## 3 System Model
### Reception State Adjustment Mechanism  
As shown in Figure 6, each node has a data buffer and a battery. Packets in the data buffer come from two sources, including newly generated packets by themselves (denoted as \(C_{i}\) ) and packets received from other nodes (denoted as \(I_{i}\)). For the packets generated by nodes following Poisson process, \(C_{i}\) obeying an exponential distribution and the value can be obtained by efficient prediction [30]. Figure 5: Forward transmission region. Figure 6: Reception state adjustment mechanism. The number of newly generated packets by node \(i\) at time slot \(t\) can be written as:  
\[IC_{i}^{t}=I_{i}^{t}+C_{i}^{t} \tag{15}\]  
The total amounts of packets in node \(i\) at the current time slot (denoted as \(t\)) can be calculated as:  
\[B_{i}^{t}=min\Big{\{}B_{i}^{t-1}+IC_{i}^{t-1}-O_{i}^{t-1},DB_{max}/L\Big{\}} \tag{16}\]  
where \(O_{i}^{t-1}\) denotes the number of packets processed by node \(i\) at time slot \(t-1\). When the data processing capacity of node reaches the upper bound (receiving too many packets), these packets have to stay in the buffer. In this case, a congestion problem would be caused easily, which accordingly increases the packet loss rate, reduces data processing efficiency and wastes energy [16; 31]. In addition, although the energy harvesting device can provide unlimited energy, the nodes would suffer from occasional energy shortage as the harvested and demanded energy do not meet. When the node runs out of energy (i.e., entering the sleep mode), these remaining packets have to wait until the energy recovers to a certain threshold. Under this certain situation, it would lead to increasing packet transmission time and diminishing of the packet processing efficiency. In order to solve the above problems, we propose a reception state adjustment mechanism, as shown in Algorithm 1. ```
0: The current node \(i\); Residual energy \(E_{r}^{i}\); Utilized cache size DB; The candidate nodes set of i \(CN(i)\); Current data receive state of i \(Flag\)
1:if\(E_{r}^{i}\geq\textit{Level 2 \&\&\leavevmode\
obreak\ DB}\leq DB_{th2}\) &&\(DB/L*E_{t}\leq E_{r}+EH_{i}*T-B\)then
2:if\(Flag=0\)then
3:\(Flag=1\)
4:endif
5:else
6:if\(E_{r}^{i}\leq\textit{Level 1}\|DB\geq DB_{th1}\|DB/L*E_{t}\geq E_{r}+EH_{i}*T+A\)then
7:if\(Flag=1\)then
8:\(Flag=0\)
9:endif
10:endif
```  
**Algorithm 1** Reception State Adjustment Mechanism. This adjustment mechanism uses \(Flag\) to indicate whether the node can receive packets. \(Flag=1\) denotes node can receive data, and \(Flag=0\) denotes cannot. There are three main parts:  
1. When the residual energy of a node is less than the minimum limit (\(\textit{Level 1}\)), the \(Flag\) would be adjusted to 0 if it is 1 (representing this node cannot receive packets). When the energy recovers to a certain threshold (\(\textit{Level 2}\)), the \(Flag\) would be adjusted to 1 if it is 0 (representing this node can continue to receive packets), that is: \[ \begin{cases}E_{r}^{i}\geq\textit{Level 2, }Flag=1\\ E_{r}^{i}\leq\textit{Level 1, }Flag=0\end{cases} \] (17)
2. When the data buffer of a node reaches the maximum limit \(DB_{th1}\), \(Flag\) would be adjusted to 0 if it is 1. When the buffer capacity is less than the lowest threshold \(DB_{th2}\), \(Flag\) would be adjusted to 1 if it is 0, that is: \[ \begin{cases}DB\geq DB_{th1},Flag=0\\ DB\leq DB_{th2},Flag=1\end{cases} \] (18)When the residual energy of a node is not enough to forward the packets in the buffer, \(Flag\) would be adjusted to 0 if it is 1. In this case, it needs to predict the total energy that the node can collect in a future period, as well as the energy required to process the packets in the buffer, so as to determine whether the node can process all packets in the buffer before its energy is exhausted. \[ \begin{cases}B*E_{t}\geq E_{r}+EH_{i}*T+c_{1},Flag=0\\ B*E_{t}\leq E_{r}+EH_{i}*T-c_{2},Flag=1\end{cases}  \tag{19}\]  
where \(B\) represents the number of packets in the buffer, \(B=DB/L\), \(c_{1}\) and \(c_{2}\) are two constants.