# Moving Towards Body-to-Body Sensor Networks for Ubiquitous Applications: A Survey
## 4 BBN Design Challenges and Open Issues
### Interference and Coexistence  
Due to the restricted number of channels of the radio spectrum, multiple wireless technologies (i.e., ZigBee, WiFi, Bluetooth, etc.) are constrained to share and manage the wireless resources. Hence, in order to preserve the link quality and do not alter the wireless signal, while implementing inter-WBAN communications, it is crucial to enable an interference mitigation mechanism to deal simultaneously with mutual and cross-technology scenarios. Three interference mitigation mechanisms are considered by the IEEE 802.15.6 standard to deal with coexisting WBANs, namely [42]:  
* Beacon shifting: by using a beacon shifting sequence not used by his neighbors, a WBAN could avoid co-channel allocation conflicts. Yet, each WBAN will periodically transmit within a scheduled interval and is characterized by its beacon shifting sequence index and the length of its beacon period. * Channel hopping: the WBAN is allocated a wireless channel for a fixed number of superframes. Then, it should hop to another channel according to his channel hopping sequence, which is different from his neighbors. Thus, WBANs could not reserve indefinitely a wireless channel. * Active superframe interleaving: two neighboring WBANs could share the same channel by interleaving their active superframes, i.e., alternating between them during their inactive periods. Also, the Bluetooth low energy (BLE) standard [44] defines an adaptive frequency hopping mechanism to avoid overlapping with neighboring wireless links. But these keep standard-specific techniques, and do not deal with cross-interference scenarios, in a BBN context. Number of recent works [38; 39; 95; 96; 97] investigate the inter-WBANs interference problem with different alternatives. For example, authors of [95] enhanced the existing frequency hopping mechanism of WPANs to ensure coexistence with WiFi devices. A WiFi system is assumed to rarely change its operating channels, thus they implemented a novel scheme that monitors the packet error rate (PER) within the network to find out the channels that should be excluded from the hop set. From another perspective, in [96], a power control game theoretical approach is implemented, considering the signal-to-interference-plus-noise-ratio (SINR) as utility function to model the interference between coexisting WBANs. An almost blank subframe (ABS) scheduling technique is proposed in [38], for a \begin{table}
\begin{tabular}{l l c c c} \hline \hline
**BBN** & **Description** & \(n\) & \(P_{0}\) & **Nakagami** & **Indoor** \\
**Scenario** & & & & **Fading** & **Shadowing** \\ \hline Stationary & Person \(A\) stands at a distance \(d=8\) m from & 2.46 & \(-43.70\) & \(\mu=0.98\) & - \\ LOS & person \(B\) and performs two forward walkings & & & \(\omega=1.8\) & \\  & and two backward walkings, into the average & & & \\  & speed of 0.8 m/s, reproducing in that way LOS & 1.91 & \(-52.46\) & \(\mu=0.87\) & - \\  & and NLOS conditions. & & & \(\omega=1.97\) & \\ \hline Rotation & Person \(B\) realizes full rotations beside person & - & - & - & \(\sigma_{s}=4.24\) \\  & \(A\), starting from \(\alpha=0^{\circ}\) to \(\alpha=180^{\circ}\), turning & & & \\  & from LOS to NLOS position, and inversely. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: BBN scenarios in indoor environment. WBAN-based machine-to-machine (M2M) medical system. Each WBAN could then avoid interference with his neighbors by specifying the type of the transmitted subframe: normal or ABS, with respect to the load status of the M2M system. In [39], the interference-aware coexistence of multiple WBANs is tackled by considering the mobility of sensor nodes within the single WBAN, and the movement of WBANs in their surrounding environment, based on social interaction detection, so as to ensure spectral reuse and power efficiency.