Zasowski, Thomas

A System Concept for Ultra Wideband (UWB) Body Area Networks

Wireless body area networks have recently gained a lot of interest due to multiple possible applications such as wireless health monitoring or wearable computing. Because of the rather simple hardware realizations and the energy efficiency, ultra wideband (UWB) communication has become one promising technology for the use in wireless body area networks (BAN). After pointing out the motivation of this work and highlighting its contribution, a definition of body area networks is presented as well as a brief introduction on UWB. There, the main promises of UWB communications are presented as well as the principles of some typical receiver structures for UWB. Since UWB communication at the human body is a brand new topic, channel measurements at the human body are performed. The frequency range for these measurements is chosen from 2 to 8 GHz. Based on 1100 channel measurements a channel model for the UWB BAN is derived. Using the Akaike information criterion (AIC) it is shown that the channel decays over the time and that the channel taps are log-normal distributed. The channel at the head is of particular interest as most human communication organs such as mouth, ears, and eyes are located there. Therefore, the ear-to-ear link, which can be regarded as a worst case scenario at the head due to the missing line-of-sight component, is considered to specify the impact of the channel on the system design. When considering the ear-to-ear link it is shown by means of theory, simulations, and measurements that the direct transmission through the head is attenuated so much that it is negligible. Therefore, antennas should be designed in a way that they do not radiate into the body but away from it or along its surface. Moreover, it is shown that the channel is robust against distance variations between the antenna and the skin, and that reflections and absorptions are caused by the body. For the ear-to-ear link the antennas should be placed behind the ears to get the smallest channel attenuations. From the measurements it can also be observed that the main energy of the channel impulse response is contained in a very small time window. Thus, non-coherent receivers with a short integration duration can capture almost the whole energy of the channel. Since UWB systems are a secondary spectrum user, the impact of existing wireless services on UWB is investigated as well. Due to the low transmit power not only the in-band but also the out-of-band interferers are harmful for UWB transmission. Based on frequency-domain and time-domain measurements it is shown that interference not close to the UWB device can be handled by using filters. However, this is not sufficient enough if an interferer is in close vicinity of the UWB device. Therefore, the temporal cognitive medium access is presented to avoid the interference from burst-wise transmitting devices. There, the UWB system listens if the channel is occupied by an interferer and it transmits only in case that no other system is active at the same time. For such a temporal cognitive MAC an expression is given to calculate the optimum UWB packet length. Assuming different interference scenarios, the packet lengths are evaluated. Moreover, it is shown that reasonable usable idle times can be achieved, which the UWB device can use for transmission, and strict latency time requirements can be met. ALOHA, 1-persistent CSMA, and non-persistent CSMA are considered as access schemes for the performance evaluation of the temporal cognitive MAC. For evaluation, two different cases are distinguished, with and without bandpass filter at the UWB receiver. It is shown that a UWB device with bandpass filter that uses the temporal cognitive MAC in conjunction with non-persistent CSMA has low packet error rates below 102 for up to about 15 active UWB links. Due to complexity reasons non-coherent receivers are the most promising solution for the use in UWB devices. Hence, the focus in this thesis lies on the en