Richard P. Gooch

The constant modulus array for cochannel signal copy and direction finding

The constant modulus (CM) array is a blind adaptive beamformer capable of recovering a narrowband signal among several cochannel sources without using a pilot or training signal. It is a conventional weight-and-sum adaptive beamformer whose weights are updated by the constant modulus algorithm. An adaptive signal canceller follows the beamformer to remove the captured signal from the array input and to provide an estimate of its direction vector. Based on a Wiener model, we investigate the steady-state properties of the CM array and the signal canceller. For mutually uncorrelated sources and noise, it is shown that the signal canceller exactly removes the source captured by the array. Thus, identical stages of the CM array and signal canceller may be used in a multistage system to recover several cochannel sources. Computer simulations are presented to verify the analytical results and to illustrate the transient behavior of the system.

Nonlinear techniques for the joint estimation of cochannel signals

Cochannel interference occurs when two or more signals overlap in frequency and are present concurrently. Unlike in spread-spectrum multiple-access systems where the different users necessarily share the same channel, cochannel interference is a severe hindrance to frequency- and time-division multiple-access communications, and is typically minimized by interference rejection/suppression techniques. Rather than using interference suppression, we are interested in the joint estimation of the information-bearing narrow-band cochannel signals. Novel joint estimators are proposed that employ a single-input demodulator with oversampling to compensate for timing uncertainties. Assuming finite impulse-response channel characteristics, maximum likelihood (ML) and maximum a posteriori (MAP) criteria are used to derive cochannel detectors of varying complexities and degrees of performance. In particular, a (suboptimal) two-stage joint MAP symbol detector (JMAPSD) is introduced that has a lower complexity than the single-stage estimators while accruing only a marginal loss in error-rate performance at high signal-to-interference ratios. Assuming only reliable estimates of the primary and secondary signal powers, a blind adaptive JMAPSD algorithm for a priori unknown channels is also derived. The performance of these nonlinear joint estimation algorithms is studied through example computer simulations for two cochannel sources.

Joint estimation algorithms for cochannel signal demodulation

Nonlinear algorithms for the joint recovery of cochannel narrowband signals are proposed. For finite impulse response channel characteristics, maximum likelihood and maximum a posteriori criteria are employed to derive cochannel demodulators of varying complexities and degrees of performance. The error rate performance of these joint estimation algorithms is examined through computer simulations.<<ETX>>

Convergence properties of the multistage CMA adaptive beamformer

The multistage CMA adaptive beamformer is capable of separating multiple narrowband sources without pilot or training signals. It is comprised of a cascade of CM (constant modulus) array subsections, each of which captures one of the signals impinging an the array. An adaptive signal canceller follows each CM array to remove captured signals from the input before processing by subsequent sections. Based an a stochastic analysis, we derive the steady-state convergence properties of the system, including its direction-finding capabilities. For mutually uncorrelated sources and noise, it is shown that the canceller exactly removes a captured signal, thereby reducing the rank of the effective array matrix of the next subsection by one.<<ETX>>

Adaptive beamforming for TDMA signals

This paper addresses the design of adaptive beamforming algorithms used to improve the reception of time division multiple access (TDMA) signals in an environment rich in multipath and co-channel interference. A formulation of the problem is presented with an emphasis on real-world issues such as frame synchronization and partial slot overlap due to co-channel interferers. Three beamformer-demodulator structures are examined along with algorithms for adaptively optimizing their performance. Simulation results are provided to show that the beamformer-demodulator structures can significantly improve the bit error rate as a function of the SINR compared to a single antenna system. The computational complexity of the various structures is compared and shown to be reasonable for implementation on a set of TMS320C40 digital signal processors.

Convergence properties of the multistage constant modulus array for correlated sources

A steady-state analysis of the multistage constant modulus (CM) array is presented for the case of correlated source signals. Using a Wiener model for the converged adaptive algorithms, it is demonstrated that because of the cascade structure, signal cross-correlation causes phantom sources to be present. The resulting signal leakage in the adaptive cancellers between stages causes an increase in the minimum mean-square error. A parallel implementation that overcomes this loss in performance is described; it requires an initialization strategy to ensure that each stage captures a different source. Computer simulations are presented to illustrate the convergence properties of the cascade and parallel CM array systems and to verify the theoretical results.

A blind adaptive antenna system for the estimation of mutually correlated cochannel sources

The constant modulus (CM) array is an adaptive beamformer that blindly recovers a cochannel source without requiring array calibration. An adaptive signal canceller removes the captured source from the array input, after which the remaining cochannel signals can be processed by a series of similar CM array stages. This multistage cascade CM array may experience a loss in performance when the sources are correlated. In this paper, we propose a modified system based on a parallel architecture. The cascade CM array is employed to appropriately initialize the parallel stages, which in turn recover the cochannel sources. Computer simulations are presented to illustrate the transient and steady-state behaviour of the proposed system.

Adaptive successive interference cancellation for the IS-95 uplink

In this paper we examine an adaptive successive interference canceler (ASIC) that can separate direct-sequence code-division-multiple-access (DS/CDMA) signals for the IS-95 (Interim Standard 95) uplink under near-far conditions, i.e., without power control. The ASIC employs a multistage architecture where each stage consists of a conventional matched filter (MF) detector with equal gain combining (EGC), followed by an adaptive interference canceler (AIC) that is adjusted by the least-mean-square (LMS) algorithm to track time-varying multipath channels. Variants of the basic ASIC architecture which exploit the specific structure of the IS-95 uplink traffic channel are also discussed. The bit-error-rate (BER) performance of the ASIC is compared with that of other interference cancelers, demonstrating the effectiveness of the ASIC for near-far scenarios.

A cyclostationary receiver for aperiodic CDMA signals

In a long-code/aperiodic direct-sequence code-division multiple-access (DS/CDMA) system, the user spreading sequences span multiple information symbols. Most interference suppression techniques have been proposed for short-code/periodic CDMA systems where the spreading sequence is confined to a single symbol interval and generally cannot be applied directly to a long-code system. In this paper, we investigate a cyclostationary receiver with a parallel architecture that converts an aperiodic CDMA signal into a set of periodic signals, one in each arm of the parallel receiver. Blind interference suppression based on Tikhonov regularization is performed by the cyclostationary receiver for real Interim Standard 95 (IS-95) signals captured inside a building and its performance is compared to that of a conventional matched filter (MF) receiver. The performance of the receiver is improved using blind recursive algorithms and a decision-directed approach.

Performance analysis of the multistage CMA adaptive beamformer

The multistage CMA adaptive beamformer is a blind adaptive antenna system capable of recovering several narrowband cochannel signals. Each stage of the system captures one source (without using a training signal), removes it before processing by subsequent stages, and provides an estimate of its direction of arrival. This system would be useful in frequency reuse applications, such as cellular radio or personal communication networks, where cochannel interference is an important consideration. We present the steady-state convergence properties of the first stage of the multistage CMA adaptive beamformer. Computer simulations are presented to verify the analytical results.<<ETX>>