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

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

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

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] PN OFDM Block Data Data interferences 4 4

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

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 6

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

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 8

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 9

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

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

MSE performance comparison 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 12

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

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

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 .

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