Gareth A. Conway

Antennas for Over-Body-Surface Communication at 2.45 GHz

In this paper, the on-body performance of a range of wearable antennas was investigated by measuring |S 21| path gain between two devices mounted on tissue-equivalent numerical and experimental phantoms, representative of human muscle tissue at 2.45 GHz. In particular, the study focused on the performance of a compact higher mode microstrip patch antenna (HMMPA) with a profile as low as lambda/20. The 5- and 10-mm-high HMMPA prototypes had an impedance bandwidth of 6.7% and 8.6%, respectively, sufficient for the operating requirements of the 2.45-GHz industrial, scientific, and medical (ISM) band and both antennas offered 11-dB higher path gain compared to a fundamental-mode microstrip patch antenna. It was also demonstrated that a 7-dB improvement in path gain can be obtained for a fundamental-mode patch through the addition of a shortening wall. Notably, on-body HMMPA performance was comparable to a quarter wave monopole antenna on the same size of groundplane, mounted normal to the tissue surface, indicating that the low-profile and physically more robust antenna is a promising solution for bodyworn antenna applications.

A Time-Domain Approach to the Analysis and Modeling of On-Body Propagation Characteristics Using Synchronized Measurements at 2.45 GHz

Modeling of on-body propagation channels is of paramount importance to those wishing to evaluate radio channel performance for wearable devices in body area networks (BANs). Difficulties in modeling arise due to the highly variable channel conditions related to changes in the users state and local environment. This study characterizes these influences by using time-series analysis to examine and model signal characteristics for on-body radio channels in user stationary and mobile scenarios in four different locations: anechoic chamber, open office area, hallway, and outdoor environment. Autocorrelation and cross-correlation functions are reported and shown to be dependent on body state and surroundings. Autoregressive (AR) transfer functions are used to perform time-series analysis and develop models for fading in various on-body links. Due to the non-Gaussian nature of the logarithmically transformed observed signal envelope in the majority of mobile user states, a simple method for reproducing the fading based on lognormal and Nakagami statistics is proposed. The validity of the AR models is evaluated using hypothesis testing, which is based on the Ljung-Box statistic, and the estimated distributional parameters of the simulator output compared with those from experimental results.

In Situ Measurement of UHF Wearable Antenna Radiation Efficiency Using a Reverberation Chamber

The radiation efficiency and resonance frequency of five compact antennas worn by nine individual test subjects was measured at 2.45 GHz in a reverberation chamber. The results show that, despite significant differences in body mass, wearable antenna radiation efficiency had a standard deviation less than 0.6 dB and the resonance frequency shift was less than 1% between test subjects. Variability in the radiation efficiency and resonance frequency shift between antennas was largely dependant on body tissue coupling which is related to both antenna geometry and radiation characteristics. The reverberation chamber measurements were validated using a synthetic tissue phantom and compared with results obtained in a spherical near field chamber and finite-difference time-domain (FDTD) simulation.

An Antennas and Propagation Approach to Improving Physical Layer Performance in Wireless Body Area Networks

A combined antennas and propagation study has been undertaken with a view to directly improving link conditions for wireless body area networks. Using tissue-equivalent numerical and experimental phantoms representative of muscle tissue at 2.45 GHz, we show that the node to node |S21| path gain performance of a new wearable integrated antenna (WIA) is up to 9 dB better than a conventional compact Printed-F antenna, both of which are suitable for integration with wireless node circuitry. Overall, the WIA performed extremely well with a measured radiation efficiency of 38% and an impedance bandwidth of 24%. Further benefits were also obtained using spatial diversity, with the WIA providing up to 7.7 dB of diversity gain for maximal ratio combining. The results also show that correlation was lower for a multipath environment leading to higher diversity gain. Furthermore, a diversity implementation with the new antenna gave up to 18 dB better performance in terms of mean power level and there was a significant improvement in level crossing rates and average fade durations when moving from a single-branch to a two-branch diversity system.

An analytical path-loss model for on-body radio propagation

An analytical model for across the body surface communication systems is presented. The paper is focused on the calculation of the approximate path gain, both along and around planar and cylindrical geometries, representative of the human body for antennas polarized normal to the body surface. The model is validated at 2.45 GHz using finite-difference time-domain numerical analysis and in-situ measurements on an adult-male test subject.

Low-Profile Patch Antennas for Over-Body-Surface Communication at 2.45 GHz

Two low-profile patch antennas on small ground planes (0.25 lambda), suitable for over the body surface communication at 2.45 GHz are presented. On-body performance was investigated using FDTD simulations of S21 coupling of shorted microstrip patch antennas (S-MPA) and higher-order mode microstrip patch antennas (HM-MPA) placed on numerical tissue phantoms with characteristics of muscle tissue. The low-profile antenna coupling results are comparable to those achieved using a quarter wave monopole antenna on the same size of groundplane, mounted normal to the tissue surface, indicating that the low-profile antennas studied are promising for bodyworn antenna applications.

Low-Profile Microstrip Patch Antenna for Over-Body Surface Communication at 2.45 GHz

A compact higher-order mode microstrip patch antenna (HM-MPA) suitable for on-body communications at 2.45 GHz is presented. Using FDTD simulations we show that the HM-MPA had an impedance bandwidth of 150 MHz and a relatively good bodyworn efficiency of more than 60% when placed in close proximity from a lossy medium with the characteristics of muscle tissue. On-body performance was investigated by modelling S21 coupling of two HM-MPAs spaced 200 mm apart on a muscle tissue layer. The HM-MPA coupling results are comparable to those achieved using a quarter wave monopole antenna on the same size of groundplane, mounted normal to the tissue surface. Considering that the HM-MPA has the benefit of being low-profile and physically more robust, it is a promising solution for over body surface applications.

The performance of on-body wearable antennas in a repeatable multipath environment

The performance of antennas designed for on-body channels is usually evaluated in an anechoic environment. However, it is also appropriate to determine their performance under multipath conditions since the presence of off-body paths may significantly improve on-body links for some antennas. This was investigated by considering the on-body performance (|S21| path gain) of a range of wearable antennas in the repeatable multipath environment of a reverberation chamber using a tissue-equivalent experimental phantom, representative of human muscle tissue at 2.45 GHz. These results were compared with the equivalent measurements taken in an anechoic far-field chamber. The study shows that antennas which radiate tangential to the body surface, supporting a surface wave propagating mode, perform favorably in both environments, which is advantageous in reliable system design.

Improving wearable slot antenna performance with EBG structures

This paper presents a 2.8 GHz slot antenna over an EBG (Electromagnetic Band Gap) substrate with the objective of higher antenna efficiency, wide bandwidth and compact operation in close proximity to the human body. To achieve compact size, two different size EBGs are developed and the antenna return loss and efficiency are measured in a reverberation chamber. From these results, the EBG substrate demonstrates 60% improvement of the antenna efficiency compared to other methods in the wearable antenna scenario.

Compact slot-loaded patch antenna for 868 MHz wireless body area networks

The development of a compact and highly efficient antenna operating at 868 MHz for communication in ISM-band body area networks is reported. With symmetrical slots and shorting pins on either side of the feed, the antenna exhibited a radiation pattern similar to that of a monopole/dipole antenna with an efficiency of 52.3% and an impedance bandwidth of 9.3 MHz (1.1%) when placed in close proximity (1 mm) from a muscle tissue phantom. The maximum dimension of the proposed design is reduced by more than 66% compared to a conventional lambda/2 microstrip patch antenna. Furthermore, for a simulated on-body path the performance of the new antenna was within 2 dB of a lambda/4 monopole.