# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Energy Efficiency in LPSANs
#### 4.1.1 RFID
The interrogation zone proposed in [100] is then further divided into \(k\) equal sized clusters, where tags that are part of different clusters will be contacted and interrogated separately from each other. Simulation results show that this method helps reducing energy consumption and collisions between readers;
* Multi Hop Communication Multi Level Data Aggregation (MHML) protocol [80]--It includes two main phases: (1) a cluster formation phase, in which the residual battery energy of the reader is evaluated, in order to form the clusters and select the CHs, and (2) the steady state phase, in which the CHs aggregate and transmit the information gathered by the tags. The CHs uses TDMA to schedule the data transmission with the nodes (readers and tags) present in its cluster. The protocol proposed in [101] builds on the above works, and focuses on mobile scenarios, where the readers with higher energy and similar mobility patterns are likely to be selected as CHs. The scheme is composed of two phases; the first phase consists of choosing some of the readers as potential CHs, based on their mobility and energy level. During the second phase, each reader decides whether another node should be a CH or not. The readers that are not chosen as CH join the best CH, based on link connection time and energy. The nodes do not send their information individually to the base station, reducing the need to consume vast amounts of energy, but they send their information to the CH, which in turn sends the information to the base station. Using this scheme the nodes (readers) avoid energy wasting, and also retransmissions in the cases of collisions. As mentioned above, the selection of CHs is addressed through the Dragonfly algorithm [102], which follows three main principles:  
* _Selection_, so to avoid collisions between neighboring readers, by evaluating the distance between eligible CHs and other readers;
* _Alignment_, so to match eligible CHs to other neighboring readers, based on their mobility;
* _Cohesion_, so that neighboring readers count of each eligible CH in the network. This means that neighboring readers know which readers have an energy level above a predetermined threshold, making them eligible to be selected as CH. Simulation results showed that the proposed clustered network, significantly improves the energy efficiency with respect to its non-clustered counterpart. The use of the Dragonfly algorithm leads to significant energy savings, since it helps reducing the breakage of clusters, by selecting as CHs the readers having the highest residual energy and similar mobility. At the same time, it also eliminates redundant data, by sending aggregated data to the CHs. **Resource Allocation techniques. ** Resource allocation techniques for RFID readers typically rely on time division approaches, identified as an effective way to reduce energy consumption. Each reader is assigned to a time slot dedicated to its transmissions, thus triggering lower energy consumption thanks to the decrease of interference and collision between readers. A time-slotted reader transmission scheme was proposed in [105]. The slotting algorithm, referred to as SELECT, assigns neighboring readers to different time slots, by taking turns to propagate the signal. In order for the slotting algorithm to function properly, it needs to have knowledge of the positions of all the adjacent readers. SELECT tries to minimize the number of time slots, so it can avoid the risk that a transmission can not be read, leading to a tag being able to leave the system before being detected, if the time that it takes a reader to complete its transmission is too high. The performance analysis was done by placing a number of fixed readers in an interrogation area, where the readers and a large amount of tags are in close proximity to each other, and evaluating the energy saved when assigning the reader to different time slots.