| 1. |
- Parkvall, Stefan, et al.
(författare)
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Asynchronous near-far resistant DS-CDMA receivers without a priori synchronization
- 1999
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Ingår i: IEEE Transactions on Communications. - 0090-6778. ; 47:1, s. 78-88
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Tidskriftsartikel (refereegranskat)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 he near-far resistant, The near-far resistance is obtained by estimating the a priori unknown propagation delay using a subspace-based technique. Quantities obtained in the estimation procedure are used to design a filter used for suppression of interference, according to the minimum mean square error criterion. Either a decision feedback technique or a simple two-state Viterbi algorithm is subsequently used for the data demodulation in the uncoded case, By extending the trellis used in the Viterbi algorithm, error correcting coding is easily implemented.
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| 2. |
- Parkvall, Stefan, et al.
(författare)
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Sensitivity Analysis of Linear DS-CDMA Detectors to Propagation Delay Estimation Errors
- 1995
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Ingår i: Proceedings IEEE Global Telecommunications Conference. ; s. 1872-1876
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Konferensbidrag (refereegranskat)abstract
- 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. 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 estimation errors. The performance penalty these errors impose on linear detectors, especially the decorrelating detector, is evaluated
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| 3. |
- Ström, Erik G, et al.
(författare)
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DS-CDMA synchronization in time-varying fading channels
- 1996
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Ingår i: IEEE Journal on Selected Areas in Communications. - 0733-8716. ; 14:8, s. 1636-1642
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Tidskriftsartikel (refereegranskat)abstract
- Herein, 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.
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| 4. |
- Ström, Erik G, et al.
(författare)
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Propagation delay estimation in asynchronous direct-sequence code-division multiple access systems
- 1996
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Ingår i: IEEE Transactions on Communications. - 0090-6778. ; 44:1, s. 84-93
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Tidskriftsartikel (refereegranskat)abstract
- In an asynchronous direct-sequence code-division multiple access CDS-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.
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| 5. |
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| 6. |
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| 7. |
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| 8. |
- Ström, Erik G, et al.
(författare)
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Complexity Reduction of Direct Sequence Code Division Multiple Access Receivers
- 1994
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Ingår i: IEEE International Conference on Communications, 1994. SUPERCOMM/ICC ’94, Conference Record, ’Serving Humanity Through Communications.’. - IEEE. ; s. 1643-1647
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Konferensbidrag (refereegranskat)abstract
- Considers an asynchronous direct-sequence code division multiple access (DS-CDMA) communication system over an AWGN channel. The authors are interested in reducing the computational complexity of a receiver by lowering the dimensionality of the received vector. Dimension reduction has previously been done in an ad hoc manner by several researchers. The present authors derive an optimum form of dimension reduction. The optimality criteria is that the Cramer-Rao bound on estimators of propagation delay, received phase and power are invariant to the reduction transform.
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| 9. |
- Ström, Erik G, et al.
(författare)
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Propagation Delay Estimation of DS-CDMA Signals in a Fading Environment
- 1994
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Ingår i: IEEE Global Telecommunications Conference, Communication Theory Mini-Conference (1994 IEEE GLOBECOM). ; s. 85-89
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Konferensbidrag (refereegranskat)abstract
- The problem considered is of estimating propagation delays of the transmitted signals in a direct sequence code division multiple access (DS-CDMA) system operating over fading channels. Special attention is given to the near-far problem. An estimator based on subspace identification techniques is proposed. The Cramer-Rao bound is presented and used as a benchmark when evaluating the performance of the estimator.
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| 10. |
- Ström, Erik G, et al.
(författare)
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Sensivity Analysis of Near-Far Resistant DS-CDMA Receivers to Propagation Delay Estimation Errors
- 1994
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Ingår i: Proceedings of IEEE 44th Vehicular Technology Conference. - IEEE. ; s. 757-761
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Konferensbidrag (refereegranskat)abstract
- We consider an asynchronous direct-sequence code division multiple access (DS-CDMA) communication system operating over an AWGN channel. In certain 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 thereceiver. In practice, the delays need to be estimated and will thus be subject to estimation errors. We evaluate the performance penalty these errors will impose on previously proposed near-far resistant receivers.
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