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Network-assisted diversity multiple access (NDMA) is a novel random medium access method, which provides impressive throughput efficiency for multiplexing variable-bit-rate data sources. In this paper, we study the implementation of the NDMA scheme in the presence of multipath channels. A transceiver architecture and random access strategies are proposed, which are able to detect and resolve collisions in the presence of unknown propagation channels. The performance issues are studied both analytically and numerically. Index Terms--Network-assisted diversity, packet radio access, signal separation.

This paper deals with the multiuser medium access problem in the packet radio environment. Under the framework of network diversity multiple access (NDMA), a recently proposed medium access method, a blind collision resolution scheme is proposed employing rotational invariance and factor analysis techniques. The proposed approach (dubbed B-NDMA for Blind NDMA) overcomes the difficulty of orthogonal identification codes required by the original protocol, thereby improving channel utilization and system capacity, while being insensitive to multipath effects and synchronization errors. Performance issues of the proposed technique are addressed both analytically and numerically. Index Terms--Access protocols, blind signal separation, matrix decomposition, packet radio, rotational invariance.

A new method for blindly starting up a minimum-mean-squared-error receiver for a user newly entering a code-division multiple-access system has been developed. The two autocorrelation matrices generally differ by a low-rank matrix provided by the autocorrelation of the new user signal.

A general, stochastic blind maximum likelihood channel estimation algorithm for nonlinearly modulated signals based on a hidden Markov model formulation was described. The proposed approach is highly suitable for short data records appearing in time division multiple access systems. It was developed by utilizing the underlying finite alphabet structure of the continuous phase modulation signals. In addition, a modified version of the Baum-Welch algorithm was used to estimate the channel parameters in direct fashion without falling to too much parameterization of the problem.

A framework for providing solutions to constrained optimization of the code division multiple access (CDMA) receiver's variance was developed focusing on maximum signal energy that utilized all delayed copies of the signal of interest. CDMA systems are significant in the development of wireless communication devices employing receivers that suppress multiuser interference. It was discovered that the choice of the constraint vector affected the performance of constrained minimum variance CDMA receivers.

Novel linear algorithms for the blind estimation of time varying FIR channels were proposed. The proposed methods can be utilized in systems were time-varying tap coefficient can be described as a linear combination as basis of functions. Furthermore, the method utilizes time-diversity information which allows channel identification. The estimation of the expansion parameters is equal to estimating the second-order parameters of the FIR single-input-many-output processes.

A study was conducted on transmitter induced cyclostationarity for blind channel equalization. The findings indicate the possibility of simpler solutions if cyclostationarity is introduced at the transmitter instead of the receiver. The results also highlight the importance of transmitter design in facilitating the elimination of channel distortions. Novel blind equalization algorithms which exploit coding data in the transmitted signal are presented.

The interception/user acquisition problem involving the recovery of the transmitted information data stream is analyzed. Zero-forcing linear receivers/equalizers based on the estimated signal parameters are proposed to recover transmitted data. Their performance is compared with matched filter-based solutions. Among the assumptions made are that the bit period Tb, spreading factor P and code length KP are known and the sequence w(k) is a complex sequence deriving values from a finite QAM constellation.

The linear inverse filtering criteria is used to obtain the receiver parameters for linear blind code-division multiple access receivers without explicit estimation of the system/channel. The criteria of constrained output energy can help in solving the inverse multichannel problems in conditions where a priori partial information on the multichannel system is available. The receiver's output energy is minimized according to appropriate constraints.

A discrete-time multirate formulation has been introduced for asynchronous code division multiple access (CDMA) systems. The formulation can incorporate asynchronous user delays and multipath effects, and reveals links between CDMA receivers and array processing problems such as source separation and beamformer design. Linear receivers that can suppress multiuser interference, also called decorrelating receivers, can be obtained by using this framework.

An optimal P-band perfect reconstruction filter bank (PRFB) cuts errors of approximation when the original signal is given low-signal modification. The PRFB thus obtained makes input signals into low-resolution, uncorrelated principal components of decreasing variance. The results were studied for special cases of cyclostationary or stationary signals.

Time-domain chi-squared statistical tests are useful in characterizing the tradeoffs between higher-order statistics (HOS) and second-order statistics (SOS) and in validating the time-reversibility and Gaussianity of the statistical signal processing algorithms. The efficient Gaussianity statistics provides a constant-false-alarm-rate test, enables parametric modeling and helps derive the power of the time-reversibility and Gaussianity tests. Comparison of the statistical tests with the present frequency-domain method was done using simulations.

A finite set of basic sequences obtained from expansion of individual time-varying (TV) coefficients are used for parametric identification of time-varying systems. The wavelet basis is an ideal choice for the basic sequence due to flexibility over varying scales. Use of F-test and AIC based multiresolution analysis of TV results in an algorithm capable of estimating TV, AR and TVARMAX models.

The problem addressed in this paper is the detection and classification of deterministic objects and random textures in a noisy scene. An energy detector is developed in the cumulant domain by exploiting the noise insensitivity of higher order statistics. An efficient implementation of this detector is described, using matched filtering. Its performance is analyzed using asymptotic distributions in a binary hypothesis-testing framework. The object and texture discriminant functions are minimum distance classifiers in the cumulant domain and can be efficiently implemented using a bank of matched filters. They are immune to additive Gaussian noise and insensitive to object shifts. Important extensions, which can handle object rotation and scaling, are also discussed. An alternate texture classifier is derived from a ML viewpoint and is statistically efficient at the expense of complexity. The application of these algorithms to the texture-modeling problem is indicated, and consistent parameter estimates are obtained. Finally, simulations are shown for both the object and the texture classification problems. Index Terms--Cumulants, detection, object recognition, polyspectra, texture classification.

Constrained optimization methods have received considerable attention as a means to derive blind multiuser receivers with low complexity. The receiver's output variance is minimized subject to appropriate constraints which depend on the multipath structure of the signal of interest. When multipath is present, the constraint equations can be written in parametric form, and the constraint parameters jointly optimized with the linear receiver's parameters. In this paper, we develop adaptive solutions for this joint, constrained optimization problem. Both stochastic gradient and recursive least-square-type algorithms are developed. The performance of the proposed methods is compared with other blind and trained methods and turns out to be close to the trained minimum mean-square-error receiver. Index Terms--Adaptive interference suppression, blind multiuser detection, code-division multiple access, constrained optimization.

Research into the estimation of the correlation matrices of fading channels is presented. Methods which utilize multichannel autoregressive models are proposed and their efficacy shown via experimental and analytical means.

A new signal processing viewpoint is proposed for the random medium access problem in wireless networks.

The problem of simultaneously determining the Doppler shift and channel parameters in a blind case is considered using deterministic and stochastic maximum likelihood approaches. The iterative solution generated by the stochastic maximum likelihood method is based on the modified version of the Baum-Welch technique. It is shown that the proposed techniques are highly suitable for short data records found in time division multiple access systems.

Presents a study which dealt with the multiuser medium access problem in the packet radio environment under the framework of network diversity multiple access (NDMA). Associated drawbacks of NDMA; Discussion of a blind collision resolution method using rotational invariance and factor analysis techniques; Simulation results that corroborate the performance analysis conducted.

Presents information on a study which proposed a blind stochastic maximum-likelihood (SML) channel estimation algorithm for convolutionally coded signals transmitted through a multipath channel. Problem statement; Performance analysis of SML estimation technique; Simulation results.

Deals with a study that presented a technique for blindly starting up a minimum-mean-squared-error (MMSE) receiver for a user entering a code-division multiple-access system. Signal model; Details on the approach used in the blind startup of MMSE receivers; Direct startup algorithm for limited case; Simulations.