Doc.: IEEE 802.11-04/794r1 Submission Slide 1 André Bourdoux (IMEC) July 2004 Preambles for MIMO...
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Transcript of Doc.: IEEE 802.11-04/794r1 Submission Slide 1 André Bourdoux (IMEC) July 2004 Preambles for MIMO...
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
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