phase offset and phase noise we use the
error compensation algorithm and MIMO equalizer is used to
get the correct path of signal. Finally the FFT is performed to
get the original subcarriers. And ZF and SD are used to
recover original binary data from the dual polarized
subcarriers.
IV.PROJECT DESIGN
SOFTWARE DESCRIPTION
MATLAB® is a high-performance language for
technical computing. It integrates computation, visualization,
and programming in an easy to use environment where
problems and solutions are expressed in familiar mathematical
notation. Typical uses include
Math and computation
Algorithm development
Modeling, simulation, and prototyping
Data analysis, exploration, and visualization
Scientific and engineering graphics
Application development, including graphical user
interface building
The name MATLAB stands for matrix laboratory.
MATLAB was originally written to provide easy access to
matrix software developed by the LINPACK and EISPACK
projects. MATLAB has evolved over a period of years with
input from many users.
MATLAB features a family of application-specific
solutions called toolboxes. MATLAB has evolved over a
period of years with input from many users. In university
environments, it is the standard instructional tool for
introductory and advanced courses in mathematics,
engineering, and science.
In industry, MATLAB is the tool of choice for high-
productivity research, development, and analysis. Very
important to most users of MATLAB, toolboxes allow you to
learn and apply specialized technology.
Toolboxes are comprehensive collections of MATLAB
functions (M-files) that extend the MATLAB environment to
solve particular classes of problems. Areas in which toolboxes
are available include
signal processing,
control systems,
neural networks,
fuzzy logic,
wavelets,
simulation, and many others
V. RESULT
Fig .4 Simulation result – 1
Fig. 5 Simulation result – 2
Fig. 6 Simulation result – 3
Fig. 7 Simulation result – 4
Fig. 8 Simulation result – 5
VI. CONCLUSION
In this paper, a signal strength using the capacity of
the IM-DD optical broadcast channel is achieved. For this
channel, we have derived capacity region outer and inner
bounds. The inner bounds are achieved using superposition
coding and either truncated-Gaussian distributions or discrete
input distributions. We have shown that a superposition of
truncated-Gaussian inputs achieves the capacity region within
a constant gap at high SNR. We have also shown that as far as
the symmetric capacity is concerned, time-sharing between
superposition coding strategies is not necessary at high SNR.
As an extension of this work, it would be interesting to study
the impact of fading on the capacity of the OBC.
ACKNOWLEDGMENT
I would like to thank our chairman Mr.M.J.A.Jamal,
Managing Director Mr.K.Karl Mark principal
Dr.M.Ravichandran, Ph.D for encouraging and providing
necessary facilities towards the growth carrying this work.
The authors knowledge with the help of Ms.R.Ramya,
M.E., As-salam college of engineering and technology,
aduthurai, Tamilnadu in assisting me towards implemented
the project work.
REFERENCES
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user IM-DD optical broadcast channel,” in IEEE Globe com
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2258, 4th quarter 2014.
3. H.Elgala, R.Mesleh, and H. Haas, “Indoor optical wireless communication: Potential and state-of-the-art,” IEEE Comm.
Magazine, vol. 49, no. 9, pp. 56–62, Sep. 2011.
4. S.M.Moser, “Capacity results of an optical intensity channel with
input dependent Gaussian noise,” IEEE Trans. on Info. Theory,
vol. 58, no. 1, pp. 207–223, Jan. 2012.
5. A.A.Farid and S.Hranilovic, “Diversity gain and outage
probability for MIMO free-space optical links with
misalignment,” IEEE Trans. On Communications, vol. 60, no. 2, pp. 479–487, Feb. 2012.