COMM-9.1

Fading-resistant and MUI-free codes for CDMA systems
Anna Scaglione, Sergio Barbarossa (INFOCOM Dept., University of Rome "La Sapienza"), Georgios B Giannakis (Dept. of Electrical Engr., Univ. of Virginia,)

A new class of codes was proposed recently for perfect multi-user interference (MUI) suppression in Code Division Multiple Access (CDMA) systems. These so called Lagrange-Vandermonde (LV) codes offer deterministic MUI elimination without channel estimation, and convert frequency selective channels into flat fading channels. In this work, we develop dual CDMA transceivers, naturally termed Vandermonde - Lagrange (VL), with the precoder - decoder roles interchanged, in order to improve system flexibility in the code assignment. We also derive and test non-redundant and redundant techniques for combating the residual flat fading.

COMM-9.2

Zero-Forcing Blind Equalization Based on Channel Subspace Estimates for Multiuser Systems
Junqiang Shen (Dept. of Electrical Engineering, Auburn University, Auburn, AL 36849-5201, U.S.A.), Zhi Ding (Dept. of Electrical and Computer Engineering, University of Iowa, Iowa City, IA 52242-1595, U.S.A.)

The recovery of input signals in a frequency selective fading channel is a problem of great theoretical and practical interest. In this paper, we present several new blind algorithms that utilize second order statistics for multichannel equalization. The algorithms are based on the subspace extraction of a preselected block column of the channel convolution matrix. For multiuser system, user signal separation can be achieved based on partial information of the composite channel response. The equalization algorithms do not rely on the signal and noise subspace separation and therefore tend to be more robust to channel order estimation errors.

COMM-9.3

Hybrid Semi-blind Multi-user Detection: Subspace Tracking Methods
Anders Host-Madsen (TRLabs/University of Calgary), Jae-chon Yu (Kwangju Institute of Science and Technology)

We consider the problem of multi-user detection for CDMA systems where the codes of some users are known while others are unknown, called semi-blind detectors. An example is at the base station of a cellular communication systems with interference from both in-cell users, with known codes, and out-of-cell users, with unknown codes. In this paper we develop a hybrid semi-blind detector, which is partly decorrelating, partly MMSE. Subspace tracking methods is developed for on-line implementation of the detector. The performance of the detectors is compared to that of the purely blind MMSE detector and the non-blind MMSE detector, and the semi-blind detector is seen to have a considerable better performance.

COMM-9.4

Blind Channel and Linear MMSE Receiver Determination in DS-CDMA Systems
Irfan Ghauri, Dirk T. M. Slock (Institut Eurecom)

We consider p users in a DS-CDMA system operating asynchronously in a multipath environment. Oversampling with respect to the chip rate is applied to the cyclostationary received signal and multi-antenna reception is considered, leading to a linear multichannel model. Channels for different users are considered to be FIR and of possibly different lengths. We consider an individualized linear MMSE receiver for a given user, exploiting its spreading sequence and timing information. The blind determination of the receiver boils down to the blind channel identification. We explore blind channel identifiability requirements. Sufficiency of these requirements is established and it is shown that if zero-forcing conditions can be satisfied, then the CDMA channel (and hence the receiver) is identifiable with probability 1. It is also shown that linear MMSE receivers obtained by different criteria (including a new one) have the same identifiability requirements asymptotically in SNR.

COMM-9.5

Blind Separation of Linear Mixtures of Digital Signals Using Successive Interference Cancellation Iterative Least Squares
Tao Li, Nicholas D Sidiropoulos (Department of Electrical Engineering, University of Virginia, Charlottesville, VA 22903)

We consider blind separation of linear mixtures of digital communication signals in noise. When little or nothing can be assumed about the mixing matrix, signal separation may be achieved by exploiting structural properties of the transmitted signals. ILSP and ILSE are two iterative least squares (ILS) separation algorithms that exploit the finite-alphabet property. ILSE is monotonically convergent and performs very well, but its complexity is exponential in the number of signals; ILSP is computationally cheaper, but is not guaranteed to converge monotonically, and leaves much to be desired in terms of BER-SNR performance relative to ILSE. We propose two computationally efficient and provably monotonically convergent ILS blind separation algorithms based on an optimal scaling Lemma. The signal estimation step of both algorithms is reminiscent of Successive Interference Cancellation (SIC) ideas. For well-conditioned data and moderate SNR, the proposed algorithms attain the performance of ILSE at the complexity cost of ILSP.

COMM-9.6

A Whitening Approach to Blind Identification and Equalization of Multiple-Input Multiple-Output Channels
Bin Huang, Jitendra K Tugnait (Auburn University)

Channel estimation and blind equalization of MIMO (multiple-input multiple-output) communications channels is considered using primarily the second-order statistics of the data. We consider estimation of (partial) channel impulse response and design of finite-length MMSE (minimum mean-square error) blind equalizers. The basis of the approach is the design of a zero-forcing equalizer that whitens the noise-free data. We allow infinite impulse response (IIR) channels. Moreover, the multichannel transfer function need not be column-reduced. Our approaches also work when the ``subchannel'' transfer functions have common zeros so long as the common zeros are minimum-phase zeros. The channel length or model orders need not be known. The sources are recovered up to a unitary mixing matrix and are further `unmixed' using higher-order statistics of the data. An illustrative simulation example is provided.

COMM-9.7

Redundancy in Block Coded Modulations for Channel Equalization Based on Spatial and Temporal Diversity
Gregori V�zquez, Francesc Rey, Meritxell Lamarca (Depart. of Signal Theory and Communications, Polytechnic University of Catalonia; UPC-Campus Nord; Modul D-5; c/Gran Capit� s/n; 08034 Barcelona; SPAIN)

Linear block codes in the complex field can be applied in spatial and/or temporal diversity receivers in order to develop high performance schemes for (almost-) blind equalization in mobile communications. The proposed technique uses the structure of the encoded transmitted information (with redundancy) to achieve equalization schemes based on a deterministic criterion. Simulations show that the proposed technique is more efficient than other schemes that follow similar equalizer structures. The result is an algorithm that provides the design of channel equalizers in low EbNo scenarios.

COMM-9.8

MMSE Multiuser Detection in Multipath Fading Channels
Stefano Buzzi, Marco Lops (Universita' degli Studi di Napoli ``Federico II'', Dipartimento di Ingegneria Elettronica e delle Telecomunicazioni), Antonia M Tulino (Seconda Universita' degli Studi di Napoli, Dipartimento di Ingegneria dell'Informazione)

In this work, we propose an MMSE multiuser detector for asynchronous DS/CDMA systems operating over frequency-selective fading channels. It is shown that computation of a conditional MMSE estimate of the bit to be decoded may be carried out with a computational burden linear in the processing gain N. We also give a closed-form formula for the Error Probability and the Near-far Resistance of the proposed detector, and curves of such performance measures, showing that the new receiver is near-far resistant and outperforms the previously derived decorrelating detector for frequency-selective fading channels.

COMM-9.9

Comparison of Multi-Channel Adaptive MLSE Equalizers Using Different Channel Tracking Strategies
Jiunn-Tsair Chen, Joonsuk Kim, John M Cioffi (Stanford University)

In order to gain insights on equalization design in the wireless mobile communication systems, we compare the performance of several multi-channel MLSE equalizers which adaptively track the fast-fading channels. Commonly-used channel tracking schemes, Decision-Directed Recursive Least Square (DD/RLS), Per-Survivor Processing Recursive Least Square (PSP/RLS) and other reduced-complexity MLSE algorithms are considered. Simulation results that illustrate the performance of the equalizers working with various channel tracking schemes are presented.

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Last Update: February 4, 1999 Ingo Höntsch