## 5 WBAN and the Communications Protocol Stack
### Physical Layer
#### 5.2.1 Characteristics of RF Communications in and along the Human Body  
In his work on Personal Area Networks (PAN), in particular body impedance, [PERSON] [26] concluded that the human body behaves as an almost perfect conductor but the low internal body resistance is insulated from the PAN nodes by skin, hair and clothing which collectively exhibit a large impedance level. Due to the human body composition of varying amounts of water, skin and tissue, the propagation of electromagnetic signals along or in the human body is inherently variable and subject to fading and multi-path propagation. Various organs and tissues exhibit different characteristics when RF signals are transmitted through them resulting in unpredictable outcomes in the context of absorption, propagation and required power levels. On-body signal propagation consists mainly of a creeping wave diffracted from human tissue and trapped along the body surface [27]. RF signal behavior can vary between outdoor and indoor environments where reflection and propagation factors differ, for example a signal transmitted at the chest and reflected by a wall a few meters away will arrive at the spine based receiver with about the same power as a creeping wave. Optimal transmission power required for an on-body wireless link between two sensors depends on the distance of the link and its instantaneous channel condition. The distance can vary with mobility driven by human postures and the channel can vary due to unpredictable RF attenuation [13]. Generally, the propagation wave rather than passing through the human body tends to diffract around it. When transmit and receive antennas are placed on different sides of the body, path loss can be significant. In investigating the mobile path loss for on-body UWB links within hospital environments, [PERSON] and [PERSON] [28] concluded that Received Signal Strength (RSS) depended on whether transmit and receive antennas are in relative Line of Sight (LOS) or NLOS conditions and that NLOS had a significant impact on RSS. Additionally, the research showed that RSS significantly varied depending on posture position of a patient either sitting or standing. Nodes placed on or in the human body communicate via antennas. One of the main issues in the design of antennas for use in the WBAN environment is catering for how the human body interacts with electrical and magnetic fields. Antenna design is affected by patients posture, skin type and body mass. Antenna designs should consider the intrinsic on-body environment, restrictions on size, shape and material. For antennas that are placed inside a human body, only non-corrosive and bio-compatible material, such as platinum or titanium can be used for implants. However, these materials yield a poorer performance compared to a copper antenna. The shape and size of an implant antenna dependson its location inside the body, which further limits the freedom of the designer. Heating effects on fat, muscle and skin tissue need to be considered as part of antenna design [16]. In experiments researching the effects of body shape and gender on body channels, [PERSON] _et al._[29] observed that females suffer less path loss than males, but concluded that this could be due to distance variations between wrist and torso in women. Additionally his research showed that males suffer more fade than females and the weak correlation observed between distance and path loss indicates that the shadow effect due to different body shapes has much more influence on path loss. [PERSON] _et al._[30] observed that the channels around the human torso are strongly dependent on the transmission medium. It was shown that echoes off both arms can change the attenuation from the left to right side of the body by up to 20 dB, depending on the position of the arms. The torso, being the largest part of the human body, can have a significant impact on the attenuation of RF signals.