## 2 Related Work
Thus, the K-means algorithm improves the CH selection process by considering spatial proximity, while energy consumption is minimized through the use of the sleep mechanism. However, the protocol overlooks the residual energy of the nodes during CH selection. Moreover, it relies on single-hop communication between the CH and the BS. Its sleep mechanism is managed locally. The Energy-Efficient Sleep-Awake Aware (EESAA) protocol was proposed in [13]. It provides a pairing-based strategy to enhance energy efficiency in homogeneous WSNs. During the setup phase, the BS groups neighboring sensor nodes into pairs. In each round, one node from each pair becomes active and is responsible for sensing and communication, while the other remains in sleep mode. The EESAA protocol selects the CHs using a probabilistic model that considers the residual energy of the nodes. Once selected, each CH directly transmits the collected data to the BS through single-hop communication, without relying on intermediate nodes. The EESAA protocol boosts the network's stability and reduces energy consumption by minimizing redundant transmissions. It does not support multi-hop communication. Moreover, sleep-scheduling decisions are entirely made at the pair level, without any network-wide coordination. To reduce energy consumption and prolong the lifetime of the WSN, the Sleep-awake Energy-Efficient Distributed (SEED) protocol was developed in [14]. It divides the network area into three regions based on the distance from the BS. Within each region, sensor nodes with similar sensing responsibilities are grouped into sub-clusters. The sleep-scheduling mechanism used by the SEED protocol relies on local decisions. In each round, only one sensor node in each sub-cluster remains active, while the others enter sleep mode. The SEED protocol employs a distributed approach to select CHs, taking into account each node's remaining energy and the number of times it has previously served as a CH. It utilizes single-hop communication to forward the collected data from the CHs to the BS. Simulation results have revealed that the SEED protocol reduces energy dissipation and extends the lifetime of the WSN. Threshold-based LEACH with Sleep/Awake Scheduling (T-LEACHSAS) was proposed in [15] as an enhanced version of the T-LEACH protocol. The T-LEACHSAS protocol integrates a threshold-based cluster-head selection strategy with a scheduled sleep-awake mechanism. The CH selection process is based on the residual energy of each node and a predefined energy threshold. Accordingly, a CH continues to operate across multiple rounds as long as its residual energy remains above the threshold. Each CH utilizes a TDMA-based schedule, where each non-CH becomes active during its assigned time slot and remains in sleep mode during the remaining time. A single-hop communication model is used to transmit the aggregated data from the CH to the BS. The T-LEACHSAS protocol helps to conserve energy and prolong the lifetime of the WSN. To mitigate the rapid energy depletion of hotspot nodes and enhance overall energy efficiency, the Interval Type-2 Fuzzy Unequal Clustering and Sleep Scheduling (IT2 FUSS) protocol was proposed in [16]. It employs an Interval Type-2 Fuzzy Inference System (IT2-FLS) to enable each node to make local decisions based on multiple parameters, including its residual energy, its distance to the BS, and its neighborhood density. The IT2 FUSS protocol dynamically adjusts the cluster sizes according to the distance of each node to the BS, assigning smaller radii to nodes located closer to the BS. In each round, IT2 FUSS enables a node to enter sleep mode unless it is considered critical by the fuzzy logic. It is worth mentioning that the IT2 FUSS protocol leverages single-hop communication to transmit the CHs' data to the BS. Simulation results showed that the IT2 FUSS protocol effectively balances energy consumption and prolongs the overall network lifetime. The Energy and Throughput Aware Adequate Routing (ETAAR) protocol was introduced in [17].