Body-centric wireless communications: wearable antennas, channel modelling, and near-field antenna measurements

This thesis provides novel contribution to the field of body-centric wireless communications (BCWC) with the development of a measurement methodology for wearable antenna characterisation on the human body, the implementation of fully-textile wearable antennas and the on-body channel modelling considering different antenna types and user's dynamic effects. More specifically, a measurement methodology is developed for characterising wearable antennas on different locations of the human body. A cylindrical near-field (CNF) technique is employed, which facilitates wearable antenna measurements on a full-body solid anthropomorphic mannequin (SAM) phantom. This technique allows the fast extraction of the full spherical radiation pattern and the corresponding radiation efficiency, which is an important parameter for optimising wearable system design. It appears as a cost- effective and easy to implement solution that does not require expensive positioning systems to rotate the phantom, in contrast to conventional roll-over-azimuth far-field systems. Furthermore, a flexible fully-textile wearable antenna is designed, fabricated and measured at 2.4 GHz that can be easily integrated in smart clothing. It supports surface wave propagation and exhibits an omni-directional radiation pattern that makes it suitable for on-body communications. It is based on a multilayer low-profile higher-mode patch antenna (HMMPA) design with embroidered shorting vias. Emphasis is given to the fabrication process of the textile vias with conductive sewing thread that play an important role in generating the optimal mode for on-body radiation. The radiation pattern shape of the proposed fully-textile antenna was found to be similar to a copper rigid antenna, exhibiting a high on-body radiation efficiency of 50 %. The potential of the embroidery technique for creating wearable antennas is also demonstrated with the fabrication of a circularly polarised spiral antenna that achieves a broadband performance from 0.9-3 GHz, which is suitable for off-body communications. By testing the textile spiral antenna on the SAM phantom, the antenna-body interaction is examined in a wide frequency range. Finally, a statistical characterisation of on-body communication channels is undertaken both with EM simulations and channel measurements including user's dynamic movement (walking and running). By using antenna types of different polarisation, the on-body channels are examined for different propagation conditions. Four on-body channels are examined with the one part fixed on the waist of the human body while the other part located on the chest, back, wrist and foot. Channel path gain is derived, while large-scale and small-scale fading are modelled by best-fit statistical distributions

Broad-band embroidered spiral antenna for off-body communications

An embroidered wearable spiral antenna is presented in this study. The spiral antenna is compact and flexible, yet has broad-band performance. The novelty of this study includes considering the antenna–body interaction rather than just considering the antenna alone. The antenna has been simulated and measured on a specific anthropomorphic mannequin torso phantom and a real person. The far-field on-body performance of the embroidered antenna on the phantom has been measured using a novel cylindrical near-field to far-field transformation technique. This technique allows the fast extraction of the full spherical radiation pattern and the corresponding far-field antenna characteristics on the human body without the need of rotating the phantom with expensive positioning systems. The on-body antenna performance including realised gain, directivity, radiation efficiency, radiation pattern and axial ratio have been presented

Design, realisation and evaluation of a liquid hollow torso phantom appropriate for wearable antenna assessment

This paper examines the design, realization and evaluation of a lightweight and low cost hollow oval cross-section torso phantom appropriate for wearable antenna performance assessment. The phantom consists of an empty inner space (hollow) surrounded by a shell with double plastic walls between which there is a tissue simulating liquid. The phantom’s plastic shell is made of a low loss cast acrylic and the liquid is a commercially available one with properties calibrated for the frequency range of 2 - 6 GHz. The proposed phantom is compared, through simulations, with a full liquid torso phantom and a heterogeneous anthropomorphic voxel phantom. Additionally, the fabricated phantom is compared with human bodies and a homogeneous anthropomorphic solid phantom, through measurements. The phantom performance is tested in terms of electric field distribution of a wearable antenna on its surface and the path loss between two wearable antennas, on either side of the phantom. It is proved that the hollow phantom performance approximates the full liquid phantom when an RF absorbing material is placed in the central hollow region. The phantom performance in terms of S11 wearable antenna measurements is evaluated and found in good agreement with real human bodies in the examined frequency range (2 - 6 GHz). The far field wearable antenna performance of the proposed phantom shows deviation in gain less than 1.5 dB, compared with anthropomorphic phantom

On-body measurements of embroidered spiral antenna

This paper presents a compact and flexible embroidered spiral antenna that can be used for wearable applications. The antenna is embroidered by using a state of the art digital embroidery machine with multi-strand conducting thread Liberator™. The antenna has been measured on a Specific Anthropomorphic Mannequin (SAM) phantom and a real human. The measurement results show that the SAM phantom emulates the dielectric properties of the human body in a wide frequency band from 0.3 to 3 GHz. The far-field on-body performance of the antenna has been measured by placing the antenna on the SAM phantom in a tapered Anechoic Chamber. Near-field to far-field transformations have been used to produce the far-field performance including radiation pattern, directivity, realised gain and radiation efficiency