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doc.: IEEE 802.11-04/794r1 Submission Slide 1 André Bourdoux (IMEC) July 2004 Preambles for MIMO channel estimation André Bourdoux Bart Van Poucke Liesbet Van der Perre IMEC, Wireless Research bourdoux@imec.be
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doc.: IEEE 802.11-04/794r1 Submission Slide 2 André Bourdoux (IMEC) July 2004 Motivation MIMO-OFDM is key to achieve 100 Mbps at the MAC SAP Conventional SISO preamble (11.a, g) is not sufficient MIMO channel estimation requires a new preamble
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doc.: IEEE 802.11-04/794r1 Submission Slide 3 André Bourdoux (IMEC) July 2004 SISO Preamble (1) STS used for AGC, Packet detection(Power measurement) Coarse timing acquisition(Auto-correlation) Coarse Carrier freq. acquisition(Auto-correlation) SISO Preamble BBBBBBBBBB CP CC SIG CP Data LTSSTS LTS used for Fine timing acquisition(Auto/cross-correlation) Fine Carrier freq. acquisition(Auto-correlation) Channel estimation(direct, least-square) IQ imbalance estimation(specific algorithm)
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doc.: IEEE 802.11-04/794r1 Submission Slide 4 André Bourdoux (IMEC) July 2004 SISO Preamble (2) Desirable properties for STS Short periodicity: CFO acquisition range = 1/2T B = 625 kHz Long periodicity: > max excess delay (T B = 800 ns 240 m.) Low PAPR Desirable properties for LTS Low auto-correlation sidelobes Double-length CP to accommodate coarse timing estimation repeated C sequence allows Long auto-correlation for accurate CFO estimation 3 dB SNR improvement for Channel estimation input data Low PAPR
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doc.: IEEE 802.11-04/794r1 Submission Slide 5 André Bourdoux (IMEC) July 2004 MIMO Preamble Requirements: SISO requirements: AGC, packet detection CFO estimation Timing estimation MIMO requirements Detect number of TX antennas (N T ) On each RX antenna, differentiate and Estimate N T channels from one received signal Low cross-correlation between TX antenna signals Legacy requirements When N T =1, compatible with SISO transmission (11a,g)
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doc.: IEEE 802.11-04/794r1 Submission Slide 6 André Bourdoux (IMEC) July 2004 Assumptions for MIMO preamble Reuse of SISO preamble (STS, LTS, SIG) for legacy Coarse/fine timing and CFO is achieved before channel estimation AGC from TX1 only cannot be reused, second AGC needed # TX antennas is known before channel estimation AGC is settled before channel estimation CP for MIMO channel estimation can be 16 samples long Total energy available per “SISO” channel is constant STS TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 LTSSIGSIG2STS1LTS1 LTS2 LTS3 LTS4 Legacy preamble - # TX antennas - MIMO mode - … - Second AGC- Multi TX antenna Channel estimation STS2 STS3 STS4
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doc.: IEEE 802.11-04/794r1 Submission Slide 7 André Bourdoux (IMEC) July 2004 Orthogonality between TX antennas We focus on the part of the preamble for Multi-TX antenna channel estimation LTS sequences from different TX antennas must be differentiated LTS sequences can be made orthogonal in -Time:TDM -Frequency:FDM -Code:CDM -Hybrid (for N T > 2):TDM-FDM TDM-CDM FDM-CDM
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doc.: IEEE 802.11-04/794r1 Submission Slide 8 André Bourdoux (IMEC) July 2004 TDM preamble LTS CP C CData 1 2 x 3.2 µs0.8 µs TX 1 TX 2 Data 2 CP C C C 2 x 3.2 µs0.8 µs TX 1 TX 2 TX 3 Data 2 Data 3 Data 1 CP C C C C 2 x 3.2 µs0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4
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doc.: IEEE 802.11-04/794r1 Submission Slide 9 André Bourdoux (IMEC) July 2004 TDM preamble Minimum duration: N T x (16+128) samples N T x 7.2µs Processing (per RX antenna): Estimate = measurement: N T x SISO Channel estim. Least square : smoothes freq-domain channel estimate with time-domain constraint; N T x 2 x N c x L complex MACs Reuse of existing blocks (IP) Allows IQ Imbalance compensation based on preamble Requires higher average power per antenna during LTS 10log 10 (N T ) dB more TX power per TX antennas RX AGC is a problem (1 TX antenna active at a time) AGC values must be the same as during payload transmission
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doc.: IEEE 802.11-04/794r1 Submission Slide 10 André Bourdoux (IMEC) July 2004 FDM preamble 2 x 3.2 µs0.8 µs C1 C2 C3 CP C3 CP C2 CP C1Data 1 Data 2 Data 3 TX 1 TX 2 TX 3 C1 CP C1 2 x 3.2 µs0.8 µs C1 C2 C3 C4 CP C4 CP C3 CP C2Data 2 Data 3 Data 1 Data 4 TX 1 TX 2 TX 3 TX 4 TX 1 TX 2 2 x 3.2 µs0.8 µs C1 C2 CP C2 CP C1Data 1 Data 2
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doc.: IEEE 802.11-04/794r1 Submission Slide 11 André Bourdoux (IMEC) July 2004 FDM preamble Different subsets of sub-carriers used on the TX antennas For 52 sub-carriers and 4 TX antennas, only 13 sub-carriers per training symbol. … 155761 … 265862 IFFT C1 C2 IFFT TX 1 TX 2 ………
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doc.: IEEE 802.11-04/794r1 Submission Slide 12 André Bourdoux (IMEC) July 2004 FDM preamble Minimum duration: 1x(16+128) samples 1x 7.2µs Duration for same energy as TDM: 16 + N T x128 samples 0.8 + N T x6.4µs Processing (per RX antenna): Freq domain interpolator: sensitivity to phase slope Least square : 2 x N C x (N T + 1) complex MACs Same average RX power as during payload reception
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doc.: IEEE 802.11-04/794r1 Submission Slide 13 André Bourdoux (IMEC) July 2004 CDM preamble LTS CP -C CP CData 1 2 x 3.2 µs0.8 µs TX 1 TX 2 Data 2 CP C C C C C C 2 x 3.2 µs0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 CP C C C C C C -C CP -C CP -C CP -C CP -C CP -C CP C C C 2 x 3.2 µs0.8 µs TX 1 TX 2 TX 3 Data 2 Data 3 Data 1 CP C C C C C -C CP -C CP -C CP -C
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doc.: IEEE 802.11-04/794r1 Submission Slide 14 André Bourdoux (IMEC) July 2004 CDM preamble Minimum duration: 2 TX: 2x(16+128) samples 2 x 7.2µs 3 or 4 TX: 4x(16+128) samples 4 x 7.2µs Processing (per RX antenna): Complex additions/substractions for “despreading” The rest is same as TDM Same average RX power as during payload reception LTS for N T =3 must be same length as for N T =4
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doc.: IEEE 802.11-04/794r1 Submission Slide 15 André Bourdoux (IMEC) July 2004 TDM-FDM preamble CP C2 CP C1 2 x 3.2 µs0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 C1 CP C2 CP C1 C2 C1 C2 Duration for 4 TX antennas: 2x(16+256) samples 2 x 13.6µs Processing (per RX antenna ): Least square for the FDM part, the rest is same as TDM Problem of average RX power (in TDM) not completely eliminated
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doc.: IEEE 802.11-04/794r1 Submission Slide 16 André Bourdoux (IMEC) July 2004 CP C C C 2 x 3.2 µs0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 CP C -C CP -C CP -C CP -C Duration for 4 TX antennas: 4x(16+128) samples 4 x 7.2µs Processing (per RX antenna ): Complex additions/substractions for “despreading” The rest is same as TDM Problem of average RX power (in TDM) not completely eliminated TDM-CDM preamble
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doc.: IEEE 802.11-04/794r1 Submission Slide 17 André Bourdoux (IMEC) July 2004 FDM-CDM preamble CP C2 CP C1 2 x 3.2 µs0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 C1 CP -C2 CP -C1 C2 -C1 -C2 CP C2 CP C1 C2 CP C2 CP C1 C2 Duration for 4 TX antennas: 2x(16+256) samples 2 x 13.6µs Processing (per RX antenna ): Complex additions/substractions for “despreading” The rest is same as TDM Can also be used for N T =3
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doc.: IEEE 802.11-04/794r1 Submission Slide 18 André Bourdoux (IMEC) July 2004 Performance of the various preambles In principle, TDM and CDM have the same performance FDM performance degrades for N T =4 because of coarser frequency sampling Simulations show Channel Estimation Mean-squared Error for preamble options and N T =2, 4 In all simulations total power / N T is constant total energy / N T is constant (except for CPs)
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doc.: IEEE 802.11-04/794r1 Submission Slide 19 André Bourdoux (IMEC) July 2004 Channel estimation error, N T =2 Impact of zero- carriers on least- square Worse estimation without least- square
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doc.: IEEE 802.11-04/794r1 Submission Slide 20 André Bourdoux (IMEC) July 2004 Channel estimation error, N T =4 Impact of zero- carriers on least- square Impact of coarse frequency sampling (FDM) Worse estimation without least- square CDM-FDM: Best performance no problem with AGC
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doc.: IEEE 802.11-04/794r1 Submission Slide 21 André Bourdoux (IMEC) July 2004 Our advice for 802.11n Several preamble structures are possible for MIMO channel estimation Preambles with simultaneous transmission from all TX antennas are mandatory no problem from AGC Least-square solution provides better estimate, is mandatory for FDM-based preambles
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