Performance of OCDMA systems using a complementary subtraction technique Feras N. Hasoon, 1, * Mohamad Khazani Abdullah, 2 S. A. Aljunid, 3 and Sahbudin Shaari 1 1 Institute of Micro Engineering and Nanoelectronics, University Kebangsaan Malaysia, 43600 UKM, Bangi, Malaysia 2 Computer System and Communication, Faculty of Engineering, University Putra Malaysia, 43400 UPM, Serdang, Malaysia 3 Computer and Communication Engineering, Northern Malaysia University College of Engineering, 02600 Jejawi, Perlis, Malaysia * Corresponding author: feras@vlsi.eng.ukm.my Received October 27, 2006; revised April 17, 2007; accepted May 9, 2007; published June 13, 2007 Doc. ID 76526 What we believe to be a new code structure to reduce the multiple access in- terference in a spectral amplitude coding optical code-division multiple access system based on double-weight code families is proposed. Enhanced double weight (EDW) possesses ideal cross-correlation properties, and weight can be any odd number that is greater than 1. The theoretical analysis shows that EDW codes can more effectively suppress the phase-induced intensity noise and provide a significantly better performance compared with modified qua- dratic congruence and partial modified prime codes. © 2007 Optical Society of America OCIS codes: 060.2330, 060.2310. 1. Introduction In optical code-division multiple access (OCDMA) systems, the detection process affects the design of transmitters and receivers. In general, there are two basic detec- tion techniques: namely, coherent and incoherent. While coherent detection refers to the detection signals with knowledge of the phase information of the carriers, incoher- ent detection refers to the case without such knowledge. Alternatively a system con- sisting of unipolar sequences in the signature code is called an incoherent system. A system that uses bipolar codewords is called a coherent system. Because incoherent detection does not need phase synchronization, hardware complexity of the system is reduced. This is the main reason why we have chosen incoherent detection in this research. In an incoherent code-division multiple access (CDMA) system, each user is assigned a distinct codeword as their address signature based on the spectral ampli- tude only. When a user wants to transmit data bit one, he sends out a codeword cor- responding to the address signature of the intended receiver. At the receiver, all the codewords from different users are correlated. If a correct codeword arrives, an auto- correlation function with a high peak results. For incorrect codewords, cross- correlation functions are generated and they create multiple access interference (MAI). MAI can be reduced by using a subtraction technique. The most common sub- traction technique is the complementary subtraction technique, which is also known as the balanced detection technique [1,2]. Many codes have been proposed for optical spectrum CDMA such as optical orthogo- nal codes (OOCs) [3], prime codes, and modified frequency-hopping (MFH) codes [4]. Enhanced double-weight (EDW) code is easy to construct compared with OOC and MFH codes; the ideal cross correlation is compared with Hadamard and prime codes; and code length is not as long as OOC and prime code. Long code lengths are consid- ered disadvantageous in its implementation, since either very wideband sources or very narrow filter bandwidths are required [5]. In this paper we proposed a new code structure: the EDW codes. Having the cross-correlation properties of one as well as a constant weight, its performance is compared with what has been achieved in MQC and PMP codes [6,7]. This is carried out through the theoretical calculation. We also Vol. 6, No. 7 / July 2007 / JOURNAL OF OPTICAL NETWORKING 854 1536-5379/07/070854-6/$15.00 © 2007 Optical Society of America