Application of antenna arrays to mobile communications. II.Beam-forming and direction-of-arrival considerations

Array processing involves manipulation of signals induced on various antenna elements. Its capabilities of steering nulls to reduce cochannel interferences and pointing independent beams toward various mobiles, as well as its ability to provide estimates of directions of radiating sources, make it attractive to a mobile communications system designer. Array processing is expected to play an important role in fulfilling the increased demands of various mobile communications services. Part I of this paper showed how an array could be utilized in different configurations to improve the performance of mobile communications systems, with references to various studies where feasibility of apt array system for mobile communications is considered. This paper provides a comprehensive and detailed treatment of different beam-forming schemes, adaptive algorithms to adjust the required weighting on antennas, direction-of-arrival estimation methods-including their performance comparison-and effects of errors on the performance of an array system, as well as schemes to alleviate them. This paper brings together almost all aspects of array signal processing     

基于分层几何单反射原理的连续时间时变矢量信道模型

提出了一种适用于智能天线技术的新的基于分层几何单反射原理的连续时间时变矢量信道传播模型。在此基础上,直接从矢量信道冲击响应出发,得到了宽带连续时间无线信道数学模型,通过改变几何参数,可以仿真具有任意衰落特性的无线信道。*     

Antenna arrays in mobile communications: gain, diversity, andchannel capacity

Are antenna arrays in mobile communications different from arrays in other applications? The answer is yes, sometimes, and it is the purpose of this paper to explain, in a tutorial fashion, when this is the case, and what this means for path loss in link calculations. One aspect is the classical gain of an antenna, which we have to understand in a new way. Another aspect is the possibility for two arrays, in a scattering environment, to create parallel channels, and thus, in effect, act as many independent antennas at the same time, carrying much more traffic over the same bandwidth     

Switchable circular disk antenna for RCS and mobile communications applications

In this paper, we describe the operation of a low profile, 3 mm deep, circular disk antenna which requires a small ground plane, and incorporates a shorting bar which can be rotated. The position of the shorting bar determines the return loss and radiation characteristics of the antenna element. In one mode at 2.34 GHz the antenna exhibits a return loss of better than -25 dB, 5 dBi gain, and a conical/monopole beam pattern suitable for radar tracking/mobile communications applications. When the short circuit is positioned at 90 degrees the return loss at 3.52 GHz becomes -18 dB, 7 dBi gain, and the antenna exhibits normal linear polarized broadside cosine radiation characteristics. In addition when the short circuit is positioned at zero degrees at 3.52 GHz the antenna gives -1 dB return loss and by reciprocity will act as a reflector to incoming energy. Theoretical results are provided in support of the experimental results observed and are used to explain the operating mechanisms of the antenna.     

电下倾天线改善蜂窝系统性能的研究

提出了一种用于蜂窝移动通信的简单电下倾天线,将电波功率随水平距离衰减的幂指数提高到5.8,采用更紧密的复用模式成为可能,使频率复用率和系统容量得到提高.     

Performance analysis of diversely polarized antenna arrays

The performance of direction finding systems utilizing diversely polarized antenna arrays is investigated. The Cramer-Rao bound (CRB) is used to evaluate the accuracy of the estimated directions of arrival. The resolving power of the array is evaluated by analyzing the performance of a likelihood ratio detector designed to determine whether the signals observed by the array originate from a single source or from two closely spaced sources. Numerical examples and analysis of some special cases provide insights into the improvement in direction finding accuracy and in the resolving power of the array due to polarization diversity. The case of a signal in the presence of an interferer is studied in some detail. The CRB and the probability of detection are evaluated as functions of the spatial separation and the polarization difference between signal and interferer, signal-to-noise ratio, and signal-to-interference ratio. Performance comparisons are made between uniformly polarized and diversity polarized arrays. It is shown that improved direction finding performance can be obtained by using polarization diversity, with no increase in hardware complexity     

Analysis of beam-steering and directive characteristics of adaptiveantenna arrays for mobile communications

Radiation patterns of planar, circular, and three-segment adaptive antenna arrays are analyzed and compared in view of applications in mobile radio. It is assumed that each array consisted of several subarrays, which themselves present vertical linear arrays (columns) of patch radiators, with prescribed magnitude and phase distributions that determine the beam shape in the vertical (elevation) direction. Weighting signals applied to the subarray inputs provide beam steering in the horizontal (azimuth) plane. The scan-angle range and directive properties of such arrays are analyzed and simulated. For relatively small and moderate-sized base-station adaptive arrays, consisting of N=4, ..., 7 subarrays, the planar configuration may not be an appropriate choice, as the planar array has limited scan-angle range. Steering its beam toward the edges of a 120° sector leads to a rise in the grating lobe in the opposite azimuth direction, which, in turn, entails an additional decrease in antenna gain. Circular and three-segment configurations are good and relatively simple alternative variants. These have some degree of curvature, which allows beam steering in a wider angular range when compared to the planar geometry, although at the expense of a wider beam and lower directivity. Their geometrical parameters should be carefully chosen, so as to provide the required scans at the least loss in directivity. The three-segment form is closely related to its planar counterpart and, in some applications, may be more preferable than the circular form. For larger arrays, consisting of N⩾8 subarrays, the planar geometry can attain the required 120° scans, and has advantages in directivity and gain when compared to any curved configuration     

移动通信基站天线的关键技术研究进展

基站天线作为移动通信系统中的重要部件,对通信质量起至关重要的作用,其关键技术被广泛研究。通过对基站天线的电调技术、极化技术、宽带技术、阵列天线多频技术和基站天线小型化技术等的研究发展历程介绍,分析了这些关键技术在工程上的实现方法与措施,总结了这5个关键技术今后的研究重点和发展方向。在基站天线下一步的发展中,需要将多个关键技术完美地结合在一起,阵列天线多频技术和基站天线小型化技术将成为今后的研究热点。     

Positive-definite Toeplitz completion in DOA estimation fornonuniform linear antenna arrays. I. Fully augmentable arrays

This paper considers the problem of direction-of arrival (DOA) estimation for multiple uncorrelated plane waves incident on so-called “fully augmentable” sparse linear arrays. In situations where a decision is made on the number of existing signal sources (m) prior to the estimation stage, we investigate the conditions under which DOA estimation accuracy is effective (in the maximum-likelihood sense). In the case where m is less than the number of antenna sensors (M), a new approach called “MUSIC-maximum-entropy equalization” is proposed to improve DOA estimation performance in the “preasymptotic region” of finite sample size (N) and signal-to-noise ratio. A full-sized positive definite (p.d.) Toeplitz matrix is constructed from the M×M direct data covariance matrix, and then, alternating projections are applied to find a p.d. Toeplitz matrix with m-variate signal eigensubspace (“signal subspace truncations”). When m⩾M, Cramer-Rao bound analysis suggests that the minimal useful sample size N is rather large, even for arbitrarily strong signals. It is demonstrated that the well-known direct augmentation approach (DAA) cannot approach the accuracy of the corresponding Cramer-Rao bound, even asymptotically (as N→∞) and, therefore, needs to be improved. We present a new estimation method whereby signal subspace truncation of the DAA augmented matrix is used for initialization and is followed by a local maximum-likelihood optimization routine. The accuracy of this method is demonstrated to be asymptotically optimal for the various superior scenarios (m⩾M) presented     

Performance of coherent OFDM-CDMA for broadband mobile communications

Uncoded orthogonal frequency division multiplexing (OFDM) transmission technique applied in a multipath environment has a bit error rate (BER) comparable with a narrowband radio channel because the fading of each subcarrier is frequency-nonselective. To overcome this behaviour and to reduce the BER, a combination of OFDM and CDMA has been proposed recently. In an OFDM-CDMA system the energy of each information symbol is spread over several subcarriers. Therefore a diversity gain can be obtained in a broadband fading channel.In this paper we discuss the performance of OFDM-CDMA with coherent QPSK signalling over a frequency-selective Rayleigh fading channel. Channel estimation and demodulation are integral parts that determine the performance of the system. The method for channel estimation presented in this paper is based on a two-dimensional array of pilot symbols with second-order regression in the time domain and interpolation in the frequency domain. Quantitative comparison of four different detection algorithms in frequency-selective Rayleigh fading with noisy channel state information (CSI) will be presented in this paper: conventional correlation (equal gain correlation, EGC), orthogonality restoring correlation (ORC), ORC with a threshold in order to suppress subcarriers with low signal strength (TORC), and an iterative improvement based on a maximum likelihood approach. With TORC and iterative improvement a gain of approximately 9 dB over conventional OFDM can be obtained at a BER of 10^–3 in Rayleigh fading.     

Performance improvement of CCD image sensor arrays

In conventional arrays, charges are read out from photodiodes through vertically placed charge-coupled registers. In an array presented here, reading out is performed through vertical diffusion lines. As shown, such an array is more sensitive, since the photodiodes occupy a larger space. Also, this array shows a reduced level of cross-coupling, because extra charges leave the photodiodes, and a reduced level of intrinsic back-ground, because charges generated exterior to the photodiodes are removed.     

PN code acquisition using smart antenna for spread-spectrum wireless communications. I

The smart antenna (a blind adaptive antenna array) has brought much attention to its ability to improve the future code-division multiple-access (CDMA) wireless communications systems. However, Adachi et al. (1998), employed only one out of M elements that are in a smart antenna array during the pseudonoise (PN) code acquisition process due to the difficulty in obtaining the weight coefficients of the smart antenna and PN code acquisition jointly and adaptively. Yet, as the PN code is acquired, all M elements are used by Adachi et al. to improve the signal-to-noise ratio (SNR) at a reverse link. We propose an efficient PN code acquisition scheme where all elements are employed during PN code acquisition. Also, we show that by using all elements, the PN code acquisition time can be shortened by at least 40% for a given signal-to-noise ratio and M=5, compared to the single element case. Furthermore, the power of a random access user signal in the reverse link can be lowered at least 3 dB. The additional computational load and complexity to use the proposed PN code acquisition are insignificant.     

Stimulated positron emission for 3-D tomographic imaging and bone studies. I. Method feasibility and system considerations

The feasibility and inherent performance parameters of a novel method of 3-D tomographic imaging have been studied analytically. A cross section in the patient's body is excited by high-energy X-rays ( hv>1.022 MeV) to produce positron-electron pairs. The resulting annihilation quanta are detected in coincidence by two detectors placed on opposite sides of the irradiated slice. Following a coincidence, the annihilation point is determined as the intersection of the line defined by the annihilation pair and the irradiated plane. Since the photon cross section for pair production interactions is proportional to the square of the atomic number of the absorber the image thus formed will be sensitive to atomic number and density of tissues in the irradiated slice. This technique is unique among other tomographic imaging modalities in its direct 3-D imaging capability. The application of the technique has been studied for imaging using contrast agents, and bone studies; in particular, osteoporosis and ostemalacia.     

Performance evaluation of random power capture on mobile communications

Power assignment schemes are man‐made methods to enhance the capture effect of radio communications. In a previous study, Wen and Yang investigated the combined capture effect of the fixed power assignment scheme, Rayleigh fading, and near–far effect on the performance of packet radios. The performance analysis was limited to an infinite population environment. This assumption is reasonable for a conventional packet radio system with a vast service area. However, for a cellular mobile system, a finite population model should be used. In this paper, we analyse the combined natural and man‐made capture effect on the performance of a cellular system with finite population in each cell. A random power assignment scheme is adopted to produce the man‐made capture. The system throughput and delay are carried out by a Markov model. Some numerical calculations are used to demonstrate the degree of performance improvement. Copyright © 2001 John Wiley & Sons, Ltd.     

Power control mixing C/I balancing and constant-received signalstrength in CDMA mobile communications system

A power control algorithm mixing C/I balancing with constant-received power control has been proposed. It is shown that the outage probability can be reduced by using the new algorithm. The new algorithm also provides better performance than the approximated optimum centralised power control algorithm if the number of removed mobiles is small     

A mobile virtual-distributed system architecture for supporting wireless mobile computing and communications

This paper describes a novel mobile virtual-distrubuted system architecture for supporting global mobile computing and communications. the principal contribution of this paper is to innovatively apply virtual memory concepts to mobile systems by deploying mobile-floating agents to de-couple services and resources from the underlying network and allow them to move around following their mobile users. The mobile-floating agents maintain data structures associated with a mobile user. By combining the mobile-floating agent functions with a predictive mobility management algorithm and location-aware caching and prefetching, services and user data structure are pre-connected and pre-assigned at the locations to which the user is moving. Thus, the users can maintain their data structures or immediately receive service with virtually the same efficiency as at the previous location.     

移动通信中的开放性

技术创新的层出不穷,使得关于开放体系和专有体系之间的争论愈演愈烈。历史经验证明,开放体系带来长期的令人瞩目的商业价值。如开放标准通过漫游呼叫和不断增长的短信互通流量,大大提高了运营商的收入。汲取早期的成功经验,移动通信产业正在选择开放?体系,以期在多媒体信息及更先进的第三代移动服务方面取得进一步的成功。移动产业中的有关各方均从开放性中获益:消费者在终端、服务方面有更多选择,成本也更低廉;运营商能够以更低廉的成本快速部署新服务;设备制造商可以从更大的规模效益和由此带来的低成本中获益;应用开发商、供应商和转包…     

Experimental evaluation of smart antenna system performance forwireless communications

In wireless communications, smart antenna systems (or antenna arrays) can be used to suppress multipath fading with antenna diversity and to increase the system capacity by supporting multiple co-channel users in reception and transmission. This paper presents experimental results of diversity gain, interference cancellation, and mitigation of multipath fading obtained by using a smart antenna system in typical wireless scenarios. Also given are experimental results for the signal-to-interference ratio (SIR) of two moving users, comparing different beamforming algorithms in typical wireless scenarios. All of the experiments were performed using the 900-MHz smart antenna testbed at The University of Texas at Austin     

Performance analysis and advancement of self-steering arrays fornonsinusoidal waves. I

The principle of a self-steering array system which has been developed in theory for beam forming with nonsinusoidal waves is analyzed and computer-simulated. A tradeoff between the maximum-scan angle of the array system, the array length, the operating frequency bandwidth, and the signal power for a small-resolution angle is established. Such a tradeoff is desirable in practice, but is not applicable to the conventional method of beam forming with zero-bandwidth sinusoidal waves. A design criterion for the optimal reception of nonsinusoidal waves by a linear array of sensors is established. The simulation results of the signal processor of the self-steering array system show that the beam-steering mechanism based on slope processing is efficient in the absence of thermal noise. The principle of the signal processor is slightly modified to enhance its performance in practice. The design of a variable-delay circuit that uses a charge-transfer device is presented. A novel mechanism of delay-time adjustment is developed for the self-steering array system to improve its response time