## 1 Introduction  
Optical code-division multiple access (OCDMA) [1; 2; 3] has drawn much attention as a promising competitor against time-division multiple access (TDMA) [4] and wavelength-division multiple access (WDMA) [5]. In OCDMA, users are allowed to asynchronously access the network with a high degree of information capacity, security, and processing gain. Other features are the flexibility of adding and removing users in the network and the transparency in different modulation formats. Based on the above characteristics, OCDMA has been employed in a wide range of optical applications, such as metro area networks (MANs) [6], radio-of-fiber (RoF) [7], passive optical networks (PONs) [8], and optical fiber sensors [9]. Each active user expresses the information bits as a unique code sequence. The correlation properties among OCDMA codes guarantee the correct extraction of user information from the multiplexed signals. However, multiuser interference (MUI) [10] induced from the overlapping signals in a shared channel degrades the network quality. Spectral-amplitude coding (SAC) improves the practicality of OCDMA by reducing the MUI effect [11; 12; 13]. Due to the support of asynchronous communications, SAC simplifies system complexity by releasing the requirement of timing recovery. The optical spectrum is encoded by the code sequenceswith a fixed cross-correlation value. Combining the code property with the complementary subtraction algorithm forms an MUI cancellation scheme known as balanced detection [14,15]. At the receiver, the multiplexed coded signals, including the target and the interference users, are correlated with the local code. The data bit \"1\" from the target user can be interpreted as a high autocorrelation peak between the local and the target code. The cross-correlation values between the local and the interference codes are eliminated by balanced detection. Therefore, the user data in SAC-OCDMA systems can be extracted without the presence of MUI. However, the principal limitation on system performance is phase-induced intensity noise (PIIN) [16]. PIIN comes from the incoherency of broadband light sources (BLSs) adopted in SAC systems for encoding. The variance of PIIN is positively correlated with user number and bit-rate. Increasing the transmission speed results in a large noise power that causes the network to fail to reach a designated floor of bit-error rate (BER). Although employing balanced detection eliminates MUI, it is not effective in suppressing PIIN. As PIIN is proportional to the square of the received power, launching a large-intensity light source cannot mitigate this effect. Modifying the decoder structure has become one of the leading solutions to noise suppression in OCDMA [17, 18, 19]. In Reference [20], rather than balanced detection with two photodiodes, a multiphotodiode scheme was proposed. PIIN mitigation is achieved by dividing the received power into multiple photodiodes to lower the detected power. The authors of [21] studied a new receiver structure known as direct detection, where a single photodiode measures the nonoverlapping chips in the target code. Recently, many studies have addressed the interference mitigation problem for wireless CDMA systems [22, 23, 24]. The authors of Reference [25] employed chip-interleaving strategies in the transmitter of direct-sequence (DS) CDMA to suppress interference. The interleaving information is based on the feedback bit sequences traveling through the loop from the receiver to the transmitter. Two uplink power control algorithms, open- and closed-loop control were proposed to limit the interference in femto cell networks [26]. The open- and the closed-schemes varied the transmitted user power to meet the requirement of the interference threshold between femto cell users, and the threshold at the macro cell base station (BS), respectively. In [27], a game theoretical approach was adopted for adjusting the user power to minimize the interference power in wireless CDMA networks.