## 1 Introduction
Furthermore, nonadjacent placement increases the complexity of the code along with the design of encoder and decoder modules. [PERSON] et al. utilized a combination of 1D ZCC and MD codes to develop a 2D hybrid ZCC/MD code [34]. However, ZCC code employes code sequences with \(w=3\), which places an upper bound on the total number of users, as the code length is proportional to the weight of the code. Furthermore, both codes have nonadjacent placement of binary ones. [PERSON] et al. presented a 2D permutation vector (PV) code developed with the help of 1D PV code [35]. Each code is characterized by length, weight, and cross-correlation properties such that the length of the code depends on the number of users and the weight of the code. One of the drawbacks associated with this code is the limited number of code patterns that can be developed while maintaining the basic code properties. Furthermore, with a few exceptions, most such codes are developed with nonadjacent placement of ones. [PERSON] et al. proposed a 2D code sequence utilizing the 1D balanced incomplete block design (BIBD) technique [36]. The proposed code employs 2D BIBD blocks with an inherent cross-correlation property of \(\lambda_{c}=1\). Furthermore, the weight of each sequence is kept as \(w=3\). Consequently, the length of the codewords needs to be kept long in order to reduce MAI, which in turn reduces the number of users and complicates the overall design of the system. [PERSON] et al. proposed another code that utilizes the diagonal eigenvalue unity (DEU) to develop a 2D-spectral/spatial coding scheme [37]. However, the DEU code is developed with \(\lambda_{c}=1\), which requires a relatively complex receiver structure to overcome the MAI. [PERSON] et al. designed another code using the polarization technique to increase the cardinality of the system [38]. Their technique does not compress the OCDMA code. This is because polarization is applied for each user, which is not efficient for a higher number of users. Here, we report a flexible and secure OBAN of eight sensors based on FSO technology and 2D-spectral/spatial OCDMA. The sensors are distributed on the bodies of patients, and operate at the rate of 50 kbps. A 2D-spectral/spatial DW-ZCC code is used to encode the data on the transmitter side, and the combined optical signal is transmitted over a 1 km FSO channel towards the medical center. The time-delay technique is used to eliminate multiple parallel FSO paths between the transmitter and medical center. The combined signal is decoded and the data of each sensor are recovered after photodetection at the medical center. The performance of the sensors is analyzed using BER and Q-factor plots for different weather conditions considering the log-normal channel model. Based on the above proposal, the main contributions of this work are:  
* A flexible, low-cost, and secure OBAN consisting of eight medical sensors operating at the rate of 50 kbps. * A 2D-spectral/spatial DW-ZCC code for encoding the sensor data. * Time-delay in 2D encoded signals to eliminate multiple parallel FSO channels between the transmitter and medical center, which greatly reduces the system's cost. * Analysis of the performance of the sensors for various weather conditions. * Evaluation of the effect of variations in receiver aperture diameter and beam divergence on the BER and Q-factor. * Analysis of the CAPEX of the proposed architecture and comparison with a conventional 2D-spectral/spatial FSO system to determine the overall impact on the cost of implementation of introducing time-delay units. Optisystem 20 commercial software from Optiwave System Inc. [39] was employed to design the 2D-spectral/spatial DW-ZCC code and log-normal channel model.