Full Duplex by means of Electrical Balance Isolation

Full Duplex
by means of Electrical Balance Isolation
Mark Beach, Leo Laughlin, Jack Zhang,
Kevin Morris & John Haine
Communication and Networks Group,
University of Bristol, Bristol. UK
http://www.bristol.ac.uk/engineering/research/csn/
EPSRC CommNET2 Latest Advances on 5G Air-Interface
University of Surrey, Wed 22nd February 2017

Communication Systems & Networks Group
University of Bristol © CSN Group 2017
Bristol’s 5G Research on show @CommNET2
2
Massive MIMO
Millimetre Wave

Summary
• Why Full-Duplex communication
• Full duplex transceiver designs
• Full-duplex in handsets?
• Electrical balance duplexers
• Duplex isolation testing in
dynamic environments
• Combining electrical balance
isolation with active cancellation
• Conclusions and open questions
3

Bi-directional “Duplex” Communication
4
>20dBm
<-90dBm
Transmitter
Receiver
Antenna
Self Interference
Current systems avoid
Self Interference (SI):
• Time Division
Duplexing (TDD)
• Frequency Division
Duplexing (FDD)

Time Division Duplexing
5
Fc
Frequency
Time

Frequency Division Duplexing
6
Frequency
Time
F2
F1

In-Band Full-Duplex
7
Frequency
Time
Fc

Full Duplex – New Approach?
8
Single Frequency Tactical Manpack, 1980
Chris Richardson (Roke), UK patent PhD Student: S Chen, Bristol 1997
‘Duplex Free’
S. Chen, M. Beach, and J. McGeehan, “Division-
free duplex for wireless applications," Electronics
Letters, vol. 34, no. 2, pp. 147-148, jan 1998.

In-Band Full-Duplex Research
9
Rice University Stanford University
Tx chain
Tx
Signal
Rx
Signal
A
τ
Rx chain
Yonsei University
Tx chain
Tx
Signal
Rx
Signal
A
τ
Rx chain
Tx chain
Tx
Signal
Rx
Signal
A
τ
Rx chain
EBD
DUPLO Project Bristol
Tx chain
Tx
Signal
Rx
Signal Rx chain
EBDTx chain
Cancellation
Signal

In-Band Full-Duplex handsets?
10
Requirements: Low cost. Small form factor. Tuneable/multiband.
Separate antennas?
Limited space for antennas on mobile device
Does not scale well to MIMO
Isolation depends on physical separation
Isolation can be compromised by nearby objects/user
Can be multi-band
71dB isolation
57dB isolation 34dB isolation
E. Everett, A. Sahai, and A. Sabharwal, “Passive Self-Interference Suppression for Full-Duplex
Infrastructure Nodes,” Wirel. Commun. IEEE Trans., vol. 13, no. 2, 2014.

Electrical Balance Duplexer (EBD)
PA LNATx Rx
ZBAL
0ᵒ0ᵒ
0ᵒ 180ᵒ
ZANT
High isolation achieved
when ZBAL = ZANT
Self Interference
Cancellation
M Mikhemar, H Darabi and A. A. Abidi, “A multiband rf antenna duplexer on cmos: design and
performance”, Solid-State Circuits, IEEE J., vol. 48, no. 9, pp. 2067–2077, 2013.

EBD is well suited to handset application
12
PA LNATx Rx
ZBAL
0ᵒ0ᵒ
0ᵒ 180ᵒ
ZANT
High isolation can be achieved
(theoretically unlimited)
Low cost and small form factor
Single antenna
On-chip duplexer
Widely Tuneable
Loss in Tx and Rx paths, however this is comparable to alternatives
Highly accurate impedance control is required
Balancing impedance must adapt to variations in antenna
impedance due to environmental effects.
L. Laughlin, M. A. Beach, K. A. Morris and J. L. Haine, “Electrical balance duplexing for small form factor
realization of in-band full duplex”, IEEE Communications Magazine, vol. 53, no. 5, pp. 102–110, May 2015.

EBD isolation – frequency domain.
13
Limited isolation bandwidth due to frequency domain antenna impedance variation.
L. Laughlin, M. A. Beach, K. A. Morris and J. L. Haine, “Optimum single antenna full duplex using hybrid
junctions”, IEEE Journal on Selected Areas in Communications, vol. 32, no. 9, pp. 1653–1661, Sep. 2014.

EBD isolation – Environmental effects.
14
Reflections from objects in the
environment are received as
self interference and can
therefore affect the isolation
L. Laughlin, C. Zhang, M. A. Beach, K. A. Morris and J. L. Haine, “Dynamic performance
of electrical balance duplexing in a vehicular scenario”, Antennas and Wireless Propagation Letters, 2016.

EBD isolation – time domain.
 Antenna impedance is time variant due to environmental interaction.
 Balancing impedance must track antenna impedance to maintain isolation.
EBD adaptation requirements have been investigated by measuring dynamic
antenna reflection coefficient and embedding this in circuit simulations
Measure antenna S11
Simulated duplexer
Antenna S11 data

Dynamic antenna S11 measurement
16
 Due to the high vehicle speed, a vehicular scenario may present a requirement
for very fast balancing impedance adaptation.
 We set out to investigate the adaptation requirements in vehicular scenario.
Scenario was car driving in urban, motorway, and underground carpark.
 VNA measures the antenna reflection coefficient in a dynamic environment.
Measurement is repeated every 0.5 ms in order to capture time domain variation.

Dynamic antenna S11 measurement
17
 Two antenna types investigated: dipole antenna and multiband cellular antenna
 Rooftop and dashboard antenna mounting. Freqs: 730 MHz and 1900 MHz.

Circuit simulation with real antenna dynamics
18
 Measured antenna S11 time-series is played back in simulation.
 Adaptive EBD circuit simulation operates on real antenna data to determine time
domain variation in isolation for different balancing adaptation behaviours.
 Three adaptation behaviours have been
simulated
 Ideal balancing. The optimal balancing
impedance is recalculated for each antenna
measurement.
 Static balancing. Balancing impedance is
calculated based on the first antenna measurement
and then not updated.
 Limited Rate Adaptation (LRA).
Balancing impedance is updated at a given interval.
 Measurement/simulation bandwidth is 20MHz (widest LTE bandwidth)

Results – road scenario
20
 Simulation calculates mean
isolation across the 20 MHz BW as
function of time.
 Tx-Rx isolation over the duration of
the simulation can be expressed as
a CDF, showing how isolation has
varied over the duration of the
measurement/simulation
 Rooftop antennas show some
differences between environments,
but only <5 dB difference between
ideal and static.
 Only the carpark required circuit adaptation to maintain performance.
 High speed balancing adaptation is not required for this scenario.
Rooftop mounted antenna – 745 MHz

Results – road scenario
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 Even less variation observed at 1900 MHz compared to 730 MHz
730 MHz 1900 MHz
L. Laughlin, C. Zhang, M. A. Beach, K. A. Morris and J. L. Haine, “Dynamic performance
of electrical balance duplexing in a vehicular scenario”, Antennas and Wireless Propagation Letters, 2016.

22
 Measurements were conducted in
the UK; consequenctly, it started
raining.
 For the case of the dashboard
mounted antennas, the windscreen
wipers were observed to have a
substantial impact on the isolation
 In this situation balancing
impedance adaptation provides a
significant benefit.
 With adaptation, duplexer maintains
>40 dB isolation at all times.
Results – road scenario (with rain)

Conclusions – road scenario
23
 Cluttered dynamic environment does have an impact on the EBD isolation,
however the impact is small.
 In general vehicle motion does not necessitate high speed EBD adaptation.
 Windscreen wipers have a substantial impact on EBD isolation. EBD adaptation
interval of 5 ms is required to track antenna impedance changes in dashboard
mounted antennas.

Train scenario
24
 Train scenario may cause more
significant high speed antenna
impedance fluctuations due to
passing train.
 To investigate this, the dynamic
antenna measurements and circuit
simulations have been repeated
on a high speed train.

Train scenario – passing trains can be very
close to the antenna
25

Two train types measured
26
British Rail Class 43 – aka “high speed train” (HST)
Speed limit: 125 mph
Great Western Mainline. Separation between trains: ~1 m
British Rail Class 158
Speed limit: 90 mph
Wessex Mainline. Separation between trains: ~0.5 m
Neither train has metallised windows, however both are priceless antiques…

Train results - HST (1 m separation)
28
1900 MHz730 MHz
 Passing train has a measureable impact, however the effect is limited and, even
without circuit adaptation, performance is within 2 dB of ideal.
 More variation at 730 MHz due to lower propagation loss in reflected energy

Train results – Class 158 (0.5 m separation)
29
1900 MHz730 MHz
 Closer proximity of passing train leads to more variation, especially at 730 MHz
 However circuit still achieves >40 dB cancellation
L. Laughlin, C. Zhang, M. A. Beach, K. A. Morris and J. L. Haine, “Electrical Balance Duplexing: Field Trials
in a High Speed Rail Scenario”, Submitted to Transactions on Antennas and Propagation, 2017.

User interaction
30
1900 MHz
 Measurements/simulation repeated in user interaction scenario
 Without adaptation, user interaction can result in catastrophic results
 With adaptation isolation is maintained above 35 dB

Electrical Balance and Active RF Cancellation
31
 EB can be combined with
signal cancellation methods
to improve isolation and
bandwidth
 Cancellation signal is
actively injected to cancel
remaining interference
Residual self-Interference after EB duplexing stage
Self-interference is cancelled by
actively injecting another signal
prior to receiver input

Electrical Balance and Active RF Cancellation
32
L. Laughlin, C. Zhang, M. A. Beach, K. A. Morris and J. L. Haine, “Passive and active electrical balance
duplexers”, IEEE Transactions on Circuits and Systems II, vol. 63, no. 1, pp. 94–98, 2016.

Hardware prototype
33

34

Results 890 MHz and 1900 MHz
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 >44dB isolation provided by EB duplexer
 >83dB isolation from combined EB and cancellation (cancellation >38dB)
 Substantially the same performance at 890 MHz and 1900 MHz
L. Laughlin, C. Zhang, M. A. Beach, K. A. Morris and J. Haine, “A widely tunable full duplex transceiver combining
electrical balance isolation and active analog cancellation”, in Proceedings VTC (spring), Glasgow, May 2015.

Open Research Questions – FD networks…
36
New interference relationships in network
(device-to-device and basestation-to-basestation
interference). How should resources be allocated
and how does this impact on capacity?
Full-Duplex in dense network deployments –
what is the overall capacity gain?How does variable self-
interference affect link
capacity and network
capacity?
How do FD transceiver
architectures perform
against out-of-band
interference?
“EPSRC SENSE
KCL & Bristol”

Acknowledgements and Thanks to …
• Colleagues at Bristol: Leo Laughlin, Chunqing Zhang, Kevin Morris, & John Haine
• Colleagues at u-blox: Balakumar Swaminathan, Rob O’Leary, Kalim Khan, Pascal
Herczog, Mici McCullagh, Jim Connelley.
• First Group: Duncan Waugh &
Mathew Gard
37
Any Questions?

Full Duplex by means of Electrical Balance Isolation

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