1. 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

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

3. 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

4. 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

5. 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)

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

16. 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

17. 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

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

19. 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

20. 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

21. 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.

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

23. 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.

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