New Challenges in DPD Linearization for High …mtt11/workshops/IMS/2012/WML/...New Challenges in...

New Challenges in DPD

Linearization for High Efficiency,

Wideband PA Architectures

Pere L. Gilabert and Gabriel Montoro

Universitat Politècnica de Catalunya (UPC)

WML: Measurement, Design and Linearization Tech. for High-Efficiency Amplifiers

WML: Measurement, Design and Linearization Tech. for High-Efficiency Amplifiers IMS2012, Montreal, June 17-22, 2012 2

Outline

Introduction

Linearity vs. Efficiency Trade-Off

High Efficiency Polar Transmitter Architectures

Polar Transmitter

Envelope Tracking

Slew-Rate/Bandwidth Reduction of the Envelope

Digital Predistortion for Envelope Tracking PAs

Conclusion


Outline

Introduction



Polar Transmitter

Envelope Tracking



Conclusion


Introduction

Because the PA is one of the most power hungry devices of a transmitter,

several efforts have been oriented to find power efficiency structures to cope

with the inherent trade-off between linearity and efficiency.

The power efficiency improvement achieved using linear PAs with constant

supply combined with linearization techniques is limited when using OFDM-

based signals (e.g. LTE, WiMAX) presenting high PAPR.

Following the SDR concept, alternative configurations to the conventional

Cartesian transmitter have been proposed to overcome typical class AB PA

efficiency figures (ranging from 5 to 10 % when operated with significant back-

off levels).

Introducing CFR and adaptive DPD techniques, the efficiency can be improved

by a factor of 3 to 5. Moreover, by using more efficient topologies such as

Doherty amplifiers, or transmitters based on switching mode RF PAs (e.g. LINC,

EE&R, Polar Transmitters) the efficiency can be raised up to 50%. In any case

DPD is necessary to guarantee the required linearity levels of the transmitter.


Outline

Introduction



Polar Transmitter

Envelope Tracking



Conclusion



When dealing with signals presenting high PAPR, the D/A converter and power

amplifier of the transmitter require large dynamic ranges to avoid amplitude

clipping (and thus nonlinear distortion), which implies increasing both power

consumption and component cost of the transceiver.

As shown in the Table, typical PA efficiency has dropped from 65% for GSM, a

constant envelope modulation scheme, to just 30% for LTE systems.

Power Amplifier Efficiency for a Range of Modulated Schemes.[3]



A power amplifier (PA) is at its

most efficient when running at

maximum output power and it

becomes increasingly inefficient at

lower powers.

As PAPR increases, average power

is reduced relative to peak power

and so the transmitter’s average

efficiency is reduced.

0 2 4 6 8 10-2 12

10

12

14

16

18

20

8

22

Input Power (dBm)

Out

put

Pow

er (

dBm

)

outsatP

1outdBP

insatP1

indBP

0 2 4 6 8 1 0-2 1 2

2

4

6

8

1 0

1 2

0

1 4

Input Power (dBm)

PAE

(%)


Outline

Introduction



Polar Transmitter

Envelope Tracking



Conclusion


High Efficiency Tx Architectures

Limiter

PA

Power

Amplifier

x(t)

LO

Class-S

Modulator

Envelope

detector

S

Modulated

supply

DC supply

y(t)

x1(t)

x2(t)

FUNCTIONING

The highly efficient PA amplifies a phase-modulated (constant in envelope) signal,

while the AM modulation takes place in the PA itself. Therefore, the output envelope is

proportional to the supply voltage, that changes according to the envelope signal.

Polar Amplification

Envelope Elimination and Restoration (EER) Polar Transmitter

HISTORY

- 1952 L.R. Kahn


1. Sensitive to possible delays between the

amplitude-modulated (AM) signal path and

the phase-modulated (PM) signal path.

2. Carrier feed-through as a source of

nonlinear distortion. In the RF PA, the RF

signal could leak directly towards the

output generating spectral regrowth.

3. Just like in any other PA, AM-AM and AM-

PM distortion are also present in polar

modulation.


Main technological constraint: The envelope signal has a bandwidth that is 3 to 5

times the bandwidth of the RF signal. As a consequence, the Envelope Amplifier (Drain

modulator) needs to be not only highly efficient but also wideband enough to cope with

the signal’s envelope.

Drawbacks & Possible Solutions [11]

1. Digital compensating mechanisms at

baseband: Fractional Delay Alignments.

2. In most cases, the feed-through is caused

by the gate-drain capacitance. A cascode

topology, where this feed-forward path is

broken, is often used as a solution for this

problem.

3. Linearization techniques: Digital

Predistortion

Polar Amplification



Polar Amplification

Currently some key issues still difficult its commercial existence [6,11]:

- To minimize quantization noise high oversampling ratios are required �

increases switching losses and degrades overall efficiency.

- the output analog reconstruction filter present in both of these arrangements

must be a low-loss narrowband band-pass filter still difficult to implement.

Towards the ‘all digital’ Transmitter:

Pulsed (or Burst-mode) Tx [6]Polar Tx with both AM and PM signals are

handled digitally [6]



Envelope Tracking

FUNCTIONING

The amplitude and phase modulated RF signal is amplified using a linear-mode PA.

The supply voltage of the RF PA is adjusted according to the envelope of the RF carrier.

Thanks to the dynamic supply the RF PA is always operating close to saturation which

increases the power efficiency at power back-off.

Fixed supply (left) vs. envelope tracking power supply [11]



Envelope Tracking

- ET PA Gain Characteristics -

Isogain Shaping [3]

- ET PA Efficiency –

Optimum Efficiency Shaping [3]

The statistics of typical high PAPR signals are such that an ET PA typically spends

most of its time operating with relatively low supply voltage, with only occasional

high voltage excursions on high power peaks.



Envelope Tracking

1. Sensitive to possible delays between the

power supply signal and the RF signal.

However, less sensitive than in PT.

2. The efficiency of the overall system is very

dependent on the efficiency of the Envelope

Amplifier (currently limited to several

tenths of MHz.)

3. To use a slower version of the original

signal’s envelope to supply the PA causes

nonlinear distortion.

Main technological constraint: Similar to PT, the slew-rate and bandwidth

requirements of the Envelope Amplifier (Drain modulator) are very stringent when

considering current spectrally efficient modulations. However, unlike in PT, the drain

voltage does not need to follow exactly the signal’s envelope.

Drawbacks & Possible Solutions [13-17]

1. Digital compensating mechanisms at

baseband: Fractional Delay Alignments.

2. Use slower (slew-rate or bandwidth

limited) versions of the original signal’s

envelope to dynamically supply the PA �

trade-off between bandwidth and power

efficiency.

3. Linearization techniques: Digital

Predistortion


Outline

Introduction



Polar Transmitter

Envelope Tracking



Conclusion


Slew-Rate Reduction of the Envelope

One of the main challenges in ET and PT concerns the Envelope Amplifier (or

Drain Modulator). It has to efficiently supply the power required by the RF

transistor at the same speed of the signal’s envelope.

The Drain Modulator can be categorized into three types [19]:

1) Low dropout (LDO) regulator: linear but not efficient with high PAPR

signals)

2) Switched-mode power supply (SMPS): (efficiency inversely proportional to

the switching frequency�limited in BW)

3) Hybrid switching amplifier: consisting of an SMPS (current source that

provides large portions of power at low frequencies) and a class-AB amplifier

(wide-bandwidth linear voltage source)�Good efficiency/BW compromise


-20 -15 -10 -5 0 5 10 15-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

Frequency (MHz)P

ower

/freq

uenc

y (d

B/H

z)

Original EnvelopeSlow Envelope (N=30)Slow Envelope (N=100)

In ET, is possible to relax the high slew-rate and bandwidth requirements of EAs

by using a slew-rate [14] or bandwidth [15-17] reduced version of the signal’s

envelope.



However, the price for using a slew-rate or BW reduced version of the envelope

to supply the RF PA is a degradation of the power efficiency � bandwidth vs.

power efficiency trade-off [18]



In addition, because both the supply and RF input signals are not univocally

related, a time-variant nonlinear gain effect is observed at the RF output � the ET

PA presents a slow envelope-dependent (SED) nonlinear behavior.



Outline

Introduction



Polar Transmitter

Envelope Tracking



Conclusion


DPD for Envelope Tracking PAs

PADPD

Predistorter Power Amplifier



p

qq

P

p

Q

qpq nxnxny ][ ][][

0 0

ττγ −−=∑∑= =

Memory Polynomial (Original

Envelope to supply the RF PA)

SED Digital Predistortion [13](Slow Envelope to supply the RF PA)

( )LUTG ⋅

[ ]u n [ ]x n [ ]y n

ˆ[ ]x n

[ ]SE n[ ]SE n

( )0 0 0 0

[ ] [ ] [ ] [ ]QM N P q p

piqj s j i ij q i p

x n E n u n u nγ τ τ τ= = = =

= ⋅ − ⋅ − ⋅ −∑∑∑∑



DPD results considering Dynamic Power Supply with the Original Envelope [18]

DPD results considering Dynamic Power Supply with the Slow Envelope [18]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Input Amplitude

Out

put A

mpl

itude

without DPDwith memoryless DPD

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1

Input Amplitude

Out

put A

mpl

itude

without SED-DPDwith SED-DPD



( )

( )00 0 0

00 0 00 0 0

(·) (·) (·)

0

(·)

[ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] ...

... [ ] [ ] [ ]

N QLUT LUT LUT

NQLUT

P P PQp p Nom pp N pN N s s p Q

p p p

G G G

PQNom ps s N pNQ N

p

G

x n u n u n u n u n E n E u n u n

E n E u n u n

γ τ γ τ γ

τ γ τ

= = =

=

= ⋅ ⋅ + − ⋅ ⋅ − + − ⋅ ⋅ ⋅ +

+ − ⋅ − ⋅ ⋅ −

∑ ∑ ∑

∑

��

��

Signal

Generator

Towards a possible FPGA implementation:



LUT-based architecture of the SED-DPD [13]

( ) ( )0 0

[ ] [ ] [ ] [ ]Q N qNom iq

s s i LUT iq i

x n E n E u n G u nτ τ= =

= − ⋅ − ⋅ −∑∑

SED DPD implemented as a set of LUTs [13]:

without DPD

with dynamic SED- DPD


Outline

Introduction



Polar Transmitter

Envelope Tracking



Conclusion


Conclusion

Polar Techniques are promising candidates to overcome power efficiency

limitations of classical Cartesian Transmitters, yet some technological

constraints are still under research (e.g. in ‘all-digital’ Tx.).

Using reduced slew-rate versions of the envelope to perform ET is shown to

be a useful solution to cope with the aforementioned PAPR and BW

constraints in Envelope Amplifiers � Envelope BW vs Efficiency trade-off

Since these transmitter architectures are design to maximize power

efficiency, linearity levels must be meet incorporating linearization techniques

such as DPD.

In the path towards SDR, several correcting mechanism (time and amplitude

adjustments, nonlinear distortion compensation) are implemented at

baseband in Digital Signal Processors, giving the transmitter the required

flexibility and adaptability.


References

[1] Digital Front-End in Wireless Communication and Broadcasting, Editor Fa-Long Luo. Cambridge University Press 2011.

[2] H. Gandhi. Digital Predistortion Linearizes Broadband PAs. Microwaves and RF, pages 1–4,2008.

[3] G. Wimpenny, “Envelope Tracking PA Characterisation,” White Paper. Open ET Alliance. Nov. 2011.

[4] A. Birafane, M. El-Asmar, A. Kouki, M. Helaoui, F. M. Ghannouchi, ”Analyzing LINC Systems,” IEEE Microwave Magazine, vol. 11 ,

pp. 59-71, Aug. 2010.

[5] P.A. Godoy, C. SungWon, T.W. Barton, D.J. Perreault, J.L. Dawson, “A 2.5-GHz asymmetric multilevel outphasing power amplifier

in 65-nm CMOS,“ IEEE Topical Conference on Power Amplifiers for Wireless and Radio Applications (PAWR), Jan. 2011, Phoenix.

AZ, USA. pp. 57 – 60.

[6] P.M. Lavrador, T.R. Cunha, P.M. Cabral, J.C. Pedro, “The Linearity-Efficiency Compromise,” IEEE Microwave Magazine, vol. 11, pp.

44-58, Aug. 2010.

[7] F. H. Raab, P. Asbeck, S. Cripps, P. B. Kenington, Z. B. Popovic, N. Pothecary, J. F. Sevic, and N. O. Sokal, “Power amplifiers and

transmitters for RF and microwave,” IEEE Trans. Microwave Theory Tech., vol. 50, no. 3, pp. 814–826, Mar. 2002.

[8] D. Kang, D. Kim, Y. Cho, B. Park, J. Kim, B. Kim, “Design of Bandwidth-Enhanced Doherty Power Amplifiers for Handset

Applications,” IEEE Trans. on Microw. Theory and Tech., vol. 59 , pp. 3474 – 3483, Dec. 2011.

[9] R. Darraji, F.M. Ghannouchi, O. Hammi, “A Dual-Input Digitally Driven Doherty Amplifier Architecture for Performance

Enhancement of Doherty Transmitters,” IEEE Trans. on Microw. Theory and Tech., vol. 59 , pp. 1284 - 1293, May 2011.

[10] P.L. Gilabert, G. Montoro, P. Vizarreta and J. Berenguer, “Digital Processing Compensation Mechanisms for Highly Efficient

Transmitter Architectures” IET Microwaves, Antennas & Propagation, vol. 5, 963-974, June 2011.

[11] P. Reynaert, “Polar Modulation,” IEEE Microwave Magazine, vol. 12, pp. 46-51, Feb. 2011.

[12] G. Montoro, P.L. Gilabert, J. Berenguer and E. Bertran, “Digital Predistortion of Envelope Tracking Amplifiers Driven by Slew-

Rate Limited Envelopes,” IEEE International Microwave Symposium (IMS’2011), June 2011, Baltimore, USA.


References

[13] P. L. Gilabert and G. Montoro, "Look-Up Table Implementation of a Slow Envelope Dependent Digital Predistorter for Envelope

Tracking Power Amplifiers," IEEE Microw. Wireless and Components Letters, vol 22, nº 2, pp. 97-99, Feb. 2012.

[14] G. Montoro, P.L. Gilabert, E. Bertran and J. Berenguer, “A Method for Real-Time Generation of Slew-Rate Limited Envelopes in

Envelope Tracking Transmitters,” IEEE Int. Microw. Series on RF Front-ends for Soft. Defined and Cognitive Radio Solutions , Feb. 2010,

Aveiro, Portugal. pp. 1-4.

[15] D. F. Kimball, C. Hsia, P. Draxler, S. Lanfranco, W. Nagy, K. Linthicum, L. E. Larson and P. M. Asbeck,., “High-Efficiency Envelope-

Tracking WCDMA Base-Station Amplifier Using GaN HFETs,” IEEE Trans. on Microw. Theory and Tech., vol. 54, pp. 3848 - 3856, Nov.

2006.

[16] J. Jeong, D. F. Kimball, M. Kwak, C. Hsia, P. Draxler and P. M. Asbeck, “Wideband Envelope Tracking Power Amplifiers with

Reduced Bandwidth Power Supply Waveform and Adaptive Digital Predistortion Techniques,” IEEE Trans. on Microw. Theory and

Tech., vol. 57, pp. 3307-3314, Dec. 2009.

[17] C. Haiying, H.M. Nemati, A. Soltani Tehrani, T. Eriksson, C. Fager, “, Digital Predistortion for High Efficiency Power Amplifier

Architectures Using a Dual-Input Modeling Approach” IEEE Trans. Microw. Theory and Tech., vol. 60, pp. 361 - 369, Feb. 2012.

[18] P. L. Gilabert, G. Montoro, P. Vizarreta, "Slew-Rate and Efficiency Trade-off in Slow Envelope Tracking Power Amplifiers,"

German Microwave Conference (GeMiC'12), March 2012, Ilmenau, Germany.

[19] B. Kim, J. Moon and I Kim, “Efficiently Amplified,” IEEE Microwave Magazine, vol. 11, pp. 87-100, Aug. 2010.

[20] B. Kim, I Kim and J. Moon, “Advanced Doherty Architecture,” IEEE Microwave Magazine, vol. 11, pp. 72-86, Aug. 2010.

[21] V. K. Parikh, P. T. Balsara, and O. E. Eliezer, “A fully digital architecture for wideband wireless transmitters,” IEEE Radio and

Wireless Symp., Jan. 2008, pp. 147–150.

New Challenges in DPD

Linearization for High Efficiency,

Wideband PA Architectures

WML: Measurement, Design and Linearization Tech. for High-Efficiency Amplifiers

Supported by Spanish Government MICINN

TEC2011-29126-C03-02

New Challenges in DPD Linearization for High …mtt11/workshops/IMS/2012/WML/...New Challenges in...

Documents

Transcript of New Challenges in DPD Linearization for High …mtt11/workshops/IMS/2012/WML/...New Challenges in...