## 4 Wireless Transmission in WBANs
### WBAN Channel Models  
Various sensors are placed on the body or implanted into the body in WBAN health monitoring. Due to the complex environment of the human body and its surroundings, there are many interference signals in WBAN channels which will affect the quality of communication, directly affecting the performance of the WBAN system. WBAN channel characteristics are not only the key to constructing the network's architecture, but also indispensable to the design of the upper-layer network protocol. Due to the complexity of human tissues and body shapes, and the diversity of environments we occupy, the difficulty of WBAN channel modeling is great. Therefore, the wireless transmission channel in the WBAN is divided into several modes, as shown in Figure 3 and Table 9. There are three typical nodes in WBANs: implanted, body surface and external nodes [76]. The channel characteristics have four main relevant factors: the frequency factor (WBAN frequency bands may include 400 MHz, 600 MHz, 2.4 GHz and UWB), environmental factors (WBAN's environment, such as the anechoic chamber, outdoors or a hospital), antenna placement and status of human motion [77; 78; 79; 80; 81]. Environment factors mainly affect the multipath propagation caused by the surrounding complex environment. In an outdoor environment, the surface channel sees little attenuation. However, in an indoor \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Operation Bands** & **Frequency Range** & **Disadvantages** & **Application** \\ \hline Medical device radio communications [12] & 401–406, 413–419, 426–432, 438–444, 451–457 MHz & Limited bandwidth [70] & In-body and on-body \\ \hline Human body communications (HBC) & 5–50 MHz & Affected by the human posture and surroundings [71] & In-body [72] and on-body [13] \\ \hline Medical implant communication service spectrum [70] & 402–405 MHz & Limited bandwidth & In-body [13] \\ \hline Wireless medical telemetry service & 608–614, 1395–1400, 1427–1432 MHz & Limited bandwidth [70] & In-body \\ \hline  & & Not suitable for critical life situations due to coexistence with aeronautical mobile telemetry [74] & On-body \\ \cline{2-3} Industrial, scientific and medical (ISM) & 2360–2500 MHz & 2390–2400 MHz & Limited bandwidth & On-body \\ \cline{2-3}  & & Unlicensed WBAN, occupied by IEEE 802.15.6, Wi-Fi, Blue-tooth, ZigBee. & On-body \\ \hline Ultra wideband (UWB) & 3.1–10.6 GHz & Incomplete spectrum monitoring campaign [75] & On-body \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Medical body area networks’ operation bands. environment, the attenuation of a WBAN channel with 900 MHz and 2.4 GHz frequency bands can be dramatic. Therefore, a perfect channel model should introduce the three modifying factors to satisfy the simulation of different applications. Differently from common wireless communication channels, which are severely affected by frequency selective fading, WBAN channels are mainly affected by flat fading. The reason is that the multipath delay is tiny due to the short distances, so the effect of multipath fading can be ignored. Concurrently, due to the variations in the surrounding environment of the human body or movement of body parts, path loss will change violently, leading to shadowing from the mean value for a given distance. The main model is a shadow fading channel and can be adjusted dynamically by three parameters--antenna position, sensor position and personal environment. Therefore, the total path loss (\(PL\)) is defined as the following formula [82]:  
\[PL=PL_{0}+10n\ \log_{10}\left(\frac{d}{d_{0}}\right)+S \tag{1}\]  
where \(PL_{0}\) is the path loss at a reference distance \(d_{0}\), \(d\) is the distance between the antennas, \(n\) represents the path-loss exponent and \(S\) represents the shadow. The first parameter is affected by the antennas' size, shape and transmitting direction; the second parameter relies on its position in the human body; and the third parameter is based on statistical modifications for height, weight and gender.