Preambles for MIMO channel estimation Title Preambles for MIMO channel estimation André Bourdoux Bart Van Poucke Liesbet Van der Perre IMEC, Wireless Research bourdoux@imec.be FirstName LastName – Activity / Group

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

SISO Preamble (1) SISO Preamble LTS STS B C SIG Data STS used for Title SISO Preamble (1) SISO Preamble B CP C SIG Data LTS STS 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) FirstName LastName – Activity / Group

SISO Preamble (2) Desirable properties for STS Short periodicity: CFO acquisition range = 1/2TB =  625 kHz 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 dB SNR improvement for Channel estimation input data

MIMO Preamble Requirements: SISO requirements: MIMO 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 (11a,g)

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 LTS SIG SIG2 STS1 LTS1 LTS2 LTS3 LTS4 Legacy preamble - # TX antennas - MIMO mode - … - Second AGC - Multi TX antenna Channel estimation STS2 STS3 STS4

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

TDM preamble C Data 1 TX 1 TX 2 Data 2 LTS C TX 1 TX 2 TX 3 Data 2 CP C Data 1 2 x 3.2 µs 0.8 µs TX 1 TX 2 Data 2 LTS CP C 2 x 3.2 µs 0.8 µs TX 1 TX 2 TX 3 Data 2 Data 3 Data 1 CP C 2 x 3.2 µs 0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4

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 10log10(NT) 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

FDM preamble TX 1 TX 2 C1 C2 Data 1 Data 2 C1 C2 C3 Data 1 Data 2 2 x 3.2 µs 0.8 µs C1 C2 CP Data 1 Data 2 2 x 3.2 µs 0.8 µs C1 C2 C3 CP Data 1 Data 2 Data 3 TX 1 TX 2 TX 3 C1 CP 2 x 3.2 µs 0.8 µs C2 C3 C4 Data 2 Data 3 Data 1 Data 4 TX 1 TX 2 TX 3 TX 4

FDM preamble … IFFT TX 1 TX 2 5 57 61 2 6 58 62 IFFT C1 C2 TX 1 TX 2 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.

FDM preamble 1x(16+128) samples  1x 7.2µs Minimum duration: 1x(16+128) samples  1x 7.2µs Duration for same energy as TDM: 16 + NTx128 samples  0.8 + NTx6.4µs 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

CDM preamble LTS -C C Data 1 TX 1 TX 2 Data 2 C TX 1 TX 2 TX 3 Data 2 CP -C C Data 1 2 x 3.2 µs 0.8 µs TX 1 TX 2 Data 2 CP C 2 x 3.2 µs 0.8 µs TX 1 TX 2 TX 3 Data 2 Data 3 Data 1 -C CP C 2 x 3.2 µs 0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 -C

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 NT=3 must be same length as for NT=4

TDM-FDM preamble Duration for 4 TX antennas: CP C2 C1 2 x 3.2 µs 0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 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

TDM-FDM preamble Duration for 4 TX antennas: CP C 2 x 3.2 µs 0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 -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

FDM-CDM preamble Duration for 4 TX antennas: CP C2 C1 2 x 3.2 µs 0.8 µs TX 1 TX 2 TX 3 TX 4 Data 2 Data 3 Data 1 Data 4 -C2 -C1 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 NT=3

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)

Channel estimation error, NT=2 Worse estimation without least-square Impact of zero-carriers on least-square

Channel estimation error, NT=4 Worse estimation without least-square Impact of coarse frequency sampling (FDM) Impact of zero-carriers on least-square

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