Performance analysis of various PSK modulation schemes for high-speed long-haul fiber-optic communication system

Abstract

In recent years, traffic on the internet has increased rapidly because of the increase in the number of users. Use of VOIP, video conference, multimedia, online data transfer and various software applications has also increased the data rate. As a result, the backbone network should have high speed and capacity which can be provided by the optical fiber. The advantages of using fiber-optic communication over wireless communication are that it is free from Electro-Magnetic Interference (EMI), Radio Frequency Interference (RMI) and highly secured etc. On-off keying (OOK) modulation has been used in optical transmission system for a long time because of its simplicity and low cost. Currently most commercial systems use OOK format at 10 Gbps data rate. If the bit rate is increased greater than 40 Gbps then the dispersion, nonlinear effect, intra-channel effects will increase much. As a result OOK modulation is not suitable in high-speed optical communication systems. On the other hand, phase shift keying (PSK) modulation techniques may provide higher spectral efficiency and better performance than OOK since PSK signals have constant amplitude and compact spectrum. Moreover PSK signals are robust to dispersion and nonlinearity. PSK receiver with coherent detection is costly and complex. However, direct detection receiver can be used in differential multilevel PSK modulation schemes which are simpler and comparatively low cost. That is why PSK modulation schemes are becoming popular for next generation optical communication system. In this thesis, the performance of DBPSK, DQPSK and 8-DPSK modulation schemes is evaluated in high-speed long-haul fiber-optic communication system. The performance parameter is quality factor Q, which is observed by varying bitrates, input power, distance and residual dispersion. The Split-Step Fourier method is used as numerical technique for solving Nonlinear Schrodinger Equation which governs the propagation of light in optical fiber. In this simulation dispersion-managed transmission line is used in order to achieve perfect dispersion compensation for each of the three modulation schemes for different fibers and bit rates. However, practically zero dispersion is not possible, even if one channel can be made approximately perfectly compensated, other channels could find residual dispersion due to dispersion slope. The performance parameter Q is measured from eye pattern which is attained from numerical simulations. Therefore, optimum PSK format has been checked which is suitable for high-speed communication. The result obtained from this work will be useful for designing high-speed long-haul fiber-optic communication system.