Gareth LloydRohde & Schwarz, MunichNovember 2016

Linearization of Frontends for 5G

What is Distortion?ı Distortion on the transmitted (or received) signal may be quantified

using measurements such as: ACLR (adjacent channel power or spectral regrowth) EVM (error vector magnitude) Harmonics (energy at integer multiples of the intended frequency)

ı Introduced by components in the RF Frontend (e.g. amplifiers, mixers, filters, A2D, D2A).

ı Types of distortion include: Non-Linear Distortion Linear Distortion Memory Effects (self-heating, PSU) Noise (quantization, thermal)

ı Communication systems are usually designed to accommodate typical or practical levels of distortion

Source: “Linearization of RF Frontends”1MA269 White Paper, Rohde & Schwarz

Typical Frontend impairments measured on a commercial Frontend with Upconversionusing SMW and FSW.

K70 (VSA)K18 (Amplifier)K40 (Phase Noise)

Linearizationı Two ways to create sufficient linearity in a Frontend Using back-off, or the “IP3 approach” Reduce distortion with “Linearization”

ı The question to be answered is: “What is the most efficient way to generate just enough fidelity?”

ı Linearization is the efficient reduction of distortion to acceptable levels.

ı The industry buzzword is Digital Pre-Distortion (DPD) When feasible, it is a no-brainer. Problems occur for Economic (cost) or Technical

(power consumption) reasons.

ı First intentional linearization scheme invented by Black in 1925, called “Feedforward”.

Digital Pre-DistortionLinearizationSource: Maxim

Feedforward LinearizationSource: Microwave Journal

Linearization for 5G ─ Problemı The first choice “DPD” might not be appropriate for

mmWave 5G because: The Frontend will comprise multiple transmit paths

to support hybrid beamforming Each RF path will transmit lower power than 4G The signal bandwidth will be much greater than 4G

ı Each transmit path requires its own DAC and Up-Conversion (in addition to the PA). For DPD, one or more Down-Conversion and ADC paths also needed.

ı With “lite” schemes (freq. stitching, ADC sharing, sub-sampling) power consumption/cost will still spiral. Even state-of-the-art 4G/LTE mobile devices do

not use full DPD today

ı Therefore, it’s useful to view all kinds of Linearization, to make sure we’re not missing something …

Hybrid Precoding for mmWaveSource: profheath.org

Digital Pre-DistortionSource: Maxim

Classification of Linearization Techniquesı There are a plurality of linearization techniques in

the literature.

ı To help understand the Linearization techniques, they may be classified according to whether they: use Predicted or Measured distortion are applied Before or After the distortion source

ı This way, order is restored and general characteristics may be identified e.g.: “Predicted” offers unlimited linearization capability “Measured” offers designable linearization “Pre-source” are potentially more efficient than

“Post-source”

Source: “Linearization of RF Frontends”1MA269 White Paper, Rohde & Schwarz

Source: “Transmitter Linearization using Cartesian Feedback for Linear TDMA Modulation”; Johansson et. Al, IEEE, 1991

The Limits of Linearizationı Ideal linearization is represented by the “hard limiter”

or “hard clipper” case: AM-AM (gain compression) curve has two distinct

regions Constant gain (i.e. perfectly linear, signal

envelope is preserved) Constant power (i.e. saturation, signal envelope

is clipped causing distortion) No AM-PM (phase) distortion

ı Example comparison of Linearized vs Back-Off / IP3 of a component (e.g. mixer or amplifier), for an IM3 goal of: -52 dBc -72 dBc

Hard Clipping Gain Compression & Phase Distortion (AM-PM)

IP3 of an Off-the-Shelf Mixer and its Linearized variant.

Source: “Linearization of RF Frontends”, 1MA269 White Paper, Rohde & Schwarz} … the size

difference is…{ 8 dB (-84%)18 dB (-98%)

Example ─ Predictive Post-Correctionı Whole family of predictive/post-correction architectures

derived from 3 basic types (Doherty, Envelope-Tracking, and Outphasing)

ı Characterized by 2 or more signal paths (not necessarily all RF): Those 2 or more signal paths efficiently generate signal

components The signal components are combined in a controlled

way to “build” or reconstruct the desired signal

Source: “Linearization of RF Frontends”; 1MA269 White Paper, Rohde & Schwarz

Example ─ Feedforwardı As an RF-in RF-out technique, it may be

seamlessly combined with other linearization methods.

ı Feedforward uses only passive components, but achieves significant broadband linearization effect.

ı In 1MA269, a mmWave Frontend model is linearized using different methods, including Feedforward.

ı Savings of 60% in Amplifier Size, Power Supply Size and Waste Heat are achieved, even in the simple example. Source: “Linearization of RF Frontends”

1MA269 White Paper, Rohde & Schwarz

INPUTOUTPUT

Conclusionsı Linearization of mmWave Frontends for 5G will

be much more challenging than for 2-3-4G

ı Many DPD implementations might not be viable, because the RF Chains: Are lower RF power Have (one or more) Up-conversion stages Operate with wider signal bandwidths Are more numerous… causing cost and power consumption hits

ı There are many Linearization alternatives which may be suitable for stand-alone use (e.g. Doherty or Feedforward), or to combine to simplify predistortion (analog, digital, etc.)

ı The dual output SMW plus FSW combination is a powerful development environment, enabling many possibilities: Measurements using, e.g. dedicated

personalities like: FSW-K18, FSW-K40, FSW-K70

Applications, e.g.: 1MA279 - Doherty, Differential, Spatial &

Balanced Amplifier Development 1MA269 - Multiple input schemes such as

Outphasing, ET, Doherty Feedforward, etc.

Back-up Slides

1MA279: The Classic Doherty Amplifierı The Doherty amplifier is widely used in moderate bandwidth

applications, across many industries. Offers higher efficiency/narrower bandwidth than its Class

AB relative.

ı The Doherty “input network” (splitter) dictates efficiency,linearity, output power and bandwidth – i.e. the “money specs”. in the case of Doherty, it may be either “Linear” or “Non-

Linear” replacing the “industry standard” input network with

something bespoke can yield significant improvements

ı The App Note details that hardware-in-the-loop process applied to a reference design from NXP™.

Source: “Doherty, Balanced, Push-Pull & Spatial Amplifier Performance Enhancement”1MA279 App. Note, Rohde & Schwarz

1MA279: The Classic Doherty Amplifierı 10,000s of measurements were made on the DUT, with differing

frequency and emulated, linear, “input network”

ı 5 alternative example scenarios were identified in the work, engineering the linear input network, compared with the baseline:1. 7,6%-pts higher efficiency, 1,9dB average power2. 4,4dB linearity improvement, 0,8dB increase in average

power.3. 3x bandwidth with baseline efficiency, linearity, average power.4. 3%-pts increase in efficiency, -0,3dB change in saturated

power.5. 2x bandwidth, -0.6dB change in saturated power

ı Synthesis of the alternative input networks was demonstrated using CST™. Source: “Doherty, Balanced, Push-Pull &

Spatial Amplifier Performance Enhancement”1MA279 App. Note, Rohde & Schwarz

1MA279: Extension toBalanced, Differential & Spatially Combinedı The linear “Input Network” technique demonstrated for

optimizing amplifier bandwidth, may be extended to building-block multi-amplifier techniques, including: Balanced Differential (Anti-Phase/Push-Pull) Spatially Combined… in addition to the Doherty

ı The basic concept of “money spec optimization” by engineering the input network remains.

ı These 4 “building blocks” are used everywhere; agnostic to both Industry and Application, but especially for mmWave.

Balanced AmplifierSource: mwrf.com

Push-Pull AmplifierSource: nxp.com

Spatial Combined AmplifierSource: Cheng et.al (IEEE, 1999)

Other Possibilitiesı Optimum development of all these architectures is enabled by SMW200A

and (in some cases) VNA product lines; the phase coherent sources.

ı The development of higher performance Doherty, Balanced, Differential and Spatial Combined structures already shown in 1MA279.

ı Development of the “Predictive-Postcorrection” schemes is also possible using the phase coherent source product lines, including (see right): Outphasing Dual-input Doherty ET-type schemes (e.g. Load Modulation, ANT tuning) Hybrids of the above basic classes (e.g. Doherty-Outphasing or

Multilevel Outphasing)

Tests on an Integrated 5G Frontendı 5G modem chipsets are unlikely to have mmWave

outputs to the FEMiD IF output architecture may be preferred Enables one modem to support multiple link

frequencies The FEMiD will contain one (or more) upconversion

stages

ı SMW/FSW combination can support full characterization of this frequency converting architecture, e.g. measuring with a single connection: Phase noise AM-AM, AM-PM, EVM etc.… using FSW-K18/40/70/96 and SMW-K540/541

mmWave DUT with:• 1~2 GHz + DC + REF input• K-band output

Tests on a mmWave 5G Beamforming Antennaı Unique VNA (vector network analyzer)

architecture allows full testing and verification of a beamforming antenna in TX mode

ı Conventional measurement of s-parameters enables measurement of: Return loss Cross coupling etc.

ı By switching into (optional) phase coherence mode AND without reconnecting the antenna DUT: TX beam pattern measurement Codebook generation and verification

4 Radiator ANT ArraySingle connection

Pattern Codebook

S-Parameters