# Analyzing the Performance of WBAN Links during Physical Activity Using Real Multi-Band Sensor Nodes
## 2 Materials and Methods
### Scenarios
#### 2.2.1 Static Scenarios  
Two static scenarios have been tested where two regular body postures have been characterized: _standing_ and _sitting_ on a desk. In both of them, the subject has a receiver node on the left hip and sender nodes on the right wrist and right ankle. This is in line with other studies in the field, where sink/hub nodes have a central body position. In a real WBAN this is the most usual position for a smartphone, which is a usual hub in these kinds of networks [19]. The wrist is also the usual spot for a sensor node in WBANs, such as a smartband for motion sensing, SpO2 measuring or pulse rate monitoring. In turn, the ankle is a regular position for motion tracking in sports or physical activity monitoring. The nodes' placement on the subject's body can be seen in Figure 2.  
In the first scenario, the subject stands still with both arms in a resting position at her sides, palms facing inward. The straight distance between the receiver node on the left hip (LH) and the sender node on the right wrist (RW), LH-RW, is 48 cm, with the subject's arm and abdomen partially obstructing the signal path so they are not completely in line-of-sight. The straight distance from the left hip node to the one in the right ankle (RA), LH-RA is 95 cm, both being only partially in LOS. The second scenario has the subject sitting on a desk with both arms placed on the table, elbows flexed and palms facing down. The hip node is partially under the table so both nodes are NLOS, with the table--made of particle board with a thickness of 3.5 cm--fully obstructing the signal path. The straight distance LH-RW in this scenario is 62 cm. In case of the ankle node, the distance LH-RA is 75 cm, with parts of the subject's legs and the chair obstructing the signal path. This scenario could represent an office \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **SPIRIT1** & **SPIRIT1** & **CC2500** \\  & **@ 433 MHz** & **@ 868 MHz** & **@ 2.4 GHz** \\ \hline Center frequency & 433.05 MHz & 868.00 MHz & 2.41 GHz \\ Modulation scheme & GFSK [1] & GFSK & MSK [2] \\ Frequency deviation & 127 kHz & 127 kHz & - \\ (GFSK) & 540 kHz & 540 kHz & 540 kHz \\ Receiver bandwidth & 250 kbps & 250 kbps & 250 kbps \\ \multicolumn{4}{c}{Packet size} & 64 bytes & 64 bytes & 64 bytes \\ Output power range & (\(-\)30 dBm, 11 dBm) & (\(-\)30 dBm, 11 dBm) & (\(-\)30 dBm, 1 dBm) \\ Receiver sensitivity [3] & \(-\)86 dBm & \(-\)92 dBm & \(-\)100 dBm \\ \multicolumn{4}{c}{External antenna} & \(\frac{1}{4}\) wavelength whip & \(\frac{1}{4}\) wavelength whip & - \\ \hline \hline \end{tabular} \({}^{\dagger}\) GFSK corresponds to Gaussian Frequency Shift Keying modulation. [2] MSK corresponds to Minimum Shift Keying modulation. [3] Data obtained empirically with external antennas mounted when applicable.  
\end{table} 
Table 1: Configuration of the wireless parameters of the test platform. worker or a student, and its characterization can potentially be used to improve body posture or ergonomics. For each sender node position in each scenario the experiment consists of the sender node iterating through the three frequency bands, and for each frequency iterating through all the possible transmit power levels in steps of two dBm. In the case of the 433 and 868 MHz bands, the output power ranges from \(-\)30 dBm to 11 dBm, so there are 22 possible levels:  
\[\text{TxPower(@433/868 MHz)}\in \tag{1}\] \[(-30,-28,-26,-24,-22,-20,-18,-16,-14,-12,-10,-8,-6,-4,-2,0,2,4,6,8,10, 11)\;\text{dBm}.\]  
In the case of the 2.4 GHz band, the output power range is smaller, so there are only 17 possible levels:  
\[\text{TxPower(@2.4 GHz)}\in \tag{2}\] \[(-30,-28,-26,-24,-22,-20,-18,-16,-14,-12,-10,-8,-6,-4,-2,0,1)\; \text{dBm}.\]  
For each of these power levels, the sender sends 500 packets to the receiver at a rate of 50 packets/s, so each power level takes 10 s of measurements. Since the packets' payload is 64 bytes long, this results in an effective data rate of 25.6 kbps.