EuMC: 5G RF Front End Module Architectures for Mobile Applications

5G RF Front End Module Architectures for Mobile

Applications

Florinel Balteanu, Hardik Modi, Yunyoung Choi, Junhyung Lee, Serge Drogi, Sabah Khesbak

Skyworks Solutions Inc., CA92617, USA

florinel.balteanu@skyworksinc.com

Abstract — The explosive growth and adoption of

smartphones provides access to voice and data for billions of

people worldwide today and connected devices are expect to

reach 5.6 billion in 2020. This growth has been and continues to

be the engine for semiconductor industry due to required

computational power of CMOS technology in lower feature nodes

as FinFet 7nm/14nm for application processors, modems and

transceivers. The adoption of 5G will bring higher data capacity

and low latency using sub-6GHz bands and mmWave spectrum,

with the first expected to be deployed in next generation 5G

smartphones. This 5G evolution will open up new applications

where our phones will be a conduit for massive amounts of data.

With lower feature nodes for RF CMOS there is an increased

usage of digital signal processing (DSP) and RF digital

calibration which are part of modern modem technology. 5G

requires more RF bands, so there is a clear shift in terms of what

parts of the RF systems are portioned in advanced CMOS nodes

and what RF and analogue blocks are integrated with other

components such as acoustic duplexers and filters in multiple RF

front-end-modules (RF FEMs). This paper proposes a low cost

RF partitioning and architecture which will be part of 5G RF

FEMs. This paper also presents some design/measurement results

and explains how these modules can be integrated into a complex

4G/5G system RF front end (RFFE) for mobile applications.

Keywords — RF front end (RFFE), CMOS, GaAs, SiGe,

silicon on insulator (SOI), SAW, BAW, GSM, 3G, 4G, 5G, GPS,

ultra-wide band (UWB), long term evolution (LTE), LTE

advanced, WiFi 6, power amplifier (PA), envelope tracking (ET),

multimode multiband power amplifier (MMBPA), frequency

duplex division (FDD), time division duplex (TDD), digital signal

processing (DSP), MIMO, transmit (Tx), multi-chip-module

(MCM), carrier aggregation (CA), licensed-assisted access

(LAA), enhanced LAA (eLAA), high power user equipment

(HPUE), duplexer, filter, diplexer, RF switch, power

management IC (PMIC), ACLR, EVM.

I. I

NTRODUCTION

The need for high data rates in mobile applications

together with the demand for new applications is pushing the

adoption for WiFi 6 [1] and 5G long term evolution (LTE) [2].

Both will provide the new mobile applications with fast and

low latency data for ultra-reliable and low latency

communications (URLLC) services. Together with other RF

technologies such as ultra-wide band (UWB) [3], 5G will

enable other services, for example vehicle-to-everything (V2X)

communications. Low latency in mobile networks is a critical

requirement for making autonomous vehicles safe. For new

features in 5G, mobile devices such as smartphones will be a

conduit for a cloud of applications. Next generation 5G

smartphones need to support legacy voice (2G/3G) capabilities

and enable the seamless transition from 4G to 5G. To provide

more RF spectrum, new bands have been allocated for 5G as

presented in Fig.1 and 3G/4G bands will be re-farmed for 5G.

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Fig. 1. 4G/5G and WiFi 6 spectrum.

The transition from the 3G/4G FEMs to 5G FEMs poses

the following challenges:

• Wider channel bandwidth up to 100 MHz where new

techniques for envelope tracking are required.

• High power user equipment (HPUE) requires 26 dBm

at the antenna port.

• Higher peak to power ratio waveforms for uplink (UL)

such as 256 QAM; this requires more power

amplifier back-off with lower distortions and noise.

• Cost efficient and compact size for 2x2 UL-MIMO

and downlink (DL) data rate coverage.

• LAA and eLAA will be introduced as part of 5G as a

possibility for bandwidth aggregation with a licensed

anchor LTE band in UL and DL.

• New 5G dedicated bands for sub-6 GHz such as

n77/n78 (3.3-4.2 GHz), n79 (4.4-4.5 GHz) and eLAA

bands B46, B47.

• Intra-band coexistence with 3G/4G bands in 5G re-

farmed bands.

• Dual-SIM operation for voice under 2G (GSM) and

data (3G/4G/5G) which will increase the linearity

requirements for antenna switches.

• Increased number of antennas to 6-8. The

requirement is to reach these antennas from different

LTE radios which have to coexist with multiple WiFi

& WiFi 6 radios, Bluetooth, GPS and UWB.

To meet these challenges, low cost and high linearity

RFEEs together with multiple filters are required to access

Proceedings of the 49th European Microwave Conference

252

multiple antennas for 4G/5G mobile devices as shown in Fig.

Fig. 2. Sub-6GHz 4G/5G RFFE structure for a mobile device.

The 3GPP standard body requires LTE power delivered at

the antenna to be 23 dBm. 5G implementation results in power

losses from filter/duplexers, switches, diplexers, board and

Impedance/Aperture Tuners (IT, AT) and the PA is required to

deliver at least 27-28 dBm assuming less than 4-5dB total

losses. Also for LTE 5G, there is an increase from 20 MHz to

40 MHz/0 MHz for the modulation uplink bandwidth in

low/mid bands as well an increase of the output power for

HPUE to 26 dBm. There is also an increase in modulation

bandwidth up to 100 MHz for high/ultrahigh 5G bands such as

new bands n77/n78 and n79. The goal of 5G is to reach a

transmission capacity of 1 Gbps. The capacity of a wireless

system is shown by Shannon formula as

(1)

To achieve higher capacity following Eq.1 these are the

techniques which are incorporated in 5G:

• increase channel bandwidth B

; eg 100 MHz LTE

• increase spatial multiplexing level k through MIMO

• increase the transmit power; such as HPUE

• decrease noise N

and improve receive sensitivity

• reduce in band interference on link k, especially in

multiple UL Tx such as CA and MIMO

• higher order modulation such as 256QAM for UL

II. 4G/5G

FRONT END MODULE STRUCTURE

To meet 5G geographical coverage requirements,

smartphones will need to cover more than 50 bands from 600

MHz to 6 GHz. In parallel, there are other radios and bands

used at the same time such as WiFi/WiFi6 (2.4 GHz/5 GHz),

UWB (6-8 GHz), GPS (1.17 GHz-1.5 GHz), Bluetooth (2.4

GHz) and NFC(13.56 kHz). Sub-3 GHz bands provides

primary cellular coverage and 3G/4G bands will be re-farmed

to 4.5G/5G and the 3 GHz new 5G bands will provide the

primary capacity layer with multiple MIMO. All these radios

will need to share common antennas without jamming the

other device radios. In addition, old features such as 2G GSM

need to be supported together with the new features introduced

in 5G such as LAA and eLAA which use 5 GHz unlicensed

band as bandwidth aggregation with a licensed LTE anchor,

usually in low band (LB). There has been a lot of research on

single die power amplifier in different technologies such as

SiGe, CMOS and SOI [4-5]. However, with band proliferation

and coexistence requirements, the biggest share and cost is

determined by the acoustic filters which are placed together

with band and antenna SOI switches into a FEM with

integrated duplexers (FEMiD) and filters (FEMiF) for TDD

space, as shown in Fig. 3.

Sw_in

Sw_byp

Mode_Sw

Ant_Sw

Tracker

Filter

Output

Match

Digital

Coupler

Antenna Tuner

Ant1

LNA

0.3dB

1.4dB-2.1dB

0.6dB

0.4dB

0.2dB

Duplexer

Ant2

1dB

Mid Band

Triplexer

Low

MidBand

Rx_Sw

0.2dB

2G GSM

Fig. 3. Sub-6GHz 4G/5G FEMiD architecture.

These LTE modules can include two to three PAs and 10-

12 SAW and BAW filters [6]. Also WiFi modules include

BAW filters for coexistence with LTE high bands as presented

in Fig. 4.

Fig.4. WiFi 2.4GHz BAW coexistence filter characteristic.

The RF power amplifier is one of the most critical

components within the transmitter chain due to efficiency and

linearity requirements. The linearity requirements for LTE are

expressed by adjacent channel leakage power ratio (ACLR)

and for 5G EVM become a critical parameter for UL 256

QAM. Doherty power amplifiers [7] and envelope tracking [8]

(ET) have been extensively researched and used techniques to

meet the efficiency and linearity requirements. Doherty power

)1(log



253

amplifier provides high efficiency and is used in base-stations

but is less used in mobile applications due to broadband

limitation and load mismatch operation [9]. In mobile

applications such smartphones, ET is used with a broadband

PA with class E output match and typically with two stages

[10-11], as shown in Fig. 5.

Cac

Env_p

Lpa

Vbias

2fo

Class E

Tracker

Cpa

Env_n

Vdc_trck

RF_out

DC_DC

Vdc_ctrl

ET combiner

Dynamic

Bias

Vdc1

PMIC

Vdc

Vbat

Fig. 5. 5G tracker and power amplifier for mobile application.

III. E

NVELOPE TRACKING

4G smartphones have just one ET circuit which can provide

Vdc tracked to one PA/FEM at a time. With the migration to

5G (where more than two transmit PAs are active at a time)

there is the need to place the ET error amplifier (tracker)

inside of the FEMiD and to have a general PMIC which

provides 4-5 Vdc power domains. For a typical linear PA

operating in class B mode when the active device conducts

180

the maximum output power Pout_max is given by

(2)

where Rlopt is the optimum impedance which has to be

matched at the output load through the marching network and

Vkn is the knee voltage for the output transistor. Under ET the

instantaneous supply voltage Vdc for the last stage is provided

by ET. Around 80% of the energy is delivered by the DC-DC

and the rest by the error amplifier (tracker) presented in Fig. 6.

Class

Bias

Ibias

Ibias1

Ibias2

Ibias3

Ibias4

Itrans

Vdd

Vdd_MLS

out

Vin_p

Vin_n

Vctrl

Vref

Isense

Vref

Fig. 6. Current mode error amplifier (tracker).

For 5G, the error amplifier is required to deliver increased

peak voltage due to an increase in the peak-to-average-ratio

(PAPR)

(3)

To reduce the peak voltage requirement and to keep the load

line to reasonable values in HPUE case, the power amplifier

can be split into two identical structures, independently

tracked. The outputs can be combined using a RF Wilkinson

combiner. A PA structure with two or more identical tracked

structure can also use a balun to cancel second harmonic

which is a coexistence issue for multiple 5G transmitters.

IV. S

OI SWITCH CONTROL

The FEMiD and FEMiF include many SOI switches.

Together with the LNAs these switches are integrated on

several SOI dies to provide enough isolation. The insertion

loss (IL) and input intercept point (IIP3) for a series-shunt

switch are defined by the relations

(4)

(5)

Where Ron is the on-state channel resistance on the switch

(6)

Vpos is the voltage applied to turn on the series switch and n

define the number of series switches. To improve switch

performance, one method is to overdrive through Vpos voltage

increase and also to track the Vpos with the instantaneous

power given by ET signal which is available for the FEMiD as

presented in Fig. 7. Also, several clock phases are used to

reduce the charge pump clock feedthrough into SOI switches.

Fig. 7. SOI Switch-Vpos schematic.

V. M

EASUREMENTS

The conversion from 4G RFFE to 5G is an evolutionary

transition with many circuits/filters reused and some designed

especially for 5G. The progress is possible also due to more

Rlopt

VknVdd

Pout

)(

max_

−

)

log(20

rmstrckVdc

peaktrckVdc

PAPR=





































)(2

1log10

ZoRonCoff

Ron

()

log103













RonZo

ZoRonIsat

IIP

()

VthVpos

Cox

nRon

−

254

components packaged into a smaller space [12, 13]. The high

bandwidth tracker as shown in Fig. 6 has been integrated into

a dual-gate 0.18µm CMOS technology together with the GaAs

CMOS controller and tested. Fig. 8 presents the voltage swing

for the error amplifier into PA Rload=6 ohm and Cpa=120 pF

for three tones 1 MHz, 84 MHz and 85 MHz.

Output Voltage swing (V)

Fig. 8. Error amplifier voltage swing (4.8V) for Vdd=5.5V and Icq=30mA.

Fig. 9 presents the waterfall curves for 3.5 GHz, n78 5G band

for different voltage supplies. The new bands n77, n78 and

n79 will provide more spectrum and higher bandwidth (up to

100 MHz). For 80 MHz LTE bandwidth the overall efficiency

for n78 PA and tracker is 32%, with -37 dBc ACLR1.

34 363230

26242220181614121086420

1.4V

1.8V

2.2V

2.8V

3.2V

3.8V

4.2V

4.8V

5.0V

5.4V

Output Power (dBm)

Constant

Gain Shaping

56%

Fig. 9. Waterfall curves for n78 band 3.5GHz GaAs PA.

5G RFFEs will use several uplink Tx (GSM/LTE) operating

the same time and one of the main problems is the internal

noise mitigation in receive bands, especially for GSM receive

bands. Fig. 10 shows the DP3T SOI Switch and Fig. 11

presents the noise reduction using the proposed SOI charge

pump shown in Fig. 7.

Fig. 10. DP3T Antenna switch photograph.

Fig. 11. Clock feed-through spur reduction for 5G SOI switches.

VI. C

ONCLUSIONS

Fully integrated 5G RFFE architectures are presented.

The transition from 3G/4G to 5G, especially for very large

volume products such as smartphones, is an evolutionary

process. This paper presents and characterizes few novel

circuits such as high bandwidth ET, 5G new band (n78) power

amplifier and SOI switch control for lower noise and high

linearity. These will enable the 5G RFFE adoption and

integration.

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