In-body sensor networks are those networks where at least one of the sensors is located inside the human body. Such wireless in-body sensors are mainly used for medical applications, collecting and monitoring important parameters for health and diseases treatment. The IEEE Standard 802.15.6-2012 for Wireless Body Area Networks (WBAN) considers in-body communications in the Medical Implant Communication Service (MICS) band. Nevertheless, high data rate communications are not feasible at the MICS band due to its narrow occupied bandwidth. In this framework, Ultra-Wideband (UWB) systems have emerged as a potential solution for in-body high data rate communications, due to its miniaturization capabilities or low power consumption. In the last years, some open issues have determined the research about in-body propagation. Firstly, the propagation medium, i.e., the human body tissues, is frequencydependent and exhibits a large attenuation at UWB frequencies. Secondly, the behavior of the in-body antennas is highly dominated by the surrounding tissues. Thus, the in-body channel characterization in UWB depends not only on the channel behavior itself, but also on the methodology of characterization. This paper intends to outline the research performed in the field of UWB in-body radio channel characterization considering the propagation medium, as well as the methodology of analysissoftware simulations, phantom measurements, in vivo measurements-. Thus, authors provide an overall perspective of the current state of the art, limitations for the analysis of in-body propagation, and future perspectives for UWB in-body channel analysis.

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Abstract-Ultra-wideband (UWB) systems have emerged as a possible solution for future wireless in-body communications. However, in-body channel characterization is complex. Animal experimentation is usually restricted. Furthermore, software simulations can be expensive and imply a high computational cost. Synthetic chemical solutions, known as phantoms, can be used to solve this issue. However, achieving a reliable UWB phantom can be challenging since UWB systems use a large bandwidth and the relative permittivity of human tissues are frequency dependent. In this paper, a measurement campaign within 3.1-8.5 GHz using a new UWB phantom is performed. Currently, this phantom achieves the best known approximation to the permittivity of human muscle in the whole UWB band. Measurements were performed in different spatial positions, in order to also investigate the diversity of the in-body channel in the spatial domain. Two experimental in-body to in-body (IB2IB) and in-body to on-body (IB2OB) scenarios are considered. From the measurements, new path loss models are obtained. Besides, the correlation in transmission and reception is computed for both scenarios. Our results show a highly uncorrelated channel in transmission for the IB2IB scenario at all locations. Nevertheless, for the IB2OB scenario, the correlation varies depending on the position of the receiver and transmitter.Index Terms-Body area network, in-body communications, path loss, ultra-wideband (UWB) phantom, uncorrelated channel.

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