## 4 Discussion
From the experimental results and the packet loss, it can be concluded that position (8), which sits firmly on the subject's back, is too close to the human body. Consequently, the radio waves cannot reach the most distant IMUunits on the lower legs on the other side of the subject's body. For positions (2) to (5), the greatest packet loss was observed for the IMU units located on the subject's pelvis, which is the closest remote device to the Master. The reason for such a high packet loss is thought to be the aluminium casing of the laptop computer inside the backpack that was blocking the radio communications. Unfortunately, this was impossible to avoid during the experiments. The master unit's placement (6) was found to be less error prone, with a low packet-loss ratio in both cases, for the subject walking in the public hall and on the treadmill. Placement (6) avoids the limitations of UWB-WBAN, as revealed in the literature [39]. The quality of the data transmission for the real case are shown in Table 2. The WSA, as a system consisting of several separate units, must be robust and autonomous, and must work well both in laboratory and real-world conditions such as public buildings. The results of the packet-loss measurements from our experiment in a public hall and on the treadmill are not significantly different from the results of the laboratory tests. All but one of the packet-loss measurements are less than 0.05%. Is 0.05% an acceptable limit? This number would correspond to one lost packet in 20,000 samples, which would occur once in 200 s at 100 Hz. Our decision algorithms and those of our partners could work properly with such input data [9; 10; 13; 33; 34]. The literature identifies the limitations in reliable data transmission for a patient's freedom of movement and the avoidance of interference from other bands [39]. Data transmission in the presence of other microwave signals, as shown in Figure 7, was not expected to affect the WSA data transmission over UWB. The Wi-Fi operates in the 2.5-GHz and 5-GHz bands, which is a different frequency band than the WSA system uses. For example, UWB channel number 3, with a central frequency of 4449.8 MHz and a bandwidth of 499.2 MHz, is away from Wi-Fi in both the 5-GHz band with channels above 5-GHz, and the 2.5-GHz band with channels between 2496 MHz and 2690 MHz There is no overlap. Also, Wi-Fi uses a much lower bandwidth with a channel bandwidth of 20 MHz or 40 MHz, while UWB generally uses a bandwidth of 500 MHz, with two channels having a maximum bandwidth of 900 MHz The 2.5-GHz band is very busy with different services. In addition to Wi-Fi, the same frequencies are used by Bluetooth and Zigbee, for example. For comparison, the Zigbee radio used in the first project CL operates on the saturated, i.e., overpopulated 2.5-GHz band ISM and uses only 2 MHz of bandwidth. In our measurements no correlation was found between the packet loss and the Wi-Fi band population. The times of the lost packets do not correlate with the occurrence of the Wi-Fi signals measured in a public hall. For the record: Wi-Fi signal levels were sampled every 100 ms, as opposed to 10 ms at the IMU. The traces showing packet loss in Figure 7, as well as the data presented in Tables 1 and 2, lead to the conclusion that WSA based on UWB data transmission is robust and usable in a real-world environment. According to the studies discussed in [39], a WBAN based on IEEE802.15.4-2011, such as the WSA presented in this paper, can be used as a communication network for personal health applications. The technology behind this standard is now mature enough to be available in the market, making it a potential solution for dedicated low-cost WBAN systems, as shown here. The low packet-loss graphs shown in Figure 7 are not surprising. The WSA set conforms to the 802.15.4-2011 or IEEE802.15.6-2012 standard for WBAN and can be further improved [40]. The current WSA currently uses unsecured communication, as the implementation of AES256 security is possible.