# Fault Tolerant Spectral/Spatial Optical Code Division Multiple Access Passive Optical Network
## 1 Introduction  
Optical code division multiple access (OCDMA)-based passive optical networks (PONs) have gained significant attention over the past two decades due to their support for asynchronous connectivity with relatively simple architectures. OCDMA-based PONs rely on the principle that each user can access the medium simultaneously through the allocation of a unique code. Each code in the OCDMA family is designed with three basic performance parameters called code length (\(L\)), code weight (\(w\)), and cross-correlation (\(\lambda_{e}\)), respectively [1, 2, 3, 4]. Communication networks have seen exponential growth in terms of data and the number of subscribers over the past few decades, and with the introduction of networks like beyond 5 th generation (B5G) wireless communication technology, 6 th generation(6G) networks, and next-generation PONs (NG-PONs), the need for a robust underlying network has become more severe than ever [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. In order to overcome the stringent challenges of high capacity and cardinality along with affordability, OCDMA codes evolved from a one-dimensional (1D) paradigm to two-dimensional (2D), three-dimensional (3D), and pseudo-3D domains. The basic goal of this expansion is to offer the best possible combination of \(L,\ w\), and \(\lambda_{c}\) properties in order to support the ever-growing demand for the upcoming networks [5]. One-dimensional (1D) OCDMA codes, as the name suggests, utilize code sequences spread along a single dimension to facilitate asynchronous connectivity over the communication channel [1]. Figure 1 represents a basic 1D OCDMA system employing spectral amplitude coding (SAC). Each user is assigned a specific code sequence translated from binary 1s into a spectral representation at the spectral encoding arrangement. A broadband source (BBS) provides the required spectrum, and an optical coupler (CP) splits the BBS spectrum to facilitate multiple subscribers. The encoded spectrum is modulated at a Mach-Zehnder modulator (MZM) and transmitted over the optical distribution networks using an optical combiner [1; 2; 3]. The receiving optical network terminal (ONT) utilizes a spectral decoding arrangement in accordance with the spectral encoder to receive and recover the intended spectrum. The received spectrum is then converted from the optical to the electrical domain using a PIN photodiode. A low-pass filter (LPF) is employed to perform the required filtering before the signal is analyzed at the bit error rate (BER) module. The performance of 1D OCDMA systems is primarily translated by N, w, and \(\lambda_{c}\) of the coding scheme. Such schemes fail to support high-cardinality systems, owing to the fact that code length increases significantly with an increase in the number of users. Furthermore, the requirement of ideal or fixed in-phase cross-correlation also limits the support of 1D OCDMA systems for high-capacity connectivity. Consequently, 2D and 3D codes were developed to overcome the limitations of 1D code. Two-dimensional (2D) OCDMA systems employ two coding schemes along the \(X^{th}\) and \(Y^{th}\) domains to overcome the problem associated with a limited number of codes while maintaining zero or fixed in-phase cross-correlation. Such systems exhibit efficient performance, owing to the fact that they are able to negate the effects of multiple access interference (MAI) and associated phase-induced intensity noise (PIIN) more effectively in comparison to their 1D counterparts. The \(X^{th}\) and \(Y^{th}\) domains in 2D codes primarily utilize three different techniques, spectral, spatial, or temporal, for the encoding and decoding processes at the transmitter and receiver modules, respectively. On the contrary, 3D codes employ three distinct coding schemes along the \(X^{th}\), \(Y^{th}\), and \(Z^{th}\) domains to fulfil the system requirements [16]. Figure 1: One-dimensional (1D) spectral amplitude coding (SAC) OCDMA architecture.