Envelope Tracking : Fuel injection for the RF Front End

Cambridge Wireless, 25 Sep 2014

jeremy.hendy@nujira.com

226/09/2014

Nujira Company overview

• Fabless IC vendor

• 60 employees, $100M of equity funding to date

• Closed $21M funding round April 2014

• Envelope Tracking IC’s are company’s sole focus

• The only technology for broadband, high efficiency RFPAs

• Improves battery life & signal quality, cuts heat dissipation

• Applicable to 3G, 4G/LTE and 802.11ac

• >400 man-years investment in High Accuracy Tracking™

• Committed to making ET Easy for customers

• 250+ patents granted & pending

• Most extensive ET patent portfolio in the industry

• Headquartered in Cambridge, UK

• Design Centre in Cambridge

• Sales in US, Germany, Korea & China

326/09/2014

Envelope Tracking 101

Conventional Amplifier

PowerAmplifier

Baseband /

RF upconverter

DC/DC Converter

PowerAmplifier

Baseband / RF upconverter

Envelope signal

Envelope TrackingAmplifier ET

chip

Power from Battery Radio Frequency Wave

Conventional Amplifier

Heat Dissipation

Envelope Tracking Amplifier

426/09/2014

ET system context

• 50k-100k gates of logic

• Fast ET DAC (~10 bits, ~300 MSPS)

• Industry-standard MIPI RFFE serial control interface

• Changes to calibration & TX power control firmware

Transmit chipset

Power AmplifierDAC

DAC

DAC

Magnitude

calculator

RF drive

matching

Envelope

shaping

Gain &

offset

calibration

Fixed

Delay

Envelope

Tracking

Modulator

B

C

Vbatt

I

Q

A

DPD

Variable

Delay

CFR

526/09/2014

12 Phones with ET inside

Nexus 5

Nov-13

iphone 6 / 6 Plus

Sep-14

One M8

Mar-14Fire

Aug-14

Nubia-Z5S

Dec-13Grand Memo

S2 Aug-14

Experia Z2 Tablet

Mar-14

Galaxy Note 3

Sep-13

Galaxy S5

May-14

Galaxy S5 Mini

Jul-14

Galaxy Round

Nov-13

Galaxy S5 LTE-A

May-14

626/09/2014

2G / EDGE

3G WCDMA

4G LTE Uplink

QPSK

Increasing waveform complexity is pushing up PA energy consumption

WiFi 802.11ac

256-QAM

Reference:

GSM = 100%

GSM

LTE Downlink

LTE-A Uplink

40 MHz/200RB

726/09/2014

System efficiency, including ETIC

826/09/2014

2G / EDGE

3G WCDMA

4G LTE Uplink

QPSK

Higher PAPR = More benefit from ET

WiFi 802.11ac

256-QAM

Reference:

GSM = 100%

GSM

LTE Downlink

LTE-A Uplink

40 MHz/200RB

926/09/2014

Higher PAPR = More benefit from ET

2G / EDGE

3G WCDMA

4G LTE Uplink

QPSK

WiFi 802.11ac

256-QAM

LTE Downlink

LTE-A Uplink

40 MHz/200RB

1026/09/2014

ET with advanced waveforms

• LTE uplink waveforms are relatively “benign” for ET

• Designed to give low PAPR (SC-FDMA)

• What’s the impact on ET of more advanced waveforms?

• Higher order QAM

• Carrier Aggregation – LTE 20+20

• Downlink waveforms

• “Proper” OFDM – eg 802.11ac

1126/09/2014

Nujira’s ETIC architecture

Switch

mode power

supply

++AC

Amp

-

Envelope

reference signal

input from

modem chipset

RF PA

High Accuracy Tracking (HAT™) ET architecture

Fast Tracking linear

AC Amplifier:

High Bandwidth

Low Noise

Low Efficiency

Slow tracking

Switcher:

High Efficiency

High Power

Low bandwidth

High Noise

External feedback

path:

Cancels noise

Reduces output

impedance

1226/09/2014

• Motorola patent (1999)

• UCSD/Ericsson, QCOM

• AC Amp and Switcher in

series, Switcher “follows”

• “Reactive” architecture

• Senses AC amp overload and

dumps current into PA

• Main issues : bandwidth,

efficiency

• Nujira HAT™ architecture

(patented 2003)

• Switcher and AC Amp in

parallel, Switcher “leads”

• AC Amp cleans up after

switcher noise

• “Proactive” architecture

ETIC architecture comparison

Switcher

++AC

Amp

-

PA

Switcher

++AC

Amp

-

Current

sense

Very Fast

Switcher

PA

PA

• Fast switcher only

• Simplest

architecture

• Can’t hit “real

world” performance

requirements

• Main issues:

Swing Range, Noise

& output impedance

1326/09/2014

Increasing modulation orderincreases the variance in signal envelope

1426/09/2014

Envelope waveform statistics vary significantly with modulation type

LTE Uplink – low PAPR

(SC-FDMA)

LTE-A, LTE Downlink, WiFi – high PAPR

(Clustered SC-FDMA / OFDMA)

1526/09/2014

LTE Uplink, Downlink & CA Uplink

1000 ns

1626/09/2014

200RB Carrier Aggregation adds 2-3 dB to crest factor

1726/09/2014

Baseband Crest Factor Reduction achieves ~1.5-2 dB reduction in PAPR for same EVM

1826/09/2014

Impact of waveform on bandwidthof envelope signal power delivery

~250 mW

from ETIC

@ 23 dBm ant

Switcher AC amp

1926/09/2014

High frequency envelope content increases from ~5% to ~20%

2026/09/2014

More HF content = more powerdelivered by ETIC’s linear amplifier

• Ratio of HF to LF power influences ETIC efficiency

• SMPS can deliver DC + LF very efficiently (>90%)

• Linear Amp theoretical class AB efficiency ~40%

• Plus bias currents, input buffers, etc

Switch mode

power supply

++AC

Amp

-

2126/09/2014

Increasing RF channel bandwidth

• Beyond 80 MHz RF channel BW, ETIC/PA interconnect

becomes a performance limitation

• Stray inductances, bond wires, etc

• Electromechanical solution : integration of ETIC with PA

• Module level / multi-die

• Monolithic with CMOS PA

• Signal processing solution : envelope BW reduction

• Requires “2D-DPD”

2226/09/2014

80 MHz 802.11ac signal

100 ns

• Slew rate ~400V/us

2326/09/2014

2D DPD for Envelope BW reduction

• Simple LPF does not meet constraint Vsupply >Vrawenv

• Various techniques possible to reduce envelope BW

• Trade-off between Env BW and Efficiency necessary

• Technique can extend ET to systems having

instantaneous RF BW >50MHz

Envelope BW reduction

~3x RFBW→ ~1x RFBW

Efficiency penalty: ~4%

Constraint:

Reduced BW

Envelope

> Raw envelope

at all times

2426/09/2014

2D-DPD for MIMO

• Hardware simplification

• 1 ET path / DAC

• 1 ETIC supplies all

MIMO branches

• PA supply voltage just

sufficient for highest

power channel

• Memory or Non-memory

2D-DPD correction

• Modest loss of efficiency

~4-5% for 4 way Mimo

2.05 2.1 2.15 2.2 2.25 2.3 2.35 2.4

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Example results of 20MHz LTE 2D DPD MIMO

implementation (memory and memoryless)

*Yu and Zhu: Single Envelope Modulator-based ET for MIMO. IEEE Trans MTT Oct 2012

2526/09/2014

ET MIMO options

BBIC/RFIC

ET Path &

ET DAC

DPD?ET

Switcher

ET AC

Amp

Efficiency

impact

Dedicated ETIC

per branchN None / 1D N N None

Shared ETIC for

all branches1 2D-DPD 1 1 2-4%

Distributed ETIC

architectureN None / 1D 1 N Negligible

• Spatial separation favours Dedicated or Distributed

• Distributed best suited for “zero cost ET” integration

• Shared has highest baseband / signal processing cost

2626/09/2014

Take-aways

• The relative ET benefits are increasing as waveforms get more complex• But think about the waveform statistics – not just PAPR

• Higher PAPR and/or higher order modulations favour ET• Without ET, PA must be backed off and heavily biased

• Baseband CFR is better than relying on the PA to clip• Can reduce PAPR by ~1.5 dB (more output power or higher eff)

• Channel bandwidth is the limiting factor for discrete ETICs• Will drive module- or chip-level integration of PA with ETIC

2726/09/2014

Where next with ET?

• Second-generation ETICs

• Higher efficiency

• Improved backoff -> ET benefit over wider power control range

• Higher peak power for CA

• Higher bandwidth for CA / 802.11ac

• Integrated CMOS PAs

• ET overcomes linearity deficiencies of CMOS A

• Monolithic co-design of ET + PA structure

• Moves performance-critical interface on-chip

• Enables >100 MHz RF ch BW

Carburettor Fuel Injection

First invented 1893 1925 / 1974

Mass adoption 1908 – 1990 1981 – present

System complexity Low High

Component complexity High Low

Control method Analogue (foot) Digital (ECU)

Supply bandwidth ~1 Hz ~1,000 Hz

Fuel efficiency 15 mpg 50 mpg

Performance 60 HP / litre 200 HP / litre

Market status Obsolete Ubiquitous

2926/09/2014

DC supply Envelope Tracking

First invented 1885 1929 / 2002

Mass adoption 1885 – present 2013 -

System complexity Low High

Control method None (fixed) Digital (DSP)

Supply bandwidth ~1,000 Hz ~60,000,000 Hz

Fuel efficiency 20% 60%

Performance 1x (RF Watts/$) 2x (RF Watts/$)

Market status Ubiquitous Emerging