| 1. |
- Bengtsson, Daniel, et al.
(author)
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Trellis coding in a discrete multitone modulation system
- 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. 668-672
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Conference paper (peer-reviewed)abstract
- Discrete Multitone (DMT) modulation is a multicarrier technique which makes efficient use of the channel, maximizing the throughput by sending different numbers of bits on different subchannels. The number of bits on each subchannel depends on the Signal-to-Noise Ratio of the subchannel. The performance of a DMT system can be further increased by using powerful coding techniques. This paper investigates an implementation of trellis coded modulation in a DMT system intended for transmission over short copper cables, less than 1000m. We suggested trellis code is Wei's 4-dimensional 16-state coder combined with trellis shaping. A single encoder is used which codes across the tones of each DMT-symbol. At a bit error probability of 10-7, the suggested code gains 4-5 dB over uncoded transmission.
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| 2. |
- Edfors, Ove, et al.
(author)
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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. |
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| 4. |
- Johansson, Stefan, et al.
(author)
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Implementation of an OFDM Synchronization Algorithm
- 1999
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In: 42nd Midwest Symposium on Circuits and Systems, 1999.. - 0780354915 ; 1, s. 228-231
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Conference paper (peer-reviewed)abstract
- In this paper, a implementation of an OFDM synchronization algorithm is presented. Both time and frequency offsets are estimated simultaneously, using the cyclic prefix. The algorithm is implemented on a commercial DSP, an FPGA and as an ASIC. It is shown that although the algorithm is too complex to be implemented on a standard DSP, a hardware architecture that is optimized for the algorithm can be implemented with moderate complexity
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| 5. |
- Johansson, Stefan, et al.
(author)
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Silicon realization of an OFDM synchronization algorithm
- 1999
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In: The 6th IEEE International Conference on Electronics, Circuits and Systems, Proceedings of ICECS '99.. - 0780356829 ; 1, s. 319-322
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Conference paper (peer-reviewed)abstract
- n this paper a hardware architecture for an OFDM synchronizer is presented. The proposed synchronization unit can be used in any OFDM system that uses a cyclic prefix. The algorithm is based on the correlation introduced by the cyclic prefix, which is exploited in the time domain where both time and frequency offset are estimated simultaneously. The synchronization unit also performs frequency correction, which means that no feedback to the analog parts is necessary. Although the algorithm is too complex to be implemented on today's most powerful standard DSP, a hardware architecture that is optimized for the algorithm can be implemented with moderate complexity. The unit contains 32 kbit RAM and 5000 gates and the sample rate is 25 Msamples/s
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| 6. |
- Landström, Daniel, et al.
(author)
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Symbol time offset estimation in coherent OFDM systems
- 1999
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In: IEEE International Conference on Communications. - Piscataway, NJ : IEEE Communications Society. - 078035284X ; , s. 500-505
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Conference paper (peer-reviewed)abstract
- This paper presents a symbol time offset estimator for coherent OFDM systems. This estimator exploits both the redundancy in the cyclic prefix and the pilot symbols used for channel estimation. The estimator is robust against frequency offsets and is suitable for use in dispersive channels. We base the estimator on the ML estimator for the AWGN channel. Simulations of an example system indicate a system performance as close as 0.3 dB to a perfectly synchronized system. Compared to an estimator not using pilots, our estimator's performance could allow a shorter cyclic prefix and thus a more spectrally efficient system.
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| 7. |
- Landström, Daniel, et al.
(author)
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Symbol time offset estimation in coherent OFDM systems
- 2002
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In: IEEE Transactions on Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 0090-6778. ; 50:4, s. 545-549
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Journal article (peer-reviewed)abstract
- This paper presents a symbol time offset estimator for coherent orthogonal frequency division multiplexing (OFDM) systems. The estimator exploits both the redundancy in the cyclic prefix and available pilot symbols used for channel estimation. The estimator is robust against frequency offsets and is suitable for use in dispersive channels. We base the estimator on the maximum-likelihood estimator for the additive white Gaussian noise channel. Simulations for an example system indicate a system performance as close as 0.6 dB to a perfectly synchronized system.
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| 8. |
- Landström, Daniel
(author)
-
Synchronization for Multicarrier Systems
- 2004
-
Doctoral thesis (other academic/artistic)abstract
- This thesis has synchronization of Multicarrier systems, more specifically Orthogonal Frequency Division Multiplexing (OFDM) systems, as the main theme. The thesis addresses two areas, the first proposing and analyzing time and frequency estimators for OFDM systems, the second investigating more practical issues around these estimators. The practical part addresses implementation and applies the estimators in an OFDM-based mobile multiuser communication system, one of the proposals for the Universal Mobile Telecommunication System (UMTS/IMT-2000). Estimation of time and frequency offsets is a core issue in synchronizing an OFDM system. We address this issue with a family of new time and frequency offset estimators using the signal redundancy provided by the cyclic prefix. The estimators are all based on the Maximum Likelihood (ML) criterion, but for different system models, e.g., with different channels, with and without pulse shaping, with and without using pilot information, and under different implementational constraints. Furthermore, the design of an OFDM-based multiuser mobile communications system using both time and frequency division multiple access has been attempted. A standardization proposal for UMTS has been developed and submitted, suggesting, among other things, a synchronization method and parameter values describing the time and frequency grid. The work documented here has been performed at Lund Institute of Technology, Luleå University of Technology, Telia Research AB, and the Telecommunications Research Center Vienna (ftw.). Financial support has been received from NUTEK (The Swedish National Board for Industrial and Technical Development), Vetenskapsrådet (The Swedish Research Council), Telia Research AB, and VINNOVA through the Eureka/Medea+ A106 INCA and A110 MIDAS programs.
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| 9. |
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| 10. |
- Landström, Daniel, et al.
(author)
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Time and frequency offset estimation in OFDM systems employing pulse shaping
- 1997
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In: Bridging the way to the 21st century. - Piscataway, NJ : IEEE Communications Society. - 0780337778 ; , s. 279-283, s. 279-283
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Conference paper (peer-reviewed)abstract
- Time and frequency offset estimation is addressed in an OFDM system that uses a cyclic prefix and pulse shaping. Earlier work has presented a method that avoids the use of pilots by exploiting the inherent correlation in the cyclically extended OFDM symbol. In this paper this technique is extended to include systems with pulse shaping. The joint Maximum Likelihood estimator of time and frequency offsets for such systems is presented and evaluated. An implementable structure of the extended estimator is given. Simulations show that it is possible to maintain the performance when introducing pulse shaping into an OFDM system.
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| 11. |
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| 12. |
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| 13. |
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| 14. |
- van de Beek, Jaap, et al.
(author)
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A conceptual study of OFDM-based multiple access schemes : Part 4: Tracking of time and frequency offset
- 1996
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Reports (other academic/artistic)abstract
- This paper is part of a conceptual study of a multiuser OFDM system. In previous reports we have examined requirements for the air interface and presented uncoded performance (in terms of bit error rate) for the uplink of such systems. In this report we adopt the user allocation structure from the previous parts of the study and examine the role of channel coding. We present performance simulations that illustrate the effects of a convolutional code exploiting channel diversity, and channel estimator performance exploiting the channel correlation.
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| 15. |
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| 16. |
- van de Beek, Jan-Jaap, et al.
(author)
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A time and frequency synchronization scheme for multiuser OFDM
- 1999
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In: IEEE Journal on Selected Areas in Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 0733-8716 .- 1558-0008. ; 17:11, s. 1900-1914
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Journal article (peer-reviewed)abstract
- We present a multiuser synchronization scheme for tracking the mobile's uplink time and frequency offsets. It uses the redundancy introduced by the cyclic prefix and does not need additional pilots. We show performance results of an orthogonal frequency division multiplexing (OFDM)-based radio interface based on universal mobile telecommunication system (UMTS) parameters. For a UMTS-typical mobile channel environment, the performance of a coherent system employing the scheme is virtually indistinguishable from the performance of a perfectly synchronized system. In a differentially modulated system, synchronization errors decrease the system performance by about 0.7 dB compared to a perfectly synchronized system. Index Terms - Communication system, delay estimation, frequency estimation, multicarrier, multiuser system, orthogonal frequency division multiplexing (OFDM), synchronization, universal mobile telecommunication system (UMTS).
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| 17. |
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| 18. |
- van de Beek, Jan-Jaap, et al.
(author)
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Three non-pilot-based time and frequency estimators for OFDM
- 1998
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Reports (other academic/artistic)abstract
- Time-domain Maximum-Likelihood (ML) estimators of time and frequency o¤sets are derived for three Orthogonal Frequency Division Multiplexing (OFDM) signal models: a pulse-shaped one-shot OFDM signal, a stream of multiple OFDM signals and an OFDM signal in a dispersive channel environment. We then develop structures to simplify their implementation. Simulation results show the relative performance and strengths of each of these three estimators.
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| 19. |
- van de Beek, Jan-Jaap, et al.
(author)
-
Three non-pilot based time- and frequency estimators for OFDM
- 2000
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In: Signal Processing. - 0165-1684 .- 1872-7557. ; 80:7, s. 1321-1334
-
Journal article (peer-reviewed)abstract
- Time-domain maximum-likelihood (ML) estimators of time and frequency offsets are derived for three orthogonal frequency division multiplexing (OFDM) signal models: a pulse-shaped one-shot OFDM signal, a stream of multiple OFDM signals and an OFDM signal in a dispersive channel environment. We then develop structures to simplify their implementation. Simulation results show the relative performance and strengths of each of these three estimators.
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| 20. |
- van de Beek, Jaap, et al.
(author)
-
On synchronization in an OFDM based UMTS proposal
- 1997
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In: Emerging techniques for communication terminals. - Toulouse : INP-ENSEEIHT. ; , s. 315-320
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Conference paper (other academic/artistic)abstract
- In this paper we discuss three signal models and respective Maximum Likelihood estimators of time and frequency osets. We consider implementation structures for these estimators. Furthermore, we discuss the applicability of these estimators and some variants in a proposal for the future UMTS.
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| 21. |
- van de Beek, Jaap, et al.
(author)
-
Synchronization of a TDMA-OFDM frequency hopping system
- 1998
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In: Pathway to a global wireless revolution. - Piscataway, NJ : IEEE Communications Society. - 0780343204 ; , s. 1290-1294
-
Conference paper (peer-reviewed)abstract
- In this paper we present a Time Division Multiple Access-Orthogonal Frequency Division Multiplexing (TDMA-OFDM)-based radio interface proposal for the Universal Mobile Telecommunication System (UMTS). We focus on synchronization and present an implementable algorithm for the tracking of the mobiles' time and frequency offsets at the base station in the uplink. This algorithm is based on recently developed estimation methods that use redundancy introduced by the cyclic prefix. A modified Maximum Likelihood (ML) estimator for the Additive White Gaussian Noise (AWGN) channel is used to meet both implementational requirements and performance requirements for frequency-selective fading multiuser environments. Simulation results for a typical UMTS mobile channel environment show that the uncoded symbol error rate of a coherently modulated system using our synchronization scheme is virtually indistinguishable from that of a system with no time and frequency offset.
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| 22. |
- Wahlqvist, Mattias, et al.
(author)
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Description of Telias OFDM based proposal
- 1997
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Reports (other academic/artistic)abstract
- This document gives an overview of Telias OFDM based proposal for UTRA and should be cinsidered as an input to the OFDM concept group within SMG 2. The document describes the fundamental frame structure, principles of the detection, modulation, coding and synchronization. Further, the channel structure, the resource management and handover principles are outlined
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