Performance analysis of direct detection and coherent detection system for optical OFDM using QAM and DPSK

Abstract

In this paper a theoretical study is conducted regarding the effect of OOFDM in radio over fiber networks (RoF). The system was simulated at 50Gbps for a transmission distance of 1000km over standard single mode fiber. A performance analysis was conducted to compare the performance of direct detection and coherent detection schemes in terms of their BER, dispersion tolerance and Q factor. The improvement in performance with the use of polarization diversity schemes in CO-OFDM is studied. The study was conducted with QAM and DPSK modulations. The effect of local oscillator power in coherent detection, and that of the laser launch power are studied. The study also shows the slight degradation in performance with increase in subcarrier. I. INTRODUCTION There has been a drastic increase in the demand for high speed communication with mobile connectivity. High speed communication has been traditionally facilitated over wired infrastructure , particularly optical fiber, while mobile communication is supported by wireless infrastructure. But the wireless network has severe performance degradation due to interference, low data rate etc. The design of communication networks that feature high-speed and mobility is facilitated by radio over fiber networks [1]. Orthogonal frequency division multiplexing (OFDM) has been an important candidate in wireless communication since many years. Recently optical OFDM has emerged as a new trend in optical communication networks to combat the effects of dispersion in optical fiber[2][5][6]. Optical OFDM is mainly classified into direct detection system and coherent detection system. In direct detection system a single photodiode is used , While in coherent detection the principle of optical mixing is utilized with local oscillator and optical hybrid. The superior performance of CO-OFDM in terms of its dispersion tolerance and OSNR requirements makes it a suitable candidate for long haul networks. Coherent reception requires that the polarization of local oscillator should be matched with that of the received signal .Otherwise severe performance degradation results. The state of polarization of light travelling through the conventional fiber varies randomly. Hence polarization tracking device is necessary at the receiver, which is impractical in field application. Another alternative is to split the received light into two orthogonal polarizations and process in separate branches of the receiver. The performance of such a diversity receiver would be independent of the state of polarization of the received signal [3] [4]. The paper presents a performance analysis of a coherent optical OFDM system in terms of its BER. Also a performance comparison is done to show the significance of polarization diversity scheme in coherent architecture. A detailed description of CO-OFDM system is given in section 2. Section 3 details the simulations. The results and discussions are given in section 4 and the paper is concluded in section 5.