On-body Wireless Communications Research Articles

Functional analysis in the body environment for a circularly polarized hybrid reconfigurable textile antenna

This paper presents the functional analysis of a hybrid reconfigurable circularly polarized (CP) textile antenna in different on-body environments. The Tai-Chi sign inspires the current textile antenna design, where the primary radiator and ground elements are sickle-shaped structures. Hybrid reconfigurability was achieved by successfully operating two RF PIN-Diodes in the basal ground plane’s elliptical and rectangular structures. Thus, four switching states are attained by operating the textenna at 3.5, 4.5 and 5.4 GHz in the S1. 4.39 and 5.8 GHz in S2, 5 and 5.9 GHz in S3 and 2.4, 4.3, 5.8, and 9.4 GHz in S4 state. The reconfigurable textenna was devised to function on the human body. Its functional behavior in proximity to the human body was rigorously validated through testing involving bending at various vertical and horizontal angles. Additionally, a Specific Absorption Rate (SAR) analysis was conducted using a three-layered human phantom. The results revealed that the maximum SAR value is below 1.6 W Kg−1 for 1 gram of tissue and 2 W kg−1 for 10 grams of tissue. These findings establish the safe operational parameters for the proposed model. The developed antenna functionalities were also tested in other circumstances like body sweating environment and interaction with body fluids (Blood, Plasma, SBF and DMEM). In these body fluid conditions, a peak gain of 4.88 dBi at S1, 6.97 dBi at S2, 7.99 dBi at S3, and 4.13 dBi at S4, was observed while interacting with Plasma, and the measured efficiency in the above configuration is between 77%–79%. The textenna durability analysis was measured using an electrochemical workstation while interacting with different body fluids. All these results confessed the applicability of the developed antennas for on-body wireless communication applications in different on-body conditions.

Dual-Band Metamaterial-Based EBG Antenna for Wearable Wireless Devices

In this paper, a wearable dual-band T-shaped antenna using a coplanar waveguide (CPW) fed operating at 2.4 GHz and 5.2 GHz bands is proposed for onbody wireless communications applications. Without the metamaterial-based electromagnetic bandgap (EBG) layer, the original antenna covers two bands from 1.96 GHz to 2.77 GHz and from 5.07 GHz to 5.35 GHz. The antenna efficiency decreases when it is placed on the human arm due to the interference from the human body to the antenna, as shown by a high specific absorption rate (SAR) value. These SAR values are reduced to 77.1% at 2.4 GHz and 91.7% at 5.2 GHz by the proposed EBG. The antenna gain is therefore improved to 1.4 dBi at 2.4 GHz and 6.25 dBi at 5.2 GHz. The antenna prototype is evaluated using a Wi-Fi wearable device, resulting in an improved signal-to-noise ratio (SNR) of 6-12 dB.

Wearable Folded Planar Dipole Antenna: Design and assessment for on-body wireless communication devices

A folded planar dipole antenna was designed and customized to operate close to the human body. Backward radiation was reduced to protect the human body from unwanted electromagnetic (EM) emissions and increase the antenna’s operating range. The antenna design was based on the concept of a folded planar dipole, which presents favorable impedance bandwidth (BW) characteristics. For integrability and robustness during usage in real environments, the antenna was completely covered by resin, taking into account the tradeoff between the antenna’s radiation performance, protection, and isolation. Measurements demonstrated a 10-dB impedance BW of 470 MHz (18.76%) covering the 2.27–2.74-GHz band. The maximum gain at 2.45 GHz was 1.4 dB isotropic (dBi). The antenna has a nearly omnidirectional pattern, occupies 77 × 35 × 11.15 mm, and weighs 120 g, making it easy to integrate with clothes. The resin packaging increased the mechanical robustness and improved the design’s isolation from external interference and perturbations; however, it reduced the radiation efficiency to 48.35%. The presented antenna is an excellent candidate for many wireless applications, in particular, applications where withstanding exposure to external mechanical strains and EM perturbations is required.

Performance improvement of an ultra-wideband antenna using textile material with a PIN diode

Wearable antennas are critical in on-body wireless communications and have piqued scientists’ curiosity. Because wearable antennas work so near to the human body, the loading effect caused by body tissue breakdown, along with their high dielectric constants and conductivity, makes developing a high-radiation output antenna challenging. The demand that these antennas be lightweight, low-cost, maintenance-free, and require no setup adds to the problem. In recent years, the demand for wearable devices has skyrocketed. Wearable antennas are utilized in a variety of applications, including wireless body area networks (WBANs), in many circumstances. The patch antenna is one of the primary research goals of wearable antennas for WBAN applications due to the extensive ground plane employed in its construction and its relatively high directionality. In addition to directivity, microstrip patch antennas provide other benefits for on-body wearable applications. The wearable textile ultra-wideband (UWB) antenna’s frequency of changes versus gain is set. The maximum gain is about 17.2 dB at 16 GHz. The gain of the frequency of 14 GHz is obtained at 16.8 dB, with a high performance rate. The design in the present paper has low power consumption as a result of achieving gain results of less than 5 dB in most frequency bands from 2 to 16 GHz. While the reflector is present, the size of the reflector is chosen to make the antenna resistant to frequency detuning. When the distance between the monopole and the reflector is lowered, the antenna’s frequency decreases. The greater power, the closer the reflector is to the inductive near zone. However, the reflector was chosen to boost the front-to-back ratio of the proposed monopole. The reflection coefficient is derived by varying different resonating frequencies between −19.45 and −16.45 dB. The highest directivity is 11.5 dB and the lowest is 9.8 dB while varying different resonant frequencies. Gain is achieved by varying different resonant frequencies, with a maximum of 17.2 dB and a minimum of 16.2 dB. The effectiveness of varying different resonant frequencies is 45% maximum and 40% minimum. The suggested UWB antenna was found to have an acceptable agreement between models and tests, demonstrating its potential for microwave applications. The directivity, gain, Voltage Standing Wave Ratio (VSWR), and radiation pattern are all performance metrics that are measured.

Stretchable 3D Wideband Dipole Antennas from Mechanical Assembly for On-Body Communication.

The development of wearable/stretchable electronics could largely benefit from advanced stretchable antennas with excellent on-body performance upon mechanical deformations. Despite recent developments of stretchable antennas based on intrinsically stretchable conductors, they are often affected by lossy human tissues and exhibit resonant frequency shifts upon stretching, preventing their applications in on-body wireless communication and powering. This work reports a three-dimensional (3D) stretchable wideband dipole antenna from mechanical assembly to simultaneously reduce the frequency detuning and enhance on-body performance. The large bandwidth is achieved by coupling two resonances from two pairs of radiation arms, which is well-maintained even when the antenna is directly placed on human bodies or stretched over 25%. Such an excellent on-body performance allows the antenna to robustly transmit the wireless data and energy. The design of the 3D stretchable wideband dipole antenna with significantly enhanced on-body wireless communication performance was validated by an experimental demonstration that features a small difference in the wirelessly received power between the on-body and off-body use. The combination of the mechanically assembled 3D geometries and the coupled mechanical-electromagnetic properties can open up new opportunities in deformable 3D antennas and other microwave devices with excellent on-body performance and tunable properties.

Standalone stretchable RF systems based on asymmetric 3D microstrip antennas with on-body wireless communication and energy harvesting

As an indispensable component, the stretchable antenna with the potential use in wireless communication and radio frequency (RF) energy harvesting can provide future wearable electronics with a low profile and integrated functions. However, mechanical deformations applied to stretchable antennas often lead to a shift of their resonant frequency (i.e., the detuning effect), which limits their applications to strain sensing. In addition, the on-body radiation efficiency of stretchable antennas severely degrades due to lossy human tissues. In this work, we introduce stretchable microstrip antennas with varying 3D configurations for excellent on-body radiation performance. Compared to their 2D counterpart, the stretchable 3D microstrip antennas showcase a strain-insensitive resonance, improved stretchability, and enhanced peak gain. In particular, the optimized peak gain from the stretchable asymmetric 3D microstrip antenna allows it to wirelessly transmit the energy and data at an almost doubled distance, as well as a doubled charging rate from the harvested RF energy. More importantly, the integration of stretchable antenna and rectenna with stretchable sensing and energy storage units can yield a standalone stretchable RF system for future health monitoring of humans and structures. The results from this work can also pave the way for the development of self-powered units with wireless transmission capabilities for stretchable body area networks and smart internet-of-things.

A Wearable Button Antenna Sensor for Dual-Mode Wireless Information and Power Transfer.

A novel wearable button antenna sensor is proposed for the concept of simultaneous wireless information and power transfer (SWIPT). This integrates two working modes for the transfer of power and information, respectively, and optimizes transfer efficiency. An omni-directional radiation pattern is achieved in the 3.5 GHz World Interoperability for Microwave Access (WiMAX) band to support on-body wireless communications, while a circularly polarized broadside radiation pattern is obtained in the 5 GHz wireless local area networks (WLAN) band to harvest power. The measured −10 dB return loss bandwidths are 4.0% (3.47–3.61 GHz) in the lower band, and 25.0% (4.51–5.80 GHz) in the higher band, respectively. An artificial magnetic conductor (AMC) structure with wideband characteristics is applied to obtain a low-profile design and to increase the stability of the antenna sensor. A high radiation efficiency of over 80% in the whole working band is observed. The specific absorption rate (SAR) of the proposed antenna sensor is below 0.509 W/kg at 3.55 GHz, and below 0.0532 W/kg at 5.5 GHz, respectively, which is much lower than the European standard threshold of 2 W/kg. All these characteristics make the designed antenna sensor suitable for on-body information transmission and off-body energy harvesting. The antenna sensor has been prototyped. Simulations and measurements agree well, proving the validity of the new concept.

Standalone Stretchable RF Systems Based on Asymmetric 3D Microstrip Antennas with On-Body Wireless Communication and Energy Harvesting

Standalone Stretchable RF Systems Based on Asymmetric 3D Microstrip Antennas with On-Body Wireless Communication and Energy Harvesting

A Cooperative Transmission Strategy for Body-Area Networks in Healthcare Systems

Wireless body area networks (BANs) have attracted enormous attention due to the promising applications in healthcare systems. Energy saving is one of the major challenges in wireless BANs, because the sensors operating on or inside a human body are energy limited. As the applying cooperative communications offers energy saving, it is necessary to utilize the sensor devices jointly in BANs to form cooperative communication. In this paper, the energy consumption models of cooperative transmission strategies are built over in- and on-body wireless communication links for direct and relay transmission scenarios. In relay cooperative strategy, the implantable and wearable devices work together by using a cooperative multiple input multiple output (MIMO) technique. In direct cooperative strategy, two wearable devices operate as the cooperative MIMO. In these ways, the energy savings of both direct and relay transmissions are achieved during the data transmission in BANs. Moreover, the closed-form expression of end-to-end average bit error ratio (BER) is derived toward minimizing the required transmission power for relay transmission scenario. In the results, it is demonstrated that the significant energy savings of the proposed cooperative transmission strategies can be obtained compared with the existing approaches under the same conditions.

Energy Efficient Transmission Approach for WBAN Based on Threshold Distance

Energy efficiency is a key concern for wireless sensor nodes, especially for wireless body area network (WBAN) in which sensors operate in close vicinity to, on or even inside a human body. In this paper, we first present a system-level energy consumption model associated with transmission distance $d$ and transmission data rate over on-body wireless communication link. Then, based on the analysis of tradeoff between circuit energy and transmission energy on distance, a threshold distance $d_{\rm th}$ which is responsible for the proportion of transmission energy and circuit energy is derived for energy saving in WBAN. With the case of $d\leq d_{\rm th}$ , since circuit energy is comparable with transmission energy consumption, the total energy consumption can be saved by optimizing the transmission data rate $R$ . Simulation results show that a 59.77% or even more energy saving is achievable using the optimized scheme, compared with baseline scheme. With $d>d_{\rm th}$ , since the total energy consumption is monotonically decreasing with respect to time $t$ , an offline algorithm is applied to energy saving by prolonging transmission time within the deadline time. In addition, on the basis of the offline algorithm, a battery-aware transmission approach is presented for WBAN using battery electrochemical property. Experimental results show that, using the presented battery-aware approach, 71.05% and 60.81% energy saving can be obtained, in comparison with the baseline and offline schemes, respectively.

Self-Calibrating Body Sensor Network Based on Periodic Human Movements

Body sensor networks with a central gateway node can control wireless transmission and optimize the network lifetime. The central node should be placed within communication reach of other nodes spread out around the human body, such as on the chest. This paper reports a novel energy-efficient time multiplexing transmission method based on the human rhythmic movement of running for on-body wireless communication. The running style of each individual allows the network to self-calibrate the communication scheme so that transmissions occur only when high link reliability is predicted. This technique takes advantage of the periodic running actions to implement a dynamic time division multiple access strategy for a five node body network with very little communication overhead, long sleep times for the sensor transceivers, and robustness to communication errors. The results showed all wireless communications were successful, except when two nodes attempt to use the transmission medium simultaneously. An aggregated network reliability of 90% was achieved compared to 63% when employing traditional time multiplexing algorithms.

A Reduced-Size Wide Slot Antenna for Enhancing Along-Body Radio Propagation in UWB On-Body Communications

In this paper, a 6-10-GHz ultrawideband (UWB) directional reduced-size wide slot antenna (RWSA) for on-body wireless communications is proposed and investigated. The RWSA is designed to be narrow in width and can be mounted on the side of a wireless device. When a side-mounted RWSA is worn on a body, the antenna boresight is naturally directed along the body surface, and most of the energy can be efficiently transmitted by direct or diffracted waves. Consequently, the on-body channel performance can be effectively improved. After a series of on-body channel measurements, the results show that, with the side-mounted RWSAs, the path loss is significantly improved (greater than 20 dB) compared with the results of previous studies, and the sensitivity of the channel to the human body and environment is effectively lessened. An approach to estimate the variation range of path loss in real environments is also proposed at the end of this paper.

Time-varying on-body wireless channel model during walking

A novel dynamic channel model for on-body wireless communication during walking is proposed. The developed model utilizes a human walking model which provides detailed information on the movement of the human body parts. The diffraction of the signal around the body parts is used to describe the time-varying shadowing effects. Body part movements are also used to estimate the signal fading caused by angular variations of the transmitting and receiving antenna gains. A Rice distribution is used to represent the multipath fading effects caused by objects around the human body. Simulation results of the first- and second-order statistics of the received signal affected by moving body parts for 2.4 GHz signal are presented. To illustrate the capabilities of the developed model, time series were generated and used in system performance calculations. The obtained results give an insight into the potential advantages of link diversity technique in wireless body area networks (WBANs).

Analysis of Dynamic On-Body Communication Channels for Various Movements and Polarization Schemes at 2.45 GHz

On-body wireless communication channels are studied by using the finite-difference time-domain (FDTD) method and moving human models. An anechoic environment is assumed. Three movements having a different dynamical characteristic each, walking, weakly walking and running, are considered. Essentially, the results are obtained for nine different polarization schemes regarding the dipole transmitter and receiver orientations and for six receiver locations. The results include the reception level curves, mean levels and standard deviation of reception (STD); dynamic path gain (PG) of a specific antenna can be obtained, too, by, e.g., using the presented “offset method.” The results, best applicable for electrically small dipole-like antennas, can be utilized in link budget calculations, channel model development and usability evaluations of different polarization schemes with different on-body links. The effect of the polarization scheme on the mean level is much understood by the theory of radiowave propagation over a lossy ground. Based on this theory, a rough method to estimate the mean reception level (and PG) is introduced and found usable for small dipole antennas under suitable conditions. Finally, reception changes due to uncertain receiver location are studied. A body-normally polarized small dipole receiver is less sensitive to its location than a body-tangential one.

Depolarization in On-Body Communication Channels at 2.45 GHz

This paper describes an investigation into the polarization behavior of on-body wireless communication channels at 2.45 GHz. The effect of the polarization of an antenna on the channel path gain has been studied and the channel cross-polarization discrimination (XPD) has been quantified, using both simulation and measurement. In simulation, short dipole antennas, which significantly reduce the simulation time, were used to investigate path gain behavior in different channels for vertically and horizontally polarized antennas. Measurements were taken in an indoor environment using a novel dual-polarized antenna, producing normal and parallel polarizations to the body surface. The measured signals were also analyzed to extract the Rician K-factor which was defined as the ratio of the power in the line of sight (LOS) signal to the scattered power for various polarization combinations.

Wearable Endfire Textile Antenna for On-Body Communications at 60 GHz

A textile endfire antenna operating in the 60-GHz band is proposed for wireless body area networks (BANs). The permittivity of the textile substrate has been accurately characterized, and the Yagi-Uda antenna has been fabricated using an ad hoc manufacturing process. Its performance in terms of reflection coefficient, radiation pattern, gain, and efficiency has been studied in free space and on a tissue-equivalent phantom representing the human body. It is shown that the antenna is matched in the 57-64-GHz band. Its measured on-body efficiency and maximum gain equal 48.0% and 11.9 dBi, respectively. To our best knowledge, this is the first textile antenna for on-body wireless communications reported at millimeter waves.

Impedance and radiation characteristics of circular patch wearable antennas at flat and bent positions

This paper describes design, development and evaluation of three electro-textile based circular patch wearable antennas for on-body wireless communications in the Bluetooth band. Performance characteristics are determined experimentally when these antennas are kept in flat position. All the three antennas are tuned to the desired frequency and they provide gain of 6.8–7.8 dBi. The performance characteristics of one of these antennas are analysed under bent conditions too, to check compatibility with wearable applications. The investigated antenna is robust against bending, yielding gain in the range 6.4–7.8 dBi. Results demonstrate the suitability of these fabric antennas for on-body wireless communications.

Design and development of flexible fabric antenna for body-worn applications and its performance study under flat and bent positions

Modern communication systems are driven by the concept of being connected anywhere and at any time.Fuelled by the idea of a body-centric approach to modern communication technology, many research projects have been initiated to integrate antennas and radio frequency (RF) systems into clothes with regard to size reduction and cost-effectiveness; so that the wearer may not notice the existence of these subsystems. For instance, the development of wearable computer systems has been very rapid and soon one will see a wide range of unobtrusive wearable and ubiquitous computing equipment into his/her everyday wear. Rapid progress in wireless communication promises to replace all wired-communication networks in the near future, in which antennas play a vital role. This paper describes design and development of a circular patch antenna for on-body wireless communications. This antenna makes use of polyester-combined cotton [65:35] fabric as its dielectric material and the conducting parts of the antenna are made up of copper. The impedance and radiation characteristics are determined experimentally when the antenna is kept in flat position. The performance characteristics of the antenna are analyzed under bent conditions too, to check compatibility with wearable applications. Results demonstrate the suitability of this kind of patch antenna for on-body wireless communications. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26188

Transmission power assignment with postural position inference for on-body wireless communication links

This article presents a novel transmission power assignment mechanism for on-body wireless links formed between severely energy-constrained wearable and implanted sensors. The key idea is to develop a measurement-based framework in which the postural position as it pertains to a given wireless link is first inferred based on the measured RF signal strength and packet drops. Then optimal power assignment is done by fitting those measurement results into a model describing the relationship between the assigned power and the resulting signal strength. A closed loop power control mechanism is then added for iterative convergence to the optimal power level as a response to both intra-and-inter posture body movements. This provides a practical paradigm for on-body power assignment, which cannot leverage the existing mechanisms in the literature that rely on localization, which is not realistic for on-body sensors. Extensive experimental results are provided to demonstrate the model building and algorithm performance on a prototype body area network. The proposed mechanism has also been compared with a number of other closed loop mechanisms and an experimental benchmark.

BAN-BAN Interference Rejection With Multiple Antennas at the Receiver

Rejection of interference from a nearby body area network (BAN) becomes significant in BAN applications, where a number of BANs are operating in the near vicinity of each other. Two interference rejection combining (IRC) techniques, optimum combining (OC) and Weiner-Hopf (WH) solution, are applied to the on-body wireless communication channels, and a more robust interference rejection technique, applicable to on-body channels, is proposed and compared to the conventional IRC algorithms. Three on-body channels are investigated by real time measurements of the desired and interference signals in an indoor environment with the antennas mounted on two human subjects walking around each other. The output signal to interference plus noise ratio improvement achieved with each algorithm is presented for the three channels and the dependence of the interference rejection gain on the average desired signal to interference ratio has been shown along with the covariance matrices for each channel. OC gives around 3-5 dB interference rejection gain for belt-wrist and belt-head channels. The WH solution gives the worst performance for all the channels. The proposed algorithm gives a flexible handle on the interference rejection gain and gives better rejection than the two conventional algorithms with proper interval selection.