## 4 Performance Evaluation
### Simulation Configuration  
In the simulation, we considered a cluster-tree network topology consisting of five HAPs and multiple sensor nodes, in which one HAP is designated as the root, and the other HAPs maintain the same number of sensor nodes as the cluster members and operate as their CHs. The number of cluster members varies from one to 10 to investigate the  
Figure 5: Example of the three-step 6P transaction.  
performance of MCSS as the number of sensor nodes changes. Moreover, we assumed that each HAP is randomly placed within the communication range of one of the other adjacent HAPs, and each sensor node is randomly placed within a distance of 2 m from its CH.  
To investigate the effect of setting the initial structure of the TSCH concurrent slotframe suitable for the network size and topology, we considered a static cluster-tree network environment without changes in the number of sensor nodes and traffic within the cluster. We further assumed that all sensor nodes maintain the same residual energy of 140 mA at the start of the simulation. The timeslot length was set to 10 ms, and we assumed that the legacy TSCH MAC uses a single slotframe with a fixed length of 100 timeslots.  
For MCSS, the lengths of the CM slotframe and HAP slotframe were set to 331 and five timeslots, respectively. The CM slotframe length was set by referring to the default setting in the Contiki implementation [36], which is a representative TSCH+RPL network implementation, including the exchange of CMs. The HAP slotframe length was empirically set to a prime number that maximizes the aggregate throughput through experiments.  
The WPT slotframe length was determined by Algorithm 1, and its maximum value is limited to 101 timeslots, the nearest prime number to the length of a single slotframe of the legacy TSCH MAC (i.e., 100). If no limitation exists on the WPT slotframe length, then the number of sensor nodes that can be accommodated in the network is almost the same; thus, no meaningful change in the network performance can be obtained.  
TMSS uses an EB slotframe for transmitting EB frames and CMs and a TMSS slotframe for energy harvesting and the data transmission of sensor nodes. Thus, in TMSS, the EB slotframe length was set to 331 timeslots, the same as the CM slotframe of MCSS, and the TMSS slotframe length was set to 100 timeslots considering the transmission period of data packets.  
Moreover, each sensor node was assumed to transmit a certain number of data packets per second to its CH, set to one, two, and four. The data packet size and data rate were set to 127 bytes and 250 Kbps, respectively. The simulation parameters, such as the Tx power (\(P_{Tx}\)) for each HAP, path-loss parameter (\(\alpha\)), and energy harvesting efficiency factor (\(\eta\)), were set to 100 mW, 2.7, and 0.65, respectively, to calculate the number of power cells. In addition, the energy consumed when a sensor node is in \(Tx\) (\(P_{tx}\)), \(Rx\) (\(P_{rx}\)), idle (\(P_{idle}\)), and sleep (\(P_{sleep}\)) states during one timeslot was set to 20.98, 17.96, 0.001, and 0.001 mA, respectively.  
The performance of MCSS in terms of average end-to-end delay, aggregate throughput, and average harvested energy was compared to those of the legacy TSCH MAC and TMSS. In addition, the effect of WPT on the aggregate throughput was investigated. The simulation was iterated 100 times. The detailed parameters are listed in Table 1.