## 3 Wake-Up Radio Communication Protocols
### Analysis of Reviewed Mac Protocols  
The key function of the MAC layer is channel access coordination among nodes to prevent a collision. This is achieved by employing various collision avoidance and prevention mechanisms. However, problems arise in the channel allocation when the appropriate technique is not applied. The wake-up call may also be overheard by an unintended node. However, these problems are mostly concessions made by protocol designers to achieve improved values in other essential parameters such as reduced latency, energy consumption, etc. The CSMA/CA (carrier sense multiple access/collision avoidance) scheme has been used more in the passive wake-up radio MAC protocols in Table 2. In CSMA/CA, the carrier is sensed before transmitting. The CCA (clear channel assessment) scheme is also utilized in many other protocols for collision avoidance. The RTS/CTS (request to send/clear to send) handshake mechanism was used in [99] and together with CSMA/CA in [108]. This mechanism prevents collision arising from hidden node problems. TDMA scheme assigns one time slot of the whole cycle unit to each passive wake-up radio to handle the multiple access control in [111]. Each time slot should have a length at least longer than the required time for a successful wake-up packet transmission. Compared to TDMA, using CSMA/CA scheme as in [104] does not need the coordinator to reserve a time slot for a specific passive wake-up radio. CSMA/CA provides a more dynamic and efficient way to make use of the channel, especially when the slot length is much longer than the actual data transmission time. However, when the number of nodes increases, especially when too many nodes start CSMA/CA process simultaneously to compete for the medium, performance of the CSMA/CA reduces as TDMA prevents collisions more perfectly than CSMA. Different techniques with easier implementation are used in other protocols. For example, the design in [100,101] maintains a table for the wake-up schedule. The network coordinator node maintains the wakeup schedule in a table for every passive wake-up radio node in the network. Use of wake-up table by the coordinator node is easy to implement and saves a significant amount of power as all BNs in the network remain in the sleep state (i.e., switch off the main transceiver) until it is woken up by to transmit or receive data. In [107], a unique slot is set for the wake-up packet, while in [110] a unique frequency is set for the wake-up packet transmission. In [109], the CCA is used with a binary exponential back-off mechanism which schedules retransmission after a collision happens. The in-band channel, which sends the wake-up signals on the same channel with the main data channel is used in protocols [105,108]. This method reduces the complexity and implementation costs, but has lower resistance to jamming attacks to the communication stack and increases the probability of collision. Therefore, during the channel design for multiple sensor node applications, which requires high reliability and low latency such as in medical applications, the in-band channel should be avoided. The out-of-band, which is more popular and used in seven out of the eleven designs send the wake-up signal on a dedicated channel, separate from the main data channel. If separate channels are utilized, the cost increases; however, resistance to jamming attacks increases and energy consumption due to overhearing decreases. From an energy perspective, the out-of-band approach is more efficient. For instance, the out-of-band channel is more suitable for the proposed concept in [112] that emphasizes opportunistic aerial-ground sensing. However, in [99,110], multiple channels are utilized for communication of the wake-up signal. Multiple channels allow multiple nodes to transmit the wake-up signal through different channels simultaneously, which increases throughput and helps mitigate interference. The destination node ID is included in the wake-up signal of the protocols in the majority of the reviewed protocols.