This document discusses coherent optical orthogonal frequency division multiplexing (CO-OFDM) for visible light communications. It begins with an introduction describing increasing bandwidth demand and the benefits of CO-OFDM. It then explains the basic principles of coherent optical transmission and CO-OFDM. The document proceeds to describe simulations conducted using OptiSystem software to model a 4-QAM CO-OFDM system over varying transmission distances. The results show degradation of the signal constellation and optical signal-to-noise ratio as distance increases due to attenuation and chromatic dispersion. Finally, the document concludes with a review of the CO-OFDM fundamentals and simulation results.

1. MTE Project Review
Optical OFDM for Visible Light
Communications
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2. Introduction
Increased demand for high data rates has resulted from a growth in
internet traffic, which includes data, phone, and video services, owing
to the boom in online videos. According to the Cisco
Networking Index, internet traffic will almost double by
Furthermore, there are several online applications
Visual
2023.
under
development, as well as high demand for remote learning. All of this
will result in a future rise in bandwidth.
Coherent optical OFDM is the next generation of optical
communications technology, combining the benefits of both
coherent and OFDM systems. It has a wide bandwidth and a good
spectral efficiency. Many optical fiber difficulties, such as chromatic
dispersion (CD) and polarisation mode dispersion (PMD), may be
overcome by the CO-OFDM system.

3. At its most basic, coherent optical transmission is a technique that
uses modulation of the amplitude and phase of the light, as well as
transmission across two polarizations, to enable the transport of
considerably more information through a fiber optic cable.
Generally, both the amplitude and phase of the incoming signal are
used to convey information. Estimate the time-domain expression
for the signal as a single cosine term, and illustrate both the signal
and the two references in thephasor domain.
What is Coherent Communication?

5. Principle of Coherent OpticalOFDM
OFDM has been widely researched for applications in the optical realm due to its
remarkable resistance to channel dispersion. The linearity of modulation, transmission
and demodulation is the most important assumption for OFDM. As a result, the primary
purpose of OFDM implementation is a linear transformation.
1.The RFOFDM transmitter
2.The RF-to-optical (RTO) up-converter
3.The optical channel
4.The optical-to-RF (OTR) down-converter
5.The RFOFDM receiver
– are the five functional blocks of a general optical OFDM system.

6. The CO-OFDM system is similar to the DD-
OFDM system except for the real/imaginary
(I/Q) modulator and local oscillator. The optical
local oscillator is used in optical coherent
systems to generate specific wavelengths.
According to the frequency of the local
oscillator, the optical coherent detection can
be classified into two categories, heterodyne
detection and homodyne detection.
Coherent Optical OFDM(CO-OFDM)

7. The most important merit in optical communications is spectral
efficiency. Optical networks use intensity modulation and direct
detection for transmission and binary modulation in order to
reduce the complexity of the transceiver.
By comparing the modulation techniques with coherent detection
techniques the result is easy to reach of several bits/s/Hz. One of
these advanced modulation techniques is the orthogonal frequency
division multiplexed (OFDM).
Coherent optical OFDM (CO-OFDM) is the next-generation
technology for optical communications as it integrates the
advantages of both coherent and OFDMsystems.
Enhancing the SpectralEfficiency

8. Coherent Optical OFDMLayout
RF OFDM Transmitter RF to Optical Upconverter (RTO)

9. With 4-QAM, a CO-OFDM system for long-haul
transmission is built and researched. OptiSystem
simulation software is used to completely develop
and model the CO-OFDM system.
OptiSystem is a comprehensive software design
suite that allows users to plan, test, and simulate
optical links in the transmission layer of modern
optical networks. It can also be used by
telecommunications companies all over the world
for planning and implementing a full optical network,
which is a low-cost and time-saving approach, and
researchers can use it to work more efficiently.
Simulations

10. On the transmitter side, Figure 3 shows a clear
constellation diagram for the 4-QAM modulator.
Because digital telecommunications data is often
binary, the number of points in the grid in QAM is
usually a power of. The value of the OSNR is (57.064
dB). The constellation diagram depicts the
modulated signal as a two-dimensional scatter
pattern, which aids in the analysis of the distortion
and interference that will occur during transmission.
Results

11. The RF spectrum for the I/Q component of the system at
the CO-OFDM transmitter is shown in the figure above.
The RFpower is estimated to be about (-7 dBm).
The optical signal spectrum after modulating the
electrical signal with the optical carrier using two MZMs
is shown in Figure 5.

12. The constellation diagram of the system
after 100 km SMF with a 25 dB EDFA
amplifier at the receiver side is shown in the
above figure. Because of the attenuation,
chromatic dispersion, and noise, the signal
seems to be indistinct & the OSNR has
decreased to 24.4 dB.
The blue dots show the thermal and shot
noise, the shot noise from the laser source, &
the thermal noise from the photodetectors
and the fiber dispersion.

13. The constellation diagram of the CO-
OFDM system after 200 km SMF at the
receiver side is shown in the above figure.
When compared to the 100 Km SMF
findings in Figure 6, it too displays some
signal distortion. The OSNR is reduced,
and the distortion is enhanced (23.7dB).

14. The figure shows the constellation diagram after 300
Km, it seems that the system will not easily detect
the signal, and the OSNR degraded to (23.4 dB).
The constellation diagram after 400 kilometers is
shown in the above figure. The signal is warped and
completely mangled. Chromatic Dispersion causes
the signal to expand over long distances and
attenuation to rise, as previously stated.

15. The figure on the left depicts the relationship between
OSNR values and distance values; it can be observed
that as the distance increases, the OSNR value at the
receiver degrades.
The system cannot detect the signal at vast distances,
according to all prior constellation diagrams. We can
overcome this problem by increasing the power of the
EDFA amplifier within certain restrictions because the
EDFA amplifier operates best when the signal has little
power loss. However, as the transmission length rises,
the OSNR lowers and the signal becomes weaker; even
increasing the EDFApower will not enhance the signal.

16. In this project, we have first reviewed the
theoretical fundamentals for CO-OFDM. We
then presented various simulations results
performed using Optisystem software.
Conclusion

17. References
A. Jovicic, J. Li, and T. Richardson, “Visible light communication:
opportunities, challenges and the path to market,”, Dec. 2013.
P
.Pathak, X.Feng, P
.Hu, and P
.Mohapatra, “Visible light communication,
networking, and sensing: a survey, potential and challenges,”, Sep. 2015.
G. Zhang, M. De. Leenheer, A. Morea, and B. Mukherjee, “A survey on
OFDM-based elastic core optical networking,”, Feb.2013.
D. Tsonev et al, “A 3- Gb/s single- LED OFDM- based wireless VLC link
using a gallium nitride LED,”,Apr. 2014.
J. Kahn and J. Barry, “Wireless infrared communications,”, Feb. 1997.
J. Carruthers and J. Kahn, “Multiple-subcarrier modulation for nondirect-
ed wireless infrared communication,”, Apr. 1996.

18. Thank you!
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