**3. System Description**

Figure 1 illustrates the considered WPSN, which includes one HAP and a number of sensor nodes in a cell. The HAP is a power source that wirelessly transfers energy to all of the sensor nodes and also acts as a sink node that collects sensing information in the cell. Only the HAP has a stable energy supply, but the sensors do not have any embedded energy source or battery. The HAP periodically broadcasts an RF wave, and all of the nodes in the cell harvest energy from this RF signal. Considering the hierarchical structure, the sensor nodes are grouped into a number of clusters and one CH is selected from among the member nodes in each cluster. Thus, all of the sensor nodes send sensing data to their CH instead of directly to the HAP. The CH aggregates all of the sensing information received from its member sensor nodes as well as its own sensing information (i.e., data fusion), then sends the aggregated data to the HAP at once.

**Figure 1.** Considered wireless powered sensor network.

Assuming that all of the sensors perform the same sensing task, such as temperature, humidity, or fire sensing, the bit size of the sensing data transmitted by each sensor node is the same. However, the sensor nodes inherently harvest different amounts of energy depending on their distances from the HAP and consume different amounts of energy for transmission depending on their distances from the CH. Incidentally, the maximum rate of sensing data collection in the WSN is limited by the worst sensor node, which has a low harvesting energy and poor link budget [19,20]. Therefore, all of the sensor nodes only need to support the same data rate as the worst node so that some sensor nodes may have energy remaining after transmitting their sensing data to the CH. Since the CH usually requires more energy for multiple receptions, processing, and uplink transmission to the HAP, the sensor nodes transfer the remaining energy to the CH while sending their data by using the SWIPT technique, and the CH acquires additional energy while receiving data from its member nodes.
