# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Energy Efficiency in LPWANs
#### 4.2.1 LoRa and LoRaWAN  
A LoRa/LoRaWAN network formed by end devices (EDs) equipped with Wake-up receivers (WuRx) was analyzed in [139]. The network is organized as a star topology, with a Central Node (CN). The CN can wake up one or more EDs via a Wake-up Beacon, referred to as WuB. EDs spend most of their time in the sleep state, and wake-up only when they receive WuB signals.The proposed scheme uses a bidirectional long-range communication for transmission between the CN and the gateway, and unidirectional communication between ED and the gateway. It also uses Ultra-Low Power (ULP) WuRx to achieve pure-asynchronous communication between the CN and the EDs. By using this method, the EDs do not have to listen the channel periodically (as Class B devices) or continuously (as Class C devices) in order to receive data from the gateway. Simulation results show that the power consumption when using the WuRx scheme is significantly lower (up to 3000 times), with respect to legacy LoRaWAN Class A, B and C devices.  
Another analysis of LoRa energy efficiency was proposed in [140]. Normally, before transmissions, a LoRa device switches into Channel Activity Detection (CAD) to verify that the channel is not occupied by another device. It uses a frame which includes the CAD interval, the time it takes to transmit a packet, followed at the end of the frame by one second in standby. Only after this procedure is complete, a window is opened for the downlink. From the moment a preamble is detected, until the entire packet is received, the device continues to be in the reception mode, otherwise the device is on standby.  
In order to correctly evaluate the energy consumption in each mode, the method utilizes the current levels \(I\) [mA] provided in [141]. When considering uplink transmissions, the energy consumption is evaluated by taking into account the consumption in CAD and transmission modes. The model in [139] gives a more complete exposure to the energy expenditure. The energy consumed during transmission can be evaluated as follows:  
\[E_{TX}=V\cdot I_{TX}\cdot T_{a}, \tag{24}\]  
where \(E_{TX}\) is the energy spent in transmission mode, \(V\)is the voltage supply, \(I_{TX}\) is the transmission current and \(T_{a}\) is the time in the air for the uplink transmission.  
In [140], the energy efficiency for a single link is calculated, defined as the ratio between the number of bits in a payload that is successfully transmitted and the total energy spent to make possible the transmission, measured in bits per Joule per Hertz. In order to calculate the energy efficiency, the authors model the probability of success for a single link, and then use the energy consumed during each mode. The energy efficiency can thus be evaluated as follows:  
\[\eta=\frac{PL\cdot P_{S}}{BW\cdot V\cdot I\cdot T}, \tag{25}\]  
where \(PL\) is the packet length in bit, \(BW\) is the bandwidth, \(I\) is the current, \(V\) is the voltage, \(P_{S}\) is the probability that there is a successful transmission, and \(T\) is the activity interval. Results of the numerical analysis show that, at long distances, higher SF values result in higher energy efficiency. On the contrary, lower SF values give better performance for short distances.  
LoRaWAN energy efficiency is also analyzed in [142]. The study considers a fixed transmit power and the Okumura-Hata radio propagation model. Two types of energy dissipation are considered in order to measure the energy consumption: energy dissipation in the transmitter \(E_{tx}\), and in the receiver \(E_{rx}\). The energy consumption is then evaluated by considering several factors, including SF, transmission power, bandwidth, and star vs. mesh topologies. Results show that, in order to achieve low energy consumption, there is a need for adapting the transmitted power depending on the used SF. Moreover, it is also observed that signal bandwidth does not have significant impact on energy consumption.