## 1 Introduction
proposed uplink transmission between medical sensors and a coordinator based on OCDMA [16] and OFDMA [17]. It is evident from the above discussion that most of the works published to date discuss channel modelling, patient mobility, and multiple access schemes. Schemes for secure transmission of sensor data over FSO channels have not been proposed in literature except in [2; 8]. FSO systems can offer huge bandwidth, cost efficiency due to reduced deployment and maintenance costs, license-free spectrum usage, protection from EMI, environmental friendliness, and high inherent security compared to RF links [18]. However, outdoor FSO channels between the coordinator and medical center are highly vulnerable to problems such as atmospheric attenuation and turbulence, pointing errors, and eavesdroping [19]. An attacker can intercept the classified information of patients transmitted over FSO channels with little effort using simple means such as a wiretap installed in close proximity to the receiver [20], a receiver placed inside the divergence region of the beam [21], etc. Therefore, the probability of interception should be minimized by adopting effective means to enable the secure transmission of classified patient data over FSO channels using OBANs. The security of the OBANs has proven to be one of their most crucial design constraints. Various hardware based techniques have been reported exploiting optical chaos, pulse position modulation (PPM), quantum communication, and OCDMA for data security in OBANs from interception. Past works have proposed approaches on optical chaos [22; 23; 24; 25], PPM [19], quantum communication [26; 27], and OCDMA [2; 28; 29]. Encryption technologies based on optical chaos, quantum communication, and PPM usually provide good data security; however, they face issues around chaos synchronization, quantum entanglement, and the need to synchronize pulses in order to operate the timing circuits in the receiver, respectively [30]. Moreover, the implementation of these hardware-based techniques increases the overall cost and complexity of the system [30]. OCDMA is another secure communication technology that has been widely adopted to achieve absolute data privacy based on spreading the spectrum of the desired optical signal [2; 30]. Apart from the intended function of data security, OCDMA systems provide ease of implemenation, cost-efficiency, robustness in transmission, and cancellation of different adverse effects between the transmitter and receiver, such as multiple access interference (MAI) and phase induced intensity noise (PIIN) [2; 31]. The working principle of OCDMA systems is based on the conversion of zeros and ones into spectral domain as per the designed code by allowing or blocking required data bits by employing a specific setup of optical filters, splitters, and couplers on the encoder side; the information is then retrieved using a specific arrangement of optical filters, splitters, and couplers on the decoder side [31]. The codes used in OCDMA systems are typically of two types, called zero cross correlation (ZCC) and fixed in-phase cross-correlation codes [2]. This basic property of the codes is further exploited to create one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) OCDMA codes using the temporal, spatial, spectral, and polarization characteristics of signals [32]. Spectral/spatial combinations are most frequently used, leading to the development of various 2D codes for OCDMA systems [32]. To the best of our knowledge, no 2D OCDMA codes have been specifically designed to support OBANs. Nevertheless, owing to the efficient performance of the OCDMA family and their ability to cancel MAI, various 2D variants have been designed by combining 1D OCDMA codes. [PERSON] et al. developed 2D multidiagonal (MD) codes by applying ZCC 1D MD codes [33]. However, binary ones in the MD code are not placed adjacent to one another, which increases the code length as more users are added to the network.