Performance analysis of multiple-beam WDM free space laser-communication system using homodyne detection approach

Abstract

A free space optical module is used in laser communication to transport a signal from the transmitter to the receiver. Free Space Optical Communication (FSOC) is a Line of Sight connectivity that sends a highly narrow beamwidth. FSOC provides high bandwidth and data rates greater than 10 Gbps. Although FSOC technology has several advantages, it is inefficient for long-distance transmission because of many constraints caused by atmospheric variables. In FSOC connections, turbulence-induced scintillation is a severe problem that significantly reduces link performance. Keeping this problem in mind, the objective of this study is to enhance FSOC performance in terms of energy efficiency, spectral efficiency and long-distance transmission. To achieve this, a study is employed using a hybrid combination of Higher-order Gaussian filter (HGF), post-amplification and a homodyne detection method. Precisely, the simulative study of 32-channel wavelength division multiplexing (WDM) FSOC has used channel model Gamma-Gamma with single-beam (SB), dual-beam (DB), four multiple-beam (MB4) and eight multiple-beam (MB8) techniques. The proposed framework has achieved a Channel capacity of more than 320 Gbps. The transmission range enhancement of 112% and reduction in transmitted power of 100% are achieved, which are considerably more significant compared with state-of-the-art literature studies. The OptiSystem platform is used to gather the outcomes. The performance is based on parametric analysis of bit error rate (BER), Quality (Q) factor and eye height.

1. Introduction

New dimensions are being added to the optical wireless communication-based broadband access industry to meet the growing demands in cellular networks. Cable and radio-frequency lines use standard broadband access technologies. Cable provides weather-independent connections but requires point-to-point infrastructure, resulting in expensive “last-mile” [1]. On the other hand, Radio frequency links are weather-resistant and do not require any point-to-point infrastructure. The problem with present Radio frequency technology is that this is already in high demand, and license requirements are expensive. FSOC is an optical correspondence framework for information communication that uses laser light in free space. As a result of the low cost [2], simplicity of installation, significant bandwidth, and fast data rates, FSOC is superior to optical fibre systems [3]. Free Space Optics is a potential breakthrough that is more likely to happen sooner rather than later. Due to atmospheric turbulence, the receiver's signal begins to degrade. However, it is now possible to reduce the environmental effects of the optical signal in FSOC systems by using spatial differentiating quality approaches. The FSOC framework's utilisation of spatial diversity has improved system efficiency in various challenging situations [4]. With increased attenuation, the Q-factor of the FSOC framework decreases. Higher-order diversities can achieve a higher Q-factor and a high eye-opening. These techniques help improve execution in FSOC.

The impact of atmospheric conditions on an FSOC connection is the subject of this research. The scintillation phenomenon has to be the most harmful regarding atmospheric effects. Hence the authors have attempted to mitigate it using some of the strategies mentioned in this work. Disruption induced by thermal variations along the course of a laser ray is referred to as scintillation. Air velocity constantly changes, resulting in eddies transporting thermal and water vapours [5]. Temperature variations in the atmosphere generated by such eddies promote the heat of air pockets known as Fresnel zones, which have different temperatures and densities and varying refractive indexes. These pockets are constantly forming and dissolving, so it's unpredictable. The laser light is deformed by variations in the air refractive index, resulting in “beam dancing” at the reception [6]. The magnitude of fluctuations is controlled by the refractive index structural parameter (C_n²). If the value of C_n² is 10⁻¹⁶ m^−2/3, that is known as weak scintillation, and if the value is 10⁻¹³ m^−2/3, then it's called strong scintillation [6].

Various types of compensation techniques are implemented to minimise losses in FSOC. In Ref. [6], SB and MB4 techniques were presented to reduce the effect of scintillation. The time and wavelength diversity technique was used to mitigate turbulence losses in Refs. [7,8]. Turbulence effects are compensated through aperture averaging and spatial diversity in Refs. [9,10]. In Ref. [11], turbulence losses are minimised using an Erbium-doped fibre amplifier. Mehtab et al. [12] used the orbital angular momentum multiplexed beams approach to minimise turbulence losses in FSOC. The transmission range achieved in this paper was 800 m. Al-Gailani et al. [13] used the MB4 approach with a dense wavelength division multiplexing (DWDM) technique to enhance the performance of FSOC. The transmission range achieved in this paper was 1.2 km. Zhou et al. [14] used a spatial division multiplexing scheme for mitigating atmospheric turbulence losses in FSOC. The transmission range achieved in this paper was 1.3 km. Fadhil et al. [15] used SB with the DWDM technique for mitigating turbulence losses in FSOC. The transmission range achieved in this paper was 1.5 km. In Ref. [16], SB with the DWDM technique was used to enhance the performance of FSOC. The transmission range achieved in this paper was 2.4 km. Arora et al. [17] used an enhanced spectrum slicing technique to minimise turbulence losses in FSOC. The transmission range achieved in this paper was 2.5 km. In Ref. [18], the FSOC reliability was assessed, which was based on the mode division multiplexing technique. The transmission range achieved in this paper was 4 km. Grover et al. [19] used MB4 with the DWDM techniques to enhance the performance of FSOC. The transmission range achieved in this paper was 5.3 km. In Ref. [20], mitigation of turbulences by optical phase conjugation technique. The FSOC reliability was examined in Srinagar, Chandigarh, Jodhpur, and Chennai. The location used in this article is based on hilly, plain, desert, and coastal areas [21]. The transmission range and BER achieved here [21] were 10.75 km and 10⁻⁹, respectively. The polarisation division multiplexing technique was used in this paper [22] for mitigating turbulence losses. The transmission range achieved here was 15 km. Rani et al. [23] used the homodyne detection method to mitigate atmospheric turbulence in the WDM FSOC framework. The maximum transmission range and minimum transmitted power achieved here were 25 km and −3 dBm, respectively, in the MB4 approach.

Leonid [24] compared the homodyne and heterodyne detection approaches in the FSOC. Statistical channel models like Gamma-Gamma and Log-Normal were presented in Ref. [25]. Comparative studies of WDM and DWDM systems are in Refs. [[26], [27], [28]]. Ghalot et al. [29] analyzed the FSOC reliability in Amritsar and New Delhi. This article concludes that FSOC is more reliable in Amritsar than in Delhi. In Ref. [30], the FSOC reliability was assessed, which was based on Spectrum Slicing WDM techniques in city of Vellore. The communication link with 1.56 Gbps data rate was successfully achieved in clear weather conditions [30]. The FSOC reliability under clear and hazy weather conditions were examined in Ref. [31]. In Ref. [32], Coded orthogonal frequency division multiplexing techniques were used to mitigate adverse weather conditions. Gamma-gamma distribution was used for channel modelling and achieved a BER 10⁻⁴ under strong turbulence conditions [32]. The FSOC reliability was assessed in Delhi, Kolkata, Ahmedabad, and Thiruvananthapuram. This paper concludes that FSOC is more reliable in Thiruvananthapuram than in Delhi, Kolkata, and Ahmedabad [33]. In Ref. [34], a four-level enabled pulse amplitude modulation technique was used to enhance the performance of FSOC. The BER value achieved in this paper was 10⁻⁹. Hayal et al. [35] used hybrid modulation techniques for mitigating the turbulence losses. The BER value achieved here [35] was 10⁻¹².

In the above studies of FSOC, different methods were employed for mitigating turbulence losses. However, in these methods, the maximum transmission range of up to 25 km was achieved in the four multiple-beam (MB4) technique. On the other hand, the bit error rate value of 10⁻¹⁰ was accomplished using the single-beam (SB) technique. Furthermore, minimum transmitted power up to −3 dBm was achieved in the MB4 approach. If free space optical communication is to be employed as a supporting technology for the deployment of 5G, it will be crucial to examine how to enhance its performance parameters. Therefore, this study aims to enhance FSOC performance in terms of energy efficiency, spectral efficiency and long-distance transmission. In this study, we have mitigated turbulence problems in FSOC using a hybrid combination of HGF, Post-amplification and a homodyne detection technique. The analysis of this work is based on SB, DB, MB4, and MB8 approaches with a gamma-gamma distribution model. The system parameters are optimised using a single parameter optimisation technique. Furthermore, by using the proposed model, Enhancement in the transmission range, improvement in the BER, and reduction in the transmitted power are achieved; due to this, energy efficiency is enhanced.

When WDM and HGF with Coherent detection are combined, the benefits of these two technologies become diverse. Coherent detection has the potential to detect weak optical signals after their propagation. Homodyne Detection is a significantly more sophisticated detection approach than traditional heterodyne detection, yet it can provide receivers with excellent sensitivity. HGF technique with WDM homodyne detection is suitable for transmitting over long distances with improved Q-factor. Increasing the filter's order requires less power, so it's a better approach corresponding with energy efficiency. Another advantage of this technique is identifying any single channel by tuning a local oscillator. Homodyne and heterodyne receivers are two types of coherent receivers. Heterodyne receiver is not suitable for high data rates and long-distance transmission systems. In Ref. [23], they analyzed the advantages of Homodyne detection over the heterodyne detection technique.

This study proposes an HGF and post-amplification-based 32-channel WDM FSOC framework with a homodyne detection technique. Furthermore, to assist system designers, this research also creates a thorough simulation model based on SB, DB, MB4, and MB8 approaches. We have used the OptiSystem software to simulate this model. More specifically, the following are the primary contributions of this work.

• i)We have developed a homodyne detection-based WDM FSOC Framework which mitigates the low, medium and high atmospheric turbulences. The system parameters are optimised by using a single parameter optimisation technique. For the channel portion, the gamma-Gamma model is employed. Exclusively, clear climatic conditions are taken into consideration for this investigation.
• ii)We have simulated the results based on the parametric analysis of BER, Q-factor and eye height.
• iii)After that, we also compared the results with designs based on state-of-the-art literature studies.
• iv)The proposed architecture results provide a higher transmission range, energy efficiency, and spectral efficiency than previous studies.

This article is divided into five sections. First, explain the importance of the FSOC based on the literature survey in Section 1. The remainder of the paper is organised as follows: Section 2 describes the architecture of the proposed system. Section 3 discusses the parameters used in the simulation. The outcomes of this proposed model have been critically evaluated in section 4. Finally, in section 5, concluding remarks have been made.

2. Methodology analysis

2.1. Proposed HGF-based homodyne detection model

Fig. 1 depicts the block diagram of the HGF-based WDM FSOC Framework, whose theoretical analysis is covered below. An analysis of the same accompanies the step-by-step design of the framework. Where PRBS stands for a Pseudo Random Bit Sequence generator, NRZ represents non-return to zero pulse generator, MZM is a Mach-Zehnder modulator, CW Laser represents Continuous wave laser, WDM MUX stands for wavelength division multiplexer, PS is a phase shifter, WDM DEMUX represents wavelength division demultiplexer, G stands for a gain of the optical amplifier. PD stands for Photodetector.

Fig. 1.

Proposed HGF-based WDM FSOC design.

2.2. Transmitting units

In the transmitter, PRBS generates a signal greater than 10 Gbps data rate. This signal input generates a series of non-return to zero pulses created by the NRZ Pulse Generator component. On the other hand, CW Laser generates a signal that uses a wavelength of 1552.52 nm, laser power in the range of −9 dBm to 10 dBm, and Line width used in the range of 0.1–1 MHz. The Power Splitter (1 × 32) splits this optical signal into an array of 32 output signals. WDM is a technique to modulate numerous data streams on different wavelength optical carriers and send them across a particular channel. A multiplexer is used at the transmission end for multiplexing distinct modulated carriers, along with 32-channels, which are used in the system to enhance signal capacity. In addition to this, a Mach-Zehnder modulator is used to convert this optical signal generated from WDM MUX (32 × 1) and the electrical signal produced by the NRZ component into the optical domain. Finally, the free space transmitter module transmits this optical signal to the channel for propagation. This framework uses a sequence length of 128 bits, 64 bits per sample, and a total of 8192 samples.

2.3. Channel model

The channel has several turbulences, rain, fog, physical obstruction, scintillation etc. In this study, the main focus is on the scintillation phenomenon. The Gamma-Gamma model has been used for channel performance [36]. This model is used to interpret the irradiance of optical channels. The probability density function (PDF) of the irradiance is given by equation (1) [37]:

$$P_J\left(J\right)=\frac{{2\left(uv\right)}^{\frac{u+v}{2}}}{\Gamma u\Gamma v}{I}^{\left(\frac{u+v}{2}\right)-1}{K}_{u-v}\left(2\sqrt{uvJ}\right)J>0$$ (1)

where K_u-v (…) is the modified Bessel function of the second kind. Γ is the gamma function. The number of small- and large-scale eddies, respectively, are represented by the symbols u and v. J is the intensity of the received signal. The values of u and v are calculated by equation (2) and equation (3), respectively [38]:

$$u={\left[\exp \left(\frac{0.49{\sigma }_{rv}^2}{{\left(1+0.18d^2+0.56d^2{\sigma }_{rv}^{12/5}\right)}^{7/6}}\right)-1\right]}^{-1}$$ (2)

$$v={\left[\exp \left(\frac{{0.51{\sigma }_{rv}^2\left(1+0.69{\sigma }_{rv}^{12/5}\right)}^{-5/6}}{1+0.9d^2+0.62d^2{\sigma }_{rv}^{12/5}}\right)-1\right]}^{-1}$$ (3)

The parameter d is calculated by equation (4):

$$d=\sqrt{\left(k{D}^2\right)/4r}$$ (4)

where D is the diameter of the free space receiver lens aperture, and r is the link's span.

Where ${{\mathbf{\sigma }}_{rv}}^2$ is the Rytov variance that determines how strong/weak turbulence is in the channel? The parameter ${\sigma }_{rv}^2$ is calculated by equation (5):

$${\sigma }_{rv}^2=1.23C_n^2{k}^{7/6}{r}^{11/6}$$ (5)

where k is the wave number (k = 2π/λ). C_n² is the Refractive index structure parameter. SB, DB, MB4 and MB8 approaches are used to overcome the atmospheric turbulences.

2.4. Receiving units

A demultiplexer is used for the receiving end to reverse the process. The carrier signal generated by a coherent laser source is used on the receiver side. The 90-degree Phase shift and amplification of this signal are done by a phase shifter and optical amplifier (G), respectively. Then this signal goes through an optical coupler (2 × 2) to the Photodetector. PIN photo detector Responsivity 1 A/W and dark current 10 nA are used for signal detection. A subtractor block adds the Photodetector output and the coupler output. The second-order low-pass filter tunes the outcome of the subtractor. In the last block, measuring instruments like the BER analyser and Eye diagram viewer are used to check the performance parameters.

3. Proposed framework simulation design

The FSOC transmitter, FSOC channel, and FSOC receiver are all parts of the FSOC technology. In reality, various atmospheric turbulences provide varied attenuations for the FSOC system. In this paper, we consider different atmospheric turbulence scenarios. That is, low, medium and high with C_n² are 10⁻¹⁶ m^−2/3, 10⁻¹⁴ m^−2/3, and 10⁻¹³ m^−2/3, respectively. The value of C_n² is 10⁻¹³ m^−2/3 in the case of SB and 10⁻¹³ m^−2/3, 10⁻¹⁴ m^−2/3, 10⁻¹⁵ m^−2/3, and 10⁻¹⁶ m^−2/3 in the case of MB4. The simulation arrangement evaluates the performance of the proposed HGF Based WDM FSOC system. The suggested system is being compared using the SB, DB, MB4, and MB8 techniques. The simulation schematic layout of the proposed WDM FSOC framework for SB, DB, MB4 and MB8 approaches are presented in Fig. 2(a), 2(b), 2(c), and 2(d), respectively.

Fig. 2.

Simulation schematic layout of the proposed WDM FSOC framework with homodyne detection approach. (a) SB approach (b) DB approach (c) MB4 approach (d) MB8 approach.

3.1. Simulation parameters

The simulation parameter values are given in Table 1.

Table 1.

Values of a simulation Parameter.

Simulation Parameters	Values
Frequency	193.1 THz
Power	10 dBm
Information rate	10 Gbps
Number of channels	32
Attenuation	0.065 dB/km [23]
Transmitter aperture diameter	15 cm
Receiver aperture diameter	15 cm
Beam divergence	2 mrad
Range	0–100 km
Responsivity	1 A/W
Dark current	10 nA
Phase shift	90°
Turbulence channel model	Gamma-Gamma
Cn2	10−16–10−13
sequence length	128 bits
samples per bit	64

4. Results and discussion

This section is divided into three sub-sections. First, compare the performance analysis of the system based on the SB, DB, MB4 and MB8 techniques in sub-section 1. Then, in sub-section 2, compare the proposed and pre-existing designs' analysis. Finally, in sub-section 3, explain the power-saving and long-distance strategy.

4.1. Performance analysis of the system based on SB, DB, MB4 and MB8 technique

The SB, DB, MB4, and MB8 approaches are compared in Fig. 3. MB8 techniques feature enormous values for the maximum Q-factor and eye height compared to other systems. At a distance of 10 km, the Q-factor for SB is 8.86. At a distance of 10 km, the values of Q-factor are 10.67, 12.22, and 13.37 in the cases of DB, MB4, and MB8, respectively, and for MB4, the Q-factor value is 7.08 at a range of 25 km. In Ref. [23], the Q-factor is 6.13 in the case of SB at 10 km and 8.5 in the case of MB4 at 25 km. The maximum eye height in the case of an SB is 81 μm, while in Ref. [23] eye height value was 53 μm.

Fig. 3.

Performance in terms of Max. Q-factor & Eye Height.

Fig. 4 shows the comparative analysis of the minimum (min.) Log of BER values. At a distance of 10 km, the min Log of BER for SB is −18.4. At a distance of 10 km, it is −26.19, −33.93, and −40.37 in the cases of DB, MB4, and MB8, respectively, and for MB4, at a range of 25 km, the minimum value of BER is 6.86 × 10⁻¹³. In Ref. [23], the minimum BER value was 7.27 × 10⁻¹⁰ in the case of MB4 at 25 km. The Eye diagrams for the SB, DB, MB4 and MB8 approaches are presented in Fig. 5(a), 5(b), 5(c), and 5(d), respectively.

Fig. 4.

Performance in terms of Min. Log of BER.

Fig. 5.

Compare the Eye Diagrams at 10 km (a) HGF-based SB approach (b) HGF-based DB approach (c) HGF-based MB4 approach (d) HGF-based MB8 approach.

4.2. Comparative analysis of proposed and pre-existing designs

The Comparative analysis in terms of beams, Q-factor, filter order, Maximum Range, Min BER, Multiplexing technique, detection technique, minimum power, Channel capacity, post-amplification and optimisation of proposed and pre-existing designs is in Table 2.

Table 2.

Comparative analysis of proposed designs and pre-existing designs.

Parameters (Unit)	[23]		[6]		Proposed design
Beams technique	SB	MB4	SB	MB4	SB	DB	MB4	MB8
Q-Factor at 10 km	6.13	8.5	–		8.86	10.67	12.22	13.37
Gaussian Filter order	1		1		2
Maximum Transmission Range (km)	10	25	1.9	4.2	32	42	53	69
Minimum BER at 10 km	4.0 × 10−10	–	–	–	3.9 × 10−19	6.4 × 10−27	6.3.1 × 10−34	4.2 × 10−41
Multiplexing Technique	WDM		WDM		WDM
Detection Technique	Homodyne detection		Heterodyne detection		Homodyne Detection
Minimum power (dBm)	–	−3	–	10	−3	−5	−6	−8
Channel Capacity (Gbps)	320		320		>320
Post-amplification	NO		NO		YES
Optimisation used	NO		NO		YES

Table 2 shows that the proposed framework is based on the SB, DB, MB4, and MB8 approaches, while [6] and [23] analyses were based on MB4 and SB techniques, respectively. The Q-factor of the proposed framework is 12.22 in the case of the MB4 strategy, which is more than [6,23]. In the MB4 approach, the transmission range of 53 km has been achieved, while this value was up to 25 km in previous designs. Compared with state-of-the-art literature studies, the proposed framework has achieved an improvement in BER. The transmitted power is also reduced in the proposed model; thereby, energy efficiency has been improved. In the proposed framework, each channel has a data rate greater than 10 Gbps, resulting in a channel capacity of more than 320 Gbps. The proposed design has used the post-amplification technique in the receiver section; for this optical amplifier is used at the receiver section in the proposed framework. The system parameters are optimised using a single parameter optimisation technique in the proposed design; this was absent in previous studies.

4.3. Power saving with a long-distance transmission strategy in the proposed system

In the face of rising global energy demand, researchers have realised the importance of conserving energy in whatever feasible way. Decreasing the power usage of wireless technologies is a well-known approach for increasing energy savings, contributing to global energy efficiency improvements. However, the High power of the source is dangerous to the human eye. So, the HGF-based Homodyne Detection design is best for power optimisation. In the 5G era, Energy Efficiency and Spectrum Efficiency must be proficient. Fig. 6 shows laser power requirements in the case of SB, DB, MB4, and MB8 techniques. For the SB approach, the transmission over a distance of 4.5 km requires a transmitted power of −3 dBm. At a distance of 4.5 km, transmitted power is −5 dBm, −6 dBm, and −8 dBm in the cases of DB, MB4, and MB8. In Ref. [23], the laser input power was −3 dBm in the MB4 technique at 25 km.

Fig. 6.

Laser power vs Q-factor at a distance of 4.5 km.

The highest transmission range for the SB, DB, MB4 and MB8 approaches is depicted in Fig. 7. The SB approach's maximum transmission range is 32 km at a laser power of 10 dBm. Its values are 42 km, 53 km, and 69 km in the cases of DB, MB4, and MB8 techniques. In Ref. [23], the maximum transmission range was 25 km at a laser power of 10 dBm.

Fig. 7.

Range vs. Q-factor at a power of 10 dBm.

4.4. Drawbacks of the current work

This study focuses on the atmospheric turbulence problem (scintillation) in the FSOC system. Moreover, in this study, only clear climatic conditions have been taken into consideration. Environmental issues are not addressed in this paper. In the near future, we will address the combined effect of atmospheric and environmental problems in the FSOC technique. This paper used the same laser source at the transmitter and receiver ends, so the synchronisation between these sources is challenging. The proposed framework simulation has been done using OptiSystem software. In the practical scenario, the accuracy of the proposed framework may vary from the simulation results because the FSOC mostly depends on random environmental conditions. Moreover, the integration of optical fibre with the FSOC technique is not studied in the current work. Accordingly, we will address these issues in our future work.

5. Conclusion

A robust FSOC framework that offers channel capacity greater than 320 Gbps has been demonstrated. The system's performance has been compared for SB, DB, MB4 and MB8 using eye diagram parameters. The findings indicate that, when the communication range is increased, MB8 exhibits the lowest BER value, maximum Q-factor, and maximum eye height for the received data. The system's effectiveness has been examined for various intensities of atmospheric turbulence. A successful transmission range of 32 km for SB, 42 km for DB, 53 km for MB4, and 69 km for MB8 has been achieved. For better energy efficiency, results showed that the MB8 technique is better than other techniques. The results also indicate that −6 dBm and −8 dBm transmitted power in the MB4 and MB8 techniques respectively, are sufficient for the transmission. This study is considered only for clear climate conditions. Future-generation wireless connectivity networks with larger capacity and spectral efficiency can use the proposed framework for both front-haul and back-haul.

Author contribution statement

Yogesh Kumar Gupta: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Aditya Goel: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Supervision, Writing-review and editing.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data included in article/supp. material/referenced in article.

Declaration of interest's statement

The authors declare no conflict of interest.