# Energy Optimization in Cluster-Based Routing Protocols for Large-Area Wireless Sensor Networks
## 1 Introduction  
Ad-hoc wireless sensor network (WSN) is a core technology of Internet of Things (IoT) with a wide range of applications including smart home networking and industrial internet of things (IIoT) [1, 2]. In IoT environments, the WSNs have large area with many sensor nodes compared with conventional application specific sensor networks. We need more sophisticated energy-efficient routing protocols than conventional ones targeted for relatively small-area WSNs. WSN is a collection of large numbers of sensor nodes deployed over a large area with a central base station (BS) that collects sensing data. Sensor nodes are characterized by limited sensing, computing and communication capabilities. Moreover, in WSN based IoT environments, several hundreds or thousands of sensor nodes are randomly deployed with limited battery power, which are difficult to be recharged. To overcome limitations from these characteristics, a great emphasis is placed on scalability and efficient energy consumption of WSN in every design aspects including routing protocols, network topology, security key management, etc. Based on the deployment of nodes, routing protocols can be divided into two types: flat routing and hierarchical routing, which is also called cluster-based routing. In flat routing protocols, all sensor nodes play identical roles in conveying sensing data to the BS. Cluster-based or hierarchical routing protocols refer to routing protocols in which the nodes are grouped into clusters, where a cluster is composed of a cluster head (CH) and one or more member nodes. Cluster-based routing protocols take advantage of load balancing and reducing communication volume in a distributed manner to prolong the network lifespan of WSN. Low Energy Adaptive Clustering Hierarchy (LEACH) [3] is a most widely adopted cluster-based routing protocol for WSN, by virtue of its simplicity and energy efficiency as a fully decentralized scheme. There has been a lot of research work related to energy consumption in cluster-based routing protocols. In single hop communication, the CH collects data from its member nodes and directly sends these data to the BS, as in LEACH [3]. If the network area is small, then the entire sensor field can be covered by the transmission range of any node in the field, and single hop protocols are advantageous due to minimum overhead and minimum delay. However, cluster-based routing protocols with single hop protocols [4; 5; 6; 7; 8; 9] are not suitable for IoT environments based on large-area WNS, due to their short lifespan. Multi-hop communication is more energy efficient approach to routing protocols for large-area sensor networks. In multi-hop cluster-based routing protocols, the CH sends its data via some intermediate nodes to the BS, if necessary. In LEACH-D protocol [10], connectivity density factor and energy factor are considered in computing the threshold value for the CH selection phase. Although it shows improvement in terms of scalability and energy consumption, it still has hotspot problem with difference in cluster sizes. This hotspot problem is addressed in FZ-LEACH protocol [11] to enhance the lifespan of the network. However, the main drawback of FZ-LEACH is non-optimality in the selection of zone heads. This drawback is improved in IFZ-LEACH [12], in which CH divides the sensor field intofar zone and non-far zone to select the zone head from the far zone based on maximum residual energy. However, it suffers from scalability and network complexity. In DL-LEACH [13], the network is divided into several layers and the nodes in lower layers make distance-base determine whether they are to transmit data via CHs or to send data to the BS directly. It has achieved a good improvement in energy consumption with moderate-size sensor networks. However, it suffers from short node lifespan in large-scale or large-area networks. In CL-LEACH [14], the node with residual energy greater than the threshold value performs the tasks of a relay node for multi-hop communication.