# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 2 IoT Technologies
### Low-Power Short-Area Networks (LPSANs)
#### 2.1.1 RFID and NFC  
Taking roots from bar-coding mechanisms, the RFID technology allows to send and receive small amount of data, for example, Identification (ID) signals from a tag to a reader, over very short distances, not requiring in principle Line-of-Sight (LoS) propagation. RFID systems operate at Low Frequency (LF), within the 125-134 kHz and 140-148.5 kHz ranges, High Frequency (HF), at 13.56 MHz, and Ultra-High Frequency (UHF), at 868 MHz and 915 MHz, in Europe and US, respectively. Microwave RFID can also operate at 2.4 GHz and higher frequencies [29].  
RFID tags can be grouped into three categories [29]:  
* _Active tags_, which are embedded with an on-board battery, and periodically transmit an ID signal;
* _Battery-assisted tags_, which are equipped with an on-board battery, same as active tags, where the transmission is triggered only when the tags are in the coverage range of a reader;
* _Passive tags_, which are not equipped with an on-board battery, and use the energy transmitted by the reader as communication enabler. Passive tags need to be illuminated with a power level almost a thousand times stronger than the power that is needed for signal transmission in the case of active tags.  
Based on the above categories, three main transmission mechanisms can be identified [29]:  
* Active Reader Passive Tag (ARPT), where the tag does not emit radio signals, and the information is exchanged using the energy emitted by the reader. This solution is cost-effective but limits the system coverage to a few meters in terms of tag-reader distance;
* Passive Reader Active Tag (PRAT), where the tag periodically emits the signals toward the reader(s) falling in its coverage area;
* Active Reader Active Tag (ARAT), where a bidirectional data exchange is established between tags and readers, for example, for transmitting information with acknowledgment.  
In order to extend RFID functionalities, such systems have been more recently embedded with advanced communication and networking protocols, leading to so-called NFC technologies [30]. In the most common NFC setup, one of the two paired devices is also connected to the Internet, so that the other device can access and exchange data toward Internet-based services. NFC links always include a so-called initiator and a set of target devices, equipped with both passive and active communication modes [8].  
NFC can operate to a distance between devices up to tens of centimeters within the unlicensed industrial, scientific and medical (ISM) band, and particularly in the HF spectrum around 13.56 MHz, offering data transmission rates of about 106, 212, 424 and 848 kbps, depending on the adopted combination of modulation and coding techniques. With regards to the modulation, amplitude On/Off Keying (OOK) and Binary Phase Shift Keying (BPSK) schemes are mostly adopted; moreover, Amplitude Shift Keying (ASK), and Non-Return-to-Zero Level (NRZ-L) are also used, along with Manchester and Miller code [31].  
NFC devices can operate in three modes:  
* Card Emulation: In this mode, the NFC-enabled device acts like a smart card, allowing the users to perform data transactions;
* Reader/Writer: The devices read the information stored on the tags, that can be embedded on items of different nature (e.g., labels and posters). Depending on the usage, a device can also write information on tags;* Peer-to-Peer: A pair of NFC devices exchange information in both directions.