# Fault Tolerant Spectral/Spatial Optical Code Division Multiple Access Passive Optical Network
## 2 Proposed Architecture
The internal structure of \(RN_{1}\) is shown in Figure 4, which essentially houses multiple passive couplers to facilitate the formation of the ring at the feeder level along with the establishment of spatial decoding for the distribution level [25; 26; 27]. The \(FF_{cw}\) is received by a 2:2 optical coupler \(\left(CP_{RN_{1}}\right)\). Here, \(\mathrm{CP_{R}}\) is used to represent the coupler that is employed for the formation of the ring topology at the feeder level and extension of traffic towards the feeder level. Furthermore, the subscript \(\mathrm{RN_{1}}\) represents the first remote node employed across the ring topology. Ports 1 and 3 of the \(CP_{RRN_{1}}\) are utilized for formation of the ring at the feeder level, such that \(CP_{RRN_{1}}\) receives the incoming traffic through port 1 and extends it towards the feeder level via port 3. Ports 2 and 4 of \(CP_{RRN_{1}}\) are extended towards a 1:2 optical coupler \(\left(CP_{RN1 SLD_{1,1}}\right)\), as shown in Figure 4, and are used to transfer the received encoded spectrum towards the ONT module through the distribution level [25; 26]. The optical coupler \(CP_{RN1 SLD_{1,1}}\) is used as the first stage of the spatial decoding process. The basic purpose of \(CP_{RN1 SLD_{1,1}}\) is to split the incoming signal into equal parts as per the weight of the \(Y^{th}\) code sequence. Here, \(CP_{RN1}\) represents the coupler employed at the first RN, and \(SLD_{1,1}\) represents the first coupler employed at the first RN for the spatial decoding operation. Both legs of the \(CP_{RN1 SLD_{1,1}}\) are connected to two 1:2 optical couplers: \(\left(CP_{RN1 SLD_{2,1}}\right)\) and \(\left(CP_{RN1 SLD_{2,2}}\right)\), respectively. Here, \(SLD_{2,2}\) represents the first coupler that is employed to perform the second phase of the spatial decoding operation. This phase consists of splitting the signal into two equal parts so that each node at the receiving end receives its intended spectrum from multiple parallel paths. This is achieved by connecting both couplers to the distribution fibers that are extended towards the ONT modules. Figure 4: ODN and receiver section of the proposed 2D DW-ZCCC ODCMA architecture. At the receiving ONT, for subscriber \(S_{0,0}\), the distribution fibers are received by a filtering arrangement, which essentially consists of band-pass filters configured to recover the intended spectrum, as shown in Figure 4. The reception arrangement is configured such that the user \(S_{0,0}\) receives one DF from \(CP_{RN15 LD_{2,1}}\) and another from the \(CP_{RN15 LD_{2,2}}\), respectively. This is performed to accomplish the process of spatial decoding as per the encoding arrangement performed at the transmitter module. Furthermore, the setup is configured in a way to ensure the recovery of \(\lambda_{1}\) and \(\lambda_{2}\) for \(S_{0,0}\). The output of the spatial decoder is then fed to the temporal decoder that is configured in accordance with \(TDU_{1}\) to perform the required temporal decoding at the ONT module. The decoded signal is then applied to the photodiode, which converts the optical signal into the electrical domain for necessary filtering and recovery operation.