Mitigating Hidden Terminals Current networks have hidden terminals N 1 AP N 2 • CSMA/CA can’t solve this • Schemes like RTS/CTS introduce significant overhead 52
Mitigating Hidden Terminals Current networks have hidden terminals N 1 AP N 2 • CSMA/CA can’t solve this • Schemes like RTS/CTS introduce significant overhead Full Duplex solves hidden terminals N 1 AP N 2 Since both sides transmit at the same time, no hidden terminals exist 53
Mitigating Hidden Terminals Current networks have hidden terminals N 1 AP N 2 • CSMA/CA can’t solve this • Schemes like RTS/CTS introduce significant overhead Full Duplex solves hidden terminals N 1 AP N 2 Since both sides transmit at the same time, no hidden terminals exist Reduces hidden terminal losses by up to 88% 54
Network Congestion and WLAN Fairness Without full-duplex : • 1/n bandwidth for each node in network, including AP Downlink Throughput = 1/n Uplink Throughput = (n-1)/n 55
Network Congestion and WLAN Fairness Without full-duplex : • 1/n bandwidth for each node in network, including AP Downlink Throughput = 1/n Uplink Throughput = (n-1)/n With full-duplex : • AP sends and receives at the same time Downlink Throughput = 1 Uplink Throughput = 1 56
Network Congestion and WLAN Fairness 1 AP with 4 stations without any hidden terminals Throughput (Mbps) oughput (Mbps) Fairness (JFI) Fairness (JFI) Upstream Downstream Half-Duplex 5.18 2.36 0.845 Full-Duplex 5.97 4.99 0.977 Full-duplex distributes its performance gain to improve fairness 57
Talk Outline • RF Cancellation using Signal Inversion: ~50dB for 20Mhz • Adaptive RF Cancellation: ~1ms convergence • Adaptive Digital Cancellation: ~30dB cancellation • System Performance: ~73dB cancellation • Implications to Wireless Networks: Collisions, Fairness • Looking Forward 58
• Other cancellation techniques Digital estimation for analog cancellation [1] RX TX Σ RF ➔ Baseband Baseband ➔ RF Baseband ➔ RF DAC DAC ADC TX Signal Cancellation Signal RX Signal [1] Duarte et al. “Full-Duplex Wireless Communications Using Off-The-Shelf Radios: Feasibility and First Results.”, in Asilomar 2010. 59
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response 60
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response Reduce distortion: feedforward amplifiers High Power Amplifier TX Signal 61
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response Reduce distortion: feedforward amplifiers High Power Amplifier TX Signal ∑ + - Estimate Distortion 62
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation 63
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation • Single antenna solution: circulators TX Signal RX Signal Circulator 64
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation • Single antenna solution: circulators • Device precision: 1 ps resolution for delay line 65
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation • Single antenna solution: circulators • Device precision: 1 ps resolution for delay line • Going mobile: Higher cancellation, faster adaptation 66
• Other cancellation techniques Digital estimation for analog cancellation [1] • Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation • Single antenna solution: circulators • Device precision: 1 ps resolution for delay line • Going mobile: Higher cancellation, faster adaptation • MIMO full-duplex 67
Full-duplex Networking Access Point networks 68
Full-duplex Networking Cell Relay Basestation Access Point networks Cellular networks 69
Full-duplex Networking Cell Relay Basestation Access Point networks Cellular networks Multi-hop Networks 70
Full-duplex Networking Cell Relay Basestation Access Point networks Cellular networks Secure Networks [1,2] Multi-hop Networks [1] Gollakota et al. “They Can Hear Your Heartbeats: Non-Invasive Security for Implantable Medical Devices.”, in Sigcomm 2011. [2] Lee et al. “Secured Bilateral Rendezvous using Self-interference Cancellation in Wireless Networks”, in IFIP 2011. 71
Full-duplex Networking Cell Relay Basestation Access Point networks ? Cellular networks Secure Networks [1,2] Multi-hop Networks [1] Gollakota et al. “They Can Hear Your Heartbeats: Non-Invasive Security for Implantable Medical Devices.”, in Sigcomm 2011. [2] Lee et al. “Secured Bilateral Rendezvous using Self-interference Cancellation in Wireless Networks”, in IFIP 2011. 72
Thank You Questions? 73
Backup 74
Talk Outline • RF Cancellation using Signal Inversion: ~50dB for 20Mhz • Adaptive RF Cancellation: ~1ms convergence • Adaptive Digital Cancellation • System Performance • Implications to Wireless Networks • Looking Forward 75
Digital Cancellation • Create a precise “digital replica” of the self- interference signal using TX digital samples • Subtract self-interference replica from received digital signal Requires ADC not saturated: RF cancellation 76
OFDM processing Signal Band 77
OFDM processing Sub-bands 78
OFDM processing Channel Distortion 79
OFDM processing Channel Equalization Distortion 80
OFDM processing Channel Equalization Distortion RX RF Mixer Carrier Frequency ADC Offset Correction FFT Engine Packet Detect Carrier Frequency Demapping Equalization Channel Estimation 81
Step 1: Estimation Self-interference Sounding FIR Filter Training Self-interference Preamble Sequence Estimate IFFT RX RF Mixer Carrier Frequency ADC Offset Correction FFT Engine Packet Detect Carrier Frequency Demapping Equalization Channel Estimation Estimation includes effect of RF cancellation 82
Step 2: Cancellation Cancellation TX Signal Signal FIR Filter Self-interference Estimate IFFT RX RF Mixer + - Carrier Frequency ADC Offset Correction FFT Engine Packet Detect Carrier Frequency Demapping Equalization Channel Estimation 83
Step 2: Cancellation Cancellation TX Signal Signal FIR Filter Self-interference Estimate IFFT RX RF Mixer + - Carrier Frequency ADC Offset Correction FFT Engine Packet Detect Carrier Frequency Demapping Equalization Channel Estimation 30dB Cancellation 84
Talk Outline • RF Cancellation using Signal Inversion: ~50dB for 20Mhz • Adaptive RF Cancellation: ~1ms convergence • Adaptive Digital Cancellation: ~30dB cancellation • System Performance • Implications to Wireless Networks • Looking Forward 85
Phase Offset Cancellation: Block Diagram TX1 d RX d + TX2 λ/ 2 Attenuator Power Splitter TX RX RF Frontend RF Frontend Digital Processor 86
Phase Offset 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) 87
Phase Offset 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) 88
Phase Offset 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) 89
Phase Offset 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) 90
What about attenuation at intended receivers? Destructive interference can affect this signal too! • Different transmit powers for two TX helps -58 dBm 30 30 -52 dBm 20 20 y axis (meters) y axis (meters) 10 10 -52 dBm 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) Single Transmit Antenna Two Transmit Antennas 91
Sensitivity of Phase Offset Cancellation Cancellation (dB) Cancellation (dB) Higher is Higher is better better dB Error (mm) Amplitude Mismatch Placement Error between TX1 and TX2 for RX 92
Sensitivity of Phase Offset Cancellation Cancellation (dB) Cancellation (dB) dB Error (mm) Amplitude Mismatch Placement Error between TX1 and TX2 for RX 30dB cancellation < 5% (~0.5dB) amplitude mismatch < 1mm distance mismatch 93
Sensitivity of Phase Offset Cancellation Cancellation (dB) Cancellation (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) 94
Bandwidth Constraint A λ /2 offset is precise for one frequency TX1 RX TX2 d + λ /2 d f c 95
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 96
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 97
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 98
Bandwidth Constraint 5.1 GHz 2.4 GHz 300 MHz Edge frequency f c 99
Bandwidth Constraint 5.1 GHz 2.4 GHz 300 MHz • WiFi (2.4GHz, 20MHz): Max 47dB reduction • Bandwidth ⬆ => Cancellation ⬇ • Carrier Frequency ⬆ => Cancellation ⬆ 100
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