# Smart Gateway for Healthcare Networks Based on Beam Steering Technology
## 4 AOA Estimation Using a Fading Emulator  
The smart gateway for healthcare networks is designed for use in indoor environments, such as a house or hospital. In such an environment, various objects in the surrounding area, such as walls, floors, ceilings, and furniture, result in a complex radio propagation environment. Furthermore, if a sensor device mounted on a person exists in the same room as the gateway, a direct wave would arrive to the gateway, which is called the Rice channel propagation environment. To verify the accuracy of the AOA estimation of the proposed antenna in a multiple-wave propagation environment, OTA testing using a two-dimensional fading emulator was executed. A spatial fading emulator is implemented using the Monte Carlo simulation [31]. In the fading emulator, the radio waves radiated from the scatterers, which are the controlled amplitude and phases of the signal used to emulate a Rayleigh fading channel, are summed around the evaluating antennas, and the desired radio environment is then generated [37; 38]. Furthermore, when the signal radiated from the extra antenna is controlled according to the features of the direct wave, such as power and direction, a Rice propagation environment can be represented [39]. Figure 20 draws the measurement setup of the two-dimensional fading emulator to actualize the Rice propagation environment that can create the desired incoming wave direction and K-factor. To reproduce a Rice propagation environment in the fading emulator, 14 half-wavelength sleeve dipole antennas are arranged at equal angular intervals on a circle with a radius of 1.2 m to radiate scattered waves, while the remaining one half-wavelength sleeve dipole antenna emits a direct wave. Then, the desired K-factor, which represents actual indoor propagation environments, is achieved by controlling the output power of the direct wave or scattered waves using the method described below. Figure 19: Radiation gain with respect to the angle of the incoming wave. Firstly, a multiple-wave radio propagation environment with only scattered waves is implemented, and the average received power \(P_{r}\) of a dipole antenna placed in the center of the fading emulator is measured. Next, the received power \(P_{d}\) of a dipole antenna is measured when only a direct wave is emitted. The difference between the two received powers (\(P_{d}\) and \(P_{r}\)) and the K-factor are compared. The attenuators (Attenuator1 and Attenuator2 shown in Figure 20) are set using the following equation:  
\[ \begin{array}{l}\text{If }P_{d}[\text{dBm}]-P_{r}[\text{dBm}]>K[\text{dB}]\\ \text{Attenuator1}=(P_{d}-P_{r})-K[\text{dB}]\\ \text{Attenuator2}=0\\ \text{If }P_{d}[\text{dBm}]-P_{r}[\text{dBm}]<K[\text{dB}]\\ \text{Attenuator1}=0\\ \text{Attenuator2}=K-(P_{d}-P_{r})[\text{dB}]\end{array}  \tag{14}\]  
Here, Attenuator1 is connected to a sleeve dipole antenna that emulates a direct wave, and Attenuator2 is plugged in the multi-way power divider, which generates multiple scattered waves. To assess estimation accuracy, the proposed antenna is set to the center of the fading emulator, and the received signals of \(E_{\Delta}\) and \(E_{\Omega}\) are measured using a vector network analyzer. Figure 21 shows the phase of each signal by applying the MIQ method as a function of the angle of the incoming wave with the K-factor as a parameter through the fading emulator. The black lines, red lines, and blue lines show the case of \(K=10\) dB, \(0\) dB, and \(-10\) dB, respectively. The solid and broken lines indicate \(E_{\Delta}\) and \(E_{\Omega}\), respectively. As shown in Figure 21, there is no phase variation regardless of the K-factor because the effect of the scattered waves on the received signal is canceled by the MIQ method, as expressed in Figure 8. \(\angle E_{\Omega}\) used as the reference angle is constant regardless of the angle of the incoming wave, and \(\angle E_{\Delta}\) varies proportionally, depending on the angle of the incoming wave.