High-Rate Millimeter-Wave Body Networks
Recently, the millimeter-wave band, mainly 57-66 GHz range, has been identified as highly promising for future body-area network (BAN) technologies. One of the main features differentiating the 60-GHz band from lower frequency BAN is confidentiality and low interference level with neighboring networks, which has been demonstrated to be crucial for body-centric and inter-BAN communications. In addition, it provides other advantages including high data rates [typically beyond 5 Gb/s], reduced size of antennas and sensors, high accuracy for localization, that can be exploited for new applications.
ResCor project focuses on the exploration of antenna solutions in the 60-GHz BAN in the near-field region since this corresponds to the most relevant users’ scenario for most of the BAN and inter-BAN applications. It also deals with the development of the body models in the 60-GHz band and investigation of the propagation channel.
The main results and outcomes of the project are summarized hereafter.
C.1. Wearable end-fire antenna for 60 GHz BAN and study of on- / off-body propagation
A compact end-fire wearable Yagi-Uda antenna covering the 57-64 GHz frequency range was designed, characterized in free space, in the presence of a skin-equivalent phantom, and under bending conditions (Fig. 1). This is one of the very first antenna prototypes for body-centric communications in the 60-GHz band. The results demonstrated that, when placed on the body and / or bended, the antenna preserves its satisfactory performances. The possibility of its use for an on / off-body communications at 60 GHz was investigated numerically and experimentally in a representative scenario in terms of E-field and power flow distributions, as well as in terms of path gain. It is shown that this antenna is a suitable candidate for high-data-rate short-range on / off-body transmissions. The impact of the feeding type of wearable antennas on antenna / human body interactions has been also investigated.
Fig. 1. End-fire antenna: (a) 3D CAD view; (b) manufactured antenna with V-connector; (c) antenna bended in H plane on a skin-equivalent phantom.
C.2. Solid phantom for body-centric propagation measurements in the 60-GHz band
The first solid skin-equivalent phantom was developed to characterize the propagation channel for 60-GHz wireless body-centric systems (Fig. 2). This phantom is designed to emulate the same reflection coefficient at the air / phantom interface as at the air / skin interface. It is fabricated by combining carbon black powder with polydimethylsiloxane (PDMS) and metalizing the resulting flexible carbon-PDMS composite on one side. Using two open-ended waveguides, it is demonstrated, both numerically and experimentally, that the propagation along flat and cylindrical phantoms is similar to the propagation along the skin in the 58-63 GHz frequency range. The phantom was further validated for two dipoles and two wearable textile Yagi-Uda antennas demonstrating that it can be successfully used for propagation studies.
Fig. 2. Solid skin-equivalent 60-GHz phantom: (a) fabricated prototype; (b) electromagnetic properties of carbon/PDMS substrate; (c) phantom in on-body propagation measurements.
C.3. Propagation study using common textiles.
Here the effect of textiles on propagation along the body at 60 GHz was studied (Fig. 3). A Green’s function representation was used to investigate analytically the field excited by an infinitesimal dipole over a multilayer structure representing a flat skin model and clothing. The propagation between two rectangular open-ended waveguides was studied numerically and analytically in terms of path gain. Furthermore, the effect of an air gap between the textile and phantom was considered. Results show that the presence of a textile over a skin-equivalent phantom, as well as an air gap between them, induces a typical decrease of the path gain by 2–5 dB, but it does not significantly affect the path gain exponent.
Fig. 3. On-body propagation in presence of regular textiles: (a) analytical model; (b) numerical model; (c) path gain as a function of permittivity and textile/skin spacing.
C.4. Enhancement of the on-body propagation using electrotextiles.
Finally, we demonstrated for the first time that an electro textile can improve the wave propagation at 60 GHz along and around the body (Fig. 4) [4,9]. An analytical formulation was implemented to evaluate the electric field excited by an infinitesimal dipole over a flat skin model with and without an electro textile layer. To validate the analytical results, the propagation was characterized numerically and experimentally for two rectangular open-ended V-band waveguides placed over a skin-equivalent phantom. Path gain values for flat, cylindrical and elliptical cylinder phantoms were measured. Results show that placing an electro textile over a skin-equivalent phantom allows increasing the path gain by 5-15 dB. In addition, it has a shielding effect by decreasing the power absorption in the body by more than 95%.
Fig. 4. Enhancement of the on-body propagation using electrotextiles: (a) surface current on the textile for vertically (left) and horizontally polarized E field; (b) path gain.