# A Novel Bitrate Adaptation Method for Heterogeneous Wireless Body Area Networks
## 3 Novel Rate Adaptation Method
### Proposed Novel Method for Data Streams Allocation
The RDMs were performed by using the symmetrical double-sided two-way ranging (SDS-TWR) method, which is compliant with the IEEE 802.15.4-2011 standard [31; 32; 33]. Some of the physical layer parameters were experimentally adjusted to maximize the accuracy of the distance estimation, i.e., the preamble length: 256 symbols and the pulse repetition frequency (PRF) of 64 MHz [27; 28; 31; 32; 33]. The deviation of the RDMs values around the average value was additionally minimized by applying low-pass filtration (3) with \(\alpha_{\min}=~{}0.13\). The calculation of the differences in the results of successive RDMs, carried out with a constant and known repetition period, allows the speed component of the node to be determined (movement of the node with respect to the stationary one). Based on the speed value, the \(\alpha\) parameters of two low-pass filters for the RSSI and TP signals were determined. Output values of these filters were used to estimate the fading parameters of the radio channels. According to [34], the fading parameters of these signals can be determined based on the instantaneous system loss values by applying an appropriate filtration (2), where its length, \(M\), corresponds to 10\(\lambda\), with \(\lambda\) as the wavelength. Since the developed method should work in real time, it was decided to choose the length of the filter to be equivalent to a single wavelength, \(\lambda\). It causes a slight decrease in the accuracy of the spatial data filtration, but also a reduction in its delay. By using (4) it is possible to determine the \(\alpha\) parameter of both filters as a function of the user's speed for the selected transmission frequency. The speed of human motion is in the range from 0.5 mps to 1.5 mps, which implies the selection of 40 ms transmission time intervals, suitable for monitoring and tracking systems operating in indoor environments [35; 36]. As the wavelength for the UWB interface is about 4.6 cm, it is over seven times shorter than for the NB interface (34.6 cm). As was expected, the value of the \(\alpha\) coefficient for the UWB RI is higher than for the NB one, and it is contained in a three-element set {0.382; 0.534; 1}. According to (3), \(\alpha~{}=~{}1\) should be interpreted as a lack of data filtration. For the NB RI, the value of the \(\alpha\) coefficient was in the range of [0.074; 0.208]. In the case when the user is standing or moving at a speed lower than 0.5 mps, the filtration parameter \(\alpha\) was set as follows: \(\alpha~{}=~{}0.074\) for the NB RI and \(\alpha~{}=~{}0.382\) for the UWB RI. This choice is determined by the fact that the errors of RDMs affects the estimated non-zero speed of the user's movement, even when the user is standing, therefore, the minimum value was limited. For both \(\alpha\) values, it is possible to store several dozen values of filter coefficients for the estimated parameters of the person's movement, which significantly reduces the computational complexity of the proposed AMASD method. Then, the data sets are sent to separate blocks of channel parameters estimation and prediction. With the knowledge of the transmitted signal power, and according to [34; 37; 38; 39; 40], the system loss exponent, \(n\), and the mean system loss values can be determined according to the following formula:  
\[L(d)~{}=~{}L_{P}~{}+~{}10\cdot n\cdot\log_{10}(d) \tag{6}\]  
where the distance, \(d\), is the result of RDMs. To determine these parameters, linear regression was used. Since the implemented spatial filtration of the empirical data corresponds to a single wavelength, \(\lambda\), it was assumed that system loss values, \(L(d)_{\left|\mathrm{dB}\right|}\), were calculated for the RSSI' and TP' values after the filtration. It is equivalent to determining the channel parameters for empirical values, partially taking into account the radio shadowing effect [34].