1. Short Training Sequence Compatibility with Legacy 802.11g Devices
January 2005
doc.: IEEE /1590r0
January 2005
Short Training Sequence Compatibility with Legacy g Devices
Date:
Authors:
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Christopher Hansen, Broadcom
Christopher Hansen, Broadcom

2. Background 802.11a/b/g are now heavily deployed
January 2005
Background
802.11a/b/g are now heavily deployed
802.11n will operate in a legacy environment
Backward compatibility is essential
Need a method for legacy stations to know when .11n frames are on the air
PHY level protection
Multiple antenna preamble that can be received and decoded by legacy stations
Christopher Hansen, Broadcom

3. Multiple Antenna Transmission to Legacy Stations
January 2005
Multiple Antenna Transmission to Legacy Stations
802.11n stations will have multiple TX paths
Need a method for communicating with legacy stations
Transmit on 1 antenna
Less efficient
Transmit on multiple antennas
Cyclic Delay Diversity (CDD)?
What is the best delay?
Christopher Hansen, Broadcom

4. Single Antenna Inefficiency
January 2005
Single Antenna Inefficiency
Power reduction (3 to 6 dB)
Range reduction for legacy protection
802.11n MIMO TX
Legacy RX
Christopher Hansen, Broadcom

5. Multiple Antenna Benefits
January 2005
Multiple Antenna Benefits
Legacy STA sees signal from all TX paths
More TX power available
Nothing turned off
No large signal transients on TX paths
Greater chance that signal will get through
The Challenge?
What multiple antenna signal will work with legacy stations?
Christopher Hansen, Broadcom

6. Cyclic Shift of ST Sequence Across Antennas
January 2005
Cyclic Shift of ST Sequence Across Antennas
STRN1
STRN2
STRNN
Tx 1
Tx 2
Tx N
STRN1 is the legacy .11a/.11g short training sequence
STRN2 through STRNN are cyclic shifted versions of STRN1
Christopher Hansen, Broadcom

7. Choice of Cyclic Shifts
January 2005
Choice of Cyclic Shifts
Long cyclic shifts are preferable
Short cyclic shifts (50 nS or less) produce correlated signals across TX array
Produces variations in power level across tones and directions
Leads to “hidden” nodes
Long cyclic shifts are less prone to this problem
Which cyclic shift is best?
Compute power estimation error
Verify with real legacy devices
Christopher Hansen, Broadcom

8. Effect of Cyclic Shift on Power Estimation Accuracy
January 2005
Effect of Cyclic Shift on Power Estimation Accuracy
400 ns shift minimizes power estimation error
Christopher Hansen, Broadcom

9. Legacy Performance Cyclic shift works with legacy 11g devices
January 2005
Legacy Performance
Cyclic shift works with legacy 11g devices
Legacy devices are able to acknowledge packets with rotated preambles, even at high rates
Can infer that legacy devices are able to carry out preamble processing
Tested products from multiple 11g chipset vendors
Atheros
Texas Instruments
Intersil
Broadcom
Christopher Hansen, Broadcom

10. Test Setup Block Diagram
January 2005
Test Setup Block Diagram B A
Legacy
DUT
2:1 Splitter
2:1 Splitter
Attenuator
AWG
-50 dBm
MAC addr = AAA
SSID = ‘test’
Driver running
MAC addr = BBB
SSID = ‘test’
50dB pad
2:1 Splitter C
AWG = Arbitrary Waveform Generator
(Agilent E8267C)
DUT = Device Under Test
Sniffer
No driver running
MAC addr = AAA (same as packets from AWG)
SSID = ‘test’
Christopher Hansen, Broadcom

11. Test Procedure 6 Mbps test frames created in MATLAB.
January 2005
Test Procedure
6 Mbps test frames created in MATLAB.
DUT’s destination address/BSSID (unicast), data frame, 5 data bytes long
Control: Legacy SISO signal. (i.e. 1 TX antenna)
500 frames created, each convolved with randomly generated channel and saved for playback with the Aribrtrary Waveform Generator (AWG)
For each candidate n STSs: 2 TX antenna case (MISO case)
TX 1 antenna has legacy STS.
500 frames created, each convolved with randomly generated channel h1(t)
TX2 antenna has cyclically shifted STS.
500 frames created, each convolved with randomly generated channel h2(t) (Note: h1(t) and h2(t) are independent)
Signals are combined and saved for playback with the AWG
Christopher Hansen, Broadcom

12. Test Procedure Received level carefully calibrated at legacy DUT.
January 2005
Test Procedure
Received level carefully calibrated at legacy DUT.
DUT will send ACK frames when it is able to receive and correctly decode a frame
Count the percentage of ACK’d frames to ascertain performance
Since DUT is running a driver, a small number of frames may be lost because of collisions with beacons, etc.
Each legacy device will have a different sensitivity point
X axis shows relative sensitivity of MISO to legacy, single TX antenna performance
Christopher Hansen, Broadcom

13. Results Performance of different cyclic shifts is presented
January 2005
Results
Performance of different cyclic shifts is presented
25 and 50 ns rms delay spread channels tested
Results reported as a ratio of MISO ACKs (.11n TX) to SISO ACKs (legacy TX)
Christopher Hansen, Broadcom

14. Results (Vendor #1 25nsec rms)
January 2005
Results (Vendor #1 25nsec rms)
Christopher Hansen, Broadcom

15. Results (Vendor #1 50 nsec rms)
January 2005
Results (Vendor #1 50 nsec rms)
Christopher Hansen, Broadcom

16. Results Vendor #2 (25 nsec rms)
January 2005
Results Vendor #2 (25 nsec rms)
Christopher Hansen, Broadcom

17. Results Vendor #2 (50 nsec rms)
January 2005
Results Vendor #2 (50 nsec rms)
Christopher Hansen, Broadcom

18. Results Vendor #3 (25 nsec rms)
January 2005
Results Vendor #3 (25 nsec rms)
Christopher Hansen, Broadcom

19. Results Vendor #3 (50 nsec rms)
January 2005
Results Vendor #3 (50 nsec rms)
Christopher Hansen, Broadcom

20. Results Vendor #4 (25 nsec rms)
January 2005
Results Vendor #4 (25 nsec rms)
Christopher Hansen, Broadcom

21. Results Vendor #4 (50 nsec rms)
January 2005
Results Vendor #4 (50 nsec rms)
Christopher Hansen, Broadcom

22. January 2005
Conclusions
Analytical results show that 400 nsec shift provides best power estimation accuracy
Laboratory results show that 400 nsec shift provides the best legacy protection
Backward compatible MIMO preambles based on a 400 nsec cyclic shift on the 2nd antenna (2 TX case) are appropriate for n
Christopher Hansen, Broadcom