I n t e g r i t y - S e r v i c e - E x c e l l e n c e · Signal Spectrum 15th Annual ITEA Test Instrumentation Workshop. Las Vegas May 9-12, 2011 1 Approved for Public Release:

I n t e g r i t y - S e r v i c e - E x c e l l e n c e

412th Electronic Warfare Group

Edwards AFB

War Winning Capabilities… On Time, On Cost

Multipath Fading Cancellation:

Using A Tap Delay To Improve

Signal Spectrum

15th Annual ITEA Test Instrumentation

Workshop. Las Vegas

May 9-12, 2011

1

Approved for Public Release:

Distribution is unlimited.

AFFTC-PA-10993

../../../../folson elona/Local Settings/folson elona/

2

JT3 / 772 Test Squadron /EWG

771 Test Squadron/ EWG

812 TS/ ENG

Edwards Air Force Base

William Chen, Dr. James Brownlow, Erich Brownlow,

Jerry Phibbs, Charles D. Lane, Andrew Thornburg, Jeff Tartaglini*

A Team Effort To Get This Presentation Here

Jeff Tartaglini * previously worked at 772 TS

Unclassified. AFFTC-PA-10993. Distribution A: Approved for public release: distribution is unlimited.

3

The Background

Digital Tap Delay

Theoretical Approach of Digital Tap Delay

Algorithm Implementation of Digital Tap Delay

Experiment and Result

Discussion. Acknowledgements. Reference.

Acronyms


Multipath Fading Cancellation: Using A Digital Tap Delay To Improve Signal Spectrum


Using A Digital Tap Delay To Improve Signal Spectrum

The Background

4Unclassified. AFFTC-PA-10993 . Distribution A: Approved for public release: distribution is unlimited.

5Unclassified. AFFTC-PA-10993. Distribution A: Approved for public release: distribution is unlimited.

Radio wave Tx propagation #1

Radio wave Tx propagation #2

Single or multi-channel receiver

Multipath Phenomena Occurs At Wave Propagation

Figure(1) Multipath Occurs at Radar, Communication and

Other Indoor or Outdoor Wave Propagation Activities.

Especially at Air To Ground Transmission

Multipath Cause Fading At Radar,

Communication Signal Processing


Example of Slow fading

Am

pli

tud

e

Time

Example of

fast fading

Figure(2B) Fast fading is signal amplitude

changing rapidly with time.

System changes too fast to follow

Figure(2A) Slow fading is variations in

amplitude changing slowly with time.

System may have time to react in some way

Example: A 3D View of Main and Multipath Signal


Main signal (LOS)

Multipath signals

Figure (3) 3D plot of main and multipath signal



The Background

Digital Tap Delay

Theoretical and Algorithm Approach


Theoretical & Algorithm Approach of

Digital Tap Delay (1)


Averaging

tap delay

output to

canceller

Signal input from

receive channel

Weight factor

Weight factor

Tap delay #1

Tap delay #2

Figure(4) Tap Delay Cancelling of Multipath Fading Technique

Note: Weight factor applied to multipath signal only.

Keep direct path signal data intact.




Figure(5) Basic Tap Delay Line Data Flow

Note: Data flow applied to multipath signal only.

Keep direct path signal data intact.

Note: Weight factor applied to multipath signal only

Z-1 is the delay operator

b [i] is the delay weighting factor

x[n] is input data, at time n

y[n] is output data, at time n



11

Modeling the output signal as

y(n) = b(n)*x(n) =

b(n) = (R)-1W’x(n) = (R)-1S

R is the expected value of test signal covariance matrix from tap

delay operation. R=E(x * xT)

S is antenna steering vector at receive channel. Assumed unity at

the static case.

S can be expressed as the expected value of test signal and a replica of

the desired signal

Here, is the replica of the desired signal.


Simplified Expression




RF:

LNA,

D/C,

BPFRx /

Demod.A/D SP DP

Comm.

Display

Single

Channel

Receive

To build data

covariance matrix and

weighting factor

Apply weighting factor to cancel

interference

Figure(6) Receive Channel with Digital Tap Delay Signal Flow

Time Domain Digitized Data Was Used




R1

R2

R3 R is covariance

matrix by tap delay

data cube. R3 means 3

tap delay.

R-matrix is updated

by sliding data cube.

Sample by sample.

Figure(7) Data Cube to Generate Covariance Matrix R at Digital Tap Delay

Application




Figure(8) Tap delay covariance matrix R=E(x * xT) and weight factor are built by

this process.



15

Excitation

source

Load

Septum


The Background

Digital Tap Delay



Experiment and Result (1)


IFF

Transmit Tx

#1 Direct path

#2 Multipath delay path

IFF

Receive Rx

Splitter Combiner

Figure(9) Experiment data collection method set up



Direct path IQ signal

Delay multipath path IQ signal

Figure(10) Recorded Direct and One Delay Multipath IQ Signals



Undesired lobe

Direct + multipath delay spectrum

Figure(11) Spectrum of direct and one multipath.

Undesired lobe is the result of multipath interference.



180 190 200 210 220 230

4

6

8

10

12

14

16

18

20

22

Sampled bin number

dB

Simulated desired and multipath spectrum before tap delay

Desired direct path.

Blue

Unwanted multipath lobe.

Red

Figure(12) Direct signal with and without Multipath Interference from Lab Measured Data.

Blue trace is the desired direct path signal



0 0.5 1 1.5 2 2.5 3

x 105

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10ITC-Test-Data1-06-30-10. Part of test data file

Number of sample

Absolu

te v

alu

e d

B

PRI=21 u s

0 500 1000 1500 2000 2500 3000 3500-60

-40

-20

0Data set from sample 80000 to 83000

Number of sample

Absolu

te v

alu

e d

B

0 500 1000 1500 2000 2500 3000 3500-4

-2

0

2

4

Number of sample

Phase a

ngle

radia

n

Direct path

signal

Phase

Multipath signals

Figure (13A) Time sequence data as shown

from experiment set up at Figure (9).

Figure(13B) Magnifying one of the signal bundles

from Figure (13A).



1050 1100 1150 1200 1250

-60

-50

-40

-30

-20

Data set from sample 80000 to 83000

Number of sample

Absolu

te v

alu

e d

B

1050 1100 1150 1200 1250 1300-4

-2

0

2

Number of sample

Phase a

ngle

radia

n

Direct

path

signal

Multipath

signal

Constant phase at both signals

0 1 2 3 4 5 6 7 8 9 10

x 104

0

2000

4000

6000

Number of sample

Absoulte v

alu

e

Time signal of direct and multipath before tap delay application

0 1 2 3 4 5 6 7 8 9 10

x 104

0

2000

4000

6000

Number of sample

Absoulte v

alu

e

Time signal of direct and multipath after tap delay application

Magenta is multipath signal

Green is direct path signal

Multipath signal after

tap digital signal processing

Figure (14A) Examining phase transition

of both signalsFigure (14B). Time signal of before and after tap delay

digital signal processing



5900 5950 6000 6050 6100 6150 6200

0

2000

4000

Number of sample

Abs

oulte

val

ueTime signal of direct and multipath before tap delay application

5900 5950 6000 6050 6100 6150 6200

-2000

0

2000

4000

Number of sample

Abs

oulte

val

ue

Time signal of direct and multipath after tap delay application

Multipath signalDirect path

signal

Multipath signal

Figure (15) Signal magnifying from left plot at Figure (14B)



0 1 2 3 4 5 6 7 8 9 10

x 104

20

25

30

35

40

45

50

55

60

Sampled bin number

dB

Specturm of direct and multipath before tap delay application

Sidelobe spectrum due

to multipath interference

0 1 2 3 4 5 6 7 8 9 10

x 104

15

20

25

30

35

40

45

50

55

60Specturm of direct and multipath after tap delay application

dB

Sampled bin number

Improved sidelobe

spectrum

Figure (17) Improved sidelobe spectrum after tap

digital signal processing

Figure (16) Spectrum before tap digital

signal processing



The Background

Digital Tap Delay



Discussion. Acknowledgements. Reference.

Acronyms


Discussion

This presentation “Multipath Fading Cancellation:

Using A Digital Tap Delay To Improve Signal Spectrum” has shown the basic

processing technique to reduce unwanted multipath interference.

This technique is a similar concept to air borne radar to cancel clutter interference in

Ground Moving Target Indicator (GMTI) mode.

This method preserves the direct main path signal data without changing waveform

characteristic such as phase coding, modulation at direct path signal.

It can be very practical applied to post processing of flight test and anechoic chamber

data.

This technique, digital tap delay to cancel unwanted multipath signal , is another

method at post processing besides hardware gating and software gating.

Tap delay processing can be automated in software.


Acknowledgement

We owe thanks to our peers for diligent review of this paper,

paragraph by paragraph.

Thanks to Mr. Jeff Jessen, Chief of Installed Systems Test Flight,

for suggesting the experimental data collection method shown in

Figure (9).

Also thanks to the Communications/Navigation/Identification

(CNI) Lab, 772 Test Squadron, Electronic Warfare Group at

Edwards for generating and collecting the experimental data.

We also owe thanks to Col R. Kurtz, EWG Commander for

reviewing this paper.


References

1. Barton, David K., “Radar Error Analysis,” Modern Radar System Analysis, Norwood, Massachusetts, 1988,

pp 512 – 528.

2. Bertoni, Henry L., Radio Propagation for Modern Wireless Systems, Prentice Hall, Upper Saddle River, New

Jersey, 2000, pp 62-68.

3. Sklar, Bernard, Digital Communication, 2nd Edition, Prentice Hall, Upper Saddle River, New Jersey, 2001, pp

771-772.

4. Willis, Mike, “Fading and Multipath,” Propagation Tutorial, http://www.mike-willis.com/Tutorial/PF15.htm, 5

May 2007.

5. Huo, Di, “Simulating Slow Fading by Means of One Dimensional Stochastically Process,” 0-7803-3157-5/96,

IEEE, 1996, pp 620-622.

6. Zentner, E., and Zentner, R., “Smart Transmitting Antenna Arrays for Multipath Interference Reduction,” 46th

International Symposium Electronics in Marine, ELMAR-2004, June 2004, pp 374-379.

7. Denidni, Ahmed T., Delisle, Gilles Y., “An Adaptive Array for Multipath Effect Reduction,” proceedings from

the Antenna and Propagation Society International Symposium, 0-7803-0730.5192, IEEE, 1992, pp 1003-1006.

8. Gray, Steven D., “Multipath Reduction Using Constant Modulus Conjugate Gradient Techniques,” IEEE

Journal on Selected Areas in Communications, Vol. 10, No. 8, October 1992, pp 1300-1305.

9. Chen, William C., Kuffenkum, Chad, Hua, Benjamin, and Brownlow, James D., “Using a Virtual BAF in EW

Testing,” American Institute of Aeronautics and Astronautics 2010-1767, U.S. Air Force Test and Evaluation

Days, Nashville, Tennessee, 2-4 February 2010.

10. Oppenheim, Alan V., Schafer, Ronald W., Discrete-Time Signal Processing, Prentice Hall, Upper Saddle

River, New Jersey, 1989, pp 313-314.

11. Compton , R.T., Jr., “The Bandwidth Performance of a Two-Element Adaptive Array with Tapped Delay-Line

Processing,” IEEE Transactions on Antennas and Propagation, Vol. 36, No. 1, January 1988, pp 5-14.

12. Aeroflex, http://www.aeroflex.com.

13. Widrow, Bernard, Stearns, Samuel, Adaptive Signal Processing, Prentice-Hall, Upper Saddle River, New

Jersey, 1985.

http://www.mike-willis.com/Tutorial/PF15.htmhttp://www.mike-willis.com/Tutorial/PF15.htmhttp://www.mike-willis.com/Tutorial/PF15.htm

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I n t e g r i t y - S e r v i c e - E x c e l l e n c e · Signal Spectrum 15th Annual ITEA Test Instrumentation Workshop. Las Vegas May 9-12, 2011 1 Approved for Public Release: