Database Based Phase Noise Compensation in CO-OFDM Transmission FEROZ AHMED, REZWANUL HAQUE KHANDOKER, KAFIUL ISLAM, SUBAHA MAHMUDA School of Engineering & Computer Science (SECS), Independent University, Bangladesh (IUB) ABSTRACT In this paper, a new approach of establishing database to estimate phase noise due to chromatic dispersion is proposed. The database can be used to compensate phase shift without using any pilot carriers, which results in high spectral efficiency and low BER. Time variability, temperature and aging effects of channel are introduced in order to ensure the suitability of the database for stable long term performance. The proposal is confirmed by simulation using matlab. Key words: Orthogonal Frequency Division Multiplexing (OFDM), Phase Estimation, Chromatic Dispersion (CD). 1. INTRODUCTION It is important to investigate the optical phase noise impact due to fiber linear and nonlinear effects and establish the optimal approach to estimate the phase evolution of OFDM signals in order to ascertain its suitability for optical transmission. Many dispersion compensation techniques have been proposed and demonstrated. Pilot aided and data aided proposals have already proved themselves as successful phase estimation methods. Adaptive tracking scheme that reduces the effect of laser phase noise and consequently relaxes the laser linewidth requirement has been proposed [1].The impact of system nonlinearity on the OFDM symbol can be moderated by partially filling the OFDM spectrum, or assigning zero values to particular OFDM subcarrier [2]. As a result gathered spurious components generated by fiber nonlinearity, four wave mixing will be located in the unfilled (zero) subcarriers, which will have no impact on the filled (data) subcarriers. Method of dividing the subcarriers in groups and allocating different pilot for different groups is also presented. A common problem with bandwidth requirement is associated with all these methods. Here in this paper we have investigated phase shift due to different subcarrier frequencies considering chromatic dispersion and finally established a database to compensate the phase noise. As it does not require any pilot carriers, it will eventually relax the bandwidth requirement. And if the database is perfectly acquired including time variability and aging effects of fiber channel it will show drastic improvement in the BER performance also. 2. THEORETICAL BACKGROUND Due to fiber linear and nonlinear effects received CO-OFDM symbols will be affected by phase noise. We want to find out the optimum way to improve the BER performance as well as bandwidth efficient. To implement our proposed concept which is described in the next section it is important to know the dependency of this phase noise on different subcarrier frequencies and channel characteristics. Now if h l ki is the frequency response of the fiber including chromatic dispersion, PMD and PDL effects, it can be expressed as [3]: h l ki = e j ∏ M p=1 exp {(-1/2. j. β pi . f k + ½. α pi ). σ} Φ D ki = π. c. D t . f k 2 / f LD 2 … (1) Where, Ф D ki is the phase shift due to the chromatic dispersion, fk is the frequency for the kth subcarrier, Dt is the total chromatic dispersion assuming quadratic dependence on frequency, fLD is the centre frequency of the transmit/receive laser. Here we see the phase shift due to channel characteristics has a relation with subcarrier Feroz Ahmed et al. / International Journal of Engineering Science and Technology (IJEST) ISSN : 0975-5462 Vol. 3 No. 2 Feb 2011 866