1. BBN: Throughput Scaling in Dense Enterprise WLANs with Blind Beamforming and Nulling Wenjie Zhou (Co-Primary Author), Tarun Bansal (Co-Primary Author), Prasun Sinha and Kannan Srinivasan The Ohio State University

2. Changes in Uplink Traffic 2 Cloud Computing Online Gaming Sensor Data Upload Code Offloading VoIP, Video Chat Traditionally, WLAN traffic: downlink heavy less attention to uplink traffic Recently, uplink traffic increased rapidly : mobile applications

3. Can we scale the uplink throughput with the number of clients?

4. Network MIMO Huge bandwidth consumption C2C2 C1C1 C3C3 Exchange raw samples AP 1 AP 2 AP 3

5. [1] Rahul, H., Kumar, S., and Katabi, D. MegaMIMO: Scaling Wireless Capacity with User Demand. In Proc. of ACM SIGCOMM 2012. MegaMIMO 1 Does not apply to uplink : Clients do not share a backbone network Does not apply to uplink : Clients do not share a backbone network

6. [1] Cadambe, V. R., and Jafar, S. A. Interference Alignment and the Degrees of Freedom for the K User Interference Channel. IEEE Transactions on Information Theory (2008). Interference Alignment 1 4 packets, 3 slots Enough time slots, everyone gets half the cake Exponential slots of transmissions, not suitable for mobile clients Heavy coordination between clients 4 packets, 3 slots Enough time slots, everyone gets half the cake Exponential slots of transmissions, not suitable for mobile clients Heavy coordination between clients C2C2 C1C1 C3C3 AP 1 AP 2 AP 3

7. Existing interference alignment and beamforming techniques are not suitable to mobile uplink traffic. How can we bring the benefits of beamforming to uplink traffic?

8. AP Density in Enterprise WLANs 8 (140,0.5) BBN leverages the high density of access points

9. Single Collision Domain C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 Switch Omniscient TDMA Time Slot: 1Time Slot: 2Time Slot: 3 Three Packets received in Three Slots Only one AP is in use Three Packets received in Three Slots Only one AP is in use 9

10. h (1) 12 x 1 + h (1) 22 x 2 + h (1) 32 x 3 h (1) 11 x 1 + h (1) 21 x 2 + h (1) 31 x 3 Blind Beamforming and Nulling Single Collision Domain 10 Time Slot: 1 C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 Switch h (1) 13 x 1 + h (1) 23 x 2 + h (1) 33 x 3 h (1) 14 x 1 + h (1) 24 x 2 + h (1) 34 x 3 h (1) 13 h (1) 23 h (1) 33

11. Receives: a 11 x 1 + s 1 h (1) 21 x 2 + s 1 h (1) 31 x 3 Receives: a 12 x 1 + a 22 x 2 + a 32 x 3 Transmits: v 4 (h (1) 14 x 1 + h (1) 24 x 2 + h (1) 34 x 3 ) Transmits: (h (1) 13 x 1 + h (1) 23 x 2 + h (1) 33 x 3 ) 11 Time Slot: 2 Blind Beamforming and Nulling Single Collision Domain AP 1 AP 2 AP 3 AP 4 Switch v3v3

12. AP 1 AP 2 AP 3 AP 4 Switch Slot 2: a 11 x 1 + s 1 h (1) 21 x 2 + s 1 h (1) 31 x 3 Slot 2: a 12 x 1 + a 22 x 2 + a 32 x 3 Slot 1: h (1) 11 x 1 + h (1) 21 x 2 + h (1) 31 x 3 Slot 1: h (1) 12 x 1 + h (1) 22 x 2 + h (1) 32 x 3 Three Packets received in Two Slots 12 Blind Beamforming and Nulling Single Collision Domain (s 1 h (1) 11 -a 11 )x 1 Slot 2: a 11 x 1 + s 1 h (1) 21 x 2 + s 1 h (1) 31 x 3

13. Number of APs Required In a network with APs, APs in BBN can receive N uplink packets in two slots 3 clients, 4 APs 4 clients, 7 APs 10 clients, 46 APs 13

14. Throughput Improvement Previous Example Topology – APs in BBN receive three packets in two slots: an improvement of 50% General Topology – Uplink throughput in BBN scales with the number of clients (N/2 packets per slot). – Half of the cake as in Interference Alignment Always two slots No coordination between clients 14

15. BBN Highlights Leverages the high density of access points All computation and design complexity shifted to APs APs only need to exchange decoded packets over the backbone instead of raw samples 15

16. Further Optimizations to Improve SNR Which subset of APs act as transmitters and which subset as receivers? Which AP decodes which packet? C1C1 C2C2 C3C3 AP 1 AP 2 AP 3 AP 4 Switch 16 BBN Approach: x i is decoded at the AP j where it is expected to have highest SNR Transmitters Receivers x1x1 x 2, x 3

17. Challenge 1/4: Synchronization of APs To perform accurate beamforming, APs need to be tightly synchronized with each other Solution: – SourceSync (Rahul et al., SIGCOMM 2010): synchronizes APs within a single collision domain – Vidyut (Yenamandra et al., SIGCOMM 2014): uses power line to synchronize APs in the same building 17

18. Challenge 2/4 : MultiCollision Domain Not all APs may be able to hear each other directly Solution: Make smaller groups where all APs in a single group can hear each other. 18

19. 19 Distributed System Group Head Within a group, all APs can hear each other When one group is communicating, neighboring groups remain silent

20. Challenge 3/4 : Inconsistency in the AP density Number of APs may be less than Solution: Appropriate MAC layer algorithm that restricts the number of participating clients 20

21. Uplink Poll Approve A, B and C Keep Silent – Allow neighboring groups to transmit DownlinkUplink....... Time Notification Period Time Slot 1Time Slot 2 Uplink MAC Timeline Compute pre-coding vectors in the background 21

22. C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 4 AP 5 Switch Challenge 4/4 : Robustness Nulling is not always perfect. x 1, x 2, x 3 x1x1 Decoding Error Can’t Subtract x 1 22

23. C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 Switch Challenge 4/4 : Robustness What if we have extra APs AP 5 AP 6 AP 7 x 1, x 2, x 3 x1x1 x1x1 23

24. Experiments 24 C1C1 C2C2 x1x1 x 2, x 3 AP 1 AP 2 AP 3 AP 4 Switch Intended Signal = x 1 Interference from x 2, x 3 x2x2 C2C2 x3x3 USRP N210

25. Throughput BBN provides 1.48x throughput compared to TDMA 25 1.48X

26. Trace-Driven Simulation Over multiple collision domains (divided into groups) Field Size: 500m X 500m Number of clients: 1000 Vary the number of APs Residual interference distribution from experiment Other algorithms simulated – Omniscient TDMA – IEEE 802.11 26

27. 27 2000 APs 4.6X throughput gain ~76 APs near each client 2000 APs 4.6X throughput gain ~76 APs near each client Throughput BBN

28. Fairness 28 BBN achieves higher fairness Beamforming increased SINR of clients that are far away BBN achieves higher fairness Beamforming increased SINR of clients that are far away BBN

29. Summary and Future Work BBN leverages the high density of APs to scale the uplink throughput for single antenna systems – Throughput scales linearly with the number of clients – All computational and design complexity shifted to APs Future Work – Coexist with legacy network – Data rate selection 29

30. Backup Slides 30

31. Long Term Results

32. OctoClock-G Frequency Accuracy w/ out : 25 ppb Frequency Accuracy with GPS Lock : <1 ppb PPS Accuracy with GPS Lock : 50 ns Vidyut : approximately 225 ns

33. Multiple Antenna AP Assume each AP has K antennas For N clients, APs required For M APs, clients

34. Estimate SNR of C 1 at AP 2 SNR of C 1 at AP 2 is low C1C1 AP 1 AP 2 AP 3 Switch AP 4 34 No path with high SNR

35. Estimate SNR of C 1 at AP 1 SNR of C 1 at AP 1 is high C1C1 AP 1 AP 2 AP 3 AP 4 Switch 35 One path with high SNR C 1 should be decoded by AP 1 AP 1 should act as a receiver in slot 2

36. Blind Nulling in BBN 36

37. AP 1 : C 1 : AC 1 Packet 1 C 2 : AC 2 Packet 2 C 3 : AC 3 Packet 3 IACS Approve MAC Layer: Phase 1 37

38. AP 1 : AP 2 : AC 3 AP 3 : AP 4 : v 4 * Samples 4 BIFS AC 1 AC 2 SIFS AP 5 : v 5 * Samples 5 AP 6 : v 6 * Samples 6 AP 7 : v 7 * Samples 7 MAC Layer: Phase 2 38

39. Experiments Setup Performed using USRP N210 Radio Testbed of 4 APs and 3 clients Modulation Scheme: OFDM with BPSK Channel: Central Frequency 400 Mhz, Bandwith set to 500 KHz 39

40. Existing Schemes Interference Alignment – Existing IA schemes perform alignment over exponential number of time slots [Cadambe et al., IEEE Transactions on Information Theory 2007] MU-MIMO (Multi User MIMO) – Requires transmitters to exchange each other’s data before transmission MU-MIMO (Multi User MIMO) in EWLAN – All APs together act as a single AP with multiple antennas – Requires APs to exchange samples over the backbone which is cost- prohibitive [Gollakota et al., SIGCOMM 2009; Gowda et al., INFOCOM 2013] 40

41. Existing Schemes Interference Alignment – Existing IA schemes require each transmitter to transmit exponential amount of data [Cadambe et al., IEEE Transactions on Information Theory 2007] MU-MIMO – All APs together act as a single AP with multiple antennas – Requires APs to exchange samples over the backbone which is cost-prohibitive [Gollakota et al., SIGCOMM 2009] 41

42. Related Work (contd.) Interference Alignment – Existing IA schemes work over exponential number of time slots [Cadambe et al., IEEE Transactions on Information Theory 2007] – Or, work only for downlink [Suh et al., IEEE Transactions on Communications 2011] – Or, require multiple antennas at clients [Gollakota et al., SIGCOMM 2009] – Or, require APs to exchange samples over backbone [Annapureddy et al., IEEE Transactions on Information Theory 2012] 42

43. Related Work Backbone Usage – MegaMIMO (Rahul et al., SIGCOMM 2012): Works only for downlink – Symphony (Bansal et al., MobiCom 2013): Works only in multiple collision domain 43

44. Related Work (contd.) Wireless Relays – Use special relay nodes to assist high speed communication between specific transmitters and receivers – Existing algorithms do not make use of the backbone – BBN leverages the backbone to improve throughput – BBN can extend to multiple rounds to decode packets with low SNR 44

45. BBN Highlights Leverages the high density of access points Uplink throughput scales with the number of clients in the network All computational and design complexity shifted to APs APs only need to exchange decoded packets over the backbone 45

46. C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 Example Topology: What we ideally want 46 x2x2 x1x1 x3x3 Works! But can we make the requirements less strict?

47. Matching in BBN 47 C1C1 C2C2 C3C3 AP 1 AP 2 AP 3 AP 4 AP 5 AP 6 AP 7 Edge Weight = Expected SINR of C2 at AP3 Find the Maximum Weight Matching Which AP decodes which packet. Which AP transmits in the second slot.

48. Number of APs Required: Example Topology Two packets (x 2 and x 3 ) need to be nulled at AP 1 One packet (x 3 ) needs to be nulled at AP 2 Three transmitting APs required Guarantee non degenerate solution: Four APs required 48 x1x1 x2x2 x3x3

49. AP Density in Enterprise WLANs 49 CDF of number of APs observed (Measurements conducted at Ohio State University campus) Can we leverage the high density of APs to scale the uplink throughput?

50. Enterprise Wireless LAN 50 AP Internet

51. BBN Overview Leverages the high density of access points Uplink throughput scales with the number of clients in the network – Schedule length: Two Slots First slot: Clients transmit Second slot: APs perform blind nulling – APs only need to exchange decoded packets over the backbone 51

52. Contents BBN Design Experiments Simulations Challenges Related Work Conclusion 52

53. C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 Example Topology (Single Collision Domain) with BBN 53 Time Slot: 1

54. C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 Simultaneous Nulling Goal 54 Receive: x 1, x 2 Null: x 3 Receive: x 1 Null: x 2, x 3 Receive: x 1,, x 2, x 3

55. C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 Example Topology (Single Collision Domain) with BBN 55 Time Slot: 1 h 14 x 1 + h 24 x 2 + h 34 x 3 h 15 x 1 + h 25 x 2 + h 35 x 3... Time Slot: 2 v 4 * (h 14 x 1 + h 24 x 2 + h 34 x 3 ) v 5 * (h 15 x 1 + h 25 x 2 + h 35 x 3 ) v 6 * (...)v 7 * (...) a 11 x 1 a 12 x 1 + a 22 x 2 a 13 x 1 + a 23 x 2 + a 33 x 3

56. AP 1 AP 2 AP 3 Switch Example Topology (Single Collision Domain) with BBN 56 Time Slot: 2 a 11 x 1 a 12 x 1 + a 22 x 2 a 13 x 1 + a 23 x 2 + a 33 x 3 - a 12 x 1 = a 22 x 2 - a 13 x 1 - a 23 x 2 = a 33 x 3 Three Packets received in Two Slots

57. Blind Nulling in BBN (Contd.) What we want: Blindly null x 2 and x 3 at AP 1 ; and, blindly null x 3 at AP 2 Assuming nulling and packet cancellation is perfect, Compute V such that 57

58. MAC Layer 58 Uplink Poll Approve A, B and C Keep Silent – Allow neighboring groups to transmit Downlink Uplink....... Time Contention Period Phase IPhase IIPhase III

59. RSS Without Blind Beamforming and Nulling 59 InterferenceIntended Signal -8dB SINR

60. RSS After Blind Beamforming 60 InterferenceIntended Signal 13dB SINR Proper precoding increases SINR by 21 dB Blind Beamforming and Nulling is practical

61. How to boost uplink throughput? MIMO? netwrok-MIMO? Smartphones are small High bandwidth consumption Interference Alignment? Smartphones are mobile 61