Issues for Multi-Band Multi-Access Radio
Circuits in 5G Mobile Communication
Yasushi Yamao

AWCC
The University of Electro-Communications

LABORATORY

Outline Background Requirements for 5G Hardware
 Issues for Tx and Rx
 Advanced DPD techniques
 Post compensation technique for Rx
 Reconfigurable BPF
 Conclusion

1

From 4G to 5G
5G Mobile Communication will be changed

to accept the diversity of ;

 Different access protocols with wide range of spectrum
from 700 MHz to more than 6 GHz (~millimeter wave)
 Heterogeneous deployment with different cell sizes,
 Carrier aggregation (CA) and dual access from UEs,
 Cooperated multiple transmission (CoMP), massive MIMO
and distributed antenna systems (DSA).
In order to achieve more efficient and flexible use of radio
resources, separation of C-plane and U-plane has been studied.
2Requirements for 5G Hardware
High bandwidth of 10 Gbps or more
Utilizing widely-spread frequency bands from current
UHF to low SHF, high SHF and millimeter waves
Low-power/cost small base station and Dual Access toboth Macro- and small-cells with different frequency bands
High Accuracy RF signals are required to increase
spectral efficiency
5G hardware is required to be more flexible, accurate,
linear and low-cost.
3Issues for Transmitter
Keeping linearity and power efficiency in a wider unit RF
bandwidth of 100MHz or more.
Keeping linearity and power efficiency under concurrent
multiband operation
Nonlinear Compensation Techniques should be developedthat can work in a wider bandwidth and multi-bandenvironments.
We proposes advanced Digital Predistortion (DPD) techniquescalled SENF and SFFB
4

Issues for Receiver
Receiver front-end faces a variety of incoming signals
with power of wide dynamic range.
Desired signals are not always stronger than others.
Under concurrent multiband operation, near-far problem
increases chance of inter-band modulation called "crossmodulation distortion".
Receiver Nonlinearity Compensation technique andreconfigurable RF BPF as pre-selector mitigate the issues.
We proposes a Post Compensation technique andReconfigurable BPF for concurrent dual-band receiver.
5

1. Wideband DPD Design Method
Existing DPDs have been designed to feedback full bandwidth of
nonlinear output signal, requiring 3 to 5 times wideband ADC.
B

3B~5BI

DPD
Iterated
Nonlinear
Modeling withSpectralExtrapolation ofNarrowband
Feedback signal

QuadMODDACQ

B

Nonlinear
Modelingwith Spectral
Extrapolationand DPD
Training

Band-limited
feedback signal

LOPA
3-5 times wider bandwidth
for nonlinear output

QuadDEMO

ADC

~BBl

With SENF (Spectral Extrapolation of Narrowband
Feedback) technique, feedback bandwidth can besame as the signal bandwidth (or even less) [1].
Pl ΞyˆŷPuΞyˆ

6

SENF DPD Equivalent Baseband Diagramk( ); polynomial nonlinear functionx

1k1 ( )DPD2Analog Domain

k (x)βk2 ( )yPAPhk P( )ŷβIdentification
(1)

w2hw1

(3)

sExtrapolation(2)hx  yˆ7

SENF DPD by FPGA
More than 100 MHz Linearization is possible with current FPGAs
with 250 Msps ADC by SENF method.
RF: 1.75 GHz
In
Out
QDEM+
2 ch ADC
250 Msps

RF-DAC
2.5 Gsps
with SENF
DPD

WithoutDPD

8 x 20MHz LTE CA signal (160MHz)
160MHz bandwidth DPDby Xilinx Kintex7 FPGA8

SENF DPD by Experiment (1)More than 300 MHz Linearization is confirmed in Experiment [2].

320MHz320MHz feedback bandwidth DPD
by measurement set up

16 x 20MHz LTE CA signal (320MHz)

Linearization of signal with 500 MHz and beyond bandwidth
will be achieved soon by DPD.
9

SENF DPD by Experiment (2)Proposed DPD can compensate non-continuous CA signal.

4 x 20MHz LTE CA signal (240MHz)

6 x 20MHz LTE CA signal (280MHz)

Linearization of 900 MHz and beyond RF bandwidth isachieved by SENF DPD.
10

2. Concurrent Dual-Band DPD DesignExisting Dual-Band DPDs have two feedback path with two setsof Down Converter and ADC.
x1

v1

DPD1

DAC

UpConverter

fL1x2
DPD2v2DAC

UpConverter

f1
Dual Band
PACoupler

f2

fL2
y1

DPD
Parameter
Adjuster y2

ADC

Down
Converter

ADC

Down
Converter

High-speed ADCs are expensive!
11SFFB Dual-Band DPD
Spectrum Folding Feedback (SFFB) DPD multiplexes two RF
spectra into common IF [3].
x1

v1
DPD1

DAC

f1

Up
Converter

Dual‐Band
PA

fL1
x2
DPD2

v2

DAC

Up
Converter

Couplerf2( ,fL2y1

DPD
Parameter
Adjustery2

PostProcessing& PA
Modeling

Spectrum FoldingDown Converter
ADCf IF 

f 2  f1
2

DownConverter＝2Two RF signals are
down-converted
into a common IF
as a multiplexedsignal.
12

SFFB Dual-Band DPD
Spectrum Folding Feedback (SFFB) DPD multiplexes two RF
spectra into common IF [3].
B1

B2

B1

x1

v1
DAC

DPD1

B2

f1

UpConverter

Coupler

fL1

x2

v2
DAC

DPD2

Dual‐BandPA0f2

UpConverter( ,ff11f1  f 2 ff22
f 
1.75 GHz lo
2.75 GHz2frequency

max(Bmax{B , 1B, }B2)

fL2

12y1
DPD
Parameter
Adjustery2Post
Processing& PA
Modeling

ADCDown
Converter

0＝SFFB Down
Converter2

ff22 f1 f1
22

frequency
frequency

0.5 GHz

Lower RF-band signal
spectrum is inverted.

y (t )  y1* (t )  y2 (t )Compensatedin baseband by
inverting input x1
13

SFFB Multi-Band Extension
With SFFB (Spectral Folding Feedback) technique, multiband signals
can be folded into one IF bandwidth.
f fflo1  1 2
2

f1, f2, f3, f4

f lo1 0000f1  f 22f1

f lo1 f f
flo1  1 22f 2  f1
2f 3 2f lo1 f2f fflo3  4 2
2f ff  f1f f
flo1  1 2 flo2  3 1 f lo 2  3222f f
flo1  1 22

f1  f 2
2f 2

f3

f 4


2f 3

f1  f 22

f4 frequency

frequency

frequency

frequency
14

SFFB Dual-Band DPD by Experiment
AM-AM characteristics of PA
36

Output Power (dBm)

34
32
30

1.75GHz
2.75GHz28
26
24

f1 + f2

22-20-18-16

f1 = 1.75 GHz-14 -12 -10-8
Input Power (dBm)-6-4-2f2 = 2.75 GHz -10

fIF = 500 MHz

Normalized Power (dB)-20-30-40-50-60-70-80

flo = 2.25 GHz-90-40-20

0Frequency (MHz)2040

Folded feedback signal without DPD
15

SFFB Dual-Band DPD by Experiment
Scenario 1; 10 MHz LTE(1.75 GHz ) + 20 MHz LTE (2.75 GHz)-30
Without DPD
SFFB DPD
Conv. DPD

-40-50-60-70-80-90-100
1.71 1.72 1.73

1.74 1.75 1.76
Frequency (GHz)1.77 1.78

Power Spectral Density (dBm/Hz)

Power Spectral Density (dBm/Hz)-30

Without DPD
SFFB DPD
Conv. DPD

-40-50-60-70-80-90-100
2.71 2.72 2.73

1.79

2.74 2.75 2.76
Frequency (GHz)2.77 2.78

2.79

Scenario 2; 20 MHz LTE (1.75 GHz) + 4 x 20 MHz LTE CA (2.75 GHz)-30
Without DPD
SFFB DPD
Conv. DPD

-40-50-60-70-80-90-100

1.65

1.7

1.75
1.8
Frequency (GHz)1.85

Power Spectral Density (dBm/Hz)

Power Spectral Density (dBm/Hz)-30

Without DPD
SFFB DPD
Conv. DPD

-40-50-60-70-80-90-100

2.65

2.7

2.75
2.8
Frequency (GHz)2.85

163. Cross-Modulation due to Multi-Band Access
Near-far problem increases chance of inter-band modulation
Far & weakADC

L ,PL20ADC

LQ

E V M (%)

15090

macro BS

H ,PHI

105010

Near & strong

Inter-bandmodulation

E V M (%)

15105010

Self-Generated
Distortion

1000-10PH (dBm)2 L   H

L

H
Not Cared

PL (dBm)

20Q
ADC-20(a) EVM for L

I

090

Self-Generated
Distortion-10-20PH (dBm)

ADC/ /femto/ micro
macro
repeater

0-10

HLNA

100Synthesized Oscillators

2 H   L-10-20-20

PL (dBm)(b) EVM for H
17

Post-Compensation of Receiver Nonlinearity
xL(n)

ADC

|yL|2 yL

LNAyH

DDCfor ωH

ADC

H

digital domainsL(n)Compensatorfor yL(n)sH(n)

Compensatorfor yH(n)|yH | 2 yL

c'50

5th IMc'52

5th CMTime
AlignmentyH(n)ChannelFilterc'32

3rd CM|yL|4 yLyL(n)ChannelFilterc'30

3rd IM

DDC
for ωL

xH(n)

sLyL

L

By
polynomial|yH |2 |yL| 2 yL

c'54

5th CM|yH | 4 yL

c'Q,Q-1|yH|Q-1 yL

Compensator diagram for yL

Blind & Adaptive Nonlinear Compensation method is necessary [4].
18How to Determine Compensator Coefficients?
Proposed algorithm determines the compensator coefficients so as tominimize outband spectra power for each band.
Minimization

0Frequency ShiftDesiredSignal Band

This works successfully because nonlinearity generates bothinband and outband components and they are correlated.
The key is separation filter that eliminates the in-band signal
and maximizes SNR of outband distortion detection.
19

Experimental Results-40

Uncompensated
3rd order5th order
7th order-50-60-70-802320

f1 = 2.35 GHz
f2 = 2.65 GHz-102330

2340 2350 2360
Frequency (MHz)2370(a) Output spectra at L

2380

Power Spectral Density (dB)

Power Spectral Density (dB)

-30

-20

Power levels of L and Hin (a)/(b) are -10 dBm and6 dBm, respectively.

-30-40-50-60
-70
2620

2630

2640 2650 2660
Frequency (MHz)

2670

2680

(b) Output spectra at H

7th-order compensation
presents the best results.

Proposed compensator
improves EVM in bothbands.

(c) EVM for L signal(d) EVM for H signal

Maximum acceptable
input power increases
more than 4 dB.
20

4. Reconfigurable RF BPF

Power (dBm)

Reconfigurable BPF can adapt the receiver flexibly to the changeof access channels and prevent unexpected cross-modulation
distortions in multi-band/multi-access operation.

Frequency (GHz)
Example of nonlinear output spectrum under concurrent dual-band operation.
Some of them locate near the operating band.
21

SHF Dual-band Reconfigurable BPF
 Low SHF (< 6 GHz) reconfigurable BPF and High SHF wideband
BPF are integrated for concurrent dual-band access.
High SHF wideband BPF section
ICResonator

ISC

Resonator

OCDIP

DIP

ICReconfig.
Resonator1

ISCReconfig.
Resonator2

OCLow SHF Reconfigurable BPF section
DIP: Diplexer
IC: Input Coupling circuit
ISC; Inter-Stage Coupling circuit
OC: Output Coupling circuit
22

2-stage 3-bit Low-SHF Reconfigurable BPF
OPENOPEN

l23

l21

l21

SW3SW1 SW2LinVg

SW4 SW5

l22

l20

l20

Zg

SW6Linter

Lcut
l1

SHORT

1st stage

l22

Lout
Lcut

0

ZL

f8
f7

f1

l1

SHORT

2nd stage

Center Frequency; 3.1 to 4.6 GHz
Insertion loss; 1.0~2.1 dB|S21| (dB)l23-10

f2

f6

f3-20

f5

f4-30-40

High SHF BPF and diplexers are underdesign process.

1234Frequency (GHz)5623

Conclusions
 5G hardware is required to be more flexible, accurate,
linear and low-cost.
 For Txs, advanced DPD techniques called SENF andSFFB are proposed.
- SENF saves feedback bandwidth, which enables wider
compensation bandwidth.
- SFFB enables simple feedback for concurrent dual-bandoperation.
 For concurrent dual-band Rxs, Post Compensation
technique and Reconfigurable BPF are proposed.
The results will contribute to improvepractical performance of the 5G system.

24

Related Works
1) Y. Ma, Y. Yamao, Y. Akaiwa, K. Ishibashi, “Wideband digital predistortion using spectral
extrapolation of band-limited feedback signal,” IEEE Trans. Circuit and Systems-I, vol. 61,
no. 7, pp. 2088-2097, July 2014.
2) Y. Ma and Y. Yamao, “Experimental results of digital predistorter for very wideband
mobile communication system,” Proc. IEEE VTC2015-Spring, 6PB-2, Glasgow, UK, May
2015.
3) Y. Ma and Y. Yamao, “Spectra-folding feedback architecture for concurrent dual-band
power amplifier predistortion,” IEEE Trans. Microw. Theory & Tech., Vol. 63, No. 10, pp.
3164-3174, Oct. 2015.
4) Y. Ma, Y. Yamao, K. Ishibashi and Y. Akaiwa, "Adaptive compensation of inter-band
modulation distortion for tunable concurrent dual-band receivers," IEEE Trans. Microw.
Theory & Tech., vol.61, no.12, pp.4209-4219, Dec. 2013.
5) R. Kobayashi, T. Kato, K. Azuma and Y. Yamao, “Design and Fabrication of Two-Stage
Three-Bit Reconfigurable Bandpass Filter Using Brunch Line-Type Variable Resonator,”
IEICE Trans. Electronics, Vol. E98-C, No. 7, pp. 636-643, July 2015.

25

Acknowledgement
A part of this work is supported by the Ministry of Internal Affairs andCommunications (MIC) of Japan under the program “R&D for the realizationof 5G mobile communication system".

Thank you for listening!

LABORATORY

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