## 4 Performance Evaluation
### Simulation Results
When the number of packets transmitted per second is two or four, and if the number of sensor nodes is greater than 24 or 12, then the average harvested energy of the MCSS is lower than those of the legacy TSCH MAC and TMSS, as illustrated in Figure 8. Additionally, MCSS can accommodate power and data traffic for a larger number of sensor nodes than the legacy TSCH MAC and TMSS by using multiple concurrent slotframes. However, such multiple concurrent slotframes causes overlap between cells, thus, reducing the average energy harvested by individual sensor nodes in MCSS compared to the legacy TSCH MAC and TMSS. Figure 9: Aggregate throughput. When the number of data packets transmitted per second is one, the average harvested energy of MCSS decreases as the number of sensor nodes increases because an increase in the number of sensor nodes causes a decrease in the number of power cells for each sensor node allocated to the WPT slotframe. If the number of data packets transmitted per second is two or four, the average harvested energy of MCSS decreases only until it reaches a certain level. The reason is that the number of power cells successfully allocated to the WPT slotframe does not increase when the number of sensor nodes exceeds a certain number. In contrast, the legacy TSCH MAC and TMSS maintain a constant average harvested energy that is almost similar to each other regardless of the number of sensor nodes because the number of power cells for each sensor node allocated to the slotframe is the same. In TMSS, the use of a separate EB slotframe enables energy harvesting of more sensor nodes compared to the legacy TSCH MAC. However, TMSS exhibits a slightly lower average harvested energy than the legacy TSCH MAC due to the overlap between cells. When the numbers of data packets transmitted per second are one, two, and four, the average harvested energies of MCSS are 244.30% and 244.28%, 79.68% and 79.68%, and 11.90% and 11.90% higher than those of the legacy TSCH MAC and TMSS, respectively. To investigate the effect of WPT on the performance of MCSS, we compared the aggregate throughput for the case where the HAP transmits power to the sensor nodes (i.e., MCSS with WPT) and the case where it does not (i.e., MCSS without WPT). In this experiment, the initial residual energy of sensor nodes was randomly set between 70 and 140 mA, and the simulation time increased from 50 to 500 min in 50 min increments. Figure 11a,b illustrates the variations in the aggregate throughput of MCSS for varying simulation times when the numbers of sensor nodes are 12 and 36, respectively. In the case of MCSS with WPT, the aggregate throughput is maintained as constant regardless of the simulation time because sensor nodes perform energy harvesting so that the energy-depleted dead nodes do not occur. On the other hand, as the simulation time increases, the aggregate throughput in the case of MCSS without WPT remains constant until a certain simulation time, and then starts to decrease. In the part in which the aggregate throughput is maintained constant, the dead nodes do not occur because the amount of energy consumed by the sensor nodes is less than their remaining energy. In contrast, in the opposite case, as the simulation time increases, the number of dead nodes increases, and, as a result, the aggregate throughput decreases. Figure 10: Average harvested energy. In Figure (a)a, in both cases, all sensor nodes successfully allocate the required data cells in the WPT slotframe; thus, the aggregate throughput remains the same until a certain simulation time. On the other hand, in Figure (b)b, in the case of MCSS with WPT, there are sensor nodes that cannot allocate their own data cells due to the increase of the number of power cells allocated in the WPT slotframe; thus, there is a part in which the aggregate throughput in the case of MCSS without WPT is higher than that in the case of MCSS with WPT. However, in the case of MCSS without WPT, the number of dead nodes increases as the simulation time increases.