| 1. |
- Edfors, Ove, et al.
(author)
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An application of the singular value decomposition to OFDM channel estimation
- 1996
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In: Proceedings of the 1996 SNRV and NUTEK Conference on Radio Sciences and Telecommunications in Luleå and Kiruna June 3-6, 1996. - Luleå : Luleå tekniska universitet. - 9163044552 ; , s. 678-682
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Conference paper (peer-reviewed)abstract
- A new approach to low-complexity channel estimation in orthogonal-frequency division multiplexing (OFDM) systems is described. A low-rank approximation is applied to a linear minimum mean-squared error (LMMSE) estimator that uses the frequency correlation of the channel. By using the singular-value decomposition (SVD) an optimal low-rank estimator is derived, where performance is essentially preserved - even for low computational complexities. A fixed estimator, with nominal values for channel correlation and signal-to-noise ratio (SNR), is analysed. Analytical mean-squared error (MSE) and symbol-error rates (SER) are presented for a 16-QAM OFDM system.
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| 2. |
- Edfors, Ove, et al.
(author)
-
An introduction to orthogonal frequency-division multiplexing
- 1997
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Reports (other academic/artistic)abstract
- This report is an introduction to orthogonal frequency-division multiplexing (OFDM). The focus is on signal processing areas pursued by our research group at Luleå University of Technology. We present an historical background and some frequently used system models. Typical areas of applications are also described, both wireless and wired. In addition to the general overview, the addressed areas include synchronization, channel estimation and channel coding. Both time and frequency synchronization are described, and the effects of synchronization errors are presented. Different types of channel estimators are described, where the focus is on low-complexity algorithms, and in this context, advantages and disadvantages of coherent and differential modulation are also discussed. Channel coding is described, both for wireless and wired systems, and pointers are included to evaluation tools and bitloading algorithms. An extensive bibliography is also included.
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| 3. |
- Edfors, Ove, et al.
(author)
-
Analysis of DFT-based channel estimators for OFDM
- 1996
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Reports (other academic/artistic)abstract
- In this paper we analyze the performance of three low-complexity channel estimators, based on the discrete Fourier-transform (DFT), for orthogonal frequency-division multiplexing (OFDM) systems. Estimators of this type have been analyzed for discrete-time channels, and we extend this analysis to continuous-time channels. We present analytical expressions for their mean-squared error (MSE) and evaluate their complexity versus symbol-error rate (SER) for 16-QAM. The analysis shows that this type of estimators may experience an irreducible error floor at high SNRs. However, in one of the three estimators the error floor can be eliminated while the complexity stays low and the performance is maximized.
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| 4. |
- Edfors, Ove
(author)
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Low-complexity algorithms in digital receivers
- 1996
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Doctoral thesis (other academic/artistic)abstract
- This thesis addresses low-complexity algorithms in digital receivers. This includes algorithms for estimation, detection, and source coding. Low-complexity algorithms for estimation and detection, in this thesis concerns the approximation of optimal algorithms, so that a low complexity is obtained while most of the original performance is maintained. Two different problems are studied: Channel estimation in orthogonal frequency-division multiplexing (OFDM) systems and sequence detection in systems with inter-symbol interference (ISI). The work on channel estimation is focused on what can be viewed as transform-based estimators, where the estimation is transformed to a domain that allows the statistical properties of the channel to be used efficiently for complexity reduction. Estimators based both on the discrete Fourier transform (DFT) and on low-rank approximations are addressed. It is shown that these types of estimators, with proper design considerations, provide good low-complexity estimators, even though the DFT-based estimators have a tendency to suffer from approximation errors at high signal-to-noise ratios. The work on sequence detection in systems with ISI is a comparative study of designs in a class usually referred to as combined linear-Viterbi equalizers (CLVEs). The idea behind CLVEs is to shorten the impulse response of the channel by a linear prefiltering, before the Viterbi detector is applied. The comparison includes three previously known designs and a minimax design. A unified design-framework is also derived, which makes the comparison easier. The last two parts of the thesis are concerned with the design of robust variable-length codes and an analysis of the decoding speed for look-uptable based decoders for variable-length codes, respectively. By robust variable-length codes are meant fixed variable-length codes that have a low susceptibility to changes in the probability distribution of the source. Robustness measures are discussed in general and a measure called gradient robustness is introduced. It is shown that only a small loss in the degree of data compression can result in substantially increased robustness. As a side effect the proposed design methods also shorten the length of the longest code words, allowing efficient storage of code books. Finally, the design and the decoding speed of look-up table based decoders for variable-length codes are studied. These decoders are based on using the coded sequence for indexing a table, thereby providing an instantaneous decoding. Two types of decoders are studied. One allows a trade-off between decoding speed and memory requirement.
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| 5. |
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| 6. |
- Edfors, Ove, et al.
(author)
-
OFDM channel estimation by singular value decomposition
- 1996
-
In: Mobile technology for the human race. - Piscataway, NJ : IEEE Communications Society. - 0780331575 ; , s. 923-927
-
Conference paper (peer-reviewed)abstract
- A new approach to low-complexity channel estimation in orthogonal-frequency division multiplexing (OFDM) systems is proposed. A low-rank approximation is applied to a linear minimum mean-squared error (LMMSE) estimator that uses the frequency correlation of the channel. By using the singular-value decomposition (SVD) an optimal low-rank estimator is derived, where performance is essentially preserved - even for low computational complexities. A fixed estimator, with nominal values for channel correlation and signal-to-noise ratio (SNR), is analysed. Analytical mean-squared error (MSE) and symbol-error rates (SER) are presented for a 16-QAM OFDM system.
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| 7. |
- Edfors, Ove, et al.
(author)
-
OFDM channel estimation by singular value decomposition
- 1996
-
Reports (other academic/artistic)abstract
- In this report we present and analyse low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed but they suffer from poor performance when the channel is not sample-spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-QAM.
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| 8. |
- Edfors, Ove, et al.
(author)
-
OFDM channel estimation by singular value decomposition
- 1998
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In: IEEE Transactions on Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 0090-6778 .- 1558-0857. ; 46:7, s. 931-939
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Journal article (peer-reviewed)abstract
- We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).
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| 9. |
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| 10. |
- Henriksson, Anders, et al.
(author)
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Evaluation of trellis termination strategy for turbo codes in UMTS
- 1998
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Reports (other academic/artistic)abstract
- This input document proposes a trellis termination startegy for turbo codes in UMTS. The proposal is based upon the fact that the extrinsic information used in the decoding of turbo codes mitigates the effect of a truncated trellis, and that this mitigating effect can be enhanced by imposing certain restrictions on the interleaver design. The simulation parameters used in the comparison bewteen different methods complies with the parameters described in Tdoc SMG2 UMTS-L1 385/98, except in one point: they are not restricted to the reference pseudo-random interleaver provided by HNS. The reason for this is the close interaction between interleaver design and trellis termination. We recommend that the choice of trellis termination method and interleaver design should be done together, and not as separate optimizations.
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