Arvind V. Keerthi

Steady-state analysis of the multistage constant modulus array

The multistage constant modulus (CM) array is a cascade adaptive beamforming system that can recover several narrowband cochannel signals without training. We examine its steady-state properties at convergence using a stochastic analysis and computer simulations. Based on a Wiener model of convergence for the gradient adaptive algorithms, closed-form expressions are derived for the CM array and canceller weight vectors, as well as the effective source direction vectors at all stages along the cascade system. The signal-capture and direction-finding capabilities of the system are also discussed. Computer simulations for stationary and fading sources are presented to confirm the theoretical results and to illustrate the rapid convergence behavior of the adaptive algorithms.

Separation of cochannel signals in TDMA mobile radio

We propose a sequential algorithm that separates cochannel time-division multiple-access (TDMA) signals that encounter multipath interference and noise. The receiver employs a multistage architecture where each stage consists of a beamformer and an equalizer that isolates one source, compensates for intersymbol interference (ISI), and demodulates the data. A problem encountered with such bursty sources is that the beamformer/equalizer trained for a particular time slot may not be appropriate for all the data contained in that slot. This occurs because a cochannel source typically overlaps only part of the time slot of interest and may not overlap the training sequence at all. The algorithm presented overcomes this problem by processing the data forward and backward in a sequential noncausal manner. Computer simulations using signals with the IS-54 format are presented to demonstrate the properties of the sequential algorithm.

Cochannel signal recovery using the MUSIC algorithm and the constant modulus array

We describe an approach to cochannel signal recovery for correlated sources that is based on the the MUSIC (multiple signal classification) algorithm and the constant modulus (CM) array. The MUSIC algorithm provides estimates of the source angles of arrival, which are used to initialize a parallel bank of CM arrays. These adaptive beamformers are updated by the constant modulus algorithm (CMA) to capture and track the source signals, even when they are highly correlated. Computer simulations are presented to illustrate the transient behavior of the proposed algorithm.<<ETX>>

Misadjustment and tracking analysis of the constant modulus array

The constant modulus (CM) array is a blind adaptive beamformer that can separate cochannel signals. A follow-on adaptive signal canceler may be used to perform direction finding of the source captured by the array. In this paper, we analyze the convergence and tracking properties of the CM array using a least-mean-square approximation. Expressions are derived for the misadjustment of the adaptive algorithms, and a tracking model is developed that accurately predicts the behavior of the system during fades. It is demonstrated that the adaptive canceler contributes more to the overall misadjustment than does the adaptive CM beamformer. Computer simulations are presented to illustrate the transient properties of the system and to verify the analytical results.

Estimation of correlated cochannel signals using the constant modulus array

The steady-state behavior of the cascade multistage constant modulus (CM) array is analyzed for correlated cochannel signals. It is demonstrated that the capture performance of the cascade system gradually deteriorates with increasing source cross-correlation. To overcome this loss, we consider using a parallel version of the multistage CM array that is appropriately initialized by a direction-finding algorithm. The proposed method of initialization is based on a single-stage CM array and a signal canceller adapted in tandem. Computer simulations are presented to illustrate the signal capture properties of the parallel multistage CM array.

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.

A modified CM array algorithm for fading channels

A new constant modulus (CM) array algorithm with a time-varying step size is presented. It is designed for use in a multistage system where each stage recovers one cochannel signal. The new algorithm ensures that a stage tracks the same source during fast fading conditions (such as those found in mobile radio communications), thus yielding reliable signal estimation in a nonstationary environment. We present the cost function associated with the algorithm and compare it to that of the original CM array. Computer simulations for Rayleigh fading channels are presented to illustrate the improved tracking behavior of the multistage CM array system.

Analysis of the multistage CM array for digital communication signals

The multistage CM array is a blind adaptive receiver that can separate cochannel digital signals. It consists of several identical stages connected in a cascade structure. Each stage is composed of a multichannel linear equalizer (MCLE) and a multichannel signal canceller (MCSC). The MCLE generates a linear estimate of one cochannel source impinging on the antenna array. This captured source is removed from the input by the MCSC, which is a parallel connection of adaptive FIR filters-one for each antenna element. We derive the Wiener weights for the MCLE and the MCSC. It is demonstrated that the power of the source captured by the first stage is reduced to the noise floor at the input of the next stage, whereas the other sources pass through essentially unchanged.

Tracking behavior of the constant modulus array for time-varying channels

In this paper, we present a stochastic analysis of the constant modulus (CM) array for time-varying channels. An expression for the mean-square error is derived, which is based on a recursion for the weight-error correlation matrix. The analysis yields an accurate model of the tracking behavior of the CM array provided the array maintains capture on the same source. Computer simulations are presented to illustrate the tracking behavior of the CM array and to verify the theoretical model.