# Exploiting Concurrent Wake-Up Transmissions Using Beat Frequencies
## 2 Related Works
### Wireless Interference  
Unintentional interference is often called collision and occurs usually when two or more nodes start to send at the same time and a receiver node detects the signals from both transmitters. However, although the packets collide, it is possible in some cases to correctly receive the strongest signal, which is usually called the capture effect [9, 10].  
Intentionally-generated interference can lead to constructive interference that can be used to improve link quality or decrease the transmission energy required by a certain node. Usually, protocols that are based on constructive interference can be found in the area of flooding protocols, as they achieve very low latencies due to the concurrently-transmitted packets.  
[PERSON] et al. present in [11] Backcast, a link-layer protocol that relies on identical and concurrently-transmitted acknowledgments as answers to a so-called probe. A probe is sent by an initiator, and each node that receives this probe sends an identical acknowledgment exactly 192 us after receiving the probe. This means, of course, that acknowledgments from several responders might reach the initiator concurrently. According to [PERSON] et al., the observed robust detection probability of the acknowledgments cannot be explained only by the capture effect.  
Figure 1: Node-3 is out of the wake-up range due to an outage of Node-4.  
One of the first flooding protocols in IEEE 802.15.4-compliant wireless sensor networks that use constructive interference is Glossy [7]. IEEE 802.15.4 uses direct sequence spread spectrum modulation by grouping binary data into four-bit symbols that are mapped to one of 16 possible and nearly orthogonal 32-bit pseudo-noise sequences [12]. The length of one pseudo-noise sequence is 0.5 us. This also reflects the maximum temporal displacement of two simultaneously-transmitted messages allowed in Glossy to achieve constructive interference [7]. This demonstrates, and [PERSON] et al. [7] also conclude, that constructive interference is only possible because of the 802.15.4 modulation scheme and the redundancy generated by the pseudo-noise sequences.  
There exist several works that are based on or build upon and analyze the performance of Glossy, such as [13; 14; 15; 16; 17; 18; 19]. These works have all in common that they are based on radios that use the IEEE 802.15.4 standard, and constructive interference of radio packets can be achieved within the synchronization limit of 0.5 us.  
However, a synchronization of 0.5 us is still very demanding. In Glossy, these temporal constraints are met by synchronizing message transmissions based on signals from the radio to the micro-controller that indicate the reception of a complete packet and by carefully balancing the number of required clock cycles before a transmission begins.  
There do not exist many constructive interference-based protocols that are based on other modulation schemes than direct sequence spread spectrum. This is obviously due to the limitations posed by the timing constraints. Zippy, recently presented by [PERSON] et al. [8], is a flooding protocol that uses wake-up transceivers to synchronize neighboring nodes and to rapidly disseminate data. To ensure the timely waking of all wireless sensor nodes, Zippy transmits concurrent wake-up packets. As these packets are on-off keying-modulated and the wake-up receivers are of low complexity to ensure ultra-low-power consumption, destructive interference has to be avoided, and the capture effect cannot be used. Zippy solves destructive interference by using carrier frequency randomization, as will be further discussed in Section 2.3.