Switched-Capacitor Power Amplifiers for NB-IoT · 2018-02-28 · Oscillator N digital bits Antenna...

Switched-Capacitor Power Amplifiers for NB-IoT

Salvatore Levantino

Salvatore Levantino, Associate Professor, DEIB Department

Outline

Ø Internet-of-Things communicationsØDigital RF power amplifiersØHigh-efficiency digital PAsØSwitched-capacitor class-D PAs


What is Internet of Things?

www.qualcomm.com


Evolution of LTE for Internet-of-Things

LTE-M NB-IoTLTELTE-A

ScalingupinperformanceandmobilityScalingdownincomplexityandpower

Applications

Mobile Connected cars Smartwatch Smart cities Smart metering

www.qualcomm.com

3GPPRelease13+Today


Short-rangewireless(Bluetoo

th,

Wi-Fi,NFC)

LTE technologies for IoT devices3GPP Release 13+

Performan

ce

LTECat-1andabove

15dB

5dB

LTECat-M1(eMTC)BroadestrangeofIoTcapabilitieswithsupportforfutureadvancedfeatures,e.g.voicesupport

LTECat-NB1(NB-IoT)Scalabletolowestcost/powerfordelay-tolerant,low-throughputIoTusecases,e.g.remotesensors

Highe

rDataRa

teLower

Power

www.qualcomm.com

LTECat-M11.4MHz– Upto1Mbps

LTECat-NB1200kHz– 10sofKbps

Coverage


Reduced LTE complexity to enable low-cost modules

www.qualcomm.com

LTECat-1 LTECat-M1 LTECat-NB1

Peakdatarate

Bandwidth

DL:10MbpsUL:5Mbps

DL:1MbpsUL:1Mbps

DL:20kbpsUL:60kbps

20MHz 1.4MHz 200kHz

RXantenna MIMO SingleRX SingleRX

Duplexmode FullduplexFDD/TDD

Full/HalfduplexFDD/TDD

HalfduplexFDD

Transmitpower 23dBm 23or20dBm* 23or20dBm*

Reducedbaseband

complexityandmemorysize

Higherthroughput,lowerlatency,highermobility*integratedPAispossible


Summary

Ø IoT applications require low cost radios. Many use cases require km range and multi-year battery life

Ø Evolution of cellular communications for IoT leverage existing infrastructureand spectrum

Ø The standards are defined to reduce complexity and enable efficient radio transceivers with DSP and power amplifier integrated in a single chip in silicon


Outline



Why RF power amplifiers are important?

13%

44% 14%

14%

15%

CellularHandsetPowerConsumptionBreakdown

Transceiver

RFPA

AppsProcessor

Screen

Others

Ø RFpoweramplifiers(PAs)are

responsibleforlargepartofmobile

devicespowerconsumption

Ø IncreaseinPAefficiencywouldmean

longerusagetime,smallerbattery,

lowercost

Nujira


What is a digital power amplifier or a digital RF transmitter?

DigitalPhase

Modulator

DigitalSignal Processor

DigitalPower

AmplifierLocal

Oscillator

Ndigitalbits

Antenna

Mdigitalbits

FullintegrationinCMOSispossible


Why digitize an RF transmitter?

Ø To achieve high power efficiency at linear output powerØ Small die area, CMOS scaling friendlyØ Digital control: easy to reconfigure, flexible

DigitalPhase

ModulatorDPA

DigitalSignal Processor

EnablelowcostIoTradios


Peak-to-average power ratio

Ø More data throughput (Mb/s) into a finite spectrum resource (MHz) (Higher spectral efficiency)

Ø Typically, the higher the spectral efficiency the larger the peak-to-average ratio (PAPR) of the transmitted signal

Amplitud

e

Time

Average

Peak

PAPR


PA power efficiency

Ø Power efficiency degrades with back-off from peak amplitude

Ø Average signal amplitude is much lower than the peak

Ø Average efficiency:

Ø Average efficiency is lower than peak efficiency

Efficiency

Amplitude,A

!PDF "�

�$ Efficiency " $ %"

Amplitude,A

PDF


PA linearity

Ø PA experiences compressionprior achieving peak output power

Ø Compression causes signal distortion, quality degradation and spectral regrowth

Ø Additional signal back-off needs to be applied to meet the required signal quality

Outpu

tAmplitud

e

InputAmplitude

Time

Outpu

tAmplitud

e

Ideal

Real

Max


Impact of PA compressionLTE Cat-M1 signal with 16-QAM modulation and SC-FDMA

KeysightSystemVueandVSA

Spectralregrowthin

adjacentchannels

Constellationerrors

6dB 2.5dB


Drivers and Technical Challenges and Tradeoffs

CMOSintegration

LinearityPowerefficiency

Low-costpervasivenodes

Kmrange

Highdatarate

Longbatterylife

RFPAtradeoffs

Drivers

Drivers


Outline



Efficiency of digital PAs

Ø Achieve high-efficiency at back-off output power:• Switching operation

Ø Minimize transistor power dissipation:

Vdd

RC L

ID

+–

VDS

&'!()* +

�

�$ ,) + $ %+

VDSID

t

VDDIbias

ID

tVDS

t

Ibias

VDD


DPA efficiency: switching operation

Ø Input voltage is a square-wave rather than a sinusoid

Ø Amplitude is modulated controlling the number of active units

Ø Lower power dissipation (VDSID)Ø Class D-1 operation

Vdd

RC L

ID

+–

VDS

VDSID

tID

tVDS

tN units

VDD

D.Chowdhury,JSSC,May2012


Digital RF transmitters

Ø Digital PAs allow direct digital control of PA amplitude and phaseØ Enable high-efficiency nonlinear PA topologiesØ Leveraging DSP to correct for PA distortion, mismatch, spectral regrowth, etc.Ø Possible in modern CMOS processes where DSP cost is much lower

DigitalSignal Processor:• Signalconditioning

• Distortioncorrection

DigitalPA

HighefficiencyHighdistortion


Area efficiency

DigitalSignal Processor:• Signalconditioning

• Distortioncorrection

DigitalPA

AnalogPA

DAC

LC LCVGALPF Driver

Digitalinverterchain

ConventionalTX

DigitalTXAreasaving Areasaving Samearea


Summary

Ø In recent years, a new design paradigm has emerged which is based on digital PAs and digital RF transmitters

Ø It allows the adoption of high-efficiency PAs, leveraging the low cost of DSP to correct for PA nonlinearity


Outline



Class-D power amplifier

Ø A CMOS inverter driven by a square-waveproduces an inverted square-wave VDS

Vdd


Class-D power amplifier

Ø A CMOS inverter driven by a square-waveproduces an inverted square-wave VDS

Ø The series LC tank filters out odd harmonic current

Ø Output voltage across the load resistance R is a sine-wave

Ø Theoretical 100% efficiency, as VDSID is always zero

Vdd

ID,pmos

tVSD,pmos

t

VDD

ID,nmos

tVDS,nmos

t

VDD

ID,pmos

ID,nmos

RCL


Class-D DPA

Ø How to digitally control the amplitude of the sinusoid:

• Replicate device N times• Activate only n < N

device• Selection of active cell

done through gating of the phase signal

Vdd

RCL

N unitcells


Switched-Capacitor PA (or SCPA)

Ø Series cap is also split into the N unit cells

Vdd

RCi L

EN

N unitcells

RCi L

n activecells

Ci

(N-n) inactivecells


Class-D DPA vs. SCPA

Ø No dissipation in the capacitive divider

RCi

L

n activecells

(N-n)Ci

SCPA

R

L

(N-n)Ci

nCi

Class-DDPA

RC L

RC L

Rmos

n activecells

Ø Dissipation in the resistive divider


Power efficiency (Class-D DPA vs. SCPA)

Ø Class-D DPA efficiency degrades quickly at lower output power

Ø SCPA maintains high efficiency across the output power range

Class-D

DPA

Idealefficiency

[%]

Normalizedpower[dB]


Digital PAs vs. discrete PAs

Ø DPAs can beat performance of most discrete PAs

Ø DPAs can be integrated into CMOS SoC with much lower added cost

Ø Discrete HBT PAs still has performance advantage at much higher cost

Efficiency

[%]

Linearoutputpower[dBm]

3010 2015 250

5

15

10

20

25

30

35DPA HBT

SiGe


Impact of CMOS scaling on SCPA

Ø Peak efficiency increases Ø SCPA power density slightly degrades

Drai

n ef

ficie

ncy

[%]

Drai

n ef

ficie

ncy

[%]

Transistor area [µm2] SCPA area [µm2]

A.Truppi,tobepresentedat IEEEISCASConf.,2018


Conclusions

Ø Many IoT use cases require low cost devices with km range and multi-year battery life

Ø The new LTE standards for IoT exploit cellular infrastructure to increase coverage and penetration

Ø NB-LTE-radio transceivers can be integrated in a small-footprint single-chip integrated in CMOS, but require transmitters capable of delivering 20-to-23-dBm linear output power


Conclusions (Continued)

Ø A new design paradigm has emerged in recent years, which is based on the adoption of high-efficient digital PAs, which leverage the low cost of DSP to correct for PA nonlinearity

Ø SCPAs in CMOS allow power efficiency and area suitable for LTE IoT applications

Ø CMOS scaling has a positive impact on SCPA power efficiency, though it slightly degrades power density

Switched-Capacitor Power Amplifiers for NB-IoT · 2018-02-28 · Oscillator N digital bits Antenna...

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