## 1 Introduction
### Clustering for Smart Energy Efficiency  
Numerous routing techniques that tend to enhance the energy efficiency of the network have been reported. Clustering proves to be promising in preserving the energy of the nodes. It is defined as the grouping of sensor nodes to form a cluster as shown in Figure 2. Every cluster has one cluster head (CH) that assembles the data from the cluster, aggregates it and then forwards it to the next CH or the base station depending upon the routing topology employed. The main concern when adopting clustering topology is the selection of the CH. It could be based on residual energy and the distance of the node from the sink. However, the problem of selecting the most appropriate CH still exists. The important concern of energy efficiency is being addressed by the researchers in WSN [20]. Homogeneity does not exist practically, and it is observed that heterogeneity in sensor nodes brings moreenergy balance to the network. Therefore, this paper focuses on heterogeneous routing protocols. It is worth mentioning that LEACH (Low Energy Adaptive Clustering Hierarchy) was the first cluster-based routing protocol that inspired other protocols to incorporate heterogeneity in their work [21].  
The first protocol to define heterogeneity in routing was the SEP (Stable Election Protocol) [19]. It used energy heterogeneity at two levels. The CH selection in the SEP was based on the weighted probability for selecting any node as CH. However, it failed for more than two levels of heterogeneity. Since the development of the SEP, various routing protocols have been presented that mostly focused on CH selection. DDEEC (Design of Distributed Energy Efficient Clustering) improved the CH selection method by avoiding the penalization of high energy nodes [20]. It used residual energy for CH selection. After the advancements in different routing strategies, energy heterogeneity at four levels was introduced by the BEENISH (Balanced Energy Efficient Network Integrated Super Heterogeneous) protocol for wireless sensor networks. However, the BEENISH suffered from the drawback of the penalization of higher energy nodes in the process of CH selection [22]. MRA (Multiple gateway-based Routing Architecture) introduced the novel method of data collection for harsh environment monitoring. In this, the authors utilized four gateways around each periphery of the network [23]. It aimed for the early detection in harsh environment monitoring. The EEZECR (Energy Efficient Zone-based Energy Clustering Routing) protocol utilized a zone divisional method for performing routing in a WSN. It proved to be energy efficient, but it suffered from the complexities in the network [24]. Several other attempts have been made to enhance the energy efficiency of routing protocols. One proposed a unique way of saving the network from heavily burdening the relay nodes which is also termed a hot-spot problem [25]. In this work, the aforementioned issues are addressed by selecting a CH based on the residual energy, distance factor and, most importantly, the network energy. The topology remains conventional where the routing scheme does not form any sector. Rather, unequal clustering that saves a huge number of overheads is followed.  
Figure 2: Clustering illustration.