## 3 Wake-Up Radio Communication Protocols
### Review of the Existing MAC Protocols
The protocol avoids idle listening by employing node time-out and also avoids overhearing by making nodes go to sleep after they overhear an RTS or CTS packet that is not meant for them. Since both RFID impulse scheme [90] and IEEE 802.15.4 operate in the same ISM band, the IEEE 802.15.4 radio can be used as a general-purpose radio and RFID reader. Hence these technologies are designed to operate together as a nonbeacon passive wake-up MAC protocol in [109] for multi-hop networks to reduce idle listening in. For a node that intends to transmit data in this scheme, the sensor output triggers an interrupt to wake-up the microcontroller unit (MCU). The MCU wakes up the main radio and the main radio performs a random byte slot clear channel assessment (CCA). It transmits the data if it can find a free channel; otherwise, it will return to the idle mode and listen further. For the reception, the node is wakening up from deep sleep when its RFID tag is triggered which turns on the MCU. The MCU turns on the radio, which again listens to the channel in the random byte slot. It listens for a maximum of up to \(\sim\)17 ms in which if there is no packet, the MCU goes back to sleep. A similar multi-hop supporting MAC protocol for passive RFID based wake-up radio is the MH-REACH protocol [79]. The nodes functions either as a multi-hop node, which sends a wake-up signal to other node or as an edge node that only receives the signal without relaying to any other node. A mobile sink moves around the nodes and initiates the wake-up signal transmission, which wakes up the entire nodes within its range. The nodes that are activated will send their data to the mobile and otherwise, in the case of a multi-hop network, another wake-up signal is further transmitted to the nearby nodes. All nodes return and remain asleep after performing the task with their passive wake-up radios scan for an incoming signal. The waking up of all the nodes within the broadcasted wake-up signal increases the overhearing overhead which reduces the energy efficiency. SLAM [110] is another passive MAC protocol proposed, which reduce energy consumption due to listening by up to 30 to 129 times, while at the same time providing security in the network. The security is achieved by assigning some nodes as guard nodes, which supervise the network traffic and identify malicious activities by nodes. These guard nodes remain asleep and are activated by the passive wake-up radios when needed. A method for enhancing WSN performance and reducing its carbon footprint is proposed in [111]. Three schemes--wireless energy harvesting, wake-up radio, and error control coding (ECC)--are incorporated and used to maximize the WSN lifetime. The proposed scheme reduced the energy consumption and the carbon footprint in the nodes. In [112], the authors developed an RF-based out-band wake-up mechanism for aerial to ground data transmission. The RF is used as the wake-up channel. The receiver, AS933, has three wake-up channels, each connecting to an antenna, and any antenna that received the correct wake-up pattern may trigger the wake-up. An energy-efficient joint downlink and uplink passive wake-up solution for IoT devices over cellular networks was designed in [113]. In this solution, a deep sleep state similar to power saving mode (PSM) by 3 GPP is defined. However, the device under this technique will not leave the PSM upon expiry of the timer, but instead upon receiving an explicit RF wake-up signal from its serving BS. A passive wake-up receiver scheme called wireless-powered wake-up receiver (WPWRx) is also proposed in [114]. In the proposed scheme, battery consumption during the listening period is eliminated by transferring wake-up data and power for the WPWRx using time switching. Each WPWRx functions as either a wake-up signaling receiver or an energy receiver in time switching manner at any given time. The scheme can be utilized for a range of applications such as wireless sensor actuator networks, machine-to-machine communications, and the IoT.