Brian D. Jeffs

Experimental characterization of the MIMO wireless channel: data acquisition and analysis

Detailed performance assessment of space-time coding algorithms in realistic channels is critically dependent upon accurate knowledge of the wireless channel spatial characteristics. This paper presents an experimental measurement platform capable of providing the narrowband channel transfer matrix for wireless communications scenarios. The system is used to directly measure key multiple-input-multiple-output parameters in an indoor environment at 2.45 GHz. Linear antenna arrays of different sizes and construction with up to ten elements at transmit and receive are utilized in the measurement campaign. This data is analyzed to reveal channel properties such as transfer matrix element statistical distributions and temporal and spatial correlation. Additionally, the impact of parameters such as antenna element polarization, directivity, and array size on channel capacity are highlighted. The paper concludes with a discussion of the relationship between multipath richness and path loss, as well as their joint role in determining channel capacity.

A statistical model for angle of arrival in indoor multipath propagation

Multiple antenna systems are a useful way of overcoming the effects of multipath interference, and can allow more efficient use of spectrum. In order to test the effectiveness of various algorithms such as diversity combining, phased array processing, and adaptive array processing in an indoor environment, a channel model is needed which models both the time and angle of arrival in indoor environments. Some data has been collected indoors and some temporal models have been proposed, but no existing model accounts for both time and angle of arrival. This paper discusses existing models for the time of arrival, experimental data (form the 6.75-7.25 GHz band) that were collected indoors, and a proposed extension of the Saleh-Valenzuela model (1987), which accounts for the angle of arrival. Model parameters measured in two different buildings are compared with the parameters presented in the paper by Saleh and Valenzuela, and some statistical validation of the model is presented.

Signal Processing for Phased Array Feeds in Radio Astronomical Telescopes

Relative to traditional waveguide feeds, phased array feeds (PAFs) for radio telescopes can increase the instrument field of view and sky survey speed. Unique challenges associated with PAF observations, including extremely low signal levels, long-term system gain stability requirements, spatially correlated noise due to mutual coupling, and tight beamshape tolerances, require the development of new array signal processing techniques for this application. We propose a calibration and beamforming strategy for PAFs including interference mitigation with power spectral density (PSD) estimation bias correction. Key efficiency metrics for single-feed instruments are extended to the array case and used to verify performance of the algorithms. These techniques are validated using numerical simulations and experimental data from a 19-element PAF on the Green Bank 20-m telescope.

Restoration of blurred star field images by maximally sparse optimization

The problem of removing blur from, or sharpening, astronomical star field intensity images is discussed. An approach to image restoration that recovers image detail using a constrained optimization theoretic approach is introduced. Ideal star images may be modeled as a few point sources in a uniform background. It is argued that a direct measure of image sparseness is the appropriate optimization criterion for deconvolving the image blurring function. A sparseness criterion based on the l(p) is presented, and candidate algorithms for solving the ensuing nonlinear constrained optimization problem are presented and reviewed. Synthetic and actual star image reconstruction examples are presented to demonstrate the methods superior performance as compared with several image deconvolution methods.

Indoor wideband time/angle of arrival multipath propagation results

Most current indoor propagation experiments measure the time of arrival of characteristics of multipath reflections without regard to the angle of arrival. Because of the increasing number of systems that are used indoors and which use multiple antenna systems to combat multipath interference, a need exists for indoor propagation data which takes the angle of arrival into account. A system is described which was used to collect simultaneous time and angle of arrival data in two indoor environments. A total of 65 data sets were taken in two buildings of different construction. The (7 GHz) data confirmed the temporal model proposed by Saleh and Valenzuela (1987), and showed a unique clustering pattern in angle, which consisted of clusters uniformly distributed in angle, with members of the clusters following a Laplacian distribution.

Efficiencies and System Temperature for a Beamforming Array

For an active beamforming array, standard definitions for efficiencies and system temperature are not available. We use noise considerations to generalize the single-antenna conventions for aperture efficiency, spillover efficiency, radiation efficiency, and system temperature to arrays. The treatment leads to a new noise matching efficiency that quantifies the effect of mutual coupling on amplifier noise. Numerical results for a phased array feed indicate that the noise increase caused by mutual coupling can be significant.

Beamforming and imaging with the BYU/NRAO L-band 19-element phased array feed

An experimental 19-element L-band phased array feed was installed on the Green Bank 20-Meter Telescope in October 2007 and July 2008 to measure sensitivity and effciency and demonstrate signal processing algorithms for array calibration, multiple beam formation, imaging, and adaptive spatial filtering methods for interference mitigation. System noise performance was characterized using a warm absorber/cold sky Y-factor setup. The peak beam aperture effciency was 69% and the minimum beam equivalent system temperature was 66K. With a single reflector pointing, a high sensitivity image of a field of view approximately two half-power beamwidths in diameter can be produced. Measured figures of merit compare well to numerical simulations, indicating that complicating effects such as mutual coupling are understood well enough to enable the next phase of array feed development to proceed on firm grounds.

Beamformer Design Methods for Radio Astronomical Phased Array Feeds

A major emphasis in current radio astronomy instrumentation research is the use of phased array feeds (PAF) to provide radio telescopes with larger fields of view. One of the challenges of PAF systems is the design of beamformers that provide sufficient sensitivity and known, stable beam pattern structure. High sensitivity has been achieved with the maximum sensitivity beamformer without regard to beam pattern shape. Deterministic beamformers provide the desired pattern shape control, but suffer from a significant reduction in sensitivity. We present a hybrid beamforming method, which balances the tradeoff between high sensitivity and precise beam pattern shape control. A comparison of each of these beamforming methods, using measured data, confirms the advantage of the hybrid approach. The pattern distortions introduced by modeled beamformers can be mitigated with a transformation step, but ultimately it is shown that PAF beamformer design is best done using measured calibrators. A PAF calibration vector quality metric based on minimum description length is also introduced.

Model-Based Subspace Projection Beamforming for Deep Interference Nulling

This paper considers the problem of adaptive array processing for interference canceling to drive very deep nulls in difficult signal environments. In many practical scenarios, the achievable null depth is limited by covariance matrix estimation error leading to poor identification of the interference subspace. We address the particularly troublesome cases of low interference-to-noise ratio (INR), relatively rapid interference motion, and correlated noise across the receiving array. A polynomial-based model is incorporated in the proposed algorithm to track changes in the array covariance matrix over time, mitigate interference subspace estimation errors, and improve canceler performance. The application of phased array feeds for radio astronomical telescopes is used to illustrate the problem and proposed solution. Here even weak residual interference after cancellation may obscure a signal of interest, so very deep beampattern nulls are required. Performance for conventional subspace projection (SP) is compared with polynomial-augmented SP using simulated and real experimental data, showing null-depth improvement of 6 to 30 dB.

Design and Characterization of an Active Impedance Matched Low-Noise Phased Array Feed

An L-band low-noise phased array feed antenna was designed, fabricated, and characterized experimentally. The element design was optimized for an active impedance matching condition with a given set of nonuniform amplitude beamformer coefficients associated with a formed PAF beam. Using measured array S-parameters and modeled array element patterns, the predicted boresight beam equivalent system noise temperature and aperture efficiency at prime focus with uncooled low-noise amplifiers were 42 K and 61% at 1600 MHz. Experimental measurements of receiver noise using an array Y-factor technique matched modeled predictions. The array feed was mounted on the Arecibo radio telescope for on-reflector characterization. The measured boresight beam sensitivity figure of merit (Tsys/ηap) was 88 K and the 1 dB sensitivity bandwidth was 450 MHz.