# Fault Tolerant Spectral/Spatial Optical Code Division Multiple Access Passive Optical Network
## 1 Introduction
MS code offers fixed in-phase cross-correlation between the adjacent codes because balanced detection is employed at the receiving end. Furthermore, spatial encoding is performed with multiple parallel optical fiber media, which reduces the feasibility of system implementation in terms of cost and complexity. A three-dimensional code with zero cross-correlation property has been proposed in [23]. However, the temporal encoding is employed before the spatial encoding arrangement, and no mechanism is utilized to reduce the number of long-span parallel optical fiber paths between the transmitter and receiver modules. A new three-dimensional successive weight (3D-SW) code OCDMA system has been proposed in [7] for FSO application. The proposed systems offer zero cross-correlation between the adjacent codes. However, employment of conventional spatial encoding results in the utilization of several FSO links. It can be observed from the aforementioned discussion that 1D OCDMA systems are not able to fulfill the ever-growing demand for high-capacity connectivity due to their limited support for a large number of subscribers. Therefore, 2D spectral-spatial OCDMA codes were proposed to offer a large number of codes at acceptable code lengths and cross-correlation properties. However, the utilization of spatial encoding complicates the overall implementation of the system due to its inherent requirement for a large number of parallel optical fiber paths. Three-dimensional (3D) OCDMA systems utilizing spectral, spatial, and temporal encoding techniques were proposed to overcome the limitations of 2D systems regarding data, reach, and the number of subscribers. Nevertheless, no action has been taken to mitigate the problem associated with deploying parallel optical fiber paths for spatial encoding that increases overall complexity and cost of the system and reduces its feasibility of deployment despite high-capacity connectivity. The problems mentioned above have been solved in [5] by implementing a pseudo-3D OCDMA system that employs temporal encoding after the spatial encoding process to reduce multiple parallel optical fiber paths to a single fiber. However, no protection is provided at the feeder level, which decreases the reliability and associated connection availability of the system. To develop an OCDMA architecture that can provide high transmission capacity and acceptable connection availability, this work optimizes the tree-based architecture proposed in [5]. Zero cross-correlation code and the employment of temporal encoding outside the transmitter module facilitate the mitigation of spatial multiplexing, enabling paths over the distribution network (ODN). This is achieved by utilizing a pseudo-3D OCDMA code that employs time delay units (TDUs) after the spatial encoding module to overcome the problem of spectrum overlap over a single communication channel. The proposed model minimizes the network's overall cost and also provides high capacity in terms of data, reach, and the number of subscribers. Furthermore, ring-based topology is deployed at the feeder level to overcome the problem of a single point of failure associated with the tree-based architecture. The analysis of the proposed architecture is performed in two stages. The first stage of the analysis evaluates the system's transmission capacity by implementing the proposed model in Optisystem and observing bit error rate (BER) and eye diagrams against the length of optical fiber media deployed between the remote nodes. The second stage of the analysis observes the improvement in connection availability by performing a reliability analysis in comparison with the conventional structure proposed in [5].