Performance investigation and development of 112 gbit/s dual polarization 16 QAM transmission system using differential encoding

This article reports the free space optics (FSO) with generation of a high bit rate transmission link with 16 QAM signals. Optical receiver is proposed to enhance the performance of demodulator under environmental conditions. Also, to resolve the adverse channel effects on the information signal digital signal processing (DSP) techniques at the receiver end is used. The bit error rate (BER) analysis using numerical simulations for different weather conditions is bring out and the results illustrate 112 Gbps transmission link. Further, the comparison is conducted with previous techniques which shows that the system provides better data rate performance and achievable range. The reported work provides a suitable reference towards optical communication under different environmental conditions.


Introduction
Staying aware of swift evolution towards data innovation technical area, it has been watched from last decade a massive climb is changed towards higher information and transfer speed executions, which must followed to upcoming need of data rate in routine usage (Fadhil et al. 2013). Optical fiber communication (OFC) technology, which was taken advantage of to take special care of high transfer speed requests for empowering advanced video broadcasting in earthly remote access systems, mobile communication networks (MCN) and different benefits over orthodox radio frequency based connections, yet experienced various 2 Related works Different researched works towards use of FSO innovation in data transmission for different connections (Kumar and Rana 2013;Jeyaseelan et al. 2018aJeyaseelan et al. , 2018bBadar et al. 2018;Prabu et al. 2017;Amphawan 2018a, 2018b;Sarangal et al. 2017;Chaudhary et al. 2014;Kaur et al. 2019), links for space communication (Sharma et al. 2020; Page 3 of 17 70 Alipour et al. 2016;Kumar et al. 2018), links for underwater communication (Willner et al. 2018;Wang et al. 2019), and links for indoor communication (Sharma et al. 2015;Perez et al. 2017) have been accounted in research papers. The research shown in Kumar and Rana (2013) reported impact of power laser system at a longer distance for optical signal transmission. The results indicates towards 1550 nm excels 850 nm frequency. (Jeyaseelan et al. 2018a(Jeyaseelan et al. , 2018b reported the performance of different polarization shift keying for FSO link with 10 Gbps transmission till last end client. Non-return to zero WDM transmission investigation of 80 Gbit/s differential phase-shift keyed (DPSK) signals link interface is accounted in Badar et al. (2018). The use of non-return to zero (NRZ) signals WDM demultiplexer empowered range cutting strategy to accomplish 6 Gbit/s data Tran's reception utilizing in a prescribed laser framework is presented in Prabu et al. (2017).
Utilization of space division multiplexing (SDM) considering photonic gem strands for mode excitation to impart independent bit streams of 2.5 Gbit/s-5 Gbit/s over different modes with two km FSO arrived has been accounted for in Chaudhary and Amphawan 2018a; Chaudhary and Amphawan 2018b). Joining of optical code-division different access (OCDMA) and SDM in light signals to send ten autonomous up to eight km of 10 Gbit/s NRZ-signals to understand a 100 Gbit/s optical Trans receiver system is assessed in Sarangal et al. (2017). Execution of a high-limit FSO framework with moderation to inter-carrier interference and inter-symbol interference with optical orthogonally FDM (OFDM) signals in is discussed in Chaudhary et al. (2014). Analysis and Design with polarization division multiplexed (PDM) effective framework with coherently introduced recipient's, sending OFDM signals with high data rates over various natural circumstances has been examined in Kaur et al. (2019). PDM improve spectral-efficiency of the system he maximum number of bits of data that can be transmitted to a specified number of users per second while maintaining an acceptable quality of service. limitation of PDM technique in FSO transmission, first is it has Extremely difficult algorithms used for decoding the binary information which is transmitted during PSK and second is there are several times which makes it extremely sensitive to phase differences. To send two autonomous 10 Gbit/s NRZ signals using SDM method between two satellites has been proposed in Sharma et al. (2020). An ultra-high link between satellite's Trans reception with WDM architecture containing 64 channels with detailed in investigation of different modulation schemes towards connectivity links. Solid 64 × 40 Gbit/s modified transmission of duo-binary RZ is accounted for up to 251 kms between intersatellite communication systems (Alipour et al. 2016). Utilization for creating a connection between satellite links of sub-carrier multiplexing is accounted in Kumar et al. (2018).
Underwater Tran's reception interface for whole system utilizing momentum with orbital angular displaced multiplexed system is accounted in Willner et al. (2018). Creators detailed utilization with laser of blue and green combined signs to transreceive rapid information in underwater links and examined connection with different impacts. A 520 Mbit/s with 100 m underwater data transreception using NRZ-encoded link with a light emitting diode of green color which is of 520 nm (Wang et al. 2019).
A point by point investigation with effective indoor link connectivity of moving receiving data features is accounted in Sharma et al. (2015). An information high data rate indoor link interface utilizing optical links is accounted in Perez et al. (2017). Solitary channels of quadrature phase shift keyed (QPSK) signal with 4 m/100 Gbit/s indoor link interface utilizing is accounted in Khan et al. (2017).
Low time consumption makes it a resourceful work towards high need towards fast connections with higher data rates, transmission capacity with larger channels & range proficiency, formats with different modulation techniques made their need in development areas. Different shift keying's takes advantage with different components of optical laser towards communication with extended rate information (Zhou and Yu 2009;Zhong et al. 2018).
In APSK, as its name suggests, both amplitude and phase are adjusted. It varies from QAM in that the star constellation points are appropriated on concentric circles in the I/Qplane. The idea was presented for satellite frameworks where the RF power amplifiers show a nonlinear way of behaving. In this manner a plan is required for nonlinear enhancement-an idea with fewer amplitude states-so nonlinearities can be adjusted without any problem. Looking at the constellation diagrams of 16-QAM and 16-APSK in Figure, in 16-QAM there are three amplitudes while 16-APSK has just two. In 32-QAM then, at that point, there are five amplitudes versus three of every 32-APSK. Additionally note that the QAM rings are unevenly dispersed, with some nearer one to another, which makes it much more challenging to make up for nonlinearities. For velocities of 400 Gbps and beyond, 16-QAM is ideal for better optical signal-to-noise proportion execution because of the larger distance between the constellation points. What as of now appears to be clear about the impending 400-Gbps and 1-Tbps transmission frameworks is that 16-QAM will assume a significant part. Multicarrier executions will total channels likely in view of another matrix granularity of 12.5 GHz. Extra significance will be put on "Nyquist pulse shaping" to oblige the expanded measure of information in the current framework with next to no deficiency of data. .
Bitstream transmission through different types of circuits in the procedure can be un-intentionally inverted. Most of the signal executed circuits didn't find out that the entire stream is changed. This is additionally called phase ambiguity. Differential Encoding is used to protect this in the process. It is one of the easiest types of error coding done on a signal prior to the modulation of the signal. Differential coding is also used to provide phase reference in the QPSK technique. The primary reason for Differential Encoding is to protect against polarity reversals of input bit sequences. Subsequently, Differentially Encoded information sequences have a somewhat predominant error performance. Differential Encoding is likewise used to give a method for decoding a signal (Michel 2013).
In the proposed work, 112 Gbit/s Dual Polarization Transmission system with differential encoding techniques using NRZ signals has been employed. The objective of the work is to realize the transmission & reception of signals with the enhancement of link transmission capacity and the received signal quality at the receiver end in presence of weather components/attenuations. In this work DSP module has used for the last destination. Channel modelling and link modeling is briefly discussed in Section 3. Otherwise result has been shown in section 4 and conclusion has been made in section 5.

Link modeling and parameter's
The Illustrative model of 112 Gbit/s dual polarization 16 QAM transmission system using differential encoding is reported in Fig. 1. For evaluating the link performance, in this work optisystem Version, 18.0 software has been used Fig. 2.
The bit sequence generator which is pseudo-random (PBRS) has been used to generate 112 Gbps binary data as shown in Fig. 3a. Then this 112 Gbps binary data is send towards 16 QAM transmitter section. This data is further splitted, irrespectively with 56 Gbps parallel streams of two independent signals. A laser using continuous wave is  Two 56 Gbit/s parallel streams has been send to X and Y orthogonal beams using a 16-QAM modulator system and then combination of parallel streams are possible by a polarization combiner (PC). 112 Gbps DP-16 QAM signal has been produced, this time/level. A gain of 20 dB and a noise figure of 4 dB amplifies has been generated from optical amplifier (OA) which amplifies the signal before transmission through the link towards the receiver end. Mathematical link expression for received power is described as (Kolev et al. 2012): where d R -Diameter for receiving antenna which is 10 cm , d T -Diameter for transmitting antenna which is 10 cm, Ɵ -attenuation coefficient (dB/km), & -angle of divergence (0.25 mrad), P Transmitted -power for transmitted signal (15 dBm), P Received -power for received signal (dBm), Z -range of signal (km).
Different models like gamma-gamma model, negative exponential distribution, log-normal model, double weibull distribution, and so forth have been proposed in the past literature to detect the effect of turbulence on the stage and phase and intensity of the optical beam carrying information. In this work, we have used gamma-gamma model since it is able to do reliably estimating different channel conditions and is the most regularly utilized model (Mahdy and Deogun 2004;Bhatnagar and Ghassemlooy 2016). Numerically, the probability density function f (I) for gamma-gamma model is given as : where α and β represent the number of large scale eddies (Eq. 3) and number of small scale eddies (Eq. 4) respectively, the gamma function and Bessel function are denoted by Γ and K(.) respectively.
where σ 2 represents the Roytov variance and is given as: where k is the wave number and C 2 n denotes the index refractive structure. The received signal which is an optical signal is amplified as a first step using an optical amplifier and then directed towards 16-QAM receiver as second step as diagrammatic in Fig. 3b. Received data at the receiver end is splitted further in two orthogonal changed polarized signals using a polarization Splitter. Received signal has been going through demodulation system using local oscillator & QAM receiver system. Then, electrical amplifier (EA) which has gain of 20dB is used to amplify orthogonal . Also, a DSP module is used for compensate the channel effects as illustrated in Fig. 4. The DSP module executes different techniques refer to processing of signals, which includes filtering to delete the signals, resampling to generalize the signal, quadrature imbalance (QI) compensation technique for equalization to reversal of distortion incurred by a signal transmitted through a channel, frequency offset estimation (FOE) technique and carrier phase estimation (CPE) system. Use of this technique to improve the effects occurred during the signal transreception towards improvement of signals in optical links.

Results
The transmission performance of a signal as a factor regarding weather in clear form is shown in Fig. 5 and the results concludes if range increases then the BER of the system degrades. Also, constellation diagram shows the degradation of system with increasing in range (kms). For climate in clear form, with threshold FEC (Karaki et al. 2013) value (−2.46) & log of BER transreception till 85 kms is achieved. For better signal, proposed work illustrate two signals one at transmitter end which is the binary signal and other at receiving end which is demodulated binary signal in Fig. 6. The results shows the accuracy between data transreceived at 85 kms. The performance output diagram of QAM signal at variable ranges (kms) in the proposed work is shown in Fig. 7. Figure 8a, b, c shows link performance with different weather aspects like hazy climate, rainfall climate, and foggy climate. It has been observed from the diagrams that if attenuation increases then the BER of the system degrades. For hazy climate weather, the 1.537 dB/km attenuation coefficient is for low haze, 4.285 dB/km for mild haze, and 10.115 dB/km for heavy haze and the achieved range is 16.8 km with acceptable BER for low haze, 7.5 km for mild haze, and 4.0 km for heaviest haze (Majumdar 2005). For rainy weather parameter, for light rain the attenuation coefficient is 6.27 dB/km, for moderate rain attenuation coefficient is 9.64 dB/km, and for heaviest rain attenuation coefficient is 19.28 dB/km and the achieved range is 5.5 km with acceptable BER for light rain, 4 km for moderate rain and 2.3 km for heaviest rain (Fadhil et al. 2013). For foggy weather parameter, the attenuation coefficient is 9 dB/km for thin fog, 16 dB/km for thick fog, and 22  Amphawan 2018a, 2018b). The achieved range is 4.1 km with acceptable BER for thin fog, 2.5 km for thick fog, and 1.9 km for heavy fog respectively.
Further, the impact of climate on the BER transmission execution is mathematically examined. Figure 9a, b, c exhibits the link performance for refractive index structure parameter increasing from weak turbulent climate ( C 2 n = 5 × 10 −17 m −2∕3 ) to solid turbulent climate ( C 2 n = 5 × 10 −13 m −2∕ 3 ) over various environment conditions at various ranges and exhibits that the BER increments as the climate disturbance increases from weak to strong turbulence. Likewise, it tends to be seen that the impact of climatic disturbance on link performance is relatively less at the lower link range than at the higher link range. This is on the grounds that the lower link beam size is comparatively small which results in lower phase distortion.  Table 2 shows the contrast of executed work with recently published works. The table shows that the executed work exhibits a higher information rate, prolonged feasible range, and a superior figure of quality (bit rate × range) for all environment states. It has been observed that the works revealed in Jeyaseelan et al. 2018a;Jeyaseelan et al. 2018b;Badar et al. 2018;Prabu et al. 2017;Chaudhary and Amphawan 2018a;Chaudhary and Amphawan 2018b;Sarangal et al. 2017;Chaudhary et al. 2014;Kaur et al. 2019) send fast information deploying multiple channels, but in the proposed work the link reliably transmits 112 Gbps over a single channel. Under all weather conditions, the maximum transmission range with an increased bit rate of the proposed work can be accredited towards the arrangement of dual-polarization 16 QAM transmission with the coherent receiver and DSP Table 1..

Conclusion
112 Gbit/s Dual polarization 16 QAM transmission system utilizing differential encoding has been proposed. A coherent receiver is attached to the system for last-end link connectivity. For free-space losses and carrier phase estimation on the receiver side DSP algorithms are utilized. A significant improvement is found in the optical signal performance which shows in numerical results under the channel impacts using the proposed DSP module. The link is analyzed under various environmental factors and achievable 112 Gbps DP-16QAM signal transmission is reported in the proposed work. The achievable range from 95 to 1.8 km is concluded from the mathematical results shows, when there is an increase in atmospheric attenuation from clear to hazy environment the reach likewise decreased and degrades the BER. A better performance is demonstrated while comparing current work with latest published research works in terms of transmission performance. The current works gives a good reference to design different types of high capacity wireless transmission links under various atmospheric conditions.