# Augmented MIMO: Body-Mounted Antennas for Tiny Wearable Devices
## 3 Augmented MIMO Hardware Prototype
### Augmented MIMO Clothing  
A prototype of Augmented MIMO clothing was created by attaching an antenna array PCB to the cuff near the watch and two shoulder antennas, as shown in Figure 4. The shoulder antennas are a commercially available flexible printed circuit (FPC) antenna, 1461530050 (Molex, Lisle, IL, USA). According to the antenna datasheet, the antenna efficiency and maximum gain are 78% and 3.2 dBi, respectively [17]. From the measured \(S\) parameters between the two shoulder antennas, whose frequency characteristics plot is omitted here, the ECC was calculated to be less than 0.01 at 2.45 GHz. The 30 cm antenna spacing, which is significantly larger than the wavelength, results in a low ECC. Therefore, it is expected that MIMO performance can be significantly improved by using the shoulder antennas. Note that the ECC calculated here is based on the \(S\) parameters measured only between the shoulder antennas, not including the proximity coupling paths of the watch and cuff antennas. For the ease of conducting the experiment, the PCB identical to the watch PCB shown in Figure 2a was also used as the cuff PCB. The cuff antennas and shoulder antennas are connected with RG-174 flexible coaxial cables. The preliminarily measured insertion loss of the cable was 2.0 dB. The total weight increase of the clothing with antennas and cables was about 40 g.  
Figure 4: (**a**) An antenna array PCB identical to the watch PCB is attached to the cuff of the clothing. A close-up view of the stacked pair of the cuff antenna PCB and the watch antenna PCB is shown in Figure 5a. (**b**) Two FPC antennas are attached to the shoulders. The cuff antennas and shoulder antennas are connected with flexible coaxial cables. (**c**) A close-up photograph of the 35 mm \(\times\) 9 mm FPC antenna. The background is 1 mm grid paper. Its conductive pattern is clearly visible in the bottom view. Figure 3: The MIMO antenna array on a watch for the experiment. Two magnets are bonded on the PCB to align and fix the clothing-cuff antenna PCB. The cuff PCB was aligned and fixed to the watch PCB by magnet fixtures without any translational and rotational misalignment, as shown in Figure 5a. Considering a practical use case, a few millimeters' distance between the PCBs is reasonable, since these PCBs are separated by at least the watch housing. Thus, the PCB distance was determined as 3 mm. Note that, in the experiment, the watch PCB was not embedded inside the watch housing, but was mounted outside the watch for simplicity. The antenna port numbers are defined as shown in Figure 5a. The measured transmittance from the watch antenna #1 to the nearest cuff antenna #3, \(|S_{31}|\), was \(-6.8\) dB at 2.45 GHz, as shown in Figure 5b. Due to the closer antenna spacing, the proximity coupling was stronger than the intra-PCB antenna mutual coupling, i.e., \(|S_{31}|>|S_{21}|\). This design, with the distance between the watch and cuff antennas being shorter than the 20 mm intra-PCB antenna distance, aimed to achieve \(|S_{31}|>|S_{21}|\) to increase the contribution of shoulder-mounted antennas. When \(|S_{31}|\ll|S_{21}|\), the strength of the signal transmitted/received at one of the watch antennas through the shoulder-mounted antenna is significantly weaker than the signal \"forwarded\" from the other watch antenna through the unwanted mutual coupling. In the experiments above and in the throughput measurements in the following section, the distance between the watch PCB and the cuff PCB was fixed at 3 mm. In practical use cases, depending on the design of the watch and cuff antennas, the proximity coupling \(|S_{31}|\) can be further increased by decreasing the PCB spacing. The measured \(|S_{31}|\) dependence on distance is shown in Figure 6. The \(|S_{31}|\) achieved \(-3.7\) dB at the maximum. Figure 5: (**a**) Two identical antenna PCBs, the cuff PCB and the watch PCB, are aligned and fixed by the magnet fixtures. (**b**) The measured \(S\) parameters.