## 3 Wake-Up Radio Communication Protocols
### Review of the Existing MAC Protocols
It supports diverse data types of the M2M by making its superframe to have both contention-free and the contention-based schemes along with a priority. A separate channel called SC is dedicated for synchronization, which uses a CC2420 radio hardware switching capability. The network is initialized by the coordinator performing a passive scan of the SC for an SF. If the SF is received, the coordinator schedules its superframe after the SP duration from the time instance of the received SF. Otherwise, it starts its superframe immediately. Results have shown that the PSMAC protocol is fast, energy efficient, supporting the diverse QoS requirements of M2M with low latency. WUR based Transmitter Initiated Cycled Receiver WUR-TICER [105] combines the wake-up radio based on the TICER [36] protocol with the energy harvesting concept. Communication is initiated by a wake-up beacon (WUB) from the transmitter to the passive wake-up radio of the receiver. The WUB and data packets are both transmitted on the same channel using the main radio of the transmitter, which results in a low rate of packet reception. The protocol achieves a significant reduction in energy consumption and up to 82% improvement in throughput. A smart power unit coupled with a nanowatt wake-up radio receiver is developed in [106]. The hardware unit can be used for harvesting various energy sources, such as RF, wind, solar, etc. The ultralow power microcontroller in the system performs MPPT (maximum power point tracking) and optimized charging of the super capacitor at the highest efficiency. The power unit uses the serial interface (SPI) to communicate with the supplied node. DoRa [107] is a cross-layer protocol for passive wake-up radio architecture in single hop star networks. In this scheme, communication is always initiated from the base station by periodically sending a wake-up call to each node including its Mac address. The node replies by sending a data message to the BS. The BS activates transmission mode when it needs to send a wake-up call and reception mode is activated after immediately after sending the wake-up call. The scheme helps in eliminating the possibility of collision in the system. To confirm the wake-up call, the receiving node must authenticate the MAC address in the wake-up call. The signals received power must also be strong enough throughout the wake-up preamble, which is equivalent to a continuous voltage from the wake-up radio. Other symmetric multi-hop enabled protocols are proposed. A new multi-hop passive wake-up radio MAC protocol known as radio triggered sensor MAC (RTM) protocol is proposed in [108]. In this scheme, both the main and wake-up radio uses the same medium. The transmitter has first to find a free channel using the CSMA/CA protocol and transmits the wake-up signal. After waking up the receiver, the transmitter will then send an RTS and await CTS. It resends the RTS if it does not receive CTS after some certain time and will give up and return to sleep after five trials. The transmitter sends its data after receiving CTS and awaits ACK. It retransmits the data after a certain time if the ACK is not received, and then return to sleep after receiving the ACK. On the receiver side, it first harvests the energy from the received signal to wake-up the node and waits for the RTS failure of which it returns to sleep. It verifies if the RTS is destined to the node, if not it returns to sleep. It then sends a CTS and waits for data, after which it waits for ACK. It then goes to sleep after receiving the ACK; otherwise, it retransmits the data until the ACK is received. Thus, it sends the wake-up message to the nearby nodes and then selecting the right receiver by exchanging RTS/CTS wastes much energy and reduces the energy efficiency. There may also be increased latency due to the utilization of single channel by both the main and wake-up radio, which results in one having to wait for the other to finish using the channel before it can transmit.