Showing papers by "Stefan Parkvall published in 1996"
TL;DR: Several new estimators, the maximum likelihood, an approximative maximum likelihood and a subspace-based estimator, are proposed and are shown to be robust against the near-far problem.
Abstract: In an asynchronous direct-sequence code-division multiple access (DS-CDMA) communication system, the parameter estimation problem, i.e., estimating the propagation delay, attenuation and phase shift of each user's transmitted signal, may be complicated by the so-called near-far problem. The near-far problem occurs when the amplitudes of the users received signals are very dissimilar, as the case might be in many important applications. In particular, the standard method for estimating the propagation delays will fail in a near-far situation. Several new estimators, the maximum likelihood, an approximative maximum likelihood and a subspace-based estimator, are therefore proposed and are shown to be robust against the near-far problem. No knowledge of the transmitted bits is assumed, and the proposed estimators can thus be used for both acquisition and tracking. In addition, the Cramer-Rao bound is derived for the parameter estimation problem.
350 citations
TL;DR: An asynchronous direct-sequence code-division multiple-access (DS-CDMA) communication system operating over an additive white Gaussian noise (AWGN) channel is considered and the performance degradation errors impose on linear detectors, especially the decorrelating detector is the topic of this paper.
Abstract: In this paper, an asynchronous direct-sequence code-division multiple-access (DS-CDMA) communication system operating over an additive white Gaussian noise (AWGN) channel is considered. In many applications, the near-far problem can be the limiting factor for the capacity of a DS-CDMA system. Several near-far resistant receivers have, therefore, been proposed (e.g., the decorrelating receiver). These receivers assume perfect knowledge of the propagation delay from all users to the receiver. In practice, the delays are estimated and therefore subject to errors. The performance degradation these errors impose on linear detectors, especially the decorrelating detector, is the topic of this paper.
82 citations
TL;DR: An estimator based on subspace identification techniques is proposed, and the Cramer-Rao bound, which serves as an optimality criterion, is derived and is experimentally shown to be robust against the near-far problem.
Abstract: The problem of estimating propagation delays of the transmitted signals in a direct-sequence code-division multiple-access (DS-CDMA) system operating over fading channels is considered. Even though this study is limited to the case when the propagation delays are fixed during the observation interval, the channel gain and phase are allowed to vary in time. Special attention is given to the near-far problem which is catastrophic for the standard acquisition algorithm. An estimator based on subspace identification techniques is proposed, and the Cramer-Rao bound, which serves as an optimality criterion, is derived. The Cramer-Rao bound is shown to be independent of the near-far problem, which implies that there is no fundamental reason for propagation delay estimators to be near-far limited. Furthermore, the proposed algorithm is experimentally shown to be robust against the near-far problem.
77 citations
01 Jan 1996
TL;DR: A Statistical Approach to Subspace Based Estimation with Applications in Telecommunications as mentioned in this paper is a statistical approach to subspace-based estimation with applications in Telecommunications, which can be found in Section 2.
Abstract: A Statistical Approach to Subspace Based Estimation with Applications in Telecommunications
5 citations
22 Sep 1996
TL;DR: A receiver for data demodulation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system without prior knowledge of the propagation delays is proposed and the proposed schemes are experimentally shown to be near-far robust.
Abstract: A receiver for data demodulation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system without prior knowledge of the propagation delays is proposed. Special attention is paid to the near-far problem and the proposed schemes are experimentally shown to be near-far robust. The near-far robustness is obtained by estimating the a priori unknown propagation delay using a subspace based technique. Quantities obtained in the estimation procedure are subsequently used to design a filter, used for suppression of interference, according to the minimum mean-squared error criterion. A simple two-state Viterbi algorithm is used for the data demodulation.
4 citations
18 Nov 1996
TL;DR: Several receivers for data demodulation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system operating without prior knowledge of the propagation delays are proposed and compared, and the proposed schemes are numerically shown to be near-far robust.
Abstract: in this paper, several receivers for data demodulation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system operating without prior knowledge of the propagation delays are proposed and compared. Special attention is paid to the near-far problem, and the proposed schemes are numerically shown to be near-far robust. The near-far robustness is obtained by estimating the a priori unknown propagation delay using a subspace-based technique. Quantities obtained in the estimation procedure are subsequently used to design an interference suppression filter according to the minimum mean-squared error criterion. A simple two-state Viterbi algorithm is used for the data demodulation in the uncoded case. By extending the trellis used in the Viterbi algorithm, error correcting coding is easily implemented.
3 citations
01 Jan 1996
TL;DR: In this article, several receivers for data demodulation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system operating without prior knowledge of the propagation delays are proposed and compared.
Abstract: in this paper, several receivers for data demodulation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system operating without prior knowledge of the propagation delays are proposed and compared. Special attention is paid to the near-far problem, and the proposed schemes are numerically shown to be near-far robust. The near-far robustness is obtained by estimating the a priori unknown propagation delay using a subspace-based technique. Quantities obtained in the estimation procedure are subsequently used to design an interference suppression filter according to the minimum mean-squared error criterion. A simple two-state Viterbi algorithm is used for the data demodulation in the uncoded case. By extending the trellis used in the Viterbi algorithm, error correcting coding is easily implemented.
3 citations
01 Jan 1996
TL;DR: In this article, an estimator based on subspace identification techniques is proposed, and the Cramkr-Rao bound is derived, which serves as an optimality criterion, which implies that there is no fundamental reason for propagation delay estimators to be nearfar limited.
Abstract: Herein, the problem of estimating propagation delays of the transmitted signals in a direct-sequence code-division multiple-access (DS-CDMA) system operating aner fading chan- nels is considered. Even though this study is limited to the case when the propagation delays are fixed during the observation interval, the channel gain and phase are allowed to vary in time. Special attention is given to the near-far problem which is catastrophic for the standard acquisition algorithm. An estimator based on subspace identification techniques is proposed, and the Cramkr-Rao bound, which serves as an optimality criterion, is derived. The Cramkr-Rao bound is shown to be independent of the near-far problem, which implies that there is no fundamental reason for propagation delay estimators to be near-far limited. Furthermore, the proposed algorithm is experimentally shown to be robust against the near-far problem.