# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Energy Efficiency in LPWANs
#### 4.2.2 Cellular IoT (cIoT)
paging in the downlink. If there is no valid paging, then the receiver can go to sleep, without needing to decode the paging block, saving in this way million instructions per second (MIPS). Simulation results showed that a receiver using the proposed paging indicator consumes considerably less power (about 40%) to decode a paging block, compared to a receiver not using paging indicator. NB-IoT  
NB-IoT is arguably the most important cloT technology, as demonstrated by the large amount of works studying its energy efficiency and consumption aspects. Techniques belonging to all the four categories defined at the beginning of the Section were proposed, as shown in Figure 8.  
**Sleep/wake-up techniques. ** NB-IoT uses two advanced mechanisms, that is PSM and eDRX. PSM makes it possible for the device to enter in a deep sleep mode within the RRC Idle state, so to minimize its consumption. In this situation, the device is still registered to the network but cannot be reached. Then, if the Mobile Mobility Entity (MME) receives a downlink data notification, it notifies the serving base station to cache those data and delay the paging request. The device goes back to Connected mode only when it has data to transmit in uplink [59]. It activates an active timer (AT) when it transits from Connected to Idle, and goes into PSM mode when AT expires. More details on the time structure enabling PSM and eDRX mechanisms are provided in Figure 9.  
eDRX was introduced in Rel-13, in order to further extend the sleep time during Idle mode. It is an evolution of the DRX technique, already described for LTE-M, which works on both Idle and Connected modes. In Idle, eDRX extends by up to 3 hours, compared to DRX, the time a device is not actively monitoring paging messages [152]. A set of timers, defined in Table 6 controls eDRX processes. In particular, the Active Timer (T3324) which can control the time lapse for which the device is reachable by the network while in Idle. In Connected, the device alternates between listening for paging occasions and sleep periods. eDRX can be used without PSM or in conjunction with PSM. It is responsible for extending the time interval during which a device is not listening to the network. eDRX-specific cycles are reported in Table 6.  
Figure 8: Energy efficiency techniques for cloT technologies [149; 150; 151]. A technique aiming to group and synchronize NB-IoT devices for multicast transmissions, in order to improve the energy efficiency, was proposed in [153]. The method is slightly different from clustering, since in this case the devices are grouped with no CH selection. The grouped devices have a common paging message within the inactivity timer, that regulates time device is inactive, and are able to receive multicast data in a single transmission, due to the fact that they go back to sleep mode only when all of them have been paged, and not before. Three different grouping and synchronization mechanisms are proposed:  
* DRX Respecting, Standards Compliant (DR-SC): This mechanism does not affect the DRX cycle, and its main objective is to transmit the multicast data with the lowest number of transmissions. A problem with DR-SC is that it is not easy for multiple devices to occur simultaneously at a multicast transmission, leading to multiple needed transmissions and high bandwidth usage;
* DRX Adjusting, Standard Compliant (DA-SC): This mechanism synchronizes the devices by modifying their DRX cycles, in order to perform a single multicast transmission. The base station chooses a time \(t\) for the multicast transmission, with \(t\) at least equal to \(2\cdot max_{DRX}\), being \(max_{DRX}\) the longest DRX cycle across the devices joining the multicast. The devices receive the new DRX value within a RRC Connection Reconfiguration message, through the random access procedure. The base station