## 4 The WBANs Communication Stack
### Mobility and Radio Channel Modeling for WBANs
#### 4.1.1 Proposed IEEE 802.15.6 Channel Models  
The WBAN radio channel is mainly impacted by two main phenomena, _i.e._, small-scale fading and large-scale fading:  
_Small-scale fading_ refers to the rapid variations in the phase and amplitude of the signal during propagation over a small-area due to small changes in the location of the on-body device or body position/postures. It is mainly caused by small displacements of the antenna on the body and gross/minor changes in the body position/posture. _Large-scale fading_ refers to the fading due to motion over large areas; this is referring to the distance between antenna positions on the body and external node (home, office, hospital, _etc._). It is mainly caused by large displacement of the body and to major changes in the body position/posture. The IEEE 802.15.6 standard proposes different channel models for three different physical types (_i.e._, Human Body Communication (HBC), Narrow Band (NB) and Ultra Wide Band (UWB). It defines three different types of nodes with reference to channel models (_i.e._, implant, body surface, and external nodes). In addition, it describes different scenarios (as shown in Table 4) for same frequency bands, which is further, sub-divided into Line-Of-Sight (LOS) and Non-Line-Of-Sight (NLOS) links. In the context of safety and critical applications, scenarios S4 to S7 can be considered for the accurate mobility and radio modeling, whereas scenarios S1 to S3 are normally reserved for medical based applications. For example, in case of narrow band, two models are proposed to satisfy scenarios S4 and S5. The authors of [34] proposed a model based on measurements (that cover frequencies of 2.4-2.5 GHz.) conducted in hospital room and anechoic chamber (to remove the multipath effects). The path loss function is defined as follows:  
\[PL(d)[dB]=a.\,log_{10}\left(d\right)+b+N \tag{1}\]  
where \(a\) and \(b\) are the coefficients of the linear fitting, \(d\) is the distance between (transmitter and receiver in mm), \(N\) is the normally distributed variable with standard deviation \(\sigma\). Typical values of the parameters of the path loss model are provided in Table 5. The path loss model is derived using a regression line through least square fitting for each frequency band. A transmitter antenna is placed at the waist and a receiver antenna is placed on different parts of the body, including head, ear, shoulder, wrist, waist, leg, and ankle. The IMEC Research Center provided real-time measurements at 2.45 GHz of frequency [35]. The measurement setup includes antennas of \(50.5\times 28\times 8\) mm size with a weight of 4.2 g. Three planes separated by approximately 15 cm along the vertical axis around torso of the body are considered. The transmitter is placed on the front and the receivers are placed at distances of 10-45 cm in steps of 5 cm measured around the perimeter of the body. The transmitter is worn at approximately shoulder height at one of two different positions. The receiver is placed directly below the transmitter at seven positions separated by 10 cm covering the range from the shoulder to the knees. Six different locations are considered inside an office room and at each of the locations, 49 points are arranged in a height of \(7\times 7\) square grid. Six cm is the separation distance between each array element (for 2.45 GHz) and 16 cm (for 915 MHz).