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

LABORATORYLABORATORYLABORATORY

Yasushi YamaoAWCC

The University of Electro-Communications

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

2

5G Mobile Communication will be changed to accept the diversity of ;

Different access protocols with wide range of spectrumfrom 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 MIMOand distributed antenna systems (DSA).

In order to achieve more efficient and flexible use of radioresources, separation of C-plane and U-plane has been studied.

Requirements for 5G Hardware

3

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 to both 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.

Issues for Transmitter

Nonlinear Compensation Techniques should be developed that can work in a wider bandwidth and multi-band environments.

4

Keeping linearity and power efficiency in a wider unit RF bandwidth of 100MHz or more.

Keeping linearity and power efficiency under concurrent multiband operation

We proposes advanced Digital Predistortion (DPD) techniquescalled SENF and SFFB

Issues for Receiver

Receiver Nonlinearity Compensation technique andreconfigurable RF BPF as pre-selector mitigate the issues.

5

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 problemincreases chance of inter-band modulation called "cross-modulation distortion".

We proposes a Post Compensation technique and Reconfigurable BPF for concurrent dual-band receiver.

PAI

DACDPD QuadMOD

Q

QuadDEMO

LO

ADC

Nonlinear Modeling 

with Spectral Extrapolation and DPDTraining

1. Wideband DPD Design Method

6

Existing DPDs have been designed to feedback full bandwidth of nonlinear output signal, requiring 3 to 5 times wideband ADC.

Band-limited feedback signal

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

3-5 times wider bandwidthfor nonlinear output

Iterated Nonlinear

Modeling with Spectral

Extrapolation of Narrowband

Feedback signal

yyΞP ˆl yΞP ˆu

Bl

B

B 3B~5B

~B

SENF DPD Equivalent Baseband Diagram

(1)(2)(3)

PAy

y

Extrapolation

xk( ); polynomial nonlinear function

βx)(k

h

Identification

kP( )

k1( )

k2( )

1

2

P

DPD

yhx ˆ

β

Analog Domain

w1s

w2

h

7

SENF DPD by FPGA

More than 100 MHz Linearization is possible with current FPGAswith 250 Msps ADC by SENF method.

with SENFDPD

Without DPD

RF-DAC2.5 Gsps

QDEM+2 ch ADC250 Msps

RF: 1.75 GHzIn Out

160MHz bandwidth DPDby Xilinx Kintex7 FPGA

8 x 20MHz LTE CA signal (160MHz)

8

SENF DPD by Experiment (1)

More than 300 MHz Linearization is confirmed in Experiment [2].

320MHz feedback bandwidth DPDby 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.

320MHz

9

SENF DPD by Experiment (2)

Proposed DPD can compensate non-continuous CA signal.

4 x 20MHz LTE CA signal (240MHz)

Linearization of 900 MHz and beyond RF bandwidth is achieved by SENF DPD.

6 x 20MHz LTE CA signal (280MHz)

10

2. Concurrent Dual-Band DPD Design

Existing Dual-Band DPDs have two feedback path with two sets of Down Converter and ADC.

DPD1

DPD2

Up Converter

Up Converter

DownConverter

DownConverter

DPDParameterAdjuster

fL1

fL2

x1

x2

v1

v2

y1

y2

CouplerDual Band PA

DAC

DAC

ADC

ADC

f1

f2

High-speed ADCs are expensive!

11

Spectrum Folding Down Converter

SFFB Dual-Band DPD

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

ADCDPD

ParameterAdjuster

fL1

fL2

x1

x2

v1

v2

y1

Coupler

＝ 2

y2

Dual‐BandPA

Up Converter

Up Converter

DownConverter

DAC

DAC

PostProcessing

&  PA Modeling

( ,

DPD1

DPD2

f1

f2

Two RF signals are down-converted

into a common IF as a multiplexed

signal. 2

12 fffIF

12

SFFB Down Converter

SFFB Dual-Band DPD

ADCDPD

ParameterAdjuster

fL1

fL2

x1

x2

v1

v2

y1

Coupler

＝ 2

y2

Dual‐BandPA

Up Converter

Up Converter

DownConverter

DAC

DAC

PostProcessing

&  PA Modeling

( ,

DPD1

DPD2

f1

f2 221 ffflo

1.75 GHz 2.75 GHz

0.5 GHz

f1 f2frequency

B1 B2

0 f1 f2

B1 B2

frequency02 1

2f f

max{B1 , B2}

212 ff frequency

max(B1, B2)

Lower RF-band signal spectrum is inverted.

Compensated in baseband byinverting input x1*

1 2( ) ( ) ( )y t y t y t

13

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

SFFB Multi-Band ExtensionWith SFFB (Spectral Folding Feedback) technique, multiband signals can be folded into one IF bandwidth.

221

1ffflo

221

1ffflo

f1, f2, f3, f4

221

1ffflo

221

1ffflo

f1 f2 frequency0 f3

0 frequency2

211

ffflo

frequency02

211

ffflo

2

132

fff lo

224

3ffflo

213

2ffflo

f4

234 ff

3 2

2f f

212 ff

0 frequency2

211

ffflo

14

SFFB Dual-Band DPD by Experiment

15

-20 -18 -16 -14 -12 -10 -8 -6 -4 -222

24

26

28

30

32

34

36

Input Power (dBm)

Outp

ut

Pow

er

(dB

m)

1.75GHz

2.75GHz

f1 = 1.75 GHzf2 = 2.75 GHz

AM-AM characteristics of PA

-40 -20 0 20 40-90

-80

-70

-60

-50

-40

-30

-20

-10

Frequency (MHz)

Nor

mal

ized

Pow

er (d

B)

f1 + f2

flo = 2.25 GHz

fIF = 500 MHz

Folded feedback signal without DPD

SFFB Dual-Band DPD by Experiment

16

1.71 1.72 1.73 1.74 1.75 1.76 1.77 1.78 1.79-100

-90

-80

-70

-60

-50

-40

-30

Frequency (GHz)

Pow

er S

pect

ral D

ensi

ty (d

Bm

/Hz)

Without DPDSFFB DPDConv. DPD

2.71 2.72 2.73 2.74 2.75 2.76 2.77 2.78 2.79-100

-90

-80

-70

-60

-50

-40

-30

Frequency (GHz)

Pow

er S

pect

ral D

ensi

ty (d

Bm

/Hz)

Without DPDSFFB DPDConv. DPD

Scenario 1; 10 MHz LTE(1.75 GHz ) + 20 MHz LTE (2.75 GHz)

Scenario 2; 20 MHz LTE (1.75 GHz) + 4 x 20 MHz LTE CA (2.75 GHz)

1.65 1.7 1.75 1.8 1.85-100

-90

-80

-70

-60

-50

-40

-30

Frequency (GHz)

Pow

er S

pect

ral D

ensi

ty (d

Bm

/Hz)

Without DPDSFFB DPDConv. DPD

2.65 2.7 2.75 2.8 2.85-100

-90

-80

-70

-60

-50

-40

-30

Frequency (GHz)

Pow

er S

pect

ral D

ensi

ty (d

Bm

/Hz)

Without DPDSFFB DPDConv. DPD

3. Cross-Modulation due to Multi-Band Access

17

LNASynthesized Oscillators

macro BS

femto/ macro /repeater

L

H

L

I

Q

090

HI

Q090

ADC

ADC

ADC

ADC

Near-far problem increases chance of inter-band modulation

Near & strong

Far & weak

Inter-band modulationSelf-Generated

Distortion

Self-GeneratedDistortion

Not Cared

L HHL 2LH 2

-20-10

010

-20

-10

0

100

5

10

15

20

PL (dBm)P

H (dBm)

EV

M (

%)

-20-10

010

-20

-10

0

100

5

10

15

20

PL (dBm)P

H (dBm)

EV

M (

%)

,PL

,PH

(a) EVM for L

(b) EVM for H

micro /

Post-Compensation of Receiver Nonlinearity

18

DDC for ωL

sL(n)

LNA

Channel Filter

xH(n)xL(n)

Compensatorfor yL(n)

yH(n)

yL(n)Time

Alignment

sH(n)Channel

FilterCompensator

for yH(n)

L

H

DDC for ωH

digital domain

ADC

ADC

Compensator diagram for yL

yL sL

|yL|2yL

|yH|2yL

|yL|4yL

|yH|4yL

|yH|2|yL|2yL

c'30

c'32

c'50

c'52

c'54

yH

|yH|Q-1yL

c'Q,Q-1

3rd IM

3rd CM

5th IM

5th CM

5th CM

Blind & Adaptive Nonlinear Compensation method is necessary [4].

Bypolynomial

How to Determine Compensator Coefficients?

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Minimization

DesiredSignal Band

0 Frequency Shift

Proposed algorithm determines the compensator coefficients so as to minimize outband spectra power for each band.

This works successfully because nonlinearity generates both inband and outband components and they are correlated.

The key is separation filter that eliminates the in-band signaland maximizes SNR of outband distortion detection.

Experimental Results

20

2320 2330 2340 2350 2360 2370 2380-80

-70

-60

-50

-40

-30

Frequency (MHz)

Pow

er

Spectr

al D

ensity (

dB

)

2620 2630 2640 2650 2660 2670 2680-70

-60

-50

-40

-30

-20

-10

Frequency (MHz)

Pow

er

Spectr

al D

ensity (

dB

)

Uncompensated

3rd order

5th order

7th order

Power levels of L and Hin (a)/(b) are -10 dBm and 6 dBm, respectively.

(a) Output spectra at L (b) Output spectra at H

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

7th-order compensation presents the best results.

Proposed compensator improves EVM in both bands.Maximum acceptable input power increases more than 4 dB.

f1 = 2.35 GHzf2 = 2.65 GHz

4. Reconfigurable RF BPF

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

Example of nonlinear output spectrum under concurrent dual-band operation.Some of them locate near the operating band.

Frequency (GHz)

Pow

er (

dBm

)

SHF Dual-band Reconfigurable BPF Low SHF (< 6 GHz) reconfigurable BPF and High SHF wideband

BPF are integrated for concurrent dual-band access.

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Low SHF Reconfigurable BPF section

DIP

Resonator ResonatorIC OCISC

DIP

Reconfig.Resonator1

Reconfig.Resonator2

IC OCISC

High SHF wideband BPF section

DIP: Diplexer IC: Input Coupling circuitISC; Inter-Stage Coupling circuit OC: Output Coupling circuit

2-stage 3-bit Low-SHF Reconfigurable BPF

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SHORT

OPEN

l21 SW3

Lin

SW1 SW2

SW6

SW4 SW5

SHORT

OPEN

Zg

Vg

l21

Lcut

Linter Lcut

Lout

ZL

l22l20 l20

l22

l23 l23

l1 l1

1st stage 2nd stage

-40

-30

-20

-10

0

1 2 3 4 5 6

f1f2

f3f4

f8f7f6f5

|S21

| (dB

)

Frequency (GHz)

Center Frequency; 3.1 to 4.6 GHzInsertion loss; 1.0~2.1 dB

High SHF BPF and diplexers are underdesign process.

Conclusions

5G hardware is required to be more flexible, accurate, linear and low-cost.

For Txs, advanced DPD techniques called SENF and SFFB are proposed.

- SENF saves feedback bandwidth, which enables wider compensation bandwidth.

- SFFB enables simple feedback for concurrent dual-band operation.

For concurrent dual-band Rxs, Post Compensation technique and Reconfigurable BPF are proposed.

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The results will contribute to improve practical performance of the 5G system.

Related Works

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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.

Thank you for listening!

LABORATORYLABORATORYLABORATORY

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

Acknowledgement