Performance Enhancement of SAC–OCDMA System Using Modified–AND Subtraction Detection Hamza M. R. Al-Khafaji*, S. A. Aljunid, and Hilal A. Fadhil School of Computer and Communication Engineering, University Malaysia Perlis (UniMAP), Perlis, MALAYSIA * Corresponding author: eng.hamza.alkhafaji@gmail.com (Hamza M. R. Al-Khafaji). Abstract- Technology is already on the move towards exploiting the limits of the fiber’s physical properties and more sophisticated techniques will be needed to increase the throughput and counteract the transmission impairments. In this paper, we present a new detection technique called the modified-AND subtraction detection for spectral-amplitude coding optical code-division multiple access (SAC-OCDMA) systems. This detection technique is based upon decreasing the received signal strength to the decoder’s branches by dividing the weight of the utilized code sequence. The new technique is capable of mitigating the intensity and shot noise impacts, as well as eliminating the multiple access interference (MAI) influence. We simulate the system performance of the proposed detection technique using OptiSystem 9.0 software. The entire practical effects of fiber nonlinearities, dispersion and attenuation were considered in the simulation of SAC-OCDMA system. Based on modified double-weight (MDW) code, the simulation results revealed that the modified-AND subtraction detection improves SAC-OCDMA system performance significantly compared to the conventional AND subtraction detection. Keywords: Optical code-division multiple access (OCDMA); Spectral-amplitude coding (SAC); Modified-AND subtraction detection, Multiple access interference (MAI); Intensity noise. I. INTRODUCTION Recently, OCDMA systems get growing consideration because they enable many subscribers to share the available bandwidth simultaneously with truly asynchronous transmission capability and differentiated quality of service [1]. Conversely, the MAI is considered as the primary performance degradation factor in conventional OCDMA systems [2]. MAI is solely a function of the in-phase cross- correlations among the code sequences. Among all OCDMA techniques, SAC has the advantage of suppressing the first- order (in the mean) MAI when codes with fixed in-phase cross-correlation are utilized as address sequences, and balance detection at the receiver side is employed simply composed of two photodiodes connected in a balanced fashion [3, 4]. SAC-OCDMA systems are outstanding candidates for flexible implementation of optical access networks used to deliver services such as video on demand, multichannel high-definition television (HDTV), ultrahigh- speed Internet, etc., to residential customers [5]. In SAC- OCDMA technique, a spectral encoding has been applied on the output of a broad-band source by decomposing it into spectral bins and the intensity of each bin is modulated such that the bin is either ‘on’ or ‘off’ depending on the user code being applied. Each user has his own unique code, defined by a specific combination of spectral components, which should be orthogonal to all other user codes. The task of choosing the most efficient code sequence is not straightforward because some properties distinguish them. These properties are the number of frequency bins L required to generate the family, the maximum number of codes (i.e. users) available in the code family K, the weight w representing the number of occupied frequency bins in an encoder, and the in-phase cross-correlation λ, which is the maximum number of common frequency bins occupied by any two codes of the family. Therefore, several unity cross- correlation codes have been suggested for SAC-OCDMA systems. One of them, is MDW introduced in [6] with variable weights greater than two. For example, for a weight of four, the code length is given by 2 ) 3 sin( 3 8 3 ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + = π K K L (1) Incoherent broad-band sources such as, a light-emitting diode (LED) or an erbium-doped fiber source, are the best choices for SAC-OCDMA system, since they are inexpensive and provide comparable performance as coherent sources [7]. However, the intensity noise considered as the main performance limitation of these sources, which caused by the phase noise of the incoherent light fields that were mixed and incident at the detection end. This noise is power-dependent; so, increasing the signal strength does not help in improving the signal-to-noise ratio (SNR) [8, 9]. To overcome this problem, researchers have proposed several methods to mitigate the intensity noise in incoherent SAC-OCDMA systems. Among the suggested approaches, using semiconductor optical amplifiers (SOAs) to reduce the intensity noise [9] – [11]; however, this solution is costly with one SOA per occupied frequency bin per user [10]. Another solution is to use a code sequence with ideal in-phase cross-correlation between channels to reduce the total amount of interchannel beating at each detector. As such, it is of interest to design a detection scheme compatible with these codes to alleviate the intensity noise. The rest of this paper is organized as follows: In Section II we describe the SAC-OCDMA receiver based on the modified-AND detection technique; simulation setup and results appear in Section III; and we conclude in Section IV. 2011 International Conference on Computer Applications and Industrial Electronics (ICCAIE 2011) 978-1-4577-2059-8/11/$26.00 ©2011 IEEE 412