Tsinghua University, Beijing, China Time-Frequency Training OFDM with High Spectral Efficiency and Improved Performance over Fast Fading Channels Linglong Dai, Zhaocheng Wang, Jintao Wang, and Jun Wang Tsinghua University, Beijing, China

Proposed Time-Frequency OFDM Contents 1 Background 2 Proposed Time-Frequency OFDM 3 Simulation Results 4 Conclusion

OFDM Transmission Cyclic prefix OFDM (CP- OFDM) [1] Use CP to alleviate IBI and ICI Bandwidth and power penalty Zero padding OFDM (ZP-OFDM) [2] Save power Synchronization lost Time domain synchronous OFDM (TDS-OFDM) [3] High spectral efficiency (increased by about 10% due to no pilot) Fast synchronization

Performance loss and high receiver complexity due to IBI [4] Problem of TDS-OFDM Performance loss and high receiver complexity due to IBI [4] Iterative padding subtraction (IPS) has to be used The PN sequence used for channel estimation is contaminated by the OFDM symbol OFDM symbol contains contribution from PN sequence Performance lost over fast time-varying channels High receiver complexity

Solutions to TDS-OFDM Cyclic postfix OFDM [5] TS is generated by the redundant frequency-domain comb-type pilots The inserted redundant pilots have very high average power The most effective solution: Dual-PN OFDM (DPN-OFDM) [6] One extra PN is used to absorb IBI Obvious loss in spectral efficiency (90% vs. 82% if GI = 1/9)

Proposed Time-Frequency OFDM Contents 1 Background 2 Proposed Time-Frequency OFDM 3 Simulation Results 4 Conclusion 6 6

New Solution to TDS-OFDM? Different look at the IBIs in TDS-OFDM Key idea Directly use the “contaminated” time-domain training sequence (TS) without IBI cancellation to only obtain the path delays The path coefficients are acquired by small amount of frequency-domain grouped pilots Results: Decouple channel estimation and equalization, so IPS is avoided Small loss in spectral efficiency (~3%) due to pilot insertion 7 7

Proposed Time-Frequency Training OFDM (TFT-OFDM) Every OFDM symbol has time-frequency training CP-OFDM has training only in the frequency domain TDS-OFDM has training only in the time domain 8 8

Time-frequency joint channel estimation TFT-OFDM Receiver (1) Time-frequency joint channel estimation Conventional method: path delays and path gains are simultaneously estimated by the TS after IBI removal and cyclicity reconstruction Proposed step 1: time-domain path delay estimation without IBI removal and cyclicity reconstruction SNR=5 dB Key point: (6 vs. 420) 9 9

Time-frequency joint channel estimation TFT-OFDM Receiver (1)… Time-frequency joint channel estimation Proposed step 2: Frequency-domain path coefficients estimation a. The nonzero path coefficients with the delays can be modeled by the Q-order Taylor series expansion [7] where denotes the polynomial coefficient is the approximation error. 10 10

TFT-OFDM Receiver (1)… b. Since the inter-carrier-interference (ICI) is dominantly caused by the neighboring subcarriers [7], the received central pilots can be denoted as where ( ) 11 11

TFT-OFDM Receiver (1)… c. Since has unknown parameters, is necessary to guarantee the matrix to be of full column rank. So we estimate as Then, the path coefficient can be calculated based on Finally, the complete CIR is obtained based on and 12 12

Iterative ICI cancellation TFT-OFDM Receiver (2) Iterative ICI cancellation Step 1: Initial channel equalization Step 2: Iterative ICI cancellation Step 3: Iterative ICI cancellation Stop if iteration number (usually ) is reached Otherwise, return to Step 2 pilots 13 13

Performance Analysis (1) Spectral efficiency pilots are used Normally, and is used to for some margin even most channels has resolvable paths For , TFT-OFDM only requires 3% pilot density, while CP-OFDM need 12.5% pilot occupation ( is assumed in CP-OFDM according to the Karhunen-Loeve theorem [7]) 8.5% 11% TFT-OFDM has negligible lower spectral efficiency than TDS-OFDM TFT-OFDM has obvious higher efficiency than CP-OFDM and DPN-OFDM 14 14

Performance Analysis (2) SNR loss due to pilot power The negligible SNR loss in TFT-OFDM is only 0.098 dB, which is independent of the guard interval length. 15 15

Performance Analysis (3) Pilot power boosting The boosted pilot power in TFT-OFDM could be 3.86 dB higher than that in CP-OFDM, which is beneficial for more accurate channel estimation. 16 16

Performance Analysis (4) Computational complexity The computation of the full channel matrix results in the higher complexity TFT-OFDM than CP-OFDM and DPN-OFDM; But the complexity of the TFT-OFDM receiver is still 63% that of the TDS-OFDM receiver. 17 17

Transmit Diversity in TDS-OFDM Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion 18 18

BER performance comparison over AWGN channel Simulation Results (1) Parameters: Central Frequency 770 MHz Signal Bandwidth 7.56 MHz Length N=3780 M=420 S=20 Q=1 d=1 J0=3 Modulation 64QAM Channel Coding [8] LDPC, CR=2/3 Channels Vehicular B, Brazil D Receiver velocity 28/140 km/h BER performance comparison over AWGN channel TFT-OFDM, TDS-OFDM, and DPN-OFDM have very close BER performance TFT-OFDM performs 2.2 dB better than cyclic postfix OFDM 19 19

Simulation Results (2) Vehicular B channel, 28 km/h TFT-OFDM outperforms DPN-OFDM, CP-OFDM and TDS-OFDM by the SNR gain of 0.95 dB, 1.15 dB and 2.40 dB, respectively

Simulation Results (3) Brazil D channel, 140 km/h Compared with DPN-OFDM, CP-OFDM and TDS-OFDM, the SNR gain achieved by TFT-OFDM is increased to be about 1.15 dB, 2.25 dB and 4.40 dB, respectively.

Transmit Diversity in TDS-OFDM Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion 22 22

Brief Conclusions This paper proposes a novel OFDM-based transmission scheme called TFT-OFDM, whereby the training information exists in both time and frequency domains. The corresponding joint time-frequency channel estimation utilizes the time domain TS without interference cancellation to estimate the channel path delays, while the channel path coefficients are acquired by using the pilot groups scattered within the OFDM symbol. The iterative ICI removal method further improves the system performance. The grouped pilots in TFT-OFDM occupy only about 3% of the signal bandwidth. Therefore, high spectral efficiency as well as good performance over fast time-varying channels could be simultaneously realized. In addition, TFT-OFDM can be easily extended to MIMO and multiple access scenarios without obvious increase in the overhead.

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. 24 24

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