Body centric wireless communication is now accepted as an important part of 4th generation mobile communications systems and will be part of the forthcoming convergence and personalization across the various domains, which include personal area networks, (PANs), and body area networks, (BANs). Advancements in the miniaturisation of wearable hardware, embedded software, digital signal processing, and biomedical engineering have made it practically possible for human to human networking incorporating wearable sensors and communications. This can be seen as a continuation of a trend spearheaded by the mobile phone, which, over the last few decades has become smaller and more convenient for personalized operation. Alongside this trend, there have been a number of body centric communication systems for specialized occupations such as paramedics and fire-fighters, as well as continuing interest for military personnel. Use for medical sensing and support, with either skin mounted sensors or implants, is also attracting much attention. To support these developments there has been considerable research into antennas and propagation for body centric communications systems, and this paper will summarise some of it. There is much interest in the characterisation of the channel on the body, and in the optimisation of antennas for these channels. Characterisation is happening at relatively low frequencies of a few MHz and also at microwave frequencies where higher data rates can be supported. Ultra wide bandwidth systems may also be advantageous on the body and both antenna design and measurement of path loss and dispersion effects is in progress. Conventional antennas such as the monopole, patch and PIFA are being examined for these applications to achieve low link loss in the face of extreme body geometry changes. Fabric based antennas also show exciting possibilities. Similar research is also being conducted on communications into medical implants where advanced antenna design and characterisation and modelling of the channel are important research needs. In all of these areas both measurement and simulation pose very different and challenging issues to be faced by the researcher. FDTD simulation of fields radiated by a planar inverted F antenna located on the chest, used to characterise on body channel performance (Simulated using XFDTD and 'Norman' full body voxel phantom and running under MPI on 8PC cluster. Frequency = 2.45 GHz, simulation time step (T) = 9.629 ps, time step between each plot = 11T, spatial cell size = 2.5 mm)

As the explosive growth of portable computing devices continues, the implementation of many short-range wireless technologies within these devices, such as Bluetooth, WiFi, etc., is becoming commonplace. The communication efficiency between these devices in various environments is of considerable interest in modeling link-loss and other parameters pertinent to communication channels. The objective of this paper is to investigate the on-body environment in the 0.5-5.5 GHz frequency range, with a view to developing an understanding of the behavior of antenna systems mounted directly on the human body. The setup for the investigation reported in this paper comprises a pair of transmit and receive antennas, located on a human torso, and separated by a distance of 25 cm (see Fig.1). A model of the torso has been simulated via the FDTD algorithm in the presence of two rectangular microstrip patch antennas operating at 2.4 GHz. These antennas were selected because of their conformal nature and ease of implementation with many current wireless portable devices. To model the human torso, a 3D CT image, consisting of 1 cubic millimeter voxels was obtained, and reconstructed in the FDTD domain mesh as a three-layer model, namely skin, fat, and muscle. Although the results presented herein are concerned with the transmission efficiency (see Fig.2 and 3) for intra-body communication of wireless systems, they can also be used to understand untoward coupling when the two antennas are transmitting to an external receiver.

A new method for the fast estimation of the propagation loss has been applied to predict propagation loss of antennas used in on-body communications. The technique is based on the equivalent source theorem, which states that any antenna can be replaced by a "black box" with surface currents on it. The method is applicable for a variety of propagation models but limited by a far field zone observation point. It is extremely useful for practical engineering system loss calculations and offers the fast prediction or comparison between different antennas and signals applied for on-body communication. A theoretical formulation has shown that the method can be applied for any kind of input signals including ultra wideband ones. The method offers reduction of simulation time and has been applied for a variety of signals. Having a library of antenna on-body "images", a library of signals and a library of different propagation scenarios allows the fast estimation and suitability of different antennas for any given application.

Good agreement has been obtained in comparison with full conventional FDTD modelling and measurements with simple antennas such as a dipole, a monopole and a patch antenna. Fig. 1a and 1b shows the first results obtained for a half-wavelength dipole and a monopole antenna placed over a simulated arm constructed from a dielectric slab of dimensions xx,yy and length zz, with ε_r = 38 and sigma = 1.5 S/m (these being close to those of human body with dry skin). The comparison has been made in the frequency domain and normalised to free-space propagation with identical path length. It is clearly seen that the vertically polarised monopole antenna is much more preferable than the horizontally polarised half-wavelength dipole placed in parallel with the dielectric surface.

A comparison of full wave solutions with measurements has shown that a relative error of a propagation loss prediction lies within the range 0.5 – 2.3 dB, so the level of error seen here is negligible. There is some discrepancy in a frequency characteristics prediction, but again this is small. The proposed method can offer an alternative to full electromagnetic models and can potentially be a powerful instrument for practical engineering and designs.

The importance of the individual soldier as an information link will increase in the modern warfare. The development trends for new systems emphasize the future soldier's role as a link in a local surveillance net and the importance of communication between soldiers and other units of the modern battle field including armoured vehicles and unmanned aerial vehicles (UAVs). Therefore, the soldier’s personal equipment should include head mounted displays, GPS, digital radio, video sight, etc. All these together will enhance the situation awareness. It is obvious that in these systems the textile antennas will play a paramount role in optimizing the system performance.

In wearable systems flat antenna surfaces cannot be provided in general. Therefore, antennas should properly function even if they are bent. The full paper will discuss the effects on input-matching and impedance bandwidth of three different textile antennas, namely, a conventional patch antenna, a dual-band antenna, and an EBG antenna, Fig. 1.

The test setup includes two plastic cylinders with diameters of 70 mm and 150 mm. These dimensions are typical for human body, e.g., arm, leg, and shoulder. Antennas are bent around the cylinder along two principal planes xz and yz (E- and H-planes, respectively) and the S11 is measured for comparison purposes. The results for all three antennas will be presented in the full paper. Here, Fig. 2. provides the results for the EBG antenna only. It can be observed the yz-plane bending has minimal effect on the input-matching and impedance bandwidth for the EBG antenna. This is due to the fact that only xz-plane bending and not yz-plane bending affects the antennas’ resonance length. The more the antenna is bent, i.e., around smaller diameter, the more resonance length is reduced, and thus it is shifted up. This is effect is more observable for conventional patch antenna. Similar trends will be shown for the dual-band antenna, Fig. 3 - 4. These results will be presented and discussed in detail in the full paper.

Fig. 1. Textile antenna prototype.

Fig. 2. Measured S11 results of EBG antenna bending.

Fig. 3. Textile dual-band antenna prototype.

Fig. 4. Measured S11 results of dual-band antenna bending.

Wireless body area networks (WBANs) are becoming an increasingly important part of the wireless communications system. Ultra Wideband (UWB) is a promising low-power communication technology for short-range scenarios which facilitates the compatibility of such technology in WBANs.

In this paper, we investigate the UWB body area propagation channel through an extensive on-body measurement campaign performed in an anechoic chamber. Frequency-domain measurements were done on the front part of the body (torso and abdomen) in the range from 2 to 12 GHz using an HP8510C® vector network analyzer (VNA) to measure the S21 transmission coefficient between two antennas placed at various positions on the body. The antennas used for all measurements are small (33 mm x 20 mm x 1.5 mm) low-profile omni-directional UWB antennas that have been recently designed in our laboratory. Details will be given in the full paper. These antennas have an input bandwidth of [3.9-11] GHz with respect to S11 < -10 dB at different positions on the body. The white boxes in Fig. 1 indicate the antennas positions. The antennas were moved on a kind of a grid in order to investigate six different distances between the antennas (d1 = 7.5 cm, d2 = 10.6 cm, d3 = 15 cm, d4 = 21.2 cm, d5 = 30 cm, and d6 = 42.4 cm) (Fig. 1).

The measurements cover all possible scenarios on the front part of the body. Twenty measurements at each distance at various positions on the body were taken. In this way, a total of 120 measurements were carried out. The antennas were placed 6 mm away from the body using a foam block (εr = 1.08).

Based on the measurement data the path loss was calculated. The distance between the antennas was measured along the surface of the body rather than along a strict straight line as the waves travel along the body rather than passing through it. In modeling the path loss, the following empirical decay law was used:

In addition to the path loss model for the front part of the body, for each measurement we calculated the attenuation integrated over a 0.4 ns interval chosen around the arrival time of the maximum amplitude of the first echo. The distribution of the calculated attenuation (Am) is then determined (Fig. 2).

Several distributions were fit to the resulting data including Lognormal, Weibull, Rayleigh, Exponential and Gamma distributions and it has been found that the lognormal distribution fits the best (Fig. 2).

Abstract

The fading experienced by a wireless personal area network (WPAN) moving in both anechoic and indoor multipath (open office area) conditions was analysed using first and second order statistics. Synchronous signal measurements were made for six self-contained bodyworn receivers communicating with a remote transmitter in each environment (Fig. 1). When compared to anechoic conditions, the mean received signal for the six channels increased by as much as 10 dB for mobile line of sight (LOS) and 13 dB for mobile non-LOS (NLOS) conditions in the open office area. Received signal dynamic range in the multipath environment increased by an average of 20 dB compared to the anechoic chamber for both LOS and NLOS scenarios confirming that multipath effects are an important consideration for WPAN systems.

Using maximum likelihood estimation and the Akaike information criterion [1], it was determined that Rician fading distributions provided optimum fits for the majority of (18 of 24) measured propagation channels, the remainder being described by Nakagami-m. The mean Rician-K_dB factor was 9.5 dB for both LOS and NLOS scenarios in the anechoic chamber. Fig. 2 shows the fitted theoretical and empirical cumulative density functions (CDFs) for the right wrist receiver in NLOS while mobile in the open office area. K-factors were vastly reduced for mobile channels in the open office area (mean K_dB = 1.7 dB). This decrease in K was due to an increase in the scattered power component (denominator) of the Rician-K factor, caused by indoor environmental factors contributing to a much more complex spatially distributed electromagnetic field pattern than that of the anechoic chamber.

Level crossing rates indicated that upper body positions, particularly the posterior shoulder region, experienced the most frequent changes in channel state in all environments for both LOS and NLOS. In the open office area, the back right (LOS) received signal level crossed the median signal level at a rate of 14.7 Hz (maximum) with the back left position undergoing transitions in a positive direction across the median at 14 Hz for corresponding NLOS maxima. Average fade durations (AFDs) below the median increased for all body locations in the open office area. The shortest fade duration in non-anechoic conditions occurred for the back left (NLOS) receiver (36.9 ms) while the maximum (50.3 ms) was observed by the back right device (NLOS).


Fig. 1 Bodyworn measurement system and environment. Fig.2 AFDs for Back Right Receiver (NLOS) Office Area.

[1] H. Akaike, "A new look at the statistical model identification," IEEE Transactions on Automatic Control, vol. 19, pp. 716-723, 1974.

Modern electronic medical implants have reached a high degree of complexity. This has increased the demands on the communication link with the implant, both regarding the bandwidth and the communication distance. A medical communication system at RF frequencies has been standardised, the Medical Implant Communication System (MICS), which use a frequency allocation of 402- 405 MHz. This frequency band is allocated for implant use both and the US and in the EU. The EIRP is limited to -16 dBm in order to reduce the interference to existing users of the same frequency band. This low EIRP makes it necessary to have reasonable effective antennas in the implants in order to get a benefit form the switch from the classical inductive link to RF.

The normal performance measures of antennas have to be modified when applied to implant antennas. The reflection coefficient S₁₁ and the VSWR are straightforward to use also in the implant case. But the gain definition is only valid in a lossless medium. When we place an antenna in an infinite medium of lossy matter the gain will be dependent on the origin of our coordinate system [1]. This is not a problem for the implanted antenna, as it is placed in a finite body, i.e. the patient. The implant and the body carrying it will act as one larger antenna, and will have a measureable gain according to the classic definition. The drawback is that the gain will depend heavily on the size and shape of the body, which makes it hard to give a generic value for the gain from a certain antenna.

The type of antenna and the amount of isolation around the antenna will influence the amount of nearfield losses, and thus the efficiency. There is a modification of the efficiency measure which solves the problem of the gain definition in an infinite lossy medium [2]. This efficiency measure is a candidate for a quality measure of implanted antennas. The efficiency of an antenna in an infinite lossy material is evaluated by calculating the integral of the Poynting vector over a closed surface in the far zone of the antenna. The full paper will include these calculations for a sample of antennas, and make comparisons with the simulated gain from the same antennas placed in a human phantom. Estimations of the validity of using the different measures as indications for radio link reliability will be given.

[1] R. K. Moore, "Effects of a surrounding conducting medium on antenna analysis", IEEE Trans. on Antennas and Propagation, Vol. 11, 1963
[2] A.Karllsson, "Physical limitations of antennas in a lossy medium", IEEE Trans. on Antennas and Propagation, Vol. 52, 2004

Wearable computing is a new, fast growing field in application-oriented research. Steadily progressing miniaturization in microelectronics along with other new technologies enables wearable computing to integrate functionality in clothing allowing entirely new applications. Integration in textiles ideally combines such requirements since clothing offers unobtrusiveness, a large area and body proximity. However, such electronic devices have to meet special requirements concerning wearability. For wireless connectivity of wearable/on-body devices ultra-wideband (UWB) technology is believed to be a favourable choice. UWB is an emerging wireless technology, recently approved by FCC for operation between 3.1 and 10.6 GHz. In low/medium data-rate applications, like wearable computing (or medical monitoring system), UWB offers possibilities of low-power operation and extremely low radiated power, thus being very attractive for body-worn, battery-operated devices. In this paper we present performance of UWB textile antennas (Fig.1a) in on-body communication scenarios. These antennas are made entirely of textiles. As a conductor we have used metalized textile with the surface resistivity of 0:1-=sq., which offer low ohmic losses. As the dielectric substrate, very thin (0.5mm) textile with dielectric constant 2.6. This textile dielectric was chosen due to a relatively high dielectric constant and a small thickness. Fig.1 Microsrip-fed textile UWB annular slot antenna: a) photograph, b) measured and simulated return loss characteristics. S1, S2 - two prototypes of this textile antenna. The last antenna we present is the UWB annular slot antenna, fed by a short microtrip line (Fig.1a). The radiating slot was realized with a use of two conductive layers. The planar antenna size is 30x30 mm2. Fig.1b presents comparison between measured and simulated RL characteristics in free space. All antennas have good input matching, measured and simulated results agree well. On-body communication performance of presented textile UWB antennas is analysed based on measured transfer function between two body-mounted antennas. Antennas are placed on different parts on the human body. Distance between the antennas and body is also varied.

In recent years, development of the information and communication devices such as cellular phones, personal digital assistants (PDAs), digital video cameras, pocket video games, etc. has been rapidly progressing. This evolution gives us many conveniences to our daily lives. In the near future, we will begin to attach these appliances to our body such as wearable computers that can be connected to internal and external network system, and we will meet the ubiquitous computing society. However, currently there is a little methods for these personal devices to exchange data directly. We want to exchange the data of the wearable devices without physical constraint like an external wire connection that may easily be tangled. The solution for networking these personal devices has been proposed as Personal Area Networks (PANs) which uses the human body as a transmission channel. Many studies have been made on the development of such devices so far, however, most of the researches have been conducted by researchers who just want to utilize the fact and practically a little physical mechanisms have been researched until recently.

Fig. 1 shows one of the communication systems of the PANs using a 10 MHz carrier frequency which was made by Sony Computer Science Laboratory Inc. When a user wearing the transmitter touches the electrode of the receiver, a transmission channel is formed using the human body. In this case, the receiver recognizes the user's ID and it can be personalized. This communication system uses the near field region of the electromagnetic wave generated by the device which is eventually coupled to the human body by electrodes.

It has been recognized by the developers of wearable devices that there are return path through the earth ground. However, little is known how the earth ground affects the signal transmission from the view point of the interaction between electromagnetic fields and the human body. Hence, in this paper, the authors propose some calculation models of the human body equipped with the wearable device by using the FDTD method to clarify the earth ground effect. Fig. 2 shows the realistic high resolution whole-body model of Japanese adult male with average height and weight. By putting these calculation models into the FDTD method, the author investigates the electric field distribution around the whole body standing on the earth ground.

Figs.3 (a) and (b) indicate electric field distribution around the human body in free space, and on the earth ground, respectively. The electric field of Fig. 3 (a) is quite similar to Fig. 3 (b) except for the area of foot. Most part of the electric field is concentrated to the left hand of the arm. Hence, this communication system has advantage for signal transmission just by simply touching the receiver. From these investigations, it can be concluded that the earth ground does not affect signal transmission except for foot region in close proximity to the earth ground.

This paper presents on-body characterisation of a compact planar UWB antenna. The path loss and small-scale parameters (specifically, mean excess delay and RMS delay spread) for various on-body positions with different antenna polarisations (vertical and horizontal) are discussed and analysed statistically.

Fig. 1(a) shows the geometry of the antenna used in the tests which are conducted in an anechoic chamber and its radiation patterns at 3GHz and 10GHz are presented in Fig. 1(b). Fig. 2 demonstrates the various receive antenna positions with respect to the transmit antenna mounted on the right waist area. The antenna is well-matched in the UWB band of 3-11GHz.

Different from what have been published in previous literatures, the effect of antenna polarisation on the path loss is studied. Fig. 3 shows the measured and modelled channel path loss for all the polarisations against logarithmic distance with reference to 1m in the 3-10GHz band, cases Rx1-Rx26. The exponent of the fitted line shows dense multipath environment characteristics although the measurement was performed in the anechoic chamber. This shows that the small size of the antenna causes reflections from human body and small items within the BAN environment to significantly influence the channel behaviour and also the feed method applied has major effect on obtained results. The path loss exponent level is around 1-1.2 with received power level decaying at about the same rate in the 4 scenarios. This proves the proposed antenna is relatively polarisation independent and could be used for on-body communications. Fig. 4 presents the mean excess delay and RMS delay spread of the measured impulse response between two antennas placed on the human body at different locations and for different polarisations. As discussed above, the delay analysis (small-scale) also proves that the channel exhibits dense multipath environment behaviour with a long delay time for the multipath component.