# A Comparative Study of On-Body Radio-Frequency Links in the 420 MHz-2.4 GHz Range
## 1 Introduction  
Interest in Wireless Body Area Networks (WBANs) has grown in the last decade because of their potential applications in several wearable applications, predominantly in health applications [1, 2]. Previous research of WBANs led to the proposal of a standard for wireless communication (IEEE 802.15.6) [3], while other research focused specifically on different layers of the WBAN stack [4]. WBANs are faced with several challenges, including variable coverage due to dynamic aspects of the channels during movement [5], limited energy availability in wearable nodes [6], bandwidth requirements imposed by the applications [7], and desired flexibility in WBAN architecture [8]. These challenges are quite prominent in the physical layer. Propagation in WBANs is not only dependent on the environment, as in conventional wireless networks, but also on static and dynamic interactions with the body. Evidence of this is provided by several studies focussing on the (dynamic) on-body and off-body propagation of radio-frequency (RF) electromagnetic waves [9, 10, 11, 7]. A complicating factor is that WBANs face restrictions on their emitted power due to limits on the available energy in wearable nodes as well as potential exposure risks [12]. To address some of these concerns, other options of wireless communication around the human body beyond RF have been explored such as acoustic [13], optical [14], and electromagnetic millimeter-wave [15] technologies. Given the very demanding constraints in terms of energy efficiency, coverage, robustness and performance imposed on WBANs [16], it is very likely that the optimal network configuration will use a combination of physical propagation mechanisms rather than the homogeneous single-technology solution advocated in the WBAN standard of today. Hence, a fair comparison between the different available technologies and a demonstration of their advantages and drawbacks features would be of very good use. Even if we restrict the discussion to RF on-body communication only, there still exist different methods in dealing with the aforementioned challenges, each with their own benefits and pitfalls. Passive RF (backscattering) techniques allow for on-body nodes without an energy reservoir, but are limited in readout range [17]. An example of passive on-body RF communication is ultra-high frequency radio-frequency identification (UHF RFID), which operates in a frequency band around 915 MHz [17]. To improve performance, active RF communication using on-body antennas can be employed in multiple WBAN configurations. However, link performance depends on the availability of energy reservoirs in the WBAN nodes and may suffer from signal loss in dynamic scenarios such as movement and lack of line-of-sight [16], although RF beam steering and multi-hop communication using repeaters can be used to mitigate some of these effects [4; 5; 8; 16]. Body coupled communication (BCC) using RF fields can be efficient in situations where on-body RF is not (e.g. through-body links), but is - up to now- limited in range, frequency, and bandwidth [18]. Depending on the requirements imposed for particular sensor nodes on certain positions on the body, different RF methods might be more suitable to establish communication between nodes at a given location and time [16]. However, up to now there is little guidance on how to make those decisions and selections. To address this need this study presents the following novelties: first, we compare, for the first time, a diverse set of on-body RF technologies (six in total) in a fixed on-body scenario; second, to our knowledge, we are the first paper to compare on-body path loss at 915 MHz in five different on-body scenarios. These results are important because it is expected that UHF RFID at this frequency will gain importance in the future [19]; third, we present a novel capacitive BCC technique; and fourth, we demonstrate a new phased array for on-body beam steering.