A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> In this tutorial survey the principles of radio propagation in indoor environments are reviewed. The channel is modeled as a linear time-varying filter at each location in the three-dimensional space, and the properties of the filter's impulse response are described. Theoretical distributions of the sequences of arrival times, amplitudes and phases are presented. Other relevant concepts such as spatial and temporal variations of the channel, large-scale path losses, mean excess delay and RMS delay spread are explored. Propagation characteristics of the indoor and outdoor channels are compared and their major differences are outlined. Previous measurement and modeling efforts are surveyed, and areas for future research are suggested. > <s> BIB001 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> The paper presents UWB channel measurements from 3 to 6 GHz for a body area network (BAN) in an anechoic chamber and an office room. Both, transmit and receive antenna were placed directly on the body. Channel parameters as delay spread and path loss are extracted from the measurements and the influence of the body is highlighted. We show that in some situations there are significant echoes from the body (e.g. from the arms) and we observed deterministic echoes from the floor that could help to simplify a RAKE receiver structure. Finally, we consider the overall energy consumption of the BAN and give decision regions for singlehop and multihop links in relation to signal processing energy. <s> BIB002 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> A channel model for a wireless body area network at 400 MHz, 900 MHz and 2.4 GHz is derived. The electromagnetic wave propagation around the body is simulated with a finite-difference time-domain simulator. Creeping waves were identified as the propagation path around the body. Its impact on the delay spread in an indoor environment is discussed. <s> BIB003 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> Due to the ongoing miniaturization of electronic devices and due to a multitude of applications, wireless body area networks (WBANs) have gained much interest recently. Ultra wideband (UWB) communication is one promising transmission technology for WBANs due to reduced hardware complexity. To optimize receiver structures and antennas for UWB WBANs it is necessary to know the propagation mechanisms at the human body. In this paper, we focus on transmission at the head and consider direct transmission, surface waves, reflections, and diffraction as possible propagation mechanisms in the frequency range between 1.5-8 GHz. We show theoretically and by measurement results that the direct path is attenuated such that direct transmission through the head is negligible. Based on measurements we conclude by process of elimination that diffraction is the main propagation mechanism around the human body and that surface waves and reflections are negligible. Finally, we discuss the impact of the propagation mechanisms on the UWB WBAN communication system. <s> BIB004 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> Using wireless sensors placed on a person to continuously monitor health information is a promising new application. In developing these sensors, detailed knowledge of the communication channel is essential. However, there are currently very few measurements describing propagation around the body. To address this problem, we have measured electromagnetic waves traveling near the torso to derive a simple pathless law. The pathless law is then extended to include the influence of arm movements and a surrounding office environment. This paper describes our measurement campaign and the basic characteristics of the body area radio channel. <s> BIB005 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> Using wireless sensors placed on a person to continuously monitor health information is a promising new application. However, there are currently no models describing the radio channel around the human body making it difficult to design a suitable communication system. To address this problem, we have simulated electromagnetic wave propagation around the body and incorporated these results into a simple model. We then compared this model with measurements taken around the human torso and with previous studies in the literature. This paper proposes a simple statistical channel model useful for evaluating both UWB and (after resampling) narrow-band body area communication systems. <s> BIB006 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> A new empirical path loss model for wireless communication at 2.4 GHz above a flat, lossy medium, representing human tissue, is presented. The model is valid for dipole antennas for heights up to 5 cm above the phantom and for distances up to 40 cm, and was applied to muscle and brain simulating media. For antennas placed close to the lossy medium, it was found that antenna height has a major influence on path loss. The model has been validated by measurements and simulations, which show excellent agreement. <s> BIB007 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> Wireless body area networks (WBANs) offer many promising new applications in the area of remote health monitoring. An important element in the development of a WBAN is the characterization of the physical layer of the network, including an estimation of the delay spread and the path loss between two nodes on the body. This paper discusses the propagation channel between two half-wavelength dipoles at 2.45 GHz, placed near a human body and presents an application for cross-layer design in order to optimize the energy consumption of different topologies. Propagation measurements are performed on real humans in a multipath environment, considering different parts of the body separately. In addition, path loss has been numerically investigated with an anatomically correct model of the human body in free space using a 3-D electromagnetic solver. Path loss parameters and time-domain channel characteristics are extracted from the measurement and simulation data. A semi-empirical path loss model is presented for an antenna height above the body of 5 mm and antenna separations from 5 cm up to 40 cm. A time-domain analysis is performed and models are presented for the mean excess delay and the delay spread. As a cross-layer application, the proposed path loss models are used to evaluate the energy efficiency of single-hop and multihop network topologies. <s> BIB008 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> Advances in wireless communication technologies, such as wearable and implantable biosensors, along with recent developments in the embedded computing area are enabling the design, development, and implementation of body area networks. This class of networks is paving the way for the deployment of innovative healthcare monitoring applications. In the past few years, much of the research in the area of body area networks has focused on issues related to wireless sensor designs, sensor miniaturization, low-power sensor circuitry, signal processing, and communications protocols. In this paper, we present an overview of body area networks, and a discussion of BAN communications types and their related issues. We provide a detailed investigation of sensor devices, physical layer, data link layer, and radio technology aspects of BAN research. We also present a taxonomy of BAN projects that have been introduced/proposed to date. Finally, we highlight some of the design challenges and open issues that still need to be addressed to make BANs truly ubiquitous for a wide range of applications. <s> BIB009 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> On-Body Radio Propagation Model <s> In recently published integrated medical monitoring ::: systems, a common thread is the high power consumption of the radio ::: compared to the other system components. This observation is ::: indicative of a natural place to attempt a reduction in system ::: power. Narrowband receivers in-particular can enjoy significant ::: power reduction by employing high-Q bulk acoustic resonators as ::: channel select filters directly at RF, allowing down-stream analog ::: processing to be simplified, resulting in better energy efficiency. ::: But for communications in the ISM bands, it is important to employ ::: multiple frequency channels to permit ::: frequency-division-multiplexing and provide frequency diversity in ::: the face of narrowband interferers. The high-Q nature of the ::: resonators means that frequency tuning to other channels in the ::: same band is nearly impossible; hence, a new architecture is ::: required to address this challenge. A multi-channel ultra-low power ::: OOK receiver for Body Area Networks (BANs) has been designed and ::: tested. The receiver multiplexes three Film Bulk Acoustic ::: Resonators (FBARs) to provide three channels of frequency ::: discrimination, while at the same time offering competitive ::: sensitivity and superior energy efficiency in this class of BAN ::: receivers. The high-Q parallel resonance of each resonator ::: determines the passband. The resonator's Q is on the order of 1000 ::: and its center frequency is approximately 2.5 GHz, resulting in a ::: -3 dB bandwidth of roughly 2.5 MHz with a very steep rolloff. ::: Channels are selected by enabling the corresponding LNA and mixer ::: pathway with switches, but a key benefit of this architecture is ::: that the switches are not in series with the resonator and do not ::: de-Q the resonance. The measured 1E-3 sensitivity is -64 dBm at 1 ::: Mbps for an energy efficiency of 180 pJ/bit. The resonators are ::: packaged beside the CMOS using wirebonds for the ::: prototype. <s> BIB010
This section summarizes the most important findings related to the path loss and the small-scale fading of on-body radio channels. Path loss Model: According to BIB003 BIB004 BIB008 BIB002 , propagated waves are more likely to diffract around the human body, rather than penetrate it. Hence, traditional formulas for wireless communications can be applied, such as the Friis equation that caclulates the path loss as a function of the distance d BIB001 : where G t and G r are the respective transmitter and receiver antenna gains, λ is the wavelength, and n is the path loss exponent that depends on the propagation environment. In free space propagation environments, the path loss exponent is equal to n = 2. In the context of WBANs, however, n has a higher value, depending on the scenario. In particular, when the transmit and receive antennas are placed at the same side of the body (Line-of-Sight-LOS situation), it has been estimated that the values of n range between 3 and 4, depending on the position of the device (i.e., the path loss on the arm is lower that the one on the trunk). Then, in Non-LOS (NLOS) cases that occur when the antennas are placed at different sides of the body, n assumes higher values, between 5 and 7.4 BIB004 BIB002 . It is important to mention that in the context of WBANs, frequency does not significantly affect the path loss BIB005 . On the other hand, the current user activity and the antenna height (defined as the distance from the body) are the two major contributors to the path loss characteristic of wearable WBANs. Regarding the former, the impact of user activity (i.e., walking) on the path loss has been experimentally studied in BIB010 . The obtained results show that the subject's mobility causes significant deviations in the path loss, with respect to the average path loss values obtained when the subject remains static. Concerning the antenna height, the authors in BIB007 claim that the closer the antenna is to the body, the higher is the path loss. A difference of more than 20 dB can be obtained by comparing the path loss for antennas placed at 5 mm and 5 cm from the body surface. Small-scale fading: Small-scale fading is different in on-body WBANs with respect to other wireless environments. Several distributions have been proposed to model small-scale fading in different frequency bands BIB009 BIB006 . However, traditional Rayleigh and Rician distributions have been proven inefficient, while the lognormal distribution seems to be the best fit  . In addition, measurements in BIB006 have indicated that the multipath components can be divided into two clusters. The first cluster is due to the initial wave diffraction around the body and the second is due to reflections on the ground. The correlation between the bins of the two clusters is very weak, and therefore the bins from the different clusters can be considered as statistically independent. On the other hand, the different delay bins of the same cluster are highly correlated. This can be explained by considering that when the path length is very short, the multipath components have overlapping path trajectories. In a real situation, however, the reflections from the scatterers in the surrounding environment BIB002 should be also taken into account. In conclusion, the recommended body area channel model for wearable WBANs should consider a fixed two cluster mutlipath model containing: (i) a dominant direct component; (ii) a set of early reflected waves adding (possibly distructively) to the dominant wave; and (iii) a set of excessively delayed waves, adding incoherently to the dominant wave. The dominant wave amplitude is determined by an exponential path loss and a log normal fading coefficient that is strongly correlated with the fading coefficients of the early delayed waves.