Study of Performance of High speed WDM networks using different Modulation Techniques

Abstract

Over a period of last two decades, fiber optic communication system has evolved from a simple single channel, short distance communication at a data rates of few gigabits per second to multiple channel Wavelength Division Multiplexing(WDM) systems operating at as high as 100 Gb/s and beyond over thousands of kilometers. To accommodate more channels on a single fiber, the channel spacing is brought down from 200GHz to 50GHZ or even less. The data rates are ranging from 10 Gb/s, 20 Gb/s, 40 Gb/s, 100 Gb/s and efforts are going on to raise this data rates higher. At higher data rates, linear and nonlinear effects like chromatic dispersion, polarization mode dispersion, self phase modulation, cross phase modulation etc. affects on signal propagation through the fiber. These effects play a vital role in performance degradation. The efforts are made by researchers to minimize the effects of such impairments by using more tolerant modulation formats. In coherent detection technique, advanced digital signal processing is employed to increase the data rates and spectral efficiency. In this thesis, we have studied different modulation techniques suitable for high speed optical communication system. Performance evolution is carried out for NRZ and RZ OOK, DPSK, DQPSK and PM-QPSK for single channel and multi-channel WDM networks. Different modulation formats are compared in terms of data rates, distance of transmission link and finally the BER. PM-QPSK modulation with coherent detection is also used for WDM system consisting of 4-channels. The RZ-DPSK and DQPSK formats are found better in terms of the tolerance to such impairments. Our simulation results shows that for 4-channel WDM link a distance of around 180km is achieved at a data rate of 50 Gb/s. In DQPSK, the data rate can be achieved as high as 80 GB/s over a distance of 600km. PM-QPSK with coherent detection can be used for ultra high speed, long haul distance communication. Our simulation results show that a transmission distance of 3600km at a data rate of 100 GB/s is achieved for WDM system.