RF Pre-Distortion for Linearizing Power Amplifiers

Semiconductors for Wireless Communications

RF Pre-Distortion for Linearizing Power Amplifiers

Rajeev Krishnamoorthy

[email protected] May, 2013

mailto:[email protected]

Semiconductors for Wireless Communications Semiconductors for Wireless Communications

Company Background


What: Adaptive pre-distortion to linearize RF power amplifiers

• RF Power Amplifier Linearizer (RFPAL)

• Reduce distortion to increase PA efficiency and useable output power

• Lower system cost & reduce design time

• Target cellular wireless and microwave

How

• Single-chip programmable, high-performance, analog signal processing platform; lower power consumption and much lower cost (vs. DPD)

• Compute & Adapt in the digital domain, Apply in the analog/RF domain

• Programmability of digital; the simplicity, cost, size and power of analog

Why

• Higher PAR Waveforms: the move to more complex modulation schemes, multi-carrier systems, time-slotted waveforms, etc. increase signal variability (peak-to-average ratio)

• New Devices & Systems: A variety of PA architectures and transistor technologies put increasing demands on linearity & efficiency

• Cost: Push to lower CAPEX and OPEX • Electric bills is a non-trivial component of expenses

• Current digital solutions are expensive, large, and power-hungry, and require integration from BB to PA.

What do we do? And why?

3


Company Timeline

4

2001 2007 2009 2011 2012

Company Founded

Focus for 1st 6 years: Optical & Wireline Markets

Shifted focus to Wireless Infrastructure

SC1887 in production Cellular (W)CDMA market

Add Microwave P2P (up to 60 MHz signal BW)

& Broadcast segments

SC1889 in production (Low-Cost SC1869)

Cellular 4G Time-slotted waveforms Improved performance

2010

SC1894 Expanded frequency range

(168 MHz – 4.2 GHz) Performance improvements Monitoring/control funcs.

2013

SC2200 for MIMO Systems


Technology & Product


Power Amplifiers & the Need to Linearize

Efficiency (%) and Inter Modulation Product (dBc) vs. output power (dBm)

PA output power (dBm) vs. input power (dBm)

6


Predistortion Principle

7

No predistorion

With predistorion

5-10 x BW expansion


Digital Pre-Distortion (DPD)

Benefits

Programmable, customizable for particular application, control

Challenges

High power consumption, high frequency reconstruction filter, wide band up-mixer, challenging clock generation to

achieve converters SNR, lack of modularity, cost…

8


RFPAL (RF PA Linearization)

Benefits:

Lower power digital processor, easy reconstruction filter, relaxed up-mixer requirements, low power, low cost, easy to use; feedback path fully integrated; DAC only need transmit at 1x signal bandwidth

Challenges

Potentially … degree of control for end user

9


RFPAL Application

10


RFPAL in Microwave Backhaul Designs

RFPALBLN BLN

BLN

ATTENIF

Xcvr PADvr

Up-convertMixer

ALC

LO

DELAY

FeedbackDown-convert

Mixer

20 or 30dB Coupler

ATTEN

RMS EVM=4.6% RMS EVM=2.8%

128QAM, 28 MHz BW Before correction After correction

11

Easy to linearize split-mount or integrated backhaul systems


RFPAL System Architecture

12

CLK

EEPROM

RFFB SPI

DIGITAL LOGIC

Generate Spectrum Power Analysis

RFOUT RFIN

ANALOG Correction Processor

Envelope

μP Initialize Calibrate

Adapt & Track Generate Corr. Func.

QPS

0

90

RF SIGNAL PROCESSOR

Down-Convert &

Sample


RFPAL Signal Path

13

I

RF VGA

Q

QPSRFIN RFOUT

VOLTERRASERIES

VGA

VGA

VGA

CORRECTION ENGINE

· RFIN: 100MHz-3GHz· I,Q > 250MHz VBW· 50 DACs

Moves application of signal processing from digital to analog domain


Analog Volterra Series

14

5

1m

p

2.m

2.m

4

1p

OUT )-(trc(t)V ~(t)r(t)V 2

IN

C2

C4

C6

C8

C10

X2

X4

X6

X8

X10

IN

Delay 1= 1

OUT

Coefficient DAC’s

Delay 2= 2

Delay 3= 3

Delay 4= 4


15

Cost Function

Optimize Cost function

),(,log)( 2221112 ULUL PPfPPfC w

Use a Stochastic gradient search algorithm to optimize

)(min www

Copt


Cost Surface & Adaptation

16

Starting Point

Concave Area

Global Optimum

0W

4p

0p

5p

1p

2p

3p

1W

2W

3W

4W

5W

0W

4p

0p

5p

1p

2p

3p

1W

2W

3W

4W

5W

Cost Surface (artist’s rendition)


Extensible Approach

Basic idea: linearize by extracting parts of the error spectrum No need to down-convert the entire signal

Wideband Signal

Multi-band Signals


SC1887 SC1869 SC1889 SC1894

Frequency of

operation

698 – 2800MHz 698 – 2800MHz 698 – 2800MHz 168 – 4200MHz

Max. signal BW 1.2 – 40/60MHz 1.2 – 20MHz 1.2 – 60MHz 25 kHz – 60MHz

Waveforms CDMA & WCDMA Same as SC1889 CDMA, WCDMA,

LTE, TD-LTE,

WiMAX, HSDPA

Same as SC1889

PAs Class A/AB &

Doherty

Class A/AB Class A/AB &

Doherty

Class A/AB &

Doherty

PA output power Up to 50 W Up to10W 5 W – 60W Up to 60W

Added features

(paid for add-ons)

RFFB RMS power

detector

RFFB RMS power

detector

RFIN & RFFB RMS

power detector,

gate bias control,

temp. & spectral

monitoring

Product Line Summary

18

SC2200 (MIMO) … 2H 2013


Representative Results


Macro: 30W LTE 20 MHz Using Freescale MRF8S2100 Doherty

-50 -40 -30 -20 -10 0 10 20 30 40 50-50

-40

-30

-20

-10

0

10

20

Frequency Offset from Carrier Center, MHz

PA

Out

put

Pow

er,

dBm

/ 3

0 kH

z

BO=0.5 SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U

POUT-1

= 44.6 dBm

POUT-0

= 45.0 dBm

ACLR2L-1

-56.5

ACLR1L-1

-51.3

ACLR1U-1

-49.7

ACLR2U-1

-56.6

ACLR2L-0

-49.7

ACLR1L-0

-31.6

ACLR1U-0

-31.0

ACLR2U-0

-50.6

PSD-1

PSD-0

32 34 36 38 40 42 44 46 48-65

-60

-55

-50

-45

-40

-35

-30

-25

Pout, dBm

AC

LR,

dBc

SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U

WP

ACLR1-0

ACLR1-1

ACLR2-0

ACLR2-1

AC

LR d

Bc

-50 -40 -30 -20 -10 0 10 20 30 40 50-50

-40

-30

-20

-10

0

10

20

Frequency Offset from Carrier Center, MHzP

A O

utpu

t P

ower

, dB

m /

30

kHz

BO=0.5 SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U

POUT-1

= 44.6 dBm

POUT-0

= 45.0 dBm

ACLR2L-1

-56.5

ACLR1L-1

-51.3

ACLR1U-1

-49.7

ACLR2U-1

-56.6

ACLR2L-0

-49.7

ACLR1L-0

-31.6

ACLR1U-0

-31.0

ACLR2U-0

-50.6

PSD-1

PSD-0

32 34 36 38 40 42 44 46 48-65

-60

-55

-50

-45

-40

-35

-30

-25

Pout, dBm

AC

LR,

dBc

SN460 hw3.1 PAM121 25 LTE20M1%100#8.21 2140.0 MHz 11/01/24 11:01:27 U

WP

ACLR1-0

ACLR1-1

ACLR2-0

ACLR2-1

PAR = 8.2 dB, Amplifier P3dB = 53.9 dBm, hdrain=39%


LTE with low traffic loading : E-TM2.0

0 1 2 3 4 5 6 7 8 9 10-20

-15

-10

-5

0

Time, ms

Sig

nal am

plitu

de,

dB

FS

LTE20M1-1#14.34.mat, dwell = 20 s, duty factor = 0.00 dB

peak across 20 us

12 kHz LPF

Am

plit

idue

Slot (Time)

Reso

urc

e B

locks (

Fre

qu

en

cy)

Blue – Uncorrected

Yellow – 100% Loaded

Magenta – Partial Loaded Signal Shown at Right


31 32 33 34 35 36 37 38 39 40 410

5

10

15

20

25

30

35

40

45

50

Pout, dBm

Powe

r-add

ed e

ffici

ency

, %

SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2690.0 MHz 13/03/12 19:39:06

WP

PAE

31 32 33 34 35 36 37 38 39 40 41-65

-60

-55

-50

-45

-40

-35

-30

-25

Pout, dBm

AC

LR

, dB

c

SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2690.0 MHz 13/03/12 19:39:06 U

WP

ACLR1-0

ACLR1-1

ACLR2-0

ACLR2-1

5W (@ Antenna) Asymmetric Doherty Line-up

22

PA

28V

RF

Xcvr 37dBm

39.5dBm

SC1894 DVR

22

20MHz 4xWCDMA 7.8dB PAR

30 32 34 36 38 40 420

5

10

15

20

25

30

35

40

45

50

Pout, dBm

Pow

er-

added e

ffic

iency,

%


WP

PAE

45%PAE

>20dB Improved Linearity

31 32 33 34 35 36 37 38 39 40 410

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Pout, dBm

EVM

, %


WP

EVM-0

EVM-1

-20 -15 -10 -5 0 5 10 15 20-50

-40

-30

-20

-10

0

10

20


PA

Outp

ut

Pow

er,

dB

m /

30 k

Hz

SN21003 hw4.1 PAM347 25.0 WCDMA4-1111#7.75 2690.0 MHz 13/03/12 19:39:06 U

POUT-1

= 39.1 dBm

POUT-0

= 39.2 dBm

ACLR2L-1

-55.0

ACLR1L-1

-50.6

ACLR1U-1

-50.6

ACLR2U-1

-52.5

ACLR2L-0

-34.1

ACLR1L-0

-32.2

ACLR1U-0

-33.9

ACLR2U-0

-36.5

PSD-1

PSD-0


2.5W (@ Antenna) Asymmetric Doherty Line-up

23

PA

28V

28V

RF

Xcvr 34dBm

38dBm

SC1894 DVR

10MHz LTE 7.5 PAR

23

45%PAE


2x1W (@ Antenna) Reference Product Line-up

24

DVR PARFPAL

12V

DVR PARFPAL

12VDual Xcvr

30dBm

30dBm

33dBm

33dBm

*

*

* Triquint PA requires special match for 33dBm.

24 26 28 30 32 34 36-65

-60

-55

-50

-45

-40

-35

-30

-25

Pout, dBm

AC

LR,

dBc

SN8072 hw 3.1 PAM231 25.0 1.81V 3.32V WCDMA4-1111#7.75 2140.0 MHz 12/02/07 18:39:23 U

WP

ACLR1-0

ACLR1-1

ACLR2-0

ACLR2-1

24 25 26 27 28 29 30 31 32 33 340

5

10

15

20

25

30

35

40

45

50

Pout, dBm

Pow

er-

added e

ffic

iency,

%

SN8072 hw3.1 PAM231 25.0 1.81V 3.32V WCDMA4-1111#7.75 2140.0 MHz 12/02/07 18:39:23

WP

PAE

20MHz 4xWCDMA 7.8dB PAR

-20 -15 -10 -5 0 5 10 15 20-50

-40

-30

-20

-10

0

10


PA

Outp

ut

Pow

er,

dB

m /

30 k

Hz

SN8072 hw 3.1 PAM231 25.0 1.81V 3.32V WCDMA4-1111#7.75 2140.0 MHz 12/02/07 18:39:23 U

POUT-1

= 32.8 dBm

POUT-0

= 32.9 dBm

ACLR2L-1

-52.8

ACLR1L-1

-50.3

ACLR1U-1

-51.2

ACLR2U-1

-51.9

ACLR2L-0

-34.7

ACLR1L-0

-32.3

ACLR1U-0

-33.6

ACLR2U-0

-36.1

PSD-1

PSD-0


Summary

25

• Technology: Programmable analog signal processing platform is ideally suited to address a wide range of markets & applications

• Initial focus on RF PA linearization

• Markets: Target markets expanding • Broadcast, Microwave backhaul, Public Safety, White space

• Advantages: Differentiated solution addressing customer issues • Low power consumption, low cost, small form factor and high performance

• Rfin/Rfout, simple design and integration of RFPAL enables fast time to market

• Supports a wide variety of

• Waveforms & modulations (2G/3G/4G, broadcast, microwave, …)

• Range of transmit powers (0.5W-60W)

• Infrastructure (macro base stations, repeaters, small cells, …)

• Technologies (LDMOS, GaN, GaAs, etc.)

• Architectures (class A, A/B, Symmetric & Asymmetric Doherty,…)

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RF Pre-Distortion for Linearizing Power Amplifiers

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