Reference Receiver Based Digital

Self-Interference Cancellation in

MIMO Full-Duplex Transceivers

Dani Korpi, Lauri Anttila, and Mikko Valkama

Tampere University of Technology, Department of Electronics and

Communications Engineering, Finland

10th International Workshop on Broadband Wireless Access

Outline

• Introduction

• RF impairments in a full-duplex transceiver

• Reference receiver based structure

• Initial analysis with system calculations

• Reference receiver aided digital cancellation

procedure

• Waveform simulations

• Conclusion

2

Introduction

• Simultaneous trans-

mission and reception

at the same center fre-

quency is an appealing

scheme

– Increased data rate,

MAC level benefits,

etc.

3

f

t

f

t

Do

wn

link

Up

link

Uplink

Downlink

FDD TDD

f

t

Uplink & Downlink

Single channel

full-duplex

Introduction (cont.)

4

• However, it has

also its down-

sides

– Increased com-

plexity due to

self-interference

cancellation, RF

impairments,

etc.

Receiver chain

Transmitter chain

Tx

RxS

elf-in

terf

ere

nce

RF cancellation Digital cancellation

DAC

ADC

SI regenerationAttenuation

& delay

PA

RF impairments

• The self-interference (SI) signal is distorted in

numerous ways within the transceiver chain,

especially in the transmitter

– This affects the accuracy with which the SI

signal can be regenerated for cancellation in

the digital domain

– Advanced modeling is required to achieve a

sufficient amount of SI cancellation

5

Reference receiver based

structure

6

• A simple way of

avoiding the complex

modeling is to take

the reference signal

always from the PA

output

• Then, the digital

cancellation signal

would include all the

TX impairments

Receiver chain

Transmitter chain

Tx

RxS

elf-in

terf

ere

nce

RF cancellation Digital cancellation

DAC

ADC

SI regenerationAttenuation

& delay

PA

Reference

receiver

Reference receiver based

structure (cont.)

• The downside of this approach is the need for

additional receivers

– In essence, a trade-off between the

computational cost of digital cancellation and

RF hardware

• It is not obvious what is the optimal scheme

of utilizing the additional receivers

7

Detailed analysis

8

Receiver chainsAmplitude

& phase

matching

RF can-

cellation

Reference receiver chains

Transmitter chains

TX2

BPF LNA IQ Mixer LPF VGA

2x2

mu

ltip

ath

co

up

ling

ch

an

ne

l

ADC

LPFIQ MixerVGAPA DAC

Digital

cancellation

To detector

TX1

LPFIQ MixerVGAPADAC

Transmit bit

stream 1

Transmit bit

stream 2

BPF LNA IQ Mixer LPF VGA ADC

Amplitude

& phase

matching

Digital

cancellationRF can-

cellation

To detector

Attenuator

IQ Mixer VGA ADC

Attenuator

IQ Mixer ADCVGA

Attenuator

-30 dB

Attenuator

-30 dB

SI

regeneration

RX1

RX2

LPF

LPF

SOI

Initial performance comparison

with system calculations

3.12.2014 9

With the reference receiver based structure, even linear processing can

provide sufficient SI suppression with transmit powers up to 20-25 dBm.

-5 0 5 10 15 20 25-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Transmit power (dBm)

Pow

er

of diff

ere

nt sig

nal c

om

ponents

(dB

m)

Linear Digital Cancellationwith Reference Receivers

-5 0 5 10 15 20 25-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Transmit power (dBm)

Pow

er

of diff

ere

nt sig

nal c

om

ponents

(dB

m)

Traditional LinearDigital Cancellation

SI mirror image

Noise floor

Quantization noise

TX-induced nonlinearities

RX-induced nonlinearities

Signal of interest

SI mirror image

Noise floor

Quantization noise

RX-induced nonlinearities

Signal of interest

Reference receiver aided

cancellation procedure

• The digital cancellation procedure is very

straight-forward in this case

• The received SI signal in the digital domain

during the training period can be expressed as

𝐬𝑖,𝐴𝐷𝐶,𝑡𝑟 = 𝐗𝑗,𝑃𝐴,𝑡𝑟

𝑁𝑇

𝑗=1

𝐡𝑖𝑗 + 𝐳𝑖,𝑡𝑟 = 𝐗𝑃𝐴,𝑡𝑟𝐡𝑖 + 𝐳𝑖,𝑡𝑟.

• Each 𝐗𝑗,𝑃𝐴,𝑡𝑟 is constructed from the samples

obtained via the reference receivers

3.12.2014 10

Parameter estimation

• For example, least squares can be used to

estimate the SI channel:

𝐡 𝑖 = 𝐗𝑃𝐴,𝑡𝑟𝐻 𝐗𝑃𝐴,𝑡𝑟

−1𝐗𝑃𝐴,𝑡𝑟

𝐻 𝐬𝑖,𝐴𝐷𝐶,𝑡𝑟

• Using 𝐡 𝑖, a cancellation signal can be

generated for any given transmit signal,

which is then used to cancel the SI:

𝐲𝑖,𝐷𝐶 = 𝐬𝑖,𝐴𝐷𝐶 + 𝐫𝑖 − 𝐗𝑃𝐴𝐡 𝑖 ≈ 𝐫𝑖 + 𝐳𝑖 ,

where 𝐫𝑖 is the signal of interest.

4.12.2014 11

Waveform simulations

8.12.2014 12

• More comprehensive

performance analysis

using the same

transceiver model as

earlier

• Explicit baseband-

equivalent model for

each component,

resulting in realistic

distortion of the SI

signal

Receiver chainsAmplitude

& phase

matching

RF can-

cellation

Reference receiver chains

Transmitter chains

TX2

BPF LNA IQ Mixer LPF VGA

2x2

mu

ltip

ath

co

up

ling

ch

an

ne

l

ADC

LPFIQ MixerVGAPA DAC

Digital

cancellation

To detector

TX1

LPFIQ MixerVGAPADAC

Transmit bit

stream 1

Transmit bit

stream 2

BPF LNA IQ Mixer LPF VGA ADC

Amplitude

& phase

matching

Digital

cancellationRF can-

cellation

To detector

Attenuator

IQ Mixer VGA ADC

Attenuator

IQ Mixer ADCVGA

Attenuator

-30 dB

Attenuator

-30 dB

SI

regeneration

RX1

RX2

LPF

LPF

SOI

Waveform simulations (cont.)

• The proposed

scheme cancels

SI efficiently

• With the higher

transmit powers,

it is limited by

the impairments

of the receiver

chains

8.12.2014 13

-5 0 5 10 15 20 25-20

-15

-10

-5

0

5

10

15

Transmit Power (dBm)

SIN

R (

dB

)

Reference RX aided cancellation

Linear cancellation

Widely-linear cancellation

Nonlinear cancellation

Conclusion

• The proposed reference receiver based

scheme achieves a significant improvement

in the SI cancellation ability utilizing only

simple linear processing in the digital domain

• However, it requires additional hardware in

the RF domain

• Additional research is required to assess

whether this is the best way of using the

additional receivers

3.12.2014 14

Thank you

• Questions and/or comments are welcome!

3.12.2014 15