# Augmented MIMO: Body-Mounted Antennas for Tiny Wearable Devices
## 1 Introduction  
Multiple-input-multiple-output (MIMO) [1], which uses multiple antennas at the transmitter and receiver, is now a common technique for increasing communication capacity without widening the occupied radio frequency (RF) bandwidth. A physical limitation of MIMO is that its effectiveness in improving channel capacity depends on the spatial correlation of the antennas. As a rule of thumb, a half-wavelength separation between antenna elements is required for sufficient antenna independence. Antenna arrays within a deep subwavelength dimension degrade MIMO performance due to the increased mutual coupling and spatial correlation between the antenna elements [2]. In particular, very small devices such as smartwatches encounter this problem. The 2.4 GHz industrial, scientific, and medical (ISM) band is widely used for a variety of wireless communications, including wireless local area networks (WLANs) and Bluetooth, which are used in most commercially available wearable devices. The free-space wavelength of radio waves in the band is about 12 cm. Therefore, the antenna array that can be embedded in wearable devices with a watch form factor or smaller must be of deep subwavelength. Without special antenna decoupling techniques, it has been reported that typical inverted-F antennas integrated into a 40 mm square scale, comparable to smartwatches, exhibit a mutual coupling of about \(-15\) dB [3]. To counter this problem, antenna decoupling techniques have been extensively investigated [4]. Despite such a challenging issue, it has been clarified in the literature that even if the antenna spacing is reduced, increasing the number of antenna elements contributes to increasing the channel capacity [5]. In addition, one of the key technologies for next-generation high-speed mobile networks is massive and ultra-massive MIMO [6]. Thus,MIMO is an essential technology for mobile communication devices. The application of MIMO technology is also expanding to smaller scales; for example, investigations of implantable antennas [7] and antenna-on-chip [8] are in progress. Therefore, for the sustainable evolution of mobile wireless communication technologies, a wearable-friendly MIMO implementation technique is needed. We propose Augmented MIMO, in which a larger antenna array with increased antenna spacing is mounted on the human body, to improve the MIMO performance of wearable devices with a physically constrained small form factor. The key idea is that, for such very small wearable devices, the wearer's own body can be used as a spacious antenna fixture that always exists with oneself. Assuming the use of the 2.4 GHz ISM band, the human body is significantly larger than the wavelength. Therefore, the proposed scheme even opens avenues to implement massive MIMO antenna arrays with wearable devices, whose implementation is currently limited to base stations that have enough space to accommodate the large number of antennas. It must be clarified that the proposed scheme is completely different from, but not exclusive to, other antenna decoupling techniques. We can find extensive research on this type of coupling reduction in size-limited MIMO applications [4]. The state-of-the-art developments include isolation techniques based on additional parasitic elements [9], characteristic modes [10], electromagnetic band gap (EBG) [11], and metamaterials [12]. The proposed Augmented MIMO scheme provides a way to implement MIMO antenna arrays with sufficient spacing and reduced antenna coupling without relying on special decoupling techniques. On the other hand, when combined with decoupling techniques, the antenna density can be increased; therefore, the total number of antenna elements that can be implemented in clothing is significantly increased, thus paving the way for the implementation of massive MIMO systems in wearable devices. An example of Augmented MIMO implementations, in contrast to conventional MIMO, is illustrated in Figure 1. Here, we consider a MIMO link between a WLAN access point (AP) and a smartwatch.