International Journal of Scientific & Engineering Research, Volume 4, Issue 8, August 2013 341

ISSN 2229-5518

Advancement in Ultra Wideband Antennas for

Wearable Applications

Narbada Prasad Gupta, Dr. Ranjan Maheshwari, Dr. Mithilesh Kumar

Abstract- There has been a tremendous rumble in UW B research after the U.S. Federal Communications Commission (FCC) allocated a frequency range with a bandwidth of 7.5 GHz from 3.1–10.6 GHz for UWB applications. Ultra wideband (UW B) technology was earlier limited to special applications, primarily in the military area and there primarily to electronic warfare. The antennas were usually horn antennas or modified, ridged horn antennas, sometimes in combination with reflectors. Wearable intelligent textile system is an innovative fast growing field in application oriented field. In recent years, body-centric wireless communication becomes an important part of fourth generation mobile communication systems (4G). Utilization of wearable textiles in the antenna segment has been seen on the rise due to the recent miniaturization of wireless devices. Wearable and fabric-based antennas have become one of the dominant research topics in antennas for body-centric communications. The review presented here intended to disclose the unconventional antenna technology including UW B technology for wearable applications. Contributions by various researchers have been compiled keeping in mind background of UW B technology, the wearable antenna, and specification of the antenna, material for the antenna and analysis that must be done to design proper UW B wearable antennas for various applications.

Index TermsBody-centric communications, UW B, UWB wearable antenna, Wearable Antenna, 4G.

—————————— ——————————

1 INTRODUCTION

NTENNA becomes a part of electrical devices in wireless communication system after late 1888;
Heinrich Hertz (1857–1894) were first demonstrated the existence of radio waves [1]. The UWB
technology opens new door for wireless communication system, since the current wireless system increasing exponentially. The appearance of wearable intelligent textile systems revealed the necessity for WBAN (Wireless Body Area Network) system to provide standalone outfit in recent years. This network enables wearable computer devices to interact with each other and exchange digital information using the electrical conductivity of the human body as a data network [2]. While talking about wireless body area networks, suddenly it comes into mind that how the signals would be communicated? Well, an antenna, which is a fundamental part of the network, is the answer for this. Textile antenna is one of the most fascinating and cutting edge research areas of modern era. It provides a wearable interface between human and the machine. Since we are talking about wearable antennas, it is necessary to mention here that antennas for such applications should possess certain properties like light weight, conformal design, low cost, easy system integrable etc. So the design should be such that antennas performance is not deteriorated even if they are bent. UWB radio applications has gain much attention to wireless personal area networks, which address short-range, ad-hoc, and high-rate connectivity among portable electronic devices.
The Federal Communication Commission (FCC) releases
ultra-wide band (UWB) from 3.1GHz-10.6 GHz in 2002 for the
use of indoor and hand-held systems [3]. Since then UWB
antennas have gained enormous attention in both academia and industry for applications in wireless transmission systems. Figure-1 shows spectral mask for UWB applications specified by FCC.

Fig.1: FCC spectral mask for Indoor UW B Application

In Impulse-Ultra wideband (I-UWB) technology a very low power pulse is sent whose power is always kept below the noise threshold and also this system does not uses any carrier for transmission.
Generation of UWB pulse has been described in [4]. The
transmission bandwidth for such system is more than 20% of
its center frequency (>500 MHz) [5], [6]. In short, the antennas to be designed for UWB systems should have sufficiently broad operating bandwidth for impedance matching and high-gain radiation in desired directions. The UWB spectral mask was defined to allow a spectral density of -41.3 dBm / MHz throughout the UWB frequency band.

2 UWB REQUIREMENT AND SPECIFICATION

Various studies have been devoted to evaluating the performance of UWB antennas (Agrawal et al., 1998; Amman
& Chen, 2003; Hertel & Smith, 2003; Klemm et al., 2005; MaTG & Jeng, 2005).
The UWB antenna must achieve almost a decade of impedance bandwidth, spanning 7.5 GHz.
The UWB system must provide:

Broad operating bandwidths for impedance matching,

High gain transmissions in the desired direction,

Stable transmission patterns and gains,

Consistent group delays,

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High transmission efficiency, and

Low profiles.

The design considerations of the UWB antennas and source
pulses are based on investigating S parameters, transfer functions, systems efficiency, group delay and fidelity.

3 DESIGN PARAMETERS FOR UWB WEARABLE ANTENNA

There are 14 parameters to be considered while designing the conventional antenna (radiation diagram, directivity, gain, equivalent receiving area, diffraction area, input impedance, radiation resistance, equivalent height, bandwidth, polarization, front-back ratio, antenna ratio, thermal noise, efficiency). When we talk about design of UWB antenna for wearable applications certain parameter need to be calculated in addition to conventional parameters like transfer function, group delay etc.
For wearable antennas integrated into clothing Microstrip
antennas have been preferred among the candidates [7], [8]. In such uses Microstrip antennas with flexible conductors and substrates are needed and this has led to an increased demand for electrical technical textiles (ETTs). Microstrip antennas [9] have some significant advantages for on-body wearable applications the three major ones being their ease of construction, their cost effectiveness and an associated metallic ground plane that when used between the body and the radiating elements can significantly reduce energy absorbed by the body. But Microstrip antennas are low bandwidth and relatively large in size.
Typically for flexible and wearable antennas the substrate materials chosen have been textiles or plastics. Textiles tend to low relative permittivity (<2) and suffer somewhat from trapped air which may have variable electrical characteristics due to water content. Plastics, for example polypropylene, are not well suited to wearing close to the skin and also have relatively low permittivity (<2). Neoprene [10] is a material commonly seen in scuba diving suits but more recently in sporting attire. It is durable, has good thermal properties and is generally consistent in density. It also has a permittivity greater than 4. It is a good choice for wearable antennas. Liquid crystal polymer (LCP) is an emerging candidate for its flexible, light weight, low loss factor and low cost characteristics [11]. LCP is a recyclable organic substrate with a uniform relative dielectric constant of 2.9-3.1 over the entire radio frequency (RF). It has an extremely low water absorption factor of 0.004 and low dissipation factor of 0.002, which make LCP an ideal choice for circuits operating in different conditions and environments. UWB antennas for body-centric wireless communications have been presented extensively in the open literature [12]–[15]. But very few people have discussed about conformal antennas, especially about the bending effect of antenna for UWB body centric communications. In [14] discussion about the bending effect of single band meander antenna on LCP is made at 5 GHz. Effect of bending on UWB antenna performance using a
copper film and a AgHT-8 film as substrate is presented in
[15].
UWB antennas should be effective in transmitting, compact,
non dispersive, and have a good wide impedance bandwidth properties, these features are desirable for both indoor and outdoor hand-held UWB applications. To satisfy such requirements, various types of planar monopole antennas have been developed for UWB communications over the last few years. Also, variety of bandwidth enhancement techniques have been reported, to improve the impedance bandwidth of these antennas, such as the use of the multiple feeds techniques [16], beveling radiating element [17], a beveling ground pattern [18], offset feeding techniques [19], feed gap optimization [20], ground plane shaping [21], a notched ground plane [22]-[24] and so on.
Following section provides the detail about development of
various techniques proposed by different authors during last
few years. Various designs & theirs implementation for
different applications have been compiled. All these are
presented to show that how UWB technology is being taken up by various researchers and what are the future scope of work in this technology and its implementation.

4 DEVELOPMENT OF UWB WEARABLE ANTENNAS

During early 1930s many wide band antennas had already been reported. All were Non-planar wideband antennas. Later, various antennas at different frequency bands like UHF, VHF etc. were introduced having planar designs and suitable for integration with ICs. Various Ultra wideband antennas have been categorized in four forms based on four corners of Figure-2
1. Multiband, Omni UWB Antennas
2. Multiband, Directional UWB Antennas
3. Omni, Impulse Radiating UWB Antennas
4. Directive, Impulse Radiating UWB Antennas

Details of the category can be found in [25].

Fig-2: UW B antenna classification according to directivity and signal radiation characteristics

During last few years researchers have been working on various aspects in wearable antenna designs. Some researchers have proposed flexible antennas which can be easily integrated into clothing [10], [13], [26]-[29]. In the year
2007, B. Sanz-Izquierdo proposed a novel “Compact UWB
Wearable Antenna”, suitable for wireless on body
applications. In this paper Circular metal nail were employed

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in some buttons to create the radiating element of the antenna. Proposed antenna was intended for textile jackets where the button structure could be easily camouflaged and disregarded as a plastic button. Although the proposed antenna had good return loss characteristics, good radiation pattern but when employed practically the radiation pattern and matching deteriorated. The biggest challenge in designing UWB antenna is achieving the wide impedance bandwidth while maintaining high radiation efficiency. Another requirement is linear phase for optimal wave reception, which would correspond to a near constant group delay [30]. In [30], a novel optimization method for the design of antennas for UWB wireless communication systems was presented. The optimization was based on the time domain characteristics of the antenna. The correlation factors obtained for several angles were used as the optimization goals in the optimization procedure. The optimization of the antenna was performed by genetic algorithm (GA) search concepts. Generation of short electromagnetic pulse has been a challenge to research community. Before, Mithilesh kumar, Ananjan Basu, S, K. Koul proposed a new technique for the generation of such pulse [4], there were only a few practical circuits proposed for the same [31]-[35]. In the conventional methods of pulse generation, when ultra-short pulses are to be generated, they have to be generated after passing pulses through a differentiator and wave shaping circuits. But, it has been shown that they are distorted in amplitude because such pulses are highly sensitive to rise and fall time [4]. The authors in this paper proposed The HEMT-based circuit which gave the simplest circuit compatible with MMIC. Figure-4 provides the circuit proposed by the authors.

In the year 2010, a compact and conformal UWB antenna for wearable application was proposed by Md. Hasanuzzaman Sagor [37]. Proposed design, antenna and results are given in Figure-4. In this paper authors demonstrated the possibility of using LCP as emerging candidate for wearable antennas to overcome both frequency and time domain performance deterioration during bending because of subject postures and motion.

Fig. 4: Compact UW B antenna proposed by [37]

In this year only, Mai A. R. Osman [38] presented a textile antenna as shown Figure-5.

Fig. 3. UW B pulse generation circuit proposed by [4]

In the year 2010, an review on UWB antennas were presented by Qammer H. Abbasi, which was published in Proceedings of 2010 IEEE International Conference on Ultra-Wideband (ICUWB2010) [36]. In this paper various section included UWB antennas to be deployed in body-centric communications, overview of UWB on body measurement techniques and characterization of UWB on-body channels, UWB on-body channel characterization using numerical modeling, UWB on-body system modeling. The authors had given an exhaustive review on UWB antennas.
In this paper, recent advancements have been compiled,
which provide the work proposed in various areas of UWB
deployment especially in and after year 2010.

Fig. 5 UW B Wearable Textile Antenna prototype

&geometry in [38]

Radius of the radiating element was calculated by
87.94

a =
fr √εr
Where a is the radius of the circular patch antenna in
millimeter. Other parameters have usual meanings. The
radiating element with radius of 15 mm and a rotated square
slot of 6 mm x 6 mm at the centre of the circular patch was used in the paper.

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For design and tuning of UWB antennas full wave EM simulation based tuning is the only available method for different types. Unfortunately, direct EM-based optimization is normally impractical because of its high computational cost. Various metaheuristic approaches such as genetic algorithms or particle swarm optimizers, while being able to handle some issues of EM-based design (e.g., multiple local optima), are characterized by huge computational overhead. Surrogate-based optimization techniques such as space mapping or simulation-based tuning are more efficient
computationally. However, they are not particularly suitable
for antenna design. Space mapping normally requires a fast underlying coarse model, typically a circuit equivalent. Unfortunately, reliable circuit models are not available for many UWB antennas. On the other hand, tuning cannot be directly applied to a radiating structure if strong EM coupling between the structure’s parts exists; also, it is not clear how the antenna structure can be “cut” to allow the insertion of the tuning ports and physically relevant tuning components. The most common approach to EM-simulation-based design optimization is through repetitive parameter sweeps guided by engineering experience. While this approach is definitely working, it is tedious, time consuming, and it does not guarantee optimal results [39]. An efficient simulation-driven design of UWB antennas was presented in the paper.
Year 2011 and 2012 have been very important as far as
various UWB antenna design are concerned. Many
researchers proposed various dual band and multi band
antennas [40]-[42] for different applications. Design and
results are given in Figure 6,7 &8.

Fig. 8 Return loss for different gap g [41]


A new simple CPW-fed monopole antenna for UWB applications was proposed by Mohsen Koohestani and his mates in the year 2011 [43], a dome-topped, bowl-shaped radiating element and a 50 Ω coplanar feed line for excitation was used in the proposed design as shown in Figure-9:

Fig-9 Coplanar-fed Ultra-W ideband monopole antenna by [43]

Fig. 6 M-shaped patch antenna proposed by [40]

Fig. 7 Dual band Fork-shaped patch antenna proposed by [41]

A simplified geometry of a human limb was found to be accurate enough for use in input reflection coefficient simulations. The limb tissues were modeled as frequency- dependent materials.
Year 2012 has been the most promising year for design and characterization of UWB antennas for wearable applications. Many authors have tried and proposed a variety of shapes and design for such applications. Md. Shaad Mahmud, Shuvashis Dey [44] proposed compact ultra wide band logo- type textile antenna structure. It was a compact antenna available at that time. Presented antenna was designed with substrate material of permittivity, εr = 3 and thickness 1.6 mm which consist of four letters, AIUB. Hypalon coated Dacron fabric textile material was used as the antenna substrate and woven copper thread was used as the conductive part of the antenna patch. This antenna had two sets of resonance frequencies, first band from 1.2985 GHz to 1.986 GHz and second band from 3.678 GHz to 21.167 GHz. Definitely, the antenna fulfills the UWB band requirement but it doesn’t fulfill it completely, since the band has to be confined within
3.1 to 10.6 GHz. So, such a design may create interference
with existing standards.
There has been a tremendous increase in interest of
researchers to create notch in the characteristics of UWB
antenna so that various interference effects may be prevented

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among UWB and existing standards. Many authors worked on this issue and proposed their findings [45]-[53].
Shilpa Jangid and Mithilesh Kumar proposed a Novel UWB Band Notched Rectangular Patch Antenna with Square slot [45]. FR-4 substrate with dielectric constant of 4.4 and thickness of 1.6 mm was used to design the antenna. The proposed antenna had a compact structure with total size of
15×14.5 mm2 as shown in figure-11. The band-notch
characteristic in the UWB band to avoid interferences by WLAN (5.15–5.825 GHz) and WiMAX (5.25–5.85 GHz) systems was proposed by the authors. Also, band-notched filtering properties in the 5.15- 5.825 GHz were achieved by rectangular slot on the radiating element.

antenna (PIFA) with experimental results was presented. It consists of a planar rectangular monopole top-loaded with a rectangular patch attached to two rectangular plates, one shorted to the ground and the other suspended, both placed at the optimum distance on each side of the planar monopole. The fabricated antenna prototype had a measured impedance bandwidth of 125%, covering 3 to 13GHz for reflection coefficient better than 10 dB. The radiator size was 20×10×7.5 mm3, making it electrically small over most of the band and suitable for incorporation in mobile devices.

Fig-11 Proposed antenna with return loss characteristics by [64]

Fig. 10 Antenna and its return loss characteristics proposed by [45]

May be due to manufacturing or otherwise the proposed structure is not showing proper characteristics throughout the UWB band as shown in figure-10.
Some authors have presented reconfigurable antennas for wearable application [52], [54]. In [54], the radiating element was created by stitching the conductive thread instead of the copper metallization. The ground plane was created by using a Liquid Crystal Polymer (LCP) material (εr=2.9). The substrate of the designed antenna was made from jeans fabric (εr=1.7) while the radiating element was made from stainless steel coated conductive thread. By changing the shaped ground, the direction of the radiation pattern was reconfigured. Some has proposed multiple band, multi polarized structures [55], [56], multiple sector structures [57]. Recently, various authors have concentrated on changing the feed network for UWB antenna structures [58]-[63].
In January 2013, Chan Hwang See with his fellow researchers
proposed Ultra-Wideband Modified Planar Inverted-F
Antenna [64]. A miniaturized modified planar inverted-F
In the proposed model different impedance bandwidth enhancing techniques were combined together to enable coverage of the entire FCC UWB frequency band. All of the antennas presented before were very sensitive to variation of ground plane size in the lower part of the usable operating band and this is due to significant contribution of the ground plane resonance over these bands. This creates problems as the ground plane behavior will be heavily influenced by a human hand holding a mobile terminal. So, in the presented design dependability of ground on antenna characteristics by combining top-loading, off-centre rectangular plate feeding and a shorting wall techniques of impedance matching. This was the unique feature in this paper.
A unique Hybrid UHF/UWB Antenna for Passive Indoor
Identification and Localization Systems has been proposed by
Catarina C. Cruz and his fellows [65] for simultaneous
identification and centimetre-resolution localization of
multiple targets in indoor environments. A new planar antenna for hybrid passive tag systems was proposed, which operates both in the UHF-RFID band and in the FCC UWB band. The co-designed UHF and UWB antenna elements were printed back-to-back on each side of a common substrate (RT

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Duroid 5880 εr = 2.2 & loss tangent = 0.0009) as shown in figure-13 with appropriate topology for future integration with a single UHF-UWB RFID chip.
Fig-12: Hybrid RFID-UWB antenna proposed by [65]
The UHF element based on a meandered line plus loop and the UWB based on modified version of the Crossed Exponential Tapered Slot antenna (XETS) proposed in [66], [67] was used in the design proposed. The hybrid antenna presented in the paper is appropriate for potential systems that combine identification and centimeter-class indoor localization using passive and therefore very low cost technology. This type of solution does not exist in current commercial systems and encourages the emergence of new applications. But these hybrid chips do not yet exist commercially, so it can be proposed to adopt such hybrid systems for future work and research.
Recently, it has been demonstrated by various authors that fractal geometries based on space filling and self-similarity attributes can be useful to improve the performance of antenna [68]. Also, fractal-based antennas can effectively couple energy to free space [69]–[71]. In addition, different feeding methodologies can be applied on fractal antennas without degrading their performance like microstrip lines [72] and coplanar waveguide (CPW) [73]. CPW transmission- line feed method these days is becoming popular because of lower loss, low radiation leakage and more convenience with shunt and series connection on the same side of substrate avoiding via holes [74]. Latest design and analysis of UWB CPW-Fed Fractal Patch Antenna With Band-Notched Function Employing Folded T-Shaped Element has been presented in [75].

5 CONCLUSION

Generations of the UWB antennas for wearable application development have been presented. Recent advancements have been characterized by the failure of successive generations of antenna designers to benefit from the lessons of earlier pioneers. It is necessary to be familiar with the history of the antenna art for any serious antenna designer who aims to implement novel designs rather than recreate the antennas of previous generations. The paper presents recent trend in UWB technology especially for wearable applications.

ACKNOWLEDGMENT

Narbada Prasad Gupta would like to acknowledge the support extended by The Director, Ideal Institute of Technology, Ghaziabad for his continuous encouragement and support during the work done.

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Narbada Prasad Gupta is working with Ideal Institute of Technology, Ghaziabad (UP) since 2007 and currently pursuing his PhD from RTU, Kota. He obtained his M. Tech (Digital Communication) degree from MANIT, Bhopal in
2007. His research areas include RF Antennas for Ultra Wide
Band, Analog & Digital Communication.
Dr. Ranjan Maheshwari is Professor & Head at UCE, RTU, Kota Rajasthan. He obtained his ME and PhD degree from IIT, Roorkee, India. His pioneer work on “Physiological manifestations under venom interactions” has led to award of PhD degree to him. He has been working for the rescue of snakes and snakebite cases for more than 15 years in Kota city of Rajasthan. He has published many research papers, news paper articles and delivered many lectures on this topic.
Dr. Mithilesh Kumar received the BE (1992) degree in Electronics & Communication Engineering from Govt. Engineering College, Kota, University of Rajasthan, M. Tech (1998-99) degree in Communication and Radar Engineering and PhD (2011) degree in RF Communication Engineering from IIT, Delhi, India. He is Professor at RTU, Kota. His current research interests include RF Communication, UWB Active Antenna, Microstrip Antenna and Microwave Device Designing Technology.

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