# Exploiting Concurrent Wake-Up Transmissions Using Beat Frequencies
## 5 Experimental Results
### Concurrent Wake-Up Flooding  
To demonstrate the feasibility of the concurrent wake-up approach utilizing more than two simultaneously-transmitting nodes in a more realistic environment, we implemented the wake-up flooding algorithm as introduced in Section 4.2. We performed an indoor test consisting of several wireless sensor nodes as depicted in Figure 23. The initiator (Node-1), started to broadcast a seed wake-up packet. All nodes (Nodes-2, -3, -4 and -5) that received this seed packet woke up and listened for the synchronization message transmitted directly after the seed packet by the initiator. Based on the timing of the synchronization packet, the nodes then transmitted concurrent wake-up packets on different center frequencies as introduced in Section 3.2. Figure 24 visualizes the timing of the wake-up flooding algorithm. As can be seen, the nodes wake up after around 20 ms and are ready to receive data.  
Table 1 lists the frequencies that were utilized by the nodes during the flooding experiment. The third row in Table 1 indicates the type of sensor node during the flooding test, and the last two rows report the amount of sent and received wake-up packets. Types could either be initiator, sender or receiver. The initiator initiated the flooding, and the sender nodes retransmitted the wake-up packets. The receiver nodes were placed in the building at different places where they had no direct link to the initiator, except for Node-5. At each node, we counted the number of successfully received wake-ups.  
Figure 22: Visualization of the states (send and receive) of Nodes-1, -2 and -3 over time during the described concurrent wake-up protocol. The data were taken with the help of a logic-analyzer. Here, we use the abbreviations LBT for listen-before-talk, WUC for a wake-up call, WACK for the wake-up acknowledgment, REQ for the concurrent transmission request, REQ ACK for the request acknowledgment, CWUC for a concurrent wake-up call and ACK for the data acknowledgment.  
Figure 23: Deployed wireless sensor network to analyze the wake-up flooding algorithm.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Node** & **Frequency in MHz** & **Type** & **Number of Wake-Ups Sent** & **Number of Wake-Ups Received** \\ \hline
1 & 868 & initiator & 500 & – \\
2 & 868.125 & sender & 500 & 500 \\
3 & 865 & sender & 500 & 500 \\
4 & 865.125 & sender & 499 & 499 \\
5 & – & receiver & – & 1000 \\
6 & 866.125 & sender & 500 & 500 \\
7 & 866 & sender & 483 & 483 \\
8 & – & receiver & – & 500 \\
9 & – & receiver & – & 499 \\
10 & – & receiver & – & 495 \\
11 & – & receiver & – & 500 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Frequencies used by the nodes during the flooding test setup.  
Figure 24: Timing of the wake-up flooding algorithm.  
Figure 23: Deployed wireless sensor network to analyze the wake-up flooding algorithm.  
As reported in Table 1, all nodes had a high wake-up packet reception ratio of almost 100%. As Node-5 could receive all seed wake-up calls, as well as the following concurrent wake-up messages, it woke up 1000 times.