Rf Power Amplifiers Vi (2010) - Base Stations

8/12/2019 Rf Power Amplifiers Vi (2010) - Base Stations

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RF Power Amplifiers VI


Power Amplifier Solutions

for Base StationsG. Ghione, M . Pirola, R. Quaglia


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Agenda

Base stations

Doherty Power Amplif ier

L inearity Enhancement Techniques

Digital Predistortion

Feed Forward


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Base stations

Base stations are the front-end of the mobileinfrastructure

I nteract with:

Mobile TerminalDown link: Base station TX

Up link: Base station RX

Backhaul (BSC)Core of the network

Cable, fiber or microwaves


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Base stations

StandardsGSM : 950 MHz and 1850 MHz, 50-100 W

UMTS: 2100 MHz, 50-100 W

WiMax: 3500 MHz, 4 W

LTE: 2100-2500 MHz, 50-100 W

F igures of merit

Cost

Linearity

Efficiency

Re-configurability


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A basestation power budget


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Linearity/Efficiency as linked issues

Power amplif ication of variable-envelope signals(var iable-power signals):

LINEARITY: AM and also PM distortion take

place if the PA is used at its full-rated RF power

level

EFFICIENCY: Conventional design of high-

efficiency PA leads to good solution only near the

maximum rated power; if the power is backed offefficiency drops sharply

I ssues: l inearity and/or eff iciency enhancement

techniques


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Why variable-power signals

I ndependent of the variable-envelope issue, the

output power of a PA is not constant:

In single-channel PAs, because the output power is

adapted to receiving station conditions (location,

environment)

In multiple-channel PAs (basestations) because the

total multichannel power undergoes statistical

fluctuations related to location, traffic pattern,

environment

This implies back-off vs. optimum PA operating

conditions


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Defining backoff

Suppose that a reference operating conditionfor a PA occurs with a given (reference) input

power

We say that the PA operates with a given backoff

(e.g. 3 dB, 10 dB...) with respect to the reference

condition when the input power isreducedby 3,

10... dB with respect to the reference input

power

Alternatively: the PA is backed off3, 10... with

respect to the reference condition


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Two examples of backoff

A class A PA is backed off with respect to the 1dB compression pointin order to increase

linearityreduce IMPs

An amplif ier is backed off with respect to the

optimum eff iciencycondition because the slowly

varying average input power is decreased

Backoff usually improves linearity, decreases

eff iciency; the eff iciency deter ioration is

maximum in highly l inear (class A) amplif iers


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Efficiency vs. backoff: class A and B

Max.

class A

power


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The multichannel amplifier

Basestation ampli f iers for

multichannel systems (e.g. CDMA)

require IMP3 levels of the order of

60-80 dBc as compared to the usual

20-30 dBc in single-channel PAs

This requi rement is extremelysevere and can be avoided only by

using a mul tiplexdemultiplex

structure with an array of single-

channel ampl if ier in the place ofthe mul tichannel one

Cons: MUX-DEMUX design, fixed

channel, less f lexible structure

Channelized PA

Multi Channel PA


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The linearization issue

Usual ly the -30 dBc IMP3 requi rement can be satisf ied by

working in class A at 1 dB compression point, with ~30-50%

efficiency

To obtain70 dBc the class A amplif ier must work with 20 dB

backoff with respect to 1 dB compression point

Suppose a 100 W amplif ier absorbing 200 W DC power is

working at 20 dB backoff the output power wi l l be only 100W/100 = 1 W with a 0.5% eff iciency!

I f we need real 100 W output power the DC power wil l be 20

kW!!!

Therefore, simple backof f does not solve the problem in an

acceptable way we need to improve the amplif ier linearity athigh input power


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Agenda

L inearity, eff iciency tradeoff , var iable-power signals, backoff


L inearity Enhancement Techniques

Digital Predistortion

Feed Forward


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Doherty Power Amplifier

I nvented in 1936 by W.H. DohertyUsed with MW tubes and modulated signals

Multistage Power Amplif ier

H igh eff iciency also in back-off region

Based on 3 concepts

Load Modulation

Active Load Pull

Impedance Inversion


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Load Modulation

Why a Class B is ineff icient in back-off?

Maximum drive level


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Load Modulation


Maximum drive level

RL = RoptPout = PMAX 78%


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Load Modulation


Half dr ive level


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Load Modulation


Half dr ive level

RL = RoptPout = PMAX-6dB

41%


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Load Modulation


Half dr ive level

RL = RoptPout = PMAX-6dB

41%

Voltage does not

reach zero:

efficiency drop!


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Load Modulation

What happens if I change the load?Half dr ive level


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Load Modulation


RL = 2RoptPout = PMAX-3dB

78%


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Load Modulation


RL = 2RoptPout = PMAX-3dB

78%

Voltage

reaches zero!


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Load Modulation

I f the load varies from 2Roptto Roptwhen inputpasses from half drive to full drive the eff iciency

stays high (near 78%)

How can this be realized?

We need to change dynamical ly the load

Active load pull


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Active Load Pull

Principle of working

The load presented at device 1 depends on the

cur rent pumped by device 2 into the commonload

We need decreasingof Z1if input increases

1

2

1

21

1

1 1

I

IR

I

IIR

I

VZ


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Impedance Inversion

Permits the r ight modulation of the load

I2 = 0 Z1= R I2= I0 Z1= R/2Choosing R = 2Ropt gives exact results

20

2

20

0

0

20

2

1

1II

IR

II

IR

I

IIR

RZ


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Doherty Power Amplifier

Schematic Representation

90 degrees line permi ts the phase adjustmentM: main amplif ier , P: peak amplif ier

Behavior of the peak must be studied

M

P

Z Inverter

90 Degrees LOA

D


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Peak Amplifier

Peak amplif ier is turned off up to half dr ive

vin

iout

vin

vout

vin

h,Pout

78%

PMAX-6dB


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Peak Amplifier

Peak amplif ier is turned off up to half dr ive

Then i t turns on with double gmrespect to Main

At the end, the two amplif iers deliver the same

power, that is 6dB higher respect to half drive

vin

iout

vin

vout

vin

h,Pout

78%

PMAX

PMAX-6dB

f


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Peak Amplifier

Peak amplif ier is commonly realized as

Class C amplifier

Class B with input control

The class C conf iguration is more commonInput control implies more added complexity

I n both cases, a greater gain respect to the main

amplif ier is needed

Due to the slow wake up of class C amplifier,

the ratio is 2.5, not 2

P k A lifi ti


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Peak Amplifier: practice

I n the original implementation, tubes where used

gm was controllable

With solid states device two solutions are adopted

Peak periphery greater respect to Main one Gain, PAE L inear ity, Hybrid circui tsUneven input power splitting

Same devices, linearity Gain, PAEBase station realizations require high linearity

Same devices, small unbalancing of inputs

20% of PAE improvement is commonly obtained

D h t P ti l li ti


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Doherty: Practical realization

I f this is a Doherty:

D h t P ti l li ti


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I f this is a Doherty:

.is this a power amplifier?

D h t P ti l li ti


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The Doherty is an intr insical ly narrow band circui t, due

to impedance inver ters

All devices have parasitic elements that inf luence the

matching

I f an imaginary part is involved in the load of the Main,the impedance inverter does not satisfy the rules of

impedance rotation studied before

Solution:

Insertion of an offset line

The bandwidth is sti l l reduced

D h t P ti l li ti


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When the peak is off , i t is necessary that i t does not

inf luence the common load

I t has to appear as an open circui t

Presence of output parasitic change the impedance

Solution:

Insertion of an offset line

S22 of the peak must be 1

The bandwidth is sti l l reduced

Typical bandwidth of a Doherty is near to 5%

D h t P ti l li ti


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Typical bandwidth of a Doherty is near to 5%

M

P

ZInverter

90+a

Deg

L

O

AD

OMN Offset LineIMN

IMN OMN Offset Line

D h t P bl


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Doherty: Problems

Theoretical eff iciency curve is never reached in practical

realizations, for many reasons

Knee voltage

Slow wake up of Class C

Trade-off with linearity

F ine tuning is necessary in base-station to achieve the

best l inearity, in particular on the gate bias levels

Bandwidth is l imited because the eff iciency, but also and

mainly the linear ity (AM -PM ), drop fast

D h t E l


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Doherty: Examples

Jaewoo Sim et. al., Proceedings of Asia-Pacif ic

M icrowave Conference 2007

D h t E l


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Colantonio et. al., High efficiency solid state power

amplifiers, John Wiley and Sons


Doherty: Examples

A d


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Agenda



L inearity Enhancement TechniquesDigital Predistortion

Feed Forward

P di t ti


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Predistortion

Predistortion: the input signal of the power

amplif ier is conditioned to compensate its non-

l inear effects

PA

Predistortion


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Predistortion

Predistortion: the input signal of the power

amplif ier is conditioned to compensate its non-

l inear effects

I n actual base-stations, conditioning is acted on

the modulation signal, dur ing digital processing

PAPre-distortion LinearizedOutput

RF Predistortion


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RF Predistortion

~10 dB improvement of IM3, moderate eff iciency improvement,

better improvements require careful tr imming

Sti l l used at high microwaves and mm-waves

The pre-emphasis character istics can be realized by subtracting a

linear and a non-l inear signal path; sometimes only a cubic pre-

emphasis is generated to decrease IM3s (cubic predistorter)

Implementedwith diodes

Baseband Analysis


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Baseband Analysis

Baseband signal can be described by a complex

envelope analysis

Signal is down-converted from the carrier

frequency to baseband

I-Q components completely describe the signals

This description depends on the chosen center

frequency

Careful must be adopted: also the absolute

reference of power is lost dur ing conversion

Baseband models


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Baseband models

Baseband models relates the input and output

complex envelopes of a circui t, in our case of the

Power Amplif ier

Two main categor ies

No memory: output depends only on theinstantaneous input

With memory: output depends also on the past

inputs

Power Amplifier

Baseband Model

x(t) y(t)

Baseband models


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Baseband models

Extraction of the baseband models can be

performed on time-domain measurement of the

complex envelope

Time is discrete: x[k], y[k]

Training algor ithms depends strongly on the

structure of the model itself

Least square meaning

Iterative algorithms

Remember: a model has a good behavior on the

domain where it was extracted

Baseband models


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Baseband models

Examples of models

Look Up Table

No memory

Easy to extract

Memory Polynomial

Parallel structure

Odd monomia + F IR

Ef fective with LDMOS

Easy to train

Neural Networks

Base on tanh() function

Versatile

Hard training

Baseband predistortion


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Baseband predistortion

Baseband predistor tion implements a baseband

model also for the predistortion function

To extract the predistor ter two strategies are

adopted

Direct learning: first a PA model is extracted; then

the DPD model is the inverse model of the PA

Indirect learning: DPD model is extracted directly

from measurements; it is in reality a post-distorterimplemented as a predistorter

Design and testing


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Design and testing

Both for extraction and validation of the Digital

predistor ter , it is necessary to measure the

complex envelope of the signals at the ports of

the power amplif ier

A baseband setup is needed

We have to

Generate a modulated signal

Measure the output complex envelope

Extract the predistorter

Verify its effectiveness (measuring ACPR, )

Design and testing


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Design and testing

Typical baseband setup

VSA: programmable

receiver that permits

envelope measurement

ADS: microwave CAD

with instrument

interfaces

I n real i ty, al l chain

between digital signal

and output is predistorted

Implementation


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Implementation

A predistorter can be implemented on a FPGA

Constrains

Area

Consumption

Elaboration Speed

These must be considered since the design of the

Power Amplif ier : predistortability

Best solutions includes dynamic adaptation

A feedback from receiver is necessary

Design and testing


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The setup includes FPGA


Design and testing

Examples


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Examples

Meenakshi Rawat et. al., MTT Transactions, 2010

Examples


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Examples

R. Quaglia et. al., EUMC 2009

Conclusions


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Conclusions

Digital Predistortion is widely adopted in Basestations

and permits signicative amelioration in terms of

l inearity vs eff iciency tradeoff

Main constrain of its implementation is the need to act

on a power amplif ier with acceptable AM -AM and AM-PM distortion, otherwise a too complex predistorter is

needed

Costly

Power consuming

Not adaptive

Agenda


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Agenda



L inearity Enhancement TechniquesDigital Predistortion

Feed Forward

Feedforward


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Feedforward

Very old idea, invented by Black together with the negative

feedback in the 20s

Basic pr inciple: to sum to the output of the power ampli f ier an

error signal compensating for the NL part of the response

Ideal Feedforward Analysis I


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Ideal Feedforward Analysis I

Ideal Feedforward Analysis II


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Ideal Feedforward Analysis II

Ideal Feedforward Analysis III


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Ideal Feedforward Analysis III

Ideal vs real feedforward


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Ideal vs. real feedforward

Amplitude and phase control


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Amplitude and phase control

Feedforward performances I


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Feedforward performances I

Feedforward performances II


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Feedforward performances II

High linearity PA example


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High linearity PA example

Documents

Rf Power Amplifiers Vi (2010) - Base Stations