Linear & Power Efficient RF Sub-Systems

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Linear & Power Efficient RF Sub-Systems

Tommaso Cappello, Kevin Morris, Mark Beach

Communication Systems and Networks (CSN) Group

University of Bristol, UK

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

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

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Dr. Tommaso Cappello

University of Bristol © CSN Group 2021

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

2 Radio Unit (RF) Antennas Macro-Cell BS (10 to >50 W) 𝑃_𝑑𝑐𝐵𝐵 Pico-cell BS (0.25 to 1 W) 𝑃_𝑑𝑐𝑅𝐹 𝑃_𝑑𝑐𝐵𝐵 𝑃_𝑑𝑐𝐴𝑈𝑋 Micro-Cell BS (1 to 10 W) 𝑃_𝑑𝑐𝐵𝐵 𝑃_𝑑𝑐𝑅𝐹 𝑃_𝑑𝑐𝐴𝑈𝑋 𝑃𝑑𝑐𝐴𝑈𝑋 𝑃𝑑𝑐𝑅𝐹

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

Digital Baseband (DB)

Base-Station Power Budget

Auxiliary Systems (AUX) 𝑃𝑑𝑐𝐴𝑈𝑋 𝑃_𝑑𝑐𝐵𝐵 𝑃_𝑑𝑐𝑅𝐹 A B 𝑃 𝑜𝑢 𝑡 𝑃_𝑖𝑛 _𝑃_𝑖𝑛 E ffi cie nc y A B Efficiency vs. P_in P_out vs. P_in

Linearity-Efficiency

Tradeoff

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University of Bristol © CSN Group 2021 3

Linearity Impairments in the Transmitter

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

Digital-to-Analog Converter 𝑓_𝑐 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,…) Parallel to Serial … … Crest Factor Reduction IFFT Serial to Parallel Cyclic Prefix Nonlinear memory effects Nonlinear memory effects

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

Output Spectrum Input Spectrum Driver D

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

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[1] T. Cappello, Z. Popovic, K. Morris and A. Cappello, "Gaussian Pulse Characterization of RF Power Amplifiers," in IEEE Microwave and

Wireless 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 20 M H z Amp lit u d e Time (ns) 4

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

Gaussian decomposition [1]

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[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 Characterization

26th May 2021," in IEEE Int’l Microw. Symp. (IMS), June 2019. T_PK-PK Envelope BW Gain w/o pre-pulse 𝜏_𝑅

• GaN PAs, however, are still affected by trapping effects V_MAX T_PK-PK Trap state (X)

…

V_MAX DPD OFF Ideal DPD ON (𝑇𝑃𝐾−𝑃𝐾 < 𝜏𝑅) DPD ON (𝑇_{𝑃𝐾−𝑃𝐾} > 𝜏_𝑅)

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

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

Pre-pulse Measure-pulse X₁(t) Time T_PK-PK 0 X₂(t) X₃(t) X P_MAX P_OUT X_MAX

• 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

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• 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 Behavioural Model with Low-Complexity Temperature Feedback for Transmitter Arrays,“ to be presented 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.

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• 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-Band Dual-Output Power Amplifier for Carrier Aggregation," in IEEE

Transactions 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 Amplified Signal 1 ≠ Signal 2

Multi-Band Power Amplification

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Coupling

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• 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

Antenna S-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 Variable Attenuator 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

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S. Ozan, M. Nair, T. Cappello and M. A. Beach, "Low-Noise Amplifier with Wideband Feedforward Linearisation for Mid-Band 5G Receivers," IEEE

Asia 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 the LNA 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):

LNA Mixer

+ IRF

BPF G ADC+ Digital_Baseband

Feedforward amplifier Main amplifier LNA Output LNA Input

Main source of non-linearities/noise

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

10 LNA DPOD STATIC LNA CHARACTERISTIC WIDEBAND COMPENSATION f_LOW f_HIGH f_LOW f_HIGH DPOD LNA DPOD LNA f_LOW f_HIGH f_HIGH f_HIGH f_HIGH fLOW

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

• 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 Digital Predistortion," in IEEE Microwave Magazine, Feb. 2016.

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

1 𝑃_𝑜𝑢𝑡 0 𝜂_𝑑𝑃𝐴 0.5 Ideal PA 1 0.5 Reference class-AB PA Dissipated power Envelope-tracking PA PA1 PA2 Supply Modulator RF Power Amplifier 𝑃_𝑜𝑢𝑡(𝑡) 𝑃_𝑖𝑛(𝑡) 𝑉_𝑑𝑐(𝑡) 𝐹[𝑃_𝑖𝑛 𝑡 ]

Envelope-Tracking (ET) PA Architecture

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

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• 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 Broadb"Supply-and 5G Base Stations," in IEEE Transactions on Microwave Theory and Techniques, July 2019.

V_out 1 DC V =VR 2 2 DC V =VR 4 N DC V = R N V 2 b1 _b2 _bN b1 b2 _bN 2N L = ( ) 3 1 1, 0 2 R out i i O i i V V b V b = = + =

…….

O V

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

Time (μs) 0 1 2 3 4 5 6 7 8 9 10 N o rm ali zed Ampli tud e 1 0.8 0.6 0.4 0.2 0 1 0.8 0.6 0.4 0.2 0 N o rm ali zed Ampli tud e Vdd D yn am ic Bia s vo ltag e (V) 30 20 10 0 Vdd D yn am ic Bia s vo ltag e (V) 30 20 10 0 ET no DPD ET + DPD In Env Out Env In Env Out Env NRMSE=27.8% NRMSE = 4.2%

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

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Radar Pulse Shaping

RF pulse

V_d(t) Supply voltage pulse (ET)

RF pulse

V_d(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 ON DPD OFF

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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

PA C_BS C_Bank ESR I_d SW V_d PA C_BS V_BUS Id V_d Supply Modulator V_BUS 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

P ow e r-D A C PA

• Integrated MMIC in Qorvo GaN 0.15μm [1]

𝑉𝐷𝐷 2 𝑉𝐷𝐷 4 𝑉𝐷𝐷 2𝑁 PA … +-𝑉_𝑂𝑈𝑇 = ෍ 𝑖=1 𝑁 𝑏_𝑖𝑉𝐷𝐷 2𝑖 “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) P o u t (d B m ) 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%

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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:

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Active Circulator mm-Wave Front-Ends

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

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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

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University of Bristol © CSN Group 2021 P. Zurek, T. Cappello and Z. Popovic, "Broadband Diplexed Power_{Amplifier," in IEEE Microwave and Wireless Components Letters, Nov. 2020} 17

• 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:

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• 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 Base Stations," in IEEE Transactions on Microwave Theory and Techniques, July 2019.

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

CCD

F

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

2 GHz

140 MHz