# Secure PD-NOMA with Multi-User Cooperation and User Clustering in Both Uplink and Downlink PD-NOMA
## 1 Introduction
### Existing Literature on PD-NOMA
This is because it relies on the unpredictability and randomness of the physical layer and uses simple operations to make sure the data being sent is secure. Once the data is secured from the eavesdroppers, the strong users can be used as relay stations that can assist the weak nodes through cooperation. The cooperative relaying PD-NOMA (Cop-PD-NOMA) is presented for the two users system where the strong user (cell-center) is used to decode and forward the signal of the weak user (cell-edge) [23]. By carefully regulating the phases of the passive components at the re-configurable intelligent surface (RIS), a suitable strategy that establishes a re-configurable radio environments of the propagation medium and increases the received signal strength is proposed in [24]. This study has focused on combined beamforming modeling and optimization for RIS-assisted hybrid satellite terrestrial relay systems, in which the connections from the BS and satellite to numerous users were obstructed. Furthermore, in [25; 26] rate-splitting multiple access (RSMA) and NOMA are briefly investigated in satellite-terrestrial and aerial integrated networks. The fuzzy logic-based technique for relay node selection was proposed by [PERSON] et al. in [27], which significantly enhances the selection of adaptive data rates and optimal power allocation. To increase system level performance, the cooperative relaying technique uses full duplex and half duplex relaying schemes on demand to improve the performance of cell-edge users by leveraging the cell-center user as a relay node in the PD-NOMA system [28]. For security purposes, the conventional techniques for user authorization in the Internet of Things are ineffective and risky. Conventional biometrics such as face and finger prints are insufficient for security. The authors in [29] suggest finger vein-based user verification to have more highly secure authentication for the Internet of Things. For real-time wireless sensor networks, the guaranteed time slots (GTSs) allocation mechanism is used in IEEE 802.15.4 [30] and an energy-efficient sleep control technique is used in [31], respectively. Moreover, a genetic algorithm based on machine learning was proposed for pedestrian identification in [32]. User clustering or user pairing is an essential factor of the PD-NOMA. Using the clustering technique, it is easy to figure out how much power to give each PD-NOMA user. Three different techniques for sub-optimal user-clustering solutions are provided in [33], all of which improve spectral efficiency while being simple and easy to apply. The maximum energy efficiency can be achieved through the best selections among cell-edge users for user clustering [34]. The likelihood of an outage was lowest in clusters with the most space between users, and it was highest in clusters with the least space between users. In [10], a low-complexity sub-optimal user clustering technique is presented. This technique takes advantage of the fact that different users in a PD-NOMA cluster have different channel gains and splits them up into different clusters to improve the system's overall throughput.