Full Duplex Radios
Sachin Katti
Kumu Networks & Stanford University
4/17/2014 1
“It is generally not possible for radios to receive and transmit
on the same frequency band because of the interference that
results.”
- Andrea Goldsmith, “Wireless Communications,” Cambridge Press, 2005.
Why are radios half duplex?
4/17/2014 2
TX
RX
RX
TX
Radio 1 Radio 2
“It is generally not possible for radios to receive and transmit
on the same frequency band because of the interference that
results.”
- Andrea Goldsmith, “Wireless Communications,” Cambridge Press, 2005.
Why are radios half duplex?
4/17/2014 3
TX
RX
RX
TX
Radio 1 Radio 2
Self-Interference is a hundred billion times
(110dB+) stronger than the received signal
Isn’t this easy to solve?
After all we know the interfering signal, why can’t
we just “subtract” it?
4/17/2014 4
4/17/2014 5
Signal Sent
Do we know what we are transmitting?
Signal Sent T
x
PA
DAC
TX
Mixer
4/17/2014 6
Signal Sent
Do we know what we are transmitting?
Signal Sent T
x
PA
DAC
TX
Centered at Carrier Freq (2.45GHz)
Mixer
4/17/2014 7
Signal Sent Signal Sent T
x
PA
DAC
TX
Centered at Carrier Freq (2.45GHz)
Mixer
Do we know what we are transmitting?
If you were to cancel, how much do we need?
Transmitted Signal
Pow
er
in d
Bm
-10 dBm Harmonics
-90 dBm Receiver Noise floor
Cancel residual
in digital to
reach noise floor
20 dBm Average Power
-20 dBm Transmit noise
4/17/2014 8
Cancel entire 110
dB to reach
noise floor
Cancel 70 dB Tx
noise to reach
noise floor
Cancel 70 dB in
Analog in such a
way to eliminate TX
noise
-80 dBm Harmonics
Cancelled Signal
Takeaways: Require 110dB of total cancellation, of which at
least 70dB has to eliminate transmitter noise in analog.
Contributions
We have invented in-band single
antenna full duplex radios • Self-Interference cancellation that
eliminates everything to the noise floor
• Practically achieves close to expected
theoretical 2x throughput increase
4/17/2014 9
RX RF Frontend
TX RF Frontend
T
+iT
+δT
Circulator
Contributions
We have invented in-band single
antenna full duplex radios • Self-Interference cancellation that
eliminates everything to the noise floor
• Practically achieves close to expected
theoretical 2x throughput increase
Algorithms & circuits to
estimate transceiver distortion
and cancel self interference • Hybrid (analog & digital) design with RF
cancellation circuit and DSP algorithms
4/17/2014 10
RX RF Frontend
TX RF Frontend
T
+iT
+δT
Circulator
Σ Analog Cancellation Eliminates all Tx noise and Protects ADC from saturating
Digital Cancellation Eliminates all Linear and Non-Linear Distortion
(Harmonics)
Σ
Mixed RF/Digital Design: Analog + Digital Cancellation
4/17/2014 11
Circulator (-15dB)
T
T
R+iT T
TX
RF Frontend
RX
RF Frontend
Digital Baseband
Analog RF Cancellation
4/17/2014 12
R
Circulator (-15dB)
T
T
T
TX
RF Frontend
RX
RF Frontend
Digital Baseband
εT iT’
RF Cancellation Circuit
Adaptive Algorithms
R+iT
+δT
4/17/2014 13
a1
a6
a2
a5
a3
a4
a8
a7
TX
RF Frontend
RX
RF Frontend
Adaptive Algorithms
Analog RF Cancellation
d4
d1
d2 d3
d5
d6 d7
d8
εT iT RF Cancellation Circuit
4/17/2014 14
a1
a6
a2
a5
a3
a4
a8
a7
TX
RF Frontend
RX
RF Frontend
Adaptive Algorithms
Analog RF Cancellation
d4
d1
d2 d3
d5
d6 d7
d8
εT iT
Analog Theory Intuition: Branch Delays
time (delay) d1 d2
fixed delays
N delay attenuation branches
control algorithm
Σ
interference signal
Delays are fundamentally related to sampling theory
4/17/2014 15
d1 a1
d2 a2
a3 d3
a4 d4
d
Analog Theory Intuition: Branch Delays
time (delay) d1 d2 d3 d4
fixed delays
N delay attenuation branches
control algorithm
Σ
interference signal
Delays are fundamentally related to sampling theory
4/17/2014 16
d1 a1
d2 a2
a3 d3
a4 d4
d
First branch pair: positive
4/17/2014 17
Estimating Branch Attenuation How do we fix attenuation ranges?
d2 d1 d
First branch pair: positive
4/17/2014 18
Estimating Branch Attenuation How do we fix attenuation ranges?
d2 d1 d
a1 a2
Second branch pair (negative)
Estimating Branch Attenuation How do we fix attenuation ranges?
4/17/2014 19
d4 d3
d2 d1 d
a1 a2
Estimating Branch Attenuation How do we fix attenuation ranges?
4/17/2014 20
Second branch pair (negative) a4
a3
d4 d3
d2 d1 d
a1 a2
Estimating Branch Attenuation Adaptation to environmental changes: Assumption d known
4/17/2014 21
d4 d3
d2 d1 d
a1 a2
a4 a3
Estimating Branch Attenuation Adaptation to environmental changes
4/17/2014 22
d4 d3
d2 d1 d
a1
a2
a4
a3
Estimating Branch Attenuation Adaptation to environmental changes
4/17/2014 23
d4 d3
d2 d1 d
a1
a2
a4
a3
Digital Baseband Cancellation
4/17/2014 24
isolator (-15dB)
T
Σ
T
T
TX
RF Frontend
RX
RF Frontend
Digital Baseband Cancellation Eliminates 2nd+ Order Non-Linearities (e.g. Intermod Products, LO
leakage, IQ imbalance)
RF Cancellation Circuit
Adaptive Algorithms
+δT
+ iT
Σ
Digital Baseband Cancellation
4/17/2014 25
TX
RF Frontend
RX
RF Frontend
Digital Baseband Cancellation Eliminates 2nd+ Order Non-Linearities (e.g. Intermod Products, LO
leakage, IQ imbalance)
RF Cancellation Circuit
Adaptive Algorithms
Digital Self-Interference Cancellation
• Challenge: Need to cancel main signal as well as higher order
harmonics upto the 11th order
• Prior approaches only cancel main signal, ignore hamonics
• Naïve approach to non-linearities: Needs to estimate ~1200
coefficients, would require a large number of training symbols &
hardware resources, infeasible in practice
• Our approach: Compact and fast digital self-interference
cancellation algorithm (needs to only estimate ~200 coefficients,
works with existing WiFi packet format)
4/17/2014 26
Evaluation Q1: Does it work with commodity radios?
• Challenge: Extremely high transmitter noise and non-linearities
• 20MHz BW (transceiver limitation) • 25dBm max TX power • WiFi 802.11n PHY
Goal: Build a full duplex radio using a cheap $2 COTS Maxim transceiver
• Commodity
transceiver
Digital
~40dB + Total
>110dB = Analog
>70dB 4/17/2014 28
-100
-85
-70
-55
-40
-25
-10
5
20
2.43 2.44 2.45 2.46 2.47
Po
we
r in
dB
m
Freq in Ghz
WARP 20 Mhz
Tx Signal
Residual Signalafter AC
Residual Signalafter DC
Noise Floor
72 dB
38 dB
Evaluation Q1: Does it work with commodity radios?
• Commodity
transceiver
• Tunes to
environmental
changes within
8us, needs to
be re-tuned
every 100ms
Digital
~40dB + Total
>110dB = Analog
>70dB 4/17/2014 29
-100
-85
-70
-55
-40
-25
-10
5
20
2.43 2.44 2.45 2.46 2.47
Po
we
r in
dB
m
Freq in Ghz
WARP 20 Mhz
Tx Signal
Residual Signalafter AC
Residual Signalafter DC
Noise Floor
72 dB
38 dB
Evaluation Q1: Does it work with commodity radios?
How do we compare against prior designs? 20 MHz Bandwidth. WiFi OFDM waveform, 25 dBm TX power
4/17/2014 30
Compared Approaches
Our Design
Balun Cancellation (Mobicom’11)
Extra-Tx Chain Design (Sigcomm’11, Asilomar’11)
How do we compare against prior designs? 20 MHz Bandwidth. WiFi OFDM waveform, 25 dBm TX power
4/17/2014 31
Compared Approaches Cancellation in (dB)
Our Design 110
Balun Cancellation (Mobicom’11)
85
Extra-Tx Chain Design (Sigcomm’11, Asilomar’10)
80
How do we compare against prior designs? 20 MHz Bandwidth. WiFi OFDM waveform, 25 dBm TX power
4/17/2014 32
Minimum SNR required for receiving a packet
Self-interference residue over noise floor
>
Compared Approaches Cancellation in (dB)
Self-interference residue over noise floor (dB)
Our Design 110 ~1
Balun Cancellation (Mobicom’11)
85 25
Extra-Tx Chain Design (Sigcomm’11, Asilomar’10)
80 30
Evaluation Q2: Does that translate to doubling of
throughput in practice?
4/17/2014 33
Throughput of FD Throughput of HD
Gain =
• Testbed: Indoor office noisy environment, various locations for the two full duplex radios.
• Compare throughput achieved in full duplex with that achieved in half duplex
• Full duplex implemented using our approach, and prior balun and extra TX chain based approaches
4/17/2014 34
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2
CD
F
Gain vs Half Duplex
Balun Cancellation Extra Transmitter Our Design
Worse than standard Half Duplex
1.97x median gain
Evaluation Q2: Does that translate to doubling of
throughput in practice?
Our design achieves the theoretical throughput doubling
Self-interference cancellation is broadly applicable
Full Duplex Radios
4/17/2014 35
Transmitted Self-Interference
Proprietary and Confidential
Channel 1 Channel 2
In-Band Full
Duplex
Double capacity
Received Signal
Adaptive
Frequency Division
Full Duplex (FDD)
Flexibly decide which
channels to transmit &
receive on
Transmitted Self-Interference
Received Signal
Channel 1 Channel 2
Applicable To a Host of Problems
LTE Access High performance Relay
Node. Doubles spectral
efficiency for TD-LTE.
WiFi Access Dense coverage by avoiding
interference between
adjacent bands
Mobile Devices World phones supporting
any FDD channel pairs with
adaptive duplexers
PtP and PtMP
Backhaul Doubles spectral efficiency,
and mitigate interference in
unlicensed bands
Full Duplex Everywhere
36 Proprietary and Confidential
To Conclude
Key contribution: Cancellation design that eliminates all self-
interference to the noise floor
– Full duplex radio is one application of this interference
cancellation technique.
– Widely applicable (Picasso, IMDShield, WiVi, Dhwani, …)
Emphasizes the need for an interdisciplinary approach that
combines RF circuit design, signal processing and
communication algorithm design
4/17/2014 37
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