*4.1. Frame Structure*

Figure 2 shows the frame structure for the proposed ENO based on SWIPT. The frame is based on time division multiple access with time division duplexing (TDMA/TDD) and scheduling-based resource allocation is used for conflict-free transmission. Firstly, a beacon signal is broadcasted at the beginning of the frame for frame synchronization and to provide the frame configuration and scheduling information to all of the nodes in the cell. In compliance with the harvest-then-transmit protocol [21], the HAP transmits RF energy during the WET slot with length *Te*. During this WET period, all of the sensor nodes harvest energy to be used in the current frame.

**Figure 2.** Frame structure for the proposed ENO based on SWIPT.

The remainder of the frame is divided and used for each cluster. The resources allocated to each cluster consist of multiple SWIPT slots for the member nodes in the cluster and one WIT slot for the CH. The total number of SWIPT slots for a certain cluster is the same as the number of member nodes in that cluster, and the length of a SWIPT slot is set to *Ts*. Each SWIPT slot is reserved for each sensor node through a pre-scheduling mechanism so that access collision does not occur [41]. On the other hand, the last WIT slot is dedicated to the CH only for WIT and has a length of *Td*. Hence, the total length of the frame depends on the number of clusters and the number of member nodes in each cluster.

The sensor node transmits the sensing data to its CH in the SWIPT slot allocated to itself. Simultaneously, it transfers the remaining energy (if it exists) to the CH by using SWIPT based on either the PS or TS method. Each sensor node *j* uses a portion *α<sup>j</sup>* of power or time for WIT and the remaining portion (1 − *αj*) for WET. During these SWIPT slots, the CH sequentially receives the sensing data while receiving additional energy from its member nodes. Then, the CH aggregates all of the sensing information and transmits the aggregated sensing data to the HAP at the last WIT slot. Here, the CH uses not only the energy initially provided by the HAP but also the energy additionally received from its member nodes. It is noted that we do not consider the ambient energy harvesting between any nodes [42] because the amount of energy harvested is negligible due to the long distance between nodes and the sensor nodes may inherently stay in sleep state except for its transmit and receive periods.
