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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 [1–3] 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 significant ranges of FSO
benefits 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 fiber 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 significant 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): s1205–s1216
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
doesn’t 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
insufficient 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 efficiency of the current and future cellular
communications. The former research, [19], has also shown
the significance 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 flight 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 significantly 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
doesn’t 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
significantly 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 fluctuation 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 inflexibility. 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 specifically, 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 doesn’t 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, [15–20].
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-
fiers. 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 amplifiers or not.
The switching decides based on the proximity of the
receiver. More specifically, if the receiver is in closer
proximity, then the amplification of the signal may not be
required. However, when the receiver is far from the
transmitter, then this switch connects to the amplifier and
enables better signal quality. The optical amplifier in-
creases the intensity of the signal which helps to easily
fight 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 significantly 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 =Number of errors
Total number of bits sent
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 amplifier 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 amplifier 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 let’sfind the minimizer,
i.e. the point x∗such that
f(x∗)≤f(x), ∀x near x∗
again, let’s find the maximizer, i.e. the point r∗such that
g(r∗)≤g(r), ∀r near r∗
finally, let’s 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 : {R≥Rb
B≤Be
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 satisfies 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
fixed 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 finite distance.
Beyond a certain distance limit, the signal is no more useful
as the atmospheric attenuation, scattering, and reflection
erode the content of the signal. On the other hand, the
noise overwhelms the signal at the receiver which results in
difficult 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 doesn’t 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 efficiency 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 amplifier 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 filters 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, significant visibility distance enhancement is
achieved.
Tables 2–4 briefly 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 scientific
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 modified 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 specifically, 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 didn’t receive any fund.
Conflict of interest statement: The authors declare no
conflicts of interest regarding this article.
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