## 4 WBANs: The Key M2M Area Network Technology for mHealth Applications
### MAC Layer Design for WBANs  
The main functions of the MAC layer include: (i) the access regulation to the shared wireless medium among the system nodes; and (ii) the scheduling of transmissions to achieve desired performance goals, such as throughput guarantees, time-delay constraints and QoS provisioning. In addition to these functionalities that are common to all systems, a MAC protocol suitable for WBANs must handle specific challenges, associated with the WBAN topology and the node constraints. Furthermore, the MAC protocol design has a significant impact on energy efficiency, QoS provisioning, reliability and interference, which, as mentioned in Section 3, are some key requirements of mHealth applications.  
Several extensive surveys and comprehensive guidelines on MAC layer design for the wider category of sensor networks are available in the literature, such as the works in [74; 75] and more recently in [76]. Even though these works can provide helpful insights for many of the challenges also faced in WBANs, especially in the design of low-energy MAC protocols, they are not specifically addressed to healthcare applications. Two overviews on WBAN-oriented MAC protocols have been published in 2010 [77; 78],  
\begin{table}
\begin{tabular}{c c} \hline \hline
**WBAN PHY Layer Technologies** & **Overview of Proposed Schemes** \\ \hline \multirow{3}{*}{**NB Technology**} & [52] Novel modulation and demodulation scheme \\  & [53] Novel hardware design \\  & [54] Novel modulation and coding scheme \\ \hline \multirow{6}{*}{**IR-UWB**} & [56] Channel modeling and performance study \\  & [57] Comparative study of IR-UWB receivers (coherent and non-coherent) \\  & [58] Channel measurements and channel impact on performance \\  & and localization \\  & [60] Evaluation of novel receiver designs with channel state \\  & information knowledge \\  & [72] Design and evaluation of an UWB transmitter for low-power WBANs \\ \hline \multirow{3}{*}{**UWB Technology**} & [61] System design and comparison with IR-UWB \\  & [62] Integration of sensing and communication devices using FM-UWB to a single device \\  & [63] Performance study of DS-UWB and FM-UWB under two different channel models \\ \hline \multirow{6}{*}{**MB-OFDM**} & [64] Channel modeling and system level evaluation \\  & [65] Receiver design and investigation of the impact of several \\ \cline{1-1}  & \multirow{2}{*}{**UWB**} & design parameters \\ \cline{1-1}  & & [66] Impact of NB interference and a novel mitigation scheme \\ \cline{1-1}  & [67] Overview of constant envelop and carrier-frequency offset mitigation \\ \cline{1-1}  & & techniques \\ \hline \multirow{3}{*}{**HBC Technology**} & [70] Novel modulation and demodulation scheme \\ \cline{1-1}  & [71] Transceiver design and simulation study \\ \cline{1-1}  & [73] Baseband system design and simulation study \\ \hline \hline \end{tabular}
\end{table}
Table 3: Overview of physical (PHY) layer transmission technologies [18; 23].  
but are missing later advancements in this rapidly growing research field. Finally, more recent surveys on WBANs that include discussions on the MAC layer challenges can be found in [15, 16, 21, 22].  
In this section, we update and complement the existing surveys by identifying the main challenges and design approaches to tackle the aforementioned requirements of mHealth applications in WBANs. After a thorough study of the proposed MAC protocols in the literature, we provide a survey of the representative mechanisms employed for (i) energy-efficient channel access and synchronization; (ii) QoS provisioning; (iii) reliability; and (iv) interference mitigation and coexistence.