###### Abstract  
This paper presents a multiple concurrent slotframe scheduling (MCSS) protocol for wireless power transfer (WPT)-enabled wireless sensor networks. The MCSS supports a cluster-tree network topology composed of heterogeneous devices, including hybrid access points (HAPs) serving as power transmitting units and sensor nodes serving as power receiving units as well as various types of traffic, such as power, data, and control messages (CMs). To this end, MCSS defines three types of time-slotted channel hopping (TSCH) concurrent slotframes: the CM slotframe, HAP slotframe, and WPT slotframe. These slotframes are used for CM traffic, inter-cluster traffic, and intra-cluster traffic, respectively. In MCSS, the length of each TSCH concurrent slotframe is set to be mutually prime to minimize the overlap between cells allocated in the slotframes, and its transmission priority is determined according to the characteristics of transmitted traffic. In addition, MCSS determines the WPT slotframe length, considering the minimum number of power and data cells required for energy harvesting and data transmission of sensor nodes and the number of overprovisioned cells needed to compensate for overlap between cells. The simulation results demonstrated that MCSS outperforms the legacy TSCH medium access control protocol and TSCH multiple slotframe scheduling (TMSS) for the average end-to-end delay, aggregate throughput, and average harvested energy. e 202222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222221222212222122221222212221222122212221222  
In WPT-enabled WSN, these heterogeneous devices (i.e., HAPs and sensor nodes) constitute a cluster-tree network, where each cluster includes one HAP serving as a cluster head (CH) and multiple sensor nodes serving as a cluster member [12; 13]. Thus, sensor nodes harvest energy from adjacent HAPs and transmit data to the CH. The HAPs collect data from sensor nodes and forward them to the sink. In such a WPT-enabled WSN structure, for timely energy harvesting and data exchange between heterogeneous devices, comprehensively designing a medium access control (MAC) layer protocol considering a network topology and various traffic types (i.e., control messages (CMs), intra-cluster traffic, and inter-cluster traffic) is a challenging issue [14; 15; 16]. In the literature, many studies have tried to develop the MAC layer protocol for WPT-enabled WSNs. In [17; 18; 19], the authors proposed carrier sense multiple access with collision avoidance (CSMA/CA)-based MAC protocols, in which HAPs and sensor nodes competitively occupy the channel to perform WPT and data transmission. However, they cause unnecessary energy consumption of sensor nodes and nondeterministic power and data transmission using a random backoff period and retransmission mechanism. The authors in [20; 21] proposed a hybrid MAC protocol that uses a superframe structure, including CSMA/CA and time-division multiple access (TDMA) periods. The use of the CSMA/CA period still causes unnecessary energy consumption of the sensor nodes, while their work combines the benefits of both approaches. In [22; 23; 24], the authors proposed TDMA-based harvest-then-transmit protocols, in which energy harvesting and data transmission are performed in a contention-free manner. They allowed sensor nodes to perform energy harvesting and data transmission through frame scheduling, preventing unnecessary energy consumption due to collisions or idle listening. However, the existing TDMA-based harvest-then-transmit protocols may waste resources in WPT-enabled WSNs because they use a fixed-length frame regardless of network topology and traffic type [25]. Time-slotted channel hopping (TSCH) specified in the IEEE 802.15.4-2015 standard [26] can address the problems of the existing MAC protocols by allowing the use of concurrent slotframes.