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

2. 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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. TDM preamble C Data 1 TX 1 TX 2 Data 2 LTS C TX 1 TX 2 TX 3 Data 2CP 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

9. 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

10. 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

11. FDM preamble … IFFT TX 1 TX 25 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.

12. 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  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

13. CDM preamble LTS -C C Data 1 TX 1 TX 2 Data 2 C TX 1 TX 2 TX 3 Data 2CP -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

14. 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

15. 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

16. 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

17. 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

18. 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)

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

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

21. Our advice for 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