Analysis and performance improvement of high speed DP QPSK WDM systems

Abstract

Fiber-optic communication systems provide high capacity transport infrastructure that enables data services, high-speed internet applications, and telecommunication services. The 100G DP-QPSK WDM technology, where every wavelength carries a data rate of 112 Gb/s, is helping to achieve a tenfold increase in spectral efficiency (SE) in comparison to conventional 10G WDM. This thesis analyzes the performance of 100G DP-QPSK WDM systems for long-haul applications with an aim to provide solutions so as to meet future challenges of better performance over legacy infrastructure with higher SE. The adaptive equalization (AE), frequency offset estimation (FOE), and carrier phase estimation (CPE) play an important role in equalizing various channel impairments such as polarization mode dispersion (PMD), residual dispersion, and phase errors. In this research work, it has been investigated that for DP-QPSK WDM system with 50 GHz channel spacing, it is better to have FOE-AE-CPE as the process flow because, for such systems, frequency offset is more dominating due to very high carrier frequency. The effects of the proposed process flow on the behavior of the constant modulus algorithm (CMA) based AE has also been analyzed. In current literature a variable step size CMA is preferred for AE, however, for the proposed process flow it is shown that a simple fixed step size CMA can also adequately compensate for the residual dispersion. Further, a reduction in channel spacing is explored to enhance system capacity. However, in a DP-QPSK WDM system if channel spacing is reduced below 50 GHz, then transmission reach falls drastically and long-haul transmission reach becomes difficult to achieve. This complete scenario demands a different set of solutions compared to 50 GHz channel spacing. The effects of system parameters like filter order of the multiplexer, launch power per channel and filter taps of AE have been investigated and their values have been optimized to improve transmission reach at 37.5 GHz channel spacing.