Optica Applicata, Vol. LIII, No.

3, 2023
DOI: 10.37190/oa230308

Evaluation of a high-speed
hybrid OAM-OFDM-MDM multiplexed
coherent FSO system under desert conditions
SHIVAJI SINHA, CHAKRESH KUMAR*

University School of Information, Communication & Technology,

Guru Gobind Singh Indraprastha University,
Sector - 16 C, Dwarka, New Delhi - 110078, India
*Corresponding author: ckumardhan@gmail.com

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid
multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital
angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing
(OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadra-
ture (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplex-
ing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its
capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is
detected using a coherent detection-based digital signal processing (DSP) algorithm at the re-
ceiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments
using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical
power performance matrices. The simulation results demonstrate the successful transmission of
a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low
and heavy dust weather environments below the signal degradation threshold value (forward error
correction (FEC) limit) of BER 2.2 × 10–3 in strong turbulent conditions.

Keywords: atmospheric attenuation, bit error rate (BER), orbital angular momentum (OAM), optical
signal-to-noise ratio (OSNR), orthogonal frequency division multiplexing (OFDM), quadra-
ture amplitude modulation (QAM), transmission range.

1. Introduction
The increasing demand for cost-effective high-speed data streaming due to exponential
growth in end users has been emerging as a big challenge for conventional radio fre-
quency (RF) and microwave transmission networks due to a lack of infrastructure, last
-mile challenges, and regulatory issues. Due to the growth of data traffic, it is estimated
that the RF spectrum will be exhausted in the next few years. However, the use of the
optical spectrum will have a significant potential to meet the constantly expanding de-
mand for this data traffic. Free-space optical (FSO) communication, which offers very
432 S. SINHA, C. KUMAR

huge data rates (in Gb/s) in the optical domain, has been found to be an alternative
wireless technological solution that can overcome all of the above discussed challenges
by providing quick infrastructure installation, abundant license-free bandwidth, and
high-speed, secure data transmission [1-4].
The major challenges in optical signal transmission through the free space channel
are the atmospheric attenuation due to molecular absorption, scattering, and the scin-
tillation phenomenon, which highly degrade the transmission link. The performance
of the high speed FSO link has been investigated under various atmospheric conditions
such as heavy rain, smoke, snow, fog, and dust by the various research groups. Various
solutions have been proposed to enhance the data rate of the FSO link in previously
discussed weather conditions [5-9].
Recently, the work in [10] investigated the transmission of 10 Gb/s-60 GHz hybrid
fiber/FSO link using orthogonal division multiplexing (OFDM) and incorporating
doublet lens technique to enhance the transmission link and reduce the multipath fading
and dispersion effect for 50 km fiber and 0.5 km FSO links with good BER for 16-QAM
modulation. MUKHERJEE et al. presented a cost effective and secure 88 Gb/s FSO sys-
tem over 0.550 km link using 4-PPM with self-injection locked-quantum dash laser
diode (QD-LD) and Reed–Solomon (RS) channel encoding which help in decreasing the
power penalty [11]. DUTTA et al. studied the hybrid orbital angular momentum and wave-
length division multiplexing (OAM-WDM)-based FSO system to realize 100 Gb/s
FSO links over 3.2 km under clear weather, haze, rain, and fog weather conditions.
A very low BER of 5 × 10–9 and a good quality factor (Q) of 5.4 are obtained for
a 2.4 dB power penalty [12]. A 640 Gb/s data transmission over 180 m link distance
is demonstrated by multiplexing of 4 OAM modes in FSO system. A 640 Gb/s data
transmission over a 180 m link distance is demonstrated by the multiplexing of four
OAM modes in the FSO system. The remarkable results have been achieved with a very
low 2.91 dB power penalty at a BER of less than 10–9 and an excellent Q-factor more
than 5.5 [13]. ABD EL-MOTTALEB et al. has reported the performance of the spectral
amplitude coding-optical code division multiple access (SAC-OCDMA) system by
employing enhanced double weight (EDW) signature codes and multiple access inter-
ference (MAI) techniques. The implementation of MAI in the proposed SAC-OCDMA
system leads to degradation in system performance, which is further resolved by using
a single photodiode detection technique (SPD). A comparison between the EDW and
modified double weight (MDW) codes is also demonstrated in terms of BER, Q-factor,
and received optical power [14]. Recently, the hybrid SAC-OCDMA-MDM FSO sys-
tem has been investigated to increase the system capacity in which a 100 Gb/s data is
transported over 8 km turbulent link by using two Laguerre–Gaussian (LG) modes and
detected at the receiver by direct detection method [15]. SIVARAJANI et al. achieved an
improvement in data rate of about 4 Gb/s with a power of 25 dBm in a FSO system
considering an underground moving train environment for a maximum link range of
1.4 km using performance metrics of BER, OSNR, Q-factor, and received optical pow-
er [16]. Dense wavelength division multiplexing (DWDM) is a bandwidth efficient
solution which increases the capacity of the FSO link. KUMAR et al. successfully
Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system... 433

demonstrated the implementation of a 5 × 16 Gb/s DWDM FSO system with a frequen-

cy spacing of 100 GHz under clear sky conditions for a link of 38 500 km. The results
showed that with increasing transmitted optical power, the Q-factor also increases,
while it decreases with increasing atmospheric turbulence effects [17].
Recently, many researchers have focused on the space division multiplexing (SDM)
technique, in which the spatially distributed intensity pattern of the optical carrier signal
is used to carry binary information using orbital angular momentum (OAM). The laser
beam carrying a QAM signal possesses two distinct characteristics: inherent orthogo-
nality and unbounded states in principle. These features are very useful to increase the
FSO system bandwidth and the information carrying capacity by transmitting multiple
information streams simultaneously using QAM multiplexing [18].
In this paper, a new high-speed and spectral-efficient hybrid FSO system has been
proposed by incorporating the features of OAM optical beams carrying two Laguerre
–Gaussian (LG) modes LG0,0 and LG0,13, respectively. The main objective of this paper
is to achieve a data rate of 120 Gb/s, by integrating the hybrid multiplexing (OAM,
OFDM and MDM) techniques under different dust storms weather conditions with
an intensity scintillation effect. The two OAM optical beams carry independent data
stream simultaneously without any crosstalk whereas OFDM and the single carrier
16-QAM format help out to achieve high data rate with robust signal transmission.
The system performance is investigated using BER, link distance, OSNR and eye di-
agram.
The rest of the paper is structured as follows: Section 2 illustrates the proposed
model and describes the working principle of the transceiver system in detail. Sec-
tions 3 and 4 explains the mathematical analysis of the FSO channel and losses incurred
due to strong storm conditions. Finally, the system performance is investigated using
the simulation results in Section 5, followed by the conclusions drawn in Section 6.

2. Proposed schematic diagram and working principle

The design schematic of the proposed 16-QAM modulated OAM-OFDM-MDM free
space optical link has been discussed in this section. The system comprises of optical
transmitter system, atmospheric FSO channel and coherent homodyne receiver as il-
lustrated in Fig. 1.
The optical transmitter section consists of a pseudorandom bit sequence (PRBS)
generator used to generate binary information, which is then converted to a parallel
binary stream for serial-to-parallel (S/P) block output. These parallel binary sequences
are then encoded in non-return to zero binary signal in NRZ block and then converted
to OFDM modulated signal. The output from this block is fed to the OAM polarized (OP)
16-QAM modulator system. The laser beam of continuous wave (CW) spatial laser
splits into two independent OAM polarized beams, LG0,0 and LG0,13, respectively.
These two optical carriers further transport the in-phase and quadrature phase (I/Q)
components of OP 16-QAM modulated signal. The OP 16-QAM modulator block
composed of the M-ary pulse generator, lithium niobate Mach–Zehnder modulator
434 S. SINHA, C. KUMAR

Fig. 1. Schematic and simulation setup demonstrating the OAM-OFDM-MDM FSO system.

(LiNbO3-MZM), dual-drive PRBS source and 90° phase-shifter as presented in Fig. 2.

The relative phase difference in both the upper and lower arms is adjusted to 90°.
The cross-coupler output consists of optically modulated 16-QAM signal with a bias
of 4 V and – 4 V. The sequence generator maps the incoming binary data into 4 bit
16-QAM symbols. In OFDM block, 12 training symbols and 8 pilot sub-carriers are
Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system... 435

Fig. 2. Structure of OAM polarized internal 16-QAM optical modulator.

adopted for the carrier phase estimation (CPE) and FSO channel estimation. The power
coupler (MDM) block combines these two OP 16-QAM signals, and the output is
amplified in erbium-doped fiber amplifier (EDFA) to compensate for the attenuation
provided by the turbulent free space channel and finally transmits over the turbulent
channel under strong storm weather conditions.
At the receiver end, the received optical modes are isolated and fed to the upper
and lower sections of the OAM polarized coherent homodyne receiver, respectively.
The front-end section of the demodulator section shown in Fig. 3 performs different

Fig. 3. Structure of OAM polarized 16-QAM homodyne optical receiver.

436 S. SINHA, C. KUMAR

operations such as A/D conversion, dispersion compensation, cyclic prefix removal,

channel estimation, and CPE. The balanced photo-detectors (BD), 90° optical hybrid,
electronic amplifiers (EA), and a decision block are the major components of the co-
herent homodyne detector as shown in Fig. 3. Balanced photo-detector unit is used to
minimize the intensity noise fluctuations in two branches by the subtraction of the
photo-detector currents. The digital signal processing (DSP) block comprises of
down sampling, equalizer, resampling and Bessel filter unit. The blind phase search
(BPS) algorithm is implemented in DSP unit to minimize the phase difference between
the optical transmitter and receiver [19, 20]. The optical intensity variation in the lower
and upper branches are cancelled out by the balanced photo-detectors. This next
homodyne section recovers the M-ary pulses for both OAM optical modes and detects
the transmitted binary information.
The modal analysis of the QAM optical beam, the mathematics of the turbulent
FSO channel, and the different dust storm levels are discussed in the next section.

3. Modal analysis of OAM beam (Laguerre – Gaussian mode)

The helical phase front structure of an OAM optical beam is expressed by exp ( jn θ ),
where n is any integral value of 2π phase shifts in the beam phase profile and θ is the
azimuthal angle, respectively. These beams can form an infinite number of orthogonal
states when they propagate in a free space channel. Figure 4, shows the donut shaped

Fig. 4. Intensity and phase structure of donut shape OAM beams.

Fig. 5. Intensity and phase structure of Laguerre – Gaussian (LG00 and LG10) OAM beam.
Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system... 437

intensity and phase profile of the OAM beam for two states (n = 2 and n = 3), respec-
tively. Whereas Fig. 5 illustrates the 2-D intensity and phase profile of the Laguerre
–Gaussian beams carrying OAM signals considered for our proposed system.
The electric field of the LG, OAM beam can be represented mathematically in cy-
lindrical coordinate as
n
2 p! 1 r 2 2r 2
E  r θ z n p  = ---------------------------------- ----------------- ----------------- L p n -----------------
-
π  p + n ! ωz ωz ω2  z 

–r 2 j r 2 cz –1 z
 exp ------------------- – ------------------------------ exp j  2p + m + 1  tan ---------- exp  – jn θ 
ω z2 2
2  z + zR  2 zR
(1)
where r, θ and z represent radial coordinates, angular coordinates, and beam propaga-
tion direction, respectively, and ω (z ) and zR represent beam waist size and Rayleigh
range, respectively. Here p = 0 or 1 and n = 0 are the modal indices in the r and θ di-
rections for LG (Lp, n ) polynomial, respectively chosen to increase the capacity of the
proposed system using MDM. The reason of choosing only two lower order modes is
the complexity in OAM multiplexing and de-multiplexing and the higher amount of
intermodal crosstalk. The wave number is shown by c.

4. Sand/dust storm characteristics and optical link attenuation

The channel characteristics of the FSO channel are dynamic with respect to time, and
deteriorates the optical link performance under different weather conditions such as
rain, haze, fog, storm, etc. These conditions can produce atmospheric attenuation, ex-
pressed by Eq. (2). The strength of this attenuation is determined by the cross-sectional
dimensions, particle density in the channel, and transmitting wavelength.

 1 
L atm  dB/km  = 10 log --------- (2)
 Ta 
where Ta shows the atmospheric transmittance. Whereas the largest free space path loss
is expressed by
–q
 λ 
L FS  dB/km  = --------------- (3)
 4πR 
Sand and dust storms are hard weather meteorological conditions that will interfere
with the wireless optical link. They are caused by high wind velocity, which pulls
ground dust particles from the ground into free space. These dust storms are classified
as “light dust (LD)” and “heavy or moderate, and severe dust (SD)”, respectively [21].
438 S. SINHA, C. KUMAR

T a b l e 1. Visibility and attenuation for different type of dust [22].

Dust type LD MD SD
Visibility [km] 1–10 0.2–1 <0.2
Attenuation [dB/km] 25.11 107.66 297.38

Table 1 presents the visibility and the attenuation levels for different classes of dust
storms. These dust storms are responsible for link degradation during the light beam
transmission through the FSO channel. This dust attenuation depends on the link dis-
tance R, which can be given by
– 1.05
L dust  dB/km  = 52  R (4)

The irradiance intensity due to the scintillation phenomenon within the cross-sec-
tion of the detector plane is respectively defined in terms of scintillation index σ 2I [23]
and the received optical power at the receiver end [24] by the following equations:

 I 2  –  I 2
σ 2I = ---------------------------------- (5)
 I 2

– α R / 10  D 2R 
Pt = 10 Pr -------------------------- (6)
 D T + φR 
where, the transmitter and receiver aperture diameters are DT (m) and DR, respectively,
and R is transmission link range (km). Here, φ (mrad) represents the divergence angle,
and α is the attenuation coefficient in dB/km.
The Gamma-Gamma probability distribution function, which expresses the prob-
ability of the given optical intensity at the receiver end, is used to model atmospheric
fading and given by
α+β
------------------
2 α+β
2α β ------------------ – 1
P  I  = ---------------------------------- I 2 K α – β  2 α βI  (7)
Γ α Γ  β
where α and β represent the small-scale and large-scale turbulence parameters, respec-
tively, and K(ꞏ) is the modified Bessel function of the second kind [25].

5. Simulation results and discussion

The proposed system performance is elucidated in this section and tested in terms of
performance metrics BER with respect to different link distances, OSNR, and received
optical power. The proposed system is evaluated under light and heavy sand or dust
storm conditions assuming a fixed strong air turbulence scenario (5 × 10–13 m–2 / 3) to
understand the degradation of the BER and to maximize the system capacity. The signal
Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system... 439

T a b l e 2. System simulation parameters for the proposed link.

System parameters Values

Spatial laser power 20 dBm
Bit rate 120 Gb/s
Baud rate 15 Gb/s
Sequence length 65536
Laser wavelength 1550 nm
Tx/Rx laser linewidth 0.01 MHz
Tx/Rx aperture diameter 10 cm
Beam divergence 0.20 mrad
Optical receivers losses 0 dB
Refractive index structure 5 × 10–13 m–2 / 3
PD responsivity 1 A/W
Detector dark current 10 nA
Thermal noise power density 10–22 W/Hz

attenuation due to large-size objects like trees, sign boards, buildings, geometric and
misalignment losses and other weather conditions such as fog, rain, etc., is assumed
to be ideal during the simulation process. The PRBS block generates a sequence length
of 217 -1 with random seed generation at the transmitter end whereas, the spatial laser
produces a 20 dBm optical power at a wavelength of 1550 nm. The simulation param-
eters chosen for the proposed system are described in Tables 1 and 2, respectively. Both
the LiNbO3 MZM modulators are fixed at extinction ratio of 25 dB at the null point
of operation.
A discrete time delay is also introduced to model propagation delay, and the channel
fading is assumed to be constant for the coherence time of 5.22× 10– 6 s and changes

20μ 1

800 m
10μ

600 m
y (m)

0
400 m

-10μ
200 m

-20μ
0
-20μ 0 20μ
x (m)

Fig. 6. Spatial intensity profile of MDM multiplexed OAM beam.

440 S. SINHA, C. KUMAR

from one frame symbol to another frame for the proposed system. A user defined
OSNR FEC limit is also set in each simulation graph to evaluate the link performance.
Figure 6, shows the simulated spatial intensity profile of the transmitted MDM multi-
plexed OAM beams used in the proposed FSO system.
The BER performance is illustrated for different links as shown in Figs. 7–10 under
different dust conditions for our proposed model system. On comparing the results, it is

Fig. 7. BER vs. OSNR for different link range under LD condition.

Fig. 8. BER vs. OSNR for different link range under MD condition.
Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system... 441

observed that under low dust conditions, the maximum link range is achieved. The sys-
tem operation cost can be minimized by optimizing the OSNR performance and max-
imizing the data transmission rate. Also, this OSNR tolerance level increases linearly
and almost doubles with respect to data transmission rate [26].
Figures 7 and 8 illustrate the BER vs. OSNR performance comparison of the
120 Gb/s OAM polarized and 16 QAM modulated proposed homodyne FSO system
for various transmission distances under different dust environments. The log (BER)
values are decreasing exponentially with the increase in OSNR ranges from 4 to 30 dB.
As shown in both the figures, to maintain the acceptable BER FEC limit less than
2.2 × 10–3, the required OSNR is 21.9 dB for link distance of 1.5 km whereas it is
18.76 dB for link distance of 0.44 km under low and heavy dust scenario.
As illustrated in Fig. 9, the minimum link distances achieved for these OSNR under
low and heavy dust conditions are 0.9 and 0.25 km, respectively, whereas the maxi-
mum link distance achieved for both the conditions are 1.5 and 0.40 km, respectively.
The obtained result clearly demonstrates that under heavy dust condition, the system
has the lowest propagation distance of 0.25 km due to the highest attenuation of
107.66 dB in the free space channel caused by the heavy dust.
The receiver sensitivity for the proposed system is demonstrated in Fig. 10, where
the two dust scenarios are compared in terms of the received optical power at the re-
ceiver end. The simulation graph clearly shows the variation of log (BER) with respect
to received optical power under two assumed dust weather conditions. As the received
optical power increases from –40 to –4 dBm, the log(BER) performance also improves
exponentially from – 0.5 to – 4.4 due to the least amount of error during the detection
process. The receiver sensitivity archived for the proposed system is greater than

Fig. 9. BER vs. optical link range (in km) for low and heavy dust conditions.
442 S. SINHA, C. KUMAR

Fig. 10. BER vs. optical receiver sensitivity for the proposed system.

– 17.8 dBm. For the BER FEC limit less than 2 × 10–3, the received optical power in
low dust conditions is –14.9 dBm, whereas under heavy dust, it is –18.3 dBm, which
clearly demonstrates 4 dBm better performance by the proposed system under low dust
environments compared to heavy dust. The system possesses a receiver sensitivity of
–17.8 dBm.
Table 3 illustrates and compares the work implemented in this paper with the ex-
isting published work. The majority of the high-speed FSO system is tested in rain,
fog, and haze. Very few researchers have focused on different desert weather condi-

T a b l e 3. Performance comparison of the proposed system with existing literature.

Channel Attenuation Laser power Maximum Range

condition [dB/ km] [dBm] data rate [Gb/s] [km]
Rain 15 3
Hybrid PDM/OFDM [27] 10 5
Fog 10 2.3
Heavy rain 19.28 0.16
SAC-OCDMA-OAM [28] Heavy haze 10.115 15 120 0.2
Heavy fog 22 0.15
Hybrid SAC-OCDMA Heavy haze 2.4 4
15 100
FSO-MDM [29] Heavy rain 19.3 8
Light fog 9 3.6
Hybrid OFDM-MDM [30] Moderate fog 12 10 80 3
Heavy fog 16 2
Light dust 25.11 1.5
Present work 15 120
Heavy dust 107.66 0.4
Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system... 443

tions. Although the attenuation level provided by desert conditions is nearly identical
to that provided by fog weather performance, it is assumed to be higher for the proposed
system. Taking this into account, our work outperforms the reference [28] in terms of
link distances for the same transmission rate and higher attenuation levels. Compara-
bly, our proposed system shows a higher data transmission rate for the higher side of
weather attenuation levels.

6. Conclusion
In this work, we have presented and analyzed the OAM polarized 16 QAM-OFDM
-MDM FSO coherent system under the low and heavy dust environment conditions.
The system uses the two LG0,0 and LG0,10 QAM modes to carry the two independent
16-QAM and OFDM modulated data streams to enhance the system capacity. These
streams are further multiplexed using MDM and transmitted over the different dust en-
vironments. At the receiver end, a DSP based coherent receiver structure is implement-
ed to combat different signal impairments. The presented system supports a 120 Gb/s
data rate at an acceptable optical SNR and maximum transmission ranges 1.5 km, and
0.40 km in low dust and heavy dust scenario, respectively. The system improvement
can be further investigated for higher values of M-QAM modulation under other dif-
ferent conditions such as fog, haze, etc. Spatial diversity and channel coding techniques
can be included in future. The proposed system can be a solution to support the next
generation of high speed and high-capacity broadband services at a much-reduced cost.

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Received November 21, 2022

in revised form December 8, 2022