Spectrum-sliced wavelength division multiplexing based single and dual channel free space optical  communication employing differential phase modulation formats

doi:10.21203/rs.3.rs-2173577/v4

A spectrum-sliced (SS) Wavelength division multiplexing (WDM) based Free-space optical (FSO) communication system at a data rate of 2.5 Gbps per optical carrier is proposed in this research article. Utilizing the broadening of the spectrum by means of supercontinuum generation in a highly non linear Fiber (HNLF), an SS-WDM based FSO system is designed. The narrow spectrum of laser light source is broadened by means of self phase modulation (SPM). The proposed work reveals that the DQPSK based SS-WDM FSO system has higher performance compared to intensity modulation formats as well as other differential phase modulation formats. Considering various values of FSO link distances, antenna diameters of transmitter and receiver, beam divergence and thus, determining the values of performance metrics such as Q-factor and bit error rate (BER), differential quadrature phase shift keying (DQPSK) based SS-WDM FSO system is simulated. The same process is repeated for dual channel FSO system for DPSK and DQPSK.

SC

DQPSK

DPSK

HNLF

Q-factor

Dual channel FSO

Optical wireless communication systems are broadly divided into indoor and outdoor systems. Outdoor systems are also referred to as free space optical (FSO) systems. The very first implementation of a free space optical (FSO) communications link was demonstrated by Alexander Graham Bell on 3^rdJune, 1880 with his new invention known as photophone [1]. The photophone is a device which allows the transmission of sound on a light beam. FSO communication is line of sight (LOS) communication involving a light source such as a continuous wave (CW) laser and a photodetector to detect light transmitted through free space. Here, the operating frequency of CW laser will be in terahertz range. The receiver section includes a low-pass filter that eliminates the noise generated during the transmission through free space channel. Since the light beams are transmitted through free space, no fiber optic cable is required. The advantages of FSO include enormous modulation bandwidth, low cost of deployment, fast deployment time, unlicensed spectrum, and no electromagnetic interference (EMI) [4]. Highly nonlinear fiber (HNLF) used for supercontinuum (SC) generation in SS-WDM based FSO [3].

The signal from the CW Laser is fed into HNLF. HNLF molecules hop and start vibrations. Variations in refractive index (RI) of the pulses occur. Due to variations in RI, speed of light beams inside HLNF varies and phase change occurs. A nonlinearity called self phase modulation (SPM) is used in HNLF. The purpose of SPM is to widen the spectrum of the CW laser. Each WDM channel used for FSO requires a light source. Therefore, the need for multiple light sources increases the cost of the FSO system. Spectrum-sliced WDM drastically reduces this cost. SS-WDM was investigated using non-return to zero (NRZ) format at a data rate of 2.5 Gbit/s [2].

Semiconductor optical amplifiers (SOA), light emitting diodes (LEDs), highly nonlinear fibers (HNLF), and arrayed waveguide gratings (AWG) are various components for spectrum slicing [5]. Optical comb can also be used to produce spectrum slicing at a reduced cost [18]. An optical comb can be expanded to 128 coherent lines [22]. HNLF provides one of the best performances while LED provides the minimum performance. SS-FSO system was demonstrated for an FSO communication distance of over 2.5 km [6]. SS-WDM-FSO system with up to 16 channels and effects of turbulence due to various factors such as buildings’ height and wind speed reported [7]. SS-WDM-FSO was demonstrated with an advanced modulation format such as carrier suppressed return to zero (CSRZ), and various line coding techniques [8]. The line coding techniques implemented are return to zero (RZ), and non-return to zero (NRZ). For amplification of the signal in SS-WDM-FSO system, the performance of various amplifiers such as Raman amplifiers, semiconductor optical amplifiers, and erbium-doped fiber amplifiers were investigated and reported that eye-opening of the SS-WDM system using the later is the best compared to the rest of the amplifiers [9].

SS-WDM with 10Gbits/s power efficient advanced modulation format, modified duo binary return to zero (MDRZ) for the downlink and OOK (on-off keying) non-return to zero (NRZ) for the uplink was proposed [10]. In addition to spectral slicing, there are a number of factors that determine the overall performance of the SS-WDM-FSO system such as the distance of the FSO link (km), the beam divergence angle (mrad) and the antenna diameters of the transmitter as well as receiver (cm) [11]. A comparison of FSO with SS-WDM and without SS-WDM for various weather conditions using NRZ modulation format was reported and showed that SS-WDM provides better performance for different weather conditions [12].

SS-WDM has been compared with Spectrum Amplitude Coding (SAC) Optical Code Division Multiple Access (Optical CDMA) systems for different light sources [17]. The above research work shows that when compared to Optical CDMA, SS-WDM provides more spectral efficiency because of no necessity for bandwidth expansion. Strhel ratio (SR) and BER are used to analyze the performance of FSO link [19]. Many researchers provided several methods for the improvement of FSO links’ overall performance in terms of performance metrics such as BER and Q-factor. There is still scope for the betterment of FSO links’ overall performance.

Hence, in this research work, an FSO system by cost effective spectral slicing WDM (SS-WDM) with a data rate of 2.5 Gbps per carrier using DQPSK modulation is proposed. Figure 1(a) shows the block diagram of 4x2.5 Gbps SS-WDM-FSO (single channel FSO) communication system. Simulation of the SS-WDM-FSO system based on DQPSK modulation is performed at various beam divergence angles, FSO link distances and transmitter as well as receiver antenna diameters in terms of Q-factor and BER. For evaluating the performance of proposed SS-WDM FSO system, a comparison is made with different intensity modulation formats such as NRZ, RZ, CSRZ, and with differential phase modulation format DPSK. The same process is repeated for dual channel FSO system for both DPSK and DQPSK. Figure 1(b) shows the block diagram of 4x2.5 Gbps SS-WDM-FSO (dual channel FSO) communication system. On comparison with single channel FSO, dual channel FSO improves the link distance, BER and received power [20-21]. The DQPSK modulation scheme encodes two bits per symbol. This doubles the utilization of the spectrum. Using DQPSK, the symbol rate is halved, for the same bit rate. This improves tolerance to dispersion and nonlinearities [14-15].

Using the well-known Optisystem simulation tool, the proposed SS-WDM-FSO system with DQPSK modulation is implemented. The transmitter section contains a CW laser source with a transmit power of 30 dBm at 193.1 THz. The light signal of the CW laser carrier spectrum emanating from the laser source is injected into a highly non linear fiber (HNLF) of 2 km length. The HNLF parameters are- effective area of core 10 µm², non-linear refractive index 2.6e−019 m²/W. The spectrum of the CW laser carrier before feeding into HNLF is shown in Figure 2(a). Due to the nonlinear effect known as self-phase modulation, the generation of a supercontinuum takes place in the HNLF as explained in previous section. At the output port of HNLF, large widened spectrum is generated from a single light source. This widened spectrum with HNLF is shown in Fig. 2(b). This broad spectrum with high power is provided for spectrum-slicing at a frequency spacing of 75 GHz with the help of WDM demultiplexer ES (Equally spaced).

The spectrum is sliced into four equally spaced optical carriers of 193 THz, 193.075THz, 193.15THz, and 193.225 THz. The proposed model for 4x2.5 Gbps SS-WDM-FSO is shown in Fig. 3(a). Pseudorandom bit sequence generators generate random data which is fed to DQPSK transmitters. DQPSK transmitter diagram is shown in Fig. 3(b). Each DQPSK transmitter incorporates a pseudorandom bit sequence generator, a 4-dpsk pre-coder, two non-return to zero pulse generators, a sine generator and three MZ modulators. The sliced signals are multiplexed after modulation. The multiplexed signals are transmitted from the transmit antenna to the receive antenna towards the receiver via free space. DQPSK receiver comprises of two balanced detectors consisting of PIN photodiodes and low pass filters. DPSK receiver diagram is shown in Figure 3(c). The received data signals are reconstructed back to the original stream of data. For FSO communication in clear weather, attenuation conditions along with their respective value of attenuation are shown in Table 1 [8]. In clear weather condition, FSO system suffers from atmospheric turbulence. This turbulence is assumed to be in between weak to moderate. Hence Refractive index structure parameter is assumed as C_n²= 5x10 ^-15m ^-2/3. Values of C_n²are shown in Table 2 [16]. Gamma-Gamma model is considered for the proposed research work.

Table 1 Atmospheric conditions and attenuation [8]

S.No.	Atmospheric weather condition	Value of Attenuation (dB/km)
1	Clear weather	0.1
2	Haze	4
3	Mild rain	6.27
4	Medium rain	9.64

Table 2 Turbulence regimes [16]

S.No.

Parameter

Turbulence regimes

1

C_n² (m ^-2/3)

Weak

Moderate

Strong

5x10 ^-16

5x10 ^-14

5x10 ^-12

Proposed System parameters for the DQPSK modulated SS-WDM-FSO system are illustrated in Table 3.

Table 3 System parameters that are considered for the research work

S.No.	Parameter	Values
1	CW laser frequency (THz)	193.1
2	CW Laser Power (dBm)	30
3	Spectrum-sliced WDM Carriers	4
4	Frequency spacing (GHz)	75
5	FSO link distance (km)	1-10
6	Beam divergence (mrad)	0.25, 0.50, 0.75 and 1
7	Transmitter/Receiver antenna diameters (cm)	5, 10, 15 and 20
8	Attenuation for clear weather (dB/km)	0.1

The block diagram of the proposed 4x2.5 SS-WDM-FSO (single channel FSO) system incorporating DQPSK modulation scheme is shown in Figure 1.

The block diagram of the proposed 4x2.5 SS-WDM-FSO (single channel FSO) system incorporating DQPSK modulation scheme is shown in Figure 1 (a) while the block diagram of proposed 4x2.5 SS-WDM-FSO (dual channel FSO) system incorporating DPSK/DQPSK modulation scheme is shown in figure 1 (b).

Figure 2(a) shows the input CW laser spectrum. Widened spectrum at the output port of HNLF is shown in Figure 2(b).

Proposed simulation model of the 4x2.5 Gbps SS-WDM-FSO (Single channel FSO) system incorporating DQPSK is shown in Figure 3(a). Figure 3(b) displays DQPSK Transmitter while Figure 3(c) shows DQPSK receiver.

Figure 4 shows the optical spectrum slices at Optical spectrum Analyzer (OSA) before they are injected into the FSO channel for single channel FSO with DQPSK.

Different intensity modulation formats such as RZ, NRZ, and CSRZ are simulated using HNLF of 2 km length [11], and it is found that CSRZ has the best Q-factor considering all three intensity modulation formats. The differential phase modulation format DPSK has been reported using HNLF of 2 km length [13] and it was found that DPSK outperforms intensity modulation formats. Therefore, in the proposed SS-WDM-FSO system, another differential phase modulation technique DQPSK, is investigated and a comparison is made with both the investigated DPSK format [13] and the intensity modulation formats [11].

To study the performance of the high-speed SS-WDM-FSO communication system, various factors such as the distance of the FSO link, the divergence angle of the beam, different modulation formats, and the diameters of the transmit and receive antennas are considered. To investigate the quality of signal reception, the length of the FSO link is varied from 1 km to 5 km. The simulation shows that the quality of the received signal deteriorates as the link length increases. In the case of FSO communication system, different modulation formats have different values of Q factor. A suitable modulation format must be investigated where the performance degradation is the smallest. SS-WDM-FSO link performance in terms of Q-factor is shown in Table 4 for DQPSK, DPSK, CSRZ, NRZ, and RZ. Figure 5 shows Q-factor performance of the SS-WDM-FSO link for different modulation formats. Dual channel is indicated by 2 CH. The Q-factor varies from 34.38 to 31.56 for DQPSK (dual channel), 30.14 to 23.53 for DQPSK (single channel), 25.98 to 19.64 for DPSK (dual channel), 22.96 to 11.78 for DPSK (single channel), 15.20 to 9.57 for CSRZ (single channel), 14.16 to 9.44 for NRZ (single channel), and 12.86 to 9.13 for RZ (single channel) for an FSO link range of 1 to 5 km under clear weather conditions.

Table 4 Values of Q-factor at various link distances for different modulation formats.

Link distance (km)	Q-factor
	DQPSK (Dual channel)	DQPSK (Single channel)	DPSK (Dual channel)	DPSK (Single channel)	Single channel
	DQPSK (Dual channel)	DQPSK (Single channel)	DPSK (Dual channel)	DPSK (Single channel)	CS-RZ	NRZ	RZ
1	34.38	30.14	25.98	22.96	15.2	14.16	12.86
2	33.78	28.99	24.48	19.97	14.56	13.91	12.86
3	33.11	27.60	22.94	17.02	13.34	12.69	12.01
4	32.38	25.82	21.33	14.24	11.54	11.44	11.23
5	31.56	23.53	19.64	11.78	9.57	9.44	9.13

It is found that DQPSK outperforms both DPSK and various intensity modulation formats because this modulation scheme can suppress nonlinear effects to a significant extent, has higher spectral effectiveness, and has good tolerance to polarization mode dispersion and chromatic dispersion. A transmission distance of 12 km is achieved without the use of optical amplifiers for single channel SS-WDM-FSO employing DQPSK. Next to DQPSK is DPSK, followed by CSRZ, NRZ and RZ. Same distance of 12 km is achieved for dual channel SS-WDM-FSO employing DPSK while dual channel SS-WDM-FSO employing DQPSK achieves a distance of 14 km. The performance analysis of the proposed SS-WDM-FSO system is studied in terms of Q-factor by increasing the diameter of the transmit/receive aperture from 5 to 20 cm in 5cm increments under clear weather conditions. By increasing the receiver aperture diameter, an improvement in the Q-factor can be observed because a larger receiver aperture diameter couples more power from the transmitter to the receiver. Therefore, the maximum power is received when the receiver aperture diameter is 20 cm. Different modulation schemes were tested to investigate the system performance. As shown in Figure 6, DQPSK (dual channel) achieves the highest Q-factor.

With further investigation, the beam divergence angle is increased in steps of 0.25 mrad, 0.5 mrad, 0.75 mrad, and 1 mrad. BER of the received signal increases as the beam divergence angle increases and the Q-factor deteriorates. As shown in Figure 7, the maximum Q-factor is reached at a divergence angle of 0.25 mrad and the minimum at 1 mrad.

Table 5 Values of log (BER) at various link distances for different modulation formats

Link distance (km)	Log (BER)
	DQPSK (Dual channel)	DQPSK (Single channel)	DPSK (Dual channel)	DPSK (Single channel)	Single channel
	DQPSK (Dual channel)	DQPSK (Single channel)	DPSK (Dual channel)	DPSK (Single channel)	CS-RZ	NRZ	RZ
1	-259	-199	-149	-117	-52	-46	-44
2	-250	-185	-132	-89	-50	-45	-39
3	-240	-167	-116	-65	-46	-42	-36
4	-229	-147	-101	-46	-38	-37	-36
5	-218	-122	-86	-32	-26	-25	-22

Table 5 shows the values of log (BER) considering link distances up to 5 km for DQPSK, DPSK, CSRZ, NRZ, and RZ modulation schemes.

Figure 8 shows graph of log (BER) (vs) SS-WDM-FSO link distance for different modulation formats, from -259 to -218 for DQPSK (dual channel), -199 to -122 for DQPSK (single channel) , -149 to -86 for DPSK (dual channel),-117 to -32 for DPSK (single channel), -52 to -26 for CSRZ, -46 to -25 for NRZ and -44 to -22 respectively for RZ single channels with FSO link distances from 1 to 5 km. There is an increase in log (BER) value, once link distance is increased. Minimum bit errors are noticed in case of DQPSK, followed by DPSK.

Eye-diagrams of the investigated SS-FSO system at different link distances of 1 km and 5 km are displayed in Figure 9 (a) and (b) respectively for dual channel SS-FSO employing DPSK. Amplitude of the proposed SS-WDM-FSO system reduces with the increase of the link distance. At 1 km link separation, clear eye-opening is noticed and a decrease of eye-opening height can be seen at 5 km.

Eye-diagrams of the investigated SS-FSO system at different link distances of 1 km and 5 km are displayed in Figure 10 (a) and (b) respectively for single channel SS-FSO employing DQPSK. Similarly Eye-diagrams of the investigated SS-FSO system at different link distances of 1 km and 5 km are displayed in Figure 11 (a) and (b) respectively for dual channel SS-FSO employing DQPSK.

Table 6 shows the comparison in terms of results for the proposed research work with other existing works.

Table 6 Comparison of results for the proposed research work with other existing works

Author/ work	Modulation Scheme	Maximum Transmission distance	Maximum Q-factor	Minimum Log (BER)
A.Thakur et al [11]	CSRZ (Single channel)	5 km	15.2	-52
P.Ravesh et al [13]	DPSK (Single channel)	10 km	22.96	-117
Proposed research work	DPSK (Dual channel)	12 km	25.98	-149
	DQPSK (Single channel)	12 km	30.14	-199
	DQPSK (Dual channel)	14 km	34.38	-259

In this research, a spectrum-sliced WDM-FSO communication system with four uniformly distributed optical carriers at a data rate of 2.5 Gbps each is investigated. Nonlinearity, known as self-phase modulation in HNLF, is used to generate an extended carrier spectrum. The performance of the proposed SS-WDM-FSO (single and dual channel) systems integrated with DPSK and DQPSK is compared in terms of Q-factor, log(BER) at different values of transmitter/receiver antenna aperture diameters, beam divergence angles, and link spacing with different intensity modulation formats such as CSRZ, NRZ, RZ. The results show that dual channel FSO performs better than single channel FSO. The results also show that DQPSK performs better than DPSK, CSRZ, NRZ, and RZ in terms of Q-factor and BER. This is because DQPSK can suppress nonlinear effects to a significant extent, has higher spectral effectiveness, and has good tolerance to polarization mode dispersion and chromatic dispersion. A transmission distance of 12-14 km is achieved without incorporating optical amplifiers. Future work to investigate advanced modulation formats such as Binary phase-shift keying (BPSK) and Offset quadrature phase-shift keying (OQPSK) can be considered.

Ethical Approval Not Applicable

Competing interests The authors have no conflicts of interest to declare that are relevant to the content of this article.

Authors’ contributions Conceptualization, methodology, simulation, investigation, writing-original draft preparation –

N.S.V.M.Y

Writing- Review and editing, Supervision – S.R.A, S.P, E.M.K

Funding No funds, grants, or other support was received.

Availability of data and materials All the data generated/analyzed during this study are included in this published article.

Code availability Optiwave - OptiSystem is used and all the schematics generated are included in this published article.

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No competing interests reported.

Spectrum-sliced wavelength division multiplexing based single and dual channel free space optical communication employing differential phase modulation formats

Status:

Version 4

Abstract

Figures

Introduction

Structure Of The System Model

Results And Discussion

Conclusion

Declarations

References

Additional Declarations

Status:

Version 4