Phased Array Beamforming Research Articles

Deterministic RF nulling in phased arrays for the next generation of radio telescopes

A requirement of the next generation of radio telescopes for astronomy is the ability to cope with the forever increasing problem of radio-frequency interference (RFI). Unlike conventional fixed parabolic receivers, currently used in astronomy, the application of phased-array beamforming techniques opens up the possibility to spatially null RFI in the RF domain, prior to signal digitization. This paper presents results from the second phased-array experimental demonstrator, the One Square Meter Array, on calibration and RF nulling performance. The approach is to deterministically null known RFI in the RF beamforming domain, and to adaptively remove the remaining RFI in the digital beamforming domain. A novel array-calibration technique, called the multi-element phase toggle technique (MEP), is presented. This technique allows a fast and very accurate calibration of wide-band phased-array antennas. Array calibration is shown to determine the extent to which RFI can be removed by experimental verification of simulated null depths.

Modified constant modulus algorithm for digital signal processing adaptive antennas with microwave analog beamforming

This paper proposes a novel algorithm, called modified constant modulus algorithm (M-CMA), which is able to give adaptability to microwave beamforming phased array antennas. Since microwave analog beamformers basically require much fewer RF devices than digital beamformers, microwave analog beamformers based on M-CMA, that is, adaptive microwave beamformers, can be cheaply fabricated. Therefore, they are very suitable for mobile communication systems where both miniaturization and low cost are required for the mobile terminals. M-CMA obtains a gradient vector by a combination of analytical calculation and perturbation of the microwave beamforming control voltage. Though M-CMA is implemented with a digital signal processor, M-CMA controls the microwave analog beamformer by utilizing the gradient vector. The microwave analog beamformer based on M-CMA is analyzed to have the following characteristics: (1) the beamformer can point its main beam to the desired direction in additive white Gaussian noise (AWGN) channels; (2) although the beamformer may possibly fail in ill solutions in cochannel interference (CCI) channels, M-CMA can converge to the optimum solution when the desired direction is roughly a priori known.

Wideband true-time-delay unit for phased array beamforming using discrete-chirped fiber grating prism

A wideband true-time-delay (TTD) unit employing a novel fiber grating prism (FGP) for phased array beamforming is proposed in this paper. The FGP consists of a single-mode fiber delay line, a chirped grating delay line and three discrete fiber Bragg grating (FBG) delay lines. The first delay line is a length of single-mode fiber, which provides a fixed time delay for all wavelengths. The second delay line, which employs a chirped grating, is used to provide small time delays. The discrete FBG delay lines, which incorporate an array of 13 discrete FBGs, are used to provide large time delays. A 5×13 fiber-grating prism is constructed and experimented. The results show that the time delays produced by the fiber grating delay lines are independent of the microwave frequency and agree well with the calculated time delays. Based on the measured time delays, the radiation patterns of 5-element array antenna are calculated and analyzed. The beampointing direction of the antenna is independent of the microwave frequency and can be controlled by tuning the wavelength of the optical carrier. This 5×13 TTD unit can provide phased array beamforming at microwave frequencies up to 6 GHz.

Continuous true-time-delay beamforming employing a multiwavelength tunable fiber laser source

A true-time-delay system for wideband continuous phased array beamforming employing a novel multiwavelength tunable fiber laser source with equally increased or decreased wavelength spacing is proposed. The wavelength tuning is realized by stretching or compressing the fiber Bragg gratings cascaded in the laser cavity. To ensure a wavelength tuning with equally increased or decreased wavelength spacing, the gratings are mounted onto a plastic plate with angles between adjacent gratings selected such that the forces applied to the gratings has an equal force increment. Time delays with equally increased or decreased time delay difference for the equally spaced wavelengths are obtained when the modulated lightwaves are reflected at different locations of a linearly chirped grating. A four-channel chirped grating based true-time-delay system using the proposed light source has been constructed and experimented. Equally increased or decreased time delays are obtained when the wavelengths are tuned.

Photonics True-Time-Delay Unit for Phased-Array Antenna Using Multiwavelength Fiber Laser Based on a Sagnac Interferemetric Filter

A photonics true-time-delay system for phased array beamforming using a chirped grating-based time-delay element and a novel multiwavelength erbium-doped fiber ring laser source employing a Sagnac filtering mirror is proposed. The Sagnac filtering mirror consists of a polarization maintaining directional coupler and two pieces of polarization maintaining fibers. The lasing wavelength and number are determined by properly adjusting the polarization controller within the unidirectional ring cavity and are very stable. A chirped-grating-based true-time-delay system using the proposed laser source is constructed and demonstrated experimentally.

Digital phased array beamforming using single-bit delta-sigma conversion with non-uniform oversampling.

Digital beamforming based on oversampled delta-sigma (delta sigma) analog-to-digital (A/D) conversion can reduce the overall cost, size, and power consumption of phased array front-end processing. The signal resampling involved in dynamic delta sigma beamforming, however, disrupts synchronization between the modulators and demodulator, causing significant degradation in the signal-to-noise ratio. As a solution to this, we have explored a new digital beamforming approach based on non-uniform oversampling delta sigma A/D conversion. Using this approach, the echo signals received by the transducer array are sampled at time instants determined by the beamforming timing and then digitized by single-bit delta sigma A/D conversion prior to the coherent beam summation. The timing information involves a non-uniform sampling scheme employing different clocks at each array channel. The delta sigma coded beamsums obtained by adding the delayed 1-bit coded RF echo signals are then processed through a decimation filter to produce final beamforming outputs. The performance and validity of the proposed beamforming approach are assessed by means of emulations using experimental raw RF data.

Application of window theory to optimization of fiber Bragg gratings for wavelength-selectable antenna beam steering

The profile of fiber Bragg gratings (FBGs) utilized in fiber grating prism and other wavelength-selective phased-array beamformers can be optimized using the results of window theory. In this paper, the window formalism is adopted to quantify the relationship between the FBG profile and its spectral selectivity. It is shown how application of optimum and suboptimum windows affects the reflectivity, bandwidth, and sidelobe level of FBG. Also, the impact of taper on the group-delay nonuniformity, an essential issue for true time delay processors, is discussed. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 17: 161–164, 1998.

Photonic beamformer for phased array antennas using a fiber grating prism

This letter presents, for the first time, measured data on a Bragg reflection grating based fiber-optic prism true time delay processor for transmit/receive phased array beamforming. Measurements taken over a 3.5-GHz bandwidth demonstrates high resolution beamsteering and highly linear low-noise phase data. The system takes maximum advantage of component reuse and fully integrates the transmit and receive modes in one efficient hardware compressive topology.

Megalithic microwave signal processing for phased-array beamforming and steering

A microwave signal processing (MSP) architecture is presented for active phased array beam forming and steering. A large scale network, comprising 63 power dividers and 32 pairs of vector-synthetic phase/amplitude controllers, has been successfully developed in an 11 mm/spl times/13 mm GaAs monolithic microwave integrated circuit (MMIC). It has a huge integration level of 128 metal-semiconductor field effect transmitters (MESFETs), 448 spiral inductors, 527 metal-insulator-metal (MIM) capacitors, and 357 ion-implanted resistors. The expected 360/spl deg/ phase is successfully obtained at all the output ports. Vector error standard deviations exhibited are within 0.38-dB root-mean square (rms) and 2.8/spl deg/ r.m.s. over the bandwidth of 20 MHz at 2.5 GHz. This megalithic chip could mark an epoch in phased array systems.

Derivation of orthogonal beams and their application to beamforming in small phased arrays

It is a difficult problem to synthesise beams with the desired characteristics (low sidelobe levels, narrow main beam) from small phased arrays with few elements. Mutual coupling between elements means that conventional tapering algorithms are corrupted. In the paper a technique is presented whereby weights are calculated by point matching measured element radiation patterns to an ideal sinc function. It is found that a very close approximation to an ideal sinc beam can be synthesised by point matching to the nulls and main beam peak value of the ideal. Moreover, an orthogonal set of sinc beams can be synthesised using this technique, which in turn are used to construct a low sidelobe beam using the main beam gain of orthogonal sinc beams as sampling points. This technique gives insight into the limits of how far sidelobe levels can be reduced in arrays with few elements. No knowledge of the coupling coefficients of the array is required.

High capacity optical interconnects for phased array beamformers

Two new concepts for overcoming the optical interconnect losses in highly dense multibeam photonics-based beamforming networks are presented. The first is the use of erbium-doped fiber amplifiers as power boosters at the transmitters, and the second is the use of external modulation techniques at the laser transmitters. Using these concepts, results show that a system noise figure of 3.5 db and a spurious-free dynamic range of 134 dB/spl middot/Hz/sup 4/5/ can be attained for a wideband phased array which can yield around 1000 independent and simultaneous antenna beams. >

Analysis of worst-case phase quantization sidelobes in focused beamforming

The effect of phase quantization errors on the sidelobe level in a phased-array beamformer is considered. The theory of random and periodic phase errors is reviewed. Periodic phase errors occur for an unfocused array with uniform element distance and give rise to discrete sidelobes. The main result of the paper is an analysis of the periodic phase error in the case that the beamformer is focused. An estimate for the worst-case discrete sidelobe level as a function of focal depth is derived. The theory is illustrated and verified by simulations.

Optical processor for array antenna beam shaping and steering

An optical processor for phased-array antenna beamforming has been demonstrated. It is capable of providing real-time control of both antenna beam steering and beam shaping in one or two dimensions, with an rms phase linearity of 5 bits. The functions of beam shaping and steering are combined through a magneto-optic spatial light modulator (SLM), which is fast, small, and simple to use.

Design of a variable‐aperture phased array beamformer for use in echocardiography

A beamformer for evaluation of dynamic aperture control to maximize lateral resolution of an ultrasonic phased array is under development at The Applied Research Laboratory of The Pennsylvania State University. For echocardiographic applications, the beamformer offers the advantages of fast scanning with good lateral resolution at all ranges. Techniques for improving lateral resolution at unlimited depth of field over that obtainable by a fixed focus system include the farfield variable aperture and previously reported dynamic focusing approaches. The relative performance limits and the effects of these techniques on beam‐former complexity are compared. An electronic beamformer configuration for implementing the less complex variable aperture approach is described. The ultrasonic array and packaging concepts are presented along with acoustic performance data relating to the individual array elements. Measurements of damping and cross coupling and analyses of predicted performance have been made and are included. Results from computer models of the beamformer and array are also presented which compare the patterns obtained by (1) multiplicative pattern analysis, (2) taking into account the effect of different element patterns, and (3) varying the size of the element. [This work was supported in part by NIH grant R01HL20872‐02.]