Doc.: IEEE 802.11-05/1198r0 Submission Nov 2005 Uriel Lemberger, IntelSlide 1 TGT Power and EVM...

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Nov 2005

Uriel Lemberger, Intel

doc.: IEEE 802.11-05/1198r0

Submission

TGT Power and EVM measurements

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.

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Date: Sept 20, 2005Name Company Address Phone email

Uriel Lemberger Intel PO Box 1659, Matam Industrial Park, Haifa 31015 Israel

+972-4-865-5701 uriel.lemberger@intel.com

Alexander Tolpin Intel PO Box 1659, Matam Industrial Park, Haifa 31015 Israel

+972-4-865-5430 alexander.tolpin@intel.com

Neeraj Sharma Intel 13290 Evening Creek Dr, San Diego CA 92128

(858) 385-4112 neeraj.k.sharma@intel.com

Nir Alon Intel

PO Box 1659, Matam Industrial Park, Haifa 31015 Israel

+972-4-865-6621 nir.alon@intel.com

Authors:

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doc.: IEEE 802.11-05/1198r0

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Abstract

This document introduces the description of transmit power measurement and Transmit EVM measurements in Conductive Test Environment as a part of Recommended Practice for the Evaluation of 802.11 Wireless Performance. We are seeking to get feedback from TGT group on “work under progress” in this direction.

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Summary

• Purpose

• Test Equipment

• Transmit power measurement

• Transmit EVM measurement

• Conclusion

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Purpose of Power and EVM measurements

• Provide calibrated Tx level which is required for many tests.– for example, TPT vs. Attenuation requires the knowledge of Tx

power to correctly correlate the TPT to the Rx signal level.

• Provide TX EVM which can effect TPT performance tests.– for example, TPT can degrade when TX EVM is low.

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Main Test Equipment

• DUT – any wireless 802.11 device (AP or Client) that includes relevant SW running on the specific platform

• WLCP (WireLess CounterPart) - reference AP or a reference Client.• Optional Shielded enclosure for DUTs and WLCPs in order to isolate

from extraneous signals– Usually not required since the measured signal is much stronger than any possible

interferer. It is commonly used with other tests that requires shielding.• Cables

– RF-cables – connected to antenna connectors.– Wired LAN cables– Control cables

• Attenuators – to close the RF link.

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Main Test Equipment (cont.)

• Power Meter Device – to measure RF signal power per packet.Alternative options will be presented in this document.

• Calibrated combiners, splitters and couplers – to handle different RF path, including antennas entries.

• Wired Traffic Generator to generate data traffic from DUT to WLCP on top of layer 2.

• Optional - Wired Traffic Analyzer to gather delivered data payload over time through wired interface on top of layer 2.

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Main Test Equipment (cont.)

Test controller Includes the following capabilities, likely automated and controlled by dedicated SW:– The ability to control TX rates and TX power of DUT– The ability to control power meter.– The ability to control Wired Traffic Generator.– Optional - The ability to control Wired Traffic Analyzer– Optional - The ability to control attenuators

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Important Notes

• The power measurement and EVM can be performed on any output port of any 802.11 component in the network.

• It is not required to have continuous transmission. The power measuring techniques presented later should have triggering mechanism that starts measuring the power only when the signal ramps and stops when there is no signal, so that duty cycle averaging won’t effect the measurement.

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TX Power measurement techniques for WLAN

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TX Power measurement techniques for WLAN

• Power meter approach

• Spectrum Analyzer approach extracted from doc doc.: IEEE 802.11-04/0935r4

• VSA (Receiver) approach

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Spectrum Analyzer approachextracted from doc doc.: IEEE 802.11-04/0935r4

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VSA approach

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TX Power Test setup

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VSA approach

• What is measured.

• How it is measured.

• Freq domain measurement – Integration of spectral density over BW– recommended.

• Time domain measurement – not recommended, sensitive to window size errors.

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Test procedure example• Set the spectrum window in the VSA at the center of the channel. With

span wider than the channel BW.

• Set the power measurement boundaries +/-BW/2 around the center.

• Set the Receiver range to be linear (for the expected TX power).

• Coupling AC 50ohm

• Trigger on IF pos slop

• Resolution Bandwidth = 23.87kHz

• Windowing type Flat top.

• Time 90% overlap with average off.

• Synchronize on channel estimation Sequence

• Demodulation (DSSS/CCK/OFDM)

• Subcarrier select all, spacing 312.5Khz, Symbol timing adjust -3.125%

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Calibration

• TBD

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EVM measurements techniques for WLAN

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EVM Overview

• Background

• EVM IEEE minimal performance specification – OFDM example.

• EVM Test setup Block diagram

• EVM test definition from IEEE 802.11 clause 17.3.9.7

• EVM Test• EVM test procedure

• Calibration

• Results Example

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EVM (OFDM)17.3.9.6.3 Transmitter constellation error

The relative constellation RMS error, averaged over subcarriers, OFDM frames, and packets, shall notexceed a data-rate dependent value according to Table 90.

Table 90—Allowed relative constellation error versus data rate

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TX EVM Test setup

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17.3.9.7 Transmit modulation accuracy testThe sampled signal shall be processed in a manner similar to an actual receiver, according to the followingsteps, or an equivalent procedure:a) Start of frame shall be detected.b) Transition from short sequences to channel estimation sequences shall be detected, and fine timing(with one sample resolution) shall be established.c) Coarse and fine frequency offsets shall be estimated.d) The packet shall be derotated according to estimated frequency offset.e) The complex channel response coefficients shall be estimated for each of the subcarriers.f) For each of the data OFDM symbols: transform the symbol into subcarrier received values, estimatethe phase from the pilot subcarriers, derotate the subcarrier values according to estimated phase, anddivide each subcarrier value with a complex estimated channel response coefficient.g) For each data-carrying subcarrier, find the closest constellation point and compute the Euclidean distancefrom it.h) Compute the RMS average of all errors in a packet. It is given by:(28)whereLP is the length of the packet;Nf is the number of frames for the measurement;(I0(i,j,k), Q0(i,j,k)) denotes the ideal symbol point of the ith frame, jth OFDM symbol of theframe, kth subcarrier of the OFDM symbol in the complex plane;(I(i,j,k), Q(i,j,k)) denotes the observed point of the ith frame, jth OFDM symbol of the frame,kth subcarrier of the OFDM symbol in the complex plane (see Figure 121);

P0 is the average power of the constellation.The vector error on a phase plane is shown in Figure 121.The test shall be performed over at least 20 frames (Nf), and the RMS average shall be taken. The packetsunder test shall be at least 16 OFDM symbols long. Random data shall be used for the symbols.

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Equation 28 and figure 121

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EVM test procedure example• Set the spectrum window in the VSA at the center of the channel. With

span wider than the channel BW.• Set the power measurement boundaries +/-BW/2 around the center.• Set the Receiver range to be linear (for the expected TX power).• Coupling AC 50ohm• Trigger on IF positive slop• Resolution Bandwidth = 23.87kHz• Windowing type Flat top.• Time 90% overlap with average off.• Synchronize on channel estimation Sequence• Demodulation (DSSS/CCK/OFDM)• I/Q normalize• Pilot Track Phase & Timing• Equalizer training on channel estimation sequence only• Subcarrier select all,• Spacing 312.5Khz, • Symbol timing adjust -3.125%

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Calibration

• TBD

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VSA Approach Results Example

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Conclusions

• These are Important secondary metrics.

• Tx power is required in most tests in order to get correct Signal strength in different location in the Link.

• EVM results can help analyze TPT anomalies.– It is important to verify good TX EVM when testing RX

performance of the counterpart.

• The proposed methodology is applicable for testing in a full system.

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References

• [1] IEEE 802.11-1999, P802.11a -1999, P802.11b -1999

• [2] IEEE 802.11-05/0661r0 TGT Conductive Test Environment and Metrics. Alexander Tolpin.

• [3]P802.11.2-D0.4 - Draft Recommended Practice for the Evaluation of 802.11 Wireless Performance

Doc.: IEEE 802.11-05/1198r0 Submission Nov 2005 Uriel Lemberger, IntelSlide 1 TGT Power and EVM...

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