Conceptualization, analysis, and design of miniaturized EBG surfaces and antenna for WBAN applications

Abstract

A network of wearable, implantable,and body-worn devices are collectively known as Wireless Body Area Network (WBAN). These networked devices comprising the WBANs are frequently used in the monitoring and tracking of the body’s essential physiological parameters such as glucose content, temperature, blood pressure, ligament pressure (round ligament), heart rate, etc. remotely. In the WBANs, antennas essentially play the roles of sensors in a variety of scenarios for link establishment between the base stations and the concerned patient. For example, there are situations requiring placement of antennas on different body parts, and in such a scenario, it is called on-body to on-body communication. Then there are often requirements of communication between sensors on patients' bodies and base station and this situation is called on-body to off-body communication. Finally, sometimes antennas in WBANs form in-body to on-body communication links. Overall, this area has been extensively studied in the past few years still issues such as large antenna size at medical body area network (MBAN), a quintessential WBAN band, and Wi-Fi bands, significant radiation towards the body, and shift in their corresponding resonant frequencies in the presence of multiple textile layers require further investigations. Moreover, body-worn antenna exhibits sound detuning (coupling) in the presence of a body that also degrades antenna performance. The critical issues of antenna size and radiation towards the body in the body-worn sensors are often addressed by utilizing Electromagnetic Band-gap (EBG) surfaces. It is achieved by integrating the EBG with the body-worn antenna sensors. This has shown great promise in enhancing the performance in terms of gain and efficiency due to the exciting in-phase reflection features of EBG cells. Besides, the surface wave rejection ability of an EBG surface reduces the diffraction of surface currents at the edges of the PCB and hence minimizes the specific absorption rate (SAR). However, the periodic arrangement of these EBG cells is often not compatible with any of the WBAN frequency standards due to the dimension constraints imposed by these cells. Besides, WBAN requires bending and conforming of antenna sensors for ease of usage and for keeping the actual practical scenario in perspectives to seamlessly place on corresponding body parts such as arms, biceps, and legs. It essentially means that the conforming of EBG surfaces is required but, in turn, leads to a shift in the in-phase reflection frequency. As a consequence, there is degradation in the performance of the proposed and designed antenna sensors. Furthermore, WBAN requires testing of SAR on a real phantom, but these are full of challenges such as the existing technologies and tools that are sophisticated, costly, and beyond access to many labs. It is thus apparent that there is plenty of scope for innovations and inventions within the broad regime of WBAN. Therefore, to advance the tools and technologies for the WBAN infrastructure, we have to pursue multi-pronged research work. Firstly, we explored, investigated, and proposed the design and analysis of miniaturized and conformal EBG integrated low profile body-worn antenna. It has been shown that the proposed EBG integrated antenna possesses dimensions that are compatible to fit in the existing smart-watches and associated gadgets. The developed EBG cells demonstrate excellent phase stability to polarization change of incident TE-TM and TEM waves. We have also identified that the surface wave rejection ability of the proposed EBG cells and arrays can be readily comprehended using the dispersion diagram and transmission characteristics. It plays a vital role in reducing SAR values in the eventual body-worn designed antenna and antenna arrays. To assess the features and characteristics of the proposed designs for WBAN compliance, the measurements have been carried out in both inside an anechoic chamber and the ambiance environments. During this research work, we were able to observe a significant problem with the shifts in the in-phase reflection frequency of EBG unit cells and corresponding arrays due to conforming for making them suitable for placement on various body parts. To address this problem, as part of our second contribution in this thesis work, we have proposed analytical formulations to determine the shift in the in-phase reflection frequency of the conformed EBG unit cell and array. We were able to develop the formulations through analysis and experimental investigation on three distinct EBG unit cells and arrays. The proposed approach is scalable, as has been demonstrated through its implementation on various shapes and radii of the conformed surfaces. It is well known that the body-worn antennas suffer from a shift in their resonance frequencies in the presence of protective layers in the form of textiles or other dielectrics. In our third contribution, therefore, we have proposed a unique iterative model to determine the effective dielectric constant and resonant frequency of an MPA covered with multiple textile layers. We have also developed closed-form expressions to determine the effective dielectric constant of such an arrangement based on the conformal mapping approach. The demonstration of excellent consonance between the theoretically obtained results and the simulation, as well as measurement results, is a testament to the effectiveness of the proposed approach. An extremely important feature of the proposed analytical model and the formulations are that they can be readily implemented using any commercial mathematical processing tool. It is pertinent to note that the applications of the design of antennas and antenna systems in the WBAN require tests and evaluations within a realistic scenario. As a first step, it is a standard practice to carry out such evaluations using phantoms, which are rarely available with academic labs. To circumvent this challenge, and as part of the last and fourth contribution of this thesis work, we proposed a cost-effective and straightforward approach to develop liquid phantoms utilizing common materials such as salt, sugar, glycerine, and water to mimic the human tissue behavior. We have also proposed a simple and cost-effective measurement setup to characterize the developed phantom. Finally, we used the developed phantom also to propose a point SAR measurement technique that proves effective in assessing the SAR performance of the proposed antennas.