In this paper, we first propose a 3-dimensional (3-D) non-stationary geometry-based stochastic model (GBSM) for maritime massive multiple-input multiple-output (MIMO) communication systems with the uniform circular array (UCA) configuration. To reduce the model complexity and improve the mathematical tractability, a novel beam domain channel model (BDCM) is then proposed based on the transformation of the corresponding GBSM from the array domain to the beam domain for maritime communications. In the proposed BDCM, the beamforming matrices suitable for UCA structures are constructed and their invertibility is demonstrated to ensure the practicability of the BDCM. Two methods are used to characterize the array non-stationarity in maritime massive MIMO channels. First, the evolution of clusters over the large UCA is modeled by the visibility regions (VRs) attached to individual multipath components (MPCs). Second, the sphere wavefront (SWF) effect is captured by dividing the UCAs into several sub-arrays. Based on the proposed GBSM and BDCM, some important channel statistical properties are studied and compared, including channel power, power leakage, space-time-frequency correlation function (STF-CF), and root-mean-square (RMS) Doppler/beam spreads. Also, the importance of considering the array non-stationarity in maritime communication channels is revealed.

The integrated sensing and communication (ISAC) refers to the integration of radio sensing and wireless communications to realize the multiplexing of space, time, and frequency resources. In the sixth generation (6G) wireless networks, ISAC is considered as one of the most promising technologies. In this paper, a novel ISAC channel model combining forward and backward scattering is proposed. In addition to the non-stationarity caused by motions, the correlations between sensing and communication channels are investigated. The channel characteristics of sensing such as forward scattering and backward scattering are introduced into the communication channel model. Utilizing the correlations between sensing and communication channels, the communication channel model is divided into line-of-sight (LOS), forward scattering, and backward scattering components. These three components are summed according to probability weighting to obtain a more accurate channel model for sensing assisted communication systems. Moreover, important statistical properties of the proposed ISAC channel model are derived and simulated. The analytical and simulation results match well, demonstrating the correctness of derivations and simulations. Some derived/simulated statistical properties are verified by corresponding measurement data, which indicates the utility of the proposed ISAC channel model.

Characterization and modeling of massive multiple-input multiple-output (MIMO) channel has been one of the research hotspots in the field of wireless communications. One important feature of the massive MIMO channel is the spatial non-stationarity. To statistically model the spatial non-stationary massive MIMO channels, a cluster-based channel model is proposed in this paper. The model incorporates both inter- and intra-cluster properties and the cluster evolution over the large-scale array. A hybrid data processing scheme is applied to extract the multipath components (MPCs) and clustering the MPCs over a large-scale antenna array. The global angular spread, cluster angular spread, and cluster delay spread are modeled with log-normal distributions. Observed cluster length and MPC length within clusters, which are introduced to describe the cluster existence over the array and MPCs existence within the cluster, respectively, are statistically modeled with the exponential distributions. Moreover, both the cluster and MPC arrival intervals, which are used, respectively, to describe the cluster occurrence position on the array and MPC occurrence position within the cluster, can be statistically modeled with the uniform distributions. Finally, the model implementation is validated by comparing with the different channel performance metrics between measurements and simulations.

With the rapid development of wireless communications, the understanding of three-dimensional (3D) propagation channels becomes essential for design and testing of some new wireless technologies, e.g., massive multiple-input multiple-output (MIMO) and full dimensional beamforming. To not only fully exploit the 3D multiplexing but also circumvent the size limitation of base station (BS), antenna elements in massive MIMO are usually arranged both horizontally and vertically. Based on an elaborate channel measurement campaign conducted at 11 GHz in a lobby environment, a 3D extended cluster-based channel model is proposed in this paper for massive multiple-input single-output (MISO) multi-user communications. In the model, the channel characteristics in both azimuth and elevation dimensions, and the visibility regions which are parametrized by observed cluster lengths across the large-scale array in both horizontal and vertical directions, are taken into consideration. Moreover, the spherical wavefront phenomenon observed from the measurements is also incorporated in the model. Model parametrization, implementation, and validation are presented in detail. Validations show that the proposed model can accurately reflect the realistic channel, and the spatial non-stationarity and the spherical wavefront should be carefully considered in the channel models for large-scale array communications.

Massive multiple-input multiple-output (MIMO) has been selected as one of the key technologies of the fifth generation mobile communication system (5G). It can provide high spectral and power efficiency, thus it is suitable to be deployed in different hotspot scenarios. In this paper, a channel measurement campaign using a wideband channel sounder and a 256-element virtual uniform rectangular array (URA) for massive MIMO communications is presented. The measurements were respectively conducted at 6 GHz and 11 GHz, and the subway station environment was considered. The typical channel parameters, root mean square (RMS) delay spread and coherence bandwidth, are analyzed based on the measurements. Moreover, the channel characteristics in angular domain are obtained by applying the space-alternating generalized expectation-maximization algorithm (SAGE). The SAGE estimates are validated by relating them to the physical environment. The overall elevation angle distribution of multipaths is found to be fitted well with Laplace distributions. The global angular spread, including the azimuth spread of departure (ASD) and elevation spread of departure (ESD), are both fitted well with Lognormal distributions. The results in this paper can be fed into the new channel simulator for massive MIMO, and are useful for the design and application of the practical massive MIMO system in the future.

Massive multiple-input multiple-output (MIMO) is one of the key technologies of the fifth generation mobile communication system (5G). For development of massive MIMO systems, the directional properties of their wireless channel are of great importance. In this paper, directional analysis is presented based on a channel measurement campaign for massive MIMO communications in a theater environment. The measurements were conducted at 11 GHz with a bandwidth of 200 MHz. A 256-element virtual uniform rectangular array (URA) was used during the measurements, and the channel characteristics in angular domain are determined by applying the space-alternating generalized expectation-maximization algorithm (SAGE). According to the SAGE estimates, channel nonstationarity over the large-scale array is discussed. The dominant scatterers are identified, by directly relating the SAGE estimates to the physical objects in the measurement environment. Moreover, direction spread over the large-scale array at the Tx side is estimated, and channel performance on beamforming is evaluated. Corresponding analysis is given in detail. These results are useful for the design and application of the practical massive MIMO system in the future.

In this paper, the channel model is proposed to compute the path loss, delay and noise at THz band of interest (0.5 THz ~ 1.5 THz). The results shows that THz band channel is strongly dependent on both the type of the medium and the distance while the concentration of water have a lot of influence because it not only causes attenuation to the THz wave but also introduces non-white noise. Therefore, the proposed model paves the way to the further studies considering more body structures and tissue properties.

The papers in this special section were presented at three well-known conferences organized in 2014: EAI Mobihealth, IEEE HealthCom, and IEEE Biomedical and Health Informatics. EAI Mobihealth is an annually organized conference, which started in 2010, to address the demands of the rapidly evolving disciplines of wireless communications, mobile computing, and sensing technologies in healthcare. The IEEE-Healthcom is held every year since 1999 in different countries in Asia, Europe, and in America. It aims at bringing together interested parties working in the field of healthcare to exchange ideas, discuss innovative and emerging solutions, and develop collaborations. The IEEE Biomedical Health Informatics Conference started in 2013 and is organized every year providing the forum to showcase enabling technologies of computing, devices, imaging, sensors, and systems that optimize the acquisition, transmission, processing, storage, retrieval, visualization, and analysis of medical data. The aim of this special section is to present an overview of recent advances in sensing technologies, monitoring of patients, security and privacy of data transfer, provision of collaborative environments, data gathering and analysis from various sources, and predictive models, which all finally target the best strategy for patient monitoring and treatment.

The paper presents an initial study of analyzing the propagation of electromagnetic waves at terahertz frequencies inside the human skin tissues in the frequency range 0.8-1.2 THz. The skin model is represented by three layers: stratum corneum (SC), epidermis, and dermis, and the effect of the presence of sweat ducts is also studied. The path loss resulting from the signal propagation between a transmitting antenna, located at the epidermis layer, and a receiving antenna, located at the dermis layer, is analyzed for different distances and different frequencies, in addition to changing the number of sweat ducts. Results show that the path loss through the skin tissues depends on the distance between the transmitter and the receiver, the frequency used, and the number of sweat ducts as the path loss decreases by approximately 1 dB for increasing the number of sweat duct from one duct to two ducts. Moreover, the path loss is statistically studied and its distribution is approximately Gaussian.

A pair of low-power wireless sensor nodes, operating in the 2.45 GHz ISM band, are used to record the radio propagation of a particular on-body channel, the waist-chest channel. The recorded time-sampled radio link data is analysed in the frequency domain. It is shown that the wave propagation along the human body's surface embeds not only the radio channel characteristics, but also physiological features.

The paper presents a subject-specific radio propagation study and system modeling in wireless body area networks using a simulation tool based on the parallel finite-difference time-domain technique. This technique is well suited to model the radio propagation around complex, inhomogeneous objects such as the human body. The impact of different digital phantoms in on-body radio channel and system performance was studied. Simulations were performed at the frequency of 3-10 GHz considering a typical hospital environment, and were validated by on-site measurements with reasonably good agreement. The analysis demonstrated that the characteristics of the on-body radio channel and system performance are subject-specific and are associated with human genders, height, and body mass index. Maximum variations of almost 18.51% are observed in path loss exponent due to change of subject, which gives variations of above 50% in system bit error rate performance. Therefore, careful consideration of subject-specific parameters are necessary for achieving energy efficient and reliable radio links and system performance for body-centric wireless network.

The paper aims to investigate the optimum locations for ultra wideband (UWB) nodes placement on the body. The performance of UWB radio propagation links is experimentally investigated by considering communications between body-worn base units and off-body access points. Radio channel parameters are derived and analysed for different sectors of the human body. System-level modelling, based on signal to noise ratio (SNR) and bit error rate (BER) is analytically performed on the basis of multi band orthogonal frequency division multiplexed (OFDM) system.

This paper presents an experimental investigation of off-body radio propagation channels based on commercially available UWB wireless tags. The tags operate at frequency band of 5.9~7.2 GHz. A measurement campaign was performed both in the chamber and in an indoor environment for comparison. The path loss for nine different channels is shown with parameters describing the radio channel derived through statistical analysis.

With the development of miniaturized wireless wearable and implantable medical devices, pervasive monitoring is becoming a clinical reality. With an increasing drive for minimizing power utilization, optimal antenna design and radio wave propagation are important topics for BSN (body sensor networks) research. To this end, subject-specific modeling is essential for achieving a truly personalized and optimized sensor design. In this paper, a Volumetric Graph Laplacian method is used for subject-specific whole-body mesh warping for finite-difference time-domain (FDTD) simulations. Validation of the method is shown with data from both phantom and MR studies with FDTD simulations.

Radio-frequency identification (RFID) is a growing technology, with the potential for reducing medical errors and improving the quality of healthcare in hospitals. The benefits include more secure and safe access in the healthcare environment (with the possibility, for example, to track patients, personnel, and equipment), as well as providing the means to easily identify patients and their medications with low risk of error. In this paper, we present an overview of the challenges faced in antenna design, electromagnetic modeling and wave propagation for RFID implants. The performance of ultra-high-frequency (UHF) subcutaneous tag antennas was investigated numerically and validated with measurements. Furthermore, the wave propagation between an off-body reader and an implanted tag was analyzed, in both free space and a scattered indoor environment. Results demonstrated that a passive tag solution allows a very limited communication range, due to the body losses, the electrically small size of the antenna, and nulls in the radiation pattern. In comparison, a maximum communication range of 10 m was predicted as achievable for an active tag operating indoors with a limited power (-20 dBm).