# Research on Energy Harvesting Mechanism and Low Power Technology in Wireless Sensor Networks
## 3 Methods
### Neighbor Discovery Mechanism
#### 3.2.2 Sending and Receiving Separation Beacon Mechanism Based on Reply to ACK  
The neighbor discovery protocol focuses on optimizing the scheduling period when the node works, and it determines the frequency of active time-slot encounters between neighboring nodes, while the beacon mechanism determines the probability of achieving mutual discovery when neighbor nodes meet in an active time slot. At present, most of the studies focus on how to optimize the work scheduling cycle of nodes. There are few studies around the beacon mechanism. In fact, the beacon mechanism also plays a very important role in the discovery of neighbor nodes.  
Most of the existing beacon mechanisms in WSN are used for sending and receiving beacons in a single time slot. The sending of the beacon will have certain constraints on the variation range of the time slot size and the listening time in the time slot, which has serious effects on the mutual discovery between nodes, especially when the time slot is  
Figure 3: Diagram of power supply module: (**A**) solar power generation circuit, (**B**) piezoelectric ceramic power generation circuit, and (**C**) electric energy storage circuit.  
small. Therefore, we propose a sending and receiving separation beacon mechanism based on reply to ACK, which only listens to the beacon in the active time slot, and sends the beacon at the end of its previous time slot. In addition, the node immediately replies to ACK after receiving the beacon sent by the adjacent node.  
The sending and receiving separation beacon mechanism based on reply ACK is as shown in Figure 4, where nth slot represents the nth time slot, and it is active. At this point, the node will turn on the RF at the end of the (\(n-1\))th slot and transfer the RF from the launch completion state to the send beacon state. Then, the beacon is sent before the end of the (\(n-1\))th slot and the RF is converted to the listening beacon state, and the node will only listen to the beacon in the active time slot. When node A meets node B in an active time slot, node A receives the beacon sent by node B, and it immediately enters the stage of replying to ACK after analysis and verification processing. The bidirectional discovery between nodes can be completed after node B successfully receives ACK.  
Discovery probability is an important indicator to measure the performance of the discovery beacon mechanism of neighbor nodes. Improving the discovery probability between nodes can reduce the detection delay and realize the rapid discovery between nodes. When the response ACK mechanism is adopted, the bidirectional discovery can be completed by dynamically extending the time slot after realizing one-way discovery. The bidirectional discovery probability \(P_{d}\) is  
\[P_{d}=\frac{t_{slot}-t_{PR}}{t_{slot}-t_{TX}} \tag{1}\]  
where \(t_{slot}\) is the time of an active time slot for a node, \(t_{PR}\) is the time that the node takes to send the synchronization packet, and \(t_{TX}\) is the time interval between sending the beacon and listening the beacon.