# Wheel-Based MDM-PON System Incorporating OCDMA for Secure Network Resiliency
## 2 Proposed Design  
Figure 1 illustrates the conceptual diagram of wheel-based architecture, first proposed by [PERSON] in [1], which incorporates MDM-based TWDM-PON. OLT in the design is positioned in the mid of the fiber ring linked by the '_n_' number of RNs, which are further connected to the '_2n_' number of connected ONUs to build a close-loop across the optical ring with end nodes. Here, the defined wavelengths \(\lambda_{n}\) are assigned for downlink and uplink transmission to each RN with donut modes 0 and 1. OLT helps to identify, control and communicate inter RNs. Each RN is further connected to two ONUs through a fiber link, each at a single wavelength but with two different donut modes. At each RN in wheel topology, the broadcasting, completion, regeneration and rebroadcasting of wavelength for the OLT path is performed clockwise via a unidirectional ring. Moreover, in the wheel-PON architecture, the OLT is fixed with an _n_-secure number of transmitters (Tx) and receivers (Rx) employing an OCDMA code. Each RN is connected with a pair of ONUs working on distinct modes at each Tx/Rx as well as the same defined wavelength is used to handle the downlink and uplink traffic through two fixed fiber links. Additionally, switches are positioned with OLT to transfer the data to the respective queue, as decided by a scheduler for distinct dataconditions. In this architecture, the fiber capacity is utilized under the link failure scenario. The wheel topology directs the data from the failed/break path onto the alternative linked path that mainly encloses the OLT channel or nearby RN or ONU to reduce the failure consequences [1; 35; 36]. Figures 2-5 illustrate the block diagram of the proposed bidirectional MDM-PON system by incorporating MWZCC OCDMA code with generated modes. In the proposed system, there are '_n_' numbers of channels with a group of eighteen channels working at two MWZCC code patterns for two users. For the downlink, at the transmitter side, a continuous wave (CW) laser array operating at 10 dBm input power was utilized. Here, a single CW laser array generates eighteen wavelengths of 1596-1602.8 nm with 0.4 nm channel spacing, as depicted in Figure 2. An ideal 18:1 WDM MUX multiplexes all incoming input signals and transfers to the 1:18 WDM DEMUX in each downstream transmitter (Tx dn1). Each encoder incorporates a 4:1 WDM MUX to multiplex distinct input wavelengths according to MWZCC code sequence, pseudo-random bit sequence generator (PRBS) at 10 Gbit/s to induce a random bit sequence, a pulse generator non-return to zero and Mach Zehnder modulator (MZM) for modulating the MWZCC coded signal. The output of MZM is transferred to the mode generator that yields the donut mode 0. Likewise, in another encoder, the next coded modulated signals are transmitted to the donut mode generator that yields the donut mode 1. Afterward, a 2:1 power combiner was incorporated to join the incoming different modes signals. An _n_:1 power combiner was then used to transmit the '_n_' number of transmitters (Tx dn_1 to Tx dn_n) followed by a 3-port optical circulator to direct the different incoming modes downstream signals for propagation via bidirectional MMF using two ports. The third port of the circulator was used for upstream transmission at OLT. The generated modes 0 and 1 are presented in Figures 3 and 4, respectively. Two separate MMFs are used for uplink and downlink transmission simultaneously, as depicted in Figure 5.  
Figure 1: Conceptual diagram of network resilience wheel-based hybrid MDM-OCDMA-PON using modes 0 and 1. Figure 3: Generated donut mode 0 in MDM-PON system. Figure 2: Proposed design of bidirectional MDM-PON system incorporating MWZCC OCDMA code of transmitter section. For data received, a 1:2 bidirectional power splitter working as RNn was utilized to split the optical data into two segments; one for the downlink receiver and another one for the uplink transmission.