###### Abstract
From the five challenges defined, energy and communication are the ones with a higher level of interest from an electrical engineering perspective. Addressing energy concerns can be done by exploring new sources for Energy Harvesting (EH), improving the efficiency of existing sources, putting together multiple sources, advancing in Wireless Power Transfer (WPT), developing charging and storage elements, or simply by ensuring the lowest energy consumption possible. By itself, each one of these elements can contribute to improve existing applications or to enable new ones. However, the final goal is to integrate all of them to significantly enhance IoT energy systems [8, 9]. Regarding communication systems, there are multiple alternatives such as Wireless Local Area Network (WLAN), Bluetooth, Low Power Wide Area Network (LPWAN), as well as mobile networks systems more often used in commercial applications such as Wireless Highway Addressable Remote Transducer Protocol (WirelessHART) and International Society of Automation 100.11a (ISA100.11a), which are also used in industrial applications and use active radios [10, 11]. In contrast, others like backscatter communication systems do not depend on the use of active radios. Backscatter  
Figure 1: IoT technological challenges. communications are nowadays mainly used in Radio Frequency Identification (RFID) systems. However, the last years have brought many technological developments that can allow an application with this type of communication (wave reflection) on a broader range of devices and applications. Some areas of scientific interest are related to high order modulations, chipless devices, commercial protocols using backscatter (e.g., Bluetooth, LoRa), and digitally controlled modulations [12; 13; 14]. Grouping energy and communication characteristics allow the identification of three distinct IoT devices: passive, semi-passive, and active. This nomenclature, usually associated with RFID tags, describes the devices according to their need for batteries and radios. Equipment with no battery or radio are considered passive, the ones with battery but without radio are semi-passive, and the so-called active have both elements [15; 16]. The presence or absence of these modules interferes with the device's general characteristics, like communication range, processing capabilities, lifetime, or price. Figure 2 presents a comparison between the established types of IoT devices. This work synthesizes useful information to provide a complete understanding of the energy and communication challenges for IoT. It scrutinizes some aspects of different techniques such as backscatter communication, WPT, Simultaneous Wireless Information and Power Transfer (SWIPT), chipless devices, and EH, that have been developed to overcome the mentioned IoT challenges. The manuscript is organized as follows: Section 2 explores the technological developments that have been made in passive systems--backscatter communication, WPT, EH and chipless techniques; Section 3 points techniques used in semi-passive solutions--SWIPT and Wake-Up Radio (WUR); and Section 4 resumes some alternatives for active systems. Section 5 presents a discussion about the real application of passive, semi-passive, and active systems. The document ends with conclusions in Section 6.