# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 3 Energy Efficiency and Consumption Metrics and Models
### Energy and Power Consumption Models
#### LoRaWAN  
A LoRaWAN energy consumption model for Class A devices is given in [85], taking into account settings such as data rate, payload size and the impact of channel conditions as measured by BER. The model has roots in two previous works on LoRaWAN energy consumption, that is, (1) the work in [86], which focuses on modeling the consumption of sleep, transmit, and receive states, and evaluates the daily battery consumed by a device; and (2) the work in [87], which presents an evaluation of the energy consumed during the activation of a LoRaWAN node. The model was introduced to assess the difference in energy consumption between acknowledged vs. unacknowledged transmissions, taking into account the consumption during transmission, reception and sleep periods.  
A LoRaWAN testbed was used to test the model, using the MultiConnectmDot platform from Multitech [88]. Results show an almost exact match between the analytical model and the testbed measurements, and highlight the strong impact of the acknowledgment mechanisms in reducing energy consumption, as explicit acknowledgment by the receiver allows a transmitter to enter a sleep mode and save energy.  
Sigfox  
A model for the current consumption of a Sigfox device is proposed in [89], based on measurements from a real Sigfox device. It is assumed that the device periodically transmits uplink data messages, and the derived model also takes into account data frame losses.  
First, the model focuses on unidirectional communication. The average current consumption is modeled over one period, since it is assumed that the device periodically performs an information exchange. This means that each period consists of the operations that are performed by the device to transmit one data message, including its replicas. In this period, the device transits over several states, similarly to the modeling approach adopted in [85] for a LoRaWAN device. The model evaluates the current consumption of the device as follows:  
\[I_{avg\_uni}=\frac{1}{T_{Period}}\sum_{i=1}^{N_{states\_uni}}n_{i}T_{i}I_{i}, \tag{8}\]  
where \(T_{Period}\) is the time between two consecutive unidirectional communications (also, transactions), and \(T_{i}\) and \(I_{i}\) are the duration and current consumption of state \(i\). Moreover, \(n_{i}\) is the number of times state \(i\) is present in the uplink data frame transmission, and \(N_{states\_uni}\) is the total number of states, that is 5 in this case. By using the average current consumption \(I_{avg\_uni}\) and \(T_{Period}\), the model can model the energy that is consumed during the unidirectional communication, which can be obtained as follows:  
\[EC_{delivery\_uni}=\frac{I_{avg\_uni}\cdot V\cdot T_{Period}}{E\cdot[I_{delivery}]}, \tag{9}\]  
where \(V\) is the voltage of the battery, while \(E\cdot[I_{delivery}]\) represents the amount of data that is expected to be delivered by the device.  
Periodic bidirectional communication (in this case the data is transmitted in both directions: send and receive) are also studied. A bidirectional transaction consists of 9 states, and expands the unidirectional case. The average current consumption is given as follows:  
\[I_{avg\_bi}=\frac{1}{T_{Period}}\sum_{i=1}^{N_{states\_bi}}n_{j}T_{i}I_{j}, \tag{10}\]  
where the variables have similar definitions of the previous case. The results of the analysis show that that the average current consumption decreases with the transaction period, with an asymptotic value that is equal to the current consumption in the sleep state. The unidirectional communication outperforms the bidirectional communication in terms of current performance. This is related to the fact that in the unidirectional communication there are less states than in the bidirectional communication, and the communication period is lower.