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Recently, ambient backscatter communications has been introduced as a cutting-edge technology which enables smart devices to communicate by utilizing ambient radio frequency (RF) signals without requiring active RF transmission. This technology is especially effective in addressing communication and energy efficiency problems for low-power communications systems such as sensor networks. It is expected to realize numerous Internet-of-Things (IoT) applications. Therefore, this paper aims to<span class="external-identifiers"> <a target="_blank" rel="external noopener noreferrer" href="https://doi.org/10.1109/comst.2018.2841964">doi:10.1109/comst.2018.2841964</a> <a target="_blank" rel="external noopener" href="https://fatcat.wiki/release/4ffe4wfdefgfzatqstpeag3igi">fatcat:4ffe4wfdefgfzatqstpeag3igi</a> </span>
more »... e a contemporary and comprehensive literature review on fundamentals, applications, challenges, and research efforts/progress of ambient backscatter communications. In particular, we first present fundamentals of backscatter communications and briefly review bistatic backscatter communications systems. Then, the general architecture, advantages, and solutions to address existing issues and limitations of ambient backscatter communications systems are discussed. Additionally, emerging applications of ambient backscatter communications are highlighted. Finally, we outline some open issues and future research directions. Index Terms-Ambient backscatter, wireless networks, bistatic backscatter, RFID, wireless energy harvesting, backscatter communications, and passive communications. Abbreviation Description Abbreviation Description MBCSs Monostatic backscatter communications systems UHF Ultra high frequency SHF Super high frequency UWB Ultra-wideband NRZ Non-return-to-zero OSTBC Orthogonal space-time block code ASK Amplitude shift keying FSK Frequency-shift keying PSK Phase shift keying BPSK Binary phase shift keying QPSK Quadrature phase shift keying FDMA Frequency division multiple access QAM Quadrature amplitude modulation OOK On-off-keying BBCSs Bistatic backscatter communications systems IoT Internet of Things MCU Micro-controller unit CFO Carrier frequency offset BER Bit-error-rate SNR Signal-to-noise ratio TDM Time-division multiplexing CMOS Complementary metal-oxide-semiconductor CDMA Code division multiple access ABCSs Ambient backscatter communications systems AP Access point D2D Device-to-device ML Maximum-likelihood OFDM Orthogonal frequency division multiplexing WPCNs Wireless powered communication networks CRN Cognitive radio network Additionally, using ambient signals from licensed sources, the communication protocols of ABCSs have to guarantee not to interfere the transmissions of the licensed users. Therefore, considerable research efforts have been reported to improve the ABCS in various aspects. This paper is the first to provide a comprehensive overview of the state-of-the-art research and technological developments on the architectures, protocols, and applications of emerging ABCSs. The key features and objectives of this paper are • To provide a fundamental background for general readers to understand basic concepts, operation methods and mechanisms, and applications of ABCSs, • To summarize advanced design techniques related to architectures, hardware designs, network protocols, standards, and solutions of the ABCSs, and • To discuss challenges, open issues, and potential future research directions. The rest of this paper is organized as follows. Section II provides fundamental knowledge about modulated backscatter communications including operation mechanism, antenna design, channel coding, and modulation schemes. Section III and IV describe general architectures of bistatic backscatter communication systems (BBCSs) and ABCSs, respectively. We also review many research works in the literature aiming to address various existing problems in ABCSs, e.g., network design, scheduling, power management, and multiple-access. Additionally, some potential applications are discussed in Section III and IV. Then, emerging backscatter communications systems are reviewed in Section V. Section VI discusses challenges and future directions of ABCSs. Finally, we summarize and conclude the paper in Section VII. The abbreviations used in this article are summarized in Table I . II. AMBIENT BACKSCATTER COMMUNICATIONS: AN OVERVIEW In this section, we first provide an overview of wireless energy harvesting networks. After that, Backscatter communications systems and fundamentals of modulated backscatter communications are reviewed. Then, key features in designing antennas for ABCSs are highlighted. Finally, typical modulation and channel coding techniques used in ABCSs are discussed. A. Energy Harvesting for Green Communications Networks Recently, energy harvesting (EH) technique has gained a lot of attention from both academia and industry due to its promising features for green communications networks, e.g., WSNs and IoT. The key principle of EH is that it allows wireless devices to harvest energy from RF signals to support their operations. There are three main schemes of EH including wireless power transfer (WPT), wireless-powered communication network (WPCN), and simultaneous wireless information and power transfer (SWIPT) as shown in Fig. 1 . • Wireless power transfer (WPT): In this scheme, the power transmitter simply transmits energy to the user devices without information. The energy is used to charge the devices' batteries. WPT has many practical applications such as home electronics, medical implants, electric vehicles, and wireless grid . • Wireless-powered communication network (WPCN): This scheme allows the user devices to harvest energy, and then use the energy to actively transmit data. In this context, wireless devices can be developed for future applications such as IoT or, more generally, Internet of Everything . • Simultaneous wireless information and power transfer (SWIPT): By using a hybrid design, in SWIPT, the power transmitter can transfer energy and information wirelessly to the user devices at the same time. The users then can choose to harvest energy or decode information sent from the power transmitter by simply switching between harvesting and decoding modules, thereby achieving a high energy-information transmission efficiency  . Although possessing many advantages, these energy harvesting schemes still have limitations when adopted in lowcost and low-power networks, e.g., WSNs and IoT. For example, in WPCNs, the users may require a long time to harvest enough RF energy to transmit data, thereby limiting the system performance. Resource scheduling and M2M communications
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