# Fault Tolerant Spectral/Spatial Optical Code Division Multiple Access Passive Optical Network
## 2 Proposed Architecture
For instance, for the subscriber \(S_{0,0}\), utilizing the spatial code sequence \(Y_{0}=\left[1\;1\;0\;0\right]\), \(CP_{SLE_{2_{1}}}\) and \(CP_{SLE_{2_{2}}}\) are utilized to implement the spatial encoder. Similarly, for the subscriber \(S_{1,0}\), utilizing the spatial code sequence \(Y_{0}=\left[0\;0\;1\;1\right]\), \(CP_{SLE_{2_{3}}}\) and \(CP_{SLE_{2_{4}}}\) shall be used. Spectral power couplers coupled with power splitters at the spectral decoder are employed in this architecture to replace the use of a star coupler (SC), shown in Figure 2, that requires several optical fiber paths parallelly employed to enable spatial encoding and decoding. Time delay units (TDUs) are introduced to perform temporal encoding with the \(Y^{th}\) code sequence. Furthermore, TDUs are employed to further reduce the optical fiber paths coming out of the \(CP_{SLE_{2}}\) power combiners by delaying the signals through a specific time period so that they can be combined and transmitted over a single optical fiber path across the feeder level (FL). Now, for the subscriber \(S_{0,0}\), output legs of \(CP_{SLE_{2_{1}}}\) and \(CP_{SLE_{2_{2}}}\) are fed into time delays units \(TDU_{1}\) and \(TDU_{2}\), respectively. Similarly, for the subscriber \(S_{1,0}\), utilizing the spatial code sequence \(Y_{0}=\left[0\;0\;1\;1\right]\), \(CP_{SLE_{2_{3}}}\) and \(CP_{SLE_{2_{4}}}\) outputs shall be fed to the time delays units \(TDU_{3}\) and \(TDU_{4}\), respectively. The end-faces of the temporal encoders are then fed into a \(1:t\) power combiner \(CP_{1:t}\), where \(t\) represents the total number of TDUs utilized at CO. The power combiner \(CP_{1:t}\) is utilized to collect all the encoded signals and combine them for transmission over a single mode optical fiber (SMF) medium. Optimization in the existing 2D DW-ZCC OCDMA-based PON is performed at the transmitter module by utilizing a switching arrangement. This arrangement is employed to facilitate a clockwise flow of traffic under normal operating conditions and switch traffic flow between clockwise and counterclockwise under failures at the FL. The switching arrangement's anatomy is shown in Figure 3. The switching arrangement is developed with a 1:2 power coupler \(CP_{SA}\). The main function of CP\({}_{SA}\) is to split the encoded spectrum into two equal parts in order to facilitate the flow of traffic in clockwise and counterclockwise directions for implementation of the ring architecture at the feeder level. Output port 1 of the \(CP_{SA}\) is utilized to carry the traffic in a clockwise direction under normal operating conditions. Port 2 of the \(CP_{SA}\) is connected to a switch, \(SW_{SA}\), as shown in Figure 2. The switching arrangement SW\({}_{SA}\) is utilized to control the flow of traffic under normal operating conditions and in the event of failure. Port 1 of the switch is normally grounded to halt traffic flowing in a counterclockwise direction under normal operating conditions. However, in the event of failure at the FL, the switch moves from port 1 towards port 2. This feature enables the flow of traffic in both a clockwise direction through \(SW_{SA}\) port 1 and a counterclockwise direction through \(SW_{SA}\) port 2, respectively. The end-face of the switching arrangement is fed to the feeder-level ring-based fi The ring-based fiber deployed along the FL carrying traffic in the clockwise direction is received by the first remote node \((RN_{1})\), being the first node at the feeder level. The feeder fiber carrying traffic in the clockwise direction is referred to as \((FF_{cw})\). Similarly, the second half of the ring carrying traffic in the counterclockwise direction is collected by the second remote node \((RN_{2})\), being the last node at the feeder level; this section of the fiber is called \((FF_{Cw})\).