Showing papers by "John J. Shynk published in 2007"

A Short-Term Measure of Dark Energy in Sparse Atomic Estimations

Abstract: A sparse atomic estimation involves finding a representation of a signal that uses terms with finite support drawn from an overcomplete dictionary. These terms can constructively and destructively interfere with each other in the reconstruction, and some may even disappear. For this reason, we refer to the interference between atoms in the sparse estimation as "dark energy" (DE). Building upon our previous work, we present a short-term measure of DE (STDE) that can be used to examine the distribution of interference in the signal estimation. We present results for several signals, and discuss some applications of this measure.

A Multistage Hybrid Constant Modulus Array With Constrained Adaptation for Correlated Sources

Abstract: The multistage constant modulus (CM) array was previously proposed for capturing multiple received signals in a cochannel signal environment. It consists of a cascade of individual CM array stages combined with adaptive signal cancelers that remove the various signals captured across the stages. However, when the received signals are mutually correlated, the signals captured by the CM array stages are not completely canceled, and previous parallel extensions of the system do not guarantee that different signals will be captured across the stages. In this paper, we present a hybrid implementation of the multistage CM array for separating correlated signals where the canceler weights in the cascade structure provide estimates of the direction vectors of the captured signals. These estimates are then used in a parallel implementation of the linearly constrained CM (LCCM) array leading to the hybrid structure. Since the direction vectors are obtained directly from the canceler weights, the hybrid implementation does not require prior knowledge of the array response matrix and is independent of the type of antennas used in the receiver. The effect of a bias in the direction vector estimates for closely-spaced signals is analyzed, and the steady-state performance of the hybrid structure is compared to that of a conventional constrained implementation for correlated sources. Computer simulations for example cochannel scenarios are provided to illustrate various properties of the system. Mean-square-error (MSE) learning curves indicate that the proposed hybrid LCCM algorithm converges faster and has lower MSE than previous implementations

A Semi-Blind Adaptive GSM Beamformer Using Re-Encoded Data

Abstract: An adaptive beamforming algorithm for the global system for mobile communications (GSM) is presented where the weights are updated using re-encoded data and a semi-blind technique. The receiver consists of several stages operating on the synchronization channel (SCH) in an iterative manner to improve the reliability of the cochannel signal decisions. An extension of the constant modulus algorithm (CMA) with a least-squares (LS) formulation is used to implement the semi-blind technique. Iterative semi-blind processing is introduced to further refine the beamformer weights. The performance of the enhanced GSM receiver is evaluated using real and simulated data.

Stochastic analysis of an iterative semi-blind adaptive beamforming algorithm

Abstract: We present an iterative semi-blind adaptive beamformer for time-division multiple-access (TDMA) cellular systems and analyze its performance using a stochastic model. The array inputs are iteratively processed using re-encoded data with blind adaptation and several stages that include beam-forming, demodulation, equalization, and decoding. In each stage, the initial beamformer weights computed by the TDMA training data are refined by a semi-blind technique based on the constant modulus algorithm (CMA). The receiver algorithms are verified using simulated and real TDMA data collected by an eight-element antenna array. The stochastic analysis of the iterative semi-blind beam-forming algorithm is based on a fourth-order moment, and demonstrates the improved performance of the receiver.

SINR Enhancement Techniques for an IS-95 Downlink SIC Receiver

Abstract: Due to near-far effects and multiple-access interference, several types of multiuser detectors have been developed in recent years to reliably demodulate user signals in a code-division multiple-access (CDMA) system. The downlink of Interim Standard 95 (IS-95) is particularly well suited to one such detector known as the successive interference canceler (SIC). In order to keep the receiver complexity low, entire base station signals are typically canceled in the receiver in a sequential manner. However, for the SIC to operate effectively, a base station signal that is being canceled must be reconstructed with enough accuracy such that sufficient interference power is removed for the subsequent base station to be reliably detected. If this is not possible, it may be necessary to employ techniques, specific to the signal format specified in the IS-95 downlink, which enhance the received signal-to-interference-plus-noise ratio (SINR). In this paper, we explore the performance gains achieved with several SINR enhancement techniques, and present computer simulations to demonstrate this improvement for example cochannel signal scenarios.

A Cyclodespreader Architecture for Detecting Multiple Downlink Aperiodic CDMA Signals

Abstract: Although a code-division multiple-access (CDMA) system is spectrally efficient and has some immunity against intentional reception, its capacity and performance are generally limited by multiple access interference (MAI) caused by other users. For an aperiodic CDMA system, the spreading sequences span multiple symbol intervals, and the cross-correlations between these sequences, which are a measure of the MAI, are periodic over multiple symbols. In this paper, we present a receiver with a parallel architecture that converts an aperiodic CDMA sequence into a piecewise periodic sequence in each arm. We refer to this receiver as a cyclodespreader because the despreader exploits the cyclostationary property of an aperiodic signal. As a result, the transmitted data can be detected separately in each arm of the receiver using low-complexity conventional algorithms proposed for periodic CDMA systems. The goal of the receiver is to decode several signals received from different cochannel base stations. The performance of the system is evaluated using real aperiodic CDMA signals, and it is compared to that of a conventional matched filter (MF) receiver using the number of correctly decoded messages as the performance measure.