Modified GLRT and AMF framework for adaptive detectors

©2007 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE."This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder."The well-known general problem of signal detection in background interference is addressed for situations where a certain statistical description of the interference is unavailable, but is replaced by the observation of some secondary (training) data that contains only the interference. For the broad class of interferences that have a large separation between signal-and noise-subspace eigenvalues, we demonstrate that adaptive detectors which use a diagonally loaded sample covariance matrix or a fast maximum likelihood (FML) estimate have significantly better detection performance than the traditional generalized likelihood ratio test (GLRT) and adaptive matched filter (AMI') detection techniques, which use a maximum likelihood (ML) covariance matrix estimate. To devise a theoretical framework that can generate similarly efficient detectors, two major modifications are proposed for Kelly's traditional GLRT and AMF detection techniques. First, a two-set GLRT decision rule takes advantage of an a priori assignment of different functions to the primary and secondary data, unlike the Kelly rule that was derived without this. Second, instead of ML estimates of the missing parameters in both GLRT and AMF detectors, we adopt expected likelihood (EL) estimates that have a likelihood within the range of most probable values generated by the actual interference covariance matrix. A Gaussian model of fluctuating target signal and interference is used in this study. We demonstrate that, even under the most favorable loaded sample-matrix inversion (LSMI) conditions, the theoretically derived EL-GLRT and FL-AMF techniques (where the loading factor is chosen from the training data using the EL matching principle) gives the same detection performance as the loaded AMF technique with a proper a priori data-invariant loading factor. For the least favorable conditions, our EL-AMF method is still superior to the standard AMF detector, and may be interpreted as an intelligent (data-dep- endent) method for selecting the loading factor.Yuri I. Abramovich, Nicholas K. Spencer, Alexei Y. Gorokho

GLRT-based threshold detection-estimation performance improvement and application to uniform circular antenna arrays

©2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE."This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder."The problem of estimating the number of independent Gaussian sources and their parameters impinging upon an antenna array is addressed for scenarios that are problematic for standard techniques, namely, under "threshold conditions" (where subspace techniques such as MUSIC experience an abrupt and dramatic performance breakdown). We propose an antenna geometry-invariant method that adopts the generalized-likelihood-ratio test (GLRT) methodology, supported by a maximum-likelihood-ratio lower-bound analysis that allows erroneous solutions ("outliers") to be found and rectified. Detection-estimation performance in both uniform circular and linear antenna arrays is shown to be significantly improved compared with conventional techniques but limited by the performance-breakdown phenomenon that is intrinsic to all such maximum-likelihood (ML) techniques.Yuri I. Abramovich, Nicholas K. Spencer, and Alexei Y. Gorokho

Estimation semi-autodidacte par méthode sous-espace pour systèmes de réception multicapteurs

Cette article présente une analyse asymptotique de l'estimateur semi-autodidacte proposé par A. Gorokhov dans [3] combinant linéairement le critère des moindres carrés sur la séquence d'apprentissage et le critère sous-espace autodidacte [6]. Cette analyse s'inscrit dans le contexte des radiocommunications numériques à accès multiple à répartition dans le temps (AMRT) et à réception multicapteurs. Nous présentons l'analyse asymptotique de l'estimateur semi-autodidacte pour deux hypothèses asymptotiques différentes et proposons deux techniques issues des résultats de cette analyse permettant de régler de façon optimale le compromis entre le critère des moindres carrés (utilisant la séquence d'apprentissage) et le critère sous-espace autodidacte. Les simulations effectuées montrent que ces approches permettent d'améliorer significativement les performances de la méthode tant en termes d'estimation que de réduction du taux d'erreur

Robust OFDM receivers for dispersive timevarying channels: Equalization and channel acquisition

In orthogonal frequency-division multiplexing, time variations of a multipath channel lead to a loss of orthogonality between the subcarriers, and thereby limit the achievable throughput. This paper proposes a general framework for a controlled removal of intercarrier interference (ICI) and channel acquisition. The core idea behind our method is to use a finite power series expansion for the time-varying frequency response, along with the known statistical properties of mobile radio channels. Channel acquisition and ICI removal are accomplished in the frequency domain and allow for any desired tradeoff between the residual ICI level, the required training for channel acquisition, and processing complexity. The proposed approach enables a high spectral efficiency (64-quadrature amplitude modulation mode) of digital video broadcasting-terrestrial in highly mobile environments

New Equalization Approach for OFDM over Dispersive and Rapidly Time Varying Channel

The paper proposes and analyses a new receiver structure to mitigate the effect of Doppler on the reception of OFDM signals. A Discrete-Frequency channel representation is developed for the link between the input of the transmit I-FFT and the output of the receive FFT. It is based on a Taylor expansion of the time variations of the received subcarrier amplitudes. The model realistically addresses the correlation of fading at neighboring subcarriers. We study a new type of receiver which estimates not only amplitudes but also derivatives of subcarriers amplitudes. An adaptive MMSE filter is proposed to cancel the Intercarrier Interference (ICI) resulting from Doppler. This results in a substantial improvement of the link performance

1375 New Equalization Approach for OFDM over Dispersive and Rapidly Time Varying Channel

Abstract- The paper proposes and analyses a new receiver structure to mitigate the effect of Doppler on the reception of OFDM signals. A Discrete-Frequency channel representation is developed for the link be-tween the input of the transmit I-FFT and the output of the receive FFT. It is based on a Taylor expansion of the time variations of the received subcarrier ampli-tudes. The model realistically addresses the correla-tion of fading at neighboring subcarriers. We study a new type of receiver which estimates not only ampli-tudes but also derivatives of subcarriers amplitudes. An adaptive MMSE filter is proposed to cancel the Intercarrier Interference (ICI) resulting from Dop-pler. This results in a substantial improvement of the link performance

Reduced Complexity Doppler Compensation For Mobile Dvb-T

The reception of the DVB-T signal on a mobile equipment is strongly affected by Doppler spread. Most of the current proposals require the use of two or more antennas at the receiver and a computationally expensive signal processing. In this paper for a DVB-T receiver with a single antenna we propose new schemes based on the interference cancellation principle. With respect to previously studied techniques, our solution is able to deliver DVB-T services at higher speeds, while a more flexible architecture allows a wider range of trade-off between complexity and performance. In order to improve the performance in terms of maximum achievable speed, an iterative scheme is proposed which performs both the interference cancellation and the channel estimation. A decoding and reencoding of the inner convolutional code is inserted into the iteration process in order to boost the performance. Moreover, by applying the interference cancellation principle also for the estimation of the channel parameters, we obtain a scheme with a reasonable complexity for the DVB system. Simulations results for various DVB-T modes, constellations and code rates show that the proposed schemes allow the correct reception of DVB-T for moderate and high speeds

Iterative Interference Cancellation and Channel Estimation For Mobile OFDM

In mobile reception, the reliability of orthogonal frequency division multiplexing (OFDM) is limited because of the time-varying nature of the channel. This causes intercarrier interference (ICI) and increases inaccuracies in channel tracking. In this paper, we model the ICI using derivatives of the channel amplitude. This allows us to design a relatively simple receiver scheme that iteratively cancels the ICI. The design of the canceler aims at maximizing the signal-to-noise-plus-ICI ratio at the detector input. We also propose a new channel estimator, and we show that it achieves reliable mobile reception in practical situations that are relevant to terrestrial Digital Video Broadcasting (DVB-T). Extensive simulations for a receiver with one or two antennas show that a small number of iterations between ICI cancellation and channel estimation allow a reliable reception at vehicle speeds above 100 km/h