Space-time processing for wireless communications

Space-time processing can improve network capacity, coverage, and quality by reducing co-channel interference (CCI) while enhancing diversity and array gain. This article focuses largely on the receive (mobile-to-base station) time-division multiple access (TDMA) (nonspread modulation) application for high-mobility networks. We describe a large (macro) cell propagation channel and discuss different physical effects such as path loss, fading delay spread, angle spread, and Doppler spread. We also develop a signal model incorporating channel effects. Both forward-link (transmit) and reverse-link (receive) channels are considered and the relationship between the two is discussed. Single- and multiuser models are treated for four important space-time processing problems, and the underlying spatial and temporal structure are discussed as are different algorithmic approaches to reverse link space-time professing with blind and nonblind methods for single- and multiple-user cases. We cover forward-link space-time algorithms and we outline methods for estimation of multipath parameters. We also discuss applications of space-time processing to CDMA, applications of space-time techniques to current cellular systems, and industry trends     

Multidimensional tomographic imaging using volume holography

We propose the application of volume holography to four-dimensional (4-D) spatiospectral imaging. The proposed systems use materials and techniques developed for holographic data storage and interconnections to capture three-dimensional (3-D) spatial and one-dimensional (1-D) spectral information about a remote light source or scatterer. We analyze case studies of simple architectures using spherical-reference volume holograms as imaging elements in a fluorescence confocal microscope arrangement and demonstrate the equivalence of the holographic degeneracies with a slicing operation on the reconstructing incoherent source. We develop a general theoretical framework for the diffraction of random fields from volume holograms and show that the formulation can be used as an imaging design tool. Applications and future directions are also discussed     

Optical signal processing using photorefractive effect

Novel applications of photorefractive effect to optical signal processing are proposed and demonstrated. The applications of two-wave mixing (2-WM) in a bulk photorefractive crystal include logic operations and cross connect. Photorefractive crystal waveguide (PCW) is another primary concern. Holographic storage of Fourier transformed image in PCW and its application to optical neural network, two-dimensional array of PCW as a storage device, phase conjugate mirror using PCW, and structural consideration of PCW for efficient 2-WM are investigated. In the experiments,LiNbO ₃ andBaTiO ₃ waveguides are used.BaTiO ₃ waveguide will be tested here for the first time. In a long-term prospect, all these technologies will eventually find important roles in optical signal processing.     

Space-time adaptive processing using circular arrays

A direct data-domain (D³) least-squares space-time adaptive-processing (STAP) approach is presented for adaptively enhancing radar signals in a non-homogeneous environment of jammers, clutter, and thermal noise, utilizing a circular antenna array. The non-homogeneous environment may consist of non-stationary clutter. The D ³ approach is applied directly to the data collected by a circular antenna array (utilizing space), and in time (Doppler) diversity. Conventional STAP generally utilizes statistical methodologies, based on estimating a covariance matrix of the interference, using the data from various range cells of the circular array and assuming that it is a uniform linear array. However, for highly transient and inhomogeneous environments, the conventional statistical methodology may be difficult to apply. Moreover, the error in forming the covariance matrix by assuming that the data collected by the circular array is assumed to be a uniform linear array is highly problem dependent. Hence the D³ method is presented, as it analyzes the data in space and time over each range cell separately. However, it treats the antenna array as circular, i.e., it deals with the antenna structure in its proper form. Limited examples are presented to illustrate the application of this approach     

Space-time processing for broadband wireless access

We present an overview of research activities on space-time coding for broadband wireless transmission performed at AT&T Shannon Laboratory over the past two years. The emphasis is on physical layer modem algorithms such as channel estimation, equalization, and interference cancellation. However, we also discuss the impact of space-time coding gains at the physical layer on throughput at or above the networking layer. Furthermore, we describe a flexible graphical user interface attached to our physical layer simulation engine in order to explore the performance of space-time codes under a variety of practical transmission scenarios. Simulation results for the EDGE cellular system and the 802.11 wireless LAN environment are presented     

Space-time processing for multichannel synthetic aperture radar

Synthetic aperture radar (SAR) provides high-resolution images of a non-moving ground scene, but fails to indicate the presence and position of moving objects. As in airborne MTI (moving-target indication) systems the solution to this problem is to use an array of antennas or subapertures and several receiving channels (`MSAR', or multichannel SAR), and to apply multichannel clutter suppression. One of the most efficient methods is adaptive space-time processing (STAP), which can be simplified to frequency-dependent spatial processing in the Doppler domain. Some of these techniques applied to SAR are reviewed and illustrated with data gathered by the German experimental multichannel SAR system `AER-II'     

Space-time adaptive processing for manoeuvring airborne radar

Space-time adaptive processing (STAP) techniques provide simultaneous rejection of jamming and clutter in airborne radar. The greatest benefits over conventional MTI (moving target indication) approaches are in terms of a capability to detect slow-moving targets which possess the same Doppler frequency as mainlobe clutter returns. This paper examines the effects of platform manoeuvre on STAP clutter and jamming rejection performance for a forward-facing array (i.e. where the array is orientated transversally to the direction of travel). It is shown that STAP slow-target detection performance is not sensitive to the radar platform orientation. It is also demonstrated that, under conditions of manoeuvre, STAP can provide better jammer rejection performance than architectures which cascade conventional clutter filtering and spatial adaptive beamforming     

基于迭代自适应的字典校正空时自适应处理算法

本文针对稀疏恢复空时自适应处理(Space-Time Adaptive Processing, STAP)由于字典设置不合适引起的离网效应,提出了一种基于迭代自适应(Iterative Adaptive Approach, IAA)的字典校正STAP算法。首先在IAA的每次迭代中,找到原始空时导向字典中每个量化空间频率最大功率对应的原子,围绕选定的原子,将其附近的多普勒频率均匀离散成一个集合,然后通过最大化联合似然函数在局域中搜索最优原子,并将选定的原子替换为最优原子,最后通过IAA的全局迭代,选择与杂波脊匹配的原子形成新的空时导向字典。实验证明,该方法可以有效地减轻离网效应引起的杂波脊扩展,杂波抑制性能优于现有的空时导向字典均匀离散化的IAA-STAP方法。      

1550-nm volume holography for optical communication devices

The two-lambda method can provide a strategic approach to implement all-optical devices for communication wavelength division multiplexing (WDM) signal processing based on volume holography. By writing holograms at 488 nm in LiNbO/sub 3/:Fe and reading them in the third window of optical communication systems (1550 nm), the feasibility of WDM demultiplexers and holographic memories for digital bytes is here demonstrated.     

基于子带阵列盲处理的空时干扰抑制

在卫星智能天线终端,传统空时自适应滤波处理中自适应算法需要信号信息而缺乏实时性,阵列处理算法复杂而抗干扰能力不足,针对此问题,提出了一种子带盲自适应阵列处理算法,用于直扩系统空时干扰抑制技术。子带阵列处理相对纯空域处理提高了阵列自由度,相对传统空时的抽头延迟线阵列自适应结构又大大降低了算法复杂度。提出的子带指数型变步长线性约束恒模算法的自适应阵列处理算法能在低算法复杂度下提供较高的收敛速度和收敛精度,不需要发送训练序列,可实现盲自适应波束形成,易于实现实时跟踪信号变化。仿真结果表明新的空时干扰抑制方案具有更好的抗干扰性能。     

Space-time adaptive processing: a knowledge-based perspective for airborne radar

This article provides a brief review of radar space-time adaptive processing (STAP) from its inception to state-of-the art developments. The topic is treated from both intuitive and theoretical aspects. A key requirement of STAP is knowledge of the spectral characteristics underlying the interference scenario of interest. Additional issues of importance in STAP include the computational cost of the adaptive algorithm as well as the ability to maintain a constant false alarm rate (CFAR) over widely varying interference statistics. This article addresses these topics, developing the need for a knowledge-based (KB) perspective. The focus here is on signal processing for radar systems using multiple antenna elements that coherently process multiple pulses. An adaptive array of spatially distributed sensors, which processes multiple temporal snapshots, overcomes the directivity and resolution limitations of a single sensor.     

SPACE-TIME ADAPTIVE PROCESSING FOR AIRBORNE RADAR:A CONVENIENT IMPLEMENTATION APPROACH

A convenient implementation approach to space-time adaptive processing for airborne radar has been proposed, which is added by some auxiliary array elements in the area of main-lobe clutter on the basis of 2-D Capon approach . It is of practical use for its small computational load. This approach possesses the ideal performance in the area of main-lobe clutter . In addition, the approach which is added by some auxiliary beams in the area of main-lobe clutter has also been discussed.     

Space-time processing for the detection of airborne targets in IRimage sequences

The problem of detecting airborne targets in a sequence of images recorded by a long range infra-red sensor is investigated. The generalised likelihood ratio test detector is derived. The detector structure and its actual implementation are discussed. Approximated expressions of the false alarm and detection probabilities are obtained and validated by means of simulations. Example results obtained on a set of experimental data are reported     

Oscillation with photorefractive gain

We review theories and experimental demonstrations of oscillation with photorefractive gain. The unidirectional ring resonator; the linear passive phase conjugate mirror, a phase conjugate resonator (the semilinear passive phase conjugate mirror), and the double phase conjugate resonator are treated, the applications in path-length-to-frequency converting interferometers and one-way wavefront converters are described.     

Temporal bit-sequence signal storage by using angular-multiplespectral volume holography (SPLUME)

A novel method is proposed for storing temporal bit sequences based upon the angular-multiple holographic recording of optical spectral components. The key technology is the spatially resolved generation of multifrequency reference beams whose phases are synchronized to the sideband of the optical cell. A preliminary experiment is made using an acoustooptic modulator for generating the multifrequency reference beams. In the experiment, a 4-bit-long nonreturn-to-zero 200-Mbit/s sequence is successfully recorded and regenerated from a photorefractive BaTiO₃ crystal as a real-time holographic medium. The recording bit rate is limited by the bandwidth of the modulator of the reference beam, not by the temporal response of the holographic medium. Cascading modulators is useful to broaden the bandwidth of the generated reference beam, resulting in expansion of the recording bit rate. Recording of an 8-bit-long 400-Mbit/s sequence is successfully demonstrated by cascading two modulators     

Space-time codes with AM-PSK constellations

This correspondence presents a new signal mapper that maps the maximal rank distance codes to space-time (ST) codes with amplitude modulation phase-shift keying (AM-PSK) constellations. It is shown that this new mapper is rank-distance preserving. Comparing to the multiradii construction proposed by Hammons, this new mapper has linear increase in the radii and the resulting signal constellations have larger minimum distance and lower peak-to-average power ratio. Variations of this new mapper are also given to provide ST codes with rotated AM-PSK constellations     

Determination of material characteristics of bleached photographic emulsions used in volume phase holography

The intensity of the light transmitted through a volume holographic grating is measured as a function of incident angle. Material parameters are derived by fitting a theoretical curve to the experimental results.