动态MIMO散射无线信道模型及性能分析

基于多入多出(MIMO)散射无线信道模型,提出一种动态MIMO散射无线信道模型,分析散射体及其收发多天线的运动对MIMO无线信道空域相关性及其容量的影响,得出这种影响是由收发天线的初始位置、运动速度及其传播环境决定的.数值模拟验证了这种影响,并指出空域相关性随天线单元间距增大而减小,随散射信号角度扩展增大不是一致减小,存在使相关性达最小的角度扩展值.     

面向远场计算的波谱射线方法

根据天线远区场对口径波谱的局部依赖特性,引入有效近场的概念,提出一种面向远场计算的波谱射线方法,该法采用波谱射线抽取的方法计算有效近场,通过近场空域积分计算远场,适合于具有雷达罩,透镜等近区散射体的天线远场计算。文中以带罩连续口径及阵列天线的远场方向计算为例,验证了方法的正确性和有效性。     

An antenna coupling model for cross-polarized antennas for radarelectromagnetic compatibility analysis

A statistically based antenna coupling model for linearly polarized, cross-polarized antennas is presented. The antenna gains are defined as random variables. It is assumed that the probability density functions of the gains can be estimated from the antenna patterns. From the density functions, a method is developed to calculate the combined density function. This method accounts for angular motion that the antennas would experience if they were mounted on ships subject to roll. The probability of interference is defined, and a method is developed to calculate it based on the combined density function. The results of this analysis for two selected antennas are presented, and the limits of applicability, are discussed     

Circularly polarized conical patterns from circular microstrip antennas

A method is presented for generating circularly polarized conical patterns from circular microstrip antennas. These antennas are excited at higher order modes and require different feed arrangements for different mode excitations. It is determined that the peak direction of the conical pattern can be varied over a wide angular range. Modal expansion technique is employed to calculate the radiation patterns of these antennas.     

Tilting antennas to reduce line-of-sight microwave link fading

Tilting the antennas up to obtain 2 dB loss (for each antenna) over maximum gain during steady signal conditions is shown to reduce fading over this particular path. The technique should also work on other paths where sufficient angular separation between the direct and reflected rays exists, and where antennas with sufficiently "sharp" main beams are used.     

Maximum power transfer between finite antennas

The optimum illumination for maximum power transfer between two opposed rotationally symmetric antennas is determined. The antennas have spherical surfaces and dissimilar apertures. Given an angular variation of the illumination ofcos (alpha theta), withalphaintegral, the optimum radial distribution becomes solutions of a finite Hankel transform.     

高角度分辨率的信源分离算法

传统的波束形成算法在期望信号和干扰信号来向相近的情况下性能下降,不能很好地完成信源分离,针对此问题提出了一种高角度分辨率的信源分离算法。首先选取一种适合任意阵型的AR模型预测方法进行阵列扩展,然后利用基于独立分量分析的鲁棒性分离算法完成信号分离,通过结合阵列扩展和盲源分离的优点,本文提出的分离算法在不增加实际阵元数目的前提下具备比原阵列更高的角度分辨率。实际数据测试表明提出的算法能够在9元均匀圆阵上分离方位角来向间隔为0.5度的信号。     

Theoretical and experimental study of metal grid angular filters for sidelobe suppression

The synthesis, analysis, and experimental measurements of a metal grid angular filter are described. Angular filters are devices that can be added to existing antennas in order to reduce the radiation over a specified angular region while allowing radiation over other angular sectors to pass relatively unaffected. The behavior of this angular filter, both in frequency and angle, is analyzed in this paper, and these results are compared with measurements.     

Multilevel fast physical optics algorithm for radiation from non-planar apertures

A novel multilevel algorithm for computing the radiation patterns of nonplanar aperture antennas over a range of observation angles is presented. The proposed technique is directly applicable to reflector and lens antennas as well as to radomes. The multilevel computational sequence is based on a hierarchical decomposition of the radiating aperture and comprises two main steps. First, computation of the radiation patterns of all subapertures of the finest level over a very coarse angular grid. Second, multilevel aggregation of the radiation patterns of neighboring subapertures into the final pattern of the whole aperture via a phase compensated interpolation. The multilevel algorithm attains computational complexity comparable to that of the fast Fourier transform based techniques while avoiding their limitations.     

Effect of Pointing Errors and Range on Performance of Dual-Pencil-Beam Scatterometers

Short-range FM scatterometers with single antennas are plagued by interference set up by feedthrough and internal reflections. Dual-antenna systems have much lower internal interference, but there are problems associated with pointing the antennas at the same spot. This note quantifies these problems for Gaussian-shaped beams. The use of antennas with beamwidth ratios of up to 5: 1 is shown to improve performance significantly over that obtained with identical beamwidths. For an angle between antenna centers (as viewed from the beam intersection) that is three times the beamwidth of the narrower antenna, the usable spread of range for equal beamwidths is only about 1.85:1, while for an angular ratio of 5, the usable spread of range is > 20:1.     

Vehicle location in angular sectors based on signal strength

The results of field measurements carried out in center city Philadelphia and in the Whippany/Morristown, NJ suburban area to assess the accuracy of a vehicle location technique at 820 MHz based on signal strength comparison are described. Six co-sited directional antennas, each covering a 60° angular sector in azimuth, were used to detect the signal transmitted by the mobile. The angular location of the vehicle was determined by comparing the signal strengths received "simultaneously" through the directional antennas. The measured vehicle location was then compared against the true location of the vehicle at the time of the measurement to generate statistics on location accuracy. An estimate is made on the expected improvements in the locating accuracy when three co-sited directional antennas are used, each of which provides coverage for a 120° sector in the horizontal plane. Also described are the effects of signal sample "integration time" and antenna beam shaping on the accuracy of the position estimate. The measurement results presented put a bound on the expected range of accuracies in the signal-strength-based "angle-of-arrival" vehicle location technique.     

Reactance-controlled short-backfire array

Experimental data on the performance characteristics of a reactance-controlled waveguide-fed short-backfire array is described. The results indicate that this simple, compact structure, which electronically steers the beam over a limited angular region, may be useful for ship and airborne antennas.     

Effect of antenna separation on the capacity of BLAST in correlatedchannels

As the base station is usually placed above local clutter, the angular spectrum incident on the base is narrow, inducing correlation among base antenna signals, which reduces the capacity of a multiple transmit and receive antenna systems. In this work the general expression for link capacity is derived, when there is correlation among receive antennas and among transmit antennas. It is found that an antenna separation of 4 wavelengths between nearest neighbors in a linear base array of dually polarized antennas allows one to achieve 80% of the capacity attainable in the uncorrelated antenna case     

The double-helix antenna and its variants, a new class of tunable endfire antennas

Endfire antennas develop maximum gain when the phase velocity of the surface wave traveling along the structure is adjusted to its optimum value determined as a function of antenna length and operating frequency. These antennas usually have a relatively small pattern bandwidth and, if maximum gain is desired, can be used over only a small frequency range. The antennas described in this paper inaugurate a new class of antennas that are tunable for maximum gain in the endfire direction over a wide frequency range; tuning is accomplished by changing the phase velocity continuously or in prescribed steps. Such antennas include certain configurations of the double helix (a novel type of endfire antenna) and its artificial and natural dielectric variants. Useful structures are obtained through parallel displacement of two juxtaposed elements or angular displacement of a scissors arrangement. Model measurements show that maximum gain can be obtained over a frequency range of more than 2:1. Tuning effects are illustrated in detail by means of near-field plots of a tuned dielectric antenna.     

Improving MIMO capacity with directive antennas for outdoor-indoor scenarios

MIMO systems are usually associated with high scattering isotropic propagation while the use of directive antennas is associated with free space conditions. We found outdoor-indoor channels to be in between these two extremes, in the sense that we observed directivity - and - MIMO gain, for the same ensemble of channels. Our observation is based on measurements with directive (8 dB) and dipole antennas. Median MIMO capacities were found to be about 80% of the ideal (Rayleigh i.i.d.), at 5 dB Signal to Noise Ratio (SNR), for both types of antennas. Using properly aimed directive antennas, the SNR was found on average to be 5.4 dB above that obtainable with dipoles, somewhat less than the 7 dB antenna gain difference. Thus, isotropic propagation, which would have negated directivity gains, cannot be justified in general. We empirically established that aiming for largest received power is the best array pointing strategy with directive antennas. Combining MIMO processing and angular search resulted on average in gains of 70% over the median capacities obtained with dipoles. Therefore it may in some cases be convenient to arrange subgroups of antennas for beamforming, and then process the thus reduced number of radio channels for MIMO gain.     

Circular array and nonsinusoidal waves

A theory of circular array antennas based on nonsinusoidal (rectangular) pulses, is developed. Different antenna patterns such as peak amplitude, peak power, energy, and slope pattern are derived and plotted. The antenna patterns yield the resolution angle for the circular array as a function of array radius and frequency bandwidth. The effect of additive Gaussian noise on the angular resolution capability of the circular array is analyzed. The analysis is based on calculating slope patterns by using linear regression algorithm for different signal-to-noise power ratios     

On the application of adaptive antennas to microcellularenvironments: radio channel characteristics and system performance

The possible application of adaptive antennas in microcellular operational environments is studied in this paper with the help of a site-specific ray-tracing propagation model. First are examined the statistics of propagation parameters that play an important role for the performance of adaptive antennas. Particular emphasis is put on the spatial characteristics of the radio channel under line-of-sight (LOS) and non-LOS conditions. For this reason, area statistics are produced for a typical small-cell environment for parameters such as the number of multipath clusters, number of rays, angular size, and excess delay of each impulse response, as well as the number of rays, angular size and power of the multipath clusters. Results show that contrary to the common belief for such environments, the signal is not "almost" uniformly distributed in the spatial domain, instead is contained in a few narrow clusters. Then the effect of the microcellular environment on the uplink performance of some well-known techniques like conventional beamforming, maximum entropy, switched beams, maximum ratio and optimum combining, and beam-space optimum combining is examined . The results show that for typical time-division multiple-access (TDMA) scenarios with one and two users, the beam space optimum combining (strongest eigenvalue version) and the switched beams methods, offer the best performance     

Pattern translation and rotation in uncorrelated sourcedistributions for multiple beam antenna design

In the uncorrelated scenario model, a continuous source distribution illuminates a receiver with waves bearing signals that are uncorrelated with respect to their angle of arrival. This model is used for multipath situations such as scintillating atmospheres, but also can be applied to optical beams for indoor communications, acoustic beamforming, and, of particular interest here, mobile communications. For diversity antennas operating in such a scenario, patterns that produce uncorrelated signals are required. An angular separation of directive beams in multipath scenarios acts to decorrelate the received signals. It is of interest to quantify the minimum angular spacing required of beams in order to provide a framework for the design and configuration of the antennas. The approach is to consider only the main lobe of the antenna pattern and to take it as a real function. This is reasonable as long as most of the energy is conveyed via the main beam. In practice, the sidelobe structure and nonuniformity in the phase of an actual pattern act to improve the situation in the sense that the decorrelation angles become smaller. The conditions for angular diversity result in a simple rule-of-thumb for the minimum beam separation requirement, which is essentially independent of the directivity. Finally, both scalar and vector elliptic beams rotated about their boresight axes are analyzed for the decorrelation rotation angle as a function of the ratio of the ellipticity. The resulting design curves offer a guideline to beam configuration for multipath scenarios     

Hybrid perturbation scheme for wide angle circular polarisation of stacked square-ring microstrip antennas

A unique hybrid perturbation scheme is proposed for circular polarisation of stacked square-ring microstrip antennas, where both positive and negative perturbations are used. It is shown that the method broadens the angular range of circular polarisation. Measured results confirm the simulations.