MIMO Testing In A Conducted Environment November 2006 doc.: IEEE 802.11-06/1839r0 November 2006 MIMO Testing In A Conducted Environment Date: 2006-11-10 Authors: Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <stuart.kerry@philips.com> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at <patcom@ieee.org>. C. Wright, Azimuth Systems C. Wright, Azimuth Systems

November 2006 doc.: IEEE 802.11-06/1839r0 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

November 2006 doc.: IEEE 802.11-06/1839r0 November 2006 Motion Not Ready Yet! Motion to accept proposal in document 11-06/xxxxr0 into the P802.11.2 draft Move/second: Y/N/A: C. Wright, Azimuth Systems C. Wright, Azimuth Systems

November 2006 References [1] IEEE 802.11-03/940r4, “TGn Channel Models”, V. Erceg, et al [2] IEEE 802.11-97/96, “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