# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 3 Energy Efficiency and Consumption Metrics and Models
### Cellular IoT  
An energy consumption model for mMTC devices is presented in [90], consisting of a single base station providing access to a large amount of uniformly distributed, machine-type nodes. The model uses the network lifetime as power consumption metric. The uplink packet generation is modeled as a Poisson process. For the \(i\)th node, the average payload size is \(D_{i}\), and the power consumption in transmission mode is \(\xi P_{i}+P_{c}\), being \(P_{c}\) the power consumed by the electronic circuits, \(\xi\) the inverse power amplifier efficiency, and \(P_{i}\) the transmit power. Then, the average energy consumption per reporting period, that is, the period of time an mMTC device is transmitting data, is evaluated as follows:  
\[\epsilon_{d}^{i}=[P_{c}+\xi P_{i}]\frac{D_{i}}{R_{i}} \tag{11}\]where \(R_{i}\) is the average transmission rate. In [90] an energy-efficient scheduling algorithm, referred to as _ExpAlg_ is also proposed. The algorithm separately addresses two specific subproblems: (1) to satisfy the minimum resource requirement for the set of high-priority nodes to be scheduled at time \(t\) and (2) to provide resources to all nodes, taking into account the impact on the network lifetime. The goal of the _ExpAlg_ algorithm is to find the nodes that have the highest impact on the network lifetime. Once they are identified, the scheduler allocates contiguous resource elements to them, in order to maximize the network lifetime. Simulation results showed that the proposed scheme significantly extends network lifetime, at the same time taking into consideration the remaining battery lifetime of the nodes, the priority class of traffic, and transmission-dependent and independent energy sources. LTE-M  
As mentioned in Section 2, LTE-M is the cellular IoT technology most similar to LTE under several characteristics, including energy consumption aspects. For this reason, and also considering the lack of LTE-M specific models in current literature, we briefly address in the following one of the most significant works on power consumption modeling for LTE User Equipment (UE). In [91], LTE UE physical layer components are analyzed, and their impact on the total power consumption is derived. The proposed model takes into account transmit/receive power levels, uplink and downlink data rates, consumption of RRC modes (Idle vs. Connected, see Section 2.2.2) and consumption due to baseband (BB) operations, that is, transmit/receive BB phases, which focus on turbo-encoding and decoding the UE data. The proposed model was tested by using an Agilent 6705B DC Measurement Power Supply, sampling the UE consumption every 1 ms, for 30 s. The verification results showed a very high accuracy of the model in the DL case (average error of 0.8%) and a good one in the case of UL as well, with an average error of 3.2%. The model can provide a valid starting point for the development of an energy consumption model of LTE-M.  
NB-IoT  
A model for NB-IoT energy consumption was presented in [92], taking into account the impact of the Power Saving Mode (PSM, mentioned in Section 2). Uplink and Downlink packet arrival rates are modeled according to a Poisson model. The model was developed to analyze the power consumption when the device moves between RRC Idle, Connected and PSM, during three typical operative cycles:  
* Cycle 1: Idle \(\rightarrow\) Connected \(\rightarrow\) Idle, modeling the transmission or reception of a packet when the device is initially in Idle. * Cycle 2: Idle \(\rightarrow\) PSM \(\rightarrow\) Connected \(\rightarrow\) Idle, modeling the transmission or reception of a packet being scheduled after the timer that causes a device to move from Idle to PSM expires; the transmission/ reception takes then place as soon as the device leaves PSM. * Cycle 3: Idle \(\rightarrow\) PSM \(\rightarrow\) Idle, modeling the behavior of a device when there is no transmission or reception taking place.