Performance Enhancement of DWDM-FSO Optical Fiber Communication Systems Based on Hybrid Modulation Techniques under Atmospheric Turbulence Channel

: In this paper, we enhance the performance efﬁciency of the free-space optical (FSO) communication link using the hybrid on-off keying (OOK) modulation, M-ary digital pulse position modulation (M-ary DPPM), and M-pulse amplitude and position modulation (M-PAPM). This work analyzes and enhances the bit error rate (BER) performance of the moment generating function, modiﬁed Chernoff bound, and Gaussian approximation techniques. In the existence of both an ampliﬁed spontaneous emission (ASE) noise, atmospheric turbulence (AT) channels, and interchannel crosstalk (ICC), we propose a system model of the passive optical network (PON) wavelength division multiplexing (WDM) technique for a dense WDM (DWDM) based on the hybrid ﬁber FSO (HFFSO) link. We use eight wavelength channels that have been transmitted at a data rate of 2.5 Gbps over a turbulent HFFSO-DWDM system and PON-FSO optical ﬁber start from 1550 nm channel spacing in the C-band of 100 GHz. The results demonstrate (2.5 Gbps × 8 channels) 20 Gbit/s-4000 m transmission with favorable performance. In this design, M-ary DPPM-M-PAPM modulation is used to provide extra information bits to increase performance. We also propose to incorporate adaptive optics to mitigate the AT effect and improve the modulation efﬁciency. We investigate the impact of the turbulence effect on the proposed system performance based on OOK-M-ary PAPM-DPPM modulation as a function of M-ary DPPM-PAPM and other atmospheric parameters. The proposed M-ary hybrid DPPM-M-PAPM solution increases the receiver sensitivity compared to OOK, improves the reliability and achieves a lower power penalty of 0.2–3.0 dB at low coding level (M) 2 in the WDM-FSO systems for the weak turbulence. The OOK/M-ary hybrid DPPM-M-PAPM provides an optical signal-to-noise ratio of about 4–8 dB of the DWDM-HFFSO link for the strong turbulence at a target BER of 10 − 12 . The numerical results indicate that the proposed design can be enhanced with the hybrid OOK/M-DPPM and M-PAPM for DWDM-HFFSO systems. The calculation results show that PAPM-DPPM has increased about 10–11 dB at BER of 10 − 12 more than the OOK-NRZ approach. The simulation results show that the proposed hybrid optical modulation technique can be used in the DWDM-FSO hybrid links for optical-wireless and ﬁber-optic communication systems, signiﬁcantly increasing their efﬁciency. Finally, the use of the hybrid OOK/M-ary DPPM-M-PAPM modulation schemes is a new technique to reduce the AT, ICC, ASE noise for the DWDM-FSO optical ﬁber communication systems.


Introduction
Pulse position modulation (PPM) and digital PPM (DPPM) and systems are modulation schemes that can perform great performance in free-space optical (FSO) transmission links [1,2]. This format is used in a variety of applications, including FSO links, hybrid fibers, optical wireless communication (OWC), subsequent FSO systems, satelliteto-satellite systems, atmospheric turbulence (AT), interchannel crosstalk (ICC), and indoor wireless channels [1][2][3]. The modulation of DPPM improves power efficiency and does not require monitoring of decision-making circuit thresholds [4,5]. Many experiments on the hybrid fiber/FSO (HFFSO) systems and the OWC have been carried out [5][6][7][8][9]. Previous researches have demonstrated that the DPPM and PPM schemes outperform on-off keying (OOK) in terms of sensitivity and power efficiency for the HFFSO link. The M-pulse amplitude and position modulation (M-PAPM) for both pulse amplitude modulation (PAM) and PPM modulation have been investigated and studied for optical fiber (OF) communications [5][6][7][8]. M-PAPM can provide high efficiency and sensitivity in the FSO communication; because the dispersion is free [1,5,8]. Because of the increased bandwidth requirement for higher data rates, OF, AT, OWC and indoor networks [8][9][10][11][12][13][14][15] have been proposed to multiplex the wavelength division multiplexing (WDM) technique and dense WDM (DWDM) systems. The WDM technique could also be used in the HFFSO, OWC, and hybrid OF/multiple networks. For example, the proposed approach designs a high-performance system and bandwidth optimization solution with long-range potential, higher bit rates, high-speed technology, and enhanced data protection for WDM passive optical networks (WDM-PON) [8,9,13,16]. The WDM application has been presented in both OF and FSO systems [8,9,[17][18][19][20][21]. A suitable technique for representing amplified spontaneous emission noise (ASE) in an FSO transmission link is the moment generating function (MGF), although we have upper limits at the bit-error-rate (BER) using the updated techniques of Chernoff bound (CB), Gaussian approximating (GA), and modified Chernoff bound (MCB) [5,8,9,[22][23][24][25]. Both the FSO and OF systems have been presented using WDM technology [8,9,[17][18][19][20][21]. The moment generating function (MGF) represents a suitable technique for finding the amplified spontaneous emission (ASE) noise in an FSO transmission link [2,9,17]. Using the modified Chernoff bound (MCB) technique, Gaussian approximations (GA), and Chernoff bound (CB), we obtain upper bounds on the bit error rate (BER) [5,8,9,[21][22][23][24][25]. To reduce the AT impact, approaches are proposed including compensation for the mitigation techniques and adaptive optics (AO) focused on digital signal processing [22]. We highlight the main contributions, including: (1) we use the M-ary DPPM-M-PAPM modulation to deliver additional bits to improve the efficiency; (2) we add the AO to minimize the ICC interferences and enhance the reliability performances; (3) we achieve adequate BER results with a lower complexity; (4) we extract the theoretical BER expressions and provide simulation results for M-DPPM-M-PAPM modulation schemes in the WDM-PON/HFFSO scenarios; (5) the hybrid OOK/M-ary DPPM-M-n-PAPM and the AO are offered to boost the performance and system efficiency in receivers of WDM-HFFSO systems through the OOK non-return-to-zero (OOK-NRZ) modulation; and (6) we improve the power penalty (PP) performance and the system efficiency for the WDM-HFFSO systems, optical-wireless and fiber-optic communication systems. In this work, we enhance the hybrid OOK/DPPM-M-PAPM techniques and improve the signal-to-noise-ratios (SNRs) of the HFFSO systems under the AT effects, ICC, and ASE. In this work, we enhance the hybrid OOK/DPPM-M-PAPM systems and improve the signal-to-noise-ratios (SNRs) of the HFFSO under the AT, ICC, and ASE effects. We develop the proposed model in the [9,17]. Also, we enhance our calculations and evaluate them to reduce the AT, and ASE noise. The remainder of the paper is organized as follows: Section 2 describes the proposed PON/WDM-HFFSO optical fiber communication system. Section 3 discusses the M-ary DPPM-M-PAPM model for the hybrid WDM-PON/HFFSO link. The AT channel effects are analyzed in Section 4. The numerical results are presented in Section 5. Section 6 concludes this paper.

System Description
The length of slots t s = MT b /n is for the DPPM frames, n = 2 M where T b = 1/R b is for the bit cycle, R b for the data rate and M is for the coding level (CL) [9]. Figure 1a shows the HFFSO-PON systems where optical signals (OSs) suffer from the ASE noise (ASEN), beam-spreading, beam-absorption, attenuation, ICC, and splitting losses at the optical band-pass filter (OBPF)/demux. The proposed system is modeled and simulated using MATLAB software (2013). The results are based on Monte-Carlo simulations. We use eight channels WDM-DWDM starting from 1550 nm over single-mode fiber and the channel spacing in the C-band of 100 GHz of the ITU (International Telecommunication Union). The results demonstrate (2.5 Gbps data rate ×8 channels) 20 Gbit/s-4000 m transmission with favorable performance. We investigate eight-wavelength channels that have been transmitted over turbulent HFFSO links using the WDM technique. The feeder fiber path length of 20 km is proposed for the system model [8,15,17]. By using the optical amplifier (OA) gain G on the remote node, we improve the device model after receiver collection lens (RCL) as seen in Figure 1a. We propose that the OA system is used to reduce the interchannel crosstalk for the proposed system. Figure 1b shows the proposed architecture for all scenarios [9]. As seen in Figure 1, M-ary DPPM is transmitted over the OA and OBPF (b). The receiver converts the OS into an electrical signal. The module of the receivers consists of a photodetector (PD), an electric amplifier, a filter, and a compared circuitry for the decision circuit and AO. We propose the hybrid pulse modulation/M-ary DPPM-M-PAPM for the DWDM-FSO and hybrid optical fiber over the atmospheric turbulence channel. Information is transmitted as a series of DPPM-M-PAPM pulses. The modulated signal from M-ary DPPM-M-PAPM is connected to OA and OBPF to provide M-ary DPPM-M-PAPM output as shown in Figure 1b [9]. For our calculations, we assume the loss of the signal multiplexer (mux)/demultiplexer (demux) (≤3.5 dB) [8,15].

M-ary Digital Pulse Position Modulation and M-pulse Amplitude and Position Modulation (M-Ary DPPM-M-PAPM) Scheme
In this section, the random variable (RV) of the MGF describes the current Y sig ∆t where sig ∈ {0, 1} which depends on the pulses or is not transmitted for the signal pulses, Δt is the duration of the crosstalk pulse. It is written as [5,9,24,[26][27][28]: where ∆t = t s is the time slots align with the OS slots otherwise t 1 or t 2 , and ∆t = 0 for no crosstalk in the slot. Furthermore, the DPPM-PAPM pulse and ICC pulse strength are P tr , P XT , respectively for the hybrid modulation techniques over the FSO link. R ' =  hν i ⁄ ,  is the PD quantum efficiency, ν i , ν and are the optical frequencies of ICC wavelengths and signal respectively and ℎ is Planck's constant, q is the electron charge, [5,8-

M-ary Digital Pulse Position Modulation and M-pulse Amplitude and Position Modulation (M-ary DPPM-M-PAPM) Scheme
In this section, the random variable (RV) of the MGF describes the current Y sig (∆t) where sig ∈ {0, 1} which depends on the pulses or is not transmitted for the signal pulses, ∆t is the duration of the crosstalk pulse. It is written as [5,9,24,[26][27][28]: where ∆t = t s is the time slots align with the OS slots otherwise t 1 or t 2 , and ∆t = 0 for no crosstalk in the slot. Furthermore, the DPPM-PAPM pulse and ICC pulse strength are P tr , P XT , respectively for the hybrid modulation techniques over the FSO link. R = η/hv i ,η is the PD quantum efficiency, v i , ν and are the optical frequencies of ICC wavelengths and signal respectively and h is Planck's constant, q is the electron charge, [5,[8][9][10][29][30][31][32][33][34], N o = 0.5(NF × G − 1) hv is the OA power spectral density (PSD) at the single polarization ASEN. NF and G are the noise figure and OA gain G respectively, L = B o m t t s encompass the system modes for the spatial and temporal method [2,8,9,[35][36][37][38][39][40][41][42], B o is the bandwidth of the optical noise for the demux channel and m t is the number of ASEN states. N o_XT is the PSD-ASEN at the PD and the signal-to-crosstalk ratio C XT = P tr /P XT . The total MGF for Gaussian zero-mean, including the thermal noise variance (TNV) is calculated as [5,9,[35][36][37][38][39][40][41][42]: where σ 2 th−DPPM is the DPPM-PAPM TNV. The means and variances are given as [5,9]: The symbol error probability (SER) is received in the exist of ICC P ws(I i− r i ) : (5) where X j denotes the non-signal slot X o ∆t j and ∆t j is the j (empty) slot overlap with the ICC. Using the GA, the expression P X o ∆t j > X 1 (∆t) is explained from [1,2,5,9,10,29]: In the proposed CB, we have fixed threshold T th and the general form for RV (X) variable and a is P (X For the MCB [2,5] of two RVs for X o ∆t j and X 1 (∆t), P(X > T th ) ≤ M X (s)e −sT th /sσ th √ π [5,9,10]. Modifying this inequality for the diffe rence of RVs s for X o ∆t j and X 1 (∆t) which both have the same TNV then: The SER for MDPPM-MPAPM frames in the exist of ICC is written as [1,9,10]: where and I s , r s are the numbers of ICC system of duration t s occurring in the frame and signal pulse respectively [42], (10) where I 1 , I 2 , and r 1, r 2 are the ICC duration t 1 , t 2 .
where h Z is the total attenuations due to the signal h sig and interfering (h int ) and pointing errors. α and β are the scattering methods of the influence of the large and small-eddies, respectively, and Γ(·) is the gamma function, and K n (·) is the modified second kind Bessel function [8,10].

Conclusions
In this work, we studied and enhanced the HFFSO optical fiber communication network system using the hybrid modulation techniques of OOK/M-ary DPPM-M-PAPM, based on the DWDM-PON network. ICC analyses for DWDM-DPPM systems are given for the GA, CB, and MCB. Also, we consider using hybrid OOK/M-ary DPPM-M-PAPM modulation techniques to increase spectral efficiency and incorporate adaptive optics to mitigate the crosstalk interferences in the DWDM-PON/HFFSO scenarios for improved reliability. The BER performances are then theoretically analyzed. In the presence of atmo-spheric turbulences, the OOK/M-ary hybrid DPPM modulation scheme is an excellent way to increase DWDM-PON/HFFSO efficiency. Furthermore, the results obtained show that the ICC interferences could be effectively suppressed thanks to the M-ary DPPM-M-PAPM modulation and that the proposed system could achieve superior BER performance. We investigate the impact of the turbulence effect on the proposed system performance based on OOK-M-ary-DPPM modulation as a function of M and other atmospheric parameters. The proposed design of the M-ary DPPM-M-PAPM can improve the power penalty over OOK-NRZ and enhances performance efficiency. The proposed M-ary DPPM-M-PAPM architecture can enhance the receiver sensitivity and reliability over OOK-NRZ.
Author Contributions: M.R.H. made substantial contributions to the design, analysis, characterization, conceptualization, methodology, software, data curation, writing-original draft, formal analysis, writing, visualization, investigation, discussed the results, reviewed, approved the article, validation and provided the revised the article critically for important intellectual content and gave final approval of the version to be submitted. B.B.Y. and M.A.A. participated in the conception and critical revision of the article for important intellectual content, supervision, discussed the results, reviewed and approved the article, project administration, data curation, and writing-review & editing and revised it critically for important intellectual content. All authors have read and agreed to the published version of the manuscript.