# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 1 Introduction  
The Internet of Things (IoT) paradigm was introduced over two decades ago, and its deployment has been ongoing for almost one. In its most general definition, IoT is a network of devices, that is, the _things_, which gather and exchange data possibly over the Internet. The ultimate goal of IoT is to enhance existing services and applications or deliver new ones to users, with little to no human intervention [1, 2]. The extreme heterogeneity of application domains and involved devices has led to different requirements and expectations. Therefore, a large variety of wireless communication technologies has gradually emerged for enabling IoT, and is expected to connect up to 75 billion devices by 2025, with an economic impact of around $11.1 trillion per year [3, 4]. Considering the first important aspect in IoT systems, that is, the coverage area of the adopted technologies, a rough taxonomy in short-range vs. wide-range systems can be identified. Moreover, by taking into account energy efficiency and power consumption aspects, the two above categories identify so-called Low-Power Short-Area Networks (LPSAN) and Low-Power Wide-Area Networks (LPWAN) [5]. LPSAN technologies target cost/energy-efficient short coverage, aiming at wirelessly solving the so-called _\"last 100 m connectivity\"_, that is, the connection of local networks to the Internet through a more structured Wide-Area Network (WAN). The latter is usually accessed via dedicated and conveniently placed gateways [6]. A plethora of differentLPSAN standards and proprietary systems have been thus proposed over the years, mostly originated from networking schemes referred to as ad-hoc or Wireless Sensor Networks (WSNs). As said before, such schemes are gateway-terminated toward the Internet for IoT-specific purposes; moreover, the technologies that they are based on mainly work in the unlicensed spectrum. Besides extreme short-range Radio Frequency Identification (RFID) and Near Field Communications (NFC) systems [7; 8], further standards and systems are currently adopted as LPSANs, such as Zigbee, derived from the IEEE 802.15.4 standard suite and deployed by the Zigbee Alliance [9]; Bluetooth and its Low Energy extension (Bluetooth LE, or BLE), defined within the Bluetooth Special Interest Group (Bluetooth SIG) [10]; and Z-Wave, a proprietary solution developed by Zenys and managed by the Z-Wave Alliance [11]. In order to overcome heterogeneity and thus possible interoperability issues when moving ad-hoc deployments, for example, based on Zigbee, to IP-based networks, the IPv6 over Low-Power Wireless Personal Networks (6 LoWPAN) standard has been recently proposed, as a result of the activities carried out within a dedicated IETF Working Group (WG). 6 LoWPAN focuses on IoT delivery over IPv6, thus allowing a seamless integration with IP-based systems [12]. In particular, 6 LoWPAN has been the basis for another technology, named Thread, which is natively IP-addressable, different from the previous technologies, which require instead dedicated extensions [13]. LPWAN systems have emerged more recently than LPSANs, and play a key role in delivering specific IoT services, generally referred to as massive Machine Type Communications (mMTC) [14]. When compared to LPSANs, LPWANs target wide-area coverage, that is, up to several kilometers, in a low cost/power consumption manner [15]. Two main LPWAN groups can be identified, that is, proprietary systems working in the unlicensed spectrum and 3 GPP-standardized technologies, these latter exploiting licensed cellular spectrum and architecture. The first group is well-represented by the Long Range (LoRa) technology, a physical layer standard owned by Semtech and exploiting the LoRaWAN network protocol stack designed by the LoRa Alliance [16]. Moreover, it also includes Sigfox, developed by the French operator of the same name and globally provided by several associated partners [17], and the Weightless technology [18].