## 3 Wake-Up Radio Communication Protocols
### Review of the Existing MAC Protocols  
Several strategies are employed to coordinate how broadcast channels are shared between the passive wake-up radio nodes in a network. These strategies aim to prevent collisions with minimal delay in accessing the channel as well as higher energy efficiency and fairness between the nodes. In this section, we identify all the strategies that have been applied in the passive wake-up radio networks. These collision avoidance strategies are categorized in Figure 8. We also recognize the MAC layer protocols available in the literature and categorize them based on the strategies employed. Table 2 gives a summary of the existing protocols. Figure 7: WuS frame structure. Dual radio architecture for multi-hop networks was proposed in [99] to minimize the effects of ultrawide band (UWB) acquisition time. An UWB-based radio with a physical layer that is based on a single band impulse radio was used as the main radio. The secondary radio is an RF-based passive wake-up radio, which provides control signaling using an out of band narrowband control channel. To reduce the signaling complexity and avoid a collision, the system uses the dynamic channel allocation concept. The simulation results have shown that the technique achieved improved network throughput and reduced latency. In [100,101], another MAC protocol for passive wake-up radio [73] is presented. The protocol is designed for single hop star network for wireless body area networks (WBAN), which handles the normal and emergency traffic types. When there is an emergency, the BAN Nodes (BN) becomes active and transmits a wake-up message to the BAN network controller (BNC). The BNC replies to the BN by sending an ACK and then a beacon for resource allocation purpose. The BN then transmits its data to the BNC and the session is finished when the BN received an ACK from the BNC. For the normal traffic, the BNC wakes up according to a TDMA based schedule and initiate the communication. It sends the wake-up signal to wake-up the BN. The BN replies by a negative ACK (NACK) if it has no data packets to send, otherwise it will send an ACK to the BNC. The latter then replies by a beacon which contains the channel, synchronization, priority, and slot information. The data is then exchanged and the communication ends with an ACK from the BNC. The method is able to achieve increased energy efficiency and reduced latency. Figure 8: Classification of the previous works on passive wake-up radio MAC protocols. Another protocol proposed in [102] called very low power MAC (VLPM) integrates the passive wake-up transmitter [81] and receiver [103]. The scheme is designed for single-hop WBAN with the wake-up transceivers mounted on both the BNC and BN nodes. The BNC sends Res-WUP (i.e., the wake-up packet together with the resource allocation and other information). The Res-WUP is received by the BN and activates the MCU to interpret the information to know whether it contains the nodes' address. The MCU goes back to sleep after the interpretation, and the main radio is only woken up if it is the intended receiver. The BN replies by sending the data, and then the BNC again replies by IMM-ACK. The BN only initiates a wake-up packet to the BNC in case of emergency. The BNC then replies a RES-ACK, and the BN subsequently begins to send its data packets. After this, the BNC sends an IMM-ACK. Passive synchronization-based energy-efficient MAC (PSMAC) [104] is another protocol, which works by broadcasting a synchronization frame to activate the radio triggered circuit. The synchronization frame is embedded within the network information from which the nodes scans to discover their neighbors. PSMAC supports the various M2M QoS requirements as well as mobility while efficiently utilizing the energy. The protocol also offers preemptive and fast slot allocation by placing the contention access period before the beacon period.