# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 3 Hybrid Acoustic-RF Approaches for Positioning Energy-Neutral Devices
### RF Backscattering
#### 3.3.2 Backscatter Modulation  
In the system description described in Section 3.2, the acoustic signals contain the necessary data to perform the ranging measurements. RF backscattering exploits the instantaneous character of electromagnetic waves and can directly transmit the received information embedded in the acoustic signals to an anchor node for further processing. The frequency of the received acoustic signals should be modulated on an RF signal.  
The load switching of a single-tone incident wave can be seen as the most simple form of ASK modulation, i.e., OOK. It is easy to implement and therefore inexpensive. However, this modulation method has a major drawback, namely the fact that it is highly susceptible to noise. In the time domain, an RF source sends out the carrier wave \(S(f_{c},t)\), a single tone sine wave with a frequency of \(f_{c}\) and the backscatter device changes the RCS at a frequency of \(\Delta f_{a}\). This latter represents the received acoustic chirp in the time window \(\tau_{rx}\). \(A_{1}\) and \(A_{2}\) represent the path loss of the electromagnetic waves. In the bistatic setup, a separate antenna receives the sum of the carrier wave and the backscattered OOK demodulated signal:  
\[R(t)=A_{1}\,S(f_{c},t)+A_{2}\,\sigma(\Delta f_{a},t)\cdot A_{1}\,S(f_{c},t) \tag{6}\]  
As backscattering is a mixing process, the OOK backscattered signals appear on the positive and negative sides of the single-tone carrier wave in the spectrum. In other words, the reflective and incoming wave spectra overlap. These reflected waves suffer from self-interference as \(\Delta f_{a}\) is relatively small, and without proper cancellation, cannot be distinguished from the carrier wave due to the limited resolution of the receiver. With the increasing distance, these signals ultimately become too weak to be separated from the noise floor.  
Figure 5: Simplified visualization of the radar cross-section (RCS).  
A solution for the self-interference problem is achieved through frequency translation backscatter [52], where the mixing capabilities of the backscatter principle are used to its advantage. This requires an extra component on the backscatter device, a local oscillator generating a sine wave at a frequency \(f_{clo}\). In the frequency domain, this moves the signal away from the direct carrier with an offset of \(f_{c}\pm f_{clo}\) at both sides. On these two mirror frequencies, the received acoustic chirp signal again appears as a double sideband-modulated signal. In total, four mirror frequencies of this acoustic signal can be seen in Figure 6. Assuming the RCS variation is defined by a sine wave, the second term of Equation (6) can be rewritten with the product to sum identities as:  
\[\begin{split}\sin(f_{a}\,t)\cdot\sin(f_{clo}\,t)\cdot\sin(f_{c} \,t)=&\frac{1}{4}\Big{(}\sin\big{(}(f_{c}-\Delta f_{a}+f_{clo})\,t \big{)}+\sin\big{(}(f_{c}+\Delta f_{a}-f_{clo})\,t\big{)}\Big{)}\\ &-\frac{1}{4}\Big{(}\sin\big{(}(f_{c}-\Delta f_{a}-f_{clo})\,t \big{)}+\sin\big{(}(f_{c}+\Delta f_{a}+f_{clo})\,t\big{)}\Big{)}\end{split} \tag{7}\]  
The added local oscillator contributes to a higher-power consumption, however, it is an excellent compromise for the gained reading distance. Additionally, as this proposed method does not require any complex self-interference cancellation mechanism on a hardware or software level, a simple radio can be used to demodulate the backscatter signal. A major downside of this method occurs when extending the approach to a multiple-access system. By adapting the local oscillator frequency, FDMA could be adapted to localize multiple nodes at the same time. However, due to the double sideband modulation, the maximum amount of simultaneous localized nodes defined by both the acoustic chirp and RF bandwidth is halved. Complex single sideband mixers, such as in [53], with two separate loads, could cancel out one band but at the same time consume more energy on the mobile node.