Abstract
In this paper, the performance of duobinary system is analyzed by using different dispersion compensation techniques. The dispersion compensation techniques tested are Pre Compensation, Post Compensation and Mix Compensation. These techniques are applied to duobinary system, which operates at a bit rate of 50 Gbps. It is found that for 50 Gbps system, Mix Compensation technique shows better performance matrices like quality factor (i.e. 7.54 at 25 km)and bit error rate (i.e. 7.52e-15 at 25 km) as compared to other techniques.
Keywords: Mach-Zender Intensity Modulator (MZIM), Single Mode Fiber (SMF), Q-factor, Bit Error Rate (BER), Low Pass Filter (LPF)
Introduction
For higher data rates, research in optical communications is being constantly driven by requirements. At a minimum bandwidth cost, Fiber Optics has reorganized the data communication technology by examining the limits of high speed network accessibility for the end users [1]. In high speed optical communication system duobinary modulation is a valuable solution that provides the better spectral efficiency and minimizes the performance degradation due to the nonlinear effects and dispersion [2–4]. Due to the promptly growing capacity requirements for long distance transmission, fiber optic communications are advancing into higher bit rate enabled [5]. To increase the capacity of system and to reduce the performance degradation caused by transmission impairments, systematic investigation is essential [6]. Duobinary formats are known for their high tolerance to residual chromatic dispersion and low spectral occupancy [7]. These features make them very attractive for both high spectral efficiency and high data rate. For high speed systems, Duobinary signaling has become an essential transmission format as the broadband networks and the bandwidth requirement has increased. By selecting suitable pulse shaping, the selection of optical modulation format has become an essential standard in any high speed link design. This optical signal pre-distortion based pulse shaping increases the dispersion tolerance related performances considerably [8, 9].

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In long distance transmission systems, Fiber chromatic dispersion is one of the most severe limiting factor. If the fiber transmission length exceeds several tens of kilometers, dispersion effect can cause intolerable amounts of distortions that ultimately lead to errors. Therefore it is necessary to use dispersion compensation devices such as dispersion compensating fiber (DCF) [10, 11] to overcome dispersion effect and consequently decrease the nonlinear distortion. In this study, we propose three DCF compensation scheme, pre-compensation and post-compensation scheme. Simulation studies show that mix compensation scheme is the best. It can greatly reduce the influences of the fiber nonlinearity and increase the transmission distance greatly
System Setup
Duobinary Transmitter is designed with laser diodes, filters, modulators and all components which are essential to build an optical network. This simulation is carried out to observe the comparative study with various compensation techniques in the presence of chromatic dispersion. Duobinary signal is launched over DCF SMF spans of 5 km and 25 km each for post, pre and symmetric compensation schemes. Duobinary modulation is achieved by driving an external Mach-Zehnder intensity modulator. MZIM has three inputs, one for laser diode and other for data from the channels. It converts the electrical signal into optical signal. On the receiver side the output of the Lorentzian optical filter a photodiode converts the optical signal into an electrical signal an electrical low pass Bessel filter follows the PIN photodiode. This has a cut-off frequency 193.41449 THz. Finally at the output of the low pass filter visualization tool called Scope, BER estimation Q meter. It is an optical or electrical oscilloscope with numerous data processing options, eye display and BER estimation features. The system setup of 50 Gbps duobinary transmission with pre, post and symmetric compensation techniques is as shown in figure below.

Fig. 1 Duobinary system with Post Compensation technique

Fig. 2 Duobinary system with Pre Compensation technique

Fig. 3 Duobinary system with Mix Compensation technique
Pre-compensation scheme achieve dispersion compensation by place the DCF before a certain conventional single-mode fiber, or after the optical transmitter. Post -compensation scheme achieve dispersion compensation by place the DCF after a certain conventional single-mode fiber, or before the optical transmitter. Mix compensation scheme is consist of post-compensation and pre-compensation
Result and Discussion
To evaluate the performance of 50 Gbps duobinary system several measurements for Pre, Post and Symmetric compensation techniques were taken. The quality factor versus transmission distance is as shown in Fig.4. The graph shows that the performance of pre, post and mix compensation is compared by varying the distance from 5 to 30 km.

Fig.4 Quality Factor vs Transmission distance
Fig.4 depicts quality factor versus transmission distance graph. It is observed that by increasing the transmission distance from 5 to 30 km, Quality factor is decreasing. The variation in Q factor is 22.26 to 5.67 for mix compensation, 19.47 to 4.84 for post compensation and 14.04 to 4.69 for pre compensation. It is observed that maximum quality factor is shown from mix compensation technique i.e. 7.54 (at 25 km transmission distance) as compared to post and pre compensation techniques which is 6.33 and 6.12 respectively.

Fig.5 Bit error rate vs Transmission distance
Fig. 5 shows the transmission distance vs bit error rate graph. The variation in BER from different compensation techniques is 4.05e-72 to 8.20e-8 for mix compensation, 4.61e-58 to 6.57e-7 for post compensation and 4.50e-45 to 4.48e-5 for pre compensation. This simulation result shows that at 25 km transmission distance, the minimum bit error rate value is obtained by mix compensation technique which is 7.52e-15 whereas the bit error rate value for post and pre compensation technique is 3.04e-13 and 1.75e-10 respectively.

Fig. 6 Quality Factor vs Input Power
Fig. 6 display the influence of signal input power on the performance of duobinary system. From the graph, we can find that as the signal input power increases, quality factor increases upto certain limit, after which it starts falling. This can be understood from the fact that for low powers, the performance of system improves with the increase in input power. However, at higher powers, the wavelengths tend to overlap each other causing more dominance of non-linear effects and thus reduce the quality factor. From the graph it also concluded that the quality factor of mix compensation is greater than the other two kind of compensation techniques.
Conclusion
In this paper, we investigate the behavior of Pre, Post and Mix compensation techniques on the basis of quality factor and bit error rate at 50 Gbps system and conclude which compensation technique perform better. From the comparative performance analysis for different compensation techniques, it is found that mix compensation is better than pre and post compensation techniques for long haul communication system. It may also be concluded that for lower laser input power, quality factor is better for all compensation techniques.
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