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

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

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 TXRX TXRX Self-interference is millions to billions (60- 90dB) stronger than received signal

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

11. Our Approach 1.Infer interference structure –Easy, we know what we are transmitting! 2.Exploit knowledge of interference structure to subtract and decode

12. First Attempt: Antenna Cancellation d d + λ/2 TX1TX2RX Signal null at RX antenna ~30dB self-interference cancellation

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

14. Our Prototype Antenna Cancellation Hardware Cancellation Digital Interference Cancellation

15. TX1 TX2 Only TX1 Active Antenna Cancellation: Performance

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

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

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

19. Sensitivity of Antenna Cancellation Amplitude Mismatch between TX1 and TX2 Placement Error for RX dB Reduction Limit (dB) Error (mm)

20. Sensitivity of Antenna Cancellation dB Reduction Limit (dB) Error (mm) 30dB cancellation < 5% (~0.5dB) amplitude mismatch < 1mm distance mismatch Amplitude Mismatch between TX1 and TX2 Placement Error for RX

21. Sensitivity of Antenna Cancellation dB Reduction Limit (dB) Error (mm) Rough prototype good for 802.15.4 More precision needed for higher power systems (802.11) Amplitude Mismatch between TX1 and TX2 Placement Error for RX

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

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

24. Bandwidth Constraint A λ/2 offset is precise for one frequency not for the whole bandwidth fcfc f c +B f c -B d d + λ/2 TX1 TX2 RX d2d2 d 2 + λ +B /2 TX1 TX2 RX d1d1 d 1 + λ - B /2 TX1 TX2 RX

25. Bandwidth Constraint fcfc f c +B f c -B d d + λ/2 TX1 TX2 RX d2d2 d 2 + λ +B /2 TX1 TX2 RX d1d1 d 1 + λ - B /2 TX1 TX2 RX WiFi (2.4G, 20MHz) => ~0.26mm precision error A λ/2 offset is precise for one frequency not for the whole bandwidth

26. Bandwidth Constraint 2.4 GHz 5.1 GHz 300 MHz

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

28. 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

29. Our Approach 1.Infer interference structure –Easy, we know what we are transmitting! 2.Exploit knowledge of interference structure to subtract and decode

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

31. Cancellation using Phase Offset Self- Interferenc e Cancellatio n Signal ∑

32. Cancellation using Phase Offset Self- Interferenc e Cancellatio n Signal ∑ Self- Interferenc e Cancellatio n Signal ∑ Frequency dependent, narrowband

33. Self- Interferenc e Cancellatio n Signal ∑ Self- Interferenc e Cancellatio n Signal ∑ Frequency and bandwidth independent Cancellation using Signal Inversion

34. Time XtXt +X t /2 -X t /2 BALUN Second Design: Balanced to Unbalanced Conversion

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

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

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

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

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

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

41. Time Lower is better Higher is better

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

43. Time From 3 antennas per node to 2 antennas Parameters adjustable with changing conditions Attenuator and Delay Line TX RX TX Frontend XtXt +X t /2 -X t /2 ∑ RX Frontend Other advantages

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

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

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

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

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

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

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

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

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

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

54. Our Approach: Infer Interference Structure Current approach: –Measure channel SINR and pick modulation, coding rate –If channel SINR < decoding threshold, decoding fails –Collision  SINR < decoding threshold Key insight: Novel rateless codes for wireless Rateless code  no need to know SINR in advance, automatically achieves optimal throughput

55. Our Approach: Infer Interference Structure Key technique: Novel rateless codes P1 acts as interference to P2 and vice versa 1.Use rateless code to decode P1  Infer interference P1 P2 Decode P1 P1

56. P2 Our Approach: Exploit Interference Structure Key insight: Exploit rateless code to decode one packet, subtract it and decode next packet Decode P1 Subtract interference 1.Use rateless code to decode P1  Infer interference 2.Subtract P1 from received signal and decode P2 __ _ P1

57. Exploiting Cross-Network Interference Detecting Degrees of Freedom Hong, Katti “DOF: A Local Wireless Information Plane” SIGCOMM 2011

58. WiFi Zigbee Bluetooth Cross-Network Interference  Coexistence How to maximize throughput in the presence of cross-network interference?

59. Microwave Smart Transmitter Smart Receiver 1.The protocol types operating in the local vicinity 2. The spectrum occupancy of each type 3. The spatial directions of each type DOF infers coexisting interference structure WiFi AP Heart Monitor AoA Freq 2.3 GHz 2.5 GHz 0°0° 180° AoA Freq 2.3 GHz 2.5 GHz 0°0° 180° Freq 2.3 GHz 2.5 GHz Freq 2.3 GHz 2.5 GHz Our Approach: Infer Interference Structure

60. “Man-made” signals  hidden repeating patterns that are unique and necessary for operation Key Insight CP Data ……………………. Repeating Patterns in WiFi OFDM signals Repeating Patterns in Zigbee signals Time Leverage unique patterns to infer 1) type, 2) spectral occupancy, and 3) spatial directions

61. Exploit interference structure knowledge Policy 0 – Only use unoccupied spectrum WiFi Microwave Smart Tx AoA Freq 2.3 GHz 2.5 GHz Frequency 2.5 GHz Smart Rx AoA Freq 2.3 GHz PSD Policy 1 – Use unoccupied spectrum + mw oven spectrum Policy 2 – Use unoccupied spectrum + mw oven spectrum + compete for WiFi spectrum Heart Monitor (ZigBee Based) Our Approach: Exploit Interference Structure

62. To Conclude Future: dense, chaotic and limited spectrum Interference is the dominant determinant of future wireless network capacity Point to point link speeds are close to Shannon Our approach: Fundamental rethink of wireless to manage and exploit interference Increase concurrency  Increase network capacity