# Analyzing the Performance of WBAN Links during Physical Activity Using Real Multi-Band Sensor Nodes
## 1 Introduction
Postural changes modify the distance between nodes and affect the path loss due to body obstructions, affecting the RSSI and PRR [13]. These body movements can rapidly turn a wireless link from being in line-of-sight (LOS) to non-line-of-sight (NLOS), giving WBAN channels a distinctive time-variant characteristic [14]. This time-variance is very noticeable during some physical activities, such as walking or running, given the periodic characteristic of these movements. Thus, several research studies have exploited the periodicity of WBAN channels during physical activity, particularly adapting their MAC scheduling strategies [5; 14] or their TPC mechanisms [4; 15] to the gait cycle of the user. However, several of these papers are based only on simulated conditions [12]. Others only work with a single frequency band [7; 15] or custom transceivers and antennas that can be impractical in real WBAN applications [16]. Finally, several strategies make use of extra resources, such as accelerometers, that are not always readily available in WBAN nodes or, if available, incur in additional power consumption and overhead [6]. A comprehensive theoretical model for WBAN communications would be a much-desired tool for researchers and algorithm designers in this field. However, an enormous number of factors--both internal and external to the network--affect WBAN communications and its channel conditions. Primarily, body movements and posture changes modify the signal paths, creating fading and shadowing effects due to body parts obstructing the line-of-sight between nodes. While free-space path loss has been sufficiently modeled and quantitated via the Friis equation for decades, the same cannot be said for the attenuation caused by the different layers and composition of body tissues. Different types of radio propagation occur in the inside layers of body parts, through their surface, and/or even in the immediately surrounding air: space waves, superficial waves, creeping waves, body coupled communication, etc. All of them are heavily influenced by the kind of antennas used, their positioning, and mainly by the body composition and structure of the WBAN subject. A theoretical model that takes all these aspects into account and quantifies them into a reliable and utilizable on-body path loss equation is impractical and possibly unrealistic. Secondly, multipath transmission plays a huge role in WBAN communications, both in indoor and outdoor scenarios. The authors in [17] go as far as to say that the strongest signal path in WBAN communications is the one that comes from ground reflections, rather than the one corresponding to the straight line between nodes (either in LOS or NLOS). Modeling all these multipath effects is also unfeasible and impractical, given the mobility aspects and the huge variety of scenarios and locations in which WBAN communications take place. Finally, foreign interferers--either external devices or other nodes in the network--also affect WBAN communications and degrade their reliability. For instance, WBAN communications on the 2.4 GHz band, e.g., Bluetooth devices, can be heavily affected by the presence of Wi-Fi access points and devices in the vicinity. It would also be impractical to consider and integrate all these external interferences into a possible theoretical model for WBAN. In the same way, many common human activities are not solitary, but are rather shared among various individuals or at least occur in the presence of other surrounding people, e.g., going for a walk, working out, etc. If modeling the WBAN characteristics of one single isolated person is at best impractical, doing so for multiple people in motion becomes practically impossible. In summary, given the amount and variety of factors impacting WBAN channel conditions, a comprehensive and applicable theoretical model is unfeasible in this context, and an empirical approach should be taken to analyze their characteristics in real scenarios and activities.