As demand for high data rate applications on the move continues to rise, cellular network and mobile hardware designers will continue to vigorously push the boundaries on the maximum rate at which information can be transmitted over wireless communications channels. One exciting possibility to supplement mobile communications is to use network users themselves as relays by employing cooperative communications. In this article we introduce the concept of a body-to-body network, where smart communicating devices carried or worn by a person are used to form a wireless network with devices situated on other nearby persons. By using body-to-body networks operating in the popular Industrial, Scientific and Medical bands, it will be possible to complement and extend existing infrastructure networks by supporting network capacity, improving data rates and promoting green spectrum usage. We present the results of some illustrative experiments that will help to quantify the impact of the human body and related movements on signal reception and their implications in body-centric system design.

Recent years have seen significant uptake of new so-called "smart phones" where the end user has access to a host of different functionality such as e-mail, web browser, media player, positioning information and on-board high definition (HD) video camera as well as the ability to make voice calls. New innovations in this area will see the form factor of smart devices being modified, so that they may be worn on the human body (for example, Sony's Nextep concept) or integrated into clothing, in the process creating a new generation of "smart" people. This explosion of intelligent technology and its associated high bandwidth requirements will place increased demands on mobile wireless networks. This high demand for data services is already being experienced in densely populated areas, such as sports venues, rail stations and airports. Network and service providers are currently trying to meet these needs through the introduction of long term evolution (LTE) and WiMAX technologies which aim to provide individual networks users with multi-megabit data rates. Another popular measure, which aims to alleviate the strain on cellular networks and extend the range of data services into difficult to reach and densely populated areas, is the use of WiFi hotspots.

While network operators are already planning for the fourth generation (4G) of cellular wireless standards to include the implementation of LTE Advanced capable networks, another as-of-yet unexplored possibility, which may help to sustain high data rates and extend the range of infrastructure networks, is to use the network users themselves as simplified ad hoc base stations. This will be achieved by creating vast body-to-body networks of interlinked wireless devices, carried, worn or integrated into clothing. These networks will allow data to be routed from person-to-person before being forwarded to the recipient or the relevant infrastructure network if necessary. In the creation of body-to-body networks there will be many issues that will impact the design of physical and medium access layers. In this article, we briefly discuss some of these issues before focussing on how the human body itself will affect these links.

Figure 1 Data communications between two cellular network users, persons A and B, who are (a) within the same network cell and (b) using a BBN.

Body-to-Body Network Concept

In its simplest form, a body-to-body network (BBN) is a type of mobile ad hoc network in which constituent wireless nodes are either carried or worn by people. By creating vast cooperative BBNs in densely populated areas, cellular network providers have opportunity to truly achieve "anytime, anywhere" connectivity and multi-megabit or even gigabit download rates (if using ultra short-range millimeter-wave communications¹). The term "cooperative" relates to the idea that all users of a BBN should contribute a nominal amount of their bandwidth to forward data relayed by other network users, thus acting in a cooperative manner for the greater benefit of the network. To illustrate the concept of a BBN and how it could be used to support mobile communications, consider the simple, but relatively common scenario depicted in Figure 1a. Here we have two cellular network users who wish to transfer data (for example, video, music or just social networking information) to one another within the same network cell. Using traditional cellular architecture, the data originating from person A would be routed through the local base station to person B. Now consider the identical communications process, whereby person A wishes to send data to person B who is still within the locality except this time, they will use a BBN to cooperatively relay the data. This scenario is shown in Figure 1b, where instead of uploading the data to the nearby base station, person A transmits the data over a much shorter distance to other BBN users in a local vicinity. The data is then routed through the BBN until it reaches the intended recipient, person B. Among the key benefits of body-to-body networking are that it provides the opportunity for both multicasting and multiplexing of the data. In the case of multicasting, multiple copies of the data are sent purposely to multiple recipients. In multiplexing, used, for example, in large file transfer (such as, HD video), the data may be broken into smaller components by the sender's smart device before taking separate network paths through the BBN and being reassembled by the recipient.

Antenna techniques like sectorization and channel access technologies, such as wideband code division multiple access (W-CDMA) and the orthogonal frequency-division multiple access (OFDMA) scheme used in LTE, promote frequency reuse within cellular networks. In BBNs, because signal propagation is likely to occur over distances of a few meters to a maximum of 100 meters, frequency reuse can be achieved over much shorter distances, in the process promoting so-called "green" spectrum usage. Furthermore, because BBN users are transmitting over much shorter distances, the power amplifier in the smart device's transmit chain used for body-to-body communications will not be required to operate with as high an output level, in the process saving valuable battery energy. As shown in Figure 1b, BBNs will complement existing infrastructure based networks by extending their coverage into difficult to reach places such as indoor environments, as it only takes one person within a BBN to be connected to a particular infrastructure network and this person (node) can act as a gateway for the whole BBN. In the long term, this may have the added benefit of helping to reduce controversial base station densities in areas like city centers.

Issues Affecting BBNs

Applications of body-to-body networking will extend well beyond the support of cellular and Wi-Fi networks as described above. They will also be used in short-range covert military applications,¹ first responder applications,² team sports and used to interconnect body area networks (BAN).³ In this respect, the wireless technology used to interconnect body-worn devices will ultimately depend on the application. For example, low data rate applications, such as positioning and health monitoring systems likely to be used by first responders, may utilize technologies like ZigBee⁴, which operates in unlicensed worldwide frequency allocations in the 868/915 MHz and 2.45 GHz Industrial, Scientific and Medical (ISM) bands. To complement cellular and other infrastructure networks, BBNs may operate using mobile ad hoc networking based on WiFi. The main reasoning and advantage here is that many smart devices already feature WiFi chipsets that could be used for communications.

Like any mobile ad hoc or mesh network, the use of BBNs to supplement cellular and other infrastructure networks will mean that communications are susceptible to increased latency due to the additional hops required to route traffic. Security will also be a major issue as routing between multiple nodes will increase the risk of unauthorized access and compromise sensitive data. This will add complexity to the medium access layer (MAC) and network management. At candidate ultra-high frequency (UHF) and microwave frequencies, hardware designed to operate in BBNs, such as antennas, will be subject to time varying electromagnetic interaction effects. These include near-field coupling, radiation pattern distortion and shifts in antenna impedance,^5,6 which may degrade the efficiency of the body-worn system and reduce signal reliability. Successful operation of physical layer (PHY) attributes, such as antennas, transceiver hardware and modulation schemes, will also be dependent upon a thorough knowledge of the radio propagation channel, which is discussed next.

Signal Propagation in BBNs

Body-to-body communication channels² found in BBNs differ significantly from conventional fixed-to-mobile or off-body channels,⁷ where the base station is fixed, typically elevated and often relatively free of local scattering. Figure 2 shows three of the main communication scenarios likely to be encountered in body-to-body networking. In these examples we have assumed that the antennas are positioned on each person's front central chest region although, in reality, communications equipment used for BBNs could be carried or worn on any region of the body. To illustrate the impact of some of these scenarios on narrowband signal reception in BBNs, we use the results of some representative experiments conducted at the ISM frequency of 2.45 GHz.⁸ In these experiments, a novel flexible patch antenna (see Figure 3) that was designed to be resonant on the human body with a peak gain in the off-body direction of +6.2 dBi and −10 dB bandwidth of 55 MHz (2.398 to 2.453 GHz) was placed at the front central chest region of two male test subjects using a small strip of Velcro® and without the use of a dielectric spacer. The measurement hardware used in these trials consisted of the body sensor node (BSN) platform developed by Imperial College London. The transceiver section of the node utilized a Texas Instruments CC2420,⁹ which has a linear dynamic operating range of approximately 100 dB, maximum transmit power of 0 dBm and a receive sensitivity of −95 dBm. A transmitter node attached to person A was configured to transmit a continuous wave signal with a power level of 0 dBm at 2.45 GHz. The receiver node was then attached to person B and was programmed to record the 8-bit received signal strength indicator (RSSI) obtained from the CC2420 every 16 ms. To limit the influence of environmental multipath and, hence, ensure the variations in the received signal level were due to the human body itself, the measurements were conducted in a low multipath outdoor environment.⁸

Scenario A (LOS)

If we initially ignore environmental multipath, scenario A illustrates the situation where two persons face each other with an antenna positioned on the front of the body. In this case, signal propagation will be predominantly via line of sight (LOS) through free space propagation, with some supplementary multipath contributions caused by scattering and reflection from the body surface. Because of this, it is necessary to add two extra parameters to the popular log-distance path loss model commonly used by wireless engineers to estimate signal loss over distance. These two parameters account for body shadowing (X_BS), slower physiological processes and small movements (X_SM), such as, rapid fluctuations in the signal due to much smaller changes in body posture akin to small scale fading. Examples of slower physiological processes are respiration and biomechanical actions, such as movement of the limbs. In Equation 1, P₀ (dB) is the path loss measured at a reference distance (in these experiments, 1 m), n is the path loss exponent, d is the distance between the transmit and receive antennas and d₀ is the reference distance.

Figure 4 shows the measured received signal power as person A walked in a straigh line toward person B from a position 15 m away. To calculate the estimated received signal power at a particular distance in person A's journey toward person B, time was translated to distance using the estimate of person A's walking speed (∼0.88 m/s). Using Equation 1 and converting the measured received signal power to path loss, the exponent n and path loss at the reference distance were calculated and found to be n = 2.9 and P₀ = 29.6 dB.

Similar to shadow fading in mobile communications channels, the X_BS component of the received signal is assumed to follow a lognormal distribution. Lognormal random variables can be viewed as the result of a number of multiplicative factors that become additive under logarithmic transformation. The µ and σ parameters of the lognormal PDF most likely to have generated the X_BS component of the signal were found to be µ = 0 and σ = 0.2. In mobile communications channels, it is often assumed that the short-term fading component of the received signal is subject to Rayleigh fading. In Rayleigh fading, the received signal is viewed as the resultant of a large number of scattered signal components arriving at the receiver, each with random amplitude and uniform phase. In body-to-body communication channels, where a dominant signal component (LOS or strong on-body reflection) may exist, it will be more appropriate to model the received signal as exhibiting Ricean fading,¹⁰ which assumes a scattered signal component alongside a single signal component, which dominates over the scattered signal. An important figure, associated with Ricean fading is the Ricean-K factor. The Ricean-K factor is used to characterize the degree of fading in a wireless channel and is defined as the ratio of the square of the dominant component (A²) to the average scattered power (2s²), such as, K = A²/2s². When K → 0 and hence the dominant component A decreases, the fading becomes closer to Rayleigh fading, and as K → ∞, the channel no longer exhibits fading. The Ricean-K factor for this scenario was extremely large (K ∼ 200), which shows that under direct LOS conditions and in body-to-body communications channels, there is very little fading due to small body movements.

Scenario B (NLOS)

At UHF and microwave frequencies, the human body acts as an inhomogeneous interfering object. Thus, as shown in scenario B, when one of the persons turns so that his body obstructs the main LOS signal path, the communications link becomes dependent on diffracted, reflected, scattered and trapped surface wave components. As shown in Figure 2, this complex propagation scenario creates a signal shadowing region at the front of the person, a mechanism commonly referred to as human body shadowing.¹¹ Additionally, when the desired signal falls below the noise threshold of the receiver, complete shadowing will occur.

Figure 5 shows the received signal power as person A walked in NLOS from a distance of 1 m from person B to a point 15 m away. One issue that became evident from this scenario, which will have implications for the future design of hardware to be used in BBNs, is the dynamic range required for operation. Even though the receiver section of the CC2420 has a linear dynamic operating range of approximately 100 dB, for scenarios where one body shadows the direct LOS path and when the straight line distance between the two persons exceeded 6 m, the received signal power regularly entered the region beyond the noise threshold of the receiver. Because of this, only the first 6 s of this scenario were used for this analysis (inset shown in Figure 5). While it could be argued that a more omni-directional antenna may help to sustain the link in this scenario, the subsequent reduction in antenna gain may reduce the distance over which the hardware could effectively operate.Equivalently, if an extra gain stage is introduced to the transmit and receive chains, it will significantly reduce battery life or increase the size of the device if a greater capacity battery is used. Using the same procedure for treating the data as in scenario A, the parameter estimates for the path loss in this scenario were n = 1.5 and P₀ = 79.6 dB. It can be seen quite clearly that for body-to-body signal propagation in which one person's body completely shadows the direct LOS an extra 50 dB signal attenuation occurs compared to the LOS case. This undesirable consequence of body shadowing is further exacerbated by an increase in the spread of the X_BS component of the received signal (μ = 0.02 and σ = 0.28). Additionally, the Ricean-K factor of the X_SM component was significantly reduced (K ∼ 18) although this value infers that a dominant signal component still exists.

Scenario C (Dual-NLOS)

The situation depicted in scenario C, where both persons' bodies now obscure the main LOS, may be viewed as one of the most complicated signal propagation scenarios to analyze and indeed compensate for in BBNs. To examine this scenario, persons A and B stood with an initial at a separation distance of 5 m in an outdoor environment. They then proceeded to walk toward each other, meeting at the 2.5 m point before turning and walking back to their starting positions. What is immediately obvious from Figure 6 is that, even at very short separation distances, the communications link in a BBN will be heavily susceptible to dual-body shadowing events. This can be seen from the time series at approximately 4 to 6 seconds as the two persons begin to turn, the received power level immediately begins to deteriorate, eventually dropping below the noise threshold of the BSN node when the persons finally enter dual NLOS channel conditions. Therefore, not only will wireless hardware designed to operate in BBNs have to contend with significant variations in received signal levels, but protocols will have to be resilient to extended periods of outage and have the ability to readily reroute communications through other nearby BBN users.

Mitigation of Body Shadowing and Fading

Antennas: Antennas designed to operate in body centric communications systems may be broadly categorized as on- or off-body radiators, according to their radiation pattern characteristics when mounted on the human body. As shown in Figure 3, the patch antenna used for the examples described above was designed to provide maximum gain in an off-body direction. This will be advantageous in BBNs when it is desired to have communications that occur over extended distances. However, as shown for scenarios B and C, because of the relatively directive radiation characteristics, this type of antenna can suffer from the effects of body shadowing. On-body antennas, because of their application space, are typically designed to maximize electromagnetic radiation across the body surface and minimize radiation in an off-body direction. While this may seem counterintuitive for body-to-body applications, an on-body mode of propagation may prove advantageous in dual NLOS situations as electromagnetic waves will be forced out and around the side (toward the back) of the human body using trapped surface and diffracted wave modes of propagation. Thus, new hybrid on/off body antennas, such as the one proposed in Scanlon and Chandran,¹² which have the ability to electronically switch between on- and off-body modes may prove indispensible for future BBN communications, especially in dual NLOS channels where there is insufficient environmental multipath available to sustain the link.

Spatial Diversity: Another possible method of improving PHY layer performance may be to use diversity combining based on multiple, spatially separated antennas. Distributing antennas in smart garments will facilitate greater spacing, reducing negative effects associated with diversity arrangements in mobile handsets such as undesired correlation and mutual coupling. For example, consider the highly demanding application of first responders attending an emergency within a building.² Using the distributed antenna system shown in Figure 7, operating at the ISM frequency of 2.45 GHz with a maximal ratio combining scheme, up to 8.69 dB diversity gain can be made available at 90 percent signal reliability when four spatially separated antennas are used at the receiver. Empirical cumulative distribution functions (CDF) are shown for the right-head positioned antenna, four branch selection combining (SC), maximal ratio combining (MRC) and equal gain combining (EGC). Note that all CDFs are relative to the mean of the branch with the highest mean signal level (right-head). By using multiple antennas at both ends of a body-to-body communications link, it will then become possible to use multiple-input multiple-output (MIMO) communications to exploit body shadowing to provide independent signal paths, which can be used to increase throughput.

Future of BBNs

In this article we have introduced the concept of a BBN, and how these new networks could be used to complement current cellular and other infrastructure networks by supporting data transmissions and extending their operating ranges. While there will be many design challenges at the PHY and MAC layers, using the ISM frequency of 2.45 GHz likely to be used by the first generation of BBN applications, we have shown some of the effects of human body movement and orientation on received signal characteristics. Most notably, the communications link between two wearable devices in a BBN will be particularly affected when either one or both persons' bodies obstruct the main LOS path.
Mitigation of body shadowing will most likely come in the form of innovative antenna design and multiple antenna systems, such as diversity and MIMO, made possible by the fabrication of fashionable smart garments. Recent advancements in millimeter-wave technology mean that it will soon be feasible to use ISM frequencies in the 59 to 66 GHz range to provide high bandwidth capabilities for a range of BBN applications. The propagation characteristics at this part of the spectrum will provide many benefits for short-range body-to-body communications, especially in densely populated areas. For example, the higher path loss, when compared to the microwave region of the radio spectrum, will mean BBN users will benefit from even better frequency reuse due to much shorter hops as well as the potential of achieving data rates in excess of 2 Gb/s. Furthermore, the short signal wavelength (λ ≈ 5 mm), will also support the development of truly miniaturized devices that will be ideal for wearing on the human body or integration into clothing.

Acknowledgments

The authors would like to acknowledge the Royal Academy of Engineering and the UK Engineering and Physical Sciences Research Council (EPSRC) for their kind support of this work through grant reference EP/H044191/1. They would also like to thank Professors G.Z. Yang and Dr. B. Lo from Imperial College London, Dr. S. Drawer from UK Sport and Dr. R. Armitage from Adidas Wearable Electronic Systems for supporting the work under the ESPRIT project, which is also funded by EPSRC. We are also grateful to Dr. A. McKernan of ACT Wireless Ltd. for his help with the experimental work presented in this article.

References

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Simon L. Cotton received a bachelor of engineering degree in electronics and software from the University of Ulster, Ulster, UK, in 2004 and a doctorate in electrical and electronic engineering from the Queen's University of Belfast, Belfast, UK, in 2007. Cotton has worked as a Research Fellow at Queen's University Belfast, where he has investigated mobile ad hoc networking of dismounted combat personnel and low power personnel and asset tracking using RFID. He is also a Cofounder and the Chief Technology Officer at ACT Wireless Ltd. Among Cotton's current research interests are millimeter-wave technologies for personal communications and novel applications of short-range radio systems, including body-to-body networking and vehicular communications. His other research interests include radio channel characterization and modeling for wireless body and personal area networks, measurements for transceiver diversity in body-worn applications and simulation of wireless channels. He has authored and co-authored more than 46 publications in major IEEE/IET journals and refereed international conferences, one book chapter and two patents.

William G. Scanlon received a bachelor's degree in electrical engineering and the doctorate degree in electronics (specializing in wearable and implanted antennas) from the University of Ulster, UK in 1994 and 1997, respectively. He was appointed as Lecturer at the University of Ulster in 1998, Senior Lecturer and Full Professor at Queen's University of Belfast (UK) in 2002 and 2008, respectively. He is Director of Research for the Digital Communications Cluster at Queen's and he holds a part-time Chair in Short Range Radio at the University of Twente, The Netherlands. Prior to starting his academic career, he had 10 years of industrial experience, having worked as a Senior RF Engineer for Nortel Networks, as a Project Engineer with Siemens and as a Lighting Engineer with GEC-Osram. His research interests include personal and body-centric communications, wearable antennas, RF and microwave propagation, channel modeling and characterization, wireless networking and protocols and wireless networked control systems. He has published more than 175 technical papers in major IEEE/IET journals and in refereed international conferences. He was a founding Director of WirelessLAB (Ireland) and is a founding Director and Chief Scientist of ACT Wireless Ltd., a member of the IEEE International Committee on Electromagnetic Safety (ICES) and the IASTED International Committee on Telecommunications.