# Stability-Aware Geographic Routing in Energy Harvesting Wireless Sensor Networks
## 4 Stability-Aware Geographic Routing in EH-WSNs (SAGEEH)
### SAGREH Protocol  
In the paper [29], a light-weight opportunistic forwarding (LWOF) scheme and an energy efficient MAC protocol (_i.e._, LWMAC) with optimized preamble's length used in LWOF are proposed to reduce energy consumption in wireless sensor networks. As mentioned in [11], a forwarding node is selected based on the location-address semantic (_i.e._, greedy forwarding) and this location address semantic is valid in many sensor networks because sensor data are normally tagged with the location information. In our paper, we also employ greedy forwarding in selecting forwarding nodes. It should be noticed that the greedy forwarding algorithm with 60\({}^{\circ}\) sector used in [29] is for eliminating hidden forwarder. Moreover, according to the opportunistic forwarding scheme, it is required that only one forwarding node is selected among group of nodes and the forwarding node selection process gradually builds a single path from source node to sink node to deliver the data packet during the data packet transmission phase. However, in our paper, the proposed SAGREH exploits greedy forwarding to limit the forwarding area of the control packet (_i.e._, Route Request (RREQ) packet) in the route exploration phase, not data transmission. The RREQ packets reach the sink node through many paths. Then, among these paths, the sink node will select the most stable path specified by Equation (15). The details of path selection will be described as follows. In a multi-hop case, when a source wants to transmit data to sink, if it chooses a nearer neighbor, the \(PRR\) is higher, but the number of hop counts to reach the sink is larger, and _vice versa_. Specifically, for our model, we ask the question: How to perform a trade off between \(PRR\) and the number of hop counts in multi-hop routing? We will propose the SAGREH to solve this problem. The localized routing algorithm for EH-WSNs proposed here has nodes that do not require complete network topology information to perform routing tasks. It assumes that each node knows the location information for itself and the sink node. Routing paths are dynamically constructed during the packet forwarding phase, reducing the cost of control message overhead. All nodes transmit with equal transmission power, thus, the nodes have fixed and equal transmission ranges, or \(R\). Most geographic routing finds greedy forwarding impossible; (_i.e._, when approaching a hole with no relay node, routing reverts to the perimeter mode). As shown in Figure 1, when the _local minimum problem_ occurs at node A, the node will seek other routing paths to sink around the _void_ area; if a routing path to the sink area exists from node A, it does not include the node located within the _void_ area. This is called the _perimeter mode_. SAGREH takes into account the residual energy, energy harvesting, location, and link quality information of neighboring nodes to select the routing path. In the following section, the routing path from the source to the sink node is described. Figure 4 shows the influence of residual energy and location information on the process of forwarding node selection in SAGREH. The circle with radius R around the nodes indicates the transmission range. The distance between source node (S) and sink node (D) is \(d_{SD}\). If \(d_{SD}\) is greater than the transmission range R, intermediate nodes are needed to forward the data packet from S to D. The detailed process of determining forwarding nodes in our proposed SAGREH is described as follows. * **Step 1 (determining the neighbor nodes):** Firstly, the source node includes its location in a RREQ packet and broadcasts this packet. An intermediate node who receives this RREQ packet can retrieve the location of source node, then it uses this location information, its own location, and residual energy information to evaluate whether it is allowed to forward to RREQ. Specifically, an intermediate node is only allowed to forward the RREQ packet if: (i) The distance from it to previous hop is less than transmission range.