# Secure PD-NOMA with Multi-User Cooperation and User Clustering in Both Uplink and Downlink PD-NOMA
## 1 Introduction
### Existing Literature on PD-NOMA  
NOMA techniques are further categorized into power-domain NOMA (PD-NOMA) [7] and code-domain NOMA [8]. In the downlink PD-NOMA, a gNodeB transmits the multiple users' signals by superimposing them in the power domain. The superposition is performed in a way that lets each NOMA user decode the signal of interest using successive interference cancellation (SIC) at their own receiver [9]. Similarly, in the uplink PD-NOMA, each user sends a signal at maximum power (unless a power control mechanism is used), which is added to the other signals in the air and decoded at the gNodeB using the SIC method [10]. The main difference between uplink and downlink PD-NOMA is the SIC order. Based on the SIC method, each user first separate the high power signal from the composite signal. In the downlink, the near users having better channel conditions first decode and eliminate the far user signal (high powered signal) having poor channel conditions using the SIC method. Once the far user signal is eliminated from the composite signal, the near user then retrieves its signal. While in the uplink, the gNodeB first decodes the signal from the near user and then uses SIC to decode the signal of the far user. The SIC technique is used to cancel the effect of the undesired signal during multi-user detection (MUD) by first decoding it from the combined signal, regenerating and subtracting it from the received combined signal [11]. A novel sky-ground (SG) NOMA receiving structure where the aerial user (AU) and terrestrial users (TU) are communicating with the ground base station is presented in [12]. It is understood that removing the most powerful users will give better performance, but only in the case when the most powerful user's signals are fully correctly decoded at the nearest user, which is impractical. As a result of the decoding errors that occurred during the detection of the far user signal, as well as the complexity of performing SIC operations, the near user experienced performance degradation. The overall performance of the PD-NOMA system can be improved through proper power allocation strategies. The power allocation is completely based on the channel gains of each user [13]. Users with high channel gains (called \"strong users\") are assigned the least amount of power, while users with low channel gains (called \"weak users\") are assigned the most amount of power. The two-user PD-NOMA outperforms the NOMA-2000 [14]. In the two user PD-NOMA a single-level of SIC is used by the strong user to eliminate the signal of the weak user. Due to non-orthogonal nature of PD-NOMA the inter users' interference is maximum and it becomes worst if more numbers of users are involved [15; 16]. Proper power allocation strategies can help to reduce such interference. In [17], the fixed and dynamic power allocation (FPA and DPA) strategies were proposed for the static and dynamic PD-NOMA environments. The performance of PD-NOMA was analyzed for multi-user scenarios in [18]. The distance-based power allocation is carried out for two, three, and four-users per-cluster PD-NOMA, and the upper limit on the number of users per cluster was proposed in [19]. In ordinary PD-NOMA, the strong users of the system are capable of decoding the weak users' data for SIC operation. In the PD-NOMA system, there are two types of users: (1) Registered users known to the gNodeB via proper communication links. (2) unregistered users or eavesdroppers, who are unknown and can steal or misuse the information from the received composite signal. This way, the privacy of the weak user data is compromised and can be detected by an unintended user. To ensure the data security in PD-NOMA, the physical layer security (PLS) and low probability detection (LPD) methods are highlighted in [20]. Owing to the raised issue, the PLS is proposed in [21; 22] to ensure the efficient deployment of PD-NOMA. PLS has been demonstrated to be more effective and reliable than previous upper-layer security methods.