International Journal of Science and Engineering Applications Volume 6–Issue 12, 372-381, 2017, ISSN:-2319–7560 www.ijsea.com 372 Efficient Chromatic and Residual Dispersion Postcompensation for Coherent Optical OFDM Ishiwu I. Jude Department of Electrical/Electronics Engineering Modibbo Adama University of Technology, Yola Adamawa State, Nigeria Yahya Adamu Department of Electrical/Electronics Engineering Modibbo Adama University of Technology, Yola Adamawa State, Nigeria Oguche D. Onoja Department of Electrical/Electronics Engineering Modibbo Adama University of Technology, Yola Adamawa State, Nigeria Boyson Andrew Department of Electrical/Electronics Engineering Modibbo Adama University of Technology, Yola Adamawa State, Nigeria Abstract: In lieu of other impairments associated with fiber communication such as; fiber nonlinearity, fading, Intersymbol Interference (ISI), Intercarrier Interference (ICI), Chromatic Dispersion (CD) is compensated by Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) technique. This technique divides the available bandwidth into five subbands, each modulated at a low data rate and postcompensated for the Chromatic Dispersion. Implementation of the Optical OFDM involves the use of Digital Signal Processing (DSP); Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT) both at the transmitter and receiver respectively. The Residual Dispersion left after CD is compensated by Constellation Adjustment Method (CAM). Simulation results using Optisystem show 107-Gb/s single-channel transmission over 1000-km Standard Single Mode Fiber (SSMF) with polarization division multiplexing Four Quadrature Amplitude Multiplexing (4-QAM) using 128 DFT, 82 subcarriers, 5 pilot subcarriers and 16 guard intervals. Equally, the simulation analysis were done at various transmission distances, OFDM systems show a better Min. BER and Max. Q factor than the conventional Non-Return to Zero (NRZ) systems. Keywords: BER, Chromatic Dispersion, CO-OFDM, Cyclic Prefix, DFT, Residual Dispersion. 1. INTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream. OFDM is similar to Frequency Division Multiplexing Access (FDMA) in that the multiple user access is achieved by subdividing the available bandwidth into multiple channels, which are then allocated to users [1]. However, OFDM uses the spectrum much more efficiently by spacing the channels much closer together. This is achieved by making all the carriers orthogonal to one another, preventing interference between the closely spaced carriers. OFDM is a widely used and very attractive modulation and multiplexing technique for broadband wireless and wired communication system due to its spectrum efficiency and channel robustness . OFDM belongs to a broader class of Multi-Carrier Modulation (MCM) carrying data over many lower rate subcarriers[2]. OFDM, forms the basis of many of several telecommunications standards in the world, counting from Digital Terrestrial Television (DTT), Wireless Local Area Networks (LANs), digital radio broadcasting and 4G mobile communications [3]. OFDM is also the source of nearly all Digital Subscribers Line (DSL) standards, and within this context, OFDM is generally known as Discrete Multi-Tone (DMT) because of some minor peculiarities. Though, OFDM offer tremendous benefits and its wide- spread use in wireless communications, it has been considered for optical communications during the last years [4]. The need to develop a high-speed and robust communication services and the tremendous expansion of the Internet are driving the development of high-capacity and flexible optical transport networks. In recent times, many researches started to pay more interest to apply the OFDM technique with MCM, instead of Single Carrier Modulation (SCM) in optical fiber communication due to its ability to reduce the effect of selective fading, chromatic dispersion, Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI). Optical OFDM has gained much interest in recent years as it is developed for long-haul transmission network or rather longer distance transmission and has capability to equalize Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD) efficiently. OFDM technique has been applied so it can be utilized in Wavelength Division Multiplexing (WDM) system [5]. To attain a high spectral efficiency and achieve simple channel equalization, OFDM takes benefit of the Fast Fourier Transform (FFT) [6, 7]. Optical OFDM has become one of the most capable technologies that are used for designing bit rate and bandwidth variable transponders for spectral efficient optical networks. O-OFDM with phase modulation and coherent detection is also the future for suitable spectral efficient key, robust against system nonlinearities, and for transmission in elastic networks[8, 9]. 2. THEORY 2.1 Historical Background of OFDM Chang in 1966, first introduced the concept of OFDM in a seminal paper, he proposed a method to synthesize band limited signals for multi channel transmission. The idea is to transmit signals simultaneously through a linear band limited channel without Inter Carrier Interference (ICI) and Inter Symbol Interference (ISI) [1]. Based on Chang`s work, Salzberg in 1967, performed the analysis and came up with conclusion that the focus to design a multi channel transmission must concentrate on reducing crosstalk between adjacent channels rather than on perfecting the individual