# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Mechanisms for RFID Active Tags  
RFID active tags switch between sleep and wake-up modes; during the latter mode, the tags exchange data with the readers, and wait for the acknowledgments (ACKs) of correct reception from them, before entering again in sleep mode. When it sleeps, the tag is inactive and consumes very little energy. **Sleep/wake-up techniques. ** The Free2 move protocol was proposed in [106], and is functional in both slotted/synchronized and asynchronous Alohan RFID systems. The Slotted Alohan RFID system, also referred as Reader Talks First (RTF) mode, is a mode where the reader transmits beacon signals to which the tags react to.The time is divided into several slots where each tag must randomly select a slot in which it will transmit its data, making the communication between the reader and the tag synchronous. In this case the tags transmit their ID using these synchronous time slots. In the asynchronous ALOHA system, called Tag Talks First (TTF) mode, data transmissions are not coordinated. In the Free2 move protocol, in the TTF mode, the tags act independently of the readers, waking up randomly in order to deliver information to the reader. In the RTF mode, the reader creates a slotted scheme by continuously sending beacon signals, and listens for answers from the tags, in between the beacon signals. If two tags are synchronized with the reader beacon signal, the reader can listen to both tags, and is able to switch and transmit using two different channels. In the TTF mode, the tag uses four possible frequencies, which are grouped in two groups; where two transmitting tags can deliver information at the same time using each frequency group. In the cases when the information is transmitted by tags which are part of this system, these tags do not have to use synchronization before transmissions and the system functions on the basis of first come, first served. This means that the first tag that the system recognizes is the tag that is read by that reader. Simulations and measurements were performed to evaluate this method, with the results showing that the tags using Free2 move have lower power consumption in the Transmit state compared with the Receive state. The Enhanced protocol was an enhancement on the Free2 move protocol, also proposed in [106], with the main modifications on the current protocol being:  
* The introduction of a deep-sleep parameter in the ACKs sent by the reader, which specifies a variable time for the tag to enter the deep-sleep mode;
* An enhancement in the use of the radio channel in RTF mode, where all the available slots are used for beacon signals; which is not the case in the Free2 move protocol. In the RTF scenario, the energy consumption is evaluated by taking into account the energy spent for detecting the reader beacon, while being in Transmit, Receive and Sleep states, as follows:  
\[E_{RTF-Free2 move}=E_{Bacon}+E_{TX}+E_{RX}+E_{Sleep1} \tag{13}\]  
\[E_{RTF-enhanced}=E_{Bacon}+E_{TX}+E_{ACK}+E_{Sleep1}, \tag{14}\]  
where \(E_{Bacon}\) is the energy consumed to receive a beacon signal from an available reader, \(E_{TX}\) and \(E_{RX}\) are the energy consumed in Transmit and Receive states, respectively, \(E_{Sleep1}\) is the energy consumed during the sleep period after the tag has communicated with the reader, and \(E_{ACK}\) is the energy consumed to receive an ACK. When there is no reader available, the Free2 move consumption only includes the energy spent while listening for a reader, which is the same for the Enhanced protocol. Then, in this case, the energy consumption for Free2 move and Enhanced is the same, as follows:  
\[E_{RTF_{-}Free2 move_{-}enhanced}=\hat{E}_{Beacon}+E_{Sler2}, \tag{15}\]  
where \(\hat{E}_{Beacon}\) is the energy consumption when there is no reader available and the tag listens for beacon signals, while \(E_{Sler2}\) is the energy consumption for the sleep period after trying to receive a beacon signal. After successfully delivering payload packets, the tag that is using the Enhanced protocol enters the deep-sleep mode.