July 2004 doc IEEE 802 11 04794 r

July 2004 doc. : IEEE 802. 11 -04/794 r 0 Preambles for MIMO channel estimation André Bourdoux Bart Van Poucke Liesbet Van der Perre IMEC, Wireless Research bourdoux@imec. be Submission Slide André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 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 Submission Slide 2 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 SISO Preamble (1) SISO Preamble STS BBBBB LTS CP C C CP SIG CP Data STS used for § AGC, Packet detection (Power measurement) § Coarse timing acquisition (Auto-correlation) § Coarse Carrier freq. acquisition (Auto-correlation) 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) Submission Slide 3 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 SISO Preamble (2) § Desirable properties for STS § § § Short periodicity: CFO acquisition range = 1/2 TB = 625 k. Hz Long periodicity: > max excess delay (TB = 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 d. B SNR improvement for Channel estimation input data § Submission Low PAPR Slide 4 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 MIMO Preamble Requirements: § SISO requirements: § AGC, packet detection § CFO estimation § Timing estimation § MIMO requirements § Detect number of TX antennas (NT) § On each RX antenna, differentiate and Estimate NT channels from one received signal § Low cross-correlation between TX antenna signals § Legacy requirements § When NT=1, compatible with SISO transmission (11 a, g) Submission Slide 5 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 Assumptions for MIMO preamble § § § TX 1 TX 2 Reuse of SISO preamble (STS, LTS, SIG) for legacy § Coarse/fine timing and CFO is achieved before channel estimation § AGC from TX 1 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 Legacy preamble STS LTS SIG 2 STS 1 LTS 1 Data 1 STS 2 STS 3 STS 4 LTS 2 LTS 3 LTS 4 Data 2 Data 3 Data 4 TX 3 TX 4 - # TX antennas - MIMO mode -… Submission - Second AGC Slide 6 - Multi TX antenna Channel estimation André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 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 NT > 2): TDM-FDM TDM-CDM FDM-CDM Submission Slide 7 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 TDM preamble 0. 8 µs TX 1 CP 2 x 3. 2 µs C TX 2 Data 1 CP C Data 2 LTS 0. 8 µs TX 1 TX 2 CP 2 x 3. 2 µs C CP C TX 3 CP 0. 8 µs TX 1 CP TX 2 TX 3 C 2 x 3. 2 µs C Data 1 CP C TX 4 Submission Data 1 Data 2 Data 3 CP Slide 8 C Data 2 Data 3 Data 4 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 TDM preamble § Minimum duration: NT x (16+128) samples NT x 7. 2µs § Processing (per RX antenna): § Estimate = measurement: NT x SISO Channel estim. § Least square : smoothes freq-domain channel estimate with time-domain constraint; NT x 2 x Nc x L complex MACs § Reuse of existing blocks (IP) § Allows IQ Imbalance compensation based on preamble § Requires higher average power per antenna during LTS § 10 log 10(NT) d. B 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 Submission Slide 9 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 FDM preamble 0. 8 µs 2 x 3. 2 µs TX 1 CP C 1 Data 1 TX 2 CP C 2 Data 2 0. 8 µs TX 1 TX 2 CP TX 3 CP CP 0. 8 µs TX 1 CP TX 2 CP TX 3 CP TX 4 CP Submission 2 x 3. 2 µs C 1 C 2 C 3 Data 1 Data 2 Data 3 C 1 C 1 Data 1 C 2 C 3 C 4 Data 2 Data 3 Data 4 2 x 3. 2 µs C 1 C 2 C 3 C 4 Slide 10 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 FDM preamble 1 5 57 TX 1 61 … 2 TX 2 6 58 C 1 IFFT C 2 62 … … IFFT § 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. Submission Slide 11 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 FDM preamble § Minimum duration: 1 x(16+128) samples 1 x 7. 2µs § Duration for same energy as TDM: 16 + NTx 128 samples 0. 8 + NTx 6. 4µs § Processing (per RX antenna): § Freq domain interpolator: sensitivity to phase slope § Least square : 2 x NC x (NT + 1) complex MACs § Same average RX power as during payload reception Submission Slide 12 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 CDM preamble 0. 8 µs 2 x 3. 2 µs TX 1 CP C Data 1 TX 2 CP C CP -C Data 2 LTS 0. 8 µs TX 1 TX 2 CP TX 3 CP CP 0. 8 µs TX 1 CP TX 2 CP TX 3 TX 4 2 x 3. 2 µs C CP CP CP C -C C CP CP CP C C -C CP CP C -C -C Data 1 Data 2 CP C -C -C CP CP Data 3 2 x 3. 2 µs CP CP C -C C CP -C Submission C C -C CP CP Slide 13 CP Data 3 Data 4 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 CDM preamble § Minimum duration: 2 TX: 2 x(16+128) samples 2 x 7. 2µs 3 or 4 TX: 4 x(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 NT=3 must be same length as for NT=4 Submission Slide 14 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 TDM-FDM preamble 0. 8 µs 2 x 3. 2 µs TX 1 CP C 1 Data 1 TX 2 CP C 2 TX 3 CP C 1 Data 2 Data 3 TX 4 CP C 2 Data 4 § Duration for 4 TX antennas: 2 x(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 Submission Slide 15 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 TDM-FDM preamble 0. 8 µs 2 x 3. 2 µs TX 1 CP C Data 1 TX 2 CP C CP -C TX 3 CP -C Data 2 Data 3 TX 4 CP -C CP C Data 4 § Duration for 4 TX antennas: 4 x(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 Submission Slide 16 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 FDM-CDM preamble 0. 8 µs 2 x 3. 2 µs TX 1 CP C 1 C 1 Data 1 TX 2 CP CP C 2 C 1 CP TX 3 C 2 C 1 CP C 2 -C 1 Data 2 Data 3 TX 4 CP C 2 CP -C 2 Data 4 § Duration for 4 TX antennas: 2 x(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 NT=3 Submission Slide 17 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 Performance of the various preambles § In principle, TDM and CDM have the same performance § FDM performance degrades for NT=4 because of coarser frequency sampling § Simulations show Channel Estimation Mean-squared Error for preamble options and NT=2, 4 § In all simulations § total power / NT is constant § total energy / NT is constant (except for CPs) Submission Slide 18 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 Channel estimation error, NT=2 Worse estimation without leastsquare Impact of zerocarriers on leastsquare Submission Slide 19 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 Channel estimation error, NT=4 Worse estimation without leastsquare Impact of coarse frequency sampling (FDM) Impact of zerocarriers on leastsquare Submission Slide 20 André Bourdoux (IMEC)

July 2004 doc. : IEEE 802. 11 -04/794 r 0 Our advice for 802. 11 n § 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 Submission Slide 21 André Bourdoux (IMEC)