A bidirectional free space optical link for last‑mile terrestrial access links employing a novel wavelength shift keying technique

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-to-point 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 re-use the optical carriers for the transmission of uplink data. The use of WSK in the down-link 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.


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).These advancements 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, in recent 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 detect objects and measure their distance, velocity, and other characteristics.This technology can be 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 refer to 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, or other end-user locations.They are called "last-mile" because they represent the final leg of connectivity 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 over fiber-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 optical line 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, fixed microwave links have been employed in such situations.However, their capacity is limited 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 and struggle to deliver multi-gigabit capacities.In these scenarios, FSO technology offers a compelling 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 generation of light pulses, which are then collimated by a lens system before transmission through the atmosphere.The transmitted pulses are received by a dedicated lens system and converted 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, and facilitates 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 and drawbacks, 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 knowledge of 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 ).PPM eliminates the need for a dynamic threshold for signal detection and offers advantages in specific situations.Pulse Width Modulation (PWM) demonstrates improved spectral efficiency, lower peak transmit power, and superior performance in 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 the need 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 Node-2, 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.

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 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 as well 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 to the 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" ( In U ) and "Input Lower" ( In L ), respectively.These inputs are related to the corresponding upper and lower outputs ("Output Upper" ( Out U ) and "Output Lower" ( Out L )) of the X-coupler through a specific mathematical relationship mentioned as follows Ghafoor et al. (2023): To encode the data stream, the phase modulator (PM) introduces a specific phase shift.When transmitting a "one" (bit 1), a ∕2 phase shift is applied to the combined optical signal.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), a phase-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 a central 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" (bit 0), the pulse at 1554 nm is transmitted.Both filters utilize a Gaussian profile and possess a bandwidth of 50 GHz each.Subsequently, the outputs from these two filters are recombined using a power combiner (PC).The combined output undergoes amplification by an optical amplifier (OA).This amplifier exhibits a noise figure (NF) of 4 dB and provides a gain of 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) performance of 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 the data 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 (1) � 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 from the photodiodes were then combined with reversed polarity for the electrical signal representing zeros.Subsequently, the combined electrical signal underwent filtering through a low-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 using balanced 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 from each splitter is used for data extraction.The remaining outputs from the PS are combined using 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 the system 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 is then 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 is presented 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).

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 these plots for FSO link length of 1 km and refractive index structure parameters of 5e−15 m −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 transmitted using two optical pulsed signals, one carrying ones and the other carrying zeros of the data stream.Whereas, the uplink signal is transmitted by applying OOK modulation to a 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 that for low scintillation index of 5e−15 m −2∕3 , the performance of the downlink and uplink Page 7 of 12 1367 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 Bit Error Rate (BER) 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 Fig. 3 BER versus received optical power plots for FSO link of 1 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 is achieved 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 this case 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, as evident 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 shown in Fig. 5a-c.The accumulated noise includes the effect of turbulence induced intensity Bit Error Rate (BER) 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 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 constant at −24 dBm. Figure 6.shows the eye diagrams of the received signals after propagating 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 travelled by 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 diagrams of 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 the results 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 in comparison 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, ease of implementation, and robustness to alignment issues.WSK tends to offer practical advantages 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 and Earth observation capabilities.LEO satellites face several payload constraints due to the limitations 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 can accommodate.This can restrict the deployable size of instruments or antennas needed for the 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.

Conclusion
We have presented a novel WSK technique for the transmission of 10 Gbps data over the 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 Page 11 of 12 1367 for bidirectional signal transmission.Therefore, the technique supports centralization of resources by making the distant nodes carrier-less.Finally, it was observed from the results that 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 shot noise.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 be achieved 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).

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

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

Table 1
Parameters of different components used in our simulation