Introduction to Envelope Tracking - Cambridge Wireless

1 Introduction to Envelope TrackingG J WimpennySnr Director Technology, Qualcomm UK Ltd EER first proposed by Leonard Kahn in 1952 to improve efficiency of SSB transmitters ET offers similar efficiency enhancement to EER but has fewer drawbacks Technique not widely adopted for many years due to difficulty of implementation, particularly for wide bandwidth signals In the last few years the implementational issues have been overcome and ET is now widely used to improve PA efficiency in Cellular HandsetsEnvelope Tracking Historical Context2 Modern high spectral efficiency Wireless communications standards have high Peak to Average Power Ratio (PAPR) 4G/LTE, WiFi Conventional fixed supply Power Amplifier has to use supply voltage high enough to support Peak Power, but is only energy efficient at the peaks. Most of the time the voltage is much higher than needed, resulting in high PA heat dissipation In ET, the PA supply voltage is dynamically adjusted to the instantaneous amplitude of the signal, resulting in high PA efficiency at all timesEnvelope Tracking Overview3 The efficiency of a Fixed Supply PA is much lower at average Pout than at peak Pout.

2 ET PA efficiency is only slightly lower at average Pout The difference in average PAPR between fixed supply and ET PAs increases with signal PAPRPA Efficiency Curves: ET vs Fixed supply4 Need for PA efficiency enhancement techniques such as ET continues to grow Envelope Tracking Implementation difficulty increases with carrier bandwidthWireless standard trends Increasing Spectral EfficiencyIncreasing PAPRS tandardLaunchedTypical Carrier BW (MHz)Typical Spectral Efficiency (bps/Hz)Approx PAPR(dB)1G cellularWCDMA cellularGSM + cellularLTE200920165-10 WiFiIEEE cellularLTE-A201320305-105G cellular5G-NR2018100-5-10 Decreasing Conventional PA efficiency5ET System ElementsEnvelope detection: most accurate if performed in digital domainEnvelope shaping: Determines relationship between RF power and PA supply voltageEnvelope Amplifier: High BW, Low Noise, High efficiency Amplifier used to generate PA supply voltageET PA: ET can be applied to standard fixed supply PA.

3 Improved performance possible by optimising PA for ET operationPA operates in polar mode at high instantaneous power and linear mode at low instantaneous powerDelay Alignment: ET requires accurate (~ns) timing alignment between Envelope and RF paths. Most accurate / repeatable if performed in digital domain6 BW of Envelope path needs to be 2-3x RF bandwidth Delay alignment between RF and Envelope paths at PA (interfaces B and C) is essential to achieve good RF linearity (ACLR / EVM). Sub-sample timing alignment ( <ns) required for wideband signals Analogue imperfections in both Envelope and RF paths must be corrected Gain / DC Offset in Envelope path Gain vs frequency in RF pathEnvelope Tracking Signal ProcessingDifferential Analogue Envelope Processing Basics Swing Range Optimise efficiency of combined modulator /PA Prevent gross PA nonlinearity due IV curve knee Envelope Shaping Control Envelope bandwidth Optimise efficiency Can be used to linearisePA Timing Alignment Timing error leads to memory effect (AM-PM) Fine adjustment necessary (~1ns) Minimum PA supply voltage (Vmin)

4 Set by Envelope shaping table Determined by PA technology HBT Vminchosen to ensure the PA never operates in its highly non-linear region At high power, PA supply voltage swings between Min voltage defined in shaping table up to Max voltage supported by PA As power is backed off, Vmax falls but Vminremains unaltered swing range reducesET High Power operation9 ET generally used for highest 10dB operating power range At lower power there is insufficient swing range to significantly improve PA efficiency Static power dissipation of ET modulator starts to outweigh PA efficiency benefit Average Power Tracking (APT) typically used at lower powers PA DC supply voltage varied with average slot powerET Lower Power operation10 Envelope Tracking PA can be considered to be a 3 port network RF Input, Supply Input, RF output PA Gain, Phase, Efficiency influenced by PA Device technology PA Circuit design (matching, biasing) Instantaneous Supply Voltage 3D Characterisation of PA surfaces allows ET system performance to be predicted Gain, Phase, Efficiency vs Instantaneous (Pin,Vsupply)ET PA = 3 Port DeviceET PARFinRFoutVsupply11 Envelope Shaping table defines Rfinto Vsupplymapping Once shaping is defined the ET subsystem is reduced to a 2 port network Envelope Shaping determines key ET PA black box metrics Linearity (AM/AM and AM/PM)

5 Efficiency Gain System Linearity metrics for a wide range of waveforms can then be derived Allows EVM / ACLR / Efficiency trade-offs to be explored PA / system memory effects are not captured but good predicted vs measured results can still be obtained for low-med BW waveformsET PA + Shaping Table = 2 Port DeviceET PARFinRFoutVsupply12 Characterisingthe ET PATest methodology PA current measurement Supply impedance Supply bandwidth requirements ET Efficiency prediction ET Linearity prediction Parameters measured Swept CW testing Bench PSU Low (decoupling Capacitor) Low (Bench PSU) Poor, due to PA die heating Poor, due to PA die heating Gain (AM:AM), Efficiency Pulsed RF /DC testing Instrumentation grade current probe, ~5 us resolution Low (decoupling Capacitor) Low (Bench PSU) Good, if short pulses (~10 us, 10% duty cycle). Fair Gain (AM:AM), Efficiency Dynamic supply modulation Challenging high BW with high common mode voltage current sense Requires low impedance dynamic supply (no decoupling) High (~60 MHz BW) V.

6 Good V. Good Gain (AM:AM), Phase (AM:PM), Efficiency 13PA Surface Characterisation setupSupply Modulator: High BW / Low output impedancePA Supply Current: High BW / High CM rejectionDynamic measurement of:Gain, Phase, Efficiency14 Example 3D surface model of ET PATrajectory across surfaces set by choice of shaping table shown by black linesGain SurfacePhase SurfaceEfficiency SurfaceGain (dB)15 Alternative 2D views of 3D ET Gain Surface2D Shaping table view(Colour = Gain)3D view2D Waterfall viewGain vs Powerparameterised by Vsupply16 ET PA shows gain expansion Kink in AM/AM characteristic is very difficult to linearise high BW requiredET Shaping Table -Optimum Efficiency vs IsogainOptimum Efficiency ShapingIsogainshapingEfficiencyGain Flat gain characteristic No DPD required if ET PA has low AM/PM17 IsogainShaping FamiliesHigher GainLow swing rangeLow EfficiencyLower GainHigh swing rangeHigh Efficiency18 Unlike traditional PAs, low voltage gain reduction is desirable as it allows isogainoperation with high gain compression at high voltages (= high efficiency)

7 Gain peaking with fixed voltage supply is unimportant these points are not visited when in ET mode Flat gain at lowest voltage is required (PA operates in linear mode at low voltages) Best ET efficiency achieved if high peak efficiency can be maintained at low supply voltagesIdeal ET PA Gain characteristic19 Different Phase optimisation required for ET PA Best ET PA performance achieved by designing PA for ET to meet ET performance objectives from the outset, rather than by adapting an APT PAET PA AM/PM optimisationAPT optimised PA(Flat AM/PM with fixed supply) ET optimised Phase Characteristic(Flat AM/PM with isogainshaping)20 When operating in ET mode a PA is in compression and acts as a mixer Noise and distortion on ET supply mixes with RF generating unwanted sidebands Supply noise transfer is significantly higher than for fixed supply or APT PA Supply noise transfer is controlled by PA compression level set by ET shaping table To maintain full ET efficiency benefit, supply noise needs to sufficiently low to avoid need to back off compression level Some distortion mechanisms can be corrected at system level using DPD, others cannot Correctable distortion: Frequency Response, ET modulator Output Impedance Non-correctable noise/distortion.

8 Switcher noise, Slew rate limiting, Thermal noiseET Noise and Distortion mechanisms in ET90% Supply Sensitivity21 Supply Sensitivity Determines Supply Noise RF conversion Partial derivative of PA RF output voltage wrtPA supply voltage = Ideal AM mixer has S=1 (100% supply sensitivity)ET PA in hard compression S ~ 1, Fixed Supply Linear PA S ~ DC drain 40 MHz injected tone rmsCalculated RF sideband level for ideal AM RF sideband level -51dBCPA Supply Sensitivity (dB) Supply Sensitivity (%)85%envenvrfrfVVVV envenvVV rfrfVV dBm 0-10-20-30-40-50-60-70-80-90-100 SoftPlot Measurement PresentationTrace AStart: 0 HzStop: MHzRes BW: 100 kHzVid BW: 300 kHzSweep: ms17/11/2010 18:44:43 FSEB 30 Skyworks 174 isogain 25dB Avg DC drainvolts = peak = A40 MHz 0dBm injected toneMeasurement ParameterValueChannel MHzChannel MHzOn- channel dBmAdjacent channel power (channel -1) dBAdjacent channel power (channel +1) dBMkr Trace X-Axis Value Notes11 Trace A dBm2-12-1 Trace A dBm{sum}dBm 0-10-20-30-40-50-60-70-80-90-100 SoftPlot Measurement PresentationTrace AStart: GHzStop: GHzRes BW: 100 kHzVid BW: 300 kHzSweep: ms17/11/2010 18:36.

9 58 FSEB 30 Skyworks 174 ET Iso gain 25dB 5 MHz AInjected CW tone (0dBm @ 40 MHz)Mkr Trace X-Axis Value Notes11 Trace A dBm2-12-1 Trace A GHz dBm{sum} 33 Trace A dBm4-34-3 Trace A dBm{sum}55 Trace A dBm6-56-5 Trace A dBm{sum}40 MHz test tone added to Envelope Amplifier O/P(whilst amplifying 5 MHz WCDMA signal)Corresponding RF sidebands40 MHz40 MHzSupply SpectrumRF Spectrum22 Envelope Tracking Modulator Requirements Hybrid Switch-mode / Linear-modeET Modulator23 Very simple hardware ET much more amenable to open loop correction than fixed supply PA operates in compression over most of Envelope cycle Moderate linearity performance Moderate bandwidth capability No direct phase correction relies on PA designed for low ET AM/PM No memory correction high intrinsic ET modulator performance neededET CFR Via Shaping TableConfigurable soft clipping provides simple CFR for free 24 IQCFR|r| PAEnvelope DetectionEnvelope ShapingEnvelope AmplifierMemoryless DPDDPDET PA SubsystemET and Digital PreDistortionCFR applied to source waveform (both Envelope and RF are clipped)DPD only needed to clean up residual AM/AM errors and correct ET PA s AM/PM (if required)25 Thank YouQuestions