# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 3 Hybrid Acoustic-RF Approaches for Positioning Energy-Neutral Devices
### Signaling  
To obtain accurate distance measurements with low-energy consumption in the mobile device requires a well-considered design of system signaling. To this end, two classes of ToF-based hybrid RF-acoustic ranging strategies are discussed: (I) schemes performing an early wake-up; and (II) schemes realizing a late wake-up.  
Two-class I, i.e., early wake-up signaling schemes are presented in Figure 3. The distance measurements are initiated by the anchor nodes. In the first case, the anchor node sends out an RF signal to indicate the start of acoustic transmission, and the mobile device times the propagation delay. In the second case, the anchor node transmits an acoustic signal, waits for a confirmation of reception from the mobile device, and measures the ToF itself. Either way, the mobile device does not know when to expect the acoustic wave from the anchor node in time. If a maximum distance of 10 m is assumed, the mobile device may need to wait actively for a full period of 29.15 ms. From an energy consumption perspective, these scenarios are not optimal, however, they can be easily deployed in situations where there is not prior knowledge of the expected range or when the distances can significantly differ.  
Rather than actively timing the propagation of an acoustic signal, class II, i.e., late wake-up signaling schemes have been introduced to improve energy efficiency. Such an energy-efficient ranging scheme was proposed in [42; 43] and is shown in Figure 4. At a time \(T_{0}\), an ultrasonic chirp of duration \(\tau_{TX}\) is transmitted by an anchor node. Mobile devices M1 and M2 remain in a sleep mode to limit their energy consumption. At time \(T_{0}+\tau_{TX}\), the end of the acoustic transmission, the anchor node broadcasts a signal through RF, waking up the mobile devices quasi-immediately. Once active, both mobile devices capture a part of the propagating chirp signal for a time interval \(\tau_{RX}\). Depending on the distance to the anchor node, a different frequency interval of the ultrasonic chirp signal will be picked up. Each mobile device sends the sampled microphone signal back to the anchor node and goes back to sleep mode afterwards. The distance information is contained in the part of the captured chirp signal, which can be obtained through correlation with the transmitted chirp signal. Using this late wake-up signaling scheme, a mobile device no longer has to wait actively for the acoustic signal to arrive. A fixed wake-up interval \(\tau_{RX}\) of 1 ms is employed in [42], which contrasts sharply with the active time in aforementioned early wake-up signaling schemes. One critical side note, however, is that the maximum ranging distance is limited to the duration of the chirp signal \(\tau_{TX}\), however, increasing \(\tau_{TX}\) negatively affects the signal correlation due to self-interference.  
Figure 4: The hybrid acoustic-RF positioning system realizing a late wake-up of the mobile nodes. Depending on the distance to the anchor node (A), each mobile device (M1/M2) captures a different part of the emitted acoustic chirp signal at RF wake-up. In this case, M2 is located at a larger distance from the anchor node than M1. Both mobile devices only sample the acoustic signal during a short time period \(\tau_{RX}\), minimizing the energy consumption.  
Figure 3: Two hybrid RF-acoustic ranging strategies considering acoustic transmission by the anchor node(s). The ToF is either measured by (a) the mobile device M or (b) the anchor node A. In each case, the mobile device needs to actively wait for the arrival of the acoustic signal, which entails poor energy efficiency.