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Murat Torlak, The University of Texas at Austin (U.S.A.)
Guanghan Xu, The University of Texas at Austin (U.S.A.)
Brian L. Evans, The University of Texas at Austin (U.S.A.)
Hui Liu, Dept. of Electrical Eng., University of Virginia (U.S.A.)
In a multi-transmitter broadcast system, the weight vector for each message signal can provide an additional degree of freedom for signal enhancement and interference suppression by taking advantage of the spatial diversity among the users. The design of optimal weight vectors that maximize the overall channel capacity is an open problem. Under certain power constraints, the channel capacity R is a highly nonlinear function of the M-dimensional weight vectors $wb_i$, where M is the number of transmitters. Hence, a closed-form algebraic solution that maximizes R over $wb_i$ does not seem to be tractable. In this paper, we decouple the weight vectors in R to simplify the optimization problem to a search for the maxima of a smooth multidimensional function. Based on this decoupling, we derive and evaluate two algorithms for computing weight vectors for the two-user and three-user cases: orthogonal, and optimal. We also propose a near-optimum algorithm for the two-user case. The optimal algorithm requires an iterative search.
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Lin Yue, Rice University (U.S.A.)
Don H. Johnson, Rice University (U.S.A.)
Type-based receivers assume no a priori channel model, and were previously shown to be effective in direct-sequence spread spectrum communications. In this paper, we investigate the macro-diversity combining of multiple type-based receivers in single-mobile-user reception (BPSK). We present a method for the exact calculation of the bit error rate of type-based receivers in this scenario. Maximal ratio combining of type-based receivers gives near optimal performance in Gaussian noise and better performance than combining matched filter outputs in Laplacian noise. The performance gain achieved by diversity reception is significant regardless of noise statistics, especially in the presence of frequency non-selective Rayleigh fading.
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John S. Thompson, University of Edinburgh (U.K.)
Peter M. Grant, University of Edinburgh (U.K.)
Bernard Mulgrew, University of Edinburgh (U.K.)
This paper considers the performance of four channel identification techniques for code division multiple access (CDMA) antenna array receivers. These techniques are based on the assumption that the interference is spatially white: they provide a spatial "matched filter" solution. Perturbation formulae are presented for estimating the attainable signal to interference and noise ratios (SINR) for these techniques. Some simulation results are also presented to compare the convergence performance of these methods.
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David Asztély, KTH, Stockholm (Sweden)
Björn Ottersten, KTH, Stockholm (Sweden)
A. Lee Swindlehurst, Brigham Young University, Utah (U.S.A.)
In wireless communication scenarios, local scatterers in the vicinity of the mobile sources cause angular spreading. As a result, the spatial signatures will not belong to the conventional array manifold parameterized by direction of arrival (DOA) alone. In this paper, a parameterized model for spatial signatures applicable in scenarios with local scattering is presented. Several algorithms that exploit this model are proposed, and the performance of signal waveform estimators using the model is investigated via simulations. It is demonstrated that considerable gain may result as compared with using the conventional plane wave model.
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Javier Ramos, Purdue University (U.S.A.)
Michael D. Zoltowski, Purdue University (U.S.A.)
A novel wideband beamforming technique for cellular CDMA systems is presented in this paper. The proposed algorithm asymptotically provides the optimum combination of space-time samples to maximize the SINR for the Signal with the Desired Code (SDC) by optimally combining its multipath and canceling strong Multi-User Access Interference (MUAI). In contrast to previously proposed techniques, code synchronization for the SDC is not required. The algorithm presented herein asymptotically provides the exact time of arrivals of the multipaths within a bit period, and subsequently the optimum space-time weights for combining the fingers across both space and time. The instrumental property exploited in this technique is the fact that although the respective spectra of the SDC and MUAI components at the output of the matched filter are statistically equal, the respective spectra of their squared values differ.
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Per Pelin, Chalmers University of Technology (Sweden)
Jonny Eriksson, Chalmers University of Technology (Sweden)
Anders Ranheim, Chalmers University of Technology (Sweden)
Antenna arrays can be used in mobile communication systems to increase capacity and performance. However, as the number of antenna elements grows, the computational burden increases significantly. To reduce the total computational cost, a beamspace transformation is derived, based on the statistical model of the array signal. Although the transformation derived here is applicable to many algorithms, it is matched to the decoupled weighted iterative least squares with projections RAKE-combiner algorithm. Not only is the computational cost reduced, the overall performance is improved by using an appropriate beamspace transformation.
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Georgios B. Giannakis, University of Virginia (U.S.A.)
Cihan Tepedelenlioglu, University of Virginia (U.S.A.)
Hui Liu, University of Virginia (U.S.A.)
In mobile communications, often time-varying multipath is too rapid for a conventional adaptive algorithm to track. This motivates expansion of the time varying channel impulse response over a basis. Rather than estimating the time-invariant coefficients of this basis expansion, it is possible to equalize the channel output directly even when the input is not known. This requires multichannel data as well as a minimal persistence of excitation condition on the input and a coprimeness condition on the multiple channels obtained via fractional sampling and/or multiple antennas. Since the coefficients of the basis expansion will not be constant due to noise and unmodelled dynamics, the equalizer coefficients may also change slowly with time. Adaptive algorithms to track this change are proposed and basis mismatch problems are also investigated in this paper.
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Vafa Ghazi-Moghadam, University of Minnesota (U.S.A.)
Mostafa Kaveh, University of Minnesota (U.S.A.)
Interference cancelling receivers have been suggested as low complexity receivers for CDMA systems. A multi-element interference cancelling receiver was proposed in [4], and it was demonstrated that using spatial information about the users will improve the performance of the receiver. In this paper, an adaptive multi-element interference canceller is formulated, and it is shown that without requiring any additional information, the receiver can spatially discriminate between the users and improve the error performance.
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Ye Li, AT&T Labs-Research (U.S.A.)
K.J. Ray Liu, University of Maryland (U.S.A.)
Spatial-temporal equalizer can be used in the wireless communication systems with antenna arrays to improve the performance. In this article, we introduce two blind adaptive algorithms for spatial-temporal equalization, and present their convergence. Computer simulation demonstrates that the new algorithms converge faster than fractionally spaced constant-modulus algorithm (FS-CMA).
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Ariela Zeira, Signal Processing Technology, Ltd. (U.S.A.)
Benjamin Friedlander, University of California, Davis (U.S.A.)
The problem of separating and estimating signals received by an array whose array manifold has an unknown structural form is usually referred to as the blind signal copy problem. In this paper we consider the blind signal copy problem for polynomial phase signals. By deriving the Cramer Rao bound we evaluate the optimal performance achievable by any unbiased estimator. To gain additional insight into this problem we compare the CRB to the bound for the case where the functional form of the array manifold is known. We derive a computationally efficient approximate Maximum-Likelihood (ML) algorithm and compare its performance with the bound.
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Mats Viberg, Chalmers University of Technology (Sweden)
Per Pelin, Chalmers University of Technology (Sweden)
Anders Ranheim, Chalmers University of Technology (Sweden)
In recent years a number of methods for blindly separating superimposed digitally modulated signals arriving at an antenna array have been proposed. These techniques are efficient at the up-link (mobile to base) in a mobile communication system. However, for solving the base-to-mobile beamforming problem it may be necessary to also estimate the directions-of-arrival (DOAs) of the various signal paths. We present an optimal decoupled DOA estimation procedure based on information from the blind separation algorithm. Its performance is evaluated in the presence of spatially correlated noise and array modeling errors. The proposed technique has computational advantages as compared to traditional DOA estimation, because the different signal waveforms are treated in a separated fashion. Yet, the decoupled approach is shown to be substantially less sensitive to modeling errors and interference.
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Kay Iversen, Heinrich-Hertz-Institut, Berlin (Germany)
Mike Wolf, Heinrich-Hertz-Institut, Berlin (Germany)
Detlef Mämpel, Heinrich-Hertz-Institut, Berlin (Germany)
Holger Schubert, Heinrich-Hertz-Institut, Berlin (Germany)
Christian Schmidt, Technical University of Ilmenau (Germany)
In this paper we present a novel infrared communication system for large cells and low user mobility. The proposed transmitter technique allows nearly perfect direction-of-arrival (DoA) estimation in time division multiple access (TDMA) environments only with three photodiodes. Using a photodiode array we show that known algorithms for DoA-estimation can be applied in spatial division multiple access (SDMA) systems.
ic974053.pdf ic974053.pdf TOPSeth D. Silverstein, GE-CRD (U.S.A.)
This work describes an algorithm called the control circuit encoding (CCE) algorithm that is effective for the remote calibration of an N element active phased array antenna. The algorithm involves transmission of orthogonal encoded signals. CCE is ideally suited for analog beamformers as it requires no additional encoding hardware. The CCE method encodes phased array elemental signals using a Hadamard matrix to control the switching of intrinsic phase shifter delay circuits. The CCE algorithm can reduce the average measurement integration times for the complete set of calibration parameters by the order of N relative to the corresponding values for single-element calibration procedures.
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Anne Ferreol, TCC/UTTC/TSI (France)
Pascal Chevalier, TCC/UTTC/TSI (France)
Over the last decade, a great number of higher order (HO) blind source separation methods have been developed, aiming at separating several statistically independent sources without any a priori information about the latter. Recently, a performance analysis of some of these methods has shown that the latter perform very well in a great number of situations encountered in radiocommunications contexts but fail in separating several Gaussian sources. Besides, their performance degrades in the presence of a background noise which is either spatially correlated or non Gaussian, situation encountered in the HF bandwidth. In this context, the purpose of this paper is to present and to analyse the performance of a new family of HO blind source separation methods exploiting the potential cyclostationarity of the received sources to overcome most of the limitations of the classical HO blind source separation methods while keeping their good properties.
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