# A Novel Bitrate Adaptation Method for Heterogeneous Wireless Body Area Networks
## 2 Related Work  
Currently, the main goal of the conducted research is to increase the efficiency of time-frequency resources' usage, mainly in homogeneous sensor networks or wireless communication networks compliant with the 802.11 standard [16]. Most available algorithms are adapted to use with CSMA/CA and are mainly associated with wireless local area networks. There are also few algorithms that allow a dynamic allocation of data streams in sensor networks that are compliant with the 802.15.4 standard. There are some protocol solutions that consider the dynamic change of bitrate, but their usability is limited only to selected types of networks and applications [16; 17; 18; 19; 20; 21; 22; 23; 24]. Based on the available literature, several known algorithms have been selected that adaptively allocate data streams in homogeneous networks by dynamic change of the transmission bitrate between pairs of nodes:  
* Automatic Rate Fallback (ARF) [16];
* Adaptive Low Bitrate Streaming (ALBS) [19];
* Link Adaptation in Wireless Body Area Networks (LA) [21; 22]. The choice of these algorithms was determined by the possibility of their implementation in the off-body WBAN with TDMA. The selected algorithms cover the possibilities of currently used methods of adaptive data rate selection, which are based on the short-term estimation of PER and the radio channel parameters, i.e., the received signal strength and the Link Quality Indicator (LQI) parameter. Similarly to the proposed method, all algorithms listed above are designed for networks with decentralized resource allocation. The ARF algorithm is widely used in networks that are compliant with the IEEE 802.11 standard. The bitrate for the next transmission is selected based on a short-term estimation of the PER in the source node. The decision on packet loss is made in the case when no acknowledgment from the destination node was received. The bitrate change is performed stepwise, if one of two conditions are met:  
* If two successive acknowledgment packets have not been received, the next transmission rate is decreased; and
* if ten consecutive packets have been received, the next transmission rate is increased. The ALBS algorithm uses the LQI parameter, which can be used only in RI that are compliant with the IEEE 802.15.4 standard [25]. Similarly to the ARF algorithm, transmission with confirmations is required, and a feedback from the destination node affects the decision on the next transmission rate. The authors of the ALBS method defined the following algorithm for handling the transmissions rate in the network:  
* The source node (SN) starts sending the data with a nominal bitrate of \(R_{th}\) kbps;
* the destination node (DN), during the reception of the packet, estimates the quality of the radio link according to the IEEE 802.15.4 standard specification. If the LQI parameter is smaller than the adjusted threshold value, feedback information is sent to the SN, with the suggestion of reducing the bitrate for the next transmission;
* the SN decides to reduce the throughput for the next transmissions based on the comparison of the number of feedback packets, \(N_{ack}\), in the \(W\) time window, with a predetermined threshold value, \(N_{th}\), of the packets' number;
* the SN decides to increase the throughput of subsequent transmissions if, in the time window, \(X\), it has not received feedback with the suggestion of reducing the bitrate from the SN. The authors of the ALBS algorithm do not specify the method for determining the algorithms' parameters. However, their exemplary values have been indicated: \(N_{th}\ =\ 4\), \(W\ =\ 1\) s, and \(X\ =\ 5\) s.  
The last chosen algorithm is the LA algorithm, which is intended to work in the WBANs. It uses the probability of packet loss (short-term estimation of the PER) and an estimation of the radio channel quality parameter. The decision on the next transmission bitrate \(R_{b}\) is made on the basis of a single value of the Signal-to-Noise Ratio (SNR) and short-term frame error rate.