# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Mechanisms for RFID Active Tags
The ACK variable is used to specify how many cycles the tag should stay in deep-sleep state, after a successful transmission. In this way the tag does not consume energy in order to wake up and check for a beacon signal periodically, but wakes up based on the time shown by the ACK variable. In the TTF scenario, however, the consumption analysis does not take into account the energy for detecting the beacon, resulting in better performance over the RTF counterpart. Performance analysis showed that the energy consumption in the case of the Enhanced protocol decreases of about 90% when a reader is available and a tag enters into deep-sleep mode for a period of time, while it is lowered of about 34% when there is no available reader, since in this case the tag consumes energy in order to listen for beacon signals and discover an available reader. **Resource Allocation techniques. ** An algorithm that aims at reducing the energy consumption for active RFID tags using the polling techniques is presented in [107]. The protocol performs a sequence of polling exchanges. Polling is a technique that enables a continuous interrogation of the devices (tags) by a central entity, in order to check if they have data to transmit. The reader sends out polling requests, and allows the tags to respond within a time frame. The polling request carries a contention probability \(p\), with \(0<p\leq 1\), and the size of the frame \(f\). A tag participates to the polling with probability \(p\); if it decides to participate, it randomly picks a slot in the frame and \(\mathbf{to}\) transmits a response string in that slot. A slot can be labeled as _empty_, if there are no tags transmitting in it, _singleton_, if one tag transmits, and _collision_, if multiple tags transmit concurrently. Singleton and collision slots are also called _nonempty_. The first algorithm proposed in [107] is the Generalized Maximum Likelihood Estimation (GMLE) algorithm. This algorithm is able to use all the information that is gathered from different polling exchanges, to minimize the total number of exchanges that need to be done. GMLE records whether the slot in each polling is empty or nonempty. The protocol consists of two phases:  
* _Initialization Phase_, during which is estimated the number of tags;
* _Iterative Phase_, during which the real number of tags is evaluated. In order to complete these two phases, the GMLE protocol uses \(\omega\), a parameter that impacts the number of polling exchanges that can be done and the number of tags that can respond to each polling. The GMLE protocol uses the \(\omega\) parameter to establish the number of tag responses. Simulation results showed that when the \(\omega\) parameter decreases, the energy spent decreases as well, because the number of tags that respond to the polling is also lower. In the GMLE protocol, this is expressed more clearly in the case that if there are two scenarios that generate the same information, but in one scenario only one tag will respond and in the other, two tags will respond, the energy cost of the second scenario will be higher than that of the first scenario, meaning that energy cost is not dependent on the generated information, but on the number of tags that respond to a poll. An enhancement of the GMLE algorithm, called enhanced GMLE (EGMLE), is proposed to address this issue. EGMLE makes use of the history information gathered from previous polling exchanges and uses the maximum likelihood method to estimate the number of tags that will respond during the polling, to compute the number of responses in each polling, extracting more information and achieving better energy efficiency than GMLE. EGMLE and GMLE are similar in the case that both use the same polling protocol, with the difference being in the frame size that is used. The frame size that is used by the EGMLE protocol is larger than the one used by the GMLE algorithm, with the intention to reduce the probability of collision.