## 3 System Model and Detail Design  
In this paper, the proposed scheme is named AUA transmission, which is shown in the left part of Figure 1, and such a scheme can be changed to three cases according to different situations (right part of Figure 1):  
* Case 1 represents the current traditional cellular network, wherein the UE transmits (or receives) data directly to (or from) the BS without help from any cooperation node. There are no other cooperation nodes to assist the UE, and there is only one direct link between the BS and UE. Correspondingly, it is hard to guarantee the data rate or latency under such single-path transmission when the UE is far away from the BS.
* Case 2 is also named general relay, which is similar to a traditional relay network. In this case, the BS will balance the transmission number of direct links and indirect links and choose the link that can obtain lower latency or better coverage. In this paper, it is assumed that the BS does not perform reception of UE data when the UE transmits data to the relay or coordination node. This means there is no combination at the BS across different source nodes for a single packet. For easy description, this scheme is also named Case 2 without (wo) combination. Some descriptions of the general relay are summarized in Section 3.1. Similar to Case 1, such transmission is also single-path based, and it uses an indirect path between the UE and BS in Case 2.
* Case 3 explores the diversity gain or power boosting by aggregating multiple cooperation nodes to forward UE data, and the BS could combine the received data to improve transmission reliability. Different from Case 1 and Case 2, there are multiple simultaneous radio links in Case 3 (i.e., the transmission in Case 3 is multi-path based). Assuming there are \(M\geq 1\) cooperation nodes, the path number would be \(M+1\), corresponding to the direct path between the UE and BS, and \(M\) indirect paths between the UE and BS via cooperation nodes. Such a transmission scheme is named UE aggregation-based transmission, since the cooperation nodes in this paper can be normal UEs. Moreover, the link condition over different paths would vary over time, and it is natural to select appropriate paths for energy consumption efficiency without transmission reliability or latency loss. Then, the detailed theoretical analysis of fixed multi-path-based transmission (Case 3 non-AUA) and the proposed adaptive UE aggregation for multi-path-based transmission (Case 3 AUA) is described in Section 3.3.  
Without a doubt, the detailed transmission for different UEs can switch among these three cases according to the real transmission environments or requirements. Under the proposed AUA scheme, there are three different links: a link between the UE and cooperation node (UC link), a link between the UE and BS (UB link), and a link between the cooperation node and BS (CB link). In this work, a composite channel model is considered, and the signal-to-noise ratio (SNR) \(\gamma\) is used to reflect the quality of the link, which is depicted as follows [40]:  
\[\gamma=\frac{P_{\text{t}}P_{\text{L}}\Omega|g|^{2}}{\sigma^{2}} \tag{1}\]  
where \(g\) represents the fast channel fading and follows a Nakagami distribution. It can be known that \(|g|^{2}\) is distributed according to a gamma distribution, while \(\Omega\) represents the shadowing effect and is modeled by a log-normal distribution. In addition, \(P_{\text{L}}\) is the path loss of the channel, \(P_{\text{t}}\) is the signal transmit power, and the system has Gaussian random noise with zero mean and variance \(\sigma^{2}\).  
The following part of this section elaborates on the three system models and provides a detailed design of each model. These models have been developed to address specific communication challenges and optimize system performance. The design considerations take into account various factors such as latency consumption, energy consumption, spectrum efficiency, throughput, and coverage.