# Analyzing the Performance of WBAN Links during Physical Activity Using Real Multi-Band Sensor Nodes
## 4 Discussion  
The performance of WBAN channels has been empirically characterized by means of two well-known metrics: RSSI and PRR. Three different bands--433 MHz, 868 MHz and 2.4 GHz--have been used to evaluate the effect of the frequency on the WBAN channel conditions. All experiments confirm the theoretical notions derived from path loss equations, with the lower frequencies performing better in terms of RSSI. The tests presented in this work have also confirmed the correlation between the RSSI and PRR metrics for most scenarios and experimental conditions. A higher average RSSI results in a higher PRR for a given frequency and transmit power. However, as was mentioned in the introduction and stated in [7], this is not the case when the packet loss is caused by interference rather than attenuation. In our experiments, this happens on the 2.4 GHz band in several environments, e.g., the sitting scenario, where interference and multipath effects result in a decreased PRR metric even for RSSI values above the receiver sensitivity. The 2.4 GHz band also seems to be much more negatively impacted by the presence of body parts on the signal path than the other two frequencies, as seen for the two different node placements in the standing and sitting scenarios. Figure 16: Effect of the transmit power on the height of the periodic RSSI peak shape for Subject A with the sender node on the wrist location. Average maximum and average minimum values for each power level: **(top)** 433 MHz, **(middle)** 868 MHz, **(bottom)** 2.4 GHz. Two dynamic physical activities--cycling and walking--were evaluated in order to study the periodic time-variant characteristics of their WBAN channels. The walking motion has been the subject of several studies in the field [5; 14; 15]. However, to the best of our knowledge, this is the first time that cycling activity has been evaluated in the context of WBAN channel characterization. As predicted, the repetitive nature of these body motions is reflected on the RSSI variation with time of WBAN links. Our results have confirmed this, being able to extract periodic patterns for several test conditions and subjects. An important contribution of this work is introducing the frequency band and node placement as independent variables to test their impact on the RSSI periodicity patterns of the different activities. More importantly, the influence of the subject wearing the WBAN has been characterized. Our results show that the RSSI periodic pattern of the walking activity is extremely dependent on the particular subject. This should be taken into consideration by algorithms and strategies focused on exploiting these periodic patterns for energy efficiency and link reliability. It is mandatory that the particular periodic RSSI shape is extracted and characterized for each subject and activity, instead of assuming an a priori expected curve. Our study of the cycling and walking activities also demonstrated that the periodicity of WBAN links can be obtained and characterized by simply extracting the RSSI metric, without any extra information. For instance, the walking cadence can be calculated just by averaging the distance between consecutive minima in the RSSI vs. time curve. This means that periodicity-exploiting algorithms or even activity-detection applications can be used without the need for extra hardware resources, such as accelerometers. Since the RSSI is already available in most commercial transceivers from the data packets already being sent, without any extra computing, this allows the development of low-resource algorithms and strategies for WBAN that require very few control packets and have almost no overhead. All experiments have been performed with a custom hardware test platform with wireless commercial transceivers in three ISM bands. All the tests have also taken place in normal indoor and outdoor environments with regular conditions in terms of possible interfering equipment.