# Augmented MIMO: Body-Mounted Antennas for Tiny Wearable Devices
## 3 Augmented MIMO Hardware Prototype  
### Two-Element MIMO Antenna Array in Watch Form Factor  
A two-element antenna array was fabricated in a watch form factor for a well-controlled experiment, instead of using commercially available smartwatches as a black box. The array consisted of two chip antennas, TSA5N18D2G45 NV001T ([PERSON],  
Figure 1: (**a**) A MIMO link between an AP and a smartwatch. Two small chip antennas (ANTs) are embedded in the smartwatch with a spacing significantly smaller than the wavelength. (**b**) Augmented MIMO link. Chip antennas in the smartwatch are proximity-coupled to the other two antennas attached around the cuff of the clothing. The two cuff antennas are individually connected to two shoulder-mounted flexible antennas with coaxial cables. The shoulder antennas are nearly half a wavelength in size and more than a wavelength apart.  
Tokyo, Japan), and 8.2 nH series inductors for matching. These components were soldered on an FR-4 printed circuit board (PCB), as shown in Figure 2a. The antenna locations were determined as to be best positioned diagonally to maximize the antenna spacing within the watch-size PCB. The PCB pattern artwork and matching inductor were determined according to the recommendations in the antenna datasheet [14]. The antenna efficiency and maximum gain are 74% and 2.8 dBi, respectively, according to the datasheet. The onboard miniature coaxial connectors, U.FL receptacles (Hirose, Kanagawa, Japan) [15], allow the antenna array to be connected to a vector network analyzer (VNA) to measure scattering parameters (\(S\) parameters) and to an external WLAN transceiver for measuring throughput.  
The measured reflection at port 1, \(S_{11}\), and transmittance from port 1 to 2, \(S_{21}\), are shown in Figure 2b. The isolation between the two coaxial ports was 14 dB, i.e., the transmittance \(|S_{21}|\) from port 1 to 2 was \(-\)14 dB. An antenna spacing of 20 mm, less than half the wavelength, results in a low isolation.  
From the measured \(S\) parameters, we can calculate the envelope correlation coefficient (ECC), a crucial parameter to evaluate the MIMO diversity performance of the antennas [16]:  
\[\text{ECC}=\frac{S_{11}S_{12}^{*}+S_{21}S_{22}^{*}}{\sqrt{(1-|S_{11}|^{2}-|S_{ 21}|^{2})(1-|S_{22}|^{2}-|S_{12}|^{2})\eta_{1}\eta_{2}}}, \tag{2}\]  
where \({}^{*}\) represents the complex conjugate, \(\eta_{1}\) and \(\eta_{2}\) are the efficiencies of the two antennas, and zero correlation of antenna losses was assumed. From the measured \(S\) parameters and \(\eta_{1}=\eta_{2}=74\%\)[14], ECC was calculated as 0.21 at 2.45 GHz. The calculated ECC is acceptably low but is an order of magnitude, or more, higher than that of antennas designed using state-of-the-art antenna decoupling techniques [4].  
Figure 2: **(a)** The two-element chip antenna array PCB. The background is 1 mm grid paper. **(b)** The ensured \(S\) parameters. Around the 2.4 GHz band, the reflection \(|S_{11}|\) decreases to about \(-\)7 dB. Due to mutual coupling between the two antennas, the isolation between ports 1 and 2 is degraded to \(|S_{21}|\approx-\)14 dB.  
The antenna array PCB was fixed onto a watch face after the removal of the watch's internal components, as shown in Figure 3. For the ease of the experiment, the PCB was attached outside the watch face, not inside it.