| 1. |
- Hoeher, Peter, et al.
(författare)
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Channel estimation with superimposed pilot sequence
- 1999
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Ingår i: Proc. IEEE GLOBECOM '99, Rio de Janeiro, Brazil, December 1999. - 0780357965 ; 4, s. 2162-2166
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Konferensbidrag (refereegranskat)abstract
- For the purpose of various synchronization tasks (including carrier phase, time, frequency, and frame synchronization), one may add a known pilot sequence, typically a pseudo-noise sequence, to the unknown data sequence. This approach is known as a spread-spectrum pilot technique or as a superimposed pilot sequence technique. In this paper, we apply the superimposed pilot sequence technique for the purpose of channel estimation (CE). We propose and verify a truly coherent receiver based on the Viterbi algorithm, which is optimal in the sense of per-survivor processing. We also suggest a generic low-cost receiver structure based on reduced-state sequence estimation. Among the distinct advantages compared to conventional pilot-symbol-assisted CE are (i) a lack of bandwidth expansion and (ii) a significantly improved performance in fast fading environments. The proposed Viterbi receiver may also be used as an alternative receiver for pilot-symbol-assisted CE
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| 2. |
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| 3. |
- Tufvesson, Fredrik
(författare)
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Channel Related Optimization of Wireless Communication Systems
- 1998
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with different optimization problems in the design of wireless communication systems. It is mainly directed to the design of systems based on multicarrier techniques and orthogonal frequency division multiplex, OFDM, but some of the problems apply to single carrier systems as well. The influence of different pilot patterns is analyzed when pilot symbol assisted modulation, PSAM, is used in OFDM systems. It is desirable to decrease the number of required pilot symbols and it is shown that the pilot pattern used plays a major role to enable reliable channel estimates from a small amount of pilot symbols. Rearrangement of the pilot pattern enables a reduction in the number of needed pilot symbols up to a factor 10, still retaining the same bit error performance. The effect of the number of sub-channels used in an OFDM system is analyzed with respect to resulting bit error rate. An analytical expression for the bit error rate on Rayleigh fading channels when interchannel interference, ICI, caused by channel changes during a symbol and energy loss due to the cyclic prefix are regarded. This expression is used to optimize the number of sub-channels, and thereby the sub-channel bandwidth (sub-channel spacing) in the system. It is argued that the system can be optimized neglecting the effect of imperfect channel estimation and on a worst case assumption for the Doppler frequency and signal to noise ratio. The benefits of using pre-compensation (precoding) in wireless time division duplex, TDD, systems are also investigated. The uplink channel estimate is used to compensate the channel impact on the downlink symbols. This enables less complex receiver structures in the mobile terminal since channel equalization is performed in the base station. Three different methods where amplitude and/or phase are adjusted are analyzed in terms of performance limits. Closed-form expressions for the QPSK bit error rate are given assuming a fully known channel. It is shown that pre-compensation is an attractive alternative to differential decoding. Phase-only compensation is preferred at low signal to noise ratios, while at high signal to noise ratios an order of magnitude improvement in the bit error rate can be obtained by including amplitude pre-compensation. All the analyses and optimizations are general and can be applied to any OFDM system.
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| 4. |
- Tufvesson, Fredrik, et al.
(författare)
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OFDM Time and Frequency Synchronization by Spread Spectrum Pilot Technique
- 1999
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Ingår i: Proc. 8th IEEE Communication Theory Mini Conference in conjunction to ICC'99, Vancouver, Canada, June 7-10, 1999. - 0780356535 ; , s. 115-119
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Konferensbidrag (refereegranskat)abstract
- Correct time and frequency synchronization is important for the performance of orthogonal frequency division multiplex (OFDM) systems. We evaluate a system where a pseudonoise sequence (PN-sequence) is used for estimation of these synchronization parameters. The PN-sequence is superimposed on the OFDM signal. The system is evaluated by means of the variance of the frequency estimation error and the probability for correct timing synchronization. Both theoretical and simulated results are presented. The proposed technique is very flexible and works also at low signal to noise ratios. The frequency offset range has been significantly increased compared to conventional OFDM synchronizer schemes
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| 5. |
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| 7. |
- Tufvesson, Fredrik, et al.
(författare)
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Pilot Assisted Channel Estimation for OFDM in Mobile Cellular Systems
- 1997
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Ingår i: Proceedings of IEEE Vehicular Technology Conference, Phoenix, U.S.A. - 0780336593 ; , s. 1639-1643
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Konferensbidrag (refereegranskat)abstract
- The use of pilot symbols for channel estimation introduces overhead and it is thus desirable to keep the number of pilot symbols to a minimum. The number of needed pilot symbols for a desired bit error rate and Doppler frequency is highly dependent on the pilot pattern used in orthogonal frequency division multiplexed, OFDM, systems. Five different pilot patterns are analysed by means of resulting bit error rate, which is derived from channel statistics. Rearrangement of the pilot pattern enables a reduction in the number of needed pilot symbols up to a factor 10, still retaining the same performance. The analysis is general and can be used for performance analysis and design of pilot patterns for any OFDM system
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