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? Radio 1 Radio 2 TX RX RX TX 4/17/2014 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? Radio 1 Radio 2 TX RX RX TX Self-Interference is a hundred billion times (110dB+) stronger than the received signal 4/17/2014 3
Isn t this easy to solve? After all we know the interfering signal, why can t we just subtract it? 4/17/2014 4
Do we know what we are transmitting? Signal Signal T Sent Sent TX PA Mixer DAC x 4/17/2014 5
Do we know what we are transmitting? Signal Signal T Sent Sent TX PA Mixer DAC x Centered at Carrier Freq (2.45GHz) 4/17/2014 6
Do we know what we are transmitting? Signal Signal T Sent Sent TX PA Mixer DAC x Centered at Carrier Freq (2.45GHz) 4/17/2014 7
Power in dbm If you were to cancel, how much do we need? Transmitted Signal 20 dbm Average Power -10 dbm Harmonics -20 dbm Transmit noise Cancel entire 110 db to reach noise floor Cancel 70 db Tx noise to reach noise floor Cancelled Signal -80 dbm Harmonics Cancel 70 db in Analog in such a way to eliminate TX noise Cancel residual in digital to reach noise floor -90 dbm Receiver Noise floor Takeaways: Require 110dB of total cancellation, of which at least 70dB has to eliminate transmitter noise in analog. 4/17/2014 8
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 T Circulator +it +δt TX RF Frontend RX RF Frontend 4/17/2014 9
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 T Circulator Analog Cancellation Eliminates all Tx noise and Protects ADC from saturating Σ +it +δt Algorithms & circuits to estimate transceiver distortion and cancel self interference Hybrid (analog & digital) design with RF cancellation circuit and DSP algorithms TX RF Frontend Digital Cancellation Eliminates all Linear and Non-Linear Distortion (Harmonics) RX RF Frontend Σ 4/17/2014 10
Mixed RF/Digital Design: Analog + Digital Cancellation T T Circulator (-15dB) R+iT T TX RF Frontend RX RF Frontend Digital Baseband 4/17/2014 11
Analog RF Cancellation T R T Circulator (-15dB) R+iT T εt RF Cancellation Circuit it +δt TX RF Frontend Adaptive Algorithms RX RF Frontend Digital Baseband 4/17/2014 12
Analog RF Cancellation εt d1 a1 d2 a2 d3 RF Cancellation a3 d4 Circuit a4 d5 a5 d6 a6 d7 a7 d8 a8 it TX RF Frontend Adaptive Algorithms RX RF Frontend 4/17/2014 13
Analog RF Cancellation εt d1 d2 d3 d4 d5 d6 d7 d8 a1 a2 a3 a4 a5 a6 a7 a8 it TX RF Frontend Adaptive Algorithms RX RF Frontend 4/17/2014 14
Analog Theory Intuition: Branch Delays N delay attenuation branches d3 a3 d1 a1 d2 a2 Σ interference signal d4 a4 fixed delays control algorithm d1 d d2 time (delay) Delays are fundamentally related to sampling theory 4/17/2014 15
Analog Theory Intuition: Branch Delays N delay attenuation branches d3 a3 d1 a1 d2 a2 Σ interference signal d4 a4 fixed delays control algorithm d3 d1 d d2 time (delay) d4 Delays are fundamentally related to sampling theory 4/17/2014 16
Estimating Branch Attenuation How do we fix attenuation ranges? d1 d d2 First branch pair: positive 4/17/2014 17
Estimating Branch Attenuation How do we fix attenuation ranges? a1 a2 d1 d d2 First branch pair: positive 4/17/2014 18
Estimating Branch Attenuation How do we fix attenuation ranges? a1 a2 d3 d1 d d2 d4 Second branch pair (negative) 4/17/2014 19
Estimating Branch Attenuation How do we fix attenuation ranges? a1 a2 d3 d1 d d2 d4 a3 a4 Second branch pair (negative) 4/17/2014 20
Estimating Branch Attenuation Adaptation to environmental changes: Assumption d known a1 a2 d3 a3 d1 d d2 a4 d4 4/17/2014 21
Estimating Branch Attenuation Adaptation to environmental changes a2 a1 d3 d4 a3 d1 d d2 a4 4/17/2014 22
Estimating Branch Attenuation Adaptation to environmental changes a1 a2 d3 a3 d1 d d2 a4 d4 4/17/2014 23
Digital Baseband Cancellation T T Σ isolator (-15dB) + it T TX RF Frontend RF Cancellation Circuit Adaptive Algorithms +δt RX RF Frontend Digital Baseband Cancellation Eliminates 2 nd + Order Non-Linearities (e.g. Intermod Products, LO leakage, IQ imbalance) 4/17/2014 24
Digital Baseband Cancellation Σ RF Cancellation Circuit TX RF Frontend Adaptive Algorithms RX RF Frontend Digital Baseband Cancellation Eliminates 2 nd + Order Non-Linearities (e.g. Intermod Products, LO leakage, IQ imbalance) 4/17/2014 25
Digital Self-Interference Cancellation Challenge: Need to cancel main signal as well as higher order harmonics upto the 11 th 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? Goal: Build a full duplex radio using a cheap $2 COTS Maxim transceiver Challenge: Extremely high transmitter noise and nonlinearities 20MHz BW (transceiver limitation) 25dBm max TX power WiFi 802.11n PHY
Power in dbm Evaluation Q1: Does it work with commodity radios? WARP 20 Mhz Commodity transceiver 20 5-10 -25-40 -55-70 -85-100 2.43 2.44 2.45 2.46 2.47 Freq in Ghz Analog >70dB Digital 72 db 38 db + ~40dB Tx Signal Residual Signal after AC Residual Signal after DC Noise Floor Total = >110dB 4/17/2014 28
Power in dbm Evaluation Q1: Does it work with commodity radios? Commodity transceiver Tunes to environmental changes within 8us, needs to be re-tuned every 100ms 20 5-10 -25-40 -55-70 -85-100 2.43 2.44 2.45 2.46 2.47 Freq in Ghz Analog >70dB WARP 20 Mhz Digital 72 db 38 db + ~40dB Tx Signal Residual Signal after AC Residual Signal after DC Noise Floor Total = >110dB 4/17/2014 29
How do we compare against prior designs? 20 MHz Bandwidth. WiFi OFDM waveform, 25 dbm TX power Compared Approaches Our Design Balun Cancellation (Mobicom 11) Extra-Tx Chain Design (Sigcomm 11, Asilomar 11) 4/17/2014 30
How do we compare against prior designs? 20 MHz Bandwidth. WiFi OFDM waveform, 25 dbm TX power Compared Approaches Our Design 110 Balun Cancellation (Mobicom 11) Extra-Tx Chain Design (Sigcomm 11, Asilomar 10) Cancellation in (db) 85 80 4/17/2014 31
How do we compare against prior designs? 20 MHz Bandwidth. WiFi OFDM waveform, 25 dbm TX power Compared Approaches Cancellation in (db) Our Design 110 ~1 Balun Cancellation (Mobicom 11) Extra-Tx Chain Design (Sigcomm 11, Asilomar 10) 85 25 80 30 Self-interference residue over noise floor (db) Minimum SNR required for receiving a packet > Self-interference residue over noise floor 4/17/2014 32
Evaluation Q2: Does that translate to doubling of throughput in practice? 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 Gain = Throughput of FD Throughput of HD 4/17/2014 33
CDF Evaluation Q2: Does that translate to doubling of throughput in practice? 1 Balun Cancellation Extra Transmitter Our Design 0.8 0.6 0.4 0.2 Worse than standard Half Duplex 1.97x median gain 0 0 0.5 1 1.5 2 Gain vs Half Duplex Our design achieves the theoretical throughput doubling 4/17/2014 34
Full Duplex Radios Self-interference cancellation is broadly applicable Transmitted Self-Interference In-Band Full Duplex Double capacity Received Signal Channel 1 Channel 2 Received Signal Transmitted Self-Interference Channel 1 Channel 2 Adaptive Frequency Division Full Duplex (FDD) Flexibly decide which channels to transmit & receive on 4/17/2014 Proprietary and Confidential 35
Full Duplex Everywhere Applicable To a Host of Problems PtP and PtMP Backhaul Doubles spectral efficiency, and mitigate interference in unlicensed bands WiFi Access Dense coverage by avoiding interference between adjacent bands Mobile Devices World phones supporting any FDD channel pairs with adaptive duplexers LTE Access High performance Relay Node. Doubles spectral efficiency for TD-LTE. Proprietary and Confidential 36
To Conclude Key contribution: Cancellation design that eliminates all selfinterference 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