# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 2 IoT Technologies
### Low-Power Wide-Area Networks (LPWANs)
#### 2.2.1 Proprietary Technologies in Unlicensed Spectrum  
LoRa/LoRaWAN  
LoRa is a long range technology enabling IoT services in heterogeneous scenarios, for example, rural, dense urban, and deep indoor environments. It operates within the sub-Gigahertz unlicensed spectrum, including 169 MHz, 433 MHz, 868 MHz (in Europe), and 915 MHz (in the North America) frequency bands. In particular, the term LoRa identifies a specific physical layer patented by Semtech, adopting a Chirp Spread Spectrum (CSS) modulation technique, where the signal frequency linearly varies over the transmission time interval \(T\) in a frequency range \([f_{0},f_{1}]\)[49]. The CSS modulation technique is based on the chirp signals, using fixed amplitude frequency modulation. The data rate and symbol rate of LoRa depend on the Spreading Factor (SF) (the duration of the chirp), and the bandwidth that is used. A LoRa symbol is composed of \(2^{SF}\) chirps that cover the entire frequency band. The symbol rate \([symbol/s]\) is:  
\[R_{\mathrm{s}}=SF\frac{BW}{2^{SF}}, \tag{1}\]  
where \(BW=f_{1}-f_{0}\) is the bandwidth [50; 51]. Figure 3: Bluetooth Scatternet and Piconet Hybrid Network Architecture. From the above formula it can be seen that the symbol time (rate) is increased (decreased) by increasing the SF. While also considering the Coding Rate (CR), the data rate \([b/s]\) can be found as follows:  
\[R_{b}=SF\frac{\frac{4}{3+\text{CR}}}{\frac{25^{3}}{BW}}1000. \tag{2}\]  
LoRaWAN does not allow device-to-device communications, and mainly works in uplink. However, if needed and required by specific applications, network servers can send downlink data and control packets to end devices. Different functionalities for bidirectional communications lead to the definition of three main classes of LoRaWAN devices [52]:  
* _Class A_: An uplink transmission time slot is immediately followed by two timeslots, in which the device can receive downlink information from the network servers. Class A devices only receive data as a consequence of their own transmissions;
* _Class B_: With respect to Class A, the devices reserve additional timeslots for downlink communications, adopting the time synchronization promoted within dedicated beacon messages from the gateways;
* _Class C_: With respect to previous classes, these devices are always available to receive downlink messages, except when they are transmitting. As specified in the standards, Class A is mandatory for each end device, while Classes B and C are optional. The LoRaWAN network is composed of a network-side server connected to many LoRaWAN gateways, that act as APs for the end devices. End devices must be activated if they want to participate in a LoRaWAN network, via either Over-The-Air Activation (OTAA) or Activation By Personalization (ABP). Aiming at leveraging performance vs. energy efficiency trade-offs, LoRaWAN has the so-called Adaptive Data Rate (ADR) mechanism. ADR functions by adjusting the SF and thus the data rate in accordance to the estimated distance between a node and a gateway. The nodes that are closer to the gateway have a higher data rate. ADR uses two parameters that are the ADR-ACK-LIMIT and ACK-ADR-DELAY to monitor and control the number of uplink messages [51]. If there is no downlink response detected, within a certain time, it is assumed that the connectivity is lost, so the end devices must increase either SF or the transmission power to resolve this issue and establish a more reliable connection with the network. Sigfox  
Similar to LoRa (LoRaWAN), the Sigfox technology enables IoT long-range applications, with low-cost end devices exchanging small amounts of data with network applications, particularly in uplink [53]. Sigfox operates in the ISM spectrum, centered at 868 MHz in Europe and 902 MHz in the US. Sigfox signals are characterized by Ultra NarrowBand (UNB) spectrum occupation, that is, 100 Hz, DBPSK modulation and around 100 bps in uplink, and 1.5 kHz, GFSK modulation and around 600 bps in downlink [53].