# 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)
The NS3-based simulator was used to evaluate the performance of the proposed algorithm. In order to evaluate the performance of the proposed algorithm, the authors also performed measurements in real scenarios. Results showed that DRX allows to safely power downdevices, thus leading to significant energy savings, with the proposed algorithm providing lower energy consumption. **Resource Allocation techniques. ** An enhanced random access algorithm was introduced in [150], in order to reduce the power consumption of devices. LTE-M defines two access modes:  
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* Contention-based Mode, where devices attempt to access at the same time, leading to competition for the channel resources;
* Contention-free Mode, where the base station allocates the channel resources for all the access requests that are received. The contention-based random access procedure presented in [150] is based on time-slotted ALOHA. The energy consumption is divided into three parts: receiving and sending (non-sleep) and sleep energy consumption. Based on slotted ALOHA, the time is divided into time slots. If there is a data frame that arrives at a time slot, it can be retransmitted only at the beginning of the next time slot. The scheme proposed in [150] is based on the principle that when a new slot arrives, the device that has initiated the access will receive a broadcast signal, containing the success rate of the last time slot. If the last time slot access has a low success, the device selects another access slot that has a higher success probability. Simulations showed that when the network starts to congest, the proposed algorithm helps improving the success rate of devices accessing the network. An increase in the number of the devices results in a decrease in the success rate; the proposed waiting scheme helps saving the energy consumption. EC-GSM-IoT An approach aiming at decreasing power consumption of EC-GSM-IoT devices was proposed in [151]. In particular, it focuses on the use of the paging operation (described in Section 2) during the DRX mode to efficiently save battery. The method makes use of the dummy bits [56], which are used in three different ways:  
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* _Paging Indicator_, indicating if the paging block has a valid Paging Indication message (bit combination \"11\") or not (bit combination \"00\"). The paging indication message is a short indicator that tells the devices that there is a paging message on an associated paging channel;
* _Type of IoT Device_, where the base station uses the flags of two consecutive PCH bursts, to provide more information to the device, making possible for the device to decide if it can further receive PCH bursts or not. The stealing bits can also help indicating the type of IoT device, by grouping the IoT devices based on their applications;
* _Coverage Class of IoT Device_, where the base station uses the stealing bits of the first 2 PCH bursts to indicate to the receiver if for a specific coverage class there is a valid \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**DRX Parameter** & **Description** \\ \hline DRX Cycle & Duration of one ‘ON time’ + one ‘OFF time’ phase \\ \hline onDuration Timer & Duration of ‘ON time’ within one DRX full cycle \\ \hline DRX-Inactivity timer & The number of consecutive Transmission Time Intervals (TTIs) during which the UE reads the PDCCH, after having successfully decoded a PDCCH that indicates an initial UL or DL user data transmission. \\ \hline DRX-Retransmission timer & The number of consecutive PDCCH subframes for UE to monitor the channel when it is expected a transmission from eNB. \\ \hline shortDRX-Cycle & The first DRX cycle that the UE enters after the inactivity timer expires. The UE remains in short DRX cycles as long as the drxShortCycleTimer is running. \\ \hline DRXShortCycleTimer & The consecutive number of subframes for which the UE remains in short DRX cycle after the DRX Inactivity Timer has expired. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: DRX Parameters.