# 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)
NAS messages include a RAI field that allows the MME to be notified by the device if no more uplink or downlink data are expected. On the other hand, UP optimization improves the establishment of the initial RRC connection between device and base station, which configures the radio bearers and the Access Stratum (AS) security context, and enables secure transmissions. When the device transits to Idle mode, the Connection Suspend procedure comes into effect, enabling the device to retain its context. In the case of new data exchange, the device resumes the connection using the Connection Resume procedure. This is done by providing a Resume ID to the base station, so that this latter can access the device-specific context. The above procedures were tested, for uplink and downlink, with and without acknowledgment, while also using PSM. Results showed that the battery lifetime decreases significantly for short Inter Arrival Times (IATs). When the device is performing DRX, the total period of time consists of the Inactivity and Activity timers. The longer a device stays in DRX, the lower the probability of reestablishing a connection after it comes out from DRX. CP optimization makes it possible for the MME to know if there is still traffic going on, using the information gathered from the RAI notification, leading to the elimination of the need for DRX. **Network techniques. ** A routing-based method for boosting the energy efficiency of a NB-IoT system was proposed in [159], using cooperative relaying. In this scheme, NB-IoT devices help each other in communicating towards the base station, acting as relays and thus establishing device-to-device (D2D) links. This would also ultimately lead to infrastructure cost reduction for network operators. The analyzed scenario consists of a single base station, with \(N\) different devices uniformly distributed in its coverage area. Different from the NB-IoT standard, devices in idle mode act as relays for UL communications of active peers. Given the above scenario, the Optimal relay selection and Greedy approaches were proposed. The first approach tries to minimize the energy consumption of the entire relay system, referred to as \(E_{TOT}\), assuming the presence of \(N_{a}\) active and \(N_{i}\) idle devices, respectively. In order to evaluate \(E_{TOT}\), the energy required on each link between the devices and the base station has to be evaluated. Therefore, the following parameters are used:  
* Transmission power P, assumed common for each link;
* Resource Unit (RU) allocated to the \(i\)th device relaying to the \(j\)th device or directly transmitting to the base station, denoted as \(R_{i,j}\) or \(R_{i,0}\), respectively. The RU is a basic unit allocated over the 180 kHz Physical Uplink Shared Channel (PUSCH);
* Slot duration for transmission, \(t_{s}\), assumed common for each link. The energy spent on each D2D link, between devices \(i\) and \(j\), is formulated as follows:  
\[E_{i,j}=P\cdot R_{i,j}\cdot t_{s}. \tag{26}\]  
Finally, the total energy consumed by the relay system is:  
\[E_{TOT}=\sum_{i=1}^{N_{a}}(\alpha_{i}\cdot E_{i,0}+\sum_{j=1}^{N_{i}}\beta_{i, j}(E_{i,j}+E_{j,0})), \tag{27}\]  
where \(\alpha_{i}\) and \(\beta_{i,j}\) are binary coefficients. In particular, \(\alpha_{i}=1\) is when the \(i\)th device transmits the gathered data directly to the base station, while \(\beta_{i,j}=1\) when the \(i\)th device employs the \(j\)th device as a relay node. A drawback with the optimal relay selection as defined based on Equation (27) is that when there is a communication based on relay selection, it does not allow for a direct link between node \(i\) and the base station, and at the same time the only devices that can be selected as relays are the devices in Idle mode. Another drawback with the optimal relay selection is that it requires high computational efforts. To overcome these drawback, a greedy algorithm is also proposed.