# Fault Tolerant Spectral/Spatial Optical Code Division Multiple Access Passive Optical Network
## 1 Introduction
Spectral-spatial systems have been known as the most effective combination in the 2D OCMDA code family for providing high capacity in terms of data, reach, and the number of subscribers. Such systems utilize spectral conversion of binary codes along the \(X^{th}\) domain and spatial encoding and decoding along the \(Y^{th}\) domain. Figure 2 shows a conventional spectral-spatial OCDMA system with two users, U\({}_{0,0}\) and U\({}_{1,0}\), at the central office (CO) of the network. It also demonstrates the operation of the 2D spectral-spatial OCDMA system, in which spectral encoding is implemented at the first phase of the network with the help of a BBS, coupler (CP), and specifically configured filtering arrangement. User data are modulated after spectral encoding with the help of MZM [16; 17; 18; 19; 20]. The end-face of the MZM is fed to a spatial encoding arrangement that starts by splitting the incoming signal into three equal parts depending on the weight of the OCDMA code. The equally distributed spectral-spatial encoded signal is transmitted over the ODN via long-span parallel optical fiber paths. The light wave paths are then received by the corresponding star couplers (SCs) at the remote (RN). The selection of a corresponding SC depends on the OCDMA code. The output ports of the SCs are fed to distribution fibers that transmit the signal towards the ONT module. The ONT module employs a balanced detector arrangement to receive the incoming spatially decoded signal. The balanced detector is utilized to recover the intended spectrum through spectral decoding. The decoded signal is then passed through an LPF and BER module to retrieve the transmitted information. It can be observed that a two-user spectral-spatial OCDMA system with OCDMA code of \(w=3\) and \(\lambda_{c}=1\) requires a total of sixteen long-span optical fiber paths and five star couplers (\(SC_{1}-SC_{5}\)). Several combinations have been developed to implement high-capacity 2D OCDMA systems. Ref. [17] presents a spectral-spatial OCDMA system using zero cross-correlation codes and multi-mode fibers (MMF). The primary purpose of employing MMF is to reduce the overall number of optical fiber (OF) strands at the optical distribution network. However, MMF are not able to support long-reach connectivity. Furthermore, ZCC code uses code sequences with \(w=3\), which places a cap on the total number of users. The authors in [18] have proposed a 2D spectral-spatial OCDMA system for free space optical (FSO) communication while using a new two-dimensional successive weight (2D-SW) code. However, no solution is provided to reduce the number of fiber strands at the ODN level. A hybrid OCDMA/OFDM system is proposed in [19] to achieve a high data rate at extended reach and a relatively large number of subscribers. The solution can extend the system's communication capacity; however, no work is conducted toward reducing fiber strands at the feeder level. A different study in [20] uses the polarization approach to increase the system's transmission capacity. However, the polarization technique does not compress the OCDMA codes, which makes it difficult to achieve large cardinality. Three-dimensional OCDMA codes have been proposed to further elevate the cardinality and capacity of the communication system by reducing the overall complexity of the  
Figure 2: Two-dimensional (2D) spectral–spatial OCDMA architecture. OCDMA network. Ref. [21] proposes a 3D OCDMA architecture based on MD with spectral, temporal, and spatial encoding. The proposed architecture claims to reduce the overall system complexity by 50 percent and offers acceptable transmission capacity. However, no remedy is provided for the reduction in multiple parallel paths along the feeder level to implement spatial encoding. Reference [22] proposes a 3D OCDMA system based on multi-service (MS) and multi-diagonal (MD) OCDMA codes while using spectral-temporal and spatial encoding.