# 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)  
cIoT enables mMTC over the cellular system. This latter is currently transitioning from the fourth to the fifth generation, which is envisioned to fully support mMTC scenarios providing connection to a large number of IoT devices. Large amounts of devices will be thus connected through the 4G/5G cellular architecture, interacting with heterogeneous users, and providing several services, particularly in the context of smart environments and industry automation [143]. To achieve this goal, capacity and usage of the cellular system will significantly increase, thus possibly leading to high energy consumption and operational costs (OPEX), and in turn to increased environmental concerns, if appropriate mechanisms for energy efficiency are not adopted [144]. Within this context, an aspect jointly highlighted by academic and industry communities is that the capacity increment targeted by next-generation systems have to match with an appropriate power consumption balance [145; 146]. LTE-M  
This section focuses on techniques that try to optimize LTE-M energy consumption. The main methods are: the Discontinuous Reception (DRX), as part of Sleep/wake-up techniques, and methods based on random access procedure optimization, as part of Resource Allocation techniques. **Sleep/wake-up techniques. ** Methods belonging to this category typically take advantage of the DRX feature that LTE-M inherits from LTE. In general, even when there is no traffic exchange between the network and the UE, the UE has to continue listening to the network, in order to decode the Physical Downlink Control Channel (PDCCH), which carries scheduling assignments and other control information [147]. By doing so, the UE remains active (ON) all the time, thus draining the battery. A solution to decrease the consumption is for the UE to go into a sleep state, that is, RRC Idle mode (OFF) for a period of time, and wake-up sporadically to check if there is any relevant data. If not, the UE can go back into the sleep mode, and will wake-up again later, after another full inactivity period, repeating this cycle over and over. DRX regulates such periodic cycle using the parameters expressed in Table 5[148]. LTE-M energy efficiency was analyzed in conjunction with DRX in [149], which proposed a cross-layer scheme for packet scheduling, using a memetic-based algorithm. This algorithm exploits the fact that memetic algorithms require less computations, thus leading to significant power savings. DRX is used to further improve battery lifetime, by switching the LTE-M devices in Idle mode every time the devices enter an inactivity period. The scheme utilizes a scheduling algorithm that selects LTE-M devices based on channel quality and energy, thus optimizing frequency/time resource allocation while reducing the energy consumption. Before this process happens, the base station identifies the active nodes in the cell, by checking if an LTE-M device has data ready for transmission; then each device that has no data for transmission applies DRX. Each node is assigned an urgency weight which determines the time the device transmits, which is compared to a threshold value. If the urgency weight is higher than the threshold, the node will be considered an _urgent node_, unable to wait for its turn to transmit. The algorithm keeps this node in the Connected mode and will be the first to be allocated in the next Transmission Time Interval (TTI). If, the urgency value is lower than the threshold value, the node is considered a non-urgent node. It is very important to define the length of the DRX cycle, because a long DRX cycle can increase data latency. After the configuration of the DRX cycle has finished, the nodes will be in sleep mode for a DRX Cycle. When and if a node reaches its delay threshold, the DRX cycle is reset to its initial value while remaining in Connected mode. If the node has not reached its delay threshold, the DRX Cycle is configured with a new updated value and the node switches from Connected state to Idle.