Optical and Quantum Electronics (2024) 56:659

https://doi.org/10.1007/s11082-024-06315-9

Performance analysis of FSO communications in desert

environments

Manel Mrabet1,2 · Maha Sliti3

Received: 28 October 2023 / Accepted: 6 January 2024 / Published online: 12 February 2024
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024

Abstract
This paper examines the performance of free space optical (FSO) communication systems
in desert environments, focusing on the interaction between FSO technology and chal-
lenging atmospheric conditions like duststorms and fogs. The research aims to enhance
the design and operation of FSO communication systems in these environments, where
reliable connections are critical. The study evaluates the performance of the quadrature
amplitude modulation-orthogonal frequency division multiplexing (QAM-OFDM) tech-
nique to mitigate severe weather impacts on FSO communication links. We compare dif-
ferent modulation methods (AM, Duobinary RZ, Modified Duobinary RZ, OOK-RZ, and
OOK-NRZ) in different weather conditions (light fog, moderate fog, heavy fog, light dust
storm, and moderate dust storm). The results demonstrate that the 4QAM-OFDM modula-
tion technique is highly effective in different fog conditions. It maintains an acceptable bit
error rate (BER) in light fog, 0.5–1.5 km, and even at extended link distances. It achieves
a BER of 2 × 10−5 at 2.4 km. In moderate fog, it achieves a BER of 6 × 10−5 at 1.75 km. In
heavy fog, it maintains acceptable BER values within 0.5–0.8 km and a BER of 5 × 10−5 at
1.35 km. In moderate dust, specifically at a link range of 0.64 km, the achieved BER value
is 7 × 10−7. 4Qam-OFDM provides the most favorable values of received power compared
to other modulation techniques.

Keywords Free space optics (FSO) · Desert environments · Fog · Duststorm · Bit error rate
(BER) · Received power

* Manel Mrabet
M.benrashed@psau.edu.sa
Maha Sliti
slitimaha@gmail.com
1
Prince Sattam Bin Abdulaziz University, Al‑Kharj, Saudi Arabia
2
Manouba University, Manouba, Tunisia
3
LR11TIC04, Communication Networks and Security Research Lab. & LR11TIC02, Green
and Smart Communication Systems Research Lab, Higher School of Communication of Tunis
(SUP‑COM), University of Carthage, Tunis, Tunisia

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659 Page 2 of 21 M. Mrabet, M. Sliti

1 Introduction

Free Space Optical (FSO) communication, also referred to as optical wireless communica-
tion, is an emerging technology that uses modulated laser beams to transmit high-speed
data through the atmosphere, offering advantages like high bandwidth, low latency, and
immunity to radio-frequency interference. Due to factors like urban growth, economic
development, and the growing need for secure and high-capacity communication networks,
the demand for reliable FSO communication systems in arid desert regions has increased
significantly in recent years. However, the harsh and arid weather conditions have a sig-
nificant impact on the dependability and performance of the FSO communication systems.
Indeed, there are particular and challenging constraints when deploying FSO communica-
tion systems in arid desert environments. In these conditions, dust storms and fog peri-
ods are frequent, which pose distinctive challenges to optical communications. Particles of
sand and dust present in the atmosphere scatter and attenuate optical signals, resulting in a
decrease in the quality of the signal, a shorter communication range, and a higher suscepti-
bility to signal disruptions. Additionally, fog introduces further optical attenuation, scatter-
ing, and beam wander, exacerbating the difficulties in maintaining reliable communication
links. Therefore, it becomes imperative to acquire an in-depth understanding of the compli-
cated interplay between FSO technology and the particular atmospheric conditions encoun-
tered in desert environments. This study explores the performance of FSO communication
systems in arid desert environments, focusing on the interaction between FSO technology
and atmospheric conditions. In this context, we consider real-time visibility data from the
meteorological department, and Optisystem software were used to estimate attenuation
under duststorm and fog conditions in Qassim, Saudi Arabia (Fig. 1). The region’s arid
desert climate presents unique challenges, particularly fogs and duststorms, which can sig-
nificantly impact daily life, including transportation, visibility, and communication.
In this study, we use strict attenuation models to find out how duststorms and fog affect
key performance metrics like signal quality, communication range, and reliability. The
goal is to make FSO system design and operation better. To enhance the resilience of FSO
communication systems against challenging atmospheric conditions in desert environ-
ments, this study evaluates the use of the Quadrature Amplitude Modulation-Orthogonal
Frequency Division Multiplexing (QAM-OFDM) modulation approach. The QAM-OFDM
method is investigated for its effectiveness in mitigating the impact of severe weather, spe-
cifically duststorms and fogs, on FSO communication links. In this work, we compare it
with other modulation techniques, including Amplitude Modulation (AM), Duobinary RZ
(Return to Zero), Modified Duobinary RZ, On-Off Keying Return to Zero (OOK-RZ), and

Fig. 1  Duststorm in Saudi Arabia

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Performance analysis of FSO communications in desert… Page 3 of 21 659

On-Off Keying Non-Return to Zero (OOK-NRZ), under light fog (LF), moderate fog (MF),
heavy fog (HF), light dust storm (LD), and moderate dust storm (MD). The results dem-
onstrate that the 4QAM-OFDM modulation technique is highly effective in different fog
conditions. It maintains an acceptable bit error rate (BER) in light fog, 0.5 to 1.5 km, and
even at extended link distances. It achieves a BER of 2 × 10−5 at 2.4 km. In moderate fog,
it achieves a BER of 6 × 10−5 at 1.75 km. In heavy fog, it maintains acceptable BER values
within 0.5−0.8 km and a BER of 5 × 10−5 at 1.35 km. The 4QAM-OFDM modulation tech-
nique shows remarkable performance in varying dust storm conditions. It maintains accept-
able bit error rates (BERs) within 0.2−1.15 km in light dust. In moderate dust, specifically
at a link range of 0.64 km, the achieved BER value is 7 × 10−7. 4Qam-OFDM provides the
most favorable values of received power compared to other modulation techniques.
The rest of the paper is organized as follows: In Sect. 2, we discuss the existing research
and potential strategies to alleviate the negative effects of harsh atmospheric conditions on
the performance of FSO systems. In Sect. 3, we examine the attenuation processes caused
by duststorms and fog phenomena, and we explore existing models and methodologies for
quantifying their effects. Section 4 describes the proposed FSO communication system
design, and Sect. 5 investigates advanced modulation techniques like amplitude modu-
lation, duobinary RZ modulation, modified duobinary RZ modulation, OOK-RZ, OOK-
NRZ, and 4QAM-OFDM to mitigate weather-induced attenuation under duststorm and fog
conditions. In Sect. 6, we present the obtained simulation results using the optisystem soft-
ware. Finally, Sect. 7 concludes the paper.

2 Related work

Despite the potential of FSO communication for high-speed data transfer, several chal-
lenges impact its performance. Adverse weather, which is characterized by signal attenu-
ation, is the biggest obstacle for outdoor FSO systems in Esmail et al. (2016a). There is a
large range in this attenuation, from very low levels in clear weather to very high levels in
difficult circumstances, frequently exceeding hundreds of dBs/km. The main offenders are
fog and dust, which can cause link interruptions and reduce vision to only a few meters
since their particle sizes are identical to those of the wavelengths used in FSO systems.
Other conditions, including snow, rain, haze, and scintillation, have lower impacts on FSO
communications.
Several studies have contributed to the understanding and enhancement of FSO sys-
tems under diverse weather conditions using various modulation techniques. In Choyon
and Chowdhury (2020), the authors study the challenges posed by strong atmospheric
turbulence in FSO systems. Their investigation focused on various modulation formats
(OOK, BPSK, DPSK, QPSK, and 8-PSK) over different link distances. BPSK emerged
as the most robust modulation format, exhibiting superior BER performance, especially
in turbulent conditions. In Chowdhury and Choyon (2021a), the authors proposed a com-
prehensive design for an Alternate Mark Inversion (AMI)-encoded FSO communication
system. Their innovative approach involved hybridizing Polarization Division Multiplex-
ing (PDM) with Wavelength Division Multiplexing (WDM), yielding a high-performance
FSO link. The study also demonstrated the advantages of the hybrid AMI-PDM-WDM
FSO system over traditional models in terms of Q factor, received optical power, BER, eye
diagrams, and optical signal-to-noise ratio (OSNR). In Choyon and Chowdhury (2021),
another hybrid design is proposed, incorporating PDM and WDM in an FSO system. The

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659 Page 4 of 21 M. Mrabet, M. Sliti

study emphasized the maximization of link capacity and spectral efficiency, showcasing
superior performance under diverse atmospheric conditions in Bangladesh. Circular Polari-
zation Division Multiplexing (CPDM) and Coherent Optical Orthogonal Frequency Divi-
sion Multiplexing (CO-OFDM) took center stage in Chowdhury and Choyon (2021b). This
hybridized approach aimed to enhance FSO communication system performance under
turbulent conditions in Bangladesh. The study emphasized the mitigation of multipath fad-
ing through OFDM modulation. Weather-specific challenges were further addressed in Bai
et al. (2022), focusing on the impact of rainy weather on FSO system performance using
BPSK modulation. A new 120 Gbps FSO transmission scheme was described in Singh
et al. (2022). It combines Orbital Angular Momentum (OAM) multiplexing with Spectral
Amplitude Coded (SAC)-Optical Code Division Multiplexing Access (OCDMA). Their
study showcased the system’s potential to achieve longer propagation distances under vari-
ous weather conditions, providing a valuable contribution to reliable FSO communication.
The work in Amar et al. (2022) studies the severe weather conditions in Algeria by ana-
lyzing a 40 Gbps Return to Zero-Differential Phase Shift Keying-FSO (RZ-DPSK-FSO)
system under different rain conditions. The study underscored the acceptable performance
levels achievable with DPSK modulation, reinforcing the importance of modulation for-
mat in varying weather scenarios. Hybridization techniques were explored by Sharma et al.
(2023b), advocating the combination of Polarization Division Multiplexing (PDM) with
Orthogonal Frequency Division Multiplexing (OFDM) for FSO integrated data commu-
nication. Their proposed system achieved reliable 160 Gbps transmission under various
weather conditions, offering adaptability to environmental fluctuations. In Sharma et al.
(2023a), the authors proposed a 16-PSK-OFDM-FSO communication system to address
environmental challenges like fog and rain.
These works provide comprehensive insights into the intricate interplay between
modulation schemes, hybridization techniques, and atmospheric conditions in the design
and optimization of FSO communication systems. In Table 1, we compare the presented
research works and the proposed approach.

3 Attenuation models for FSO

This section presents potential attenuation models for fog and duststorms in FSO commu-
nication systems, highlighting the importance of developing models to accurately mitigate
signal degradation in adverse weather conditions.

3.1 Fog attenuation

Environmental conditions significantly impact the performance of FSO communication

systems. Adverse weather circumstances, such as fog, haze, dust, smoke, smog, snow,
and industrial emissions, reduce environmental visibility significantly, owing to increased
optical attenuation via absorption and scattering. This attenuation has a negative impact
on communication link efficiency and reduces the link margin. Fog, which is common
in desert regions, is especially important. Fog occurs when the temperature drops to 3–5
degrees Celsius and the relative humidity exceeds 85%. A variety of fog attenuation pre-
diction models, including those developed by Kim, Kruse, and Al-Naboulsi, are widely
used in this context to estimate optical attenuation in atmospheric channel visibility during

13
 Table 1  Comparison of recent approaches for mitigating turbulent weather conditions
Reference Modulation technique Weather conditions/attenuation (dB/ Data bit rate (Gbps) Authenticated meteorological data
km) source

Choyon and Chowdhury (2020) OOK, BPSK, DPSK, QPSK and Strong atmospheric Turbulence 1 Gbps Not considered
8-PSK
Chowdhury and Choyon (2021a) Hybrid AMI-PDM-WDM Clear weather (0.233 db/km), light 320 Gbps Not considered
haze (0.55 db/km), heavy haze (8 × 40 Gbps) with
(2.37 db/km), light rain (6.27 db/ 200 GHz channel
km), moderate rain (9.64 db/km) spacing
Choyon and Chowdhury (2021) PDM-WDM FSO Light rain (6.27 db/km), moder- 16 × 40 Gbps with Yes, Bangladesh Weather conditions
ate rain (9.64 db/km), light 200 GHz frequency
fog (12.47 db/km), heavy rain spacing
(19.28 db/km)
Chowdhury and Choyon (2021b) Hybrid CPDM-CO-OFDM FSO Light rain (6.27 db/km), moder- 200 Gbps (4 channels) Yes, Bangladesh Weather conditions
ate rain (9.64 db/km), light
fog (12.47 db/km), heavy rain
Performance analysis of FSO communications in desert…

(19.28 db/km)
Bai et al. (2022) BPSK Modulation Light rain (6.27 db/km), moder- Not mentioned Not considered
ate rain (9.64 db/km), heavy rain
(19.28 db/km)
Amar et al. (2022) RZ-DPSK Clear weather (0.233 db/km), light 40 Gbps Yes, Algeria weather conditions
rain (6.27 db/km), moderate
rain (9.64 db/km), heavy rain
(19.28 db/km)
Singh et al. (2022) SAC-OCDMA-OAM-based FSO Heavy rain (19.28 db/km), heavy 120 Gbps Not considered
transmission system haze (10.115 db/km), heavy fog
(22 db/km)
Sharma et al. (2023b) Dual-polarized-32-1eve1-quadrature Clear sky (0.233 db/km), heavy fog 160 Gbps Yes, Chandigarh city weather condi-
amplitude modulated (DP- (22 db/km) tions
32-QAM)
Sharma et al. (2023a) 16-PSK-OFDM-FSO Cloudy and rainy weather condi- Not mentioned Not considered
tions
Page 5 of 21 659

13
 Table 1  (continued)
659

Reference Modulation technique Weather conditions/attenuation (dB/ Data bit rate (Gbps) Authenticated meteorological data
km) source

13
The proposed approach AM, OOK-RZ, OOK-NRZ, Duobi- Light fog (9 db/km), moderate fog 15 Gbps Yes, Qassim weather conditions
nary RZ Modified Duobinary RZ, (16 db/km), heavy fog (22 db/km),
Page 6 of 21

4QAM-OFDM light dust storm (25.11 db/km),

moderate dust storm (50–100 db/
km)
M. Mrabet, M. Sliti
Performance analysis of FSO communications in desert… Page 7 of 21 659

foggy circumstances. Visibility graph illustrated in Fig. 2 confirms the incidence of fog
events.

3.1.1 Kim & Kruse models

The Kruse (Kruse 1962) and Kim model (Isaac et al. 2001) are wavelength-dependent
models that are used to calculate specific optical attenuation from visibility data. Visibility
is defined as the distance during which 5% or 2% of 550 nm collimated light of an initial
power is attenuated (AbdElKader et al. 2022). The attenuated power is measured in dB/km,
whereas visibility is measured in kilometers. The attenuation coefficient can be computed
by employing the Kim and Kruse models for a 2% transmittance, as described by the math-
ematical formula in Eq. 1.
17 𝜆 −q
𝛼= ( ) , dB∕km
V 𝜆0 (1)

where 𝜆 represents the wavelength, l𝜆0 denotes the reference wavelength, and 𝛼 indicates
attenuation coefficient.
For transmittance of 5%, the attenuation coefficient is given by Eq. 2:
13 𝜆 −q
𝛼= ( ) , dB∕km
V 𝜆0 (2)

In Eqs. 1 and 2, visibility is represented by V, 𝜆 denotes the wavelength of the transmitted

signal, with 𝜆0 set at 550 nm, and the size distribution of scattering particles is denoted
by q (Farouk and Mazin 2021). The Kruse model specifies the values of q in Eq. 3 (Singh
et al. 2022).

⎧ 1.6 if V > 50 km
⎪
q = ⎨ 1.3 if 6 km < V < 50 km (3)
⎪ 0.585V 1∕3 if V < 6 km
⎩

where q indicates the size of distribution of scattering particle.

In contrast, Kim’s model employs distinct size distribution values without offering a
visibility advantage for distances below 0.5 km. These specific size distribution values for
the Kim model are detailed in Eq. 4.

Fig. 2  Visibility graph during

March 2018 in Qassim

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659 Page 8 of 21 M. Mrabet, M. Sliti

⎧ 1.6 if V > 50 km
⎪ 1.3 if 6 km < V < 50 km
⎪
q = ⎨ 0.16V + 0.34 if 1 km < V < 6 km (4)
⎪ V − 0.5 if 0.5 km < V < 1 km
⎪0 if V < 0.5 km
⎩

3.1.2 Al‑Naboulsi model

Al Naboulsi model, as described in Al Naboulsi and Sizun (2008), is a comprehensive

tool for predicting fog attenuation within the spectral range of 0.69–1.55 μ m and cover-
ing visibilities from 50 ms to 1 km. It distinguishes between convection and advection
fog, which are linked to different atmospheric conditions. The model provides equations
for attenuation prediction for both types, with a specific equation for advection fog and
a dedicated equation for convection fog. The model recognizes the distinctions between
these two types of fog, namely that convection fog results from local ground cooling
while advection fog is a result of moist air movement over a cooler surface. This model
aids in understanding and managing fog-related challenges in optical communication
systems.
0.11478𝜆 + 8367
Aadv =4.343( ), dB∕km (5)
V

0.18126𝜆2 + 0.3709𝜆 + 3.7502

Aconv =4.343( ), dB∕km (6)
V
The model is wavelength-dependent, exhibiting an increase in attenuation with increasing
wavelength, unlike the Kruse and Kim models, which show a decrease in attenuation with
increasing wavelength.
Figure 3 shows that at a wavelength of 1550 nm, optical attenuation for maximum
visibility of 16 km is 0.0154 dB/km for Kim, 0.0043 dB/km for Kruse, and 0.1012 dB/
km for the Al-Naboulsi model advection. The Kruse model is more sensitive to wave-
length and shows the lowest attenuation values.

Fig. 3  Attenuation graph resulted

from fog periods in Qassim dur-
ing March 2018

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3.2 Duststorm attenuation

Arid regions, such as Saudi Arabia, face a distinct challenge when it comes to free-
space optical (FSO) communications due to the prevalence of duststorms. Saudi Ara-
bia’s expansive desert landscapes and arid climate make it a prime location for dust-
storm formation, primarily during transitional seasons. These environmental factors
contribute to the transport of fine dust particles over long distances during dust storms.
In this context, the impact of duststorm attenuation on FSO communications becomes
particularly relevant. Duststorms in arid regions can significantly degrade FSO signals.
The fine dust particles suspended in the atmosphere scatter and absorb optical signals,
leading to signal loss, unstable links, and reduced communication range. This attenu-
ation can adversely affect various FSO applications, including data transmission and
terrestrial networking. To address these challenges, several mitigation strategies are
employed. Advanced signal processing techniques, such as error correction coding and
dynamic power control, help optimize signal quality and maintain link reliability in the
face of duststorm attenuation. Implementing dust-proof enclosures for FSO equipment
is crucial to protecting sensitive optical components during duststorms. Additionally,
building FSO networks with multiple links and diverse paths enhances network resil-
ience, providing redundancy to ensure communication continuity even during dust-
storms. Consequently, ongoing research efforts in arid regions, including Saudi Arabia,
are focused on understanding the dynamics of duststorms, developing predictive mod-
els, and exploring innovative technologies. In arid and semi-arid regions susceptible
to dust storms, the estimation of FSO signal attenuation is performed using Eq. 6 as
outlined in reference (Esmail et al. 2016b). This model is specifically applicable to a
wavelength of 1550 nm, a commonly employed wavelength in FSO systems due to its
minimal absorption loss.

𝛼 = 52 ∗ V −1.05 , dB∕km (7)

Dust storms are categorized into four classes according to their visibility range, as pre-
sented in Table 2 (Yaping 2008). The most severe storms exhibit a visibility range of less
than 200 ms, while the second type extends from 200 ms to 1 km. Dust with a visibility
range spanning 1 to 10 kms falls into the category of blowing dust or haze. Blowing dust
occurs when wind transports dust particles, whereas haze results from a dust storm occur-
ring at a distance from the observation point. Figures 4 and 5 illustrate respectively the vis-
ibility and the optical signal attenuation during March 2108 in Qassim in the presence of
duststorm events. We note that the maximum attenuation is 55 dB/km.

Fig. 4  Visibility graph due to

Duststorm events during May
2018 in Qassim

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659 Page 10 of 21 M. Mrabet, M. Sliti

Fig. 5  Attenuation due to Duststorm events during May 2018 in Qassim

Table 2  Dust storm classification established on a visibility basis

Type of dust Severe dust storm Dust storm Blowing dust Dust haze

Depiction Dense Moderate Light Very light

Visibility (km) < 0.2 0.2–1 1–10 < 10

Fig. 6  Attenuation comparison of

fog and duststorm events

3.3 Comparison of fog and dust attenuation effects

In the literature, fog stands out as the foremost obstacle to optimal FSO performance. This
stems from the close alignment in size between fog particles and the wavelength of signals
employed in FSO communications. The examination of signal attenuation in fog primarily
relies on field observations in regions, notably in Europe and East Asia, where harsh desert
climates are not prevalent. As a result, fog has been identified as the primary challenge
confronting FSO systems.
Figure 6 illustrates that in dusty conditions, the FSO signal suffers significantly higher
attenuation. Fog and dust particles have sizes that align with FSO signals’ wavelengths,
with fog particles exhibiting less scattering. Both fog and dust storms can negatively
impact FSO communications by scattering and attenuating optical signals. Fog, being
water-based, has different scattering effects from dust particles. Scattering involves signal

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Performance analysis of FSO communications in desert… Page 11 of 21 659

direction changes due to interactions with particles or irregularities, resulting in dispersion

and changes in the signal’s trajectory. Attenuation reduces signal intensity as it propagates
through the medium, caused by factors like absorption, scattering, and obstacles in the
transmission path (Fig. 7).
FSO communications encounter challenges in adverse weather conditions such as dust
storms and fog. Dust storms, characterized by solid particle composition, exert a more pro-
nounced negative impact on FSO signals, resulting in higher attenuation and posing greater
communication challenges. In contrast, fog, composed of small water droplets, induces less
severe attenuation. Mitigating these effects is crucial for sustaining reliable FSO communi-
cation, particularly in the face of signal strength reduction and visibility challenges caused
by dust storms and the less severe attenuation induced by fog.

4 FSO communication system design

In this section, we propose an optimized design solution for FSO systems operating under
conditions of fog and duststorm attenuation using Optisystem software. To this end, we
explore an effective modulation technique to reduce fog and duststorm attenuation in high-
data-rate links. We also investigate bit error rate (BER), quality factor (Q), and received
optical power versus transmission range for several modulation schemes (AM, Duobinary
RZ, Modified Duobinary RZ, RZ, NRZ) to alleviate weather-induced attenuation under
duststorm and fog conditions. The simulation parameters and their values are described in
Table 3.
The simulated FSO communication system is depicted in Fig. 8. The transmitter side
features a continuous-wave (CW) laser operating at 34dBm power. Data bits are generated
using a pseudo-random bit sequence generator and subsequently modulated with an opti-
cal modulator. The modulated signal is transmitted through an FSO channel with varying
attenuation levels corresponding to specific meteorological conditions. At the receiver, a
5-meter erbium-doped fiber amplifier (EDFA) amplifies the received signal. A photodetec-
tor with a sensitivity of 1 A/W, a gain of 3, and an ionization ratio of 0.9 is then employed.
APDs were chosen for their heightened sensitivity, a crucial attribute in Free-Space Opti-
cal (FSO) systems where reliable communication is imperative, especially in challenging
atmospheric conditions. The unique avalanche multiplication feature of APDs significantly
enhances their performance, ensuring robust signal detection even in adverse scenarios
such as fog and dust storms. The adequate selection of APD sensitivities plays a pivotal
role in improving the reliability and efficacy of our FSO communication system when
faced with atmospheric challenges, as illustrated by Fig. 7.

5 Modulation approaches for the proposed FSO communication

system

For a reliable FSO communication system operating at a challenging data rate of 10

Gbps, the choice of modulation technique is pivotal for ensuring the reliability and effi-
ciency of data transmission, particularly in the presence of adverse weather conditions
like duststorms and fogs. In this section, we focus on advanced modulation techniques like
amplitude modulation, duobinary RZ modulation, modified duobinary RZ modulation,

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659 Page 12 of 21 M. Mrabet, M. Sliti

Fig. 7  BER versus APD respon-

sitivity

Table 3  Simulation parameters Parameters Value

Bit rate 15 (Gbits/s)

Distance 0.2–2.5 (km)
Wavelength 1550 (nm)
Sequence length 512 bits
Samples per bit 16
Number of samples 8192 bits
Transmitter optical power 34 dBm
Transmitter/receiver aperture 5 cm/20 cm
Beam divergence 2 mrad
Photo detector APD (ava-
lanche
photodiode)

Fig. 8  FSO communication system design

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Performance analysis of FSO communications in desert… Page 13 of 21 659

Fig. 9  Transmitter design with

AM modulation

Fig. 10  Transmitter design with

OOK-NRZ modulation

OOK-RZ, OOK-NRZ, and 4QAM-OFDM, to mitigate weather-induced attenuation in

urban desert environments.

5.1 Amplitude modulation

AM modulation uses a pseudo-random bit sequence (PRBS) generator to generate a 10

Gbps binary data stream. A NRZ encoder is used to convert these binary bits into electri-
cal signals. Following that, these signals are processed and modulated by a continuous-
wave (CW) laser with a wavelength of 1550 nm. As shown in Fig. 9, the modulated signal
is subsequently broadcast across a Free-Space Optical (FSO) channel, with its amplitude
adjusted in accordance with the NRZ-encoded data.

5.2 OOK‑NRZ modulation

As shown in Fig. 10, OOK-NRZ modulation employs a PRBS generator to create a 10

Gbps binary bitstream. Following that, these binary bits are transformed into electrical sig-
nals using an NRZ encoder, which employs a bipolar non-return-to-zero approach that uses
high values (+5) for 1 s and low values (-5) for 0 s. The signals are then modulated with a
Mach-Zehnder modulator (MZM) utilizing a continuous-wave (CW) laser with an optical
power of 34 dBm at a wavelength of 1550 nm. The modulated optical signal’s amplitude
varies in accordance with the NRZ-encoded data.

5.3 OOK‑RZ modulation

As shown in Fig. 11, OOK-RZ modulation employs a pseudo-random bit sequence (PRBS)
generator that generates binary bits at a rate of 10 Gbps. An RZ encoder converts these
binary bits into electrical impulses. This bipolar return-to-zero method resets the signal to
zero after encoding each bit, requiring more processing time and wider bandwidth. After

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659 Page 14 of 21 M. Mrabet, M. Sliti

Fig. 11  Transmitter design with

OOK-RZ modulation

encoding, a Mach-Zehnder modulator (MZM) is used to modulate the signals using a con-
tinuous wave (CW) laser operating at 34 dBm with a wavelength of 1550 nm. Following
that, the amplitude of the modulated optical signal is adjusted in accordance with the RZ-
encoded data.

5.4 Duobinary RZ modulation

A 10 Gbps data rate pseudo-random bit sequence (PRBS) generator is used in conjunction
with a duo-binary precoder, NRZ generator, and pulse generator to generate a duo-binary
return to zero (DB-RZ) modulated signal in the FSO communication system. The first
Mach-Zehnder modulator (MZM) was generated by a continuous-wave (CW) laser with
an optical power of 34 dBm and a frequency of 1550 nm. As shown in Fig. 12, the second
MZM is cascade-connected to the first MZM and is powered by a 10 GHz signal generator
with an electric gain of 1.

5.5 Modified Duobinary RZ modulation

Figure 13 shows the Modified Duo-Binary Return to Zero (MDB-RZ) modulation. For
bit 1, the phase of the signal in MDB-RZ alternates between 0 and 180 degrees. To
generate a 10 Gbps bit sequence, a pseudo-random bit sequence (PRBS) generator is
used, together with a duo-binary precoder delay and a subtractor circuit to drive the
first Mach-Zehnder modulator (MZM) with an electric gain of 1. This first modulator is
linked to a second MZM, which similarly has an electric gain of one and is controlled

Fig. 12  Transmitter design with Duobinary RZ modulation

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Performance analysis of FSO communications in desert… Page 15 of 21 659

Fig. 13  Transmitter design with Modified Duobinary RZ modulation

by a signal generator with a −90◦ phase shift. The MZMs are used for electronic sig-
nal modulation and utilize a continuous-wave (CW) laser with an optical power of 34
dBm at a wavelength of 1550 nm. The modulated signal is then delivered over the FSO
channel.

5.6 QAM OFDM modulation

The foundation of QAM-OFDM lies in its dual modulation scheme. QAM, known for
its ability to encode multiple bits per symbol by varying both amplitude and phase, is
integrated with OFDM, which divides the data stream into multiple orthogonal subcar-
riers. In the transmitter section, digital data undergoes QAM modulation, and the result-
ing symbols are mapped onto subcarriers using OFDM. The inverse process occurs at
the receiver, where the received signal is demodulated using QAM, and the OFDM
demodulator extracts the original data. The FSO communication system using QAM-
OFDM modulation is illustrated by Fig. 14.

Fig. 14  FSO communication

system using QAM-OFDM
modulation

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659 Page 16 of 21 M. Mrabet, M. Sliti

6 Simulation results

In this section, we evaluate the performance of the proposed FSO communication system
under fog and dust storm conditions.

6.1 System performance in foggy conditions

In this subsection, we assess the performances of the modulation techniques (AM, Duo-
binary RZ, modified Duobinary RZ, OOK-RZ, OOK-NRZ, 4QAM-OFDM) in light fog
(attenuation: 9 db/km), moderate fog (attenuation: 16 db/km), and heavy fog (22 db/km).
Figure 15 illustrates the obtained bit error rates under the influence of light fog (9 dB/
km). Notably, for link distances ranging from 0.5 to 1.5 km, all modulation techniques
exhibit acceptable BER values. As the link range extends to 1.5−2.4 km, acceptable BER
values are observed specifically for duobinary RZ, modified duobinary RZ, and 4Qam-
OFDM. For a link range of 2.4 km, the BER values are 1.3 × 10−5 for Modified duobinary
RZ, 4 × 10−7 for duobinary RZ, and 2 × 10−5 for 4QAM-OFDM.
Figure 16 illustrates the bit error rates acquired under the influence of moderate fog (16
dB/km). Notably, for link distances ranging from 0.5 to 1 km, all modulation techniques
exhibit satisfactory BER values. However, within the 1.5−1.7 km range, Duobinary RZ,
modified Duobinary RZ, and 4Qam-OFDM show acceptable BER values. Specifically, at a
distance of 1.5 km, the achieved BER values are 10−9 for modified duobinary RZ, 3 × 10−12
for duobinary RZ, and 0 for 4Qam-OFDM. At a distance of 1.75 km, the 4Qam-OFDM
method yields a BER of 6 × 10−5.
Figure 17 illustrates the acquired bit error rates in the presence of heavy fog (22 dB/km).
It is noteworthy that within link distances of 0.5−0.8 km, all modulation techniques exhibit
acceptable BER values. However, as the link range extends to 0.8−1.2 km, duobinary RZ,
modified duobinary RZ, and 4Qam-OFDM demonstrate satisfactory BER values. Specifi-
cally, at a distance of 1.35 km, the 4Qam-OFDM method yields a BER equal to 5 × 10−5.
Figure 18 depicts the relationship between optical received power (in dBm) and link range
within the context of heavy fog (22 dB/km). Notably, the modulation methods of OOK-NRZ,

Fig. 15  Ber versus link range for light fog

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Performance analysis of FSO communications in desert… Page 17 of 21 659

Fig. 16  Ber versus link range for moderate fog

Fig. 17  Ber versus link range for heavy fog

OOK-RZ, and AM exhibit extremely low optical received power values. In comparison, duo-
binary RZ, modified duobinary RZ, and 4Qam-OFDM modulation techniques give improved
results. For instance, at a link range of 1.5 km, the optical received power values are
−4 28 dBm for duobinary RZ, − 30 dBm for modified duobinary RZ, and − 25 dBm for
4Qam-OFDM.
In conclusion, the modulation techniques studied demonstrate varying degrees of robust-
ness in fog conditions. While they generally perform well in light fog, their effectiveness
diminishes in moderate and heavy fog.

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659 Page 18 of 21 M. Mrabet, M. Sliti

Fig. 18  Received power versus link range for heavy fog

6.2 System performance in duststorm conditions

In this subsection, we assess the performances of the modulation techniques (AM, Duo-
binary RZ, modified Duobinary RZ, OOK-RZ, and OOK-NRZ, QAM-OFDM) for light
dust storms (attenuation: 50db/km).
Figure 19 illustrates the acquired bit error rates in the presence of light dust (25.11
dB/km). It is noteworthy that within link distances of 0.2−0.7 km, all modulation tech-
niques exhibit acceptable BER values. However, as the link range extends to 0.7−1.2
km, duobinary RZ, modified duobinary RZ, and 4Qam-OFDM demonstrate satisfac-
tory BER values. For instance, at a link range of 1.15 km, the obtained BER values
are 2 × 10−5 for duobinary RZ, 10−4 for modified duobinary RZ, and 1.4 × 10−4 for
4Qam-OFDM.
Figure 20 illustrates the obtained bit error rates in the presence of moderate dust
(50 dB/km). Notably, within link distances ranging from 0.2 to 0.67 km, duobinary RZ,
modified duobinary RZ, and 4Qam-OFDM demonstrate acceptable BER values. Specif-
ically, at a link range of 0.64 km, the achieved BER values are 10−11 for duobinary RZ,

Fig. 19  Ber versus link range for light dust

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Performance analysis of FSO communications in desert… Page 19 of 21 659

Fig. 20  Ber versus link range for moderate dust

4 × 10−7 for modified duobinary RZ, and 7 × 10−7 for 4Qam-OFDM. However, at a link
distance of 0.7 km, 4Qam-OFDM yields a BER equal to 4 × 10−3.
Figure 21 illustrates the relationship between optical received power (in dBm) and link
range in the context of moderate dust (50 dB/km). Notably, within link distances ranging
from 0.2 to 0.8 km, the 4Qam-OFDM modulation technique stands out by providing the
most favorable optical received power values. For example, at a link range of 0.5 km, the
optical received power values are − 11 dBm for duobinary RZ, − 13 dBm for modified
duobinary RZ, and − 8 dBm for 4Qam-OFDM.

7 Conclusion

This research focuses on the development of FSO communication systems in desert

regions, particularly in duststorms and fogs. The study evaluates various modulation tech-
niques, with a particular emphasis on Quadrature Amplitude Modulation-Orthogonal Fre-
quency Division Multiplexing (QAM-OFDM). The results demonstrate that the 4QAM-
OFDM modulation technique is highly effective in different fog conditions. It maintains
an acceptable bit error rate (BER) in light fog, 0.5 to 1.5 km, and even at extended link

Fig. 21  Received power versus link range for moderate dust

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659 Page 20 of 21 M. Mrabet, M. Sliti

distances. It achieves a BER of 2 × 10−5 at 2.4 km. In moderate fog, it achieves a BER of
6 × 10−5 at 1.75 km. In heavy fog, it maintains acceptable BER values within 0.5−0.8 km
and a BER of 5 × 10−5 at 1.35 km. In moderate dust, specifically at a link range of 0.64 km,
the achieved BER value is 7 × 10−7. 4Qam-OFDM provides the most favorable values of
received power compared to other modulation techniques. As perspective, we can study
more advanced modulation methods, such as Polarization Division Multiplexing (PDM)-
QAM-OFDM. This could make the system more robust and reliable, opening up more
ways to improve FSO communication in a range of difficult weather conditions.

Funding The authors extend their appreciation to Prince Sattam bin Abdulaziz University for funding this
research work through the Project Number (PSAU/2023/01/25098).

Data availability The data used and/or analyzed during the current study are available from the correspond-
ing author on reasonable request.

Declarations
Conflict of interest The authors declare no competing interests.

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