Interference Centric Wireless Networks

1. Interference Centric Wireless Networks Sachin Katti Assistant Professor EE&CS, Stanford University

2. Interference is Everywhere Zigbee WiFi How to maximize throughput in the presence of interference? Bluetooth

3. Current Approach to Interference • Fears & avoids interference at all costs • Impacts all aspects of wireless design • Radios are half duplex • MAC protocols try to schedule one link at a time • Coexisting networks use different channels if possible • …….

4. Current Approach Cannot Scale Dense and chaotic wireless deployments  Interference is unavoidable • Hidden terminals cause collisions • Coexisting networks interfere with each other • Legacy interferers (e.g. microwave) ….. Moreover, Limited spectrum + interference avoidance design  Achievable capacity is fundamentally limited

5. This Talk Fundamental rethink: Exploit interference instead of avoiding it High-Level Approach • Infer interference structure • Exploit structure to better decode interfered packets and increase throughput

6. Exploiting Interference in All Contexts • Exploiting In-Link Interference • Full Duplex Radios (Mobicom 10,11) • Exploiting In-Network Interference • Rateless & Collision Resilient PHY (Sigcomm11) • Exploiting Cross-Network Interference • Detecting Degrees of Freedom (Sigcomm11)

7. Exploiting In-Link Interference:Full Duplex Radios Jain et al, “Practical Real Time Full Duplex Wireless” Mobicom 2010, 2011

8. “It is generally not possible for radios to receive and transmit on the same frequency band because of the interference that results. Thus, bidirectional systems must separate the uplink and downlink channels into orthogonal signaling dimensions, typically using time or frequency dimensions.” - Andrea Goldsmith, “Wireless Communications,” Cambridge Press, 2005.

9. In-Link Interference  Half Duplex Radios Self-interference is millions to billions (60-90dB) stronger than received signal TX RX TX RX

10. In-Link Interference  Half Duplex Radios ADC max Analog Self Interference Digital Self Interference Analog Received Signal Digital Received Signal Tx Rx - max Self-interference drowns out received signal

11. Our Approach • Infer interference structure • Easy, we know what we are transmitting! • Exploit knowledge of interference structure to subtract and decode

12. TX1 RX TX2 d + λ/2 d First Attempt: Antenna Cancellation • Signal null at RX antenna • ~30dB self-interference cancellation

13. Bringing It Together RX Antenna Cancellation TX Signal Hardware Cancellation QHX220 RF ADC Baseband + - Digital Cancellation ∑ TX Samples Clean RX samples

14. Our Prototype Antenna Cancellation Digital Interference Cancellation Hardware Cancellation

15. Antenna Cancellation: Performance TX1 TX2 Only TX1 Active

16. Antenna Cancellation: Performance TX1 TX2 Only TX1 Active Only TX2 Active

17. Antenna Cancellation: Performance TX1 TX2 Both TX1 & TX2 Active Only TX1 Active Only TX2 Active Null Position

18. Antenna Cancellation: Performance TX1 TX2 Both TX1 & TX2 Active Only TX1 Active Only TX2 Active ~25-30dB Null Position

19. TX1 RX TX2 d + λ/2 d Bandwidth Constraint A λ/2 offset is precise for one frequency fc

20. TX1 RX TX2 d + λ/2 d Bandwidth Constraint A λ/2 offset is precise for one frequency not for the whole bandwidth fc fc+B fc -B

21. TX1 TX1 TX1 RX RX RX TX2 TX2 TX2 d1 + λ-B/2 d + λ/2 d2 + λ+B/2 d1 d d2 Bandwidth Constraint A λ/2 offset is precise for one frequency not for the whole bandwidth fc fc+B fc -B

22. TX1 TX1 TX1 RX RX RX TX2 TX2 TX2 d1 + λ-B/2 d + λ/2 d2 + λ+B/2 d1 d d2 Bandwidth Constraint A λ/2 offset is precise for one frequency not for the whole bandwidth fc fc+B fc -B WiFi (2.4G, 20MHz) => ~0.26mm precision error

23. Bandwidth Constraint 300 MHz 2.4 GHz 5.1 GHz

24. Bandwidth Constraint 300 MHz 2.4 GHz 5.1 GHz • WiFi (2.4GHz, 20MHz): Max 47dB reduction • Bandwidth⬆ => Cancellation⬇ • Carrier Frequency⬆ => Cancellation⬆

25. First prototype gives 1.84x throughput gain with two radios compared to half-duplex with a single radio. Limitation 1: Need 3 antennas Limitation 2: Bandwidth constrained (802.15.4 works) Limitation 3: Doesn’t adapt to environment

26. Our Approach • Infer interference structure • Easy, we know what we are transmitting! • Exploit knowledge of interference structure to subtract and decode

27. 300 MHz 2.4 GHz 5.1 GHz Poor Man’s Subtraction

28. Cancellation using Phase Offset Self-Interference ∑ Cancellation Signal

29. Cancellation using Phase Offset Self-Interference ∑ Cancellation Signal Self-Interference ∑ Cancellation Signal Frequency dependent, narrowband

30. Cancellation using Signal Inversion Self-Interference ∑ Cancellation Signal Self-Interference ∑ Cancellation Signal Frequency and bandwidth independent

31. Time +Xt/2 Xt -Xt/2 BALUN Second Design: Balanced to Unbalanced Conversion

32. Traditional Design aT R R+aT T TX Frontend RX Frontend

33. 1. Invert the Signal aT R +T -T R+aT 2T balun TX Frontend RX Frontend

34. 2. Subtract Signal aT R R+aT +T -T Σ R+aT-T 2T balun TX Frontend RX Frontend

35. 3. Match Signals aT R attenuator and delay line R+aT +T -T Σ v R+aT-vT -vT 2T balun TX Frontend RX Frontend

36. Can Receive If v = a! aT R attenuator and delay line R+aT +T -T Σ v R+aT-aT -vT 2T balun TX Frontend RX Frontend

37. Time Signal Inversion Cancellation: Wideband Evaluation • Measure wideband cancellation • Wired experiments • 240MHz chirp at 2.4GHz to measure response λ/2 Delay +Xt/2 Xt +Xt/2 TX RF Signal Splitter Xt ∑ RX RX TX ∑ -Xt/2 +Xt/2 Phase Offset Cancellation Setup Signal Inversion Cancellation Setup

38. Time Higher is better Lower is better

39. Time Higher is better Lower is better ~50dB cancellation at 20MHz bandwidth with balun vs ~38dB with phase offset cancellation. Significant improvement in wideband cancellation

40. Time Other advantages RX TX Attenuator and Delay Line ∑ -Xt/2 +Xt/2 Xt RX Frontend TX Frontend • From 3 antennas per node to 2 antennas • Parameters adjustable with changing conditions

41. Adaptive RF Cancellation RX TX • Need to match self-interference power and delay • Can’t use digital samples: saturated ADC RF Reference Attenuation & Delay Σ RF Cancellation Balun Wireless Receiver Wireless Transmitter TX Signal Path RX Signal Path

42. Adaptive RF Cancellation RX TX • Need to match self-interference power and delay • Can’t use digital samples: saturated ADC RF Reference Attenuation & Delay Σ RSSI RF Cancellation Balun Wireless Receiver Wireless Transmitter TX Signal Path RX Signal Path RSSI : Received Signal Strength Indicator

43. Adaptive RF Cancellation RX TX Control Feedback • Need to match self-interference power and delay • Can’t use digital samples: saturated ADC RF Reference Attenuation & Delay Σ RSSI RF Cancellation Balun Wireless Receiver Wireless Transmitter TX Signal Path RX Signal Path Use RSSI as an indicator of self-interference

44. Adaptive RF Cancellation RX TX Control Feedback RF Reference Attenuation & Delay Σ RSSI RF Cancellation Balun Wireless Receiver Wireless Transmitter TX Signal Path RX Signal Path Objective: Minimize received power Control variables: Delay and Attenuation

45. Adaptive RF Cancellation Objective: Minimize received power Control variables: Delay and Attenuation ➔ Simple gradient descent approach to optimize

46. Bringing It All Together RX TX Control Feedback RF Reference Attenuation & Delay Σ RSSI RF Cancellation Balun Baseband ➔ RF RF ➔ Baseband DAC ADC Digital Interference Cancellation FIR filter ∆ Digital Interference Reference Encoder Channel Estimate Decoder TX Signal Path RX Signal Path

47. Performance • WiFi full-duplex: with reasonable antenna separation • Not enough for cellular full-duplex: need 20dB more

48. Full Duplex Implications • Breaks a fundamental assumption in wireless • Could eliminate the need for paired spectrum • Impacts higher layer design • Reduce control overhead (Radunovicet al) • Other applications • Security & Privacy (Gollakota et al) • Many more …..

49. Exploiting In-Network InterferenceRateless & Collision-Resilient Codes Gudipati, Katti “Strider: Automatic Rate Adaptation” SIGCOMM 2011

50. In-Network Interference  Collisions Carrier sense failure  Packet collisions and loss Current Approach: Conservative backoff, RTS/CTS