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Research Project

Optimized pulse shaping for efficient performance of digital backward propagation in 112 Gbit/s DP-QPSK transmission

by Muhammad Usman Sadiq under supervision of Prof. Dr.-Ing. Bernhard Schmauss, Lehrstuhl für Hochfrequenztechnik (LHFT), University of Erlangen-Nürnberg (FAU) (31.10.2012)

Since 1990's optical communication systems have been deployed all over the world but with the passage of time information traffic is increasing mainly due to excessive internet usage. To accommodate for current traffic requirements, high speed optical transmission systems and new detection technologies, i.e. coherent receivers, have become really important. In the previous decade wavelength-division-multiplexing (WDM) techniques have been used to increase the capacity of on/off keying (OOK) and direct detection (DD) up to 10 Gb/sec. To realize high capacity long-haul optical transmission systems the following enabling technologies are used which are summarized in Figure 1.

Fig.1: Enabling technologies for high capacity optical transmission systems[Bildunterschrift / Subline]: Fig.1: Enabling technologies for high capacity optical transmission systems

For example technologies such as OOK and DD have been replaced by advanced modulation formats and coherent receiver. After transmission through the optical fiber an optical signal suffers from the linear, e.g. dispersion (D), and non-linear impairments, e.g. self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM). These impairments must be compensated to achieve sufficient quality of the received signal. Bisides actually implemented filter based concepts, Digital Backward Propagation (DBP) algorithm is gaining importance as it allows the joint compensation of linear and non-linear impairments of the fiber. DBP propagation is applied at the coherent receiver after the coherent detection of the optical signals.
In this master project, mainly the effect of root raised cosine (RRC) pulse shaping is evaluated on the performance of DBP algorithm for the 112 Gbit/s dual polarization (DP) QPSK system. The performance of the DBP for non-return-to-zero (NRZ), return-to-zero (RZ) and RRC pulse shape is also compared. It is shown that applying RRC pulse shaping with help of finite impulse response (FIR) filters increases the non-linear tolerance of the system by 2 dB of input power as compared to simple RZ and NRZ modulation formats. Increase in non-linear tolerance of the system means that higher input power can be injected into the DBP based optical communication system without causing significant nonlinear signal distortions. Thus increased transmission distances can be realized. Figure 2 shows a complete system diagram including the integration of pulse shaping FIR filter in a DBP based optical communication system.
Furthermore a modified concept of the optimized RRC pulse shape is investigated. This idea is implemented again using RRC pulse shaping filters. The pulse is optimized by controlling the duty cycle of the digital pulse. Several factors of the RRC pulse shaping filters are required to be optimized to achieve an optimum performance. Performance of DBP algorithm is again tested for both the optimized RRC and the standard RRC pulses. In terms of the non-linear performance the optimized RRC pulse outclassed the simple RRC pulse in both single channel and 3-channel WDM system.

Fig.2: Implementation DBP with FIR pulse shaping filters[Bildunterschrift / Subline]: Fig.2: Implementation DBP with FIR pulse shaping filters

A multi-span DBP analysis is also performed for the same of 112 Gb/sec DP-QPSK system. It has been shown that by employing RRC pulse shaping in multispan-DBP the computational efforts can be reduced by 80% as compared to per-span DBP compensation. Using the different transmission scenarios the robustness of DBP is also tested and it is proven that algorithm works even with ±50 Km mis-information about the total transmission distance.
In a nutshell, a comprehensive study has been performed to evaluate the performance of DBP from different aspects especially together with an optimized pulse shaping. Although DBP is an offline processing technique until now it has been shown that DBP is one the potential DSP algorithms to be implemented in real time for the next generation optical communication systems.

Muhammad Usman Sadiq
* 1987
Education/ Qualifications
10/2010-02/2012 M.Sc. Advanced Optical Technologies Friedrich Alexander University (FAU) Erlangen-Nürnberg ,Germany
2004-2008 B.Sc. Communication Systems Engineering Institute of Space Technology (IST), Islamabad Pakistan

Employment History
Alcatel-Lucent Nürnberg, Germany
Student Internship (Optical Networks Division
Fraunhofer IIS, Erlangen, Germany
Student Research Assistant (High Frequency Department)
National Space Agency of Pakistan
Research Engineer (RF Communications Group)