# IoT: A Long-Reach Fully Passive Low-Rate Upstream PHY for IoT over Fiber
## 2 Materials and Methods
### IoToF Prototype Implementation
#### 2.5.4 Experimental Setup and System Characterization  
The implemented testbed, according to the IoToF architecture in Figure 2, comprised of a single AOM with remote excitation provided by a broadband source (ALS-CL-17-B-FA, Amonics). In the receiver stage, the edge filtering was performed by an in-line Fabry-Perot interferometer detailed in a previous work [12]. The CMD stage was emulated by an arbitrary waveform generator (AFG3021, Tektronix), which allows up to 1 GS/s arbitrary waveform with 14-bits DAC resolution, being the modulation signals generated by a script. Both the modulation CMD signal used to feed the speaker and the modulated signal measured at the receiver stage were monitored in real time at 5 kS/s and saved for posterior analysis. In order to validate the maximum distance between the AOM and the receiver, a variable optical attenuator was used to emulate the optical fiber loss (0.2 dB/km). Since the modulation bandwidth was reduced, the dispersion effect of the optical fiber can be neglected [37].  
To characterize the AOM wavelength shift imposed by the diaphragm excursion (Figure 3, \(M_{1}\) and \(M_{2}\) in back to back configuration), the backscattered FBG spectrum was monitored using a spectrometer (I-MON 512E-USB, Ibsen) with a sampling rate of 945 S/s. A sinusoidal signal with unitary amplitude, and a frequency sweep from 10 Hz to 1000 Hz, in steps of 10 Hz, was used. The maximum measurable frequency was \(\sim\)300 Hz (limited by the optical spectrometer). This parameter allowed us to optimize the edge-filter bandwidth and slope. The system transfer function response was analyzed, under the same conditions that the characterization stage described above, just with the edge-filter receiver (Figure 3, \(M_{2}\) in back to back configuration).  
The system temporal response (pulse response) study was carried out in order to analyze the diaphragm dumping response (Figure 3, \(M_{2}\) in back to back configuration). To obtain a smooth and continuous diaphragm movement, a filtered version of a square signal (Gaussian shape), applying a first order low-pass Butterworth filter with \(\sim\)500 Hz frequency cut-off, was used. The period of the injected signal was decreased from 100 ms to 10 ms, in steps of 10 ms, with a duty cycle of 10 %, which means a pulse width from 10 ms to 1 ms.  
Finally, in order to validate both the maximum bit rate and transmission distance achieved with the proposed approach (Figure 3, \(M_{3}\) input to output bits), M-ary FSK modulation was considered (2-, 4- and 8-FSK). To analyze the maximum bit rate, the selected values for \(f_{c}\) and \(\Delta f\) were both 100 Hz. The frequency separation value was selected in order to guarantee continuous phase modulation, with a minimum bit or symbol period of \(T=1/f_{c}\). Hence, the expected bit rates were 100 bps, 200 bps and 300 bps. This analysis was performed for different values of optical attenuation between the AOMs and the receiver stage. No acoustic isolation was provided to the prototype, which was subjected to typical background acoustic noise from a 60 m\({}^{2}\) optical lab with equipments running (\(\sim\)80 dB).