# Opportunities and Challenges for Error Correction Scheme for Wireless Body Area Network--A Survey
## 4 Channel and Network Models  
In WBAN, there are three types of nodes: implant node (placed inside human body or below the skin surface), body surface node (on the skin surface or at most two centimeters away from the body surface), and external node (not in contact with human body i.e., between a few centimeters to up to five meters away from the body surface). Therefore, the communication between these nodes creates different types of communication links, such as communication through body, Line of Sight (LOS), and Non Line of Sight (NLOS) scenarios. The possible communication links for WBAN is shown in Figure 4[32] and there are various types of channel model (CM) to present these communication links. The channel models CM\({}_{1}\) and CM\({}_{2}\) characterize the communication channel  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Priority** & **User Priority** & **Traffic Designation** & **Frame Type** \\ \hline Lowest & 0 & Background (BK) & Data \\  & 1 & Best Effort (BE) & Data \\  & 2 & Excellent Effort (EE) & Data \\  & 3 & Video (VI) & Data \\  & 4 & Voice (VO) & Data \\  & 5 & Medical or network control & Data or management \\  & 6 & High-priority medical data or & Data or management \\  & & network control & Data or management \\  & 7 & Emergency or medical implant & Data \\  & & event report & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Quality of Service (QoS) for IEEE802.15.6[30].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Modulation/Error** & **Default Mode** & **High QoS Mode** \\
**Correction Technique** & **Dead** & **Dead** \\ \hline Modulation & Mandatory: on-off keying & Mandatory: DPSK \\  & Option: DPSK & Option: on-off keying \\ FEC & (63,51) BCH code & (126,63) Shortened BCH code \\ HARQ & Not use & Use \\ \hline \hline \end{tabular}
\end{table}
Table 2: Modulation, Error-Correcting Codes (ECC), and use of Hybrid Automatic Repeat Request (HARQ) for each mode in Ultra-Wide Band-Physical (UWB-PHY).  
between implant-to-implant node and implant node to body surface or external node, respectively. CM\({}_{3}\) channel model characterizes the communication link between body surface node to body surface node, including LOS and NLOS scenarios. The CM\({}_{4}\) channel model describes the communication link between body surface node to external node, including LOS and NLOS scenarios. The possible scenarios with description, used frequency band, and channel model are listed in Table 3[32]. The scenarios are determined by the types of nodes involved in the communicating link and whether or not there is LOS between the nodes. Afterwards, the scenarios are grouped into classes as CMs.  
All nodes are in and around the human body with a maximum range of five meters and star topology is adapted as a network topology in WBAN [30]. In this topology, one of the communication node acts as a hub and the rest of the communication nodes are used as nodes. The hubs are responsible for medium access and power management. Figure 5 illustrates that one-hop star WBAN topology for single WBAN. In a one-hop star WBAN, frame exchanges occur only between nodes and the hub of the WBAN. In practical settings, there may be more than one WBAN that may cause interference to each other. One-hop star WBAN extended to two-hop extended star WBAN, except in the Medical Implant Communication Service (MICS) band i.e., 402-405 MHz, as illustrated in Figure 6. In the extended two-hop extended network, the hub and node exchange frames optionally via a relay-capable node [30].