Potential of Full Duplex for Future Wireless Systems

Creating the Living Network

Alain Mourad

Cambridge Wireless

25 September 2014

POTENTIAL OF FULL DUPLEX FOR FUTURE WIRELESS SYSTEMS

© 2014 InterDigital, Inc. All rights reserved. 2Creating the Living Network

•Introduction

•Self-Interference Cancellation

•Usage Scenarios

•Merits and Challenges

•Conclusions

Outline


•What do we have today?

• TDD (Time Division Duplex): Transmit and Receive on the same radio

channel, but not simultaneously Half Duplex

• FDD (Frequency Division Duplex): Transmit and Receive simultaneously,

but not on the same radio channel Half Duplex

•Full-Duplex: The same carrier frequency used for simultaneous

transmission and reception at the radio transceiver.

Introduction

Radio transceiver

Channel f1

Channel f1

Transmit

Receive

time

Radio transceiver

Channel f1

Channel f1

Transmit

Receive

time

Radio transceiver

Channel f1

Channel f2

Transmit

Receive

time

Full Duplex TDD (Half Duplex) FDD (Half Duplex)


•Why we haven’t thought about it before? Simply because it wouldn’twork, due to a killing TX-to-RX self-interference.

• In full duplex, TX and RX are NOT orthogonal, as opposed to Half Duplexwhere orthogonality is guaranteed in time (TDD) or frequency (FDD).

• The local TX interference signal can be billions of times stronger (>100 dB)than the desired RX signal!

•What makes it now possible? Recent advances in self-interferencecancellation, exceeding the bar of 100 dB!

Introduction (Cont’d)

Radio transceiver

Channel f1

Channel f1

Transmit

Receive

time

Interference


Self-Interference Cancellation


Self-Interference

TX data DAC

ANT

PA

Duplexer

LO

ADC LNALO

Modulator(BB)

Demodulator(BB)

RX data

f1

TX signal

RX signal

Due to practical limitations inthe duplexer design andadditional mismatch at theduplexer’s output connectionto the transmission lineconnecting it to the antenna.

Due to mismatch between thetransmission line impedanceand the antenna’s inputimpedance.

Due to reflections in thesurrounding environment.

Duplexer Leakage Antenna Reflection Multipath Reflections


•Fortunately, self-interference cancellation is possible thanks to the knowledge of the transmitter’s signal. BUT, such knowledge is only perfectly available in the digital BB domain!

• The transmitter’s signal in the analogue domain suffer from both linear and non-linear distortions that are difficult to model accurately!

•Hence, any cancellation approach to be successful, must account for the non-linear distortions at the TX.

•Moreover, the cancellation approach must account for the RX ADC finite resolution in order to avoid RX saturation.

• The TX self interference at the ADC input must be kept below a certain level in order to allow ADC resolution for the relatively weak RX signal

•The cancellation approach must also include isolation mechanisms at the Duplexer and Antenna to reduce TX leakage and reflection.

Cancellation Approach


Cancellation Approach (Cont’d)

Isolation

TX data DAC

ANT

PA

Duplexer

LO

ADC LNALO

Modulator(BB)

Demodulator(BB)

RX data

f1

TX signal

RX signalDigital

CancellationAnalogue

Cancellation

- -

Post-LNA digital cancellation(Subtract a digital TX copyfrom the RX input signal at thedemodulator).Shall account for both linearand non-linear TX distortions.

Pre-LNA analogue cancellation(Subtract an analogue TX copyfrom the RX input signal at theLNA).Shall account for the ADCresolution to avoid saturation.

Antenna and Duplexer designto minimize leakage andantenna reflection

Both use knowledge of TX signal

25-40 dB20-30 dB20-30 dB


Recent Advances in Cancellation Techniques


Recent Advances in Cancellation Techniques (Cont’d) Important for the consolidation of FD and MIMO gains


Usage Scenarios


•Pairwise full-duplex (i.e. both nodes are in full-duplex)

• Use cases: Device-to-Device, AP-to-AP (mesh backhaul)

•Master full-duplex (i.e. forwarding node in full duplex)

• Use cases: Small cell AP, relays

Usage Scenarios

Station Access point

UL

DLSICSIC Symmetric

UL/DL traffic

Station 1 Access point

UL

DL

SIC

Station 2

CCI SIC at AP, but also CCI

at STA2


Merits & Challenges


•Full Duplex can achieve up to 2 times the link spectral efficiency

of Half Duplex (TDD or FDD).

•The actual spectral efficiency gain depends on the UL/DL traffic

distribution (i.e. needs to transmit and receive simultaneously).

Merits – Up to double the link spectral efficiency

(UL + DL)/T*BW

Full Duplex

(UL + DL)/2T*BW

FDD - HD

(UL + DL)/T*2BW

Station

Channel f1UL

Access point

UL

DLDL Channel f1

Time = T

Station

Channel f1UL

Access point

UL

DLDL Channel f2

Time = T

Station

Channel f1UL

Access point

UL

DLDL Channel f1

Time = T Time = T

TDD - HD

DL DL DL DL

UL

DL DL DL DL

UL

In this example, FD is operated 25% of the time, hence the gain from having the FD capability is around 1.25 times


•Simultaneous reception of feedback information (e.g., control

channel signaling) while transmitting data.

•Instantaneous re-transmission/forwarding (no need to wait

until the data is fully received before starting to forward it).

Merits – Reduced air interface delay vs. TDD

Full DuplexHD TDD

PDU1 PDU3PDU2

ACK1 ACK2 ACK3

Elapsed time

PDU1 PDU3PDU2

ACK1 ACK2 ACK3

Elapsed time

Transmit

Receive

Full DuplexHD TDD

PDU1 PDU3PDU2

Elapsed time

PDU1 PDU2 PDU3

Receive

Transmit

PDU1 PDU3PDU2

Elapsed time

PDU1 PDU2 PDU3


•Alleviate the hidden node problem (e.g. STA2 will be exposed

through the AP occupying the channel in DL as soon as it

receives from STA1 in UL).

•Fast Collision Detection thanks to the ability of the transmitter

to detect collision during its own transmission.

Merits – Additional advantages (1)

STA1

Occupy channel

UL

Access point

UL

Time = T

STA2

Occupy channel

UL UL

Time = T

Collision

STA1

Occupy channel

UL

Access point

UL

DLDL Occupy channel

Time = T

STA2

Occupy channel

DL DL

Time = T

Full Duplex

DATA

Wait for ACK

CD timeTransmit DATATransmit

ContinuousSensing

busy

Full Duplex

Abort transmission as soon as channel is detected busy


•Increased trunking efficiency vs. FDD, since full-duplex does not divide the channels between UL and DL (i.e. a large common pool of frequencies for UL/DL).

•Better spectrum sharing vs. FDD (e.g. higher likelihood to find one vacant primary channel than two at a given time).

•Better UL/DL decoupling vs. TDD (relaxed time dependency for UL/DL scheduling).

•Enhanced adaptation and coordination (thanks to faster reception of feedback information).

•Enhanced wireless security thanks to DL/UL jamming over the air (complicated eavesdropping).

Merits – Additional advantages (2)


•Design of low-power, low-cost, small-form-factor SIC circuitry

(including antennas, analog and digital cancellation).

•Design of effective and practical SIC for large bandwidth (>100

MHz) transmissions, and for massive MIMO (>32 antennas).

•Accurate statistical modelling of the effective channel, in

particular with MIMO systems and accounting for the non-

linear distortions at TX/RX, and derivation of fundamental

performance limits (e.g. information theory metrics).

•Design of optimized PHY layer techniques (e.g. coding,

modulation, synchronization, detection, estimation,

equalization, etc.), in particular with MIMO systems.

Challenges (I)


•Design of optimized MAC protocols (e.g. Sensing, Collision

Avoidance, ACK, etc.), in particular for high density scenarios.

•Optimized resource management across the power-space-time-

frequency dimensions, accounting for co-channel interference

and traffic distribution, and considering distributed vs

centralized architectures.

•Optimized control schemes for flexible and dynamic switching

between half-duplex and full-duplex, including adaptive frame

formats and control signaling with minimized overhead.

• Co-existence with legacy (half duplex) systems.

Challenges (II)


•In-band full duplex is a disruptive technology made possiblethanks to recent advances in Self-Interference Cancellation.

•The technology has attractive merits such as:• Up to 2x link spectral efficiency vs. TDD/FDD

• Reduced AI latency and UL/DL decoupling vs. TDD

• High trunking efficiency and flexible spectrum sharing vs. FDD.

•Though, it still presents a number of challenges before itbecomes a reality, mainly in terms of:• Practical low-cost small-form factor SIC (in particular for large BW)

• Combination with MIMO (in particular massive)

• Efficient MAC protocols for dense deployments (with CCI)

It is a matter of when, rather than if, FD will become a reality!

Conclusions

Technology

Potential of Full Duplex for Future Wireless Systems