The new generation of telemedicine systems enables healthcare service providers to monitor patients not only in the hospital but also when they are at home. In order to efficiently exploit these systems, human information collected from end devices must be sent to the medical center through reliable data transmission. In this paper, we propose an adaptive relay transmission scheme to improve the reliability of data transmission for wireless body area networks. In our proposal, relay nodes that have successfully decoded a packet from the source node are selected as relay nodes in which the best relay with the highest channel gain is selected to forward the failed packet instead of the source node. The scheme addresses both the data collision problem and the inefficient relay selection in relay transmission. Our experimental results show that the proposed scheme provides a better performance than previous works in terms of the packet delivery ratio and end-to-end delay.

Analog and digital collaborative design techniques for wireless SoCs are reviewed in this paper. In wireless SoCs, delicate analog performance such as sensitivity of the receiver is easily degraded due to interferences from digital circuit blocks. On the other hand, an analog performance such as distortion is strongly compensated by digital assist techniques with low power consumption. In this paper, a sensitivity recovery technique using the analog and digital collaborative design, and digital assist techniques to achieve low-power and high-performance analog circuits are presented. Such analog and digital collaborative design is indispensable for wireless SoCs.

Accidental falling among elderly people has become a public health concern. Thus, there is a need for systems that detect a fall when it happens. This paper presents a portable real-time remote health monitoring system that can remotely monitor patients' movements. The system is designed and implemented using ZigBee wireless technologies, and the data is analysed using Matlab. The purpose of this research is to determine the acceleration thresholds for fall detection, using tri-axial accelerometer readings at the head, waist, and knee. Seven voluntary subjects performed purposeful falls and Activities of Daily Living (ADL). The results indicated that measurements from the waist and head can accurately detect falls; the sensitivity and reliability measurements of fall detection ranged between 80% and 90%. In contrast, the measurements showed that the knee is not a useful position for the fall detection.

Broadcasting is the process of sending a message from one node to all the other nodes in a network. Simple flooding is the simplest form of broadcasting in ad hoc wireless networks. Simple flooding provides important control, route discovery, and network information update functionality for unicast and multicast protocols. However, simple flooding generates too many broadcast message duplications in ad hoc wireless networks. Minimum spanning tree (MST)-based flooding has traditionally been used in networks to reduce the broadcast duplications by determining broadcast trees using global topology information. However, MST-based flooding still generates a lot of broadcast traffic duplications. In this paper, we propose an efficient type of flooding, called “minimizing re-transmissions” (MRT), to significantly reduce the broadcast duplications. The purpose of MRT is to minimize the number of retransmitting nodes in an ad hoc wireless network based on the network's link state information. This advantage of minimizing the number of retransmitting nodes significantly reduces broadcast message duplications in ad hoc wireless networks. The performance of MRT is analyzed, evaluated, and compared to that of the simple flooding and the MST-based flooding. Simulations are conducted using the OMNet++ Simulator in order to validate the traffic performance analysis. For our sample network, analytical and simulation results show that MRT reduces broadcast message duplications by about 80% compared to simple flooding and by about 68% compared to MST-based flooding, thus saving a significant amount of network bandwidth and energy. MRT can be used in static or mobile ad hoc wireless networks and in wired networks to implement scalable broadcast communications.

Circular Polarized Optical OFDM (CPO-OFDM) is a system that applies OFDM to optical wireless communications. This system separates OFDM signals into positive and negative signals and converts these signals into left-handed and right-handed polarization and then multiplexes the resulting polarized signals. In CPO-OFDM, the separated signals must be combined at the receiver. Then, as a noise-reduction method, the comparison method compares the signal amplitudes of the positive and negative signals and uses the signal having the larger amplitude as the received signal. However, if we use the comparison method when the received signals have background light, the combined signals are distorted. In the present paper, we herein report a method by which the receiver estimates the amplitude of the background light and then removes the background light, which is easily accomplished. Furthermore, we also report a theoretical method for analyzing the bit error rate (BER). We develop a closed form of the theoretical formula for the BER in an additive white Gaussian noise (AWGN) channel. By using this formula and through numerical integration, we investigate the theoretical BER for a scintillation channel. We compare the results of the theoretical analysis with those of the simulations. As a result, the theoretical BER is generally coincident with the BER obtained through simulation. Even if we use the closed-form formula, we can derive the BER with sufficient accuracy.

Determination of wordlength is essential for designing digital circuits because the wordlength affects system performance, hardware size, and power consumption. Variable wordlength methods that a system dynamically and effectively changes the wordlength depending on surrounding environments have been studied for power reduction in wireless systems. The conventional variable wordlength methods induce communication performance degradation when compared with a floating-point representation in time-varying fading channels. This paper discusses rapid wordlength control on packet basis and proposes a new method based on monitoring subcarrier SNRs in an OFDM receiver. The proposed method can estimate signal quality accurately and can decrease the wordlength decision errors. The simulation results have indicated that the proposed method shows better PER performance compared with the conventional methods.

Time synchronization is of paramount importance in wireless sensor networks (WSNs) due to the inherent distributed characteristics of WSNs. Border surveillance WSNs, especially, require a highly secure and accurate time synchronization scheme to detect and track intruders. In this paper, we propose a Secure and Efficient Time synchronization scheme for Border surveillance WSNs (SETB) which meets the requirements of border surveillance WSNs while minimizing the resource usage. To accomplish this goal, we first define the performance and security requirements for time synchronization in border surveillance WSNs in detail. Then, we build our time synchronization scheme optimized for these requirements. By utilizing both heterogeneous WSNs and one-way key chains, SETB satisfies the requirements with much less overhead than existing schemes. Additionally, we introduce on-demand time synchronization, which implies that time synchronization is conducted only when an intruder enters the WSN, in order to reduce energy consumption. Finally, we propose a method of deploying time-source nodes to keep the synchronization error within the requirement. Our analysis shows that SETB not only satisfies the performance and security requirements, but also is highly efficient in terms of communication and computation overhead, thus minimizing energy consumption.

Recently, wearable wireless devices or terminals have become hot a topic not only in research but also in business. Implantable wireless devices can temporarily be utilized to monitor a patient's condition in an emergency situation or to identify people in highly secured places. Unlike conventional wireless devices, wearable or implantable devices are used on or in the human body. In this sense, body-centric wireless communications (BCWCs) have become a very active area of research. Radio-frequency or microwave medical devices used for cancer treatment systems and surgical operation have completely different functions, but they are used on or in the human body. In terms of research techniques, such medical devices have a lot of similarities to BCWCs. The antennas to be used in the vicinity of the human body should be safe, small and robust. Also, their interaction with the human body should be well considered. This review paper describes some of the wearable antennas as well as implantable antennas that have been studied in our laboratory.

To design a wireless sensor network for farms, it is necessary to understand and predict the effect of vegetation. In this study, the change in the propagation loss characteristics in 920-MHz band is examined during the growth of mulberry bushes. The received signal strength indicator (RSSI) is measured as a function of the distance between the transmitting antenna (Tx) and the receiving antenna (Rx) in a 50×50m mulberry field. The Tx and Rx are placed at a height of 1.5m. Moreover, the horizontal and vertical polarizations are measured and the differences are shown. Three empirical vegetation attenuation models are introduced, and the measured data have been fitted to each model. The results show that the non-zero gradient model is the best model at predicting the vegetation attenuation in a mulberry farm regardless of the polarization or mulberry growth. It is found that the attenuation dependence on the plant height is linear. Furthermore, the results have revealed that the horizontal polarization had about 1.5 times as large an effect on the vegetation attenuation as the vertical polarization.

This paper presents a design methodology for positioning sub-platform from the viewpoint of positioning for smartphone-based location-based services (LBS). To achieve this, we analyze a mechanism of positioning error generation including principles of positioning sub-systems and structure of smartphones. Specifically, we carry out the experiments of smartphone positioning performance evaluation by the smartphone basic API (Application Programming Interface) and by the wireless LAN in various environments. Then, we describe the importance of considering three layers as follows: 1) the lower layer that caused by positioning sub-systems, e.g., GPS, wireless LAN, mobile base stations, and so on; 2) the middle layer that caused by functions provided from the platform such as Android and iOS; 3) the upper layer that caused by operation algorithm of applications on the platform.

A method for measuring the magnetic field strength for human exposure assessment closer than 20cm to wireless power transfer (WPT) systems for information household appliances is investigated based on numerical simulations and measurements at 100kHz and 6.78MHz. Four types of magnetic sources are considered: a simple 1-turn coil and three types of coils simulating actual WPT systems. A magnetic sensor whose cross sectional area is 100cm2 as prescribed in International Electrotechnical Commission 62233 is used. Simulation results show that the magnetic field strength detected by the magnetic sensor is affected by its placement angle. The maximum coefficient of variation (CV) is 27.2% when the magnetic source and the sensor are in contact. The reason for this deviation is attributable to the localization of the magnetic field distribution around the magnetic source. The coupling effect between the magnetic source and the sensor is negligible. Therefore, the sensor placement angle is an essential factor in magnetic field measurements. The CV due to the sensor placement angle is reduced from 21% to 4% if the area of the sensor coil is reduced from 100 to 0.75cm2 at 6.78MHz. However, the sensitivity of the sensor coil is decreased by 42.5dB. If measurement uncertainty that considers the deviation in the magnetic field strength due to the sensor placement angle is large, the measured magnetic field strength should be corrected by the uncertainty. If the magnetic field distribution around the magnetic source is known, conservative exposure assessments can be achieved by placing the magnetic sensor in locations at which the spatial averaged magnetic field strengths perpendicular to the magnetic sensor coils become maximum.

We propose the concept of an integrated coherent photonic wireless transmitter based on the simultaneous injection locking of two monolithically integrated distributed feedback (DFB) laser diodes (LDs) using an optical frequency comb (OFC). We characterize the basic operation of the transmitter and demonstrate that two injection-locked integrated DFB LDs are sufficiently stable to generate the carrier signal using a uni-traveling-carrier photodiode (UTC-PD) for a real-time error-free (bit error rate: BER < 10-11) coherent transmission with a data rate of 10 Gbit/s at a carrier frequency of 97 GHz. In the coherent wireless transmission, we compare the BER characteristics of the injection-locked transmitter with that of an actively phase-stabilized transmitter and show that the power penalty of 8-dB for the injection-locked transmitter is due to the RF spurious components, which can be reduced by integrating the OFC generator (OFCG) and LDs on the same chip. Our results suggest that the integration of the OFCG, DFB LDs, modulators, semiconductor optical amplifiers, and UTC-PD on the same chip is a promising strategy to develop a practical real-time ultrafast coherent millimeter/terahertz wave wireless transmitter.

This paper deals with the inefficient channel utilization of wireless LANs that use rate adaptation. Recently, wireless LANs are being utilized in various environments. However, inefficient channel utilization is still a serious problem. The effective solutions include to decrease the frequency of packet loss and to transmit packets at a higher rate. While the backoff algorithm in IEEE 802.11 avoids only the packet loss caused by collision, other previous works tackle the packet loss caused by channel fading by means of transmission at a lower rate. This approach is called rate adaptation and a simple rate adaptation scheme is widely diffused in commercial 802.11 wireless LAN devices. However, utilizing lower transmission rate degrades transmission efficiency because the channel is occupied for a longer time. In this paper, decreasing transmission rate is avoided with novel transmission scheduling. Specifically, the proposed scheduling interrupts packet transmission to receiver stations under fading channel condition until the condition improves. Instead, other packets to other stations are transmitted in advance. To implement this proposed scheduling, only access points (APs) need to be modified. Hence, legacy wireless stations can benefit from higher communication bandwidth simply by introducing the modified APs. Moreover, although wireless stations must also be modified, an extended RTS/CTS handshake is also proposed to quickly detect the improvement of channel condition and to minimize the wasted time even if fading loss occurs. Here, wireless stations must also be modified to adopt the extended RTS/CTS handshake but further bandwidth increase is achievable. Evaluation results demonstrate that network throughput is improved without degrading the throughput fairness among receiver stations and packet transfer delay of interrupted stations.

Wireless local area networks (LANs) based on the IEEE802.11 standard usually use carrier sense multiple access with collision avoidance (CSMA/CA) for media access control. However, in CSMA/CA, if the number of wireless terminals increases, the back-off time derived by the initial contention window (CW) tends to conflict among wireless terminals. Consequently, a data frame collision often occurs, which sometimes causes the degradation of the total throughput in the transport layer protocols. In this study, to improve the total throughput, we propose a new media access control method, SP-MAC, which is based on the synchronization phenomena of coupled oscillators. Moreover, this study shows that SP-MAC drastically decreases the data frame collision probability and improves the total throughput when compared with the original CSMA/CA method.

This paper studies the impact of integrating pricing with connection admission control (CAC) on the congestion management practices in contention-based wireless random access networks. Notably, when the network is free of charge, each self-interested user tries to occupy the channel as much as possible, resulting in the inefficient utilization of network resources. Pricing is therefore adopted as incentive mechanism to encourage users to choose their access probabilities considering the real-time network congestion level. A Stackelberg leader-follower game is formulated to analyze the competitive interaction between the service provider and the users. In particular, each user chooses the access probability that optimizes its payoff, while the self-interested service provider decides whether to admit or to reject the user's connection request in order to optimize its revenue. The stability of the Stackelberg leader-follower game in terms of convergence to the Nash equilibrium is established. The proposed CAC scheme is completely distributed and can be implemented by individual access points using only local information. Compared to the existing schemes, the proposed scheme achieves higher revenue gain, higher user payoff, and higher QoS performance.

The dramatic growth in wireless data traffic has triggered the investigation of fifth generation (5G) wireless communication systems. Small cells will play a very important role in 5G to meet the 5G requirements in spectral efficiency, energy savings, etc. In this paper, we investigate low complexity millimeter-wave communication systems with uniform circular arrays (UCAs) in line-of-sight (LOS) multiple-input multiple-output (MIMO) channels, which are used in fixed wireless access such as small cell wireless backhaul for 5G. First, we demonstrate that the MIMO channel matrices for UCAs in LOS-MIMO channels are circulant matrices. Next, we provide a detailed derivation of the unified optimal antenna placement which makes MIMO channel matrices orthogonal for 3×3 and 4×4 UCAs in LOS channels. We also derive simple analytical expressions of eigenvalues and capacity as a function of array design (link range and array diameters) for the concerned systems. Finally, based on the properties of circulant matrices, we propose a high performance low complexity LOS-MIMO precoding system that combines forward error correction (FEC) codes and spatial interleaver with the fixed IDFT precoding matrix. The proposed precoding system for UCAs does not require the channel knowledge for estimating the precoding matrix at the transmitter under the LOS condition, since the channel matrices are circulant ones for UCAs. Simulation results show that the proposed low complexity system is robust to various link ranges and can attain excellent performance in strong LOS environments and channel estimation errors.

This paper analyzes the impact of directional antennas in improving the transmission capacity, defined as the maximum allowable spatial node density of successful transmissions multiplied by their data rate with a given outage constraint, in wireless networks. We consider the case where the gain Gm for the mainlobe of beamwidth can scale at an arbitrarily large rate. Under the beamwidth scaling model, the transmission capacity is analyzed for all path-loss attenuation regimes for the following two network configurations. In dense networks, in which the spatial node density increases with the antenna gain Gm, the transmission capacity scales as Gm4/α, where α denotes the path-loss exponent. On the other hand, in extended networks of fixed node density, the transmission capacity scales logarithmically in Gm. For comparison, we also show an ideal antenna model where there is no sidelobe beam. In addition, computer simulations are performed, which show trends consistent with our analytical behaviors. Our analysis sheds light on a new understanding of the fundamental limit of outage-constrained ad hoc networks operating in the directional mode.

This contribution addresses optimum wireless link selection in a 5G heterogeneous Multicomm environment in which multiple distinct Radio Access Technologies can be operated simultaneously by a given Mobile Device. The related decision making mechanisms are proposed to be part of the Mobile Device User Equipment which identifies the preferred Radio Access Technologies to be operated as well as the preferred Modulation and Coding parameters and finally selects the optimum choice either independently or through negotiation with the Network Infrastructure Equipment. In this context, the concerned Mobile Device is able to manage the bandwidth to be employed for communication — the bandwidth per Radio Access Technology is typically defined by the Network Infrastructure but the possibility for aggregating a multitude of distinct technologies introduces a new degree of freedom enabling the choice of the bandwidth selection. Simulation results demonstrate that the Mobile Device driven selection of a Multicomm configuration may either substantially reduce Mobile Device power consumption (75% in a typical example) or increase the aggregate throughput at an identical power consumption level compared to the single link case (a 3x factor in a typical example).

Wireless sensor networks (WSNs) are ubiquitous in a wide range of applications requiring the monitoring of physical and environmental variables, such as target localization and identification. One of these applications is the sensing of ferromagnetic objects. In typical applications, the area to be monitored is typically large compared to the sensing radius of each magnetic sensor. On the other hand, the RF communication radii of WSN nodes are invariably larger than the sensing radii. This makes it economical and efficient to design and implement a sparse network in terms of sensor coverage, in which each point in the monitored area is likely to be covered by at most one sensor. This work aims at investigating the sensing potential and limitations (e.g. in terms of localization accuracy on the order of centimeters) of the Honeywell HMC 1002 2-axis magnetometer used in the context of a sparse magnetic WSN. The effect of environmental variations, such as temperature and power supply fluctuations, magnetic noise, and sensor sensitivity, on the target localization and identification performance of a magnetic WSN is examined based on experimental tests. Signal processing strategies that could enable an alternative to the typical “target present/absent” mode of using magnetic sensors, such as providing successive localization information in time, are discussed.

Combining heterogeneous wireless networks that cross licensed and unlicensed spectra is a promising way of supporting the surge in mobile traffic. The unlicensed band is mostly used by wireless LAN (WLAN) nodes which employ carrier sense multiple access/collision avoidance (CSMA/CA). Since the number of WLAN devices and their traffic are increasing, the wireless resource of the unlicensed band is expected be more depleted in 2020s. In such a wireless environment, the throughput could be extremely low and unstable due to the hidden terminal problem and exposed terminal problem despite of the large resources of the allocated frequency band and high peak PHY rate. In this paper, we propose user equipment (UE) centric access in the unlicensed band, with support by licensed band access in the mobile network. The proposed access enables robust downlink transmission from the access point (AP) to the UEs by mitigating the hidden terminal problem. The licensed spectrum access passes information on the user data waiting at the AP to the UEs and triggers UE reception opportunity (RXOP) acquisition. Furthermore, the adaptive use of UE centric downlink access is presented by using the channel utilization measured at the AP. Computer simulations confirm that licensed access assistance enhances the robustness of the unlicensed band access against the hidden terminal problem.