IEEE NJ Coast Seminar, 12/01/2008Walid Ahmed et. al. Performance Analysis of a Novel Real- Time Closed-Loop Technique for Non- Linearity Correction of.

IEEE NJ Coast Seminar, 12/01/2008Walid Ahmed et. al.

Performance Analysis of a Novel Real-Time Closed-Loop Technique for Non-

Linearity Correction of Power Amplifiers

Walid K. M. Ahmed, Senior Member IEEE

Qing Li, Member IEEE

Ajit Reddy, Senior Member IEEEThis work has been done while the authors were with Tyco Electronics

IEEE NJ Coast Seminar, 10/28/2008Walid K. M. Ahmed

Outline

General Closed-Loop Block Diagram. Motivation: The EDGE/GSM Challenge! Predictive Algorithm. DTx System Model and PA Closed-

Loop Correction. Results.


Closed-Loop Approach: General

Generic Pictorial Illustration of a Block Diagram for the Proposed Technique

Target/Reference

Signal

Non-LinearDevice

Detection/Measurement

Module

Pre-DistortionModule


Motivation: The EDGE/GSM Challenge

Highly efficient PA implies severe non-linearity. Hence, PA characteristics need to be corrected for, or linearized, in baseband, which involves pre-distortion of the baseband signal prior to modulating the PA. Otherwise, acceptable spectral and waveform quality performance can not be achieved for the transmitted signal.

The severe PA non-linearity is also expected to change versus many parameters, such as temperature, PA power level setting (PA bias), PA load (VSWR), PA part-to-part tolerance and aging.

Open-loop (factory calibration) of such a non-linear PA behavior is difficult since it is not possible to store such a large number of PA correction table versus so many parameters.

On-chip non-real-time closed-loop solutions, e.g., calibrate the PA before every call or when turning the phone on and use the generated tables for the entire transmission duration, may not work if the PA is expected to change its AMAM/AMPM behavior during transmission due to, for example, load and VSWR changes.


Motivation: The EDGE/GSM Challenge Hence, frequent calibration is needed, which implies

the need for an adaptive closed-loop solution. Two challenges come with such an approach:

When to calibrate? Standards usually do not allocate off-air time for such an operation to be done!

How to maintain signal quality while calibrating and transmitting at the same time?



In EDGE/GSM, for example, the switching-transient requirements are very stringent. It is not possible to let the PA run free of correction (i.e., without linearization) at any point in time. It is also not possible to have inaccurate correction tables applied to the PA. Moreover, a high-efficient PA will have a high level of non-linearity that will change from one EDGE/GSM burst to the next due to (at least) load/VSWR changes. Hence, pre-stored (or pre-loaded) correction tables will not suffice.

The question then becomes: How and when do we calibrate the highly-non-linear PA in an EDGE/GSM application EVERY BURST?!!



The answer to the question of WHEN is attained by calibrating the PA on the EDGE/GSM burst ramp-up.

In order to do that, the power ramp-up profile must be designed in a special way to ensure all amplitude levels are covered, yet a smooth rising ramp is achieved in order not to violate the tough EDGE/GSM switching-transients requirements. A simple, novel, robust and arbitrarily tunable ramp-up

design technique has been proposed by Ahmed (see Walid K. M. Ahmed, “Method and Apparatus for Signal Power Ramp-Up in a Communication Transmitter”. Patent pending, U.S. Patent Serial #

11/385,212).



Augm

ente

d Sym

bols

for R

ampin

g up

Tail Symbols of a Normal Burst



The answer to the question of HOW is the focus of this presentation.



...The chicken and egg problem: A major challenge in calibrating on the ramp is that while collecting the PA

response data, we are also transmitting! Hence, we must have correction data available to ensure quality transmission…but didn’t I just say we don’t even have correction yet since we are still calibrating!!

Moreover, the attempt at performing such an on-the-fly correction/linearization of the PA while ramping up will suffer from memory effects, that is, the collected measurements of the PA AM/AM and AM/PM non-linearity lag behind the current bias point of the PA due to the inevitable forward and feedback loop path delay. Hence, resulting-in inaccurate correction (or inversion of the PA non-linearity), which clearly leads to un-acceptable performance that fails the stringent requirements of the EDGE/GSM specifications.

Accordingly, one must use a mechanism that can break into the future in some sense and predict what the next non-linearity point on the PA curve would look like based on previous on-the-fly measurements!

A novel algorithm has been proposed by Ahmed and Li (see Walid K. M. Ahmed and Qing Li, “Method and Apparatus for a Non-Linear Feedback Control System”. Patent pending)



An Example of EDGE time-burst/time-mask/ST graph



ST performance without AM/PM correction during ramp-up


Predictive Algorithm

PA Output

(y)Actually

measured

PA Input

(x)

PA Output

Actually

measured

Required Point

PA Input

( )yfx 1-=

Begin Prediction

Path Delay Elapsed?

Path Delay Elapsed?

Extrapolate Corrected PA Input

Extrapolate Corrected PA Input

Apply PA Input Uncorrected, or from Preloads

Apply PA Input Uncorrected, or from Preloads

Prediction Time Over?Prediction

Time Over?

Stop and Use Measured Data as Correction LUT for Modulation

Stop and Use Measured Data as Correction LUT for Modulation

Simplified Pictorial Illustration of the Non-Linear Predictive Algorithm


Predictive Algorithm

Xi-1 Xi

dX = Xi– Xi-1

dy = yi – yi-1yi-1

yi

Xi

Yidy/dx

If for any xi in set X, there is a yi in set Y following the functionyi = f(xi) (i = 1, 2, …n),

then we can find Yi = f(Xi) = f(xi) + f ’(xi)*(Xi – xi) + 1/(2!) f ’’(xi)*(Xi – xi)2 + …

If (Xi – xi) is small enough, we can simplify it to linear (1st order) extrapolating:Yi = f(Xi) f(xi) + f ’(xi) * (Xi – xi) = yi + dy/dx * (Xi – xi)

yn

xi yi

yi = f(xi)x1

x2

xn y1 y2

Xi Yi


DTx System Model

AmplitudeCorrection

+ PhaseModulator

AFE

Antenna

PA

Low Band

High Band

PA

PhaseCorrection

AmplitudeEstimation

PhaseEstimation

Detector

Multi DAC

Sine/CosineGeneration

Phase

Amplitude AmplitudeCorrection

+ PhaseModulator

AFE

Antenna

PA

Low Band

High Band

PA

PhaseCorrection

AmplitudeEstimation

PhaseEstimation

Detector

Multi DAC

Sine/CosineGeneration

Phase

AmplitudeA “differential-phase” down-converter is used, i.e., RF input and RF output of the PA are mixed to save an extra un-modulated LO and to obtain AMAM while “zoom” into AMPM (see Walid K. M. Ahmed and Dale Douglas, “Multi-Mode Selectable Modulation Architecture Calibration and Power Control Apparatus, System, and Method for Radio Frequency Power Amplifier”. Patent pending, U.S. Patent Serial # 11/347,455).


PA Closed-Loop Correction

EDGE AMSignal

EDGE PMSignal

Extrapolation/PredictionAlgorithm

(Used duringRamp-up onlyuntil full LUTs

are built.Then LUTs are

used for modulationand Ramp-down)

AMPath

PMPath

PA

AMAMand

AMPMDetection

CorrectedAM

CorrectedPM


Performance Results

Amplitude Predictor

Phase Predictor

Numerical noise is filtered by a 3rd order Bessel filter


Performance Results

DTx EDGE/GSM PA Characteristic Examples:Low Band

High Band

AMAM AMPM

AMAM AMPM0 100 200 300 400 500 600

0

5

10

15

--> States

-->

Vol

ts

0 100 200 300 400 500 600-40

-20

0

20

40

60

80

100

120

--> States

-->

Deg

rees

0 100 200 300 400 500 6000

2

4

6

8

10

12

14

16

--> States

-->

Vol

ts

0 100 200 300 400 500 6000

50

100

150

200

250

--> States

-->

Deg

rees

Low Band

High Band

AMAM AMPM

AMAM AMPM0 100 200 300 400 500 600

0

5

10

15

--> States

-->

Vol

ts

0 100 200 300 400 500 600-40

-20

0

20

40

60

80

100

120

--> States

-->

Deg

rees

0 100 200 300 400 500 6000

2

4

6

8

10

12

14

16

--> States

-->

Vol

ts

0 100 200 300 400 500 6000

50

100

150

200

250

--> States

-->

Deg

rees


Performance Results EDGE/GSM Switched Transient and ACPR Specification

Center Frequency (kHz)

Bandwidth (kHz)

ACPR Limit (dBm)

ST Limit

(dBm)

400 30 -54 -23

600 30 -60 -26

1200 30 -60 -32

1800 100 -65 -36


Performance Results

GSM Low Band Performance:

0 5 10 15 20 25 30 35 400

5

10

15

20

25

30

35

40

45

50Switched Transient Margin at Antenna

Power Setting in (dBm) -->

Mar

gin

in (

dB)

-->

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35 40-110

-105

-100

-95

-90

-85

-80

-75

-70ACPR


AC

PR

in (

dBc)

-->

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35 40-5

0

5

10

15

20

25

30

35Peak, Average Power


Out

put

Pow

er in

(dB

m)

-->

Peak Power

Average Power

0 5 10 15 20 25 30 35 40-10

0

10

20

30

40

50

60PAE


PA

E in

(%

) --

>

PA

ANT

0 5 10 15 20 25 30 35 400

5

10

15

20

25

30

35

40

45



Mar

gin

in (

dB)

-->

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35 40-110

-105

-100

-95

-90

-85

-80

-75

-70ACPR


AC

PR

in (

dBc)

-->

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35 40-5

0

5

10

15

20

25

30



Out

put

Pow

er in

(dB

m)

-->

Peak Power

Average Power

0 5 10 15 20 25 30 35 40-10

0

10

20

30

40

50

60PAE


PA

E in

(%

) --

>

PA

ANT


Performance Results

GSM High Band Performance:

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40

45



Mar

gin

in (

dB)

-->

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35-110

-105

-100

-95

-90

-85

-80

-75

-70ACPR


AC

PR

in (

dBc

) --

>

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35-5

0

5

10

15

20

25

30



Ou

tpu

t P

ow

er i

n (d

Bm

) --

>

Peak Power

Average Power

0 5 10 15 20 25 30 35-10

0

10

20

30

40

50

60

70PAE


PA

E in

(%

) --

>

PA

ANT

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40

45



Mar

gin

in (

dB)

-->

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35-110

-105

-100

-95

-90

-85

-80

-75

-70ACPR


AC

PR

in (

dBc

) --

>

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35-5

0

5

10

15

20

25

30



Ou

tpu

t P

ow

er i

n (d

Bm

) --

>

Peak Power

Average Power

0 5 10 15 20 25 30 35-10

0

10

20

30

40

50

60

70PAE


PA

E in

(%

) --

>

PA

ANT


Performance Results

EDGE Low Band Performance:

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40

45



Mar

gin

in

(dB

) --

>

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35-90

-85

-80

-75

-70

-65

-60

-55ACPR


AC

PR

in

(dB

c) -

->

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35-5

0

5

10

15

20

25

30



Out

put

Pow

er

in (

dB

m)

-->

Peak Power

Average Power

0 5 10 15 20 25 30 35-10

0

10

20

30

40

50PAE


PA

E in

(%

) --

>

PA

ANT

0 5 10 15 20 25 30 350.54

0.56

0.58

0.6

0.62

0.64

0.66

0.68

0.7

0.72

0.74EVM


EV

M in

(%

) --

>

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40

45



Mar

gin

in

(dB

) --

>

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35-90

-85

-80

-75

-70

-65

-60

-55ACPR


AC

PR

in

(dB

c) -

->

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35-5

0

5

10

15

20

25

30



Out

put

Pow

er

in (

dB

m)

-->

Peak Power

Average Power

0 5 10 15 20 25 30 35-10

0

10

20

30

40

50PAE


PA

E in

(%

) --

>

PA

ANT

0 5 10 15 20 25 30 350.54

0.56

0.58

0.6

0.62

0.64

0.66

0.68

0.7

0.72

0.74EVM


EV

M in

(%

) --

>


Performance Results

EDGE High Band Performance:

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40



Ma

rgin

in (

dB)

-->

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35-90

-85

-80

-75

-70

-65

-60

-55

-50ACPR


AC

PR

in (

dBc

) --

>

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35-10

-5

0

5

10

15

20

25

30



Ou

tput

Po

we

r in

(dB

m)

-->

Peak Power

Average Power

0 5 10 15 20 25 30 35-10

0

10

20

30

40

50PAE


PA

E i

n (%

) --

>

PA

ANT

0 5 10 15 20 25 30 350.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5EVM


EV

M i

n (

%)

-->

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40



Ma

rgin

in (

dB)

-->

ST400

ST600ST1200

ST1800

0 5 10 15 20 25 30 35-90

-85

-80

-75

-70

-65

-60

-55

-50ACPR


AC

PR

in (

dBc

) --

>

ACPR400

ACPR600

ACPR1200

ACPR1800

0 5 10 15 20 25 30 35-10

-5

0

5

10

15

20

25

30



Ou

tput

Po

we

r in

(dB

m)

-->

Peak Power

Average Power

0 5 10 15 20 25 30 35-10

0

10

20

30

40

50PAE


PA

E i

n (%

) --

>

PA

ANT

0 5 10 15 20 25 30 350.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5EVM


EV

M i

n (

%)

-->


Performance Results

GSM Low Band Performance:

0 1 2 3 4 5 6 7

x 10-4

-20

0

20

40Switching Transient vs. Time Mask

dBm

/ST

Mar

gin

dB

time

0 5 10 15 20 25-200

-150

-100

-50

0

Frequency (MHz)

PS

D (

dBc/

Hz)

Spectral Performance

Ideal

PA Output

Model Name: SPCM PredictDate/Time: 11-May-2006 15:11:00Mode: GSM Low BandPAR Burst: 0.001009 dBPAR Modulation: 0.002405 dBPipeline Stgs: 8AmpFilter BW: 1.2 MHzPhFilter BW: 1.6 MHzDriver BW: 2.5 MHzI/Q ADC Res: 10 BitsMutiDAC Res: 11 BitsClock: 192

FreqOffset ST Marg(dBm) ACPR dBc400kHz PA:1.8 Ant:3.6 -75.2600kHz PA:0.86 Ant:2.7 -891200kHz PA:3.9 Ant:5.7 -1061800kHz PA:0.088 Ant:1.9 -117Rx 20MHz PA:-171.5 Ant:-173.3

Power pk: PA:35.3dBm Ant:33.5dBmPower Avg: PA:35.3dBm Ant:33.5dBmMod. PAE PA:57.9% Ant:38.2%Avg Current 1685mA

EVM 0.343%


Performance Results

EDGE Low Band Performance:

0 1 2 3 4 5 6 7

x 10-4

-20

0

20

40Switching Transient vs. Time Mask

dBm

/ST

Mar

gin

dB

time

0 5 10 15 20 25-200

-150

-100

-50

0

Frequency (MHz)

PS

D (

dBc/

Hz)

Spectral Performance

Ideal

PA Output

Model Name: SPCM PredictDate/Time: 11-May-2006 15:19:37Mode: EDGE Low BandPAR Burst: 3.095 dBPAR Modulation: 2.839 dBPipeline Stgs: 8AmpFilter BW: 1.2 MHzPhFilter BW: 1.6 MHzDriver BW: 2.5 MHzI/Q ADC Res: 10 BitsMutiDAC Res: 11 BitsClock: 192

FreqOffset ST Marg(dBm) ACPR dBc400kHz PA:4.1 Ant:5.9 -63.9600kHz PA:1.4 Ant:3.2 -65.51200kHz PA:3.7 Ant:5.5 -77.11800kHz PA:10 Ant:12 -84Rx 20MHz PA:-129.5 Ant:-131.3

Power pk: PA:33.9dBm Ant:32.1dBmPower Avg: PA:30.8dBm Ant:29dBmMod. PAE PA:45.6% Ant:30.0%Avg Current 750mA

EVM 0.649%


Conclusions

A novel predictive real-time closed-loop non-linearity correction technique has been proposed and tested.

Despite the severe non-linearity of the PA, the predictive algorithm has been able to achieve healthy performance margins compared to the stringent limits set by the EDGE/GSM standard.

The results indicate that the predictor based closed loop approach is very promising compared to other linearization techniques.

It offers a simple, yet robust, efficient and intelligent solution that is not-only independent of modulation/transmission protocol constraints, but is also flexible enough to handle a wide range of PA non-linearity levels, in order to make the PA correction reliable and robust against variations due to temperature, process, power and aging.

It should be noted that this technique can be applied to any PA modulation architecture (e.g., polar or IQ).


Conclusions, continued

Also, this technique can be applied during actual modulation/data-transmission periods (i.e., not necessarily on a ramp-up time such as in EDGE/GSM) if the performance specifications allow for that. This is possible in interference-averaging modulation schemes such as CDMA technologies, where switching transients are not of critical importance. Hence, we can afford longer prediction periods in order to allow for sometime during modulation such that the signal covers all possible modulation signal levels (to collect a complete correction table). Clearly, in such a scenario, we can design the predictor to predict in both directions (i.e., forward and backwards prediction).

This technique can also be enhanced to perform PA compression prediction. Hence, in addition to correcting for the PA non-linearity on-the-fly, the algorithm can also predict when the PA gets into compression and prevents such an event to avoid future signal clipping if the PA is driven hard into compression (see Walid K. M. Ahmed, “Algorithm, Method and Apparatus for Real-Time Adaptive Compression-Control in Power Amplifiers”. Patent

Pending).

IEEE NJ Coast Seminar, 12/01/2008Walid Ahmed et. al. Performance Analysis of a Novel Real- Time Closed-Loop Technique for Non- Linearity Correction of.

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