1. 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

2. Time-Frequency Training OFDM
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] PN
OFDM Block
Data
Data
interferences 4 4

5. Time-Frequency Training OFDM
Outline 1
Background 2
Time-Frequency Training OFDM 3
Simulation Results 4
Conclusion 5 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 6

7. 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

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 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 9

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

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

12. 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

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

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17. Thanks for your attention !