# Exploiting Concurrent Wake-Up Transmissions Using Beat Frequencies
## 2 Related Works
### Zippy  
Zippy, introduced by [PERSON] et al. [8], is a flooding protocol that uses wireless sensor nodes with wake-up receivers without address correlation, as introduced in Section 2.2. The wake-up receiver tests the frequency of the demodulated signal, and in case it matches 125 kHz, it interrupts the main controller from deep sleep. As illustrated in Figure 4, the node then transmits a wake-up call, here called the preamble, by itself. Figure 4 schematically shows a two-hop Zippy network consisting of the initiator Node A and participating Nodes B and C [8]. As Node A continues to send its wake-up packet after Nodes B and C are awake, all three nodes transmit part of their wake-up packets concurrently to generate a one-hop neighbor time synchronization in the order of milliseconds [8].  
After sending the preamble, all nodes go to sleep state for a certain period of time that depends on the number of hops in a multi-hop network. Then, the initiator Node A wakes up again to transmit a synchronization bit (SYNC) as visualized in Figure 4. Node A transmits the SYNC bit in the same matter as the preamble, by using the on-off keying-modulated carrier signal. This SYNC bit then is received by the wake-up receiver of Node B. As the internal automatic gain control unit of Node B's wake-up receiver has already settled during preamble reception, the detection of the SYNC bit is achieved in the order of microseconds [8]. After receiving the SYNC bit of Node A, Node B itself starts to transmit a SYNC bit. This sequence then continues until no further nodes participate. Using this method, Zippy achieves a mean per-hop synchronization of 34 us in a two-hop network [8].  
After achieving neighborhood synchronization, Node A starts to transmit data using the same on-off keying modulation scheme as before. To ensure fast relaying of the data, the data are transmitted bit-wise and not byte-wise. After Node B receives a bit, it immediately forwards it to Node C.  
Figure 3: Schematic of a low-frequency wake-up message on-off keying modulated on the high-frequency carrier signal.  
To mitigate destructive interference during concurrent transmissions, Zippy uses carrier frequency randomization [8]. Figure 5 illustrates an exemplary on-off keying-modulated message that implements carrier frequency randomization. Each low-frequency one-bit consists of several high-frequency periods, which are transmitted at \(q\) different frequencies. In the example illustrated in Figure 5, each low-frequency bit consists of \(q=4\) frequencies \(f\) that can be calculated by \(f=f_{c}+\beta\Delta\), with \(f_{c}\) being the center frequency of 446.8 MHz, \(\Delta=135.41\) kHz resembling a minimum offset between two frequencies and \(\beta>=2\) being a uniformly-distributed random variable. Using carrier frequency randomization, [PERSON] et al. experimentally verified a packet reception rate of almost 100% by using six concurrent senders. Using an indoor testbed, Zippy achieves an end-to-end latency between 17.8 ms and 24.4 ms using eight-bit packets and between 29.8 ms and 41.6 ms for 16-bit packets. Here, end-to-end latency is the time elapsed between the start of flooding by the initiator and the end of flooding at each participant [8]. An unsolved challenge of Zippy remains the existence of false positive wake-ups, as Zippy wake-up receivers cannot use address correlation.