Vector channels for smart antennas. Measurements, statisticalmodeling, and directional properties in outdoor environments

In wireless communications, smart antenna systems that employ antenna arrays coupled with adaptive signal-processing techniques at the basestation improve capacity, coverage, and trunking efficiency. However, design and performance analysis of smart antenna systems strongly depend on channel propagation characteristics of signals present at the antenna array, the so-called vector channels. Here, variation of narrow-band vector channels (spatial signatures) due to a moving terminal is studied in typical suburban settings. Vector channel measurements are taken using a real-time smart antenna system with a uniform circular array at the basestation and a mobile transmitter at several locations. Two different wireless scenarios, namely, pedestrian and car mobile, are implemented to emulate the random movement of the mobile user. In each scenario, the mobile transmitter locations are chosen so that there exists line-of-sight (LOS), nonline-of-sight (NOLOS), or both LOS and NOLOS (mixed) propagation to the basestation. We find that in all cases, the Beta distribution can be used to empirically represent the spatial signature correlations and that large spatial diversity exists in NOLOS cases compared to LOS cases. Also, direction-of-arrivals mostly do not change much with movement in a suburban environment     

Smart antennas in wireless systems: uplink multiuser blind channeland sequence detection

Space-division multiple-access (SDMA) schemes have been proposed to increase the capacity of wireless communication systems by simultaneously transmitting and receiving multiple co-channel signals through different spatial channels. We address the uplink (remote users to the base station antenna array) blind channel and sequence identification problem for an SDMA system. We show that multiuser blind identification can be accomplished by exploiting the spatial and temporal diversities of an antenna array system. In particular, a recursive estimation algorithm is developed to recover multiple signals from intersymbol interference (ISI) and co-channel interference (CCI) by taking advantage of a special structure of the array output and the finite-alphabet property of digital communication signals. The implementation of the proposed approach in practical applications is discussed, and field experiments have been conducted to demonstrate the effectiveness of the proposed algorithm     

Dual-polarized array for signal-processing applications in wirelesscommunications

A novel dual-polarized antenna array designed for a spatial division multiple access (SDMA) system working in the 1850-1990-MHz band is designed and built. The antenna is designed to have similar element patterns, and measurements of S-parameters and radiation patterns are presented. The array signal processing performance of the array using all elements and with no compensation for mutual coupling or differences in element patterns is studied through direction-of-arrival (DOA) estimation using total least squares estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT). The results show that the accuracy of the DOA estimates is quite acceptable for wireless communication applications     

Experimental studies of direction of arrivals using a smart antenna testbed in wireless communication systems

This paper presents some preliminary results from experimental studies on space‐division‐multiple‐access (SDMA) for wireless communications. A smart antenna system utilizing direction‐of‐arrival (DOA)‐based beamforming techniques can enhance signal quality by reducing co‐channel interference from mobiles located at angles spatially distinct from the base station. Adopting both smart uplink and downlink beamforming, a communication system with an antenna array can increase the cell coverage of a base station and significantly boost capacity compared with conventional antenna systems. However, successful implementation of DOA‐based beamforming techniques depends on the DOA characteristics. This paper presented the feasibility of direction finding and DOA variation with respect to frequency. Furthermore, the angle spread was studied for selected environments. The results demonstrate the feasibility of applying proposed smart antenna system utilizing DOA‐based beamforming algorithm for increasing channel capacity and improving system performance in frequency‐division‐duplex (FDD) wireless communication systems. Copyright © 2003 John Wiley & Sons, Ltd.     

Spatial signature estimation for uniform linear arrays with unknownreceiver gains and phases

The problem of spatial signature estimation using a uniform linear array (ULA) with unknown receiver gain and phase responses is studied. Sufficient conditions for identifying the spatial signatures are derived, and a closed-form ESPRIT-like estimator is proposed. The performance of the method is investigated by means of simulations and on experimental data collected with an antenna array in a suburban environment. The results show that the absence of receiver calibration is not critical for uplink signal waveform estimation using a plane wave model     

Characteristics of vector propagation channels in dynamic mobilescenarios

In wireless communications, the performance of a smart antenna system depends heavily upon vector channels describing channel propagation between an antenna array and a mobile subscriber. The smart antennas perform quite well in stationary mobile environments in which channel propagation characteristics are stable. However, in dynamic wireless environments where the mobile user is in motion, knowledge of how vector channels are affected is necessary for the proper operation of smart antennas. Here, we experimentally investigate the variation of vector channel parameters such as spatial signatures, directions-of-arrival (DOAs), and complex path attenuations with small movement (2λ) of the mobile under typical line-of-sight (LOS), line-of-sight with local scatterer (LOSLS), and nonline-of-sight (NLOS) propagation scenarios. The experiments are conducted using a 1.8-GHz smart antenna testbed developed at The University of Texas at Austin and a mobile transmitter. The results show that with small displacements, DOAs remain approximately unchanged and spatial signatures change due primarily to complex attenuations. Spatial signatures are very susceptible to the movement in the NLOS scenario, reaching up to 90% relative angle change within 2λ displacement. However, in the LOS scenario, they exhibit small and periodic fluctuations with a period of 0.6λ     

Performance Comparison of MIMO-STBC Systems with Adaptive Semiblind Channel Estimation Scheme

Wireless communication is now a part of everyday life in the urban areas. Wireless LAN is mostly utilized communication system as an example. These wireless devices are data rate and range limited, for which the scientists are spending great efforts on finding ways to overcome these limitations. Multi input multi output (MIMO) antenna systems are the example through which these limitations have been reduced upto great extent which provides multilayer beamforming, diversity, and spatial multiplexing. Analysis of adaptive semiblind channel estimation scheme for MIMO antenna array systems with different code rate space time block coding (STBC) has been performed using the adaptive pilot assisted modulation scheme proposed earlier. Semi blind channel estimation method provides the best trade-off in terms of bandwidth overhead, computational complexity and latency. The result after using MIMO systems shows higher data rate and longer transmit range without any requirement of additional bandwidth or transmit power. This paper presents the detailed analysis of diversity coding techniques using MIMO antenna systems. Different STBC schemes have been explored and analyzed with the different code rate STBC using MATLAB environment and the simulated results have been compared in the semiblind environment which shows the improvement even in highly correlated antenna arrays, and is found close to the condition when channel state information is known to the channel.     

Antenna Array Designs for OFDM WLAN Indoor Transmission

Wireless local area networks (WLAN??s) based on the 802.11 standards are ubiquitous. However, the popularity of WLAN??s has made interference between WLAN users an issue. Spatial division multiple access (SDMA) is one way to orthogonalize these users and reduce interference. The contribution of this paper is to use real antenna array prototypes to determine the best array design for implementing indoor SDMA. Two SDMA antenna array prototypes are constructed and used to collect propagation measurements in an indoor environment. The propagation data is then incorporated into an SDMA orthogonal frequency division multiplexing (OFDM) analysis. This approach is able to very accurately predict how the two array designs will influence SDMA-OFDM performance in the indoor environment where the measurements are collected. The results indicate that the compact sectorized antenna array prototype performs better than the linear array prototype for in-room communication and that the reverse is true for inter-room communication.     

太赫兹超大规模3D MIMO系统中的波束斜视补偿技术

太赫兹通信技术凭借超大带宽的优势成为未来6G的关键技术之一。超大规模天线技术可以提供巨大的空间分集,提升频谱效率,同样在6G无线通信系统中起到关键作用。在基于移相器的大规模MIMO（multiple-input multiple-output）混合预编码中,由于太赫兹频段的超大带宽,不同频率的子载波信道具有不同的等效空间方向,发射端波束形成时,将带来严重的波束斜视问题。与此同时,随着天线规模的不断增长,超大规模天线技术的应用更进一步扩大了波束斜视造成的影响。针对超大规模天线阵列带来的波束斜视现象放大问题,本文利用3D MIMO平面天线阵列来改善这一状况。为进一步改善太赫兹频段超大带宽引起的波束斜视,在3D大规模MIMO系统的基础上,本文提出了基于两层移相器结构的混合预编码方案,利用第二层移相器,对不同频率的子载波进行补偿。实验结果表明,本方法可以有效地弥补波束斜视带来的阵列增益损失,实现接近最优的系统性能。      

Closed‐Loop Transmit Diversity Techniques for Small Wireless Terminals and Their Performance Assessment in a Flat Fading Channel

Closed‐loop transmit diversity is considered an important technique for improving the link budget in the third generation and future wireless communication standards. This paper proposes several transmit diversity algorithms suitable for small wireless terminals and presents performance assessment in terms of average signal‐to‐noise ratio (SNR) and outage improvement, convergence, and complexity of operations. The algorithms presented herein are verified using data from measured indoor channels with variable antenna spacing and the results explained using measured radiation patterns for a two‐element array. It is shown that for a two‐element array, the best among the proposed techniques provides SNR improvement of about 3 dB in a tightly spaced array (inter‐element spacing of 0.1 wavelength at 2 GHz) typical of small wireless devices. Additionally, these techniques are shown to perform significantly better than a single antenna device in an indoor channel considering realistic values of latency and propagation errors.     

The performance evaluation of spatial–temporal algorithms in W‐CDMA systems

Differing from FDMA, TDMA, and CDMA, space division multiple access (SDMA) uses space resources to improve communication system performance. Utilizing the smart antenna system is an approach to realize the SDMA technique. Smart antenna systems using the beamforming technique can reduce the co‐channel interference and multipath fading to increase the channel capacity and communication quality. In this study the smart antenna system and rake receiver are integrated. The performance of spatial–temporal structure applied to the W‐CDMA system is evaluated. From the cumulative distribution function simulation results, W‐CDMA system with spatial–temporal algorithm can exactly provide SINR gain to improve the system performance and capacity. Copyright © 2005 John Wiley & Sons, Ltd.     

具有谐波抑制功能的多极化微带天线阵设计

本文提出了一种基于切换馈电网络的极化可重构微带天线阵。阵列单元采用正方形辐射贴片,利用微带线缝隙耦合进行馈电,通过在馈电结构中心加载短路柱,可获得宽带性能的同时,有效抑制偶次谐波。综合采用串联组阵和并联组阵技术,优化设计了一款2×2的天线阵列,将阵列端口分别与微带分支线耦合器及微带混合环相连,最终实现了水平和垂直线极化,左旋和右旋圆极化四种模式的切换。实验结果表明,所提出的天线在ISM频段2.4~2.5 GHz频率范围内获得了良好的匹配和多极化特性。该天线阵设计简单、易于操作,可潜在应用于无线通信系统中天线极化分集的场合。     

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     

Diversity eigenbeamforming for coded signals

Transmit antenna diversity without feedback has been widely investigated for various wireless communication systems. Especially, space-time codes are extensively studied to exploit the spatial diversity induced by multiple transmit antennas. Apart from these approaches, there can be different methods that provide a diversity gain using multiple antennas with conventional channel coding. In this paper, diversity eigenbeamforming is studied to exploit the spatial diversity when the channel covariance matrix is available at the transmitter. Diversity eigenbeamforming is based on eigenmode (of the spatial covariance matrix) switching that converts the spatial diversity into the temporal diversity which is exploited by channel coding. Optimized diversity eigenbeamforming is considered to take into account the spatial correlation. In addition, the trade-off between diversity and multiplexing gains is addressed. It is shown that the diversity eigenbeamforming can achieve optimal trade-off for the case of one receive antenna. Although, for simplicity, binary phase shift keying (BPSK) is used to derive diversity eigenbeamforming, any higher-order modulation scheme can also be used with diversity eigenbeamforming.     

Successive beamforming,multiuser detection and diversity reception for multicarrier DS-CDMA system with antenna array

Code-Division Multiple-Access (CDMA) systems are interference limited,and therefore efficient interference management is necessary to enhance the performance of a CDMA system.In this paper,a successive beamforming (spatial filtering),linear decorrelating MultiUser Detection (MUD, temporal filtering) and diversity reception structure for uplink multicarrier Direct Sequence CDMA (DS-CDMA) system with antenna array are proposed.By beamforming,the antenna array suppresses interference according to the distinct array signature.Subsequently,linear decorrelating MUD is ap- plied to separate the signals of different users and eliminate Multiple Access Interference (MAI).Finally, the decorrelated signals at different subcarriers that belong to the same user are combined to achieve frequency diversity.Simulation results show that the proposed structure offers significant Bit Error Rate (BER) performance improvement by successively exploiting the space-time-frequency processing.     

基于nRF905的协作分集实验研究

无线信道衰落特性是影响信道容量和传输质量的关键因素,协作分集技术允许一组无线终端互相中继信号,形成虚拟天线阵列,实现空间分集,进而得到相应的分集增益。针对不同的协作方法进行比较,设计协作实验系统,选择易于实现的解码前传协议,在STC5410单片机和nRF905的硬件环境下完成实验,并参考室内路径损耗模型对实验结果进行了理论分析,实验结果表明该协议可以减小系统中断概率。     

TD-SCDMA HSPA+中的单双流MIMO技术的研究

多输入多输出(MIMO)技术是近年来移动通信的热门研究领域,它的特征在于无线发射机和接收机都引入了多根天线.MIMO技术除了通过空间分集方式获得容量的提升外,还可以通过空间复用方式,利用不同天线信道的独立性建立多个空间子信道来提高容量.但是,无论是基于空间分集的单流模式还是基于空间复用的双流模式,单独在TD-SCDMA...     

Traffic Models and Blocking Probabilities for Two-Fold and Three-Fold SDMA Communication Systems

Spectrum efficiency is a constant challenge in the design of wireless networks. Space-division-multiple-access (SDMA) is a promising approach to achieve higher spectral efficiency which reuses bandwidth via multiplexing signals based on their spatial signature. Several different studies have shown that SDMA can effectively improve system capacity in a mobile environment. In this paper, we present a new Markov chain traffic model for a duplicate-at-last (DL) approach [IEE Proceedings on Communication 146 (1999) 303] in two-fold and three-fold SDMA systems. Simplified blocking probability formulations for two-fold and three-fold SDMA are also derived. Simulations based on a common method of spatial separation check for channel allocation in SDMA are presented to evaluate the probability of successfully creating two-fold and three-fold SDMA channels. The simulation, as well as analytical, results indicate that the SDMA system can reduce the blocking probability of the calls and result in more traffic loading than a traditional cellular system. The results also show that our simplified approaches not only can reduce the computational complexity, but can also accurate approximate two-fold and three-fold SDMA performance. Wen-Jye Huang received the B.S. degree in electrical engineering from Tatung Institute of Technology, Taipei, Taiwan, in 1991, the M.S. degree in electrical engineering from Ohio University, Athens, OH, in 1997, and the Ph.D. degree in electrical engineering from The Pennsylvania State University, University Park, PA, in 2001. Since 2002, he joined the Department of Electrical Engineering, Chang Gung University, Kwei-San, Tao-Yuan, Taiwan, as an assistant professor. His research activities include smart antenna, SDMA, and MC-CDMA techniques. John F. Doherty received the B.S. degree (with honors) in engineering science from the College of Staten Island, City University of New York, in 1982, the M.Eng. degree in electrical engineering from Stevens Institute of Technology, Hoboken, NJ, in 1985, and the Ph.D. degree in electrical engineering from Rutgers University, New Brunswick, NJ, in 1990. He was an integrated circuit reliability engineer with IBM, from 1982 to 1984. From 1985 to 1988, he was member of the technical staff at AT&T Bell Laboratories, working in sonar signal processing. In 1990, he joined the Electrical and Computer Engineering Department, Iowa State University, Ames, as an assistant professor and Harpole Entair fellow. He is currently an associate professor of electrical engineering with The Pennsylvania State University, University Park. His current research activities include interference rejection in wireless communication systems, spatial-division multiple-access techniques, and radar target detection techniques. He is a former AFOSR summer faculty research fellow at the Rome Laboratory, Rome, NY, and an Army Research Office Young Investigator.     

Adaptive interference suppression in multiuser wireless OFDMsystems using antenna arrays

This paper considers the problem of mitigating fading and interference in wireless orthogonal frequency division multiplexing (OFDM) multiple access communication systems. Applications include cellular mobile radio, wireless local loop, and wireless local area networks. The effect of interchannel interference (ICI) arising from time-selective fading and frequency offsets and co-channel interference (CCI) is analyzed. A loop-timing method that enables a synchronous uplink between multiple mobile transceivers and a base-station is described. Adaptive antenna arrays are utilized at the base for uplink reception, and optimum array combining based on the maximum SINR criterion is used for each subchannel over slowly time-varying channels. For operation over fast time-varying channels, a novel two-stage adaptive array architecture that incorporates combined spatial diversity and constraint-based beamforming is presented. While ICI alone is most effectively overcome by spatial diversity, combined beamforming and diversity are most effective to combat CCI in the presence of fading. The overall method is suitable for real-time implementation and can be used in conjunction with traditional coding schemes to increase the link-margin     

分集阵列式水下激光通信光学接收天线设计

针对水下无线激光通信系统中对准困难的问题,提出了一种分集阵列式光学接收天线,在光学设计软件Zemax中分别设计出了复合光学接收天线和分集阵列式光学接收天线的光学结构,分析了复合光学接收天线和分集阵列式光学接收天线的视场角、聚光效率以及光源移动范围,并且通过实验和Matlab仿真给出两种光学接收天线的聚光效率随光源径向移动范围和光源入射角的关系,结果表明:当光源尺寸10 mm时,复合光学接收天线的聚光效率是0.06%,接收视场角是6,光源径向移动范围是6 mm;分集阵列式光学接收天线的聚光效率是0.06%,接收视场角是16,光源径向移动范围是22 mm。因此分集阵列式光学接收天线更适用于水下激光通信系统。