doc.: IEEE 802.11-04/794r1

Submission Slide 1 André Bourdoux (IMEC)

July 2004

Preambles forMIMO channel estimation

André Bourdoux

Bart Van Poucke

Liesbet Van der Perre

IMEC, Wireless Research

bourdoux@imec.be

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

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

B B B B B B B B B B CP C C CP 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)

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 = 1/2TB = 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

Low PAPR

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 (NT) On each RX antenna, differentiate

and Estimate NT channels from onereceived signal

Low cross-correlation between TX antenna signals Legacy requirements

When NT=1, compatible with SISO transmission (11a,g)

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

STSTX 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

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 NT > 2): TDM-FDMTDM-CDMFDM-CDM

doc.: IEEE 802.11-04/794r1

Submission Slide 8 André Bourdoux (IMEC)

July 2004

TDM preamble

LTS

CP C

CP C Data 1

2 x 3.2 µs0.8 µs

TX 1

TX 2 Data 2

CP C

CP C

CP C

2 x 3.2 µs0.8 µs

TX 1

TX 2

TX 3

Data 2

Data 3

Data 1

CP C

CP C

CP C

CP C

2 x 3.2 µs0.8 µs

TX 1

TX 2

TX 3

TX 4

Data 2

Data 3

Data 1

Data 4

doc.: IEEE 802.11-04/794r1

Submission Slide 9 André Bourdoux (IMEC)

July 2004

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

doc.: IEEE 802.11-04/794r1

Submission Slide 10 André Bourdoux (IMEC)

July 2004

FDM preamble

2 x 3.2 µs0.8 µs

C1 C1

C2 C2

C3 C3CP C3

CP C2

CP C1 Data 1

Data 2

Data 3

TX 1

TX 2

TX 3

C1CP C1

2 x 3.2 µs0.8 µs

C1 C1

C2 C2 C2

C3 C3 C3

C4 C4 C4CP C4

CP C3

CP C2 Data 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

C2CP C2

CP C1 Data 1

Data 2

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.

…

1 5 57 61

…

2 6 58 62

IFFT C1

C2IFFT

TX 1

TX 2

… … …

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 + 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

doc.: IEEE 802.11-04/794r1

Submission Slide 13 André Bourdoux (IMEC)

July 2004

CDM preamble

LTS

CP -C

CP C Data 1

2 x 3.2 µs0.8 µs

TX 1

TX 2 Data 2

CP CCP C

CP C

CP CCP C

CP 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 CP C CP C

CP C

CP C CP C

CP -C

CP -C CP -C

CP -C

CP -CCP -C

CP CCP C

CP C

2 x 3.2 µs0.8 µs

TX 1

TX 2

TX 3

Data 2

Data 3

Data 1CP C CP C CP C

CP C CP C

CP -CCP -C

CP -CCP -C

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

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

doc.: IEEE 802.11-04/794r1

Submission Slide 16 André Bourdoux (IMEC)

July 2004

CP C

CP C

CP 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 CCP -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

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 C1

C2

CP C2

CP C1 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 NT=3

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 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)

doc.: IEEE 802.11-04/794r1

Submission Slide 19 André Bourdoux (IMEC)

July 2004

Channel estimation error, NT=2

-30 -20 -10 0 10 20 300

0.2

0.4

0.6

0.8

1

1.2x 10

-3

sub-carrier index

MS

E

Channel Estimation MSE at SNR=30dB, NT=2

TDM + LSTDMFDM +LSCDM +LSCDM

Impact of zero-carriers on least-square

Worse estimation without least-square

doc.: IEEE 802.11-04/794r1

Submission Slide 20 André Bourdoux (IMEC)

July 2004

Channel estimation error, NT=4

-30 -20 -10 0 10 20 300

0.5

1

1.5

2

2.5x 10

-3

sub-carrier index

MS

E

Channel Estimation MSE at SNR=30dB, NT=4

TDM + LSTDMFDM +LSCDM +LSCDMCDM-FDM +LSTDM-FDM +LS

Impact of zero-carriers on least-square

Impact of coarse frequency sampling (FDM)

Worse estimation without least-square

CDM-FDM:

Best performanceno problem with AGC

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