Achieving Single Channel Full-Duplex Wireless Communication Jung Il Choi, Mayank Jain, Kannan Srinivasan, Philip Levis and Sachin Katti 1
Can a wireless node transmit AND receive at the same time on a single band? 2
Can a wireless node transmit AND receive at the same time on a single band? Status quo: NO 3
Current wireless radios • In-band half-duplex • Full-duplex through other dimensions • E.g. different frequencies • Bandwidth is a precious resource 4
Why not full-duplex on the same band? 5
Why not full-duplex on the same band? • Very strong self-interference TX RX TX RX • ~70dB stronger for 802.15.4 • Analog to Digital converter (ADC) saturates 6
Existing Techniques • Digital cancellation: Subtracting known interference digital samples from received digital samples. ZigZag [1] , Analog Network Coding [2] etc. • Hardware cancellation: RF noise cancellation circuits with transmit signal as noise reference Radunovic et al. [3] [1] Gollakota et al. “ZigZag Decoding: Combating Hidden Terminals in Wireless Networks”, ACM SIGCOMM 2008 [2] Katti et al. “Embracing Wireless Interference: Analog Network Coding”, ACM SIGCOMM 2007 [3] Radunovic et al. , "Rethinking Indoor Wireless: Lower Power, Low Frequency, Full-duplex", WiMesh (SECON Workshop),, 2010 7
Existing Techniques • Digital cancellation: Subtracting known interference digital samples from received digital samples. ZigZag [1] , Analog Network Coding [2] etc. Ineffective if ADC is saturated • Hardware cancellation: RF noise cancellation circuits with transmit signal as noise reference Radunovic et al. [3] [1] Gollakota et al. “ZigZag Decoding: Combating Hidden Terminals in Wireless Networks”, ACM SIGCOMM 2008 [2] Katti et al. “Embracing Wireless Interference: Analog Network Coding”, ACM SIGCOMM 2007 [3] Radunovic et al. , "Rethinking Indoor Wireless: Lower Power, Low Frequency, Full-duplex", WiMesh (SECON Workshop),, 2010 8
Existing Techniques • Digital cancellation: Subtracting known interference digital samples from received digital samples. ZigZag [1] , Analog Network Coding [2] etc. ~15dB Ineffective if ADC is saturated • Hardware cancellation: RF noise cancellation circuits with transmit signal as noise reference Radunovic et al. [3] ~25dB These are not enough 25dB +15dB < 70dB 9
Our innovation: Antenna Cancellation TX1 RX TX2 d + λ /2 d 10
Our innovation: Antenna Cancellation TX1 RX TX2 d + λ /2 d ~30dB self-interference cancellation Enables full-duplex when combined with Digital (15dB) and Hardware (25dB) cancellation. 11
Can a wireless node transmit AND receive at the same time on a single band? 12
Can a wireless node transmit AND receive at the same time on a single band? YES, IT CAN! Full-duplex prototype achieves 92% of the throughput of an “ideal” full-duplex system 13
Talk Outline • Design of Full-Duplex Wireless • 3 Techniques: Antenna, Hardware and Digital Cancellation • Analyzing Antenna Cancellation • Performance Results • Implications to Wireless Networks • Limitations of Design, Future Work 14
Talk Outline • Design of Full-Duplex Wireless • 3 Techniques: Antenna, Hardware and Digital Cancellation • Analyzing Antenna Cancellation • Performance Results • Implications to Wireless Networks • Limitations of Design, Future Work 15
Three techniques give ~70dB cancellation • Antenna Cancellation (~30dB) • Hardware Cancellation (~25dB) • Digital Cancellation (~15dB) 16
Antenna Cancellation: Block Diagram TX1 d RX d + TX2 λ / 2 Attenuator Power Splitter TX RX RF Frontend RF Frontend Digital Processor 17
Hardware and Digital Cancellation Hardware Cancellation • Use existing interference cancellation circuits (QHx220) * Digital Cancellation • Subtract known transmit samples from received digital samples * Radunovic et al. , "Rethinking Indoor Wireless: Lower Power, Low Frequency, Full-duplex", MSR Tech Report, 2009 18
Bringing It Together RX Antenna Cancellation TX Signal Hardware QHX220 Cancellation RF ADC Baseband + - Digital ∑ TX Samples Cancellation Clean RX samples 19
Bringing It Together RX Antenna Cancellation TX Signal Hardware QHX220 Cancellation RF ADC Baseband + - Digital ∑ TX Samples Cancellation Clean RX samples 20
Bringing It Together RX Antenna Cancellation TX Signal Hardware QHX220 Cancellation RF ADC Baseband + - Digital ∑ TX Samples Cancellation Clean RX samples 21
Bringing It Together RX Antenna Cancellation TX Signal Hardware QHX220 Cancellation RF ADC Baseband + - Digital ∑ TX Samples Cancellation Clean RX samples 22
Our Prototype Antenna Cancellation Digital Interference Cancellation Hardware Cancellation 23
Talk Outline • Design of Full-Duplex Wireless • 3 Techniques: Antenna, Hardware and Digital Cancellation • Analyzing Antenna Cancellation • Performance Results • Implications to Wireless Networks • Limitations of Design, Future Work 24
Antenna Cancellation: Performance TX1 TX2 -25 Only TX1 Active -30 -35 RSSI (dBm) -40 -45 -50 -55 -60 0 5 10 15 20 25 Position of Receive Antenna (cm) 25
Antenna Cancellation: Performance TX1 TX2 -25 Only TX1 Active -30 Only TX2 Active -35 RSSI (dBm) -40 -45 -50 -55 -60 0 5 10 15 20 25 Position of Receive Antenna (cm) 26
Antenna Cancellation: Performance TX1 TX2 -25 Both TX1 & Only TX1 Active TX2 Active -30 Only TX2 Active -35 RSSI (dBm) -40 -45 -50 Null -55 Position -60 0 5 10 15 20 25 Position of Receive Antenna (cm) 27
Antenna Cancellation: Performance TX1 TX2 -25 Both TX1 & Only TX1 Active TX2 Active -30 Only TX2 Active -35 RSSI (dBm) -40 -45 ~25-30dB -50 Null -55 Position -60 0 5 10 15 20 25 Position of Receive Antenna (cm) 28
Sensitivity of Antenna Cancellation Reduction Limit (dB) Reduction Limit (dB) dB Error (mm) Amplitude Mismatch Placement Error between TX1 and TX2 for RX 29
Sensitivity of Antenna Cancellation Reduction Limit (dB) Reduction Limit (dB) dB Error (mm) Amplitude Mismatch Placement Error between TX1 and TX2 for RX 30dB cancellation < 5% (~0.5dB) amplitude mismatch < 1mm distance mismatch 30
Sensitivity of Antenna Cancellation Reduction Limit (dB) Reduction Limit (dB) dB Error (mm) Amplitude Mismatch Placement Error between TX1 and TX2 for RX • Rough prototype good for 802.15.4 • More precision needed for higher power systems (802.11) 31
Bandwidth Constraint A λ /2 offset is precise for one frequency TX1 RX TX2 d + λ /2 d f c 32
Bandwidth Constraint A λ /2 offset is precise for one frequency not for the whole bandwidth TX1 RX TX2 d + λ /2 d f c f c -B f c +B 33
Bandwidth Constraint A λ /2 offset is precise for one frequency not for the whole bandwidth TX1 RX TX2 d 1 + λ - B /2 d 1 TX1 RX TX2 d + λ /2 d f c f c -B f c +B TX1 RX TX2 d 2 + λ +B /2 d 2 34
Bandwidth Constraint A λ /2 offset is precise for one frequency not for the whole bandwidth TX1 RX TX2 d 1 + λ - B /2 d 1 TX1 RX TX2 d + λ /2 d f c f c -B f c +B TX1 RX TX2 d 2 + λ +B /2 d 2 WiFi (2.4G, 20MHz) => ~0.26mm precision error 35
Bandwidth Constraint z H M 0 0 3 z H G 4 . 2 z H G 1 . 5 36
Bandwidth Constraint z H M 0 0 3 z H G 4 . 2 z H G 1 . 5 • WiFi (2.4GHz, 20MHz): Max 47dB reduction • Bandwidth ⬆ => Cancellation ⬇ • Carrier Frequency ⬆ => Cancellation ⬆ 37
What about attenuation at intended receivers? Destructive interference can affect this signal too! 38
What about attenuation at intended receivers? Destructive interference can affect this signal too! • Different transmit powers for two TX helps Deep Nulls at 20-30m 30 30 20 20 y axis (meters) y axis (meters) 10 10 0 0 -10 -10 -20 -20 -30 -30 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 x axis (meters) x axis (meters) Equal Transmit Power Unequal Transmit Power 39
What about attenuation at intended receivers? Destructive interference can affect this signal too! • Different transmit powers for two TX helps -52 dBm -100 dBm -58 dBm 30 30 20 20 y axis (meters) y axis (meters) 10 10 0 0 -52 dBm -10 -10 -20 -20 -30 -30 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 x axis (meters) x axis (meters) Equal Transmit Power Unequal Transmit Power 40
What about attenuation at intended receivers? Destructive interference can affect this signal too! • Different transmit powers for two TX helps • Diversity gains in indoor environments 41
Talk Outline • Design of Full-Duplex Wireless • 3 Techniques: Antenna, Hardware and Digital Cancellation • Analyzing Antenna Cancellation • Performance Results • Implications to Wireless Networks • Limitations of Design, Future Work 42
Experimental Setup • 802.15.4 based signaling on USRP nodes • Two nodes at varying distances placed in an office building room and corridor 43
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