Achievable Rate Regions andSelf-interference Channel Estimationin Hybrid Full-Duplex/Half-DuplexRadio Links

Dani Korpi, Taneli Riihonen, and Mikko Valkama

3:45 pm, Thursday, March 19, 2015Session B3L-B: “Applications of Information Theory”49th Annual Conference on Information Systems and SciencesBaltimore, Maryland

• Introduction

• Flexible hybrid transmission and channel estimation

• Achievable rates and waveform simulations

• Numerical results and discussion

• Conclusions

Outline

2

3

Introduction• Hot research area: in-band full-duplex

communications; a novel 5G paradigmin the field of wireless communications

• The basic idea: simultaneoustransmission and reception (“STAR”)reusing the same frequency bands– Possible by cancelling the full-duplex

transceiver’s own transmit signal whichis self-interference (SI) for reception

• In this work: the practical throughputgain of full-duplex (FD) communications

• An important aspect: throughputrequirements are usually asymmetricbetween downlink (DL) and uplink (UL)

Downlink

Uplink

• focus: a single-cell systemcomprising a basestation (BS) and amobile user (UE)

• full-duplexoperationcould doublespectralefficiencyin the best case

– no residual SI– DL/UL = 1/1

4

Flexible hybrid transmission scheme• Fixed continuous full-duplex

operation is not necessarilyoptimal for the overall rate (!)

• In this work, a hybrid FD/HDtransmission scheme is analyzed– Two-way full-duplex transmission

occurs only part of the time– In addition, the BS and UE are

allowed to transmit alonein their half-duplex (HD) phases

• Benefits:1) This type of a flexible framestructure allows for balancing ofthe downlink and uplink data rates

2) Now both the BS and the UEhave a silent period for estimatingtheir SI loopback channels– An accurate estimate is crucial

for efficient SI cancellation

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Achievable rates• The respective downlink and uplink

data rates can be written as

= + 1 − −

= + 1 − −

• is the proportion of timewhen only the BS is transmitting

• is the proportion of timewhen only the UE is transmitting

• and are the rates overthe downlink- and uplink-onlyhalf-duplex periods

• and are the rates overthe full-duplex period

• The rates during the half-duplextransmission periods dependonly on transmit power, channelconditions, receiver noise flooretc. (the usual factors)

• The rates during the full-duplextransmission period depend alsoon the achieved cancellationperformance, i.e., the amountof residual SI– The efficiency of SI cancellation

is in turn determined bywhether or not the SI loopbackchannel can be estimatedwhen the other communicatingparty is silent

6

Evaluating achievable rates• System performance is assessed by

plotting achievable rate regions– Transmission performance shown

separately for downlink and uplinkas well as all of their ratios

• The numerical values for the ratesare obtained with realisticwaveform simulations

Parameter BS value UE value

Signal bandwidth 12.5 MHz

Number of antennas (TX/RX) 4/4 2/2

PA gain 27 dB 22 dB

Total transmit power 20 dBm 15 dBm

SNR per receiver 10 dB 15 dB

Analog SI attenuation 70 dB 65 dB

ADC bits 12 12

Parameter estimation sample size 10 000 7500

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tect

orTr

ansm

itda

ta

• Determining the rates with fullwaveform simulations ensures that

– the received signal’s effect onthe SI cancellation accuracy isimplicitly included in the results

– the rate under full-duplex operationis realistic because all the prominent(analog RF) circuit impairments areexplicitly modeled

Simulation parameters and the used in-band full-duplex transceiver model (simplified illustration)

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Numerical results 1

0 2 4 6 8 10 120

2

4

6

8

10

12

Upl

ink

rate

(bps

/Hz)

Downlink rate (bps/Hz)

SI channel coherence time: 1 ms

Flexible schemeHalf-duplex schemeFull-duplex schemeFixed hybrid scheme withinterference-free SI channel estimation

• Rate regions are shown in the figuresfor four different schemes:

– The flexible hybrid scheme proposed hereinwhere the lengths of the uplink-only ( ),downlink-only ( ) and full-duplex periodscan be freely adjusted

– A half-duplex scheme where + = 1,i.e., there is no full-duplex operation

– A full-duplex scheme where = = 0,i.e., simultaneous transmission and receptionoccurs continuously all the time

– A fixed hybrid scheme with interference-free SI channel estimation, where and

are always set to match the length of theSI channel estimation period

• The flexible scheme achieves the bestrates in both uplink and downlink

• With such a short SI channel coherencetime, it does not pay off to haveinterference-free SI channel estimation

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Numerical results 2• With a longer SI channel coherence

time, the flexible scheme againoutperforms the other schemes

• However, now the fixed hybridscheme with interference-freeSI channel estimation performsmuch better

• With a slowly changing SI channel,the penalty of interference-free SIchannel estimation is quite small– Continuous full-duplex operation

still does not pay off

0 2 4 6 8 10 120

2

4

6

8

10

12

Upl

ink

rate

(bps

/Hz)

Downlink rate (bps/Hz)

SI channel coherence time: 100 ms

Flexible schemeHalf-duplex schemeFixed hybrid scheme withinterference-free SI channel estimationFull-duplex scheme

99

Numerical results 3• Rate regions for three different

schemes and three different SNRvalues at the UE and the BS

• A higher SNR for the signal of interestpotentially means more interferencefor SI channel estimation

• However, based on this figure, itseems that it is actually still betterto have as high SNR as possible

– Improved SI cancellation performancecannot by any means compensate for theweaker signal of interest

• Also, the flexible hybrid scheme stilloutperforms the reference schemes,regardless of the SNR level

– It can adjust the lengths of the differenttransmission periods to maximize the rate

0 2 4 6 8 10 12 14 160

2

4

6

8

10

12

14

16

Upl

ink

rate

(bps

/Hz)

Downlink rate (bps/Hz)

SI channel coherence time: 100 ms

Flexible, SNRBS = 15 dB, SNRUE = 20 dB

Half-duplex, SNRBS = 15 dB, SNRUE = 20 dB

Full-duplex, SNRBS = 15 dB, SNRUE = 20 dB

SNRBS = 10 dB, SNRUE = 15 dB

SNRBS = 5 dB, SNRUE = 10 dB

1010

Numerical results 4• This figure shows the rate regions

for two different schemes withdifferent SI channel coherence times

• The basic observation is thata shorter coherence timerenders lower achievable rate– The SI channel estimate is valid

for a shorter period of time andmore resources are needed toestimate it repeatedly

• Also, with the longer coherencetimes, the fixed hybrid scheme canalmost match the flexible scheme– The optimal solution is to estimate

the SI channel without interference

0 2 4 6 8 10 120

2

4

6

8

10

12

Upl

ink

rate

(bps

/Hz)

Downlink rate (bps/Hz)

Flexible scheme, coherence time: 50 msFixed hybrid scheme, coherence time: 50 msCoherence time: 5 msCoherence time: 0.5 ms

Conclusion• In general, continuous full-duplex communication

does not maximize rates in a cellular network

• Separate half-duplex periods help in adjusting the rates betweenuplink and downlink, and also in obtaining a more accurateself-interference channel estimate

• Inclusion of half-duplex periods with adjustable lengths results insignificantly broader achievable rate regions, providing especiallyhigh gains with asymmetric downlink/uplink rate requirements

• Adjusting the ratio between uplink and downlink data rates is animportant feature in practical cellular networks, as requested ratesare typically significantly higher in the downlink direction

Thank you