Analysis of phase-focused near-field testing for multiphase-centeradaptive radar systems

Airborne or spaceborne radar systems often require tests before deployment to verify how well the system detects targets and suppresses clutter and jammer signals. The radar antenna diameter can be large and thus the conventional far-field test distance is impractical to implement. The theory and simulations of phase-focused near-field testing for adaptive phased array antennas is discussed. With near-field source deployment, standard phased-array near-field phase focusing provides far-field adaptive nulling equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. Both main beam clutter sources and sidelobe jammer sources are addressed. The phased array antenna elements analyzed are one-half wavelength dipoles over the ground plane. Bandwidth, polarization, array mutual coupling, and finite array edge effects are taken into account. Numerical simulations of an adaptive antenna that has multiple displaced phase centers indicate that near-field and far-field testing can be equivalent     

A comparison of adaptive algorithms based on the methods of steepest descent and random search

This paper compares the performance characteristics of three algorithms useful in adjusting the parameters of adaptive systems: the differential (DSD) and least-mean-square (LMS) algorithms, both based on the method of steepest descent, and the linear random search (LRS) algorithm, based on a random search procedure derived from the Darwinian concept of "natural selection." The LRS algorithm is presented here for the first time. Analytical expressions are developed that define the relationship between rate of adaptation and "misadjustment," a dimensionless measure of the difference between actual and optimal performance due to noise in the adaptive process. For a fixed rate of adaptation it is shown that the LMS algorithm, which is the most efficient, has a misadjustment proportional to the number of adaptive parameters, while the DSD and LRS algorithms have misadjustments proportional to the square of the number of adaptive parameters. The expressions developed are verified by computer simulations that demonstrate the application of the three algorithms to system modeling problems, of the LMS algorithm to the cancelling of broadband interference in the sidelobes of a receiving antenna array, and of the DSD and LRS algorithms to the phase control of a transmitting antenna array. The second application introduces a new method of constrained adaptive beamforming whose performance is not significantly affected by element nonuniformity. The third application represents a class of problems to which the LMS algorithm in the basic form described in this paper is not applicable.     

BER performance of CMA adaptive array for high-speed GMSK mobilecommunication-a description of measurements in central Tokyo

This paper describes the performance of an adaptive array as a countermeasure to multipath fading for a 256 kbps Gaussian-filtered minimum shift keying (GMSK) mobile communication system operating in the 1.5 GHz band. An adaptive array having four antenna elements is implemented using the digital beam forming concept. The constant modulus algorithm (CMA) is employed for the adaptation process to ease the implementation. Measurements in central Tokyo of the bit error rate (BER) performance and an array pattern arising in the multipath environment are presented. Analysis of the array pattern confirms that the array succeeds in directing nulls to the delayed signals. BER performance shows an improvement in E_b/N₀, compared with that of a single antenna system, of 17.5 to 22 dB at a BER of 1.0×10^-2 in a frequency-selective fading channel     

一种自适应通道均衡技术及其实现

自适应天线接收通道间的幅相不一致会其以抑制干扰的零点深度变浅、方向图副瓣电平升高,严重影响自适应天线的抗干扰性能,因此在其工作之前必须对其各通道的幅度和相位特性进行均衡。提出了一种基于变步长最小均方算法（LMS）的自适应通道均衡技术,并给出了其现场可编程门阵列（FPGA）实现方案,仿真结果表明,该方案可以在很宽的频率范围内获得高均衡精度,可以为提高宽带自适应天线系统的抗干扰性能提供参考和借鉴。     

A nonlinear algorithm for output power maximization of an indooradaptive phased array

An adaptive receiving phased-array antenna is considered as one of the desirable features of the next generation of personal wireless indoor system. Such a feature will render this communication system much less sensitive to the signal fluctuations due to multipath propagation. This paper presents a prototype adaptive phased array based on an original nonlinear method. This adaptive algorithm which uses as performance criterion the maximization of the array output power to adjust a phase angle, is developed and applied to antenna system in order to control automatically its directivity pattern. Numerical results for a linear eight element array are presented. To test the algorithm experimentally, an eight element 20-GHz array, constructed using horn antennas and analog phase shifters, was used. The results obtained so far with such an array are discussed in the context of its eventual, implementation in MHMIC     

Adaptive processing using real weights based on a direct data domain least squares approach

In traditional, adaptive signal processing algorithms one change both the amplitude and phase of the weight vectors associated with an array at each of the antenna elements. The use of complex weights offers greater control over the array response at the expense of system complexity. However, it is easier if one requires only amplitude variation with a fixed phase for all the weight vectors associated with all the antenna elements. Because one uses only real arithmetic operations to find the amplitude of the weights connected to the antenna, the computational complexity is reduced considerably. Hence, this paper addresses the use of real weights in an adaptive system. In this paper we describe a new direct data domain least squares (D/sup 3/LS) method using real weights, which utilizes only a single snapshot of the data for adaptive processing. This technique may be useful for real time implementation of the D/sup 3/LS method on a chip.     

Performance enhancement of CDMA cellular systems with augmentedantenna arrays

The use of base station adaptive antenna arrays is an attractive way to increase the capacity of code division multiple access cellular systems. In this paper, a system with an adaptive minimum redundancy array (MRA) at the base stations is proposed. This system uses the high-resolution signal angle of the arrival estimation algorithm, ESPRIT, in conjunction with array augmentation techniques, and linear least squares adaptation. For the MRA, we propose to use virtual array elements at the locations where there is no sensor element. All real and virtual sensor outputs are weighted and combined to extract the desired signal components and suppress interference. To simplify the evaluation of the interference from adjacent cells, the concentric circle cell geometry is employed in place of the common hexagonal cell geometry. The performance of the proposed system is quantified by comparison with the omnidirectional antenna, and the adaptive uniform linear array (ULA) with the same number of elements and same array aperture, using realistic simulations. It is shown that, for a four-element array, there is about a 4- and a 1-dB improvement in the despread output signal-to-interference-plus-noise ratio of the proposed system over the omnidirectional antenna and the conventional ULA, respectively     

A cross-layer approach to transmit antenna selection

In this paper, we investigate a cross-layer approach to transmit antenna selection capable of adapting the number of active antennas to varying channel conditions. We address a cross-layer methodology in the sense that the criterion for the selection of antenna subsets is the maximization of link layer throughput which takes into account characteristics both at the physical and link layers. In order to enhance system performance, adaptive modulation is included to jointly perform antenna selection and rate adaptation. Performance assessment is conducted in terms of link layer throughput and transmission delay.     

Adaptive antenna systems

A system consisting of an antenna array and an adaptive processor can perform filtering in both the space and the frequency domains, thus reducing the sensitivity of the signal-receiving system to interfering directional noise sources. Variable weights of a signal processor can be automatically adjusted by a simple adaptive technique based on the least-mean-squares (LMS) algorithm. During the adaptive process an injected pilot signal simulates a received signal from a desired "look" direction. This allows the array to be "trained" so that its directivity pattern has a main lobe in the previously specified look direction. At the same time, the array processing system can reject any incident noises, whose directions of propagation are different from the desired look direction, by forming appropriate nulls in the antenna directivity pattern. The array adapts itself to form a main lobe, with its direction and bandwidth determined by the pilot signal, and to reject signals or noises occurring outside the main lobe as well as possible in the minimum mean-square error sense. Several examples illustrate the convergence of the LMS adaptation procedure toward the corresponding Wiener optimum solutions. Rates of adaptation and misadjustments of the solutions are predicted theoretically and checked experimentally. Substantial reductions in noise reception are demonstrated in computer-simulated experiments. The techniques described are applicable to signal-receiving arrays for use over a wide range of frequencies.     

Capacity of multiple-antenna systems with both receiver and transmitter channel state information

The capacity of multiple-antenna systems operating in Rayleigh flat fading is considered under the assumptions that channel state information (CSI) is available at both transmitter and receiver, and that the transmitter is subjected to an average power constraint. First, the capacity of such systems is derived for the special case of multiple transmit antennas and a single receive antenna. The optimal power-allocation scheme for such a system is shown to be a water-filling algorithm, and the corresponding capacity is seen to be the same as that of a system having multiple receive antennas (with a single transmitter antenna) whose outputs are combined via maximal ratio combining. A suboptimal adaptive transmission technique that transmits only over the antenna having the best channel is also proposed for this special case. It is shown that the capacity of such a system under the proposed suboptimal adaptive transmission scheme is the same as the capacity of a system having multiple receiver antennas (with a single transmitter antenna) combined via selection combining. Next, the capacity of a general system of multiple transmitter and receiver antennas is derived together with an equation that determines the cutoff value for such a system. The optimal power allocation scheme for such a multiple-antenna system is given by a matrix water-filling algorithm. In order to eliminate the need for cumbersome numerical techniques in solving the cutoff equation, approximate expressions for the cutoff transmission value are also provided. It is shown that, compared to the case in which there is only receiver CSI, large capacity gains are available with optimal power and rate adaptation schemes. The increased capacity is shown to come at the price of channel outage, and bounds are derived for this outage probability.     

Experimental investigation of an adaptive feedback algorithm forhot spot reduction in radio-frequency phased-array hyperthermia

A computer-controlled adaptive phased array radiofrequency hyperthermia system for improved therapeutic tumor heating is experimentally investigated. Adaptive array feedback techniques are used to modify the electric-field in hyperthermia experiments with a homogeneous saline phantom target. A hyperthermia phased-array antenna system has been modified to implement adaptive nulling and adaptive focusing algorithms. The hyperthermia system is a ring phased-array antenna applicator with four independently controlled RF transmitter channels operating at a CW frequency of 100 MHz. The hyperthermia phased array is made adaptive by software modifications which invoke a gradient-search feedback algorithm that controls the amplitude and phase of each transmitter channel. The gradient-search algorithm implements the method of steepest descent for adaptive nulling (power minimization) and the method of steepest ascent for adaptive focusing (power maximization). The feedback signals are measured by electric-field short-dipole probe antennas. The measured data indicate that with an adaptive hyperthermia array it may be possible to maximize the applied electric field at a tumor position in a complex scattering target body and simultaneously minimize or reduce the electric field at target positions where undesired high-temperature regions (hot spots) occur     

Phase-only adaptive Processing based on a direct data domain least squares approach using the conjugate gradient method

Generally, adaptive antenna arrays operate by changing the complex adaptive weights consisting of both magnitudes and phases applied at each of the antenna elements. However, it is easier to require the adaptive weights have only phase variation with a fixed magnitude at each of the antenna elements. Hence, This work addresses the phase only adaptive systems whose weights have a fixed magnitude through a new phase only adaptive method based on a direct data domain least squares approach (D/sup 3/LS), which utilize only a single snapshot of the data for adaptive processing.     

Adaptive crossed dipole antennas using a genetic algorithm

Antenna misalignment in a mobile wireless communications system results in a signal loss due to a decrease in antenna directivity and a polarization mismatch. A genetic algorithm (GA) is used to adaptively alter the polarization and directivity of a crossed dipole receive antenna in order to increase the link budget. The three orthogonal dipole configuration works better than only two crossed dipoles, but both improved the link loss as the angular pointing errors increased. A GA with a high mutation rate works best for a noiseless open loop adaptation, while a GA with a low mutation rate works best for noisy fully adaptive system.     

DS-CDMA capacity enhancement with adaptive antennas

A new ray-based simulation methodology for an adaptive antenna system is presented. Results for a typical microcellular environment highlight the behaviour of the adaptive antenna, and DS-CDMA capacity analysis illustrates the capacity enhancement attainable. For the cases considered, a minimum five fold increase in the overall spectrum efficiency is predicted for an eight-element adaptive antenna array employing the RLS algorithm     

The root-trajectory method--A tool for analyzing linear adaptive arrays

A root-trajectory method is described that seeks to obviate the need for an extensive set of antenna patterns to describe the behavior of an adaptive array during its adaptation process. This aim is achieved by use of Schelkunoff's idea of describing a linear array in terms of an array polynomial. The roots of this polynomial have a direct interpretation in obtaining the array gain, and the trajectories of these roots during the adaptation process give a simple visualization of the dynamic behavior of the adaptive array.     

The effects of reflector antenna diffraction on the interference cancellation performance of coherent sidelobe cancellers

While adaptive antenna technology has undergone significant development, little attention has been given to the impact of antenna design on the performance of the adaptive system. A need exists to factor the details of the antenna system response into the analysis of adaptive system performance, particularly in the case where adaptive cancellation is required over a broad bandwidth. At extremely high frequency (EHF) where wide bandwidth allocations exist, reflector antenna technology used with an adaptive sidelobe canceller design is appropriate. This paper uses a simple diffraction model to compare the adaptive performance differences between Cassegrain and offset reflector designs. The reduced diffraction of the offset reflector design results in improved cancellation performance. These analyses also provide the opportunity to explore the impacts of antenna design parameters and interference power levels and arrival directions.     

A Multiuser Detection Receiver Using Blind Antenna Array and Adaptive Parallel Interference Cancellation

In this paper, the problem of multiuser detection for synchronous code division multiple access systems in both additive white Gaussian noise and multipath channels is addressed. A new multiuser detection receiver that uses an adaptive blind array along with an adaptive parallel interference canceler is proposed. The replacement of a conventional antenna array with a two-dimensional RAKE receiver is also considered for frequency selective Rayleigh fading channels. By using a constrained optimization criterion along with the gradient-projection algorithm, a blind algorithm for the adaptation of the array response vector is proposed. The new algorithm is superior to a few typical blind algorithms in the literature in terms of both performance and computational complexity. The proposed receiver has the ability to cancel very strong multiple access interference coming from the same direction as the desired signal. Simulation results are presented to show the excellent performance of the proposed combination scheme in comparison to that of using either a multiuser detection or adaptive antenna arrays in a severe near-far situation.     

智能天线跟踪波束与切换波束的比较

首先从波束形成或控制的角度界定了2类智能天线—跟踪波束智能天线(以下简称跟踪波束系统)和切换波束智能天线(以下简称切换波束系统)。其次分别介绍了其中关键部分:跟踪波束系统的自适应方法和切换波束系统的波束选择方法的设计考虑。跟踪波束系统的自适应方法是利用基于Lagrange公式的自适应算法跟踪最强信号源,而切换波束系统的波束选择方法则是基于正确波束接收的信号必然比任何其他波束接收的信号强这个事实。最后,依据其性能和所需的复杂度对跟踪波束系统和切换波束系统作了比较。     

CDMA移动通信系统中基于RLS算法的天线阵列有源校正

基于递归最小二乘(RLS)自适应滤波理论,本文提出了一种天线阵列的幅度和相位不一致性的有源在线 校正方法。该方法具有快速稳健的收敛性能,不受天线阵列的几何形式限制,也适用于移动用户数大于阵元数的实际通信 环境中。计算机仿真证实了该方法的有效性。