1. 20us effective preambles for MIMO-OFDM
Seigo Nakao,
Yoshiharu Doi,
SANYO Electric Co., Ltd. Japan
Yasutaka Ogawa,
Hokkaido University Japan
Presented by Jon W. Rosdahl

2. Contents Background Detecting the Antenna number in STS
AGC performance of new STS
A proposal for new LTS and channel estimation techniques
Conclusions

3. Background:
PHY overhead is one bottleneck that prevents high throughput.
Overlapped LTS can reduce the PHY overhead.

4. Issues for overlapped LTS
Preferably, the number of antennas should be known before overlapped LTS.
Consideration of the best LTS for MIMO channel estimation
Example of overlapped STS and LTS
TX1
STS1
LTS1
Sig
DATA1
TX2
STS2
LTS2
Sig
DATA2

5. Assumption: STS is used for LTS is used for AGC Timing detection
Rough frequency offset estimation
Detecting the number of antennas
LTS is used for
Channel estimation
Fine frequency offset estimation

6. Issues for STS: Each TX antenna should have an unique STS.
The cross-correlation of 1 STS cycle for any pair of STSs should be 0 for
Easy synchronization,
Good frequency offset estimation, and
Optimum AGC implementation.
(As concluded in IEEE r2)
STSs should be used to distinguish the number of TX antennas.

7. Detecting the Number of Antennas

8. Detecting the Number of antennas
If the number of antennas can be known during the STS time,
we can easily employ “overlapped STS and LTS”
Example of overlapped STS and LTS
TX1
STS1
LTS1
Sig
DATA1
TX2
STS2
LTS2
Sig
DATA2
Example of overlapped STS, staggered LTS
TX1
STS1
LTS
Sig
Sig2
DATA1
TX2
STS2
LTS
Sig
DATA2
Detection timing for number of TX antennas

9. Proposed STSs (4antenna max)
Using 6 carriers of
legacy STS
1TX
Legacy STS
2TX
STS1
STSa
3TX
STS1
STS2
STSb
4TX
STS1
STS2
STS3
STSc
Using the other 6 carriers of legacy STS
Low cross-correlation
Zero cross-correlation

10. Examples of Proposed STSs (3 antenna max case)
Carrier no.
Legacy
STSa
STSb
STS1
STS2
-24 j j j
-20 j j j
-16 j j j
-12 j j j -8 j j j -4 j j j 4 j j j 8 j j j 12 j j j 16 j j j 20 j j j 24 j j j
sqrt(13/6)=1.472, sqrt(13/3)=2.082

11. Correlator for Legacy STS detection
Carrier no.
Legacy STS
sequence
Correlator for
legacy STS
(6carrier)
-24 j
This correlator can detect the legacy STS without any effects from STS1 and STS2.
-20 j
-16 j
-12 j j -8 j j -4 j 4 j j 8 j 12 j j 16 j j 20 j j 24 j

12. How to detect the number of antennas (3 antenna max case)
Correlation
peak
Received signal
Correlator for
legacy STS
(6carrier)
Max
Select
Number of
antennas
Correlator for
STSa
Correlator for
STSb

13. Detecting Number of Antennas
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5 10 20 30 40
SNR [dB]
detection error
3 antenna case
Channel model B

14. Backwards compatibility
Proposed STSs take analogous approach to .11G
The 11g document defines the same preambles as 11b, but no venders actually provide the same preambles - They only use pure OFDM (like 11a) for throughput.
The proposed STS can't be directly received by legacy STA either.
However, an 11g STA and 11b STA can know the signals exist by calculating receiving power.
Thus this proposal has the same degree of backwards compatibility as 11g does to 11b.

15. Summary Proposed STSs can be used for any number of antenna detection.
To reduce the cost of correlators, STSa and STSb can be defined such that the correlator implementation cost can be much less than the cost of n separate correlators.
(e.g. STSa_I = STSb_Q, STSa_Q=STSbI)

16. AGC performance of new STSs

17. AGC performance
Generally, AGC calculates the power of 16 FFT points (i.e. 1 STS).
The power of 16 FFT points of STS term should be the same as that of DATA term.

18. Other candidates of new STSs
Frequency cyclic use of legacy STS
Which doesn’t have 16 FFT samples cycle
(IEEE r1)
Polarity changed STS of legacy STS
(IEEE r1)

19. AGC performance of proposed STS in channel model B

20. AGC performance of other candidates in channel model B
Polarity changed STS of legacy STS
Frequency cyclic use of legacy STS

21. Summary
Proposed STSs have good performance characteristics for AGC implementations.
Proposed STSs have an advantage over frequency cyclic use of legacy STS because the proposed STS is a 16 FFT points cycle.
No disadvantage results from using 6 carriers (vs. 12) for STS (for MIMO)
For 1 tx, 12 carriers are better than 6
For 2+ tx, 6 carries are better than 12 because of cross correlation advantages.

22. Issues of overlapped LTS:
LTS should be used for fine frequency offset estimation
LTS should have good performance of MIMO channel estimation

23. One proposal for new LTS and channel estimation
Ogawa, et al. “A MIMO-OFDM System for High-Speed Transmission,” VTC2003-Fall, Oct

24. Proposal of new LTS and Channel Estimation:
TX1
GI21
T1,1
T1,2
(32)
(64)
TX2
GI22
T2,1
T2,2
(32)
(64)
The Impulse response between each TX and RX antenna pair is estimated by the minimum mean square error scheme using 2 LTSs in the time domain (128 samples).
Channel at each subcarrier is obtained by FFT of the zero padded impulse response.

25. Channel and Frequency Offset (Df ) Estimation:
Coarse Df estimation is carried out using 3 cycles of the STS.
Phase rotation is compensated by the coarse Df estimator before next steps.
Channel estimation is done using the 2 LTS portions (Ti,1 and Ti,2) assuming that Df = 0.
Replica of the time-domain LTS sequences with Df = 0 is calculated by the channel estimator.
Fine Df is estimated from the phase rotation in the LTS period using the replica.
Phase rotation is compensated again using the fine Df estimator.
Channel estimation is done again using the 2 LTS portions.

26. Average BER Performance of Proposed method:/ N [ d ] D f s t i m W h p c k n o w l 1 2 3 4 - 5
TX 4, RX 4
Df = 50kHz
QPSK
9 data symbols / subcarrier
16 multipath signals (Average power of the multipath signals decreases successively by 1 dB.)
No channel coding
Spatial filter (MMSE)

27. Conclusion
New STSs enable the easy use of overlapped LTS, therefore PHY overhead can be reduced dramatically.
20 [us] preamble can be employed for MIMO-OFDM
Overlapped LTS can be used for
Fine frequency offset estimation
Channel estimation

28. Reference: IEEE 802.11-04-0002r2 IEEE 802.11-04-0046r1
Ogawa, et al. “A MIMO-OFDM System for High-Speed Transmission,” VTC2003-Fall, Oct
IEEE r2
IEEE r1
IEEE r1

29. Questions?