Showing papers on "Antenna array published in 1998"

Overview of spatial channel models for antenna array communication systems

Abstract: Spatial antenna diversity has been important in improving the radio link between wireless users. Historically, microscopic antenna diversity has been used to reduce the fading seen by a radio receiver, whereas macroscopic diversity provides multiple listening posts to ensure that mobile communication links remain intact over a wide geographic area. In later years, the concepts of spatial diversity have been expanded to build foundations for emerging technologies, such as smart (adaptive) antennas and position location systems. Smart antennas hold great promise for increasing the capacity of wireless communications because they radiate and receive energy only in the intended directions, thereby greatly reducing interference. To properly design, analyze, and implement smart antennas and to exploit spatial processing in emerging wireless systems, accurate radio channel models that incorporate spatial characteristics are necessary. In this tutorial, we review the key concepts in spatial channel modeling and present emerging approaches. We also review the research issues in developing and using spatial channel models for adaptive antennas.

Efficient use of side information in multiple-antenna data transmission over fading channels

Abstract: We derive performance limits for two closely related communication scenarios involving a wireless system with multiple-element transmitter antenna arrays: a point-to-point system with partial side information at the transmitter, and a broadcast system with multiple receivers. In both cases, ideal beamforming is impossible, leading to an inherently lower achievable performance as the quality of the side information degrades or as the number of receivers increases. Expected signal-to-noise ratio (SNR) and mutual information are both considered as performance measures. In the point-to-point case, we determine when the transmission strategy should use some form of beamforming and when it should not. We also show that, when properly chosen, even a small amount of side information can be quite valuable. For the broadcast scenario with an SNR criterion, we find the efficient frontier of operating points and show that even when the number of receivers is larger than the number of antenna array elements, significant performance improvements can be obtained by tailoring the transmission strategy to the realized channel.

Blind multiuser detection: a subspace approach

Abstract: A new multiuser detection scheme based on signal subspace estimation is proposed. It is shown that under this scheme, both the decorrelating detector and the linear minimum-mean-square-error (MMSE) detector can be obtained blindly, i.e., they can be estimated from the received signal with the prior knowledge of only the signature waveform and timing of the user of interest. The consistency and asymptotic variance of the estimates of the two linear detectors are examined. A blind adaptive implementation based on a signal subspace tracking algorithm is also developed. It is seen that compared with the previous minimum-output-energy blind adaptive multiuser detector, the proposed subspace-based blind adaptive detector offers lower computational complexity, better performance, and robustness against signature waveform mismatch. Two extensions are made within the framework of signal subspace estimation. First, a blind adaptive method is developed for estimating the effective user signature waveform in the multipath channel. Secondly, a multiuser detection scheme using spatial diversity in the form of an antenna array is considered. A blind adaptive technique for estimating the array response for diversity combining is proposed. It is seen that under the proposed subspace approach, blind adaptive channel estimation and blind adaptive array response estimation can be integrated with blind adaptive multiuser detection, with little attendant increase in complexity.

Subscriber location in CDMA cellular networks

Abstract: Subscriber radio location techniques are investigated for code-division multiple-access (CDMA) cellular networks. Two methods are considered for radio location: measured times of arrival (ToA) and angles of arrival (AoA). The ToA measurements are obtained from the code tracking loop in the CDMA receiver, and the AoA measurements at a base station (BS) are assumed to be made with an antenna array. The performance of the two methods is evaluated for both ranging and two-dimensional (2-D) location, while varying the propagation conditions and the number of BS's used for the location estimate.

Recent advances in dielectric-resonator antenna technology

Abstract: This paper features some of the advances in dielectric-resonator antenna technology at the Communications Research Centre. Several novel elements are presented that offer significant enhancements to parameters such as impedance bandwidth, circular-polarization bandwidth, gain, or coupling to various feed structures. Several linear and planar arrays are also presented, to illustrate the performance of dielectric-resonator antenna elements in the array environment.

Highly bandwidth efficient communications

Abstract: A discrete multitone stacked-carrier spread spectrum communication method is based on frequency domain spreading including multiplication of a baseband signal by a set of superimposed, or stacked, complex sinusoid carrier waves. In a preferred embodiment, the spreading involves energizing the bins of a large Fast Fourier transform (FFT). This provides a considerable savings in computational complexity for moderate output FFT sizes. Point-to-multipoint and multipoint-to-multipoint (nodeless) network topologies are possible. A code-nulling method is included for interference cancellation and enhanced signal separation by exploiting the spectral diversity of the various sources. The basic method may be extended to include multielement antenna array nulling methods for interference cancellation and enhanced signal separation using spatial separation. Such methods permit directive and retrodirective transmission systems that adapt or can be adapted to the radio environment. Such systems are compatible with bandwidth-on-demand and higher-order modulation formats and use advanced adaptation algorithms. In a specific embodiment the spectral and spatial components of the adaptive weights are calculated in a unified operation based on the mathematical analogy between the spectral and spatial descriptions of the airlink.

Practical space-time radio method for CDMA communication capacity enhancement

Abstract: A practical way to enhance signal quality (carrier to interference. C/I) in both up and downlink of wireless point to multi-point CDMA service implements basic radio direction finding techniques to allow for optimal diversity combining in an antenna array employing large number of elements. This approach is facilitated through the use of very small bit counts arithmetic and capitalizing on finite alphabet signal structure (Walsh symbols, for example in IS-95 CDMA) or a known training sequence. Alternate implementations can use floating point data representations. The method facilitates ASIC implementation, thereby enabling distributed processing to achieve the required computation practicality. The method utilizes the uplink channel data to determine the downlink spatial structure (array beams) to enhance downlink C/I and hence, increase downlink capacity. The preferred embodiment is optimized to IS-95 , however, any signal that has either a finite alphabet or a training sequence built in can utilize the same idea. The use of the known signal structure facilitates simple array response vector determination and eliminates the necessity for covariance matrix calculation and analysis. Hence, this approach can be utilized for GSM and TDMA wireless air-interfaces as well.

Estimation of multipath parameters in wireless communications

Abstract: In a parametric multipath propagation model, a source is received by an antenna array via a number of rays, each described by an arrival angle, a delay, and a fading parameter. Unlike the fading, the angles and delays are stationary over long time intervals. This fact is exploited in a new subspace-based high-resolution method for simultaneous estimation of the angle/delay parameters from multiple estimates of the channel impulse response. A computationally expensive optimization search can be avoided by using an ESPRIT-like algorithm. Finally, we investigate certain resolution issues that take the fact that the source is bandlimited into account.

Maximum likelihood methods in radar array signal processing

Abstract: We consider robust and computationally efficient maximum likelihood algorithms for estimating the parameters of a radar target whose signal is observed by an array of sensors in interference with unknown second-order statistics. Two data models are described: one that uses the target direction of arrival and signal amplitude as parameters and one that is a simpler, unstructured model that uses a generic target "spatial signature". An extended invariance principle is invoked to show how the less accurate maximum likelihood estimates obtained from the simple model may be refined to achieve asymptotically the performance available using the structured model. The resulting algorithm requires two one-dimensional (1-D) searches rather than a two-dimensional search, as with previous approaches for the structured case. If a uniform linear array is used, only a single 1-D search is needed. A generalized likelihood ratio test for target detection is also derived under the unstructured model. The principal advantage of this approach is that it is computationally simple and robust to errors in the model (calibration) of the array response.

Injection- and phase-locking techniques for beam control [antenna arrays]

Abstract: Applications of millimeter-wave radar, imaging, and communication technology requires cost-effective implementation of intelligent scanning antenna systems Injection-locking and phase-locked-loop (PLL) techniques can be used to achieve synchronous operation of a number of antenna array elements, and allow for the manipulation of the phase distribution without additional phase-shifting circuitry, suggesting a potential for low-cost beam-scanning systems This paper describes a number of techniques, with an assessment of some remaining technical challenges for practical implementation

Performance monitor for antenna arrays

Abstract: An antenna monitor collects statistics about the received signal strengths of each antenna in a cooperating antenna array over long periods of time. In cellular telephone base station applications for a base station using an array of antenna elements to obtain spatial diversity, samples of the received signal strength are taken during particular times whenever an active mobile subscriber is operating in the area. Periodically, such samples are averaged and added to a running total that is maintained in a long-term accumulator for each antenna. The accumulated totals are then compared after a statistically sufficient number of samples have been collected and averaged. Faulty antennas, including broken ones and ones with bad connections, will be revealed by those corresponding signal paths having relatively low accumulated totals. Given long enough collection periods, the spatial diversity of the antennas in the array will have only a minimal impact on favoring one antenna over another.

Performance analysis of optimum combining in wireless communications with Rayleigh fading and cochannel interference

Abstract: Optimum combining for space diversity reception is studied in digital cellular mobile radio communication systems with Rayleigh fading and multiple cochannel interferers. This paper considers binary phase-shift keying (BPSK) modulation in a flat Rayleigh-fading environment when the number of interferences L is no less than the number of antenna elements N(L/spl ges/N). The approach of this paper and its main contribution is to carry out the analysis in a multivariate framework. Using this approach and with the assumption of equal-power interferers, it is shown that the probability density function of the maximum signal-to-interference ratio (SIR) at the output of the optimum combiner has a Hotelling T/sup 2/ distribution. Closed form expressions using hypergeometric functions are derived for the outage probability and the average probability of bit error. Theoretical results are demonstrated by Monte Carlo simulations.

Algebraic methods for deterministic blind beamforming

Abstract: Deterministic blind beamforming algorithms try to separate superpositions of source signals impinging on a phased antenna array by using the deterministic properties of the signals or the channels such as their constant modulus or directions-of-arrival. Progress in this area has been abundant over the past ten years and has resulted in several powerful algorithms. Unlike optimal or adaptive methods, the algebraic methods discussed in this review act on a fixed block of data and give closed-form expressions for beamformers by focusing on algebraic structures. This typically leads to subspace estimation and generalized eigenvalue problems. After introducing a simple and widely used multipath channel model, the paper provides an anthology of properties that are available, as well as generic algorithms that exploit them.

Time delay and spatial signature estimation using known asynchronous signals

Abstract: This paper considers the problem of estimating the parameters of known signals received asynchronously by an array of antennas. The parameters of primary interest are the time delays of the signals and their spatial signatures at the array. Estimates of the signal directions of arrival are also considered but are of secondary importance in this work. Maximum likelihood algorithms and more computationally efficient approximations are developed for both the case where all received signals are identical (the channel estimation/overlapping echo problem) and where they are all distinct. Conditions are also derived under which the standard matched filter approach yields consistent and statistically efficient parameter estimates. The issue of solution uniqueness is addressed, and in particular, an upper bound on the number of signals whose parameters may be uniquely identified is derived for a number of different cases. Typically, the bound is far greater than the number of sensors and is limited only by the number of data samples collected. Some representative simulation examples are also included to illustrate the algorithms' performance relative to the Cramer-Rao bound.

Base-station tracking in mobile communications using a switched parasitic antenna array

Abstract: Base-station tracking in mobile communications benefits from a directional antenna and so requires direction finding technology. A novel technique for electronically directing the radiation pattern of an antenna array employs a directional array with only one active element and three parasitic elements operating near resonance. Three different methods of direction finding are assessed; a coarse angular location method, a precise angular location method assuming one incident beam, and a precise angular location method with multiple incident beams. An array with n elements, if used in conjunction with a relatively simple controller, can be used to resolve n-1 signals. This technology can be implemented using both wire and patch antenna-array elements and either linear or circular polarization can be used, lending the technology to applications in both terrestrial and satellite communications systems.

Method for sequentially transmitting a downlink signal from a communication station that has an antenna array to achieve an omnidirectional radiation

Abstract: This invention relates to a method and apparatus for transmitting a downlink signal from a communication station to one or more subscriber units to achieve a desired radiation level over a desired sector (e.g., everywhere), the communication station including an array of antenna elements and one or more signal processors programmed (in the case of programmable signal processors) to weight the downlink signal according to one of a sequence of complex valued weight vectors. The method includes sequentially repeating transmitting the downlink signal, each repetition with a different weight vector from the sequence until all weight vectors in the sequence have been transmitted with. The sequence is designed for achieving the desired radiation level during at least one of the repetitions. In this way, every user in the desired region is transmitted to in the time period.

Line-of-sight angle of arrival estimation in the outdoor multipath environment

Abstract: Angle of arrival (AOA) estimation utilizing the MUltiple Signal Identification and Classification (MUSIC) algorithm is investigated for land vehicle location systems. A "virtual" antenna array is used to overcome problems inherent to MUSIC when operating in a highly correlated signal environment. Based on assumptions regarding the multipath radio environment, triangulation is possible by estimating the AOA of the line-of-sight (LOS) component in the received signal. Computer simulations are used to analyze the ability of techniques incorporating MUSIC to achieve this. Since triangulation is achieved using a narrowband signal, no new spectrum is required. Instead, the current cellular telephone bands as well as the existing basestation hardware could be exploited resulting in a cost-efficient and easily implemented system. A further cost reduction may be realized by processing received signal data while travelling along a nonlinear trajectory. In this way, an unambiguous angular spectrum is obtained with one antenna.

The application of characteristic-mode techniques to vehicle-mounted NVIS antennas

Abstract: Radio communications over relatively short distances, but beyond line-of-sight, present few problems nowadays, except in certain situations when no suitable, artificial means of relaying the signals exist. A mode of propagation, known as NVIS, or near vertical-incidence skywave, which relies only on the ionosphere, provides a solution. However, to exploit it requires careful choice of operating frequency, as well as the use of appropriate antennas. If the terminals are mobile, then the antennas, which are usually electrically small, can be the limiting factor. A method of optimizing antenna performance is presented that exploits the natural or characteristic modes of the structure on which the antenna is mounted. Extensive use is made of the method of moments, in the form of both the NEC and MININEC codes, to compute these modes, and to design and test antenna systems that exploit them. Measurements, made using an instrumentation package flown from a tethered balloon, compare very favorably with computed results.

Spatio-temporal radio channel characteristics in urban macrocells

Abstract: The performance assessment of new mobile radio base station (BS) transceiver concepts with adaptive intelligent antennas requires realistic directional radio channel models. An appropriate model is derived from the results of an extensive uplink sounding campaign performed at 1.9 GHz in a urban macrocell within the city of Frankfurt. In the experiments the wideband sounder RUSK XL was applied for synthetic aperture reception. The directional multipath propagation is characterised by means of delays, directions of arrival (DOA) at the BS antenna array and mean powers of the multipath components. The coupled delay-DOA-power information is determined by an adaptation of the 2-D unitary ESPRIT algorithm. It can be used directly to configure fading simulators for multiple antenna reception which reflect real-world small-area radio channel properties in simulations at the link level. Additionally, probability distribution functions for excess delays, DOAs and mean powers are derived from the measurement results. They are useful for consideration of directional propagation effects in investigations of the cellular system behaviour.

Estimating directivity and gain of antennas

Abstract: Gain is the most important performance parameter of an antenna. However, in many practical situations it is not possible to measure or calculate the gain of an antenna. Also, the interest in wireless applications has increased the need of system engineers to accurately estimate antenna gain. Many simple formulas are available for estimating gain. But, each formula has a range of applicability, and inappropriate use of these formulas will result in inaccurate gain values. This paper explains the process of gain evaluation, and gives several simple gain-estimation formulas, together with recommendations for their use.

Radiation patterns of dual concentric conductor microstrip antennas for superficial hyperthermia

Abstract: The finite difference time domain (FDTD) method has been used to calculate electromagnetic radiation patterns from 915-MHz dual concentric conductor (DCC) microwave antennas that are constructed from thin and flexible printed circuit board (PCB) materials. Radiated field distributions are calculated in homogeneous lossy muscle tissue loads located under variable thickness coupling bolus layers. This effort extends the results of previous investigations to consider more realistic applicator configurations with smaller 2 cm-square apertures and different coupling bolus materials and thicknesses, as well as various spacings of multiple-element arrays. Results are given for practical applicator designs with microstrip feedlines etched on the backside of the PCB antenna array instead of previously tested bulky coaxial-cable feedline connections to each radiating aperture. The results demonstrate that for an optimum coupling bolus thickness of 2.5-5 mm, the thin, flexible, and lightweight DCC antennas produce effective heating to the periphery of each aperture to a depth of approximately 1 cm, and may be combined into arrays for uniform heating of large area superficial tissue regions with the 50% power deposition contour conforming closely to the outer perimeter of the array.

Radio antenna system

Abstract: The invention relates to an apparatus and a method for simultaneously generating, with the same radio antenna apparatus (10), a number of narrow beams and a wide beam, covering substantially the same area covered by the individual pointed beams together. The radio antenna apparatus (10) comprises an antenna array (3), a Butler matrix (2) connected to the antenna array and a set of amplifying modules (1a,...,1h). The activation of each of the inputs (L1,...,L8) of the radio antenna apparatus corresponds to a radiation pattern characterized by a narrow beam with a high antenna gain from the antenna array (3). By simultaneously activating the beam ports with the same signal with suitable phase relationships a superimposition of the radiation patterns to which the activated beam port corresponds is achieved in such a way that a wide beam is generated. Since all amplifying modules (1a,...,1h) are used simultaneously, the lower antenna gain of the wide beam will be compensated by a corresponding higher amplification. The wide beam will therefore have substantially the same range as the narrow beams.

Downlink beam forming architecture for heavily overlapped beam configuration

Abstract: An antenna beam forming for generating a plurality of broad overlapping radiation beams for a cellular radio application comprises an antenna array having a plurality of individual antenna elements, each individual antenna element connected by a cable to a respective diplexor, each diplexor fed by a multi-channel power amplifier receiving an input from an output port of an eight way Butler matrix. Transmission signals are input to seven input ports of the Butler matrix, and output transmission signals are produced at four of the output ports of the Butler matrix. A remaining four output ports are each terminated with a respective matched load. A 3 dB power loss incurred in the Butler matrix is compensated by provision of a plurality of pre-amplifiers prior to the input ports of the Butler matrix, with the power amplification stage being carried out on the outputs of the Butler matrix. Side lobe suppression is maintained by ensuring phase and amplitude integrity across the output channels. A set of phase and amplitude adjusters operate on the output transmission signals from the Butler matrix output ports. The phase and amplitude adjusters may be electronically controlled.

Determination of jammer directions using multiple antenna beam patterns

Abstract: A radar or GPS receiving system uses an antenna array to produce multiple antenna beams directed toward multiple targets or spacecraft. Stand-off jammers produce signals which tend to be received on the sidelobes of the antenna beams and to disrupt system operation. An array of autonomous or adaptive nullers process the received signals to produce weights which adjust the antenna nulls to direct them toward the jammers. The antenna nulls of one beam pattern are compared with the nulls of other beam patterns. Normal between-sidelobe nulls will not occur at the same angle on all of the patterns, but those nulls generated by the nullers will occur at the same angle. The directions to the jammers are deemed to be those directions in which all beam patterns have nulls.

Weighted subspace fitting for general array error models

Abstract: Model error sensitivity is an issue common to all high-resolution direction-of-arrival estimators. Much attention has been directed to the design of algorithms for minimum variance estimation taking only finite sample errors into account. Approaches to reduce the sensitivity due to array calibration errors have also appeared in the literature. Herein, one such approach is adopted that assumes that the errors due to finite samples and model errors are of comparable size. A weighted subspace fitting method for very general array perturbation models is derived. This method provides minimum variance estimates under the assumption that the prior distribution of the perturbation model is known. Interestingly, the method reduces to the WSF (MODE) estimator if no model errors are present. Vice versa, assuming that model errors dominate, the method specializes to the corresponding "model-errors-only subspace fitting method." Unlike previous techniques for model errors, the estimator can be implemented using a two-step procedure if the nominal array is uniform and linear, and it is also consistent even if the signals are fully correlated. The paper also contains a large sample analysis of one of the alternative methods, namely, MAPprox. It is shown that MAPprox also provides minimum variance estimates under reasonable assumptions.

Method and system for transmitting and demodulating a communications signal using an adaptive antenna array in a wireless communication system

Abstract: In a transceiver for transmitting a communication signal from an antenna array in a wireless communications system, a different element pilot is transmitted from a plurality of antenna elements in an antenna array. Thereafter, transmitter control data based upon measured characteristics of the element pilots as measured at a subscriber unit is received at the transceiver. In response to the transmitter control data, a plurality of adaptive array weights for modifying a plurality of element communication signals are determined. Next, the transceiver determines that the subscriber unit is ready to receive a communication signal transmitted from the antenna array and weighted according to the plurality of adaptive array weights. In response to determining that the subscriber unit is ready, the transceiver modifies the plurality of element communication signals according to the determined plurality of adaptive array weights.

Phased array antenna

Abstract: A phased array antenna includes a plurality of planar panels each having opposing sides and top and bottom edges and forming an antenna face. Each planar panel is disposed circumferentially about a central axis and is connected to adjacent panels along the sides thereof. Each panel is inclined toward the central axis from the bottom edge. A plurality of antenna elements are positioned on each panel. Each panel includes substantially the same number of antenna elements as adjacent panels and forms an antenna array on each antenna face. A plurality of transmit/receive modules generates and receives signals, and each transmit/receive module corresponds to a respective antenna element on an antenna array. Means connects each of the transmit/receive modules to respective pluralities of the antenna elements within each array. A switch collectively controls the connection of the transmit/receive modules to respective antenna elements of one selected antenna array so that the transmit/receive modules are collectively connected to respective antenna elements of a selected antenna array at a selected time.

Antenna array calibration in wireless communication systems

Abstract: A method and apparatus are provided for easily and accurately calibrating an antenna array in a multipath environment. A calibration table containing spatial signature data together with corresponding location data is used to calibrate the antenna array, i.e. to determine the array response in various directions. The calibration table includes a set of calibrated signal covariances R 1 , . . . , R N together with a set of N corresponding geographical locations. From this data, a set of array calibration vectors {a(θ)} is determined, where each vector a(θ) characterizes the complex signal response of the antenna array in the direction θ. Once the array calibration vectors are determined, this information can be used for direction finding, beamforming, and other enhancements to the performance of the communication system.

Deterministic approaches for blind equalization of time-varying channels with antenna arrays

Abstract: In this paper, we study the blind equalization problem of time-varying (TV) systems where the channel variations are too rapid to be tracked with conventional adaptive equalizers. We show that using a finite Fourier basis expansion, a TV antenna array system can be cast into a time-invariant multi-input, multi-output (MIMO) framework. The multiple inputs are related through the bases, thereby allowing blind equalization to be accomplished without the use of higher order statistics. Two deterministic blind equalization approaches are presented: one determines the channels first and then the equalizers, whereas the other estimates the equalizers directly. Related issues such as order determination are addressed briefly. The proposed algorithms are illustrated using simulations.

Generalised array manifold model for wireless communication channels with local scattering

Abstract: The authors propose the use of a generalised array manifold for parameterised spatial signature estimation in wireless communication channels with local scattering. The array manifold commonly used for point sources is generalised to include linear combinations of the nominal array response vectors and their derivatives. The motivation behind this idea is to obtain better estimates of the spatial signatures for direction of arrival (DOA) based signal waveform estimation. The estimators proposed exploit the orthogonality between the so-called noise and signal subspaces, leading to a separable solution for the derivative coefficients. As a result, a search is required for the DOAs only. For uniform linear arrays, the spatial signatures are shown to be approximately Vandermonde vectors with damped modes, and a closed-form estimator such as ESPRIT may be used in this case. Simulation examples are included to compare the signal estimation performance obtained using the proposed generalised manifold and the conventional array manifold.