## 4 Wireless Transmission in WBANs
### Physical (PHY) Layer and Medium Access Control (MAC) Layer  
The IEEE 802.15.6 standard defines new PHY and MAC layer specifications for WBANs, providing ultra-low-power, low-cost and short-range wireless communication which operates in or around the human body. IEEE 802.15.6 standard outlined three different PHY schemes: narrowband (NB), UWB and HBC. NB and UWB PHY are based on radio frequency (RF) propagation, and HBC is based on a new non-RF technique. NB PHY included seven frequency bands from 402 to 2483.5 MHz for a total of 230 channels, of which the 402-405 MHz band is used for implantable devices, three different frequency bands (863-956 MHz) are used for wearable applications and 2360-2400 MHz is used for medical demands. UWB PHY included 11 frequency bands ranging from 3494.4 to 9984.0 MHz. Due to the high signal attenuation and severe shadow effect through the human body, these radio frequency bands are not suitable for HBC. As a signal transmission medium, the frequency band of HBC ranges from 5 to 50 MHz; the center frequency band is at 21 MHz. The WBAN standard demands an ultra-short range communication mechanism limited to 2 m. As the lowest layer, PHY, determines the high-layer protocols, which requires minimizing power consumption and bit error rate (BER) [91]. Ideally, power consumption and BER increase linearly as the data rate increases from 1 kbps to 10 Mbps, resulting in consistent energy use per bit of information. In order to acquire low power consumption, the PHY signal processing algorithms at the receiver need to be designed carefully. Future works should focus on seamless connectivity in dynamic environments to minimize performance degradation in terms of data loss, latency and throughput. WBAN is a severely resource-constrained network in which most of the power consumption is caused by the transceiver, whose duty cycle is controlled by the MAC layer. Therefore, it is very important to design an energy-saving MAC protocol to ensure efficient and reliable transmission of data using limited wireless channel resources in WBAN. The unique challenges of MAC protocol design are mainly focused on energy consumption, QoS and transmission efficiency. Early studies focused on addressing each individual problem, such as the dynamic traffic pattern [92; 93] or the energy efficiency [94; 95]. [PERSON] et al. [96] proposed a TDMA-based MAC protocol that can adjust the duty cycle according to the  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Model Descriptions** & **Scenarios** & **Method** & **Propagation Effects** & **Mobility** & **Link Type** \\ \hline \multirow{3}{*}{Dynamic channel model [84]} & on-body, off-body, and body-to-body & finite-difference time-domain & \begin{tabular}{c} fade variation and \\ their corresponding \\ amplitude \\ distributions \\ \end{tabular}  & walking & hand and thigh \\ \hline \multirow{2}{*}{ \begin{tabular}{c} A filter based probabilistic model [85] \\ \end{tabular}  } & \multirow{2}{*}{Intra-WBAN} & orthogonal frequency-division multiplexing & fading and dynamic static sitting and dynamic walking & hand \\  & & & & & \\ \hline \multirow{3}{*}{ \begin{tabular}{c} Simulations-based space-time \\ dependent channel model [86] \\ \end{tabular}  } & \multirow{3}{*}{Intra-WBAN, Indoor or Anechoic Chamber} & Combination of frequency, distances in free space and around the body & \begin{tabular}{c} Spatial and temporal \\ characteristics-based \\ fading, Shadowing \\ due to body parts \\ length and size. \\ \end{tabular}  & \multirow{3}{*}{\ \begin{tabular}{c} Standing, walking \\ and running \\ \end{tabular}  } & \multirow{3}{*}{
\begin{tabular}{c} Hip to \\ Wrist/Foot/Thigh. \\ A from to Foot and \\ Headtypes of nodes in the WBAN to guarantee both QoS and energy efficiency. [PERSON] et al. [69] proposed an adaptive TDMA MAC protocol which could automatically detect the shadowing effect and adjust the superframe parameters; it provided good results in latency outage and energy consumption. [PERSON] et al.