# Enhanced Spectral Amplitude Coding OCDMA System Utilizing a Single Photodiode Detection
## 1 Introduction  
Due to its large bandwidth and low attenuation, the optical code division multiple access (OCDMA) system is used for high-quality video transmission [1]. It is a multiplexing technique in which each user in the communication channel is assigned a distinguishable and unique optical code (rather than a wavelength or time slot) at the transmitter [2]. Using low-cost incoherent broadband light sources and complete cancellation of multiple-access interference (MAI) makes the spectral amplitude coding (SAC) OCDMA a system of special interest among all the OCDMA techniques. The reasons for this are its ability to restrain the negative effects of MAI and its flexibility in phase cross-correlation, which can be utilized as address sequence and balance detection at the receiver side [3] when using a suitable detection approach [4], as well as the availability of cost-effective broadband sources such as light emitting diodes (LEDs) [5]. In a previous work [6], the use of broadband sources results in phase-induced intensity noise (PINN), which degrades the communication quality. Recently, a single photodiode (SPD) detection technique using a PIN photodiode was implemented to reduce both PINN and MAI [7], and enhance theperformance compared with other detection techniques such unmodified-AND subtraction detection [8]. Many different codes are developed in the literature for the SAC-OCDMA networks, such as a modified frequency-hopping (MFH) code [9], modified quadratic congruence (MQC) code [10], optical orthogonal code [11], prime code [12], Khazani-Syed (KS) code [13], random diagonal (RD) code [14], and dynamic cyclic shift (DCS) code [15]. As some of these codes have limitations in length, code construction is limited by code parameters, as is the case for MQC [9], DCS [15], and MFH [9] codes, and cross-correlation increases with weight number, as is for the prime and RD codes. In addition, some of the SAC-OCDMA codes do not support a large number of simultaneous users or high data rates [9; 10; 11; 12; 13; 14; 15]. Modified double weight (MDW) codes and enhanced double weight (EDW) codes are utilized as signature sequences for our SAC-OCDMA systems. They are characterized by unity cross-correlation, which is the ideal cross-correlation value, and can support a large number of users and high data rates [16]. For high-bit-rate long-haul fiber optic communications, the use of avalanche photodiode (APD) is preferred because of its internal gain, which provides a relatively better sensitivity margin than PIN photodiodes. APDs can achieve a sensitivity 5-10-dB higher than PINs, provided that the multiplication noise is low and the gain bandwidth product is sufficiently high [17; 18]. For high-speed receivers, researchers used SAC-OCDMA with MDW as signature codes and APD in modified-AND subtraction detection to detect the received signal [19]. The SAC-OCDMA system was compared using EDW codes as signature codes with the SPD detection technique and modified-AND subtraction detection technique using an APD detector at 622 Mbps; the SPD detection technique with an APD was found to improve the performance of the system [8]. In this study, a comparison was performed for a SAC-OCDMA system using EDW codes as signature codes and the SPD detection technique with different detectors (APD and PIN) at different bit rates. The system is simple, as it uses only one photodiode (APD or PIN) in the SPD detection technique to overcome both PIN and MAI. Furthermore, the effect of adding erbium-doped fiber amplifier (EDFA) and dispersion compensating fiber (DCF) on the system performance was studied. Finally, a comparison was performed for SAC-OCDMA using MDW and EDW codes with different detectors (PIN and APD) at different bit rates and transmission distances in the presence of both EDFA and DCF. The performance evaluation characteristics were investigated, including the quality factor (Q-factor) of the detected signal at the receiver end, bit error rate (BER), optimum received power, and eye opening.