RF Channel Research Articles

Brain imaging with improved acceleration and SNR at 7 Tesla obtained with 64-channel receive array.

Despite the clear synergy between high channel counts in a receive array and magnetic fields ≥ 7 Tesla, to date such systems have been restricted to a maximum of 32 channels. Here, we examine SNR gains at 7 Tesla in unaccelerated and accelerated images with a 64-receive channel (64Rx) RF coil. A 64Rx coil was built using circular loops tiled in 2 separable sections of a close-fitting form; custom designed preamplifier boards were integrated into each coil element. A 16-channel transmitter arranged in 2 rows along the z-axis was employed. The performance of the 64Rx array was experimentally compared to that of an industry-standard 32-channel receive (32Rx) array for SNR in unaccelerated images and for noise amplification under parallel imaging. SNR gains were observed in the periphery but not in the center of the brain in unaccelerated imaging compared to the 32Rx coil. With either 1D or 2D undersampling of k-space, or with slice acceleration together with 1D undersampling of k-space, significant reductions in g-factor noise were observed throughout the brain, yielding effective gains in SNR in the entire brain compared to the 32Rx coil. Task-based FMRI data with 12-fold 2D (slice and phase-encode) acceleration yielded excellent quality functional maps with the 64Rx coil but was significantly beyond the capabilities of the 32Rx coil. The results confirm the expectations from modeling studies and demonstrate that whole-brain studies with up to 16-fold, 2D acceleration would be feasible with the 64Rx coil.

Doppler Shift Time Expansion Resolution and Spectral Performance in Wideband Real-Time RF Channel Emulators

The possibility to test a radio frequency transceiver through the use of appropriate channel emulators allows evaluating their performance under various operating conditions. Many systems are able to operate with a relatively limited instantaneous bandwidth by applying known statistical models. Sometimes, it is necessary to evaluate the performance of a RF transceiver installed on high or very high speed platforms over predictable trajectories using optionally DEM (Digital Elevation Model) data of the terrain to estimate the number of stimulated paths and their contributions during the flight. When the instantaneous bandwidth of the signal becomes high (over several hundreds of MHz), one of the most important phenomena to consider is the Doppler spread induced by the channel and traditional narrowband models become useless. This paper presents some results when a time expansion is adopted to emulate the transceiver dynamic and the consequent Doppler spread with the aim of controlling the spectral purity of the emulated propagation channel.

Privacy preserving with adaptive link selection for hybrid radio-frequency and free space optical networks.

In order to improve the secrecy performance, we propose a secure hybrid radio frequency/free space optical (RF/FSO) transmission scheme that takes advantage of both the RF and FSO channels to protect the privacy messages. In the proposed scheme, Alice adaptively selects RF link or FSO link for information transmission according to the secrecy performance of each link. Considering the priority of the FSO link, we propose two secure transmission policies: the FSO dominant secure (FDS) policy and the secrecy rate optimal (SRO) policy. In the FDS policy, we assign high priority to the FSO link due to its high secrecy performance. Therefore, in this policy, Alice transmits the privacy information through the FSO link when the FSO link is reliable. When the FSO link cannot provide successful transmission, Alice will consider the RF secure transmission. In the SRO policy, Alice optimally selects FSO link or RF link according to the secrecy rates of both the FSO and RF links in each time slot. For both FDS and SRO policies, we analyze the secrecy performances and derive closed-form expressions for the average secrecy rate. Numerical results demonstrate the performance improvement of the proposed policies when compared with the current RF or FSO secure schemes in terms of the average secrecy rate.

Sec-D2D: A Secure and Lightweight D2D Communication System With Multiple Sensors

Device-to-Device (D2D) communication is a promising method for the emerging Internet of Things. Secure information exchange plays a key role in the application of D2D communication. Considering that the wireless devices are powered by batteries, in this paper, a lightweight secure D2D system is designed by using multiple sensors on mobile devices. Specifically, by leveraging an acceleration sensor equipped in two wireless devices, a lightweight and efficient key distribution scheme for secure D2D communication is proposed. Based on the distributed secure key, an efficient near-field authentication is developed with a speaker and a microphone to determine whether these two devices are physically close; and a secure information exchange scheme with high efficiency, which includes message encryption/decryption and message authentication, is presented over the audio channel and the RF channel. The Extensive experiments are provided to demonstrate that our system can achieve a secure information exchange between two wireless devices with low energy consumption and computing resources.

A Software Defined Radio Evaluation Platform for WBAN Systems.

In recent years, the Wireless Body Area Network (WBAN) concept has attracted significant academic and industrial attention. WBAN specifies a network dedicated to collecting personal biomedical data from advanced sensors that are then used for health and lifestyle purposes. In 2012, the 802.15.6 WBAN standard was released by the Institute of Electrical and Electronics Engineers (IEEE), which regulates and specifies the configurations of WBAN. Compared to the prevailing wireless communication technologies such as Bluetooth and ZigBee, the WBAN standard has the advantages of ultra-low power consumption, high reliability, and high-security protection while transmitting sensitive personal data. Based on the standard specification, several implementations have been published. However, in terms of evaluation, different designs were implemented in proprietary evaluation environments, which may lead to unfair comparison. In this paper, a Software-Defined Radio (SDR) evaluation platform for WBAN systems is proposed to evaluate the RF channel specified in the IEEE 802.15.6 standard. A narrowband communication protocol demonstration with a security scheme in WBAN has been performed to successfully validate the design in the proposed evaluation platform.

Adaptive resource allocation in FSO/RF multiuser system with proportional fairness for UAV application

The combination of free space optic (FSO) and unmanned aerial vehicle (UAV) can be a promising solution to last-mile problem because of FSO high bandwidth and flexibility of UAV. However, FSO is vulnerable to the atmospheric turbulence and transmitter configuration is limited on UAV. Therefore, in this paper, we investigate an adaptive resource allocation strategy to provide relative fair and high transmission capacity for users. In the proposed network model, the downlink scenario is considered and ground station communicates with UAV and its local users by FSO and RF links, respectively. According to the channel conditions and rate requirements from users, channel and power assignments should satisfy the capacity demand in reasonable. To this end, we decompose the original channel and power allocation problem with high computational complexity into low-complexity subproblems, corresponding to channel allocation in RF transmission phase and channel matching and power allocation in FSO transmission phase. Then, a heuristic efficient resource assignment algorithm is proposed to achieve the optimal capacity distribution for users. Numerical results show that the proposed method can asymptotically achieve optimal throughput. Furthermore, system throughput is higher at low transmitting power requirement than those of other existing methods.

Physical-Layer Security for Mixed <inline-formula> <tex-math notation="LaTeX">$\eta$</tex-math> </inline-formula>–<inline-formula> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\mathcal {M}$</tex-math> </inline-formula>-Distribution Dual-Hop RF/FSO Systems

In this correspondence, we investigate the physical-layer security of a mixed radio frequency/free space optical (RF/FSO) dual-hop communication system for both fixed-and variable-gain relaying schemes. More specifically, we only assume that the eavesdropping happens at the RF link because the optical link has high security. We assume that all RF channels suffer from η-μfading, while the FSO link experiences M-distributed fading. Then, we derive some analytical results for the average secrecy rate and the secrecy outage probability.

Hyperwideband Microwave Crossover Using Bridged Suspended Substrate Line Configuration

A planar microwave crossover is presented. The structure uses a suspended substrate transmission line configuration to cross two independent RF channels. In the central crossed region, one channel is bridged by a conductor on the upper side of the substrate, while the second channel is bridged on the lower side. Two conductive stubs, realized as screws in the fabricated prototype, are placed on both sides of the crossed region to adjust the capacitance in this section and improve the impedance match of each channel. Measurements show that the overall bandwidth of the crossover with less than 0.5-dB insertion loss is from dc to 12 GHz (200% fractional bandwidth). Over this band, the measured return loss and isolation between the two channels are more than 15 and 17 dB, respectively.

Improving Receiver Close-In Blocker Tolerance by Baseband <inline-formula> <tex-math notation="LaTeX">$G_m-C$ </tex-math> </inline-formula> Notch Filtering

This paper presents a receiver front end with improved blocker handling implemented in a 65-nm CMOS technology. Since close-in blockers are challenging to reject at RF, the receiver features a baseband (BB) notch filter, which effectively sinks close-in blocker current directly from the output of an LNTA and passive mixer structure. The notch-filter frequency can be tuned to match the blocker offset frequency, and the measurements indicate a significant improvement in the overall front-end interference robustness, while sensitivity remains unaffected. To optimize notch performance, the BB impedance is analyzed in detail. The front-end RF range is 750 MHz–3 GHz with an RF channel bandwidth of 20 MHz corresponding to 10-MHz BB bandwidth. The notch frequency is programmable from 16, which is less than one octave from the channel edge, up to 160 MHz. The gain-compression improvement is upto 9 dB, while IIP2 can be increased by more than 26 dB without calibration and IIP3 is 1 dBm. The current overhead for the notch function is between 7.5 and 30 mA, but it only exists under strong blocker conditions as the notch filter can be switched OFF if strong blockers are absent. The total front-end current consumption excluding the notch filter varies with LO frequency from 31 to 44 mA from a 1.2-V supply.

EVALUATION OF DYNAMIC AD-HOC UWB INDOOR POSITIONING SYSTEM

Abstract. Ultra-wideband (UWB) technology has witnessed tremendous development and advancement in the past few years. Currently available UWB transceivers can provide high-precision time-of-flight measurements which corresponds to range measurements with theoretical accuracy of few centimetres. Position estimation using range measurement is determined by measuring the ranges from a rover or a dynamic node, to a set of anchor points with known positions. However, building a flexible and accurate indoor positioning system requires more than just accurate range measurements. The performance of indoor positioning system is affected by the number and the configuration of the anchor points used, along with the accuracy of the anchor positions.This paper introduces LocSpeck, a dynamic ad-hoc positioning system based on the DW1000 UWB transceiver from Decawave. LocSpeck is composed of a set of identical nodes communicating on a common RF channel, forming a fully or partially connected network where the positioning algorithm run on each node. Each LocSpeck node could act as an anchor or a rover, and the role could change dynamically during the same session. The number of nodes in the network could change dynamically, since the firmware of LocSpeck supports adding and removing nodes on-the-fly. The paper compares the performance of the LocSpeck system with commercially available off-the-shelf UWB positioning system. Different operating scenarios are considered when evaluating the performance of the system, including cases where collaboration between the two systems is considered.

Joint Impact of RF Hardware Impairments and Channel Estimation Errors in Spectrum Sharing Multiple-Relay Networks

In this paper, we investigate the performance of a spectrum sharing decode-and-forward multiple-relay network (SSDMN) with direct link under the joint impact of two practical and detrimental imperfections, called transceiver hardware impairments (HIs) and channel estimation errors (CEEs). Here, we consider the hardware distortions induced by all the non-ideal secondary nodes and residual interferences originating from imperfect channel estimations. By taking these imperfections into account, we derive an exact closed-form expression for the outage probability of the considered SSDMN employing selection cooperation over independent and non-identical Rayleigh fading channels. We identify three important ceiling effects invoked by HIs, namely relay cooperation ceiling (RCC), direct link ceiling (DLC), and overall system ceiling. Specifically, we highlight the impact of these prevailing effects on the system’s performance and provide some useful insights about the tolerable level of HIs while designing the practical systems for a given rate requirement. We also illustrate that a potential direct link and a relaying link can, respectively, partially compensate for the incurred performance losses due to RCC and DLC effects. Moreover, we investigate the asymptotic behavior of outage probability to highlight the impact of HIs and CEEs on the achievable diversity and coding gains.

Performance Analysis of Opportunistic Transmission in Downlink Cellular DF Relay Network With Channel Estimation Error and RF Impairments

In this correspondence, we analyze the effects of radio frequency impairments and channel estimation error in a cellular decode-and-forward relay network, where multiple antennas are deployed at the base station (BS). Here, a single relay that has the best end-to-end signal-to-noise-plus-distortion-and-error ratio is selected among the available relays, which decode the BS information correctly in each transmission session. Furthermore, we derive an exact closed-form expression for outage probability and expected spectral efficiency in multiuser and multirelay scenario. In addition, simple asymptotic expression at high signal-to-noise ratio (SNR) regime is also obtained, which facilitates the characterization of the achievable diversity order of the practical cellular relay network. To validate the derived analytical expressions, we have presented simulation results, which are sufficiently tight across the entire range of SNRs. Finally, we examine the diversity order for the ideal as well as practical relaying systems.

Portable ultrasound imaging system with super-resolution capabilities

This paper discusses an ultrasound technique where the echo signals from the array of transducer elements are compressed to as few as two RF channels while still in analog domain, with a much simplified front-end electronics. The method can achieve resolutions well beyond the diffraction limit, which is set by the excitation signal wavelength and numerical aperture of the imaging system. The fundamental principle that underlies this model based imaging technique is the preservation of the spatial frequency information content of the recorded echo signals with the help of pseudo-random apodization function followed by summation. A Verasonics V1 ultrasonic scanner is used to conduct experiments using an anechoic cyst made from gel phantom, immersed in degassed water. The estimated images were compared to those obtained using traditional B-mode delay-and-sum imaging available with the Verasonics V1 ultrasound machine. The estimated images using the proposed imaging technique showed a contrast ratio of 0.96 and Full-Width-Half-Maximum (FWHM) of about half the wavelength at a depth of 9.1 cm and at 1.875 MHz center frequency while the traditional delay and sum images had a contrast ratio of 0.62 and FWHM of about 5.5 wavelengths.

Tunable optical-microwave filters optimized for 100 MHz resolution.

New continuously tunable RF-spectrum analyzers, RF receivers, and RF signal generators are proposed and analyzed for the silicon-on-insulator integrated-photonic platform at the ~1550 nm wavelength. These RF system-on-a-chip applications are enabled by a new narrowband 2x2 Mach-Zehnder interferometer (MZI) tuned filters for reconfigurable multiplexing, demultiplexing and RF channel selection. The filter can be optimized for ~100 MHz 3-dB bandwidth (BW) by utilizing N closely coupled Bragg-grating resonators to form one effective waveguide resonator in the single-mode silicon nanowire used for each MZI arm. The number of periods M within each individual resonator is selected to engineer BW in the 0.1 to 1 GHz range. Butterworth design is employed. Continuous tuning of the 100 MHz-BW devices over 18.6 GHz has been simulated by using local micron-scale thermo-optical heater stripes on the MZI arms with a temperature rise from 0 to 48K. For the case of N = 3 and 100-nm silicon side teeth, some representative performance predictions are: insertion loss (IL) = -10.7 dB, BW = 80.5 MHz and L = 113 μm for M = 58; while IL = -0.74 dB, BW = 1210 MHz and L = 86 μm for M = 44.

An efficient radio frequency channel distribution in 5g heterogeneous cellular networks for avoiding cross-tier interference in macro and small cells

Abstract Recently, the need for user data rate traffic has increased for running high-bandwidth applications. Therefore, the way forward lies in 5G heterogeneous cellular networks. The 5G network is comprised of two network hierarchies. As the first hierarchy, there are MBSs with large macro cells for macro users. As the second hierarchy, there are FBSs referred to as small cells for femto users. The 5G networks encourage the use of large macro and small cells for efficient utilisation and distribution of channel resources. In this study, the authors have proposed an efficient RF channel distribution mechanism on the basis of the current SINR levels of FUEs and MUEs. On the basis of the users’ present SINR levels, the channels will be allocated by the central MBS to MUEs and FUEs via FBSs. The major obstacle in RF channel allocation to FUEs and MUEs is cross-tier interference at the downlink channel at the MUEs and FUEs from the transmitted signals of MBSs and FBSs. In this study, an efficient RF channel allocation scheme is proposed on the basis of channel modelling constraints, which will minimise the cross-tier interference at the downlink channel at the MUEs and FUEs during RF channel allocation to FUEs and MUEs present in the same coverage area

Direction Finding by Time-Modulated Linear Array

The direction finding method by the time-modulated linear array (TMLA) is analyzed and verified. The direction of arrival (DOA) can be calculated out by analyzing the spectrum characteristic of the received signal after the time modulation. Its principle is as follows. Because of the periodic modulation, the fundamental and harmonic components are generated in each element simultaneously. After the combiner, the harmonic components’ characteristic of the combined signal is influenced by the DOA. Through analyzing their mathematical relationship, the DOA can be calculated out directly. Compared with the existing direction finding methods, the proposed method has two main advantages. One is that only a single RF channel is used, and the other is that its precision can be improved by the time accumulation. The main calculation amount of the proposed method locates in the spectrum estimation for the received signal and the matrix inversion for the harmonic characteristic matrix. Numerical simulations are provided to verify the proposed method, and the related factors influencing on the direction finding precision are analyzed. A simple $S$ -band eight-element TMLA is constructed and tested to verify its effectiveness. Finally, some conclusions are drawn, and the further research is sketched out.

Direction Finding of BPSK Signals Using Time-Modulated Array

A direction-finding algorithm by the time-modulated array (TMA) is proposed for binary phase shift keying (BPSK) signals. In the proposed scheme, only two antenna elements and a single RF channel are required, and the switch period has no relation with the symbol period of the BPSK signal. Numeric simulations are provided to examine the performance of the proposed scheme, and a two-element TMA test platform is constructed to verify its effectiveness.

Ultra-high resolution blood volume fMRI and BOLD fMRI in humans at 9.4 T: Capabilities and challenges

Functional mapping of cerebral blood volume (CBV) changes has the potential to reveal brain activity with high localization specificity at the level of cortical layers and columns. Non-invasive CBV imaging using Vascular Space Occupancy (VASO) at ultra-high magnetic field strengths promises high spatial specificity but poses unique challenges in human applications. As such, 9.4 T B1+ and B0 inhomogeneities limit efficient blood tagging, while the specific absorption rate (SAR) constraints limit the application of VASO-specific RF pulses. Moreover, short T2* values at 9.4 T require short readout duration, and long T1 values at 9.4 T can cause blood-inflow contaminations.In this study, we investigated the applicability of layer-dependent CBV-fMRI at 9.4 T in humans. We addressed the aforementioned challenges by combining multiple technical advancements: temporally alternating pTx B1+ shimming parameters, advanced adiabatic RF-pulses, 3D-EPI signal readout, optimized GRAPPA acquisition and reconstruction, and stability-optimized RF channel combination. We found that a combination of suitable advanced methodology alleviates the challenges and potential artifacts, and that VASO fMRI provides reliable measures of CBV change across cortical layers in humans at 9.4 T. The localization specificity of CBV-fMRI, combined with the high sensitivity of 9.4 T, makes this method an important tool for future studies investigating cortical micro-circuitry in humans.

A Low-Power Interference-Tolerance Wideband Receiver for 802.11af/ah Long-Range Wi-Fi With Post-LNA Active <inline-formula> <tex-math notation="LaTeX">$N$ </tex-math> </inline-formula>-Path Filter

A low-power interference-tolerance wideband receiver with post-LNA active N-path filter for RF channel selection is proposed for 802.1af/ah long-range Wi-Fi standards. By leveraging gain, noise, and filtering characteristics among the wideband LNA, high-Q active N-path filter, and reconfigurable analog baseband circuits, the proposed receiver achieves high gain, low noise, and high linearity with low power dissipation. The receiver provides narrow-bandwidth RF filtering with a small chip area. Implemented in a 40-nm CMOS process with the chip size of 1.1 mm $\times2.25$ mm, the full receiver shows a conversion gain of 84 ± 1 dB and achieves noise figure from 3.4 to 3.9 dB in subgigahertz frequency bands. The measured in-band IIP3, out-of-band IIP3, and out-of-band IIP2 are −5.9, −0.5, and +62.5 dBm, respectively. The proposed receiver dissipates an average power of 41 mW.

RF Channel Modeling for Implant-to-Implant Communication and Implant to Subcutaneous Implant Communication for Future Leadless Cardiac Pacemakers.

Propagation of radio-frequency signals inside human body is demanding to analyze as it is a highly complex medium consisting of different frequency-dependent lossy materials of varying thickness. Moreover, experimental analyses are also unfeasible because that requires probes to be placed inside a human body to collect the signals. This paper focuses on in-body to in-body implant communication for future multinodal capsule-like leadless cardiac pacemaker technology. The frequency range of 0.3-3GHz is analyzed using very detailed numerical simulations of digital human models. The results show that the Industrial, Scientific, and Medical radio band of the frequency range of 2.4-2.5GHz is optimal, having the least attenuation of signals considering the size constraints of the implant antenna. Furthermore, the placement of an additional subcutaneous implant transceiver is studied. The analysis shows that the abdominal wall is the optimal position for the placement of the implant compared to shoulder and lateral side of the body. This result is further validated by an in vivo experiment on an adult pig. The other novelty of the study is the investigation of the channel behavior based on ventricular blood volume of the heart to find out the appropriate timing of the transmission of signals between the implants. The results show that the attenuation of the signal increases with the increase in blood volume inside the heart.