linear & power efficient rf sub-systems

Linear & Power Efficient RF Sub-Systems

Tommaso Cappello, Kevin Morris, Mark Beach

Communication Systems and Networks (CSN) GroupUniversity of Bristol, UK

http://www.bristol.ac.uk/engineering/research/csn/

6G: Technology Enablers for Spectrum & Energy Efficient Wireless Access

University of Bristol with the support of DCMS/SPF, 26th May 2021

http://www.bristol.ac.uk/engineering/research/csn/

Dr. Tommaso CappelloUniversity of Bristol © CSN Group 2021

• Base-station transmitters are classified based on their output power required to cover a certain area.

2

RadioUnit (RF)

Antennas Macro-Cell BS(10 to >50 W)

𝑃𝑑𝑐𝐵𝐵

Pico-cell BS (0.25 to 1 W)

𝑃𝑑𝑐𝑅𝐹


𝑃𝑑𝑐𝐴𝑈𝑋

Micro-Cell BS(1 to 10 W)

𝑃𝑑𝑐𝐵𝐵 𝑃𝑑𝑐

𝑅𝐹

𝑃𝑑𝑐𝐴𝑈𝑋 𝑃𝑑𝑐

𝐴𝑈𝑋𝑃𝑑𝑐𝑅𝐹

• BS overall power consumption is dominated by the RF radio unit.

Digital Baseband (DB)

Base-Station Power Budget

Auxiliary Systems (AUX) 𝑃𝑑𝑐

𝐴𝑈𝑋


𝑃𝑑𝑐𝑅𝐹

A

B𝑃𝑜𝑢𝑡

𝑃𝑖𝑛 𝑃𝑖𝑛

Efficiency

A

B

Efficiency vs. PinPout vs. Pin

Linearity-EfficiencyTradeoff

• The RF power amplifiers (PAs) are the most power-hungry components in the radio.

Dr. Tommaso CappelloUniversity of Bristol © CSN Group 2021 3

Linearity Impairments in the Transmitter

Nonlinear compression, I/Q offset, LO leakage,…

Digital-to-Analog Converter

𝑓𝑐

Digital-to-Analog Converter

𝐼(𝑛)

PA BPF

𝑄 𝑛

INL/DNL distortion

Major source of nonlinear memory effects

Frequency Distortion

Bit Source

Modulation(BPSK, QPSK,…)

Parallelto

Serial

… … Crest Factor Reduction

IFFT

Serialto

Parallel

Cyclic Prefix

Nonlinear memory effects

Nonlinear memory effects

• Transmitter linearity and efficiency is limited by the RF frontend.

Output Spectrum

Input Spectrum

DriverD

Starting from 4G, RF hardware capabilities are becoming more and more a performance limitation

90°


[1] T. Cappello, Z. Popovic, K. Morris and A. Cappello, "Gaussian PulseCharacterization of RF Power Amplifiers," in IEEE Microwave andWireless Components Letters, April 2021.

LTE 1.4MHz signal partition

Gaussian pulse (FWHM = 930ns)

• Gaussian pulse decomposition is equivalent to the response with an LTE signal (not a-priori known).

• More powerful than the sinusoidal and time-domain techniques.

• Gaussian pulse is optimal for capturing NL memory effects.

0 250 500−500 −250

1

1

2

0 20MHz

Am

plit

ud

e

Time (ns)

4

• It is necessary to rethink the way transmitters are characterized → hybrid time-frequency characterization.

Gaussian decomposition [1]

Hybrid Time-Frequency Transmitter Characterization


5

[1] T. Cappello, C. Florian, A. Santarelli and Z. Popovic, "Linearization of a 500-W L-band GaN Doherty Power Amplifier by Dual-Pulse Trap Characterization26th May 2021," in IEEE Int’l Microw. Symp. (IMS), June 2019.

TPK-PK

EnvelopeBW

Gain w/o pre-pulse

𝜏𝑅

• GaN PAs, however, are still affected by trapping effects

VMAX

TPK-PK

Trap state (X)

… …

VMAX

DPD OFF

IdealDPD ON (𝑇𝑃𝐾−𝑃𝐾 < 𝜏𝑅)

DPD ON (𝑇𝑃𝐾−𝑃𝐾 > 𝜏𝑅)

• New finding [1]: If TPK-PK separation is longer than the dominating trap time constant (𝜏𝑅), a memory-less DPD is effective.

• This is particularly evident with high-PAPR signals (e.g., OFDM):

Pre-pulse Measure-pulse

X1(t)

Time TPK-PK0

X2(t)

X3(t)

X

PMAX

POUT

XMAX

• New characterization technique that mimics the envelope signal

• GaN is a candidate technology to improve the bandwidth/efficiency of RF frontends.

• Doherty architecture is common in macro-cell base-stations. GaN Doherty PA

Trapping Effects in GaN Doherty PAs


6

• In MIMO arrays, large heatsinks are used and PAs experience wide temperature variations.

• Thermal coupling in the array • GaN RF PA testbed (10-W Wolfspeed at 3.5GHz)

G. Jindal, G. Watkins, K. Morris, T. Cappello, "An RF Power Amplifier BehaviouralModel with Low-Complexity Temperature Feedback for Transmitter Arrays,“ to bepresented at IEEE Int’l Microw. Symp. (IMS), June 2021.

• Linearized PA with thermal correction:

PA gain amplitude and phase vs. temperature:

• Because of the high power density of GaN transistors, temperature effects are particularly evident.

TemperatureSensor

PA Digital Pre-Distortion (DPD) with Temperature Correction


• The congested spectrum requires flexible hardware solution capable to transmit on multiple bands.

• Normally, a PA for each band is required. The DB-PA allows to transmit on multiple bands simultaneously:

T. Cappello, A. Duh, T. W. Barton and Z. Popovic, "A Dual-BandDual-Output Power Amplifier for Carrier Aggregation," in IEEETransactions on Microwave Theory and Techniques, July 2019.

𝑧1,𝑛 =

𝑚=0

𝑀

𝑖=0

𝐾

𝑗=0

𝑖

𝑐1,𝑚,𝑖,𝑗𝑥1(𝑛 − 𝑚)|𝑥1(𝑛 − 𝑚)|𝑖−𝑗|𝑥2(𝑛 − 𝑚)|𝑗

𝑧2,𝑛 =

𝑚=0

𝑀

𝑖=0

𝐾

𝑗=0

𝑖

𝑐2,𝑚,𝑖,𝑗𝑥2(𝑛 − 𝑚)|𝑥2(𝑛 − 𝑚)|𝑖−𝑗|𝑥1(𝑛 − 𝑚)|𝑗

• Linearity is recovered with 2-D Digital Pre-Distortion*

* S. A. Bassam, “2-D digital predistortion (2-D-DPD) […],” IEEE TMTT, 2011.

Input power at 3.5 𝐺𝐻𝑧 (dBm)Input power at 5.5 𝐺𝐻𝑧 (dBm)

• Characterization of the channel cross-Modulation

CH 1

CH 2

CH 1 CH 1 CH 2

CH 2

(a)

(a) (b) (c)

Channel 1

(b)

Channel 2

(c)

DB-PA

𝑃𝑂1

𝑃𝑂2

𝑃𝐼𝑁

Channel 1

Channel 2

Channel 1 Channel 2

Concurrently AmplifiedSignal 1 ≠ Signal 2

Multi-Band Power Amplification

7


Coupling

8

• MIMO is used for high-capacity data links and it requires an high number of antennas in small volume.

• Typical MIMO antenna array • Coupling (|S21|) between two nearby elements

AntennaS-parameters

PA1

PA2

• The PAs experience active load-pulling because of the coupling between the antenna elements [1].

[1] F. M. Barradas, P. M. Tomé, J. M. Gomes, T. R. Cunha, P. M. Cabral and J. C.

Pedro, "Power, Linearity, and Efficiency Prediction for MIMO Arrays With Antenna

Coupling," in IEEE Transactions on Microwave Theory and Techniques, Dec. 2017.

• Antenna coupling hardware simulator (most PAs are not available in simulators).

PA1

PA2

VariableAttenuator

VSG1

VSG2 VSA2

VSA1

• Synthesizable coupling factors (S21):

-12 dB↓

-21 dB @4.5GHz

-12 dB↓

-21 dB @1.5GHz

• PA gain variation for varying coupling:

PA1 PA2

CF (dB) = -21, -16, -12

Antenna Coupling in MIMO Arrays

Dr. Tommaso CappelloUniversity of Bristol © CSN Group 2021 9

S. Ozan, M. Nair, T. Cappello and M. A. Beach, "Low-Noise Amplifier withWideband Feedforward Linearisation for Mid-Band 5G Receivers," IEEEAsia Pacific Conference on Circuits and Systems (APCCAS), 2020.

• Similarly to the RF transmitter, where the main limiting factor is the linearity-efficiency tradeoff in the PA, in receivers theLNA sets the sensitivity-efficiency limit.

• LNA with feedforward linearization is a candidate architecture to improve linearity without extra digital complexity.

• LNA w/ feedforward performance (NF and OIP3):

LNAMixer+ IRF

BPFADC+ Digital

BasebandG

Feedforward amplifier

Main amplifier

LNAOutput

LNAInput

Main source of non-linearities/noise

3-4 GHzbandwidth

14 dBgain

NF <1.2 dB

OIP3 > 30dB

LNA Linearization with Analogue Techniques


• When extra digital complexity can be afforded, digital post-distortion (DPOD) can be used to simplify the RF hardware.

10

LNA DPOD

STATIC LNACHARACTERISTIC

WIDEBANDCOMPENSATION

fLOW fHIGH

fLOW fHIGH

DPOD

LNA

DPOD

LNA

fLOW

fHIGH

fHIGH

fHIGH

fHIGHfLOW

• Wideband linearization can be achieved without the addition on noise by characterizing the LNA from fLOW to fHIGH.

• Digital complexity requirements and added power consumption need to be carefully investigated.

FPGA

A. Katz, J. Wood and D. Chokola, "The Evolution of PA Linearization:From Classic Feedforward and Feedback Through Analog and DigitalPredistortion," in IEEE Microwave Magazine, Feb. 2016.

LNA Linearization with Digital Techniques (DPD)


11

• ET is candidate for improving the linearity-efficiency limitations of next generation transmitters.

1

𝑃𝑜𝑢𝑡0

𝜂𝑑𝑃𝐴

0.5

Ideal PA

10.5

Reference class-AB PA

Dissipated power Envelope-tracking PAPA1

PA2

Supply Modulator

RF Power Amplifier

𝑃𝑜𝑢𝑡(𝑡)

𝑃𝑖𝑛(𝑡)

𝑉𝑑𝑐(𝑡)

𝐹[𝑃𝑖𝑛 𝑡 ]

Envelope-Tracking (ET) PA Architecture

• Still research needs to be done to improve the supply modulator bandwidth.

• Unlike Doherty, ET can enhance the efficiency of very broadband PAs because the supply modulator operates at low frequencies and so independent from the RF carrier.


12

• ET PAs are suitable for medium to high power PAs in the sub-6GHz spectrum.

• LTE compliant ET-PA @ 1.84GHz using the Power-DAC supply modulator [1].

[1] T. Cappello, P. Pednekar, C. Florian, S. Cripps, Z. Popovic and T. W. Barton, "Supply-and Load-Modulated Balanced Amplifier for Efficient Broadband 5G Base Stations,"in IEEE Transactions on Microwave Theory and Techniques, July 2019.

Vout

1

DCV = RV

2

2

DCV = RV

4

N

DCV = R

N

V

2

b1 b2 bNb1 b2 bN

2NL =( )3

1

1,02

Rout i O ii

i

VV b V b

=

= + =

…….

OV

• PA characteristics at variable supply level • FPGA DPD and ET control

Time (μs)

0 1 2 3 4 5 6 7 8 9 10

No

rmal

ized

Am

plit

ud

e

1

0.8

0.6

0.4

0.2

0

1

0.8

0.6

0.4

0.2

0

No

rmal

ized

Am

plit

ud

e

Vd

dD

ynam

ic B

ias

volt

age

(V)

30

20

10

0

Vd

d D

ynam

ic B

ias

volt

age

(V)30

20

10

0

ET no DPD

ET + DPD

In Env

Out Env

In Env

Out Env

NRMSE=27.8%

NRMSE = 4.2%

• LTE waveforms with and without DPD

ET-PA for Base-Station Transmitters (5G)


• Pulse shaping in radars is used to reduce the detectability and/or allow the radar operation in a congested spectrum.

13

Radar Pulse Shaping

RF pulse

Vd(t)

Supply voltage pulse (ET)

RF pulse

Vd(t)Supply voltage pulse

Vd(t)Supply voltage pulse

RF pulse

1st sidelobe at -13dB(high spectral leakage)

Frequency Offest (Hz)

1st sidelobe at -30dB(low spectral leakage)

High PAE Low PAE Recovered PAE

DPD ONDPD OFF

Frequency Offest (Hz)


14

ET for Radar Pulse Shaping• ET is an effective candidate to increase the efficiency of standalone or array PAs used with pulse-shaped waveforms.

• Typical and proposed way for PA supply

PACBS

CBank

ESRId

SW

VdPA

CBSVBUS

Id

Vd

Supply ModulatorVBUS

RF pulse input

RF pulse input

a. Rectangular supply voltage pulse

RF pulse output

RF pulse output

b. Shaped supply voltage pulse

• High-efficiency supply modulator

Po

we

r-D

AC

PA

• Integrated MMIC in Qorvo GaN 0.15μm [1]

𝑉𝐷𝐷2

𝑉𝐷𝐷4

𝑉𝐷𝐷2𝑁

PA

…

+-

+-

+-

𝑉𝑂𝑈𝑇 =

𝑖=1

𝑁

𝑏𝑖𝑉𝐷𝐷2𝑖

“Power-DAC”

• PA characterization and DPD

0 50 100 150 200 250 300-10

-5

0

5

10

15

20

25

30

35

40

Time (s)

Po

ut (d

Bm

)

PD

• Arbitrary radar pulses generation with high efficiency

T. Cappello, C. Florian, D. Niessen, R. P. Paganelli, S. Schafer and Z. Popovic, "Efficient X-Band Transmitter With Integrated GaN Power Amplifier and Supply Modulator," in IEEE Transactions on Microwave Theory and Techniques, April 2019.

65%45%49%52%


15

Conclusions & Take Aways• Since 4G, hardware performance are becoming more and more a limiting factor.

III-V semiconductor technologies (GaN/GaAs)

DPD-friendly RF hardware

RF architectures(multi-LNA, multi-PA, ET,…)

• Superior transceiver bandwidth, linearity, efficiency, noise figure, and operating frequencies compared to Silicon technologies.

• Most effective way to achieve large bandwidths and linearity.

• Reconfigurable and ‘upgradable’ RF hardware performance.

• Improve the single device technological limits by using multi-transistor architectures to achieve the required specifications.

Solutions to move forward:

Fundamental research is required to develop next generation RF transceivers


Active Circulator mm-Wave Front-Ends

• Magnetic circulators cannot be integrated in MMIC circuit operating at mmWaves.

16

L. Marzall, S. Verploegh, T. Cappello, M. Roberg and Z. Popović, "Active

MMIC Circulator Performance in a Phased-Array-Like Environment," 2020

50th European Microwave Conference (EuMC), 2021

• A possible alternative is to exploit the isolation introduced by transistors.

GaAs MMIC X-band

3dB gain Between 8-12GHz

7dB isolation over a very large bandwidth (simulation predicts >10dB)


17P. Zurek, T. Cappello and Z. Popovic, "Broadband Diplexed Power

Amplifier," in IEEE Microwave and Wireless Components Letters, Nov. 2020

• When two PAs can be afforded, diplexed PA architecture is an alternative to improve the bandwidth

• Input and output diplexer need to be designed accordingly

• High efficiency over an extended bandwidth is achieved

• Because of the isolation between the two PAs introduced by the diplexer, a simple 1-D pre-distortion is sufficient:

Multi-Band Power Amplification


18

• With 5G, base-station PAs need to achieve wider bandwidths and efficiency with high PAPR signals.

• Solution: load-modulated balanced amplifier (LMBA) for bandwidth + ET for high PAPR [1]

[1] T. Cappello, P. Pednekar, C. Florian, S. Cripps, Z. Popovic and T. W. Barton,"Supply- and Load-Modulated Balanced Amplifier for Efficient Broadband 5G BaseStations," in IEEE Transactions on Microwave Theory and Techniques, July 2019.

• >20W between 1.8-3.8GHz with >40% average efficiency

CC

DF

• PAPRs 1-2dB higher than in LTE/LTE-A

2 GHz

140 MHz

• Record instantaneous bandwidth of 140MHz in ET mode

ET-PA for Base-Station Transmitters (5G)

linear & power efficient rf sub-systems

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