The problem of blind channel identification for Direct-Sequence/Code-Division MultipleAccess (DS/CDMA) multiuser systems is explored. For wideband DS/CDMA signals, multipath distortion is well modeled by a a finite-impulse response model. In this work, a blind channel identification technique based on second order statistics is investigated. The method exploits knowledge of the spreading code of the user of interest via matched filtering, as well as properties of spreading codes. The current scheme focuses on a method appropriate for randomized long sequence DS/CDMA. This access scheme poses special challenges as the spreading codes are time-varying. An analytical approximation of the mean-squared error is derived using perturbation techniques. The performance of the algorithm is studied via simulation and through the mean-squared error approximation, which is observed to be tight. Submitted to the IEEE Transactions on Signal Processing, May 12, 1999 EDICS Category 3-WIRL Permission to...

A RAKE receiver using a decorrelating matched filter is proposed for the joint channel and symbol estimation for long-code wideband CDMA. Channel parameters and data symbols are estimated jointly by least squares via rank-one decompositions. An identifiability condition is established which suggests that channels unidentifiable in short code CDMA systems are almost surely identifiable when aperiodic spreading codes are used. An efficient implementation of the decorrelating matched filter is proposed based on time-varying state space realizations which makes the complexity of the proposed decorrelating RAKE comparable with that of the conventional RAKE receiver. The mean square error of the estimated channel is compared to the Cramr-Rao bound.

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) eectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The eectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The eect of error in decision feedback from the multistage detector (used in the abs...

A Generalized Frequency Hopping (GFH) OFDMA system is developed in this paper as a structured long code DS-CDMA system in order to bridge frequency hopped multi-carrier transmissions with long code DS-CDMA. Through judicious code design, multiuser interference is eliminated deterministically in the presence of unknown frequency-selective multipath channels. Thanks to frequency hopping, no single user suffers from consistent fading effects and constellation-irrespective channel identifiability is guaranteed regardless of channel nulls. A host of blind channel estimation algorithms are developed trading off complexity with performance. Two important variants, corresponding to slow- and fast-hopping, are also addressed with the latter offering symbol recovery guarantees. Performance analysis and simulation results illustrate the merits of GFHOFDMA relative to conventional OFDMA and long code DS-CDMA with PN codes and RAKE reception. Keywords--- Frequency-Hopping, Multi-Carrier CDMA, Spread Spectrum Systems, Multipath Fading Channels, Blind Channel Estimation I.

The problem of blind and semiblind channel estimation and symbol detection is considered for long code wideband CDMA systems, including systems with multirate and multicode transmissions. A decorrelating matched filter, implemented efficiently in state space, eliminates multiaccess interference and produces a bank of vector processes. Eachvector process spans a one-dimensional subspace from whichchannel parameters and data symbols of one user are estimated jointly by least squares. Anewidentifiability condition is established, which suggests that channels unidentifiable in short code CDMA systems are almost surely identifiable when aperiodic spreading codes are used. The decorrelating matched filter is implemented efficiently based on time-varying state space realizations that exploit the structure of sparsityofthe code matrix. The mean square error of the estimated channel is compared to the Cram'er-Rao bound, and a bit error rate (BER) expression for the proposed algorithm is presented.

Multiuser CDMA systems with aperiodic spreading codes have received considerable attention. One major difficulty in multiuser detection is the user's time-varying signature. In a multipath communication environment, this signature is not a priori known, making direct design of the detector more intractable. In this paper, multiuser detection is performed by a two-step approach: estimate the unknown channel coefficients and construct the detector based on the MMSE criterion. We model the interference as colored Gaussian noise with unknown correlations and estimate the channel parameters based on the correlation matching idea. The spreading codes are assumed random with known up to the fourth order statistics, resulting in computationally efficient method. In the case of unknown statistics however, the method is still applicable by estimating those statistics from given codes. To guarantee identifiability, both the channel's output and the output of the matched filter for the desired user are considered in one cost function. Comparisons with previously proposed method and other existing methods are made by simulations.

This paper chronicles more than 40 years of evolution in the design of such modems.