Chair: Guanghan Xu, The University of Texas at Austin (USA)
Yiping Wang, University of Oklahoma (USA)
J.R. Cruz, University of Oklahoma (USA)
The use of an adaptive antenna array at the base station in the reverse link of cellular communication systems is proposed. Two kinds of adaptive methods are considered. First, the recursive least squares (RLS) algorithm, a reference signal based method, is used to adjust the weight coefficients of the array. For eigenstructure based methods, the ESPRIT algorithm is used for angle of arrival (AOA) estimation and the linear least squares estimate (LLSE) is used to determine the adaptive array weights. The improvement in performance is quantified by comparison with the omnidirectional antenna system using realistic simulations. The comparisons of different adaptive arrays are discussed and analyzed.
Guanghan Xu, University of Texas at Austin (USA)
Hui Liu, University of Texas at Austin (USA)
Most wireless communication systems use different carriers for uplinks and downlinks, hence the downlink beamforming can only be performed based on the Directions-of-Arrival (DOA) information of the uplink signals. In this paper, we propose an effective downlink transmission scheme for TDMA mobile communication systems via the integration of direction finding and blind signal estimation techniques. With an M-element antenna array, our new scheme can estimate up to $2M^2/3$ DOAs of direct path and multipath signals, while a conventional DOA estimation algorithm can resolve no more than M DOAs. RF experiments show that by incorporating these additional DOA estimates, much improved interference suppression was obtained.
Yeheskel Bar-Ness, New Jersey Institute of Technology (USA)
Nadir Sezgin, New Jersey Institute of Technology (USA)
Vast research was recently performed on signal detection of multiuser Code Division Multiple Access (CDMA). Particularly for uplink (users to base station) the signals are asynchronous and the near-far problem is an important issue to deal with. All near-far resistant detectors, adaptive or non-adaptive, assume knowledge of the relative delays of the different users' signals. Among these are the one-shot detectors suggested by Verdu. In this paper we suggest an adaptive algorithm to decorrelate the outputs of the one shot matched filters that assumes no knowledge of the relative delay. The performance of this approach is shown to be better than the non-adaptive zero-forcing method previously used and which implements linear transformation via the inverse of matched filters' output cross-correlation. For simplicity a two-user case is presented. Extension to a higher number of users is relatively simple.
Montse Najar, Universitat Politecnica de Catalunya (SPAIN)
Miguel A. Lagunas, Universitat Politecnica de Catalunya (SPAIN)
A new architecture for adaptive arrays using Frequency Hopping modulation is addressed in this paper. The resolution of the array and the interference rejection increase substantially applying random processing to the carrier frequency of the signals. The proposed framework is composed of to different stages. The Anticipative stage, devoted to minimize the noise and fixed interferences contribution and the GSLC stage which provides cancellation of follower jammers and solves the multiuser collision problem. The developed system requires neither temporal nor spatial reference for its implementation, only the frequency sequence must be known. An adaptive approach has been implemented, following a fast convergence to the optimal behavior.
Y.C. Pati, Stanford University (USA)
Gregory G. Raleigh, Stanford University (USA)
A. Paulraj, Stanford University (USA)
A simple adaptive technique is proposed for separation and demodulation of multiple cochannel frequency modulated (FM) signals received at an antenna array. The proposed method, which for FM signals is embodied in an architecture referred to as a Multitarget Adaptive Phase-Lock Loop (MADPLL), exploits known signal structure through a complete demodulation and remodulation of the signals. The two properties of the signal that are exploited here are the known bandwidth of the information signal and the constant-modulus (CM) property of FM signals. It is shown that the proposed method can lead to significant improvements in performance over methods that exploit only the CM property.
Michael D. Zoltowski, Purdue University (USA)
Javier Ramos, Polytechnic University of Madrid (SPAIN)
The problem addressed is that of narrowband digital cellular communications in the presence of multipath and co-channel interferers having the same symbol rate and same overall signal characteristics as the desired source. For a given signal source, an algorithm is presented for blindly estimating the weight vector yielding the optimum SINR at the beamformer output. The instrumental quantity, denoted S_xx(f), is the Fourier Transform of the expected value of the zero-lag autocorrelation matrix truncated to one symbol period. The proposed scheme employs the PRO-ESPRIT algorithm to exploit the relationship between the timing offset and optimum beamforming weight vector for each source and the principal generalized eigenvalues and eigenvectors of the matrix pencil ( S_xx(1/T_o ) , S_xx(0)), where 1/T_o is the symbol rate. Simulations are presented showing the rapid convergence of the algorithm and an improvement of several dB over the subspace-constrained Phase-SCORE algorithm.
Jeffrey B. Schodorf, Georgia Institute of Technology (USA)
Douglas B. Williams, Georgia Institute of Technology (USA)
John R. Barry, Georgia Institute of Technology (USA)
An adaptive space-time processing system for dynamic spatial channels in a CDMA mobile communications network is presented. The proposed system consists of an adaptive sensor array that estimates the desired directions and waveforms, followed by adaptive linear equalizers that refine the waveform estimates by compensating for the mobile radio uplink channel. Capacity gains are achieved through code reuse, which is made possible by performing the spatial processing after the codes have been decorrelated with one another. Simulation results for a two sensor array in a variety of interference scenarios are presented in the form of bit error rate curves.
Jen-Wei Liang, Stanford University (USA)
Arogyaswami J. Paulraj, Stanford University (USA)
An approach to mitigate forward link signal fading in a FDD communication system by using an adaptive transmit antenna array and feedback on the reverse link is presented. In a personal communication system environment, multipath propagations can lead to severe space selective fading. Cordless phones and similar devices which can not conveniently provide multiple antennas at the receiver can suffer from long term fading and therefore have unacceptable quality. Using multiple adaptive transmit antennas, we can adjust the transmission weights to ensure the user is kept out of deep fades. This is achieved by using feedback of the received signal level on the reverse link and adapting the transmission weights. Simulation shows the significant gain against the fading characteristics can be achieved.
Hui Liu, University of Texas at Austin (USA)
Guanghan Xu, University of Texas at Austin (USA)
In this paper, we study two of the fundamental operations of a TDD smart antenna system, namely, the uplink channel and sequence identification and downlink selective transmission. For uplink, our focus is on the development of a blind estimation algorithm which is capable of resolving a multiuser system without the use of training sequence or any input statistics. For downlink, we propose a spatial channel pre-equalization scheme which simultaneously eliminates the intersymbol interference (ISI) and the co-channel interference (CCI) for all users using FIR filters. Both algorithms were validated by RF experiments using the smart antenna testbed developed in the University of Texas at Austin.
H.P. Lin, University of Texas at Austin (USA)
S.S. Jeng, University of Texas at Austin (USA)
I. Parra, University of Texas at Austin (USA)
G. Xu, University of Texas at Austin (USA)
W.J. Vogel, University of Texas at Austin (USA)
G.W. Torrence, University of Texas at Austin (USA)
This paper presents some preliminary results of experimental studies of Space-Division-Multiple-Access (SDMA) systems for wireless communications to expand capacity, increase coverage, and improve quality. Although the SDMA schemes have been studied by a number of researchers, most of these studies are based on theoretical analyses and computer simulations. Very few real RF or microwave experiments have been conducted to validate the feasibility of various signal processing algorithms, such as direction finding and signal copy techniques. Also, no extensive experiments have been conducted to study the channel propagation associated with multiple antennas. The purpose of this paper is to present our preliminary experimental results using our recently developed antenna array testbed. We will also discuss the implications of these results on various array signal processing algorithms.