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Spectrally Efficient Time-Frequency Training OFDM for MIMO Systems

Spectrally Efficient Time-Frequency Training OFDM for MIMO Systems. Linglong Dai and Zhaocheng Wang Tsinghua University, Beijing, China. Outline. 1. Motivation. 2. Time-Frequency Training OFDM. 3. Simulation Results. 4. Conclusion. Motivation 1.

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Spectrally Efficient Time-Frequency Training OFDM for MIMO Systems

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  1. Spectrally Efficient Time-Frequency Training OFDM for MIMO Systems Linglong Dai and Zhaocheng Wang Tsinghua University, Beijing, China

  2. Outline 1 Motivation 2 Time-Frequency Training OFDM 3 Simulation Results 4 Conclusion

  3. Motivation 1 • OFDM and MIMO in current and future wireless communcaitons [1] • One key problem: pilot overhead is high[2] ~25% in LTE-A !

  4. Motivation 2 • Cyclic prefix OFDM (CP- OFDM) • Use cyclic prefix (CP) to alleviate IBI • Frequency-domain pilots for channel estimation • Time domain synchronous OFDM (TDS-OFDM) [3] • Use known training sequence (e.g., PN) to alleviate IBI • No pilotincrease the spectral efficiency by about 10% • Key technology of international DTV standard DTMB [4] • Hard to be extended to MIMO systems due to interferences [5] OFDM Block PN Data Data interferences 4

  5. Outline 1 Background 2 Time-Frequency Training OFDM 3 Simulation Results 4 Conclusion 5

  6. Time-Frequency Training OFDM (TFT-OFDM) • Every OFDM symbol has time-frequency training information • Key idea • Two-domain processing outperforms one-domain processing • Orthogonal time-domain training sequence and frequency-domain pilots 6

  7. TFT-OFDM Receiver • Wireless channel model number of active paths channel length path gain path delay Key point: (6vs. 420 in DTMB standard) 7

  8. TFT-OFDM Receiver • Time-frequency joint channel estimation • Step 1: only path delay estimation using the time-domain training sequence without interference cancellation • Averaging can be used to improve the accuracy • Inaccurate path gains SNR=5 dB • Accurate path delays 8

  9. TFT-OFDM Receiver • Time-frequency joint channel estimation • The number of unknown parameters in the CIR has been substantially reduced from to • Step 2: path gain estimation using only ( ) • frequency-domain pilots under ML criterion 9

  10. Performance Analysis • Cramer-Rao lower bound (CRLB) • Spectral efficiency channel sparsity noise level number of pilots 17 % 10

  11. Outline 1 Background 2 Time-Frequency Training OFDM 3 Simulation Results 4 Conclusion 11

  12. Simulation Results MSE performance comparison Parameters: (Based on DTMB standard) • Central Frequency 770 MHz • Signal Bandwidth • 7.56 MHz • Length • N=3780 • M=420 • G=20 • MIMO Encoding • Alamouti code • Modulation 64QAM • Channel Coding • LDPC, CR=0.6 • Channels • Brazil D TFT-OFDM outperforms CP-OFDM and TDS-OFDM by more than 4 dB 12

  13. Simulation Results BER comparison with mobile speed of 140 km/h TFT-OFDM outperforms CP-OFDM and TDS-OFDM by 0.75 dB and 1.60 dB at the BER of 10^−4

  14. Outline 1 Background 2 Time-Frequency Training OFDM 3 Simulation Results 4 Conclusion 14

  15. Conclusions • In this paper, we propose a spectrally efficient TFT-OFDM transmission scheme for MIMO systems • Each TFT-OFDM symbol has training information in both time and frequency domains, and the frequency-domain grouped pilots occupy much fewer pilots than that in standard OFDM MIMO systems • This is achieved by the joint time-frequency channel estimation scheme, whereby the path delays are firstly acquired by the time-domain training sequence without interference cancellation, while the path gains are acquired by by the substantially reduced number of frequency-domain pilots. • TFT-OFDM can easily extend TDS-OFDM in MIMO scenarios, and increase the spectral efficiency of CP-OFDM MIMO by about 17% • Two-domain processing outperforms one-domain processing • .

  16. References F. Adachi and E. Kudoh, “New direction of broadband wireless technology,” Wirel. Commun. Mob. Com., vol. 7, no. 8, pp. 969–983, Oct. 2007. B. Muquet, Z. Wang, G. Giannakis, M. De Courville, and P. Duhamel, “Cyclic prefixing or zero padding for wireless multicarrier transmissions?” IEEE Trans. Commun., vol. 50, no. 12, pp. 2136–2148, Dec. 2002. C. yen Ong, J. Song, C. Pan, and Y. Li, “Technology and standards of digital television terrestrial multimedia broadcasting,” IEEE Commun. Mag., vol. 48, no. 5, pp. 119–127, May 2010. J. Wang, Z. Yang, C. Pan, and J. Song, “Iterative padding subtraction of the PN sequence for the TDS-OFDM over broadcast channels,” IEEE Trans. Consum. Electron., vol. 51, no. 11, pp. 1148–1152, Nov. 2005. J. Kim, S. Lee, and J. Seo, “Synchronization and channel estimation in cyclic postfix based OFDM system,” in Proc. IEEE 63rd Vehicular Technology Conference (VTC’06-Spring), Melbourne, Vic, May 2006, pp. 2028–2032. J. Fu, J. Wang, J. Song, C. Pan, and Z. Yang, “A simplified equalization method for dual PN-sequence padding TDS-OFDM systems,” IEEE Trans. Broadcast., vol. 54, no. 4, pp. 825–830, Dec. 2008. W. Song and J. Lim, “Channel estimation and signal detection for MIMO-OFDM with time varying channels,” IEEE Commun. Lett., vol. 10, no. 7, pp. 540–542, Jul. 2006. Frame Structure, Channel Coding and Modulation for a Second Generation Digital Terrestrial Television Broadcasting System (DVB-T2). ETSI Standard, EN 302 755, V1.1.1, Sep. 2009. 16

  17. Thanks for your attention !

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