Optical and Quantum Electronics(2024) 56:1367
https://doi.org/10.1007/s11082-024-07279-6

A bidirectional free space optical link for last‑mile terrestrial
access links employing a novel wavelength shift keying
technique
Changjia Qu1 · Salman Ghafoor2 · Arsalan Ahmad2 · Jawad Mirza3 ·Abdulah Jeza Aljohani4 · Imran Aziz5
Received: 15 April 2024 / Accepted: 15 July 2024
© The Author(s) 2024

Abstract
In this study, we propose a novel wavelength shift keying (WSK) technique that is combined with the conventional intensity modulation scheme for the transmission of point-topoint bidirectional data at the rate of 10 Gbps in each direction. We observe that WSK
technique has not been investigated for implementation in point-to-point free space optical(FSO) links. Therefore, to the best of our knowledge, our study is the first one to perform
this investigation. Our proposed link uses WSK in the downlink direction while we reuse the optical carriers for the transmission of uplink data. The use of WSK in the downlink direction enables us to perform balanced detection at the receiver, resulting in 3 dB
improvement in receiver sensitivity compared to simple direct detection. We present bit
error rate (BER) results for the signals transmitted in both the directions under different turbulence conditions and FSO link lengths. It was observed that the downlink signals generally perform better compared to the uplink due to the use of balanced detection
and higher intensity fluctuations induced over the re-used optical carrier transmitted in the
uplink direction.
Keywords Wavelength shift keying · Free space optical communication · Wirelesscommunication · Beyond 5G

Changjia Qu, Salman Ghafoor, Arsalan Ahmad, Jawad Mirza, Abdulah Jeza Aljohani, Imran Aziz havecontributed equally to this work.
* Imran Aziz
imran.aziz@physics.uu.se
1

Department of Network Engineering, School of Information and Communication Engineering,
University of Electronic Science and Technology of China, Chengdu, China

2

National University of Sciences and Technology (NUST), H‑12, Islamabad, Pakistan

3

SEECS, Photonics Research Group, Islamabad, Pakistan

4

Department of Electrical and Computer Engineering, King Abdulaziz University, Jeddah,
Saudi Arabia

5

Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden

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1 Introduction
As 5G networks continue to roll out and mature, researchers and industry experts are
already looking ahead to the next generation of wireless communication technology, often
referred to as 6G. 6G is expected to offer even higher data rates, lower latency, greater
reliability, and more efficient use of spectrum compared to 5G (Sun et al. 2024). Theseadvancements could lead to new possibilities for last-mile wireless access networks, enabling immersible virtual reality (VR), augmented reality (AR), holographic communication, and other bandwidth-intensive applications (Shafique et al. 2021). Furthermore, inrecent years, autonomous vehicles (AVs) equipped with photonic radars have emerged as
intelligent modes of transportation, capturing the attention of researchers and innovators
globally (Chaudhary et al. 2024). Photonic radar utilizes light waves in free space to detectobjects and measure their distance, velocity, and other characteristics. This technology canbe seamlessly integrated with 5G networks, offering enhanced and precise environmental
awareness for both autonomous vehicles and IoT devices (Chaudhary et al. 2024). Looking
beyond 5G communication systems, the future of last-mile access networks holds promise for even more advanced technologies and innovative solutions to meet the evolving
demands of users and applications (Singh 2023). Last-mile terrestrial access links referto the final segment of telecommunication networks that connect end-users or subscribers
to the core network infrastructure. These links typically traverse relatively short distances,often extending from a local exchange or central office to individual homes, businesses, orother end-user locations. They are called "last-mile" because they represent the final leg ofconnectivity before reaching the end-user (Singh 2023).
Passive Optical Networks (PONs) are a type of fiber-optic network architecture commonly used for delivering high-speed broadband services to end-users, such as residential homes, businesses, and institutions (Bekkali et al. 2022). PONs rely on passive optical components, such as optical splitters and combiners, to enable data transmission overfiber-optic cables without the need for active electronic components, such as repeaters or
signal amplifiers, in the distribution network. With the proliferation of high-bandwidth
applications such as video streaming, online gaming, cloud services, and IoT devices,
there’s a significant increase in the demand for broadband bandwidth (Dos Santos et al.
2018). PONs can provide the necessary bandwidth to meet these demands. Modern PON
technologies such as Gigabit PON (GPON) and 10 Gigabit PON (10GPON) offer impressive downstream and upstream bandwidth capabilities. For instance, GPON can provide
downstream speeds of up to 2.5 Gbps and upstream speeds of up to 1.25 Gbps per opticalline terminal (OLT) port. 10GPON further increases these speeds to 10 Gbps downstream
and 2.5 Gbps upstream per port (Zhang et al. 2020). Additionally, advancements in PON
technology, such as wavelength division multiplexing (WDM), time division multiplexing(TDM), and higher-split ratios, further enhance the capacity and efficiency of PONs, making them capable of meeting the growing bandwidth demands of users for the foreseeable
future (Xu et al. 2018).
The growing demand for high-performance 5G networks necessitates a deeper
penetration of optical signals within access networks, particularly for inter-connectivity
between small cell base stations (Xu et al. 2018). However, deploying fiber optics
can be cost-prohibitive in certain scenarios. For example, as shown in Fig. 1, rapidly
augmenting network capacity for temporary events or serving sparsely populated
remote areas may not justify the high upfront cost of fiber installation. These challenges
necessitate alternative wireless access solutions (Rodrigues et al. 2023). Traditionally,

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Fig. 1  Application scenario for our proposed design FSO-Bidirectional link to enhance access network
capacity for an event

fixed microwave links have been employed in such situations. However, their capacity islimited due to spectral congestion of the radio frequency band. Transitioning to higher
frequencies reduces the range of the signal due to higher atmospheric attenuation.
Furthermore, microwave point-to-point links are susceptible to heat generation andstruggle to deliver multi-gigabit capacities. In these scenarios, FSO technology offers acompelling alternative (Qu et al. 2022). FSO operates on a relatively simple principle:
a high-powered laser source is modulated by electrical data, resulting in the generationof light pulses, which are then collimated by a lens system before transmission throughthe atmosphere. The transmitted pulses are received by a dedicated lens system andconverted back into electrical data by a high-sensitivity photodetector. Essentially,
FSO leverages air as the transmission medium instead of optical fiber. For enhanced
eye safety and lower atmospheric attenuation, FSO systems typically employ lasers
operating at an infrared wavelength of 1550 nm. Leveraging the benefits of extensive
coverage and low-latency broadband in a highly cost-effective way, the FSO based
space-air-ground integrated network is considered a promising solution. Space-airground integrated network supports massive connections, ensures high reliability, andfacilitates seamless communication, surpassing the capabilities of traditional terrestrial
mobile communication systems (Qu et al. 2022).
The choice of modulation technique in Free-Space Optical (FSO) communication
is dependent upon the specific application. Each scheme offers unique advantages anddrawbacks, particularly in terms of spectral efficiency (data rate per unit bandwidth)and energy efficiency. Like any other communication system, spectral efficiency is paramount in FSO systems as it directly influences the speed of electronic switching circuits,
despite the vast bandwidth available in the unlicensed optical band (Yue et al. 2020).
Accurate knowledge of real-time channel characteristics is crucial for optimal signal
detection at the receiver. This can be achieved through a limited number of pilot carriers
for high-fidelity channel state estimation (Yue et al. 2020). Alternatively, symbol-bysymbol maximum likelihood detection can be employed, leveraging partial knowledgeof channel fading characteristics and statistical analysis of joint temporal fading coefficients (Le and Kim 2019). On-Off Keying (OOK) is a prevalent modulation scheme in
FSO systems, utilizing binary data (0 or 1) (Aljohani et al. 2021). However, OOK’s limitations in spectral and energy efficiency make it less suitable for ultrafast data transmission and complex system architectures. In such scenarios, intensity modulation schemes
like Pulse Position Modulation (PPM) or its variant, Variable PPM, are preferred, especially for energy-constrained applications such as deep space communications (Ghafoor

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et al. 2023). PPM eliminates the need for a dynamic threshold for signal detection andoffers advantages in specific situations. Pulse Width Modulation (PWM) demonstrates
improved spectral efficiency, lower peak transmit power, and superior performancein handling inter-symbol interference (ISI) compared to PPM (Ebrahimi et al. 2018).
However, PWM requires an additional guard interval to prevent overlapping positive
pulses. Conversely, Digital Pulse Interval Modulation (DPIM) offers superior spectral
efficiency compared to PPM and PWM, operating asynchronously and eliminating theneed for symbol and slot synchronization.
In this study, we investigate the implementation of a relatively less researched modulation scheme known as wavelength shift keying (WSK). Binary WSK utilizes two different
wavelengths of light to transmit ones and zeros within the digital data stream (Hung et al.
2004), Chi et al. (2008). WSK has not been previously demonstrated for terrestrial FSO
links. Therefore, we propose a novel technique for implementing WSK in FSO links for the
point-to-point access link depicted in Fig. 1. A major disadvantage of WSK is the use of
two optical sources for the transmission of OOK data. We have overcome this disadvantage
by reusing the downlink transmitted WSK signal for the transmission of uplink data. As
shown in Fig. 1, Node-1 transmits the WSK modulated signals towards Node-2. At Node2, the received WSK signal is divided into two parts, one for extracting the downlink data,
while the other part is remodulated for transmission of uplink data. This approach enables
centralized light sources and cost-efficient link design. The details of the proposed technique are discussed in the following sections. We have used the commercial tool known as
Optisystem to simulate the WSK based FSO link.

2 The proposed WSK link
The setup of our proposed bidirectional free-space optical (FSO) link utilizing wavelengthshift keying (WSK) is depicted in Fig. 2. Two independent optical sources, each generating
Gaussian pulses with a center wavelength of approximately 1550 nm and 1554 nm, are
combined using a power combiner. These sources operate at a repetition rate of 10 GHz
and produce pulses with a duration of 25 picoseconds. The choice of these two wavelengths

Fig. 2  The setup of the proposed WSK based bidirectional FSO link. PRBSG: Pseudo Random Bit
Sequence Generator, NRZ-PG: NRZ-Pulse Generator, OBPF: Optical bandpass filter, OGPG: Optical
Gaussian Pulse Generator, MZI: Mach-Zehnder Interferometer, PC: Power Combiner, PS: Power Splitter,
PD-PIN: Photo-detector-PIN, LPF: Low Pass Filter, OA: Optical Amplifier, FSO: Free Space Optics, PM:
Phase Modulator, X-Coupler: Cross Coupler, MZM: Mach-Zehnder Modulator

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and the pulse width should be such that the two optical signals do not overlap spectrally
when combined. The choice of the pulse width is also dependent upon the data rate aswell as the length of the FSO link. We will use a data rate of 10 Gbps, therefore, the pulse
duration should be less than 100 ps. Furthermore, it has been observed that pulses of
longer durations travel longer distances over the FSO link due to higher energy (Aljohani
et al. 2021; Ghafoor et al. 2023). The combined signal is then split into two separate paths.
One path is modulated by a phase modulator (PM) before being directed to one of the
inputs of a 2x2 directional coupler (X-coupler). The other path is directly connected tothe other input of the X-coupler. The downlink data, carrying information at a rate of 10
Gbps, is generated using a pseudo-random bit sequence (PRBS) generator (refer to Fig. 2).
The output of this generator is converted into a non-return-to-zero (NRZ) electrical signal,
which is subsequently used to drive the PM. The combination of an optical coupler, a
PM, and an X-coupler effectively forms a Mach-Zehnder interferometer (MZI) structure.
As illustrated in Fig. 2, the upper and lower inputs of the X-coupler are designated as"Input Upper" ( InU ) and "Input Lower" ( InL ), respectively. These inputs are related tothe corresponding upper and lower outputs ("Output Upper" (OutU ) and "Output Lower"(OutL )) of the X-coupler through a specific mathematical relationship mentioned as follows
Ghafoor et al. (2023):
� �� �
��
OutU
1 1 j InU
= √
(1)
OutL
2 j 1 InL

To encode the data stream, the phase modulator (PM) introduces a specific phase shift.
When transmitting a "one" (bit 1), a 𝜋∕2 phase shift is applied to the combined opticalsignal. Conversely, for a "zero" (bit 0), no phase shift occurs. This manipulation leverages
the operating principle of the directional coupler. As discussed in Ghafoor et al. (2023), aphase-shifted signal exits the upper output port of the coupler, while the unshifted signal
exits the lower port. At the upper output port, an optical bandpass filter (OBPF) with acentral wavelength of approximately 1550 nm is utilized. Similarly, at the lower output
port, another OBPF centered at 1554 nm is used. This filtering scheme ensures that for a"one" (bit 1), the pulse with a wavelength of 1550 nm is transmitted, while for a "zero" (bit0), the pulse at 1554 nm is transmitted. Both filters utilize a Gaussian profile and possess abandwidth of 50 GHz each. Subsequently, the outputs from these two filters are recombinedusing a power combiner (PC). The combined output undergoes amplification by an opticalamplifier (OA). This amplifier exhibits a noise figure (NF) of 4 dB and provides a gainof 20 dB, as illustrated in Fig. 2. The amplified signal is then transmitted over the freespace optical (FSO) channel, which is modeled using the Gamma-Gamma channel model
as described in Aljohani et al. (2021). The channel length is varied within the study, as
detailed in the results section, to analyze its impact on the bit error rate (BER) performanceof the transmitted signals.
After transmission over the FSO link, the signal is amplified and connected to a power
splitter (PS), as shown in Fig. 2. One of the output of the splitter are connected to OBPFs
having similar parameters as the OBPFs at the transmitter side. One output port of the PS
is connected to a power meter (PM) for observing the power of the received optical signal.
At the output of the OBPF centered at 1550 nm, a OOK signal representing ones in thedata stream is obtained. Similarly, at the output of the OBPF centered at 1554 nm, a OOK
signal representing zeros of the data stream is obtained. This means that for all the zero
bits, an optical pulse centered at 1554 nm is present, while for the one bits, there is no
optical pulse. Both these pulsed signals at the output of the OBPFs are split into two paths

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using a PS, as shown in Fig. 2. At this stage, we can retrieve the transmitted data stream
from the optical pulsed signal centered at 1550 nm, since it represents all ones in the data
stream. However, to achieve a 3-dB sensitivity gain at the receiver, we employed balanced
detection, akin to detecting a differential phase shift keying (DPSK) signal (Ghafoor et al.
2023). In our implementation, pulsed optical signals at 1550 nm and 1554 nm, representing
ones and zeros of the data stream, respectively, were directed through splitters. Each splitter output was photodetected using PIN photodiodes (PDs). The electrical outputs fromthe photodiodes were then combined with reversed polarity for the electrical signal representing zeros. Subsequently, the combined electrical signal underwent filtering through alow-pass filter (LPF) to eliminate higher-order harmonics. As demonstrated in the results
section, the resulting eye diagrams closely resemble those of a DPSK signal detected usingbalanced detection (Ghafoor et al. 2023).
Prior to photodetection, a 1 × 2 power splitter (PS) divides the received optical signals
centered at 1550 nm and 1554 nm into two paths each (refer to Fig. 2). One path fromeach splitter is used for data extraction. The remaining outputs from the PS are combinedusing a power combiner (PC). This combined optical signal at the PS output consistently
contains a pulse within each time slot due to its composition of pulses representing both"ones" and "zeros." However, the pulses corresponding to "ones" are centered at 1550 nm,
while those representing "zeros" are centered at 1554 nm. We leverage this combined optical pulsed signal as the source for transmitting uplink data from Node-2 to Node-1 in thesystem depicted in Fig. 1. As shown in Fig. 2, the signal is directed to the optical input of
a Mach-Zehnder modulator (MZM). A pseudo-random bit sequence generator (PRBSG)
generates an NRZ electrical signal representing the uplink data. This electrical signal isthen connected to the MZM’s electrical input, thereby modulating the reused optical pulsed
signal. The resulting modulated optical signal is transmitted over a free-space optical(FSO) link identical to the downlink FSO link, enabling simulation of bidirectional transmission between the two nodes. The uplink signal received at Node-1 undergoes demodulation and is subsequently provided to a BER analyzer for visualization of eye diagrams
and analysis of the relationship between bit error rate (BER) and received optical power.
A comprehensive summary of all key parameters employed within our simulation study ispresented in Table 1. These parameters have been adapted from multiple studies related to
FSO links (Bekkali et al. 2022; Aljohani et al. 2021; Ghafoor et al. 2023).

3 Results and discussion
To observe the performance of the proposed novel bidirectional FSO link based on WSK
modulation, we measure the BER versus received power plots. Figure 3 shows theseplots for FSO link length of 1 km and refractive index structure parameters of 5e−15m−2∕3, 5e−14 m−2∕3 and 5e−13 m−2∕3, representing low, medium and strong turbulence
conditions, respectively. The figure shows BER plots for both the downlink and uplink
directions. As discussed in the previous section, the downlink signal is transmittedusing two optical pulsed signals, one carrying ones and the other carrying zeros of thedata stream. Whereas, the uplink signal is transmitted by applying OOK modulation toa re-used pulsed optical signal obtained by combining the two downlink optical signals.
Furthermore, the downlink data is demodulated using balanced detection while the uplink
data is demodulated using simple OOK demodulation. It may be observed from Fig. 3 thatfor low scintillation index of 5e−15 m−2∕3, the performance of the downlink and uplink

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Table 1  Parameters of different
components used in our
simulation

Parameter details

Value

Bit rate per channel

10 GbpsNumber of bits transmitted
Center wavelength of OGPG-1
Center wavelength of OGPG-2
Pulse width of the Gaussian pulses
Noise figure of optical amplifiers
Gain of optical amplifiersReceiver aperture diameter
Beam divergence of transmitter
FSO channel attenuation
Bandwidth of each OBPFs
Cut-off frequency of LPF
Length of FSO channel
Refractive index structure parameter of the FSO channel
Attenuation of FSO channel
Transmitter aperture diameter
Responsivity of PIN PDs
Extinction ratio of MZMs

512 Bits
1550 nm
1554 nm
25 ps
4 dB
20 dB
20 cm
2 mrad2 dB/km50 GHz10 GHz1 km

10-2

Bit Error Rate (BER)

10

1367

5e−15 m−2∕3
2 dB/km
5 cm
0.9 A/W
30 dB

Downlink, Scintillation Index=5e-15
Downlink, Scintillation Index=5e-14
Downlink, Scintillation Index=5e-13
Uplink, Scintillation Index=5e-15
Uplink, Scintillation Index=5e-14
Uplink, Scintillation Index=5e-13

-4

10-6
10-8
10-10
10-12
10-14
-35

-30

-25

-20

-15

-10

-5

Received Optical Power (dBm)

Fig. 3  BER versus received optical power plots for FSO link of 1 km and three different refractive index
structure parameters

signals are almost the same, with downlink performing better by 0.5 dB for lower received
optical powers. There is a significant difference between the performance of the downlink
and uplink signals for stronger turbulence conditions. This may be attributed to the 3-dB
gain provided by balanced detection applied to downlink signals.
Similarly, Fig. 4 shows the BER versus received power plots for the downlink
and uplink signals that are transmitted over FSO link distance of 2 km. The plots are
obtained at three different values of refractive index structure parameters, that are

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10-2

Downlink, Scintillation Index=5e-15
Downlink, Scintillation Index=5e-14
Downlink,Scintillation Index=5e-13
Uplink, Scintillation Index=5e-15
Uplink, Scintillation Index=5e-14
Uplink, Scintillation Index=5e-13

Bit Error Rate (BER)

10-4

10-6

10-8

10-10

10-12
-35

-30

-25

-20

-15

-10

Received Optical Power (dBm)

Fig. 4  BER versus received optical power plots for FSO link of 2 km and three different refractive index
structure parameters

5e−15 m−2∕3, 5e−14 m−2∕3 and 5e−13 m−2∕3, representing low, medium and strong
turbulence conditions, respectively. It can be observed that as the link distance increases,
the performance disparity between the downlink and uplink signals becomes more
pronounced, with the downlink outperforming the uplink. For example, for the refractive
index structure parameter of 5e−15 m−2∕3, representing small turbulence, BER of 10−9 isachieved for received optical power of −26 dBm. While for the uplink, the same BER for
small turbulence is achieved for received optical power of -23 dB. Therefore, the difference
between the receiver sensitivity is exactly 3 dB, highlighting the gain provided by balanced
detection of the downlink signal. Similarly, let us consider the refractive index structure
parameter of 5e−13 m−2∕3, representing strong turbulence. For this case, the minimum
BER that the uplink signal can achieve is 10−7 at a received optical power of −16.5 dBm.
The downlink signal can achieve the same BER for a received optical power of −23 dBm.
Thus, the receiver sensitivity difference between the downlink and uplink signals for thiscase is a significant value of 6.5 dB. This significant difference may be attributed to the
3 dB gain provided by the balanced detection of the downlink signal as well as the low
quality of the re-used optical carrier employed for the uplink signal transmission.
To visualize the difference between downlink and uplink signals in the time domain, we
have obtained the eye diagrams under different FSO link conditions discussed previously,
while keeping the received optical power constant at −24 dBm. Figure 5 shows the eye
diagrams of the received signals after propagating the FSO link of 1 km for downlink
signals (a), (b) and (c) and uplink signals (d), (e), (f), at refractive index structure
parameter values of 5e−15 m−2∕3, 5e−14 m−2∕3 and 5e−13 m−2∕3, respectively. Firstly, it
can be observed that the eye diagrams for the downlink signal resemble those of a DPSKmodulated signal detected using balanced detection (Ghafoor et al. 2023). Therefore,
the eye opening of the downlink signals is generally wider than the uplink signals.
Furthermore, the eye diagrams become worse as we increase the turbulence by increasing
the refractive index structure parameter. However, as observed from the BER plots as well,
the performance of the downlink signals is generally better compared to the uplink, asevident from wider eye openings. The effect of accumulated noise on the downlink signal
may also be observed from the intensity variations observed in the eye diagrams shownin Fig. 5a–c. The accumulated noise includes the effect of turbulence induced intensity13

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Fig. 5  Eye diagrams of the received signals after propagating FSO link of 1 km for downlink signals (a),(b) and (c) and uplink signals (d), (e), (f), at refractive index structure parameter values of 5e−15 m−2∕3,5e−14 m−2∕3 and 5e−13 m−2∕3, respectively

fluctuations as well as the receiver thermal and shot noise. The intensity fluctuations areslightly higher for the uplink signals, as observed from Fig. 5d–f. The reason for this higher
intensity fluctuations over the uplink signals may be attributed to further accumulation of
turbulence induced noise as well as the receiver thermal and shot noise.
We also obtained the eye diagrams of the downlink and uplink signals for FSO link
length of 2 km, as shown in Fig. 6. Again, the received optical power is kept constantat −24 dBm. Figure 6. shows the eye diagrams of the received signals after propagating

Fig. 6  Eye diagrams of the received signals after propagating FSO link of 2 km for downlink signals (a),(b) and (c) and uplink signals (d), (e), (f), at refractive index structure parameter values of 5e−15 m−2∕3,5e−14 m−2∕3 and 5e−13 m−2∕3, respectively

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FSO link of 2 km for downlink signals (e), (f) and (g) and uplink signals (h), (i), (j), at
refractive index structure parameter values of 5e−15 m−2∕3, 5e−14 m−2∕3 and 5e−13 m−2∕3,
respectively. It may be observed that the opening of the eye diagrams of Fig. 6 are lower
compared to the eye diagrams of Fig. 5. The reason for this is the longer distance travelledby the optical signals shown in Fig. 6. For longer FSO link, the turbulence induced
intensity fluctuations over the optical pulses is higher. The effect of noise accumulation
due to turbulence and receiver thermal and shot noise can be seen from the eye diagramsof Fig. 6. Furthermore, this noise is higher for the uplink signals compared to the downlink
signals due to the re-use of optical carriers for the transmission of uplink data. From theresults discussed above, it may be deduced that the proposed WSK technique is suitable for
implementation in point-to-point FSO links. It may be argued that the benefit of balanced
detection provided by the WSK technique presented in this study can also be provided
by the DPSK modulation. While this is correct, however WSK offers further benefits incomparison to DPSK, especially in the context of FSO communication systems. WSK
is advantageous in FSO communication due to its resilience to atmospheric turbulence,
simpler and more cost-effective receiver design, lower sensitivity to phase noise, easeof implementation, and robustness to alignment issues. WSK tends to offer practicaladvantages in terms of robustness and implementation simplicity in the often challenging
conditions of FSO communication, while DPSK may be chosen for applications demanding
higher spectral efficiency and sensitivity when phase coherence can be reliably maintained.
Our proposed scheme is also suitable for bidirectional Ground to Low Earth Orbit(LEO) satellite optical link (Hauschildt et al. 2017). Ground-to-LEO bidirectional optical links are a significant development and offer many advantages over the traditional RF
links. Compared to traditional RF links, optical links offer significantly higher bandwidth.
This allows for much faster data transfer between the ground and satellites in LEO, which
can be crucial for applications that generate large amounts of data, like Earth observation
satellites capturing high-resolution imagery or video. Ground-to-LEO optical links are a
key technology for future constellations of mega-constellations with large numbers of LEO
satellites. These constellations have the potential to revolutionize global internet access andEarth observation capabilities. LEO satellites face several payload constraints due to thelimitations of their launch vehicles and the environment they operate in. LEO satellites are
typically launched by smaller rockets compared to those used for geostationary satellites.
This limits the total weight a satellite can carry, impacting the size and complexity of the
payload. Launch vehicles also have limitations on the physical size of the payload they canaccommodate. This can restrict the deployable size of instruments or antennas needed forthe mission. Under such constraints, it is of paramount importance to have light weight
LEO satellites. Since our proposed link does not employ a separate high power laser source
at Node-2 (which can act as the LEO satellite), it reduces the overall payload of the satellite
while maintaining high data rates.

4 Conclusion
We have presented a novel WSK technique for the transmission of 10 Gbps data overthe FSO link. To the best of our knowledge, the WSK technique has not been previously
implemented for point-to-point FSO links. Therefore, this study explores the feasibility of
WSK technique for point-to-point FSO links. The apparent disadvantage of using two optical carriers associated with the WSK technique has been mitigated by reusing the carriers

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for bidirectional signal transmission. Therefore, the technique supports centralization of
resources by making the distant nodes carrier-less. Finally, it was observed from the resultsthat due to the use of balanced detection, our proposed technique is suitable for longer FSO
links. The factor limiting the length of the FSO link in our proposed scheme is the accumulation of noise due to atmospheric turbulence as well as the receiver thermal and shotnoise. This limitation mainly applies to the uplink signal, since it is transmitted by re-using
the downlink optical carrier. It was observed that the BER for uplink signal transmitted
over a length of 2 km under high channel turbulence does not show significant improvement when the received optical power is increased. In future research, we aim to explore
the potential for increasing data rates while minimizing component counts. This can beachieved by employing a combination of low-level modulation formats, which will facilitate simpler designs for both the transmitter and receiver. Additionally, to mitigate noise
accumulation and fluctuations induced by turbulence, we propose incorporating a regeneration mechanism before reusing the optical carriers (Aljohani et al. 2021).
Author contributions All authors contributed equally to this work.
Funding Open access funding provided by Uppsala University.
Data availability The datasets generated during and/or analyzed during the current study are available fromthe corresponding author upon reasonable request.

Declarations
Conflict of interest The authors declare that they have no conflict of interest.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as longas you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article
are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to thematerial. If material is not included in the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directlyfrom the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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