## 2 Theoretical Study
### Optimal Relay Node Selection  
Cooperative communication consists of two phases: signal broadcasting and signal forwarding. Figure 1 shows a schematic diagram of how cooperative communication data transmission works. The first phase of work is the broadcast of signals by the source node [18]. The data signal is transmitted to the surrounding nodes in broadcast form, \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Innovation Points** & **Research Content** & **Lead Author** \\ \hline \multirow{3}{*}{Research framework} & An automatic retransmission request protocol framework for cooperative communication is proposed. & [PERSON] \\  & A relay model with three nodes is proposed. & [PERSON] \\  & Cooperative communication theory is first introduced into wireless communication network. [PERSON] \\  & Random coding, packet and packet communication are proposed to improve the performance of CARQ. & [PERSON] \\  & Power adjustable, packet communication and other CARQ methods are proposed, K-mean and other methods are integrated into CARQ. & [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26] \\ Cluster communication & A packet multi-relay retransmission strategy is proposed, and data processing and multiplexing techniques are studied. & [27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A comparison of classical literature on automatic retransmission request strategies for cooperative communication. and the surrounding nodes obtain a diversity copy of the signal directly from the source node. The second phase of work is the relay node forwarding the signal [19]. The optimal relay node forwards a copy of the signal to the destination node and its surrounding nodes are also able to receive a copy of the signal it transmits. The destination node decodes the accepted signal and transmits a feedback signal to the relay node. The relay node decides whether to retransmit the signal based on the feedback signal. The two-stage communication of cooperative communication broadcasts the emitted signal, so the surrounding nodes are also able to receive a copy of the signal, and the study defines the non-relay nodes as listening nodes. The listening node receives the signals emitted by the surrounding nodes [20]. Referring to the research results of [PERSON] [3], the optimal relay node is determined by Equations (1)-(5). The expressions for the signals of each phase in cooperative communication are presented. \[y_{sr}=h_{sr}\sqrt{E_{s}}x+z_{sr} \tag{1}\]  
Destination node receives signal  
\[y_{rd}=h_{rd}\sqrt{E_{r}}y_{sr}+z_{rd} \tag{2}\]  
In the above two equations, the symbol y represents the receiving signal, the symbol h represents the channel gain of the communication link, and the symbol \(E\) is the energy consumed during the signal transmission. The symbol \(z\) is the expression of noise. The subscripts \(s\),\(r\),\(d\) represent the source node, relay node and destination node, respectively. Relay node selection is a key technique for cooperative multi-node communication in reload requests. Since the distribution of nodes is random, the commonly used relay selection methods can no longer meet the demand of retransmission competition [21; 22; 23]. A novel relay selection algorithm is proposed to solve the problem of the increasing number of forwarding nodes, which are divided into different groups according to their spatial distribution [24]. Within the communication group, the best relay node is determined by a competition rule to be used to forward the signal. To improve the stability of the communication, the link quality is defined as the competition used to design the competition function. Since cooperative communication contains two phases of communication, the link quality consists of two links, the SR link and RD [25]. Considering the signal constraint, the minimum value of the channel quality of both links is defined as the whole link channel quality.