## 1 Introduction  
Wireless sensor networks (WSNs) have become an increasingly popular research topic in recent years. WSNs have produced promising solutions for several applications, such as intrusion detection, target detection, industrial automation, environmental monitoring, surveillance and military systems, medical diagnosing systems, and tactical systems [1]. WSNs consist of small sensor nodes disseminated in a targeted area to monitor the events for collecting the data of interest. WSNs also experience many challenging problems, including large energy consumption, network scalability, mobility, coverage, and uniformity [2]. These problems affect the lifetime of the network, increase the latency, and reduce the throughput. The limited battery life and harsh operating conditions cause further complications, which can lead to node failure [3]. Although significant research has been conducted on WSNs to maintain high communication standards (especially coverage), the issue of high power consumption remains unresolved [4]. The radio is one of the major power-consuming sections of the sensor in WSNs that can be handled using energy-efficient medium access control (MAC) protocols. Several MAC protocols, introduced to reduce the energy consumption, improve the lifetime of WSNs [5]. Unfortunately, most of the application-dependent [6] MAC protocols for WSNs are not energy efficient and thus do not effectively improve the lifetime of WSNs. The protocols should be scalable to adjust to changes in the network, such as the insertion of new nodes and the deletion of existing nodes [7, 8]. The reduction in energy achieved by the MAC protocols increases the latency, particularly in multi-hop data communication [9]. These design constraints must be considered when developing new MAC protocols. MAC protocols are classified into different categories, such as schedule-based, contention-based, mobility-aware, and hybrid protocols [10, 11], however, many of the contention-based MAC protocols are based on sensor-MAC (S-MAC), which are designed for specific WSN applications [12]. Contention-based protocols have free access to acquire the medium [13]. The nodes, which follow contention-based mechanisms, are not required to follow the cluster. These protocols are network adaptable to allow for the insertion and removal of sensor nodes from the network. However, in contention-based MAC protocols, when nodes are available on channel but do not know the activities (schedule) of each other, nodes do not know when to turn on/off the radio, thus increasing the energy consumption. Schedule-based MAC protocols are more suitable for reducing idle listening [14]. However, in such protocols, node problems occur due to the presence of a tight schedule; once a node misses its schedule, then it must wait for the next turn, thus increasing the energy consumption. Additionally, schedule-based MAC protocols are not adaptable due to changes in network topology [15]. Hybrid protocols leverage the characteristics of time division multiple access (TDMA) and carrier sense multiple access (CSMA) [16]. Existing hybrid MAC protocols are based on the clustering approach [17, 18], where time is divided into different time slots for each node in the cluster. Each node is responsible for using its own allotted time slot. Clustering reduces the idle listening and collisions. The transceiver also receives the sleep schedule without any additional overhead. However, such a mechanism experiences several drawbacks, as discussed in [16]. First, it is critical to determine an effective time schedule in a scalable manner. A centralized node is often needed to determine a collision-free schedule. It is extremely difficult to create an effective schedule with channel reuse or a high degree of concurrency (the ideal solution is NP-hard) [19]. Second, TDMA requires clock synchronization, which is an important feature of several sensor applications. However, tight synchronization results in energy overhead because it necessitates recurring message exchanges.