Suresh Kumar*, Akshaya Dhingra and Payal Arora Evaluation of Proposed Coherent Optical OFDM Link Using X-QAM with Polarization Division Multiplexing https://doi.org/10.1515/joc-2019-0088 Received April 04, 2019; accepted April 30, 2019 Abstract: Orthogonal Frequency Division Multiplexing (OFDM) is one of the popular techniques used for 4 G, 5 G and backbone optical communication networks to meet the large data transfer requirement applications. Optical Communication model based on Coherent Optical OFDM (CO-OFDM) using 4-Quadrature Amplitude Modulation (QAM), 16-QAM and 64-QAM has been evaluated by differ- ent researchers. To understand and evaluate the effects of polarization, we have proposed an Optical Communication model with Polarization Division Multiplexing (PDM). The proposed CO-OFDM PDM system has been evaluated for different link lengths of 50, 100 and 150 km at varying data rates of 10, 20 and 30 Gbps. The performance of the designed link has been evaluated in terms of Q-factor, Bit Error Rate (BER), Optical Spectrum and constellation dia- gram. The results clearly exhibit that the Q-factor, BER and constellations are superior when PDM is incorporated with the CO-OFDM link using X-QAM. Keywords: polarization division multiplexing (PDM), multi-carrier modulation (MCM), QAM, MZM, polarization beam splitter (PBS), polarization beam combiner (PBC), pseudo random bit sequence (PRBS) 1 Introduction With the advancement in cellular network architecture, there is a tremendous rise in the number of mobile sub- scribers. Therefore, we require a network which provides seamlessly fast services to users without any interruption. To resolve this problem, a modulation scheme namely Orthogonal Frequency Division Multiplexing (OFDM) is preferred which supports optical as well as wireless infrastructure utilizing lower system bandwidth [1]. OFDM is a Multi-Carrier Modulation scheme where the whole transmission bandwidth is divided into multiple narrow- band subcarriers modulated with the help of Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) schemes over the transmission channel. OFDM system utilizes Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) algorithms for bet- ter transmission and reception respectively. This scheme also mitigates the effect of Intersymbol Interference (ISI), Intercarrier Interference, Polarization Mode Dispersion, Chromatic Dispersion, etc. with the help of guard band insertion [2]. The major limitations of OFDM are its sensitiv- ity to phase noise (i. e. self-phase modulation and cross- phase modulation), frequency offset and high Peak to Average Power Ratio (PAPR) [3]. The OFDM modulation scheme is utilized in a wide range of high data rate wireless applications such as LTE, Wireless LAN, WiMax, Digital Audio/Video Broadcasting systems and Fibre-to-the-Home (FTTH) services. CO-OFDM is a technique which combines advantages of coherent detection and OFDM. The CO-OFDM provides robustness against polarization dispersion and high power and spectral efficiency. The coherent reception requires the polarization of Local Oscillator (LO) to match with the received signal which otherwise leads to degradation of system performance [4]. The block diagram of CO-OFDM with homodyne detec- tion or direct up/down conversion architecture shown in Figure 1 consists of five functional blocks: Radio Frequency (RF) OFDM transmitter, RF to Optical (RTO) Up-Converter, Optical Link, Optical to RF Down-Converter and RF OFDM receiver [5, 6]. The RTO Up-Converter basically uses parallel I/Q structure consisting of two Mach Zehnder Modulator (MZM) for up conversion of transmitted signal s(t) into real and imaginary parts. At the receiver, direct down architecture uses pair of balanced receivers and a 90° optical hybrid block for I/Q detection. The signal is then passed through Discrete Fourier Transform modulator for data recovery [7]. The CO-OFDM with homodyne detection design eliminates the need for image rejection *Corresponding author: Suresh Kumar, UIET, ECE, Maharshi Dayanand University, Rohtak, Haryana, India, E-mail: skvashist_16@yahoo.com Akshaya Dhingra, akshayadhingra@gmail.com, and Payal Arora, payalarora325@gmail.com, UIET, ECE, Maharshi Dayanand University, Rohtak, Haryana, India J. Opt. Commun. 2019; aop Brought to you by | American University of Beirut Authenticated Download Date | 6/15/19 10:05 AM