In wireless environments, transmission and 1 reception costs dominate system power consumption, motivating 2 research effort on new technologies capable of reducing the 3 footprint of the radio, paving the way for the Internet of 4 Things. The most important challenge is to reduce power 5 consumption when receivers are idle, the so called idle-listening 6 cost. One approach proposes switching off the main receiver, 7 then introduces new wake-up circuitry capable of detecting 8 an incoming transmission, optionally discriminating the packet 9 destination using addressing, then switching on the main radio 10 only when required. This wake-up receiver technology represents 11 the ultimate frontier in low power radio communication. In 12 this paper, we present a comprehensive literature review of 13 the research progress in wake-up radio (WuR) hardware and 14 relevant networking software. First, we present an overview of 15 the WuR system architecture, including challenges to hardware 16 design and a comparison of solutions presented throughout the 17 last decade. Next, we present various medium access control and 18 routing protocols as well as diverse ways to exploit WuRs, both 19 as an extension of pre-existing systems and as a new concept to 20 manage low-power networking.

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Energy efficiency is crucial in the design of battery-powered end devices, such as smart sensors for the Internet of Things applications. Wireless communication between these distributed smart devices consumes significant energy, and even more when data need to reach several kilometers in distance. Low-power and long-range communication technologies such as LoRaWAN are becoming popular in IoT applications. However, LoRaWAN has drawbacks in terms of (i) data latency; (ii) limited control over the end devices by the gateway; and (iii) high rate of packet collisions in a dense network. To overcome these drawbacks, we present an energy-efficient network architecture and a high-efficiency on-demand time-division multiple access (TDMA) communication protocol for IoT improving both the energy efficiency and the latency of standard LoRa networks. We combine the capabilities of short-range wake-up radios to achieve ultra-low power states and asynchronous communication together with the long-range connectivity of LoRa. The proposed approach still works with the standard LoRa protocol, but improves performance with an on-demand TDMA. Thanks to the proposed network and protocol, we achieve a packet delivery ratio of 100% by eliminating the possibility of packet collisions. The network also achieves a round-trip latency on the order of milliseconds with sensing devices dissipating less than 46 mJ when active and 1.83 μW during periods of inactivity and can last up to three years on a 1200-mAh lithium polymer battery.

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