1. MIMO Testing In A Conducted Environment
November 2006
doc.: IEEE /1839r0
November 2006
MIMO Testing In A Conducted Environment
Date:
Authors:
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C. Wright, Azimuth Systems
C. Wright, Azimuth Systems

2. November 2006
doc.: IEEE /1839r0
November 2006
Abstract
This presentation describes how to augment the TGT conducted environment to measure the performance of MIMO WLAN systems with and without the presence of simulated multipath conditions.
C. Wright, Azimuth Systems
C. Wright, Azimuth Systems

3. November 2006
Summary
Multipath is an important factor influencing the performance of WLANs
Also, MIMO systems present some interesting challenges to conducted testing
How do you cable together two MIMO systems?
The current TGT draft does not contain any recommendations on how to make repeatable tests of WLAN devices in the presence of multipath
This proposal contains
A recommendation that the TGn channel models be used as the baseline channels
A description of a conducted test environment for measuring multipath performance of two WLAN devices
C. Wright, Azimuth Systems

4. Current Throughput vs Attenuation Test Configuration
November 2006
Current Throughput vs Attenuation Test Configuration
A traffic source sends traffic through, e.g., the AP to the STA (downstream) and throughput is measured
Attenuator in RF path is stepped to new setting
Plot of throughput vs. total path loss is created
Tx power is measured to enable comparing different devices
C. Wright, Azimuth Systems

5. MIMO version of Throughput vs Attenuation
November 2006
MIMO version of Throughput vs Attenuation
Traffic generator and analyzer required, as usual
Need to have up to 4 controllable attenuators
Assuming 4x4 MIMO is maximum
A MIMO channel is introduced
More to come on next slide
Otherwise same procedure and same set of results
C. Wright, Azimuth Systems

6. Possible Choices for MIMO Channel, H
November 2006
Possible Choices for MIMO Channel, H
Fixed channels
Identity matrix
Butler matrix
Other
Time-varying channels
TGn channel models
C. Wright, Azimuth Systems

7. Identity Matrix Fixed Channel
November 2006
Identity Matrix Fixed Channel
Channel matrix representation is identity matrix
Consists simply of connecting antennas directly between devices
Drawbacks
Can’t adequately connect devices with dissimilar numbers of antennas
Does not exercise receiver diversity
C. Wright, Azimuth Systems

8. Butler Matrix is A Better Choice
November 2006
Butler Matrix is A Better Choice
Butler matrix forms the “spatial Discrete Fourier Transform”
Passive Butler matrices are readily available
Channel matrix elements are unit gain with a phase angle:
Important features:
Well-conditioned channel
Unity gain
Can connect dissimilar number of antennas
C. Wright, Azimuth Systems

9. Example Test Setup with Butler Matrix
November 2006
Example Test Setup with Butler Matrix
MIMO version of Fig 14 of D0.14 for 3x2 MIMO setup
Ports used on Butler matrix are arbitrary
Must measure power of transmitters if intention is to compare throughput curves of different systems
Reverse positions of Traffic Gen/Analysis and power meter for upstream test
C. Wright, Azimuth Systems

10. Butler Matrix is not the only possibility
November 2006
Butler Matrix is not the only possibility
Main requirement is the channel matrix should be:
Equal gain for each transmit antenna in reaching a receiver antenna
Full rank and well-conditioned
Here is another possibility:
The problem is there are many possibilities, so it would be nice to choose a single one
2x2 version
C. Wright, Azimuth Systems

11. For testing effect of multipath, should use TGn channel models
November 2006
For testing effect of multipath, should use TGn channel models
Models representing five environments of progressively longer range between endpoints
Models also specify several combinations of Tx and Rx antenna spacings
l/2, 1l, 4l
Also Rician component on 1st tap is possible
Grand total of 6*3*3*2 = 108 different possible models
C. Wright, Azimuth Systems

12. General case requires a bidirectional channel emulator
November 2006
General case requires a bidirectional channel emulator
Channel emulation and attenuation in both directions
Most accurate representation of reality
Fading between forward and return must be synchronized for so as not to create unrealistic conditions for the protocol
Required for transmit beamforming
Example implementation Others are also possible
Attenuators are set so the overall path gain is the same in both directions
4x4 mode shown; all others possible as well
C. Wright, Azimuth Systems

13. Unidirectional channel emulator is less useful
November 2006
Unidirectional channel emulator is less useful
Useful for most current SISO systems
Useful for low-level system/device/algorithm debugging
Possibly useful in non-beamformed MIMO systems
Problem that it’s hard to tell whether a MIMO system will do better with a unidirectional
One major drawback:
Shouldn’t use for measuring performance of arbitrary devices because you don’t necessarily know whether the device will perform its best under these circumstances
Example implementation Others are also possible
Attenuators are set so the overall path gain is the same in both directions
4x4 mode shown; all others possible as well
C. Wright, Azimuth Systems

14. Example throughput curve for commercially available Draft-N devices
November 2006
Example throughput curve for commercially available Draft-N devices
Bidirectional emulation
2 minutes per data point
Single-vendor AP and NIC
C. Wright, Azimuth Systems

15. Performance difference for unidirectional vs. bidirectional emulation
November 2006
Performance difference for unidirectional vs. bidirectional emulation
2 minutes per data point
Same single-vendor AP and NIC
C. Wright, Azimuth Systems

16. The TGn models also apply to SISO
November 2006
The TGn models also apply to SISO
The models were developed for the MIMO case
But multipath for SISO systems is a special case of MIMO where the number of antennas on both nodes is one
But also, many “SISO” systems are actually “SIMO”: one transmit antenna and two receive antennas
Switched diversity, or
Maximal Ratio Combining (MRC)
The TGn models naturally apply to these situations as well
Existing commonly-used SISO models [2], [3] are truly SISO-only and are not capable of accurate diversity testing
C. Wright, Azimuth Systems

17. November 2006
Proposed changes to the TGT draft for implementing MIMO throughput tests
Changes are mainly implemented as an augmentation to the conducted test environment (subclause 5.3)
Several new figures are added to describe the MIMO setup
Guidelines are described for proper use of these setups
TGn models specification
This text is not yet complete, but submitted to give an idea of how this would be implemented in the draft
C. Wright, Azimuth Systems

18. November 2006
doc.: IEEE /1839r0
November 2006
Motion
Not Ready Yet!
Motion to accept proposal in document 11-06/xxxxr0 into the P draft
Move/second:
Y/N/A:
C. Wright, Azimuth Systems
C. Wright, Azimuth Systems

19. November 2006
References
[1] IEEE /940r4, “TGn Channel Models”, V. Erceg, et al
[2] IEEE /96, “Tentative Criteria for Comparison of Modulation Methods”, N. Chayat
[3] 3ERI085B, “Channel models for HIPERLAN/2 in different indoor scenarios”, J. Medbo, et al
C. Wright, Azimuth Systems