Energy Efficiency in Wireless Sensor Networks

WSN is one of the main elements of IoT for sensing environments. A number of reviews of IoT [9, 11, 39, 52] have described the challenges of WSN concern- ing energy efficiency. Moreover, numerous energy-efficient approaches for IoT scenarios were proposed in [12, 155] to improve the lifetime of the sensor data- collection processes. The current author summarised the existing approaches into a few groups based on their common features, as follows.

Clustering-based Approaches

To increase the lifetime of the sensor nodes, many cluster head selection schemes have been developed during the last decade. Many of these were based on extend- ing the low-energy adaptive clustering hierarchy (LEACH) [66] algorithm, which forms a cluster of wireless sensors and selecting cluster head based on signal straight. In the framework proposed in [79], the base station first collects all sensor nodes’ energy and location information, and then elects the cluster heads using the fuzzy rule according to the collected fuzzy variables. In addition, an energy-aware multi-objective fuzzy-clustering algorithm for WSNs [123] provides a solution to prevent the early death of cluster heads close to the sink. The proposed fuzzy- clustering algorithm uses the three parameters of remaining energy, distance to the sink and density of the nodes to assign appropriate ranges to tentative cluster heads. The final cluster heads are then selected from them via an energy-based competition.

Scheduling Approaches

Scheduling schemes ensure that sensor nodes only function when necessary, and remain in sleep mode at all other times to reduce energy consumption. One of the conventional approaches involves dynamically scheduling the duty cycles of sensors by using sleep scheduling algorithms. The connected k neighbourhood

(CKN) [103] algorithm turns off the redundant nodes if a node has more than k- neighbours in the network. The groups of active nodes are selected periodically, and when the network does not satisfy the k-neighbours, other nodes should be in active mode until the particular node has more than k-neighbours. Correspond- ingly, the algorithm discussed in [162] is an extension of CKN that also considers nodes’ residual energy to decide whether a node should be active or asleep. This approach enables the nodes with more power to participate actively and the nodes with less energy to participate infrequently during the sensing period.

In mobile cloud computing, a mobile client can preserve its energy by offload- ing its computation task to the cloud if the cost of local computation is higher than the uploading. In [157], the authors designed an adaptive energy optimal scheduling policy based on the size-controlled collaborative execution model to reduce the energy consumption on the mobile client. Moreover, the authors pro- posed a low-complexity threshold adaptation scheme that exploits the degenerated Monte Carlo method to estimate the threshold of locally executed data. Similarly, a collaborative execution scheduling policy presented in [164] adopted the La- grangian relaxation-based aggregated cost algorithm. The proposed model pre- pares an energy-efficient task-scheduling policy to preserve the energy on the mo- bile device.

To overcome issues deriving from the energy limitations of the nodes and the distributed nature of WSNs, an energy-aware scheduling strategy to allocate com- putational tasks on small devices in WSN was presented in [37]. The proposed framework is established on the two-phase heuristic-based algorithm that initially assigns a task locally to the cluster, and, if the task cannot be completed locally, it is migrated to the most suitable node. Moreover, the focus of the scheduling strategy exploits the network lifetime by increasing the number of alive nodes with balanced energy load. In addition, efficient resource allocation for multi-hop IoT infrastructure presented in [2] introduced an energy-efficient context-aware traffic-scheduling (EE-CATS) algorithm that reduces nodes’ total awake time by applying an adaptive duty cycling approach.

Correspondingly, generally in WSNs, each sensor node has a limited buffer space to hold the sensed data, and data may have overflows if not forwarded to the data collectors on time. However, the sensor nodes in different areas have different sampling rates that cause complications in designing a unique schedule to upload data from nodes to collectors. With help from mobile data collectors, a mobile element scheduling algorithm [133] enables the mobile element to visit nodes promptly to gather data to avoid data loss at the sensor nodes.

Similarly, properly scheduling the time-division multiple access (TDMA) slots in WSNs will improve the nodes’ lifetime and network efficiency. The authors who presented a distributed TDMA scheduling algorithm for the IoT [84] stated that the energy and topology information of network nodes is essential for extend- ing the network lifetime. With this in mind, they proposed an energy–topology (E-T) factor that uses the residual energy and topology information to formu-

late the scheduling algorithm. Further, the algorithm prioritises time slots that minimise the execution time and energy consumption of the whole network. Ad- ditionally, research on TDMA scheduling algorithms for WSNs [51] proposed two centralised scheduling algorithms—node-based scheduling and level-based scheduling—established on the colouring of the linear network.

Accordingly, another approach to enhance the energy efficiency in WSNs in- volves selecting a set of representative nodes that periodically provide sensor data, which will minimise the number of active nodes and messages in the network. Based on this strategy, [34] introduced an energy-efficient node-scheduling algo- rithm using Markov random field. In addition, [145] presented an asynchronous wake-up scheme for energy conservation in underwater acoustic sensor networks. The authors offered a combinatorial design asynchronous wake-up scheme to min- imise the active duty cycle of sensor nodes.

Moreover, the research in [33] proposed a Service-oriented Node Scheduling Scheme, Energy-aware Centralised Heuristic Scheme (ECHS) and Energy-aware Distributed Heuristic Scheme (EDHS). An energy-aware benefit function used in the ECHS determines the active sensor nodes and rotate sensor nodes by peri- odically reconstructing the scheduling scheme. In addition, the feature improves the network performance by considering the nodes’ capability of providing ser- vices and residual energy. Moreover, the EDHS provides a distributed solution by assigning one header to each service. In addition, the scheduling scheme is performed in a distributed way that, for the active node selection process, only requires local information.

2.6.2. Using Public Fog Networking Services for Energy-efficient