# A Novel Bitrate Adaptation Method for Heterogeneous Wireless Body Area Networks
## 2 Related Work
The LA algorithm assumes that the SN estimates the SNR based on the received broadcast packets or a transmission confirmation packet. The decision to change the bitrate is made in two stages:  
* if the estimated SNR is below the specified \(Th_{SNR}\) threshold, then the next transmission rate is reduced;
* otherwise, a short-term probability of the correct transmission of the packet, \(P_{F}\), is determined. It is defined as the ratio of the number of lost acknowledge packets to all sent packets in the 20-frame window. The calculated \(P_{F}\) value is compared with the \(Th_{P_{F}}\) threshold value, and, based on the comparison result, the throughput for the next transmission may be increased. It is completed based on the assumption that, since the channel quality is good, the losses occur due to the interferences. Thus, to reduce the packet transmission time and the probability of collision, the bitrate should be increased. As far as the \(Th_{P_{F}}\) threshold value is fixed during the network operation, the \(Th_{SNR}\) value must be changed depending on the RI parameters and the designated noise characteristics. It should be noted that the radio modems can usually estimate only the Received Signal Strength Indicator (RSSI) instead of the SNR. In this research, the RSSI parameter should be understood strictly as the value of the received signal power. In this case, the algorithm had to be adapted to the capabilities of the used radio module. The principles of operation of the LA algorithm is presented in Figure 1.  
_Modification of the Known Methods for Adaptive Data Streams Allocation_  
In the ALBS and LA algorithms, the decision on the next transmission bitrate is based on the current LQI or RSSI values, which--in high frequency, fading environments--may lead to inefficient usage of the available resources by decreasing the bitrate. For this reason, an original modification of both algorithms, i.e., ALBS MOD and LA MOD, was proposed to counter the consequences of the fast fading phenomenon. In the proposed modification, the principles of operation of the ALBS and LA algorithms have not been changed, besides the method of the input data processing. The modification is that a shift register, with a length of \(C\), is created, and the output value is calculated as a mean of its current state. This operation is understood as a filtration of the instantaneous values of the LQI and RSSI values in the moving average filter (MAF) [26]. In the proposed modification, the determination of the length, \(C\), of the shift register is crucial. Too small of a value results in the final result being more influenced by sudden deviations of the input values, while the too large a length may cause an inertia of the algorithms' operation. The behavior of the algorithms can be observed, especially, when rapid changes of instantaneous values occurs, for example, during switching between the Line-of-Sight (LOS) or the Non-Line-of-Sight (NLOS) conditions. It was empirically determined that the length of the MAF, \(C~{}=~{}10\), is an acceptable value from the analyzed WBAN point of view, where periodic transmission is assumed. The results of comparative research on the effectiveness of time-frequency resources using the ALBS and LA algorithms, as well as the modified versions of the ALBS MOD and the LA MOD, are presented in Section 5 of this article. It should be noted that these methods are designed for homogeneous networks. Thus, they were tested in the heterogeneous simulator with independent bitrate adaptation for each RI. Figure 1: The principle of operation of the LA algorithm.