enabling high efficiencies digital signal conditioning in modern rf transmitters

All Rights Reserved © Alcatel-Lucent 2006, 2008

Enabling high efficiencies

Digital signal conditioning in modern RF transmitters

Thomas BOHN

June 2008

All Rights Reserved © Alcatel-Lucent 2006, 20082 | Enabling high efficiencies | June 2008

Agenda

1. Overview

2. Peak-to-Average-Power-Ratio (PAPR) Reduction

3. Power amplifier linearisation

4. Image suppression and DC-offset compensation

5. Conclusions


1 Overview

Architecture of a modern RF transmitter


Transmitter architecture

Digital-in analogue-out RF amplifier– Signal conditioning right in front of the analogue PA chain

Easiest (cheapest) possible analogue design Highest possible power efficiency

Clipping Predistortion

Digital-Module

D/A-conv.

Conversion-Module

Image- &Carrier-Suppr.

PA chain

PA-Module

Wide bandDown-conversion

A/D-conv. Time Delay

Compens.

Modulation &Wide bandUp-conversion


2 PAPR Reduction


PAPR ReductionWhat is PAPR reduction ? Clipping !

Reducing signal power peaks to a certain threshold– increasing the EVM

Power @ Soft Clipping Power @ Hard Clipping


PAPR Reduction Why do we perform PAPR reduction ?

PA has to be designedfor the signal peak driving into saturation

produces intermodulation

PA delivers an averageoutput power

Black: PAPR: 10.9 dB 43dBm (12% efficiency)

Blue: PAPR: 5.5 dB 45.2 dBm (16% efficiency)

Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes)

Ref 50.6 dBm Att 10 dB *

Offset 50.5 dB

AVG

A

LVL

Center 2.14 GHz Span 25.5 MHz2.55 MHz/

*

*

*1 RM

RBW 30 kHz

VBW 300 kHz

SWT 500 ms

3DB

*

*2 RM

VIEW

*

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH PWR 45.19 dBm

ACP Low -45.90 dB

ACP Up -46.11 dB

ALT1 Low -58.45 dB

ALT1 Up -58.34 dB

Date: 9.JUN.2008 17:19:10


3 PA linearisation

Digital predistortion for higher efficiencies


PA linearisationHow to linearise a PA ? With (digital) predistortion !

Principle of PA linearisation:

Amplitude distortions can be compensated up to the PA’s saturation point

Phase distortion can be compensated for arbitrary input amplitudes

Phase

x in PD x

act

y des

y sat

y

x

y

x


PA linearisationDigital predistortion - the PA

Complex baseband model for the PA:

Quasi-static nonlinear model (static in the complex baseband) AM-AM and AM-PM distortions Complex gain model:

– x, y: Complex envelope of RF input and output signal

PA(|x|) Complex gain depending on the magnitude of the input signal

– G(|x|) Magnitude of the gain (|x|) Phase shift

– ai Complex coefficients


PA linearisationDigital predistortion - the linearisation

PA linearisation with inverse PA characteristic:

Linear PA transfer characteristics up to saturation point:

– x, y: Complex envelope of RF input and output signal Constant real gain

Linearised PA transfer characteristics up to saturation point:

– G(|x|) Magnitude of the gain (|x|) Phase shift

– ai Complex coefficients


PA linearisationDigital predistortion - the non-linearity (1)

The intervals over the input signal power:

Pout

PA transfer characteristic just for illustration The “r” depicts the power of the current input sample in the 2nd interval

r1 r r2 r3 (max.) PIN



In the interval : 1 ii rrr

)()()()()( 32322222121222 rarararar iiiiiiiiPD

4

27

4

27

s

s

-

s

s

0 2

10

1

10

10

3

1

0

11



Block diagram of the PIPOP:

PD calculation foreach input sample

Predistortion ofeach input sample

Issues to ease theimplementation: f(x) (= 1 ?)

Base functionsgranularity

Interval widths(equal ?)

Input signalx

Output signaly

Coefficient input

| . |2

f(x)

Delay

Determine

interval no. addresses

offsetCalc.

Calc. coeff.

Function

Function

Function

Coefficient

storage

0

Function

1

2

3

i

r

s

Determineinterval width

a a2i 2i+3


PA linearisationDigital predistortion - the adaptation

Coefficient update formula:

For each of the four ( 0 k 3) coefficients within one interval

c - coefficient - step width of the algorithm - hermite base function y’* - complex conjugated PD output signal (y’ is the delayed y) e - error (y’ - w) - PD gain

)()()(')|)((|)()1( *2 nnenynxmcmc kkkk


PA linearisationDigital predistortion - the system

Schematic view:

PA

MEM MLN

MLN(1)coeffs

I/Q (out_data)

int_delay

TD

OUT

Y_ IN

(FB)

Base functionsI/Q (in_data)

Z (OUTPUT)

FB gain

fb_factor

INPDtap delay

PD Error

PDtap delay

MLN(1)Adaptation

Base functions

Error

IN (I/Q)

MLN(2)Adaptation

Base functions

Error

IN (I/Q)

MLN(2)coeffs

X (INPUT)

Y

Integer delay

Data PD

BFMLNMLN(1) +MLN(2)

MLN(3)

MLN(4)

PDtap delay

PD Gain ()


PA linearisationEfficiency and spectral distortions in case of 1 carrier UMTS (1)

Blue: 46.0 dBm 18.2% efficiency out of spec

Black: 43.0 dBm 11.9% efficiency in spec

Green: 40.6 dBm 8.35% efficiency in spec

Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR)


Offset 50.5 dB

A

LVL


*

*

RBW 30 kHz

VBW 300 kHz

SWT 500 ms

*3 RM

VIEW

3DB

*

*

*2 RM

VIEW

*1 RM

VIEW

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH PWR 45.87 dBm

ACP Low -41.57 dB

ACP Up -41.59 dB

ALT1 Low -56.92 dB

ALT1 Up -56.18 dB

Date: 3.JUN.2008 17:40:01


PA linearisation Efficiency and spectral distortions in case of 1 carrier UMTS (2)

Green: 46.0 dBm 18.2% efficiency out of spec

Blue: 46.0 dBm memory-less PD in spec with 5dB margin



Offset 50.5 dB

A

LVL


*

*

RBW 30 kHz

VBW 300 kHz

SWT 500 ms

*3 RM

VIEW

3DB

*

*

*1 RM

VIEW

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH PWR 45.95 dBm

ACP Low -54.01 dB

ACP Up -53.82 dB

ALT1 Low -63.61 dB

ALT1 Up -63.23 dB

Date: 3.JUN.2008 18:01:29




Blue: 47.0 dBm memory-less PD in spec



Offset 50.5 dB

A

LVL

*


*

*

RBW 30 kHz

VBW 300 kHz

SWT 500 ms

*

*2 RM

AVG

1 RM

AVG

3DB

*

*3 RM

VIEW

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH PWR 46.88 dBm

ACP Low -45.02 dB

ACP Up -47.69 dB

ALT1 Low -53.14 dB

ALT1 Up -55.31 dB

Date: 3.JUN.2008 18:17:26



Blue: 46.0 dBm 17.8% efficiency out of spec

Black: 31.7 dBm 1.2% efficiency out of spec


Set-up: PA rated for 46.0 dBm; 4 x 3GPP test model 1 (64 codes, 5.5 dB PAPR)


Offset 50.5 dB

A

LVL

Center 2.14 GHz Span 50 MHz5 MHz/

*

*

3DB

RBW 30 kHz

VBW 300 kHz

SWT 160 ms*

*3 RM

VIEW

*2 RM

VIEW

*1 RM

VIEW

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH1 39.81 dBm

CH2 39.94 dBm

CH3 39.97 dBm

CH4 40.10 dBm

Total 45.98 dBm

ACP Low -34.01 dB

ACP Up -34.05 dB

ALT1 Low -35.26 dB

ALT1 Up -34.97 dB

Date: 3.JUN.2008 17:49:55




Black memory-less PD out of spec

Green PD with memory effect

compensation in spec

Set-up: PA rated for 46.0 dBm; 4 x 3GPP test model 1 (64 codes, 5.5 dB PAPR)


Offset 50.5 dB

A

LVL

Center 2.14 GHz Span 50 MHz5 MHz/

*

*

RBW 30 kHz

VBW 300 kHz

SWT 160 ms

*3 RM

VIEW

3DB

*

*2 RM

VIEW

*1 RM

VIEW

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH1 39.91 dBm

CH2 40.00 dBm

CH3 39.99 dBm

CH4 39.90 dBm

Total 45.97 dBm

ACP Low -50.46 dB

ACP Up -49.94 dB

ALT1 Low -51.82 dB

ALT1 Up -51.31 dB

Date: 3.JUN.2008 17:53:53


4 Image suppressionand DC-offset compensation


Image suppression and DC-offset compensation Zero-IF transmitter and efficiency

Simple architecture with small amount of analogue components:– 1 analogue quadrature modulator (QM, also known as IQ modulator)

– 1 local oscillator (LO)

Half of the signal bandwith @ DACs, Performance of DACs is best around 0 Hz

No frequency selective filters

But: QM has imperfections resulting in DC feed trough and image

DCI

DCQ

r(t)

90°+p

k

QM Modulator

DAC

DAC

v

LO(t)

z(t)

I

Q


Image suppression and DC-offset compensation Spectral distortions in case of 1 carrier UMTS (1)


– ACP both

– ALT low

Blue: memory-less PD out of spec

– DC @ ACP low

– Image @ ALT low

Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR) @ 5MHz


Offset 50.5 dB

A

LVL

*1 RM

AVG


*

*

3DB

RBW 30 kHz

VBW 300 kHz

SWT 500 ms*

*3 RM

VIEW

*2 RM

VIEW

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH PWR 46.04 dBm

ACP Low -22.50 dB

ACP Up -46.01 dB

ALT1 Low -41.00 dB

ALT1 Up -52.56 dB

Date: 3.JUN.2008 18:05:57


Image suppression and DC-offset compensation Spectral distortions in case of 1 carrier UMTS (2)

Green : 46.0 dBm 18.2% efficiency out of spec

Black: memory-less PD out of spec

Blue: memory-less PD Image suppression and

DC-offset compensation in spec

Set-up: PA rated for 46.0 dBm; 3GPP test model 1 (16 codes, 5.5 dB PAPR) @5MHz


Offset 50.5 dB

A

LVL


*

*

3DB

RBW 30 kHz

VBW 300 kHz

SWT 500 ms*

*3 RM

VIEW

*2 RM

VIEW

*1 RM

VIEW

-50

-40

-30

-20

-10

0

10

20

30

40

50

SWP 3 of 3

CH PWR 46.04 dBm

ACP Low -54.57 dB

ACP Up -53.87 dB

ALT1 Low -61.79 dB

ALT1 Up -62.63 dB

Date: 3.JUN.2008 18:07:59


5 Conclusion


Conclusion

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