## 5 Open Research Issues, Challenges and Opportunities
### Mobility and Channel Modeling Challenges  
The modeling of the on-body radio channel has attracted more attention in the research community, however, only a few studies have addressed the issue of the time-variant channel model. The human body mobility should be taken into account to enable the design of efficient cooperative communication protocols for WBANs applications. Moreover, the characterization of these phenomena is difficult in terms of channel sounding. This is why VNAs (VNA: Vector Network Analyzer.) are often exploited to characterize the static WBAN channel by means of frequency domain measurements [139].  
From IEEE 802.15 TG6 only few studies have targeted the characterization of the dynamic on-body channel (namely CM3 in [140]). For instance, NICT have performed measurements using a channel sounder at 4.5 GHz [141], while NICTA characterized the on-body narrowband channel on two ISM (ISM: The industrial, scientific and medical (ISM) radio bands.) frequency bands: 820 MHz and 2.36 GHz [142]. The high dispersion of results presented in literature, even in the same channel model; e.g., [140], is due to the fact that the proposed distance-dependent path-loss models are inconsistent for on-body channels [143,144].  
In future, works such as [145] are needed further to investigate the impact of realistic WBAN radio channel and mobility conditions on the network topology dynamics and the performance of network and routing architectures. Similarly, biomechanical cylinder-based channel models, such as [38], are important to generate realistic mobility patterns as a function of time.  
With respect to communication protocols, for example, to evaluate Intra-WBAN communication performance, a time varying channel model is necessary. In particular, temporal and spatial correlation and associated features are important to accurate model the mobile WBAN channel. Whereas, for inter WBAN 'body-to-body' channel variations have to be considered which are much more mobility and environment dependent. In such cases, specific mobility models such as group mobility of rescue team or mobile workforce are necessary to be validated first, and then to evaluate their impact on the performance of the body-to-body communications.  
Concerning the worker safety scenario discussed from the application perspective in Section 5.1, Table 16, presents different sensors and their possible locations on the human body in various body postures along with corresponding movement of individual sensor. Four different body positions (_i.e._, sitting, standing, walking, and running) can be envisioned for a typical working environment. The positioning of the sensors and devices is important, first, it should be comfortable to the worker; second, the corresponding links between each sensor should be reliable. From past different studies and experiences (as explained in Section 4.1.2), positions presented in Table 16 are considered as one of the examples, though real experiments are necessary to validate them. Finally, mobility column reflects individual sensors mobility in different body positions.  
In order to design or select appropriate channel model and mobility patterns, radio link between each sensor is necessary to identify. In this regard, cross-radio-links (between sensors) under workers safety scenario for sitting, standing, walking and running positions can be analyzed for accurate modeling. For the given application, three types of links can be considered which are, line-of-sight (LOS), non-line-of-sight (NLOS) and periodic LOS/NLOS (P-LOS/NLOS).