# AOA-Based Three-Dimensional Multi-Target Localization in Industrial WSNs for LOS Conditions
## 6 Testbed Development and Field Experiments  
### Prototype System and Testbed Development  
The prototype system of the ATML scheme is implemented according to the MT-SIMO signal design and AOA ML estimation method in Section 4. The system consists of an Rx (to emulate an AN) and two Tx's (to emulate MSNs). The system is shown in the photos in Figure 10a,b.  
In each Tx, a baseband module (BBM) based on an field programmable gate array (FPGA) generates circularly the PN-sequence is the same as in the simulations in Section 5 and pre-stored in the FLASH memory. The chip rate is 20 Mcps, resulting in the chip duration of \(\Delta_{c}=50\) ns and signal bandwidth of 20 MHz. The duration of the spreading sequence of \(T_{u}=50\times 1023=51\),150 ns. The BBM also performs the waveform shaping and sampling (for digitalization) to produce the modulated zero intermediate frequency (ZIF) signal. The digital waveform samples are input into the radio frequency (RF) front-end model (FEM). The FEM first performs the frequency-up-conversion to shift the ZIF signal to the carrier of 3.5 GHz, and then drives the RF signal by an amplifier with the output power of 10 mW. Finally, the signal is transmitted by an omnidirectional dipole antenna with the maximum antenna gain of 4 dBi. The modules are controlled by an onboard microcontroller unit (MCU). All the devices are integrated circuits and suitable for practical sensor nodes due to the miniaturization.  
Figure 9: Network deployment and CDFs of the localization errors of different methods.  
The Rx is equipped with an \(8\times 4\) rectangular array of antenna pairs where each pair contains two \(\pm 45^{\circ}\) polarized dipole antennas. The spacings between the adjacent antenna rows or columns is half a wavelength. The 64 antenna elements are connected to a microwave switch with 64 input and eight output ports. Eight antenna elements in the same polarization in one column of the array are selected by the switch and the signals received on them are output. Then, the signals pass through low noise amplifiers (LNAs) and bandpass filters, and finally are input into two vector signal transceivers (VSTs) where they are sampled at 60 MHz and stored for offline processing. The microwave switch connects eight antennas for signal capture for 320 \(\mu\)s and then switches to another column of eight antennas. Since there are a total of eight columns in both polarizations, the Rx takes 2.56 ms for all the 64 antenna elements to capture the signals. Although the antennas receive signals in a TDM manner, the duration is so short that the signals on all the antennas are captured almost simultaneously.  
We have measured the 3D radiation pattern of each dipole in an anechoic chamber for \(\phi\in[0^{\circ},360^{\circ})\) and \(\theta\in[0^{\circ},180^{\circ}]\) with the step of \(1^{\circ}\) in the azimuth and elevation domains. For example, the pattern diagrams, \(s_{i,m}(\phi,\theta)\), of the dipoles in \(+45^{\circ}\) and \(-45^{\circ}\) polarizations at the position of \((1,1)\) in the array are plotted in Figure 11. We put the whole antenna array including the back support structure into the chamber. Hence, the mutual coupling and impact among the antenna elements have been included in the measured radiation patterns. Then, the steering vectors are directly utilized in the AOA ML estimation as described in Section 4. Since the resolution of the measured pattern diagrams is \(1^{\circ}\), the grain of the angle estimation error in one measurement is \(1^{\circ}\). Thus, the error in one measurement is \(0^{\circ},\pm 1^{\circ},\pm 2^{\circ},\cdots\). However, we can perform multiple measurement trails for one angle and take the average of multiple results. Thus, the error of final estimate may be a decimal fraction.  
Figure 10: Testbed of the ATML scheme and field experiment setup.