20us effective preambles for MIMO-OFDM Seigo Nakao, snakao@gf.hm.rd.sanyo.co.jp Yoshiharu Doi, doi@gf.hm.rd.sanyo.co.jp SANYO Electric Co., Ltd. Japan Yasutaka Ogawa, ogawa@ice.eng.hokudai.ac.jp Hokkaido University Japan Presented by Jon W. Rosdahl

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

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

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

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

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 802.11-04-0002r2) STSs should be used to distinguish the number of TX antennas.

Detecting the Number of Antennas

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

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

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

Correlator for Legacy STS detection Carrier no. Legacy STS sequence Correlator for legacy STS (6carrier) -24 1.472+1.472j This correlator can detect the legacy STS without any effects from STS1 and STS2. -20 -1.472-1.472j -16 1.472+1.472j -12 -1.472-1.472j -2.082-2.082j -8 -1.472-1.472j -2.082-2.082j -4 1.472+1.472j 4 -1.472-1.472j -2.082-2.082j 8 -1.472-1.472j 12 1.472+1.472j 2.082+2.082j 16 1.472+1.472j 2.082+2.082j 20 1.472+1.472j 2.082+2.082j 24 1.472+1.472j

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

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

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.

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)

AGC performance of new STSs

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.

Other candidates of new STSs Frequency cyclic use of legacy STS Which doesn’t have 16 FFT samples cycle (IEEE 802.11-04-0046r1) Polarity changed STS of legacy STS (IEEE 802.11-04-0087r1)

AGC performance of proposed STS in channel model B

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

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.

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

One proposal for new LTS and channel estimation Ogawa, et al. “A MIMO-OFDM System for High-Speed Transmission,” VTC2003-Fall, Oct. 2003.

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.

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.

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)

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

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. 2003. IEEE 802.11-04-0002r2 IEEE 802.11-04-0046r1 IEEE 802.11-04-0087r1

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