ArticlePDF Available

Free Space Optics Communication System Design Using Iterative Optimization

Authors:

Abstract and Figures

Free Space Optics (FSO) communication provides attractive bandwidth enhancement with unlicensed bands worldwide spectrum. However, the link capacity and availability are the major concern in the different atmospheric conditions. The reliability of the link is highly dependent on weather conditions that attenuate the signal strength. Hence, this study focuses to mitigate the weather and geographic effects using iterative optimization on FSO communication. The optimization maximizes the visibility distance while guaranteeing the reliability by minimizing the Bit Error Rate (BER). The wireless optical communication system is designed for the data rate of 10 Gbps. The performance of the proposed wireless optical communication is compared against the literature in terms of visibility distance, quality factor, BER, and Eye diagram at different atmospheric conditions. The simulation results have shown that the proposed work has achieved better performance.
This content is subject to copyright. Terms and conditions apply.
Gebrehiwet Gebrekrstos Lema*
Free space optics communication system design
using iterative optimization
https://doi.org/10.1515/joc-2020-0007
Received January 8, 2020; accepted June 9, 2020
Abstract: Free Space Optics (FSO) communication pro-
vides attractive bandwidth enhancement with unlicensed
bands worldwide spectrum. However, the link capacity
and availability are the major concern in the different at-
mospheric conditions. The reliability of the link is highly
dependent on weather conditions that attenuate the signal
strength. Hence, this study focuses to mitigate the weather
and geographic effects using iterative optimization on FSO
communication. The optimization maximizes the visibility
distance while guaranteeing the reliability by minimizing
the Bit Error Rate (BER). The wireless optical communica-
tion system is designed for the data rate of 10 Gbps. The
performance of the proposed wireless optical communi-
cation is compared against the literature in terms of visi-
bility distance, quality factor, BER, and Eye diagram at
different atmospheric conditions. The simulation results
have shown that the proposed work has achieved better
performance.
Keywords: BER; FSO; optical link design; quality factor.
1 Introduction
The wireless communication has shown pragmatic
development. This increased customer attraction has led
to significant demand for high Quality of Service (QoS).
Though optical communication has been providing
tremendous data rates in a glass guided communication
link, the benefits of Free Space Optics (FSO) was mot
exploited even though it has significant data rate, secu-
rity, and reliability benefits over the ordinary RF wireless
communications. The optical communication is not
accessibleinremoteareasbecauseofboththedeploy-
ment difficulty and the cost-ineffective. Recently, FSO
communication [13] has shown an attractive alternative
solution that replaces the radio and microwave commu-
nication with Gigabits data rate. FSO provides local area
network unlicensed spectrum, simple deployment, free
electromagnetic signal interference, and extremely high
data rate [4]. However, these signicant ranges of FSO
benets are challenged by its high susceptibility to
attenuation because of the weather and turbulence con-
ditions [5]. The light beam loss happens because of the
absorption due to molecular diffusion and scattering
caused by fog, rain, snow, and haze [6]. The atmospheric
turbulence happens because of the scattering, absorp-
tion, and dispersion due to fog,haze,mist,snow,and
rain.
The need for high-speed Internet is significantly
growing with the fast expansion of smartphones.
The customers can use it on different online app-
lications, audio/video streaming, videoconferencing,
online messaging, and web browsing [7, 8]. To estimate
the average usage by these applications, the average
number of passengers on one train ranges from 500 to
1300 [9] which requires several Gbps data rates. For
example, a resolution of 1280 ×720 pixels YouTube video
user requires 2500 Kbps data rates [10]. The collective 500
users video demand requires a 1.25 Gbps data rate.
However, imagine how this data rate demand is not easy
to achieve using the usual RF communications because of
the Doppler effect due to movement, frequent handovers,
and operational frequencies and bandwidths [11]. In
general, the FSO connections are becoming a fascinating
alternative for copper, RF, and ber optic communication
techniques, in terms of speed, costing, distance, and
mobility.
Recently, 2018 [12], An adaptive beam has proposed
that adapts its divergence angle according to the receiver
aperture diameter and its communication distance to
improve the received power. However, neither the Bit
Error Rate (BER) reduction nor the visibility distance
enhancement was signicant enough. For different at-
mospheric turbulences, the digital modulations
including amplitude shift keying and pulse position
modulation techniques [13] are evaluated. However, the
data rate was limited to 2.5 Gbps and the visibility dis-
tance was limited. Besides, there was no adaptive
*Corresponding author: Gebrehiwet Gebrekrstos Lema, School of
Electrical and Computer Engineering, Mekelle University, Mekelle,
Ethiopia, E-mail: g.jcool.com@gmail.com. https://orcid.org/0000-
0001-5703-1391
J. Opt. Commun. 2023; 44(s1): s1205s1216
Open Access. © 2020 Gebrehiwet Gebrekrstos Lema, published by De Gruyter. This work is licensed under the Creative Commons Attribution
4.0 International License.
concept that enhances the visibility distance according to
the atmospheric conditions.
The overall wireless optical communication has
tremendous benefits over the RF communication as it has a
higher operating frequency and hence better data rate [14],
however, the atmospheric condition prone problems are
challenging. In 2017 [15], attractive data rate (10 Gbps) has
been achieved at different optical bands, however, the
visibility distance was limited to only 500 m.
[16] evaluates three optical transmission windows
performance on bad weather conditions, however, it
doesnt propose any atmospheric turbulence mitigating
technique. In 2018 [17], again other types of digital modu-
lations (namely amplitude shift keying and phase shift
keying) were compared and the latter has shown better
performance. However, it has limited increment and it is
insufcient for the breaking multimedia requirement of the
generation and beyond.
Quite recently 2019 [18], a transmit power adapting
transmitter and receiver design was used to combat the
atmospheric problems, how8ever, the design applies
expensive parameters to overcome the channel impair-
ments. Increasing the transmit power contradicts the en-
ergy efciency of the current and future cellular
communications. The former research, [19], has also shown
the signicance and challenges of the transmit power for
FSO. More particularly, the aviation regulatory authorities
(the United States federal aviation administration) regulate
the use of outdoor lasers in aircraft ight paths and prohibit
visible lasers or high-powered non-visible lasers from be-
ing aimed at aircraft pilots, [20]. Hence, increasing the
transmit power has a number of drawbacks. The packet
size optimization was also proposed to enhance the data
rate of the FSO, [21]. Though optimizing the packet size can
guarantee the communication reliability, the overall rate
cannot signicantly increase because the amount of
payload sent when the Signal to Noise Ratio (SNR) is low is
lower than the ordinary packet size. On the other hand,
introducing a technique that enhances the SNR provides a
better data rate than decreasing the packet size when there
is more signal deterioration.
To mitigate the atmospheric turbulences, a closed-
form of mathematical expression is derived, [22], however,
it cannot be applied to multiple objectives, for example, it
doesnt discuss how to increase the range of communica-
tion. A self-healing Bessel beams accompanied by adaptive
compensation techniques have proposed [23] which can
reduce the inter-channel crosstalk and BER. It is good that
the phase distortions caused by atmospheric turbulence
are solved by integrating an adaptive compensation unit,
however, as the optical coupler and polarizer can introduce
additional processing complexity it cannot be suitable for
low latency optical communications.
On the other hand, the performance of the FSO dis-
tance is evaluated for which the transmitted signal can be
received without any error, [24]. Though the research has
shown the understanding of the distance limit of the
transmitted signal that can be received without (almost)
error, the techniques to increase the distance without
signicantly affecting the error rate were not discussed.
Though fiber optic has tremendous advantages, for
several years it was limited to the backhaul networks. The
physical connection between the access network and the
end-user was not benefited from the reliable and high
bandwidth optical fiber nature. Finally, the FSO systems
provide an innovative solution to this problem, however,
the atmospheric challenges have continued as the main
challenges to widely deploy the FSO, [25]. Hence, this
research focuses on mitigating atmospheric problems.
Recently, OFDM-based radio over free-space optics
have proposed and they have shown attractive link-
distance enhancement on different atmospheric condi-
tions, [26, 27]. However, both multiplexing to enhance the
data rate and using low order modulation to enhance the
reliability have the drawbacks of higher processing time
and lower data rate, respectively, for future communica-
tions, 5G. Besides, the OFDM-based communication has
peak-to-average power uctuation problem, [28]. The
network management technique was also proposed to
enhance the communication performance, [29], however,
the proposed BER and data rate are not achievable without
the introduction of the FSO. The BER performance of
wavelength selection has studied, [30], however, miti-
gating the atmospheric turbulences using the wavelength
selection introduces operating spectrum inexibility. To
combat the FSO atmospheric turbulences, the robust
modulation (BPSK) and spatial diversity techniques are
used [31]. However, the robust modulation limits the
throughput of the transmission and the spatial diversity
introduces receiver computational complexity.
The coded-orthogonal frequency division multiple
access (OFDM) has used to address the FSO atmospheric
turbulences, [32], and of course, the coded and low-level
modulation has shown better BER performance, however,
this method of BER performance enhancement is well
known. More specically, increasing the reliability of the
communication (BER reduction) at the expense of
throughput reduction (because of the coding and
decreasing the modulation order) is trivial engineering
solution.
Hence, motivated by the attractive FSO characteristics
an adaptive communication system is designed using an
s1206 G.G. Lema: FSO communication system design
iterative optimization. The proposed technique adapts the
atmospheric conditions including haze, fog, snow, mist,
and rain conditions. More specifically, when the visibility
of the transmission distance changes due to the atmo-
spheric conditions, the amplifier works in a manner that
covers the distance. On the other hand, the reliability of
communication is guaranteed by the proposed iterative
optimization technique. Quality of Service (QoS) is guar-
anteed by specifying the BER to a minimum possible value.
The BER constancy is kept by the iterative optimization that
maximizes the possible transmission distance without
increasing the transmit power while still guaranteeing the
QoS by minimizing the BER to an acceptable level.
2 System model
The FSO has attractive applications including better data
rates, better security, cheap network installation, and
license-free spectrum. Besides, it has better immunity from
the electromagnetic interference because it cannot be
detected using the RF meter, it is neither visible nor health
hazardous, it can easily achieve very low BER, unlike RF
antennas it doesnt have side lobes and the deployment is
both cheap and quick. In contrary to their attractive ben-
efits and applications, the FSO is often prone to atmo-
spheric absorption, beam dispersion, rain, fog, snow, and
shadows, [1520].
Generally, the wireless optical link contains the
transmitter, the atmospheric channel, and the receiver,
Figure 1. The transmitter part transmits the signal in the
wireless media by converting the electrical signal into the
optical one using the optical modulator. Then, the optical
signal propagates via the wireless medium and it is
collected by the receiver and converted into a useful elec-
trical signal. The transmitter subsystem consists of a pulse
generator, line coder, modulator, optical power meter,
spectrum analyzer, switching system, and optical ampli-
ers. The pulse generator generates pulses that carry the
information in electrical form. The modulator converts the
baseband signal into a high frequency that is suitable for
transmission. The optical power meter measures the
amount of power ready for transmission. The spectrum
analyzer displays the input signal against the frequency.
The fork and switching subsystems are used to select the
path in which the circuit has to connect. This helps to
decide whether to use one or more optical ampliers or not.
The switching decides based on the proximity of the
receiver. More specically, if the receiver is in closer
proximity, then the amplication of the signal may not be
required. However, when the receiver is far from the
transmitter, then this switch connects to the amplier and
enables better signal quality. The optical amplier in-
creases the intensity of the signal which helps to easily
ght the atmospheric effects. This enables better distance
coverage without increasing the transmit power of the
transmitter. The major signal attenuation happens in the
wireless channel. This is because the atmospheric effects
can signicantly attenuate the signal. The overall attenu-
ation is calculated as, [16]:
αTotal =αFogyγ+αSnowγ+αHazeγ+αRainyγ
+αMistγ,dB/km
where, αis the attenuation and γis the operational wave-
length in μm.
On the other hand, the receiver is comprised of optical
amplifiers, photodetector, low pass filter, power meter,
and BER analyzer. Similar to the optical amplifier used in
the transmitter, it improves the received signal strength.
The photodetector perceives the received optical signal
and it converts it into electrical form. The low pass filter
reduces the total environment noise by allowing to pass
only a certain frequency of the signal. Finally, the BER
analyzer determines the accuracy of the received signal.
The BER averages the probability of correctly received bits
out of the total transmitted bits.
BER =Numberof errors
Totalnumberof bitssent
On the other hand, the BER can be calculated from the
SNR of the received information, [16]:
BER =2
π×SNR ×e(SN R
8)
The output of the system is evaluated for three different
optical transmission atmospheric conditions with the
attenuation values 20 dB/km, 30 dB/km, and 70 dB/km
using a BER analyzer.
The atmosphere is the gaseous layer that surrounds the
planet. Fog is a thick cloud of tiny water droplets sus-
pended in the atmosphere that restricts visibility. On the
other hand, Smoke is a visible suspension of particles in the
air, typically one emitted from a burning substance, for
example, carbon. The Haze is another atmospheric phe-
nomenon where the dust, smoke, and other dry particles
make the sky unclear. Dust is a fine powder made up of very
small pieces of earth or sand. The common atmospheric
conditions and the corresponding signal attenuation
values are summarized in Table 1.
In this paper, the optical amplifier operates without
the need for conversion to electrical signals. Since the
G.G. Lema: FSO communication system design s1207
optical content of the signal is amplified, the SNR and
hence the BER performs better (See Figure 2).
The optical amplifier boosts the average power of the
laser output and it also amplifies the weak signals before
the photodetector detects the optical signal. This reduces
the detection noise and hence decreases the BER of the
communication. Especially in longer visibility distance
optical communication, the optical power level should be
raised before the information is lost in the noise. This is a
big deal with wireless optical communication because the
atmospheric conditions severely affect the signal. It is also
well known that amplifiers do not only amplifies the
amplitude or phase of the input signal but also introduces
some noise. Hence, with this trade-off, the less the BER it
results the better the significance of the amplifier.
As it is shown in Figure 3, the fork/switch circuit is
used to adapt the distance of the receiver. If the commu-
nication distance is short enough, then the switch selects
the circuit without the additional optical amplier and it
Figure 1: Overview of the optical wireless communication.
Table :FSO atmospheric conditions and the corresponding signal
attenuation magnitudes.
Atmospheric conditions Attenuation (dB/km)
Haze ..
Rain .
Mist ..
Snow 
Fog 
Figure 2: Overview of optical amplifier.
s1208 G.G. Lema: FSO communication system design
switches to the amplier when the receiver is far from the
transmitter. This adapting the visibility distance is espe-
cially important when the receiver and/or transmitter are
mobile in nature.
2.1 Optimization technique
The optimization technique is a mathematical procedure
which applies a random starting test parameters to
generate ordered improving approximate solutions for a
certain problem. The optimization technique proposed in
this paper is an iterative optimization that starts with
termination criteria and constraint boundaries. The pro-
posed iterative algorithm utilizes the problems whose so-
lution fairly constrained by many service requirements and
it avoids significant computational downsides.
The objective of this study is to maximize visibility
distance and minimize error rate while the reliability and
data rate are kept guaranteed. The proposed iterative
optimization problem is to minimize the BER, given as a
function fof Nvariables. The proposed iterative optimiza-
tion is also going to maximize the visibility distance, given
as a function gof Mvariables. Now letsnd the minimizer,
i.e. the point xsuch that
f(x)f(x),xnearx
again, lets find the maximizer, i.e. the point rsuch that
g(r)g(r),rnearr
finally, lets express the overall problem as
min
xf(x)max
rg(r), then we can have a combined func-
tion, hwith the xand rvariables: h(xr)=g(r)
f(x). Hence, the
overall problem will be to maximize h:
max h(xr)
xr
=
max
rg(r)
min
xf(x)
Subjected : {RRb
BBe
Where Ris the data rate that is constrained to be not less
than R
b
and Bis the BER that is constrained to a minimum
acceptable value, B
e
, that satises the service requirement.
This also guarantees the reliability of the optical
communication.
3 Results and discussions
Using the proposed FSO, the end to end optical design is
constructed as shown in Figure 3. The optical transmitter is
xed to a data rate of 10 Gbps. It is then encoded with the
NRZ pulse generator. As an optical source, the CW laser is
used and it is given 60 dBm of power and 1550 nm wave-
length. With this in mind, the proposed visibility distance
maximizing and BER minimizing mechanism have evalu-
ated under different atmospheric conditions including
Haze, rain, mist, and fog. The Q factor, BER, and received
power are evaluated using OptiSystem 16.
Figure 3: Schematic view of the adaptive FSO design.
G.G. Lema: FSO communication system design s1209
3.1 Under haze atmospheric conditions
It is not surprising that the quality factor and signal power
are reducing over the distance, shown in Figure 4(a) and
(b). However, it is interesting that very long-distance
propagation is achieved without increasing the transmit
power of the transmitter. Since the optical signal has a lot
of safety problems (including aviation exposure problems)
and this effect increases when the transmit power increase.
Hence, the proposed design is ideal as a reliable FSO
communication is achieved without increasing the trans-
mit power.
Figure 4: (a) Q factor, (b) received power and (c) eye pattern performances over distance.
s1210 G.G. Lema: FSO communication system design
Though the haze atmospheric wireless environment
significantly attenuates the optical signal, the proposed
adaptive and iterative algorithm has enabled 4685 m dis-
tance propagation without any repeaters. As can be shown
inFigure 4(c), this long-distance optical signal propagation
has achieved without spoiling the quality of service
(i.e., the BER). The Q-factor measures the quality perfor-
mance of the optical link. The longer the distance the more
the Q-factor is decreased because the signal strength de-
creases with increasing link distance.
3.2 Under rain and mist atmospheric
conditions
Similar to the Haze atmospheric wireless environment, the
rain and mist have significant optical signal attenuation
effects. Even the rain and mist have higher atmospheric
effects than the haze atmospheric conditions. Though the
atmospheric signal attenuation is increased, the optimal
design has enabled us up to 3314 m distance propagation
while still, the BER is almost zero, shown in Figure 5(a)(c).
As the signal power decreases fast, as shown inFig-
ure 5(b), the optical link is limited to a nite distance.
Beyond a certain distance limit, the signal is no more useful
as the atmospheric attenuation, scattering, and reection
erode the content of the signal. On the other hand, the
noise overwhelms the signal at the receiver which results in
difcult signal regeneration. Hence, the Q-factor decreases
with distance, as shown inFigure 5(a).
Even at this adverse environment (rainy and misty),
the new optical communication design has empowered us
to communicate more than 3.3 km in an unlicensed spec-
trum. Unlike the many RF and optical communications,
this remarkable distance coverage is not attained at the
expense of throughput or at the expense of communication
Figure 5: Q factor (a) & received power
(b) performances under rain and mist
conditions (c): Eye diagram under rainy and
misty atmospheric conditions.
G.G. Lema: FSO communication system design s1211
reliability. This unlicensed spectrum enables attractive
distance coverage and it is also characterized 10 Gbps data
rate and no error.
3.3 Under fog atmospheric conditions
The fogy atmospheric condition attenuates the optical
signal much more than the rainy and misty atmospheric
conditions (more than twice worse effect). This results in
faster signal falldown, as shown in Figure 6(a) and (b). If the
usual unlicensed optical wireless communication link is
used, then the usual design doesnt enable longer distance
communication. Hence, the optical communication design
should adapt its parameters and circuitry to enable reliable
communication when the atmospheric conditions are
changing. The maximum distance possible for reliable
communication has reached 1550 m with the proposed op-
tical design. Though reliable communication is possible
beyond this distance, it may not be achieved at a low cost.
For example, it is well known that further distance coverage
will be achieved at the expense of increased transmit power.
However, the increase in transmit power contradicts the
energy efciency use case of the 5G communication net-
works and it introduces safety problems.
The Q-factor of the rainy and misty is still fine while the
1550 m distance is covered,Figure 6(c). Furthermore, the
BER is negligible at this adverse condition.
Though the fogy atmospheric condition significantly
attenuates the FSO signal (70 dB/km), with the help of the
Figure 6: Q factor (a) & received power
(b) performances under rainy and misty
conditions (c): eye diagram under rainy and
misty atmospheric conditions.
s1212 G.G. Lema: FSO communication system design
proposed iterative optimization and new design, more than
1.5 km distance FSO communication is possible without the
need for additional repeaters. It is also good to notice that
the proposed design adapts the atmospheric conditions
while collecting channel state conditions.
The literature [17] has increased the visibility distance
by applying three concatenated couplers. However,
though it has used more optical devices, the distance
increment was just a small amount. In this paper, the
optimization of the amplier length has increased the
visibility distance while the QoS and data rate are not
deteriorated by the increase in distance.
As we can observe in Table 2 and Figure 7, there are
different visibility distance enhancement mechanisms.
From the literature, we can clearly observe that different
operating bands (S,C,L) have different atmospheric effect
resistances. Different lters and different modulation
technics also result in different atmospheric effect toler-
ance and hence different visibility distances. However, the
visibility distance enhancements were only limited incre-
mental effects. The proposed solution has optimized to
maximize the distance while the throughput and QoS are
guaranteed. While still, the Q factor is better than the
literature, signicant visibility distance enhancement is
achieved.
Tables 24 briey present the performance enhance-
ment achieved in this paper compared to the literature.
Mainly, the proposed work has conducted to enhance the
visibility distance of wireless optical communication.
However, increasing the visibility distance at the expense
of the BER, data rate, and Q-factor is not sounding scientic
design. Hence, the BER, data rate, and Q-factor are made
better than or equal to the literature. In all cases, the pro-
posed work has overperformed to the existing works
because of the optimization and modied optical link
designs.
Figures 8 and 9 have shown performance enhance-
ment in terms of the visibility distance. In all the rainy,
misty and foggy, the proposed work has achieved longer
distance coverage without affecting the signal quality. The
Figures can also indicate that the threshold for reliable QoS
communication is limited to these maximum distances
according to the proposed design. More specically, when
the atmospheric condition is known to be in the hazy re-
gion, then the proposed link budget enables 4685 m dis-
tance without deteriorating the signal quality. A similar,
conclusion holds for the misty, rainy, and foggy atmo-
spheric conditions.
To sum up, the proposed transceiver design enables
better communication performance in the FSO scope. Both
Figure 7: Visibility distance comparison
under Hazy atmospheric conditions.
Table :Summary results under Haze atmospheric conditions.
Reference Attenuation (dB/km) Visibility distance (m) Q factor BER Data rate (Gbps)
[],  (ASK & PPM)   . .e
[],    . .
[],  (S,C&Lbands)   . 
[],  (ASK & PSK)   . 
Proposed (Optimization)   . 
G.G. Lema: FSO communication system design s1213
Table :Summary results under rainy and misty atmospheric conditions.
Reference Attenuation (dB/km) Visibility distance (m) Q factor BER Data rate (Gbps)
[],  (ASK & PPM)   . .e.
[],    . .
[],  (S,C&Lbands)   . 
[],  (ASK & PSK)   . .e 
Proposed (Optimization)   . .e 
Table :Summary results under Fogy atmospheric conditions.
Reference Attenuation (dB/km) Visibility distance (m) Q factor BER Data rate (Gbps)
[],  (S,C&Lbands)   . . 
[],  ( , & nm windows)  . . .
[],  (ASK & PSK)   . .e 
Proposed (Optimization)   . .e 
Figure 8: Visibility distance comparison under
rainy &misty atmospheric conditions.
Figure 9: Visibility distance comparison
under fogy atmospheric conditions.
s1214 G.G. Lema: FSO communication system design
the iterative optimization and adaptive characteristics of
the transceiver made the design handy for the FSO in any
atmospheric environment. For several years, the signifi-
cance of FSO was limited because of the atmospheric
challenges. Of course, if the atmospheric problems are
solved or reduced, the extremely low BER, high bandwidth,
and license-free long-distance communication are
possible. The optical fiber was also very expensive and it
requires a lot of deployment costs. However, the FSO is
quite simple in deployment while still an affordable cost.
Due to its incapability to the front wireless communication,
the fiber optics communication was limited to the backhaul
networks before the introduction of the FSO. Hence, the
FSO is an attractive solution to the alarmingly increasing
traffic demand for 5G.
4 Conclusion
Even though the FSO enables attractive communication
characteristics, the wireless link easily deteriorates the
optical signal. The data rate and BER are highly weather
conditions dependent which severely attenuate the signal.
To decrease the atmospheric conditions, this study has
proposed an iterative optimization algorithm and an
enhanced optical communication link design.
The optimized optical link design is evaluated its per-
formance in terms of visibility distance, quality factor, BER,
and Eye diagram at hazy, misty, rainy, and foggy atmo-
spheric conditions. The performance evaluation has con-
ducted while the QoS is guaranteed using the reliability
and data rate. Keeping the BER, data rate, and Q-factor are
greater than or equal to the recent researches, the visibility
distance is maximized by advancing the optical link design
and by optimizing the optical amplifier length. For any
given atmospheric condition, the maximum possible
guaranteed QoS visibility distance is determined and
adapting the atmospheric condition is possible using the
newly proposed design. In general, the simulation results
have shown that better visibility distance, Q factor, and
less BER are achieved at the expense of little system
complexity.
Author contribution: All the authors have accepted
responsibility for the entire content of this submitted
manuscript and approved submission.
Research funding: The article didnt receive any fund.
Conict of interest statement: The authors declare no
conicts of interest regarding this article.
References
1. Leitgeb E, Gebhart M, Birnbacher U. Optical networks, last mile
access and applications. J Opt Fiber Commun Rep 2005;2:5685.
2. Son K, Mao S. A survey of free space optical networks. Digit
Commun Netw 2017;3:6777.
3. Stotts LB, Stadler B, Lee G. Free space optical communications:
coming of age. Int Soc Opt Eng, Proc SPIE 2008;695I:15.
4. Khalighi MA, Uysal M. Survey on free space optical
communication: a communication theory perspective. IEEE
Commun Surv Tutorials 16;223158, 2014.
5. Alexander V, Sandalidis HG, Varoutas D. Weather effects on FSO
network connectivity. J Opt Commun Netw 2012;4:73440.
6. Ijaz M, Ghassemlooy Z, Perez J. Brazda V, Fiser O. Enhancing the
atmospheric visibility and fog attenuation using a controlled FSO
channel. IEEE Photon Technol Lett 2013;25. https://doi.org/10.
1109/lpt.2013.2264046.
7. Organization WR. High speed trafc in the world, 2017. [Online].
Available: https://goo.gl/3k9trm.
8. Masson E, Berbineau M, Lefebvre S. Broadband internet access
on board high speed trains, a technological survey. In:
International workshop on communication technologies for
vehicles: Springer; 2015:16576 pp.
9. Ollivier G, Bullock R, Jing Y, Sondhri J, Zhou N. Chinese high-
speed: an evaluation of trafc. Int Railw J 2015;55. https://www.
railjournal.com/in_depth/chinese-high-speed-an-evaluation-of-
trafc.
10. YouTube. Live encoder settings, bitrates, and resolutions; 2018.
Last Access: 6/11/2018.[Online]. Available https://support.
google.com/youtube/answer/2853702?hl=en.
11. Kaltenberger F, Byiringiro A, Arvanitakis G, Ghaddab R,
Nussbaum D, Knopp R, et al. Broadband wireless channel
measurements for high speed trains. In: 2015 IEEE International
conference on communications (ICC): IEEE; 2015.
12. Kaymak Y, Fathi-Kazerooni S, Rojas-Cessa R, Feng JH, Ansari N,
Zhou MC, et al. Beam with adaptive divergence angle in free-
space optical communications for high-speed trains. arXiv
preprint arXiv:1812.11233; 2018.
13. Das S, Chakraborty M. ASK and PPM modulation based FSO
system under varying weather conditions. In: 2016 IEEE 7th
Annual ubiquitous computing, electronics & mobile
communication conference (UEMCON): IEEE; 2016.
14. Kadhim DS, Shakir AA, Mohammad DA, Mohammad NF. System
design and simulation using (OptiSystem 7.0) for performance
characterization of the free space optical communication system.
Int J Innovat Res Sci Eng Technol 2015;4:482331.
15. Jain D, Mehra R. Performance analysis of free space optical
communication system for S, C and L band. In: 2017 International
conference on computer, communications and electronics
(Comptelix): IEEE; 2017.
16. Gupta A. Comparative analysis of free space optical
communication system for various optical transmission windows
under adverse weather conditions. Procedia Comput Sci 2016;89:
99106.
17. Ashraf M, Baranwal G, Prasad D, Idris S, Beg MT. Performance
analysis of ASK and PSK modulation based FSO system using
coupler-based delay line lter under various weather conditions.
Optic Photon J 2018;8:277.
G.G. Lema: FSO communication system design s1215
18. El-Nayal MK, Aly MM, Fayed HA, AbdelRassoul RA. Adaptive free
space optic system based on visibility detector to overcome
atmospheric attenuation. Results Phys 2019;14:102392.
19. Chaudhary S, Amphawan A. The role and challenges of free-space
optical systems. J Opt Commun 2014;35:32734.
20. Gies D. Safety of free-space optical communication systems. In:
2019 IEEE International symposium on product compliance
engineering (ISPCE): IEEE; 2019.
21. Alnwaimi G, Boujemaa H, Arshad K. Optimal packet length for
free-space optical communications with average SNR feedback
channel. J Comput Netw Commun 2019;2019:18.
22. Sunilkumar K, Anand N, Satheesh SK, Moorthy KK, Ilavazhagan G.
Performance of free-space optical communication systems: effect
of aerosol-induced lower atmospheric warming. Optic express
2019;27:1130311.
23. Li S, Wang J. Adaptive free-space optical communications
through turbulence using self-healing Bessel beams. Sci Rep
2017;7:43233.
24. Parween S, Aruna T. Free space optic communication using
optical AM, OOK-NRZ and OOK-RZ modulation techniques. In:
2019 3rd International conference on electronics, materials
engineering & nano-technology (IEMENTech): IEEE; 2019.
25. Khan MS, Ghafoor S, Mirza J, Zaidi SH. Review of studies that
integrate the free space optics with ber optics. In: 2019 IEEE 16th
International Conference on Smart Cities: Improving Quality of
Life Using ICT & IoT and AI (HONET-ICT): IEEE; 2019.
26. Singh M, Malhotra J. Long-reach high-capacity hybrid MDM-
OFDM-FSO transmission link under the effect of atmospheric
turbulence. Wireless Pers Commun 2019;107:154971.
27. Singh M, Malhotra J. Performance comparison of M-QAM and
DQPSK modulation schemes in a 2×20 Gbit/s-40 GHz hybrid
MDM-OFDM-based radio over FSO transmission system. Photonic
Netw Commun 2019;38:37889.
28. Gebrekrstos Lema G, Baweke Reda T, Hadush Hailu D. LTE quality
of service enhancement under OFDM Modulation Techniques.
Wireless Pers Commun May 2020;112. https://doi.org/10.1007/
s11277-020-07264-8.
29. Gebrekrstos Lema G. Telecommunication customer satisfaction
using self organized network based heuristic algorithm. Int J
Commun Syst 25 May 2020;33. https://doi.org/10.1002/dac.4421.
30. Miglani R, Singh ML. Performance evaluation of free space optical
link using mid and far infrared wavelengths in turbulent
atmospheric conditions. 4th International workshop on ber
optics in access network (FOAN). Almaty; 2013:315 pp.
31. Miglani R, Malhotra J. Statistical analysis of FSO links employing
multiple transmitter/receiver strategy over double-generalized
and gammagamma fading channel using different modulation
techniques. J Opt Commun 2019;40:295305.
32. Miglani R, Malhotra JS. Investigation on RS coded coherent
OFDM free space optical (CO-OFDM-FSO) communication link
over gammagamma channel. Wireless Pers Commun 2019;109:
41535.
s1216 G.G. Lema: FSO communication system design
... Several performance evaluation factors are considered by the authors, such as BER, distance, and atmospheric turbulence impact. The author of [15] illustrated iterative optimization techniques used for functionality and dependability enhancement of FSO systems; this approach involves creating a mathematical model for the FSO system under environmental constraints, with optimization aimed at improving performance metrics like data rate and link availability. The author of [16] identified various challenges faced by both wireless and free-space optical systems. ...
Article
Full-text available
Free space optical (FSO) systems offer an attractive and cost-effective solution for providing communication services in remote regions, as they allow secure transmission without the need for licensing and with lower deployment costs. However, the performance of FSO systems can be significantly impacted by atmospheric turbulences, creating considerable challenges to their deployment. To meet the expanding bandwidth requirements in optical networks, dense wavelength division multiplexing (DWDM) has emerged as a viable option. The development of a 400-Gbps DWDM-FSO system with advanced modulation formats is the subject of this paper. To ensure efficient energy conservation in such a system, power consumption needs to be minimized while maintaining performance level; this calls for optimization of different components within the system. The system is made up of 10 channels and each channel can transmit data at 40 Gbps. Various modulation schemes like carrier-suppressed return-to-zero, modified duo binary return-to-zero, differential phase shift keying, and duo binary return-to-zero are studied for their impact on system performance parameters Q-factor and bit error rate (BER) in C-band around 1550 nm wavelengths. The assessment is also extended to the effects that changing FSO length, input power, and data rate have on these two parameters as well as an evaluation regarding how differing atmospheric conditions influence the FSO system’s effectiveness.
... The modifications required primarily involve the addition of polarized filters and calibration of existing FSO equipment to accommodate the polarization states. This ensures an efficient upgrade to a more secure communication framework [48]. ...
Article
Full-text available
In the burgeoning era of quantum communications, Internet of Things (IoT) devices in high-speed train (HST) environments encounter formidable challenges. These devices are constrained by limited power and computational capabilities while needing to safeguard their data and communications against adversaries equipped with quantum-grade computational power. To counter such threats, Quantum Key Distribution (QKD) emerges as a vital solution, facilitating secure communication between servers and IoT controllers, thereby shielding the more vulnerable IoT sensors. This paper delves into the application of QKD within the unique scenario of HSTs, employing Free Space Optics (FSO) to establish high data rate communication channels. Our experimental setup involves the integration of FSO links for photon exchange essential to QKD. We meticulously explore the QKD process in the context of HSTs, detailing our methodology that involves the alignment of FSO transceivers on ground base stations with those on the moving trains, thereby enabling efficient photon exchange. The study presents quantitative results demonstrating that this approach allows for the exchange of a substantial number of keys, with negligible impact on FSO data throughput. These findings highlight that our proposed method can significantly enhance IoT communication security in HSTs without compromising the Quality of Service (QoS) offered to train passengers. Furthermore, we assess the system’s performance under various visibility conditions, which is crucial for FSO viability. Our results indicate the robustness of the proposed QKD method in diverse operational scenarios, underlining its practical applicability in securing IoT communications within HST environments. Through this study, we provide a comprehensive understanding of implementing QKD in high-speed mobile settings, contributing valuable insights into its effectiveness and feasibility.
Article
Full-text available
The rapid advancement of terahertz (THz) communication systems has positioned this technology as a key enabler for next-generation telecommunication networks, including 6G, secure communications, and hybrid wireless-optical systems. This review comprehensively analyzes THz communication, emphasizing its integration with free-space optical (FSO) systems to overcome conventional bandwidth limitations. While THz-FSO technology promises ultra-high data rates, it is significantly affected by atmospheric absorption, particularly absorption beyond 500 GHz, where the attenuation exceeds 100 dB/km, which severely limits its transmission range. However, the presence of a lower-loss transmission window at 680 GHz provides an opportunity for optimized THz-FSO communication. This paper explores recent developments in high-power THz sources, such as quantum cascade lasers, photonic mixers, and free-electron lasers, which facilitate the attainment of ultra-high data rates. Additionally, adaptive optics, machine learning-based beam alignment, and low-loss materials are examined as potential solutions to mitigating signal degradation due to atmospheric absorption. The integration of THz-FSO systems with optical and radio frequency (RF) technologies is assessed within the framework of software-defined networking (SDN) and multi-band adaptive communication, enhancing their reliability and range. Furthermore, this review discusses emerging applications such as self-driving systems in 6G networks, ultra-low latency communication, holographic telepresence, and inter-satellite links. Future research directions include the use of artificial intelligence for network optimization, creating energy-efficient system designs, and quantum encryption to obtain secure THz communications. Despite the severe constraints imposed by atmospheric attenuation, the technology’s power efficiency, and the materials that are used, THz-FSO technology is promising for the field of ultra-fast and secure next-generation networks. Addressing these limitations through hybrid optical-THz architectures, AI-driven adaptation, and advanced waveguides will be critical for the full realization of THz-FSO communication in modern telecommunication infrastructures.
Article
Full-text available
In this paper, we demonstrated a novel bidirectional high-speed transmission system integrating a free-space optical (FSO) communication with a 5G wireless link, utilizing a high-power erbium-doped fibre amplifier (EDFA) for enhanced loss compensation. The system supports downlink rates of 1-Gb/s/4.5-GHz and 10-Gb/s at 24-GHz and 39-GHz, and an uplink rate of 10-Gb/s/28-GHz. The high-power EDFA boosts signal strength, facilitating reliable long-distance transmission through the FSO transmission link while compensating for losses and reducing bit error rates (BER) and error vector magnitude (EVM). Fibre Bragg grating sensors are employed as wavelength selectors for both downlink and uplink, offering a simpler, cost-effective solution compared to previously utilized reflective semiconductor optical amplifiers and multiple laser sources. The system successfully transmits 16-quadrature amplitude modulation orthogonal frequency division multiplexing signals across various carrier frequencies, achieving total data rates of 21-Gb/s for downlink and 30-Gb/s for uplink across a 1.6-km FSO transmission link integrated with 10/15-m 5G links. Performance metrics, including low EVM and BER, and well-defined constellation patterns, indicate the reliability and effectiveness of the system. This bidirectional FSO-5G wireless communication system offers a high-speed and cost-effective solution for extending 5G coverage in both densely and sparsely populated areas.
Article
Full-text available
The adaptive single-mode fiber (SMF) coupling technique is normally adopted since the coupling efficiency (CE) significantly determines the performance of the free-space optical communication (FSOC) systems. The stochastic parallel gradient descent (SPGD) algorithm is the most commonly used control algorithm in adaptive SMF fiber coupling system. This paper proposes an improved SPGD algorithm named estimation-based stochastic parallel gradient descent (ESPGD) algorithm to accelerate the sytem convergence when applied to a practical adaptive SMF coupling system based on fast steering mirror (FSM). Applying the perturbed voltages, FSM dynamic response and then recording the performance metrics is the basic and most time-consuming process in actual adaptive SMF coupling system control. The ESPGD algorithm uses a different gradient estimation method based on adaptive parameter estimation method. The algorithm only needs to perform this process once in one iteration while the original SPGD algorithm needs to perform it twice to obtain the estimated gradient. This greatly reduce the time consumed by one iteration of the algorithm, thereby reducing the convergence time. The simulation and experimental results show that the ESPGD algorithm reduces the system convergence time by nearly half when correcting static angular errors and more than doubles the control bandwidth when correcting sinusoidal angular jitters.
Article
In recent years, the evolution of optical wireless communication (OWC) system has emerged as a viable alternative to radio frequency communication. These technologies provide an effective solution for addressing the need for point-to-point communication, offering benefits such as higher bandwidth, faster data rates, no licensing requirements, low power usage, quick and simple installation, enhanced security, and resistance to electromagnetic interference. In this article, we analyze two wireless optical communication systems: one using an FSO channel and the other using an OWC channel. The analysis focuses on range and quality factor as performance metrics. We examine the performance of one-to-many Tx/Rx FSO/OWC channel under three different atmospheric conditions: clear weather, haze, and fog, using eye diagrams. The system analysis includes mathematical models for the received optical power and the pointing error. Additionally, we investigate the impact of spatial diversity on the performance of FSO/OWC channel with configurations of 1 × 1, 2 × 2, 4 × 4, and 8 × 8. Our findings indicate that the 8 × 8 FSO/OWC configurations yield better results compared to other configurations and the OWC channel performs well over long distances up to 110 km, while the FSO channel is more suitable for short range communication up to 37 km.
Preprint
Full-text available
The utilization of Free-Space Optical (FSO) communication in underwater environments is gaining momentum, particularly in Software Defined Underwater Wireless Sensor Networks(SDUWSNs). However, challenges such as high-energy loss and limited propagation distance persist in data transmission within SDUWSNs. In addressing these issues, this study introduces an innovative approach known as Software Defined Free Space Optical Underwater Wireless Sensor Networks, where FSO communication is seamlessly integrated with SDUWSNs to enhance network longevity. To optimize the performance of SD-FSO-UWSNs, the implementation of clustering and routing is explored as an effective strategy for energy-efficient data delivery. Nevertheless, the selection of optimal control nodes (CNs) in clustering poses a significant challenge. In response, a novel self-adaptive cheetah optimization-based clustering approach (SACO-CA) is proposed by incorporating self-adaptive inertia weights to identify optimal CNs based on a devised fitness value. The fitness function considers important parameters such as energy levels and distances among network devices, aiming to balance cluster sizes effectively. Moreover, the NS3 simulator is used to run network simulation while, SDN policies are implemented through the Open Network Oper-ating System (ONOS) controller. The simulation result metrics, including stability period,alive nodes, average residual energy, packets transmitted to the control server, and averagedelay, indicate that SACO-CA outperforms existing state-of-the-art methods. The results underscore the efficacy of the nature-inspired SACO-CA approach in optimizing CNs and improving overall network performance in SD-FSO-UWSNs.
Article
Full-text available
The emerging network including long‐term evolution‐advanced (LTE‐A) aims at enhancing the telecommunication customer satisfaction in numerous aspects including system capacity, network coverage, handover management, and quality of service (QoS). Effective handover (HO) management reduces HO failure and hence enhances the data rate and supports user mobility. There are numerous challenges that increase the call drop rate. Among the main challenges that increase the call drop rate, the variable user speeds and variable traffic loads are the major ones. Hence, the time to trigger (TTT), HO margin (HOM), and HO offset (HOO) are used to evaluate the HO management using self‐organized network‐based heuristic algorithm under variable user speeds and variable traffic loads. In recent researches, different HO management techniques were applied to manage the HO decision including fuzzy‐logic tactics and Q‐learning. However, they did not apply intelligent optimization techniques that adapt the variable user speeds and dynamic traffic loads. This paper aims at increasing the telecom customer satisfaction by decreasing the call drop rate using particle swarm optimization (PSO), which adaptively manages the handover control parameters according to the user speed and traffic loads. The simulation results have shown that the proposed optimization tool results in significant call drop rate reduction compared to the ordinary HO management.
Article
Full-text available
Recently, to satisfy the ever-growing data needs, Long Term Evolution (LTE) has been deployed by many operators. LTE enables better spectrum management and avoids the adjacent channel interference by using the Orthogonal Frequency Division Multiple Access (OFDMA) for downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for uplink connection. Even though the spectrum is limited, the number of active devices is increasing which requires the need for extra spectrum. OFDM is capable of better Quality of Service (QoS) as it avoids the multipath fading using equalization and immune to inter-symbol interference, in addition to its attractive spectral efficiency. However, the LTE system is suffered from Peak to Average Power Ratio (PAPR) which reduces the QoS of the communication. Therefore, in this paper, we have proposed a QoS enhancement using a hybridized clipping-selective mapping (Clip-SLM) technique. Different QoS enhancement methods are evaluated for different modulation techniques, phase sequences, and number of sub-carriers. From the simulation results, we have shown that the proposed hybridized technique results in significant PAPR reduction which results in substantial Bit Error Rate (BER) reduction and hence QoS enhancement.
Article
Full-text available
This work is focused on the modeling and performance investigation of a 2 × 20 Gbit/s–40 GHz hybrid mode division multiplexing–orthogonal frequency division multiplexing-based radio over free space optics (RoFSO) transmission system under the influence of different weather conditions. The performance of the proposed system has been compared for 4-quadrature amplitude modulation (QAM), differential quadrature phase-shift keying, 16-QAM, and 32-QAM modulation schemes using error vector magnitude, optical signal-to-noise ratio requirement, and maximum link reach as the performance metrics. The results show that 4-QAM scheme demonstrates the best performance. The proposed RoFSO transmission system incorporating 4-QAM modulation demonstrates a successful transmission of 2 × 20 Gbit/s–40 GHz information over 104 km link range under clear weather conditions. Also, the maximum link range using the proposed system is reported as 4.52 km under light fog, 2.78 km under moderate fog, and 2.11 km under heavy fog conditions. Further, the performance of the proposed system has been compared with the previously reported literature which shows that the proposed system has a better figure of merit (information rate × transmission distance). The presented work can be used to implement a spectrum efficient, high-speed, long-haul information transmission system for future wireless networks.
Article
Full-text available
Free space optics (FSO) provides high data rate by allowing transmission of signal through atmosphere. FSO is affected by different weather conditions. The atmospheric attenuation is caused by two main factors; absorption and scattering. Fog, rain, snow and cloud cause the scattering of optical signals transmitted in free space. Fog is the most critical attenuating factor. In this paper, a new idea to overcome the atmospheric attenuation is discussed using adaptive transmitter and receiver design based on the transmitted power equation that depends on the weather conditions. The obtained results show a huge performance enhancement in the signal to noise ratio (SNR) and bit error rate (BER). Keywords: Free space optics, Atmospheric attenuation, Adaptive receiver
Article
Full-text available
In this article, a method to enhance data rates of free-space optical (FSO) systems using packet length optimization is proposed. The average signal-to-noise ratio (ASNR) is measured at the receiver and sent back to the transmitter to optimize packet length. In addition, the length of packet is optimized to enhance the average throughput. We concluded that packet length can be reduced at low ASNR. However, packet length should be increased at higher values of received ASNR. For each ASNR, we also choose the optimal modulation and coding scheme (MCS) and optimal packet length to maximize the throughput. Different MCSs are investigated such as 4-pulse amplitude modulation (PAM) with and without channel coding, 8-PAM, 16-PAM, and 32-PAM. The proposed method gives 0.8–1.9 dB gain with respect to conventional FSO with adaptive modulation and coding (AMC) and fixed packet length. This is the first paper to deal with packet length optimization for FSO systems.
Article
Full-text available
Atmospheric turbulence is known to significantly degrade the efficiency and reliability of free space optical communication link. Use of coded-orthogonal frequency division multiplexing (OFDM) technique to mitigate the effect of adverse atmospheric conditions on free space optical (FSO) communication link has been proposed here. With Gamma–Gamma distribution for channel modeling, the error performance of the proposed RS8 (Reed Solomon) coded, 128 sub-carrier OFDM link has been investigated using coherent BPSK and QPSK modulation scheme. The results obtained from this analysis have also been compared with intensity modulated/direction detection (IM/DD) based OOK-OFDM FSO link. In case of strong turbulence and for target BER of 10⁻⁴, it was observed that BPSK and QPSK modulated OFDM FSO link achieve a descent coding gain of 18.2 dB and 12.6 dB respectively over non coded OOK-OFDM FSO link. Also, it was observed that as the link conditions worsened from weak to strong turbulence, the effect of atmospheric impairments on FSO link becomes significantly pronounced. Additionally, in terms of BER performance, the BPSK modulated link out-performed QPSK and OOK under all the considered channel conditions.
Article
Free space optical (FSO) communication has emerged as a viable technology for broadband wireless applications which offers the potential of high bandwidth capacity over unlicensed optical wavelengths. This research is focusing on FSO system designs and simulation study using a mathematical and computational program based on mathematical relationships of weather factors and their impact on the movement of the beam to evaluate the performance of Free Space Optical communication systems by study of the impact of weather factors affecting the transmission of laser beam carries the information within different ranges. The laboratory-measured data and the data obtained from the Iraqi Meteorological website (visibility) are used in calculations. Processed data and obtained results were identical to scientific publications and the results could be adopted as inputs for the preparation of an advanced computer course where results have been compared with some scientific software (optiSystem 7.0) which is a program for optical communication systems design and evaluation and the results were identical. This research considering the first step to create a scientific base for optical and electro-optical designs operating in communication field and with self-help efforts.