Alejandro Fornes-Leal

Experimental Path Loss Models for In-Body Communications Within 2.36-2.5 GHz

Biomedical implantable sensors transmitting a variety of physiological signals have been proven very useful in the management of chronic diseases. Currently, the vast majority of these in-body wireless sensors communicate in frequencies below 1 GHz. Although the radio propagation losses through biological tissues may be lower in such frequencies, e.g., the medical implant communication services band of 402 to 405 MHz, the maximal channel bandwidths allowed therein constrain the implantable devices to low data rate transmissions. Novel and more sophisticated wireless in-body sensors and actuators may require higher data rate communication interfaces. Therefore, the radio spectrum above 1 GHz for the use of wearable medical sensing applications should be considered for in-body applications too. Wider channel bandwidths and smaller antenna sizes may be obtained in frequency bands above 1 GHz at the expense of larger propagation losses. Therefore, in this paper, we present a phantom-based radio propagation study for the frequency bands of 2360 to 2400 MHz, which has been set aside for wearable body area network nodes, and the industrial, scientific, medical band of 2400 to 2483.5 MHz. Three different channel scenarios were considered for the propagation measurements: in-body to in-body, in-body to on-body, and in-body to off-body. We provide for the first time path loss formulas for all these cases.

Ultra wideband propagation for future in-body sensor networks

Body area network (BAN) technology can enable the real-time collection and monitoring of physiological signals for personalized healthcare. Implantable biomedical sensors transmitting continuously clinical information to an external unit can facilitate the involvement of the patients in the management of chronic diseases. In addition, ingestible sensors like the wireless capsule endoscope (WCE) have been proven extremely useful as clinical diagnostic tools. It is envisaged that such devices will evolve to also perform in-body therapeutic procedures. Future medical applications may require the interconnection of two or more of these in-body devices to interchange information for better diagnostics or to relay data from deeply implanted sensors. In this context, ultra wideband (UWB) radio links can be used for the communication interfaces of in-body sensors due to their large bandwidth and low power consumption. Nevertheless, little is known about the behavior of the in-body to in-body (IB2IB) radio channel in the UWB spectrum. This paper aims to fill this gap by providing insight into the behavior of the IB2IB channel based on propagation measurements in 3.1-8.5 GHz. Because of the impossibility to conduct in-body measurements with human subjects, we used a phantom that emulated the dielectric characteristics of the human muscle tissue. The path loss as a function of the distance between antennas and the frequency are thoroughly discussed.

Experimental ultra wideband path loss models for implant communications

Ultra wideband (UWB) signals possess characteristics that may enable high data rate communications with deeply implanted medical sensors and actuators. Nevertheless, this application could be hindered in part by international spectrum regulations, which restrict UWB communications to 3.1–10.6 GHz where propagation conditions through the human body are rather unfavorable. Therefore, for the proper feasibility assessment and design of implant communications using UWB signals, accurate models of the radio channel are of utmost importance. Hence, we present UWB path loss models for the two most commonly used implant communication scenarios, i.e., in-body to on-body (IB2OB) and in-body to off-body (IB2OFF). These models were extracted from in vivo measurements in the abdominal cavity within 3.1–8.5 GHz using a living porcine subject. A thorough comparison between this modeling approach and channel measurements using a homogeneous phantom, which mimics the electromagnetic behaviour of muscle tissue, is presented too. Measurements in a homogeneous propagation medium are simpler to perform, but they fail to capture several physiological effects observed in a living subject. Thus, we measured the deviation between the phantom-based and in-vivo-based path loss models. In general, phantom measurements yielded a more pessimistic estimation of the path loss. We provide the correction factors to adjust easy-to-perform phantom-based measurements to more realistic path loss values, which can assist the biomedical engineer in the early stages of design and testing of wireless implantable devices.

Dielectric characterization of healthy and malignant colon tissues in the 0.5-18 GHz frequency band.

Several reports over the last few decades have shown that the dielectric properties of healthy and malignant tissues of the same body organ usually show different values. However, no intensive dielectric studies of human colon tissue have been performed, despite colon cancers being one of the most common types of cancer in the world. In order to provide information regarding this matter, a dielectric characterization of healthy and malignant colon tissues is presented. Measurements are performed on ex vivo surgery samples obtained from 20 patients, using an open-ended coaxial probe in the 0.5-18 GHz frequency band. Results show that the dielectric constant of colon cancerous tissue is 8.8% higher than that of healthy tissues (p  =  0.002). Besides, conductivity is about 10.6% higher, but in this case measurements do not have statistical significance (p  =  0.038). Performing an analysis per patient, the differences in dielectric constant between healthy and malignant tissues appear systematically. Particularized results for specific frequencies (500 MHz, 900 MHz, 2.45 GHz, 5 GHz, 8.5 GHz and 15 GHz) are also reported. The findings have potential application in early-stage cancer detection and diagnosis, and can be useful in developing new tools for hyperthermia treatments as well as creating electromagnetic models of healthy and cancerous tissues.

Spatial In-Body Channel Characterization Using an Accurate UWB Phantom

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.

Spectrum Sharing for LTE-A and DTT: Field Trials of an Indoor LTE-A Femtocell in DVB-T2 Service Area

In the near future, many applications such as environmental sensors, smart objects, health sensors, and personal devices will be connected to mobile networks requiring additional spectrum. Studies have been made to demonstrate a low occupancy time and locations on the digital terrestrial television (DTT) band. This available or unused spectrum have been called “TV-White-Spaces.” In the 2015 World Radiocommunication Conference, the ITU decided to re-allocate the 700 MHz band (694-790 MHz) for International Mobile Telecommunications (IMT) services and also emplaced the discussion of the future use of the DTT band (470-694 MHz) to 2025, for which studies have been requested. In this paper, we study a particular case which goes a step beyond the previous ones, as it aims at sharing the same frequency band in the same area between long term evolution-advance (LTE-A) and digital video broadcasting-terrestrial second generation (DVB-T2) technologies. Those geographical areas that are not covered because the useful DTT signal is obstructed by the environment or it has a limited coverage by the network design can be called “micro-TVWS.” We assume that a DVB-T2 transmitter provides coverage for fixed rooftop reception as a primary service, to a building in which a LTE-A femtocell is installed indoors for local coverage, as a secondary service. The results have been obtained by laboratory emulation and validated through field measurements using professional equipment. Our results provide the technical restrictions of the LTE-A femtocell, mainly on the maximum allowable effective isotropic radiated power that could transmit on the DTT band in terms of carrier separation, from co-channel to adjacent band. These results meet the need of spectrum for IMT-Advanced technologies, so spectrum sharing is proposed in this paper as a new solution to make an efficient use of this resource.

Tailor-Made Tissue Phantoms Based on Acetonitrile Solutions for Microwave Applications up to 18 GHz

Tissue-equivalent phantoms play a key role in the development of new wireless communication devices that are tested on such phantoms prior to their commercialization. However, existing phantoms cover a small number of tissues and do not reproduce them accurately within wide frequency bands. This paper aims at enlarging the number of mimicked tissues as well as their working frequency band. Thus, a variety of potential compounds are scanned according to their relative permittivity from 0.5 to 18 GHz. Next, a combination of these compounds is characterized so the relation between their dielectric properties and composition is provided. Finally, taking advantage of the previous analysis, tailor-made phantoms are developed for different human tissues up to 18 GHz and particularized for the main current body area network (BAN) operating bands. The tailor-made phantoms presented here exhibit such a high accuracy as would allow researchers and manufacturers to test microwave devices at high frequencies for large bandwidths as well as the use of heterogeneous phantoms in the near future. The key to these phantoms lies in the incorporation of acetonitrile to aqueous solutions. Such compounds have a suitable behavior to achieve the relative permittivity values of body tissues within the studied frequency band.

Accurate broadband measurement of electromagnetic tissue phantoms using open-ended coaxial systems

New technologies and devices for wireless communication networks are continually developed. In order to assess their performance, they have to be tested in realistic environments taking into account the influence of the body in wireless communications. Thus, the development of phantoms, which are synthetic materials that can emulate accurately the electromagnetic behaviour of different tissues, is mandatory. An accurate dielectric measurement of these phantoms requires using a measurement method with a low uncertainty. The openended coaxial technique is the most spread technique but its accuracy is strongly conditioned by the calibration procedure. A typical calibration is performed using an open circuit, a short circuit and water. However, this basic calibration is not the most accurate approach for measuring all kinds of materials. In this paper, an uncertainty analysis of the calibration process of openended coaxial characterization systems when a polar liquid is added to the typical calibration is provided. Measurements are performed on electromagnetically well-known liquids in the 0.5 - 8.5 GHz band. Results show that adding methanol improves the accuracy in the whole solution domain of the system, mainly when measuring phantoms that mimic high water content tissues, whereas ethanol is more suitable for measuring low water content tissue phantoms.

Formulas for easy-to-prepare tailored phantoms at 2.4 GHz ISM band

Emerging integration of communication networks into wearable or implantable body devices involves a challenge due to the transmitting medium, the body itself. This medium is heterogeneous and lossier than air, so devices that are supposed to work on it should be tested in tissue-equivalent materials. A number of materials with the electromagnetic response of body tissues have been proposed. Most of them are sucrose aqueous solutions that are supposed to simulate humans muscle tissue mainly within medical frequency bands. However, these recipes are restricted to a single tissue and it is difficult to adapt them to fit the permittivity values of different body tissues. The significance of this study lies in the development of a mathematical relationship that models the dielectric properties of an aqueous solution according to the concentration of sugar and salt at 2.4 GHz, the frequency around which an Industrial, Scientific and Medical (ISM) band is placed. Thus, it becomes possible to create custom-made phantoms with simple and accessible ingredients that are easy to prepare in any laboratory.