## 3 Wake-Up Radio Communication Protocols
### Review of the Existing MAC Protocols
Table 2 provides a comparison and summary of the MAC protocols that are utilized for passive wake-up radios. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Network** & **Implementation** & **Ref.** & **Key Innovation** & **Downside** & **Channels** & **Wake-Up Signal** & **Collision Avoidance** \\ \hline \multirow{8}{*}{Single Hop} & \multirow{8}{*}{Simulation} & [9] & Dynamic channel allocation \& & & & & \\  & & narrowband channel for RSTCTS & \multirow{2}{*}{Channel allocation overhead} & Multiple & & & & \\  & & & narrowband channel for RSTCTS & \multirow{2}{*}{Channel allocation overhead} & \multirow{2}{*}{Multiple} & & & & \\  & & & & & & & & \\ \cline{3-8}  & & [100,101] & TDMA scheme for multiple access & - & Out-of-band & Beacon with ID \& & \\  & & and resource allocation & & & & & \\ \cline{3-8}  & & [102] & Introduced the piggyback-oppsach & - & Out-of-band & Beacon with ID & Unique slot allocation, unique frequency for sending a wake-up packet. \\ \cline{3-8}  & & [103] & Wake-up radio that supports mobility & - & Out-of-Band & Beacon with ID & CSMACA \\  & & & & & & \\ \cline{3-8}  & & [104] & Combining Walk with energy & More time needed to harvest & \multirow{2}{*}{In-band} & Beacon with ID \& & \\  & & harnessed through energy causing delay & & & & & \\ \cline{3-8}  & & [105] & Combining Walk with energy & More time needed to harvest & \multirow{2}{*}{Out-of-Band} & & & \\  & & harvesting & & & & & \\ \cline{3-8}  & & [107] & Combining Walk with energy & & & & & \\ \cline{3-8}  & & harvesting & & & & & \\ \cline{3-8}  & & [113] & Joint downlink/update RF wake-up & \multirow{2}{*}{In-band} & & & & \\  & & (receive the Wwa in ID, and transmit data in UL) & & Increased take wake-up rate & \multirow{2}{*}{Out-of-Band} & & & \\  & & & & & & & \\ \cline{3-8}  & & [114] & Transfer of wake-up data and power & Repeated preamble needs to & \multirow{2}{*}{Out-of-Band} & & & \\  & & for wireless-powered wake-up & & be received before & \multirow{2}{*}{TDMA} & & \\  & & receiver WPRN by utilizing time & determining the exact WPo & & & & \\  & & switching & & & & & \\ \hline \multirow{2}{*}{Test-bed} & \multirow{2}{*}{[112]} & Nodes utilite three channels receiver & Delay due to time for MCU to & \multirow{2}{*}{Out-of-Band} & & & \\  & & which generates a wake-up signal & & wake-up from sleep. & & & \\  & & upon detection of a data signal. & & & & & \\ \hline \multirow{8}{*}{Multi Hop} & \multirow{2}{*}{Simulation} & [108] & RSTCTS mechanism used to check & Uses the same channel for & \multirow{2}{*}{In-band} & & \\  & & the problem of hidden terminal & & wake-up preamble and signal & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & Nodes only wake-up for each packet & Energy waste due to long idle & \multirow{2}{*}{Out-of-Band} & & & \\  & and Eng back to sleep & & Inserting a channel & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & \\ \hline \multirow{2}{*}{Multi Hop} & \multirow{2}{*}{Simulation} & [110] & Employs redistribution pairwise key & Delay due to the warm-up & \multirow{2}{*}{Multiple} & & \\  & & management protocol for security & time required to wake-up & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & \\ \hline \multirow{2}{*}{Simulation} & \multirow{2}{*}{[111]} & Readmental residue number systems & Delay due to CC overhead & \multirow{2}{*}{-} & & \\  & & (RRS)S ICC is used to improve & in ending and decoding of & - & - & TDMA \\ \cline{3-8}  & & reliability and reduce retransmission. & & & & \\ \hline \multirow{2}{*}{Test-bed} & \multirow{2}{*}{[9]} & Instended mobile sink, which moves around to wake-up other nodes, & All nodes within the wake-up & \multirow{2}{*}{Out-of-Band} & & \\  & & & & & & \\ \cline{3-8}  & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Key features and characteristics of the MAC protocols for the wake-up radio.