# Secure PD-NOMA with Multi-User Cooperation and User Clustering in Both Uplink and Downlink PD-NOMA
## 3 Proposed Secure Communication, Multi-User Cooperation, and User Clustering Strategy in PD-NOMA
### Proposed Solution of Multi-User Cooperation in PD-NOMA  
Cooperative relaying has been receiving a lot of attention in wireless communication because it can provide spatial diversity to minimize fading and get around the problems of putting many antennas on small communication stations. Multiple strong users are used as relay stations and are designated to aid weak users using the D&F technique in cooperative communications. As a result, combining cooperative communications with PD-NOMA can further improve system performance. In this method, users with good channel conditions D&F the signals for other users. This way, weak users with poor channel conditions to the gNodeB can transmit and receive data more reliably.  
A downlink cooperative PD-NOMA (Cop-PD-NOMA) is shown in Figure 4, where four users are communicating with the gNodeB. The gNodeB and all the users are equipped with single input and single output (SISO) antennas. Users \(R_{1}\), \(R_{2}\), and \(R_{3}\) are considered strong users, and \(R_{4}\) as a weak user. The \(R_{4}\) signal is attenuated more due to the greater distance separation to the gNodeB. To improve the received signal performance at \(R_{4}\) we use multi-user cooperation from the strong users. In the time slot (\(t_{1}\)), all the users receive a composite signal from the gNodeB, which is given as follows.  
\[Y_{i,g}^{r}(t_{1})=h_{i,g}\sum_{i=1}^{4}\sqrt{P_{i}}X(t_{1})+n_{i,g}^{r}(t_{1}), \tag{9}\]  
where \(h_{i,g}\) is the channel gain and \(n_{i,g}^{r}(t_{1})\) is the AWGN between the gNodeB and the \(i\)th user. Superscript \(r\) denotes signal from gNodeB at \(i\)th receiver. Each receiver \(R_{1}\), \(R_{2}\), and \(R_{3}\) first decode the \(R_{4}\) signal due to its higher power. The decoded \(R_{4}\) signal is then used for SIC operation to decode the other low powered signals such as \(R_{3}\), \(R_{2}\), and then \(R_{1}\). Users \(R_{1}\), \(R_{2}\), and \(R_{3}\) are used sequentially as relay stations that use the (\(D\&F\)) technique to assist the cell-edge user \(R_{4}\) in the time slots (\(t_{j}\)) where \(j=2,3,\text{and }4\). More explicitly, the previously decoded \(R_{4}\) signal for SIC operation is restored and transmitted by each cell-center user. Multiple cell-center users can be used as relay stations, but this means that more bandwidth is being utilized. This way, in the proposed Cop-PD-NOMA system, user \(R_{4}\) receives three forwarded copies of his signal from relay stations. The received D&F signals from each relay station at \(R_{4}\) is given by  
\[Y_{4,i}^{d}(t_{j})=l_{4,i}\sqrt{P_{4}}x_{4}(t_{j})+n_{4,i}^{d}(t_{j}), \tag{10}\]  
where \(l_{4,i}\) denotes the channel gain and \(n_{4,i}^{r}(t_{j})\) is the AWGN between \(R_{4}\) and the \(i\)th user. Superscript \(d\) denotes the D&F signal from \(i\)th relay station to \(R_{4}\). Now user \(R_{4}\) have four received signals: one from the gNodeB and three from the relay stations: \(R_{1}\), \(R_{2}\), and \(R_{3}\). \(R_{4}\) uses the maximal ratio combining (MRC) scheme to decode its message signal. The performance improvement due to Cop-PD-NOMA can be observed in the BER curves in the Results section. The generalized equation for D&F Cop-PD-NOMA signal is given by  
\[Y_{n,j}^{d}(t_{j})=\sum_{i=1}^{I}\left(l_{n,i}\sqrt{P_{n}}x_{n}(t_{j})+n_{n,i} ^{d}(t_{j})\right),\qquad\forall\quad n=1,2...N \tag{11}\]  
here, \(n\) represents the user which receives the \(D\&F\) signals and \(i\) is the number of relay stations used in the Cop-PD-NOMA system. This way, the Cop-PD-NOMA significantly improve the performance at cell-edge users.