Doc.: IEEE 802.11-05/0943r0 Submission September 2005 Dr. Michael D. Foegelle, ETS-LindgrenSlide 1...

September 2005

Dr. Michael D. Foegelle, ETS-Lindgren

Slide 1

Doc.: IEEE 802.11-05/0943r0

Submission

Conducted Power and Sensitivity 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.

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.

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Date: 2005-9-20

Name Company Address Phone email Dr. Michael D. Foegelle ETS-Lindgren 1301 Arrow Point Drive

Cedar Park, TX 78613 (512) 531-6444 [email protected]

Authors:

September 2005


Slide 2

Doc.: IEEE 802.11-05/0943r0

Submission

Abstract

This presentation demonstrates techniques for measuring the transmit power and receiver sensitivity of individual DUTs using off-the-shelf test equipment with traceable calibrations.

The results of measurements made using these techniques are compared to conducted rate vs. range measurements made using a variable attenuator inserted between to DUTs.

September 2005


Slide 3

Doc.: IEEE 802.11-05/0943r0

Submission

Overview

• Introduction

• Transmit Power

• Receiver Sensitivity

• Individual DUT tests compared to variable attenuator rate vs. range tests.– RF Link Calculations

– Rate vs. Range measurement

– Comparisons

– Problem Diagnostics

• Conclusions

September 2005


Slide 4

Doc.: IEEE 802.11-05/0943r0

Submission

Introduction

• The biggest limitation to most of the existing techniques proposed for measuring the RF performance of 802.11 devices is that these techniques rely on system level testing.

– Devices are tested as a pair or more.

• Testing the transmit power and sensitivity of each individual DUT allows prediction of the resulting system performance for any combination of DUTs.

September 2005


Slide 5

Doc.: IEEE 802.11-05/0943r0

Submission

Introduction

• There are a number of important issues these tests must be able to address to obtain desired results.– Measure DUT in native operational mode (ensure testing can be

performed by independent parties and discourage use of test modes that may not mirror normal operation).

– Control the data rate being tested.

– Keep the DUT at full power.

– Generate necessary traffic, ideally without modifying DUT.

• Possible to meet these criteria with most DUTs.

September 2005


Slide 6

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• Conceptually simple procedure:– Generate traffic from DUT at full power with desired data rate and

packet size.

– Measure average power of one or more packets.

• Generating traffic from the DUT:– A simple technique that will work for most 802.11 devices is to

use the ICMP Echo request (ping the DUT with the desired packet size). Doesn’t require client software, etc.

– Operate DUT in typical use case to test packets from normal traffic.

September 2005


Slide 7

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• Controlling Data Rate– Interface to traffic generator with dot11SupportedDataRatesRx table

set to desired data rate.– Send Echo request at desired data rate (doesn’t work in all cases)– Configure DUT to transmit desired data rate (requires some level of

DUT control)– Selectively acknowledge packets to force DUT to desired data rate.

• Measuring power of packets:– Use calibrated wide bandwidth device such as vector signal analyzer

or wideband power meter.– Detect pulse from DUT and calculate average power.

September 2005


Slide 8

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• Sample Echo traffic at 11 MBPSTime Response

Res

po

nse

(d

Bm

)

Time (ms)0 30.5 1 1.5 2 2.5

-60

20

-50

-40

-30

-20

-10

0

10

September 2005


Slide 9

Doc.: IEEE 802.11-05/0943r0

Submission

Time Response

Res

po

nse

(d

Bm

)

Time (ms)1.5 2.41.6 1.7 1.8 1.9 2 2.1 2.2 2.3

-16

-14.8

-15.8

-15.6

-15.4

-15.2

-15

Transmit Power

• Anatomy of an 11 MBPS 802.11 PPDU Packet

September 2005


Slide 10

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• PLCP Preamble and Header not necessarily at same power level as PSDU.– Differences in modulation schemes and data content can result in

different levels.

• Power level of PSDU (more complex modulation) determines relationship with sensitivity of receiver.

• Ideally only measure power of PSDU.

• Also need to stay away from edges (rise/fall) of packet.– Typically use center 85% of packet for wireless protocols (GSM &

TDMA).

September 2005


Slide 11

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• Effect of modulation on transmit power.– Largest amplitude (peak power) determines limits on output power.

– Constant envelope modulations can use total power of output amplifier (average power = peak power).

– Non-constant modulation schemes such as QAM result in average power less than maximum output power of amplifier for same peak signal.Constant Amplitude Modulation (0 dB)

Sig

nal

Am

plit

ud

e

Bit Number0 81 2 3 4 5 6 7

-1

1

-0.6

-0.2

0.2

0.6

Non-Constant Amplitude Modulation (-2 dB)

Sig

nal

Am

plit

ud

e

Bit Number0 81 2 3 4 5 6 7

-1

1

-0.6

-0.2

0.2

0.6

September 2005


Slide 12

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• Effect of modulation on transmit power.802.11a Average Packet Power vs. Data Rate

Ave

rag

e P

ow

er (

dB

m)

Data Rate (MBPS)6 5410 15 20 25 30 35 40 45 50

-28

-24.5

-27.5

-27

-26.5

-26

-25.5

-25

September 2005


Slide 13

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• Conducted Transmit (Output) Power Measurement Configuration.

DUT

Shielded Enclosure(as needed)

TrafficGenerator

Directional Coupler

CalibratedReceiver

Communication Path

CalibratedMeasurement Path

Variable Attenuator

September 2005


Slide 14

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power• Conducted Transmit (Output) Power Measurement

Configuration.– Cabled connection to DUT required.– Traffic generator (PC with wireless NIC) pings DUT

• If possible, DUT can also ping TG. Simplifies pulse triggering slightly.

– Variable attenuator keeps path loss between TG and DUT at a point where input stages are not overloaded.

• If necessary, can also be used to help ensure full power and/or control TX data rate of DUT.

– Directional coupler allows DUT signal to be routed to measurement receiver/power meter at higher level than that from TG.

– Calibrated measurement path from DUT to receiver allows correction for path loss introduced by cabling and directional coupler.

– Optional shield around DUT eliminates unwanted extraneous signals and/or inter-device coupling.

September 2005


Slide 15

Doc.: IEEE 802.11-05/0943r0

Submission

D-Link DWL-AG530 PCI NIC 11 MBPS Transmit Power vs. Time (Avg = 16.7 dBm)

Tra

nsm

it P

ow

er (

dB

m)

Time (s)0 6010 20 30 40 50

16

18

16.2

16.4

16.6

16.8

17

17.2

17.4

17.6

17.8

Transmit Power

• Component 1: PCI NIC.

September 2005


Slide 16

Doc.: IEEE 802.11-05/0943r0

Submission

D-Link DWL-AG700AP AP 11 MBPS Transmit Power vs. Time (Avg = 15.3 dBm)

Tra

nsm

it P

ow

er (

dB

m)

Time (s)0 6010 20 30 40 50

14

16

14.2

14.4

14.6

14.8

15

15.2

15.4

15.6

15.8

Transmit Power

• Component 2: “Single Antenna” AP.

September 2005


Slide 17

Doc.: IEEE 802.11-05/0943r0

Submission

Transmit Power

• Varying levels of signal stability.– Different packet content can result in different power levels for

non-constant envelope modulations.

– Other internal oddities of a given design

• Points to the need to average multiple packets to get a valid reading of transmit power.

• For DUTs with marked max/min ratio (i.e. 1 dB or more) it may be desirable to use the minimum value for link budget calculations.– At a minimum, deviation should be reported.

• Other interesting effects observed.– 20 dB drop in TX signal level at low path loss (explanation later)

September 2005


Slide 18

Doc.: IEEE 802.11-05/0943r0

Submission

Receiver Sensitivity

• Again, conceptually simple procedure:– Perform Packet Error Rate (PER) test by:

• Sending packets to DUT at a fixed level with desired data rate and packet size.

• Counting ACKs received from DUT.

– Repeat PER test at continually lower packet power levels until desired sensitivity level is detected (typically 10% PER).

• Does not require a continuous search curve. Can be optimized starting with larger steps and narrowing down on target.

September 2005


Slide 19

Doc.: IEEE 802.11-05/0943r0

Submission


• PER tester requires some special capabilities:– Packet source must be separate from receiver used to count ACKs.

• Must be able to reduce the PER tester output power across a broad dynamic range without affecting the receiver.

• Receiver needs its own attenuator (and possibly amplifier) to keep received ACKs in optimum range of receiver.

• Ensures that missed ACKs are due to the DUT failing to decode the packet and not due to the PER tester missing the response.

• Can be same radio with separate transmit/receive paths.

– Packet source needs to offer calibrated output power levels.• Build around stable calibrated source (VSG, Golden Radio).

• Use power meter to measure output packet power real-time.

September 2005


Slide 20

Doc.: IEEE 802.11-05/0943r0

Submission


• Possible conducted PER setup:

DUT

Shielded Enclosure(required)

CalibratedPacket

Generator

Directional Coupler

ACKCounter Low-Loss

ACK Detection Path

Variable Attenuator

CalibratedMeasurement Path

September 2005


Slide 21

Doc.: IEEE 802.11-05/0943r0

Submission


• Shielded enclosure around DUT is a MUST. – Most DUTs have insufficient shielding to ensure that only signal

reaching DUT is conducted on cable.

– Need to isolate DUT to ensure that external interference/collisions don’t affect measured PER.

• Packet generator and detector must have a high level of shielding as well.– Use of non-specialized test equipment (off-the-shelf 802.11

components) requires additional shielding.

• Several off-the-shelf and system solutions available.– Anritsu MT8860A used here.

September 2005


Slide 22

Doc.: IEEE 802.11-05/0943r0

Submission


• Component 1: PCI NIC.PER vs. RX Power, D-Link DWL-AG530 PCI NIC 11 MBPS, (10% PER = -86.2 dBm)

Pac

ket

Err

or

Rat

e (%

)

Receive Power (dBm)-90 0-80 -70 -60 -50 -40 -30 -20 -10

0

100

20

40

60

80

September 2005


Slide 23

Doc.: IEEE 802.11-05/0943r0

Submission

PER vs. RX Power, D-Link DWL-AG700AP AP 11 MBPS, (10% PER = -87.7 dBm)

Pac

ket

Err

or

Rat

e (%

)

Receive Power (dBm)-92 -10-80 -70 -60 -50 -40 -30 -20

0

100

20

40

60

80



September 2005


Slide 24

Doc.: IEEE 802.11-05/0943r0

Submission

RF Link Calculations.

• Combining the Transmit Power of Device 1 with the Sensitivity of Device 2 gives the NIC to AP link performance:

16.7 dBm – (-87.7 dBm) = 104.4 dB

• Combining the Transmit Power of Device 2 with the Sensitivity of Device 1 gives the AP to NIC link performance:

15.3 dBm – (-86.2 dBm) = 101.5 dB

September 2005


Slide 25

Doc.: IEEE 802.11-05/0943r0

Submission

Rate vs. Range

• Throughput from Device 1 to Device 2.

Throughput vs. Attenuation DWL-AG530 to DWL-AG700AP (Falloff starts at ~103 dB)

Th

rou

gh

pu

t (

Mb

ps)

Attenuation (dB)28.2 109.335 40 45 50 55 60 65 70 75 80 85 90 95

2

11

3

4

5

6

7

8

9

10

September 2005


Slide 26

Doc.: IEEE 802.11-05/0943r0

Submission

Comparison of Conducted Power/Sensitivity with Throughput vs. Attenuation

101.5 dB < 103 dB < 104.4 dB

September 2005


Slide 27

Doc.: IEEE 802.11-05/0943r0

Submission


• ~1.5 dB uncertainty in result not unexpected. – Attenuation step size is 1 dB for thoughput test and sensitivity test.

– Power measurement uncertainty >0.5 dB

• It should be expected that the NIC to AP link will be the primary contribution given that the traffic is from NIC to AP.

• Interesting to look at this link vs. data rates…

September 2005


Slide 28

Doc.: IEEE 802.11-05/0943r0

Submission


• PER can be measured for each data rate.PER vs. Received Power for Each 802.11b Data Rate.

Pac

ket

Err

or

Rat

e (%

)

Received Power (dBm)-96 -70-90 -85 -80 -75

0

100

20

40

60

80

1 MBPS 2 MBPS 5.5 MBPS 11 MBPS

September 2005


Slide 29

Doc.: IEEE 802.11-05/0943r0

Submission

Expected Throughput vs. Received Power for Each 802.11b Data Rate.

Th

rou

gh

pu

t (M

BP

S)

Received Power (dBm)-96 -70-90 -85 -80 -75

0

12

2

4

6

8

10

1 MBPS 2 MBPS 5.5 MBPS 11 MBPS


• Scaling PER by data rate gives throughput.

September 2005


Slide 30

Doc.: IEEE 802.11-05/0943r0

Submission

Expected Throughput vs. Path Loss for Each 802.11b Data Rate.

Th

rou

gh

pu

t (M

BP

S)

Path Loss (dB)86.7 112.790 95 100 105 110

0

12

2

4

6

8

10

1 MBPS 2 MBPS 5.5 MBPS 11 MBPS Max


• Adding TX Power gives Throughput vs. Path Loss.

September 2005


Slide 31

Doc.: IEEE 802.11-05/0943r0

Submission

Comparison between Predicted and Measured Throughput vs. Path Loss

Th

rou

gh

pu

t (M

BP

S)

Path Loss (dB)86.7 108.790 95 100 105

2

11

4

6

8

Predicted Measured


• Comparing prediction to measured values…

September 2005


Slide 32

Doc.: IEEE 802.11-05/0943r0

Submission


• Throughput from Device 1 to Device 2.

Throughput vs. Attenuation DWL-AG530 to DWL-AG700AP (Falloff starts at ~103 dB)

Th

rou

gh

pu

t (

Mb

ps)

Attenuation (dB)28.2 109.335 40 45 50 55 60 65 70 75 80 85 90 95

2

11

3

4

5

6

7

8

9

10

September 2005


Slide 33

Doc.: IEEE 802.11-05/0943r0

Submission

PER vs. RX Power, D-Link DWL-AG700AP AP 11 MBPS, (10% PER = -87.7 dBm)

Pac

ket

Err

or

Rat

e (%

)

Receive Power (dBm)-92 -10-80 -70 -60 -50 -40 -30 -20

0

100

20

40

60

80



September 2005


Slide 34

Doc.: IEEE 802.11-05/0943r0

Submission

Pulse Power vs. Time

Po

wer

(d

Bm

)

Time (ms)0 40.5 1 1.5 2 2.5 3 3.5

-60

20

-50

-40

-30

-20

-10

0

10



September 2005


Slide 35

Doc.: IEEE 802.11-05/0943r0

Submission


• Component 2: “Single Antenna” AP has a sensitivity problem and apparently a transmit signal level problem!

• Turns out the internal circuitry is based on a diversity reference design. At high RX power, the diversity channel (20 dB down from the test channel as seen through the diversity switch) leaks through and begins trying to communicate, resulting in packet errors. Above a certain level, the leakage from the unused diversity channel takes over (TX Power -20 dB from max!) and is used for all communication.

• Points to a possible problem using this AP in close proximity to the client.

September 2005


Slide 36

Doc.: IEEE 802.11-05/0943r0

Submission

Conclusions

• This presentation shows clearly that it is possible to measure transmit power and sensitivity of individual DUTs using off-the-shelf components and accurately reproduce the results of rate vs. range type measurements performed on pairs of devices.

• The ability to qualify individual devices provides desired performance criteria for a specific product by itself.

• This testing also provides needed diagnostic information.

• This measurement method can now be extended for OTA testing.

Doc.: IEEE 802.11-05/0943r0 Submission September 2005 Dr. Michael D. Foegelle, ETS-LindgrenSlide 1...

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