Multiple Receiver Channels Research Articles

A Local Oscillator Phase Compensation Technique for Ultra-Wideband Stepped-Frequency Continuous Wave Radar Based on a Low-Cost Software-Defined Radio.

The paper discusses a way to configure a stepped-frequency continuous wave (SFCW) radar using a low-cost software-defined radio (SDR). The most of high-end SDRs offer multiple transmitter (TX) and receiver (RX) channels, one of which can be used as the reference channel for compensating the initial phases of TX and RX local oscillator (LO) signals. It is same as how commercial vector network analyzers (VNAs) compensate for the LO initial phase. These SDRs can thus acquire phase-coherent in-phase and quadrature (I/Q) data without additional components and an SFCW radar can be easily configured. On the other hand, low-cost SDRs typically have only one transmitter and receiver. Therefore, the LO initial phase has to be compensated and the phases of the received I/Q signals have to be retrieved, preferably without employing an additional receiver and components to retain the system low-cost and simple. The present paper illustrates that the difference between the phases of TX and RX LO signals varies when the LO frequency is changed because of the timing of the commencement of the mixing. The paper then proposes a technique to compensate for the LO initial phases using the internal RF loopback of the transceiver chip and to reconstruct a pulse, which requires two streaming: one for the device under test (DUT) channel and the other for the internal RF loopback channel. The effect of the LO initial phase and the proposed method for the compensation are demonstrated by experiments at a single frequency and sweeping frequency, respectively. The results show that the proposed method can compensate for the LO initial phases and ultra-wideband (UWB) pulses can be reconstructed correctly from the data sampled by a low-cost SDR.

Decoupling the Doppler Ambiguity Interval From the Maximum Operational Range and Range-Resolution in FMCW Radars

Classical saw-tooth Frequency Modulated Continuous Wave (FMCW) radars experience a coupling between the maximum unambiguous Doppler-velocity interval, maximum operational range, range-resolution and processing gain. Operationally, a trade-off is often necessarily made between these parameters. In this paper, we propose a waveform and a processing method that decouples the aforementioned parameter dependencies at the price of using multiple receiver channels within the radar. The proposed method exploits the fact that beat-frequency signals have the same baseband frequency, even if the transmitted and received chirps occupy different radio frequency bands, and have different center-frequencies. We concatenate those baseband signals in the time-frequency domain to restore the range-resolution and processing gain. An overview of FMCW parameters trade-off for related waveforms and a feasibility and limitations analysis of implementing the proposed processing method are presented. The method is verified by simulations and experiments with an FMCW radar for stable, moving and extended-moving targets. We additionally have highlighted its non-idealities in the simulations and experiments. We found that the proposed method indeed alleviates the trade-off between FMCW operational parameters and allows the extension of the Doppler ambiguity interval without compromising on those parameters.

Multichannel Staggered SAR Azimuth Processing

State-of-the-art and future spaceborne synthetic aperture radar (SAR) systems increasingly often face the requirement of providing high-resolution images with reduced revisit times, requiring coverage of wide swaths. Since these are contradicting drivers in terms of system design, different alternatives for high-resolution wide-swath SAR imaging have been investigated, relying on digital beamforming and the use of multiple receiver channels, both in elevation and azimuth dimensions. In this context, staggered SAR, which operates with a pulse repetition frequency (PRF) variation, using a single channel in azimuth proves itself as a promising alternative for covering wide continuous swaths with moderate azimuth resolution, whereas the use of multiple azimuth receiver channels bears the potential of improving the azimuth resolution over a given swath, but has yet only been applied to systems with a fixed PRF. This paper introduces and analyzes in detail processing techniques suitable for the combination of these techniques, leading to novel multichannel staggered SAR imaging modes with the potential for very fine azimuth resolution over ultra-wide swaths. A system concept with 2-m azimuth resolution over a 400-km swath in quad-pol is provided as an example.

HF Radio Angle-of-Arrival Measurements and Ionosonde Positioning

Since 2010 a 2 nd generation NOAA MF/HF radar, also referred to as the VIPIR ionosonde, has been operated at Hualien, Taiwan (23.8973°N, 121.5503°E). The Hualien VIPIR ionosonde is a modern ionospheric radar, fully digitizing complex signal records and using multiple parallel receiver channels for simultaneous signal measurements from multiple spaced receiving antennas. This paper considers radio direction finding based on interferometric phase measurements from a horizontal antenna array in the Hualien VIPIR ionosonde system. We applied the Hermite normal form method to solve the phasemeasurement aliasing and least squares problems and improve the radio angle-of-arrival (AOA) measurements. Backward ray-tracing simulation has been proposed to determine radio transmitter position. This paper presents a numerical, step by step ray-tracing method based on the IGRF superimposed onto a phenomenological ionospheric electron density model, the TaiWan Ionospheric Model (TWIM). The proposed methodology is successfully applied to locate two experimental HF radio transmitters at Longquan and Chungli with distance errors within 5 km and less than 5% of the great circle distances.

A modularized pulse programmer for NMR spectroscopy

A modularized pulse programmer for a NMR spectrometer is described. It consists of a networked PCI-104 single-board computer and a field programmable gate array (FPGA). The PCI-104 is dedicated to translate the pulse sequence elements from the host computer into 48-bit binary words and download these words to the FPGA, while the FPGA functions as a sequencer to execute these binary words. High-resolution NMR spectra obtained on a home-built spectrometer with four pulse programmers working concurrently demonstrate the effectiveness of the pulse programmer. Advantages of the module include (1) once designed it can be duplicated and used to construct a scalable NMR/MRI system with multiple transmitter and receiver channels, (2) it is a totally programmable system in which all specific applications are determined by software, and (3) it provides enough reserve for possible new pulse sequences.

Moving Target Indication via RADARSAT-2 Multichannel Synthetic Aperture Radar Processing

With the recent launches of the German TerraSAR-X and the Canadian RADARSAT-2, both equipped with phased array antennas and multiple receiver channels, synthetic aperture radar, ground moving target indication (SAR-GMTI) data are now routinely being acquired from space. Defence R&D Canada has been conducting SAR-GMTI trials to assess the performance and limitations of the RADARSAT-2 GMTI system. Several SAR-GMTI modes developed for RADARSAT-2 are described and preliminary test results of these modes are presented. Detailed equations of motion of a moving target for multiaperture spaceborne SAR geometry are derived and a moving target parameter estimation algorithm developed for RADARSAT-2 (called the Fractrum Estimator) is presented. Limitations of the simple dual-aperture SAR-GMTI mode are analysed as a function of the signal-to-noise ratio and target speed. Recently acquired RADARSAT-2 GMTI data are used to demonstrate the capability of different system modes and to validate the signal model and the algorithm.

Ｓｉｎｇｌｅ−ｓｈｏｔ ＥＰＩにおけるＳＥＮＳＥ折り返し展開ＥＲＲＯＲについて（３．０Ｔ装置での検討）

Magnetic field inhomogeneity causes artifacts in MRI. For example, in single-shot echo-planar imaging (EPI), they often appear as severe geometric distortions along the phase-encoding direction. Sensitivity encoding (SENSE) is useful in reducing the distortion in EPI since it only acquires partial k-space data using multiple receiver channels. In SENSE, a reference scan usually needs to be performed to create a sensitivity profile of each receiver channel. Gradient echo (GRE) sequences are often used in the reference scan. In diffusion-weighted imaging (DWI) using single-shot EPI with SENSE, non-negligible aliasing artifacts often remain in the reconstructed images. We suppose that these artifacts result from misregistration between the reference images acquired using GRE sequences and the DWI acquired using EPI. In this study, we used two types of acquisition methods to create sensitivity profiles, GRE sequences and EPI, and have compared the residual artifacts in the reconstructed images. The sensitivity profiles were created from the data acquired using the GRE and EPI sequences. Artifacts were reduced when EPI sensitivity profiles were used. This resulted from the fact that off-resonance effects, e.g., magnetic field inhomogeneity, susceptibility, and chemical shift, often cause severe image distortion in EPI, and, therefore, there is misregistration between images reconstructed from the data acquired using GRE and EPI sequences. Our study suggests that EPI sensitivity profiles be used when imaging data are acquired using a single-shot EPI with SENSE, although GRE sensitivity profiles have often been used in practice.

Total-body MR-imaging in oncology

Although MRI is an effective modality in oncology, state-of-the-art total-body MRI (TB-MRI) in the past was infeasible in the diagnostic work-up, due to the need for repeated examinations with repositioning and separate surface coils to cover all body parts. To overcome this limitation, either a moving table platform in combination with the body-coil or a special designed rolling table platform with one body phased-array coil have been implemented with promising results for both tumor staging and metastases screening. Since 2004, state-of-the-art TB-MR imaging with high spatial resolution has become feasible using a newly developed 1.5 Tesla TB-MRI system with multiple receiver channels. This review gives an overview based on the recent literature as well as our own experience concerning the possibilities, challenges, and limitations of TB-MRI in oncology, emphasizing both oncological staging and early tumor detection in asymptomatic subjects.

Is Magnetic Resonance Image Guidance the Key to Opening Chronic Total Occlusions?

Through progressive plaque growth or fibrotic organization of occlusive thrombus, atherosclerosis may result in chronic total occlusion (CTO) of a major arterial conduit. If CTO develops slowly, collateral pathways may supply sufficient perfusion to retain tissue viability despite occlusion of the major inflow conduit. However, such collateral-mediated perfusion rarely matches that provided by an open main conduit, particularly during the peak demand of regional muscular activity. Although strategies to limit regional oxygen demand (eg, β-blockers for the treatment of angina pectoris) or to enhance collateral function (eg, a walking program for superficial femoral occlusion, experimental angiogenesis) may be of some benefit, these approaches generally offer less complete return of normal physiology and relief of ischemic symptoms than does direct revascularization of the occluded vessel. With surgical bypass, the nature of the occlusion (its length, duration, tortuosity, calcification) is less important than the presence of a suitable distal anastomotic target. On the other hand, with catheter-based techniques, success depends heavily on these lesion characteristics. In fact, crossing such occluded segments with a guidewire remains the dominant barrier to catheter-based treatments of occluded arterial segments. Once the guidewire has been passed successfully into the distal true lumen, the process of dilating and stenting the segment is usually straightforward and durable, at least with the use of drug-eluting stents in coronary CTOs.1,2 Article p 1101 Devices designed to cross CTOs should have 3 key features: an ability to distinguish a true luminal path from one created within (dissection) or through (perforation) the vessel wall of the occluded segment, an ability to change direction (steer) to correct deviations from the desired path through the occlusion, and an ability to penetrate the frequently fibrotic and focally calcific substance of the occlusion through the use of either mechanical stiffness or an alternative energy modality (laser, …

Interventional MRA using actively visualized catheters, TrueFISP, and real-time image fusion.

An integrated system for performing interventional magnetic resonance angiography (MRA) with actively visualized instruments and real-time image fusion was implemented on a 1.5 T scanner. True fast imaging with steady precession (TrueFISP) imaging provided high acquisition speed paired with high signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for the simultaneous visualization of active instruments and arterial morphology. The system enabled simultaneous image reconstruction and image postprocessing of multiple receiver channels, with subsequent image fusion display in real time. Optional interleaved image acquisition in two planes provided additional important information for biplanar instrument guidance. Various vascular interventions, including selective catheterization and subsequent selective MRA of the abdominal aorta, renal arteries, superior mesenteric artery (SMA), hepatic artery, and aortic arch, were performed on 10 pigs under MR guidance. In terms of instrument contrast, image acquisition, reconstruction, and fusion speed, the setup represents a powerful platform for performing interventional MRA procedures.

Multiple field of view MR fluoroscopy.

This work describes a real-time imaging and visualization technique that allows multiple field of view (FOV) imaging. A stream of images from a single receiver channel can be reconstructed at multiple FOVs within each image frame. Alternately, or in addition, when multiple receiver channels are available, image streams from each channel can be independently reconstructed at multiple FOVs. The implementation described here provides for real-time visualization of the placement of guidewires and catheters on a dynamic roadmap during interventional procedures. The loopless catheter antenna, an electrically active intravascular probe, was used for MR signal reception. In 2D projection images, the catheter and surrounding structures within its diameter of sensitivity appear as bright signal. The simplicity of the resulting images allows very-narrow-FOV imaging to decrease imaging time. Very-narrow-FOV images are acquired on MR receiver channels that collect guidewire or catheter data. These very-narrow-FOV images provide very high frame rate continuous, real-time imaging of the interventional devices (25 fps). Large-FOV images are formed from receiver channels that collect anatomical data from standard imaging surface coils, and simultaneously provide a dynamic, frequently updated roadmap. These multiple-FOV images are displayed together, improving visualization of interventional device placement.

Angle and space diversity comparisons in different mobile radio environments

The angle diversity performance of two types of high-gain multibeam antennas-24 vertically polarized 15/spl deg/ beams and 12 vertically polarized 30/spl deg/ beams-were tested and compared to the space diversity performance of traditional sector antenna configurations. The antennas were tested at 850 MHz in dense urban and rural cellular mobile radio environments. A vehicle equipped with a mobile transmitter was driven in the coverage area, while the received signal strength (RSS) was recorded on multiple receiver channels attached to multibeam and sector antennas at the base site. The RSS data recorded included fast (Rayleigh) fading and was averaged into local means based on the mobile's position/speed. The fast fading was extracted from the recorded RSS and the fading distributions of the two multibeam antennas tested were studied in two distinctly different mobile environments. Fading cumulative distributions for the angular diverse antennas were compared to those of spatially diverse antennas. Diversity gain was calculated and compared to traditional space diversity in these mobile environments. Results in urban environments indicated that the angular diversity performance was comparable to the space diversity (/spl sim/8 dB improvement). Rural tests typically suggested that both space diversity and angular diversity provided little or no (<2 dB) fading reduction. A description of the experiment, data reduction and analyses, and calculation of diversity gain are presented. The motivation for this experiment is the application of fixed multiple beam antennas (FMBA) in cellular radio and digital personal communication systems.

The Polarization Characteristics of Lidar Scattering from Snow and Ice Crystals in the Atmosphere

The effects of ice crystals on the backscatter polarization in lidar returns have been measured. Scattering from ground-level snow of varying density, high-altitude cirrus clouds and ice pellet shower clouds has been studied using a linearly polarized ruby laser system with multiple receiver channels. The space and time dependence of the total intensity and the linear depolarization ratio δ of the lidar return have been investigated. It has been found that considerable variations occur in snow with δ varying between 0.2 and 0.8. In cirrus and ice pellet shower clouds δ is typically found to lie in the range 0.3 ± 0.1. The factors involved in the use of polarization data for ice-water discrimination in the atmosphere are presented and discussed.

Lidar polarization studies of the troposphere

The polarization properties of lidar backscattering from the atmosphere have been measured using a ruby system operating at 694.3 nm and 347.2 nm. Linearly polarized signals are transmitted, and multiple receiver channels simultaneously sample the return with polarizers oriented parallel and perpendicular to the transmitted polarization. It is found that the polarization of the scattered signal varies substantially with meteorological conditions and that the signals can be employed to derive useful information about the atmospheric conditions.