# A Novel Bitrate Adaptation Method for Heterogeneous Wireless Body Area Networks
## 1 Introduction  
Wireless Body Area Networks (WBANs) can be classified according to the type of radio link between nodes, i.e., in-body, on-body, and off-body communication. In-body communication is realized in radio links, where at least one node is placed inside the human body. Many conducted research works on radio channel analysis in such networks investigate the impact of the internal structure of the human body on radio wave propagation, or the penetration of the radio wave into the human body. The on-body radio link considers communication between the nodes located close to the surface of the same human body. In this article, the research focuses on off-body communication, where the human body interacts with one side of the radio link and the other node is placed at a greater distance outside of the body. This is one of the out-of-body communication cases. The second one is body-to-body communication, where two nodes operate at a distance between two human bodies [1]. WBANs' topology is usually formed in the so-called star topology, characterized by centralized management, low scalability, limited coverage, and a high failure rate. The failure of the coordinating node implies the malfunction of the entire managed subnet. The typical operational range of such a network is in the range of 3 m to 6 m, and, very rarely, even up to several dozen meters. The other characteristic factor of WBANs is the multiple access method to the transmission medium. The two main methods are: Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and Time Division Multiple Access (TDMA). In this article, the TDMA method is considered, due to the assumption of periodic and organized data transmission between the nodes in the network [1; 2; 3; 4; 5]. Another type of WBAN classification factor can be the number of radio interfaces (RIs) used in the network node, known as homogeneous and heterogeneous WBANs. A heterogeneous network is defined as a network composed of nodes with many simultaneously operating RIs, where generateddata streams should be allocated before transmission via available radio links. Considering this, the effective usage of available time-frequency resources during their work becomes one of the key aspects. The analyzed issue is characteristic not only for the WBANs, but also for Wireless Sensor Networks (WSNs), cellular networks, or cognitive radios [6; 7; 8; 9; 10; 11; 12; 13]. In WSNs and WBANs, a static allocation of data streams with fixed physical layer parameters during the operation of the node is usually used. This ensures the reduction of control data redundancy exchanged between nodes in the network, and simplification of the medium access control protocols. Despite these advantages, it is known that the efficiency of using the available time-frequency resources can be increased, for example, by adaptation of transmission bitrate, depending on the variable radio channel parameters or the short-term packet error rate (PER) [14; 15; 16]. It should be noted that protocol solutions on the network level are already investigated and described in the literature, including dynamic time slots scheduling with distributed or centralized control methods. However, these investigations are limited to homogeneous radio links, which may limit their applicability in heterogeneous networks. Papers [17; 18] focus on changes in the rules of timeslot's assignment in the TDMA network, but without any changes in physical layer parameters, and are also limited to one radio interface only. In contrary, in this paper, the authors proposed an algorithm of adaptive data rate selection without changing the timeslot arrangement. In the second section, the authors describe algorithms for the adaptive selection of the data rate. The three most commonly known algorithms are briefly presented, indicating their principle of operation and the key characteristics. Additionally, a modification of two algorithms is proposed to counter the consequences of the fast fading phenomenon and to increase their effectiveness.