1. Co-BCast: High-Rate WiFi Broadcasting in Crowded Scenarios via Lightweight Coordination of Multiple Access Points
Hang Qiu, Konstantinos Psounis, Giuseppe Caire, Keith M. Chugg, and Kaidong Wang

2. WiFi?

3. Inter-cell Interference
WiFi?
Live Video?
SuperBowl 2015
900 WiFi APs
Support users
6TB/s
SuperBowl 2016
1300 WiFi APs.
Support users
10TB/s
Inter-cell Interference

4. Fix the bandwidth crunch?
MU-MIMO (AirSync, MegaMIMO, OpenRF, etc.)
Concurrent transmission for multiple different data stream.
Heavy Encoding and Decoding.
Heavy Coordination.
Tight clock synchronization.
Constantly monitoring the channel state.
Scalability?

5. Fix the bandwidth crunch?
What we need?
Concurrent transmission for single broadcast data stream.
Light Coordination / No clock synchronization.
No inter-stream level coding.
Coordinated Transmission
DATA PKT
DATA PKT

6. Coordinated Transmission in WiFi
CoMP for LTE.
Barrage Relay Network, e.g. Trellisware.
Co-BCast
A light weight WiFi-compatible coordinated transmission solution.
Addresses several WiFi-specific PHY layer challenges.
Designs a MAC protocol that is WiFi-compatible.
Demonstrates >10x gains in WiFi broadcast rates.

7. Co-BCast Outline Basic Challenges of WiFi Coordinated Transmission
PHY Layer Techniques
WiFi Experiments (WARP WiFi reference design)
MAC design
Large Scale Simulations

8. Co-BCast Testbed

9. Co-BCast Challenges Block Boundary Detection
Cooperative Time Offset (CTO)
Cooperative Carrier Frequency Offset (CCFO)
CTO & CCFO

10. Challenges: Block Detection
10x STS
2.5x LTS
64 Samples
WiFi Packet Preamble

11. Challenges: Block Detection32
Block
Boundary
LTS1
LTS2
64?

12. Challenges: Cooperative Time Offset (CTO)
CTO = 200ns

13. Challenges: Cooperative Frequency Offset (CCFO)
Traditional CFO scenario:
CFO = 1000Hz

14. Challenges: Cooperative Frequency Offset (CCFO)
CCFO = 1000Hz
CCFO scenario:

15. Challenges: CTO & CCFO
CTO= 600ns & CCFO = 1000Hz

16. Co-BCast Applying Coding
Forward Error Correction Codes
Convolutional Codes (implemented in SDR & commercial chipsets)
LDPC codes (part of n/ac, not often seen in chipset yet)

17. Co-BCast Constellation Demapping
Hard constellation de-mapper
Soft constellation de-mapper
Describe the error as a drift.
Soft-in Decoding
Decode over the drift instead of 0, 1.
2.5-4 dB gain in RX sensitivity.
Tolerate channels with 2.5-4dB
Lower SINR
0000
0001
0011
0010
0100
0101
0111
0110
1100
1101
1111
1110
1000
1001
1011
1010

18. Co-BCast PHY Evaluation
16QAM requires
< 400ns CTO
< 1000Hz CCFO
40MHz 10ppm oscillator:
400Hz
16 sample cyclic prefix:
800ns
Avg. BER of 16QAM Coordinated Transmission with LDPC and Different CTO and CCFO

19. Time to make it real: Co-BCast MAC

20. Time to make it real: Co-BCast MAC
CSMA contention
SENDNOW
SENDNOW
Over-the-air Trigger
ACK Auto-responder
Buffer Management
DATA PKT
DATA PKT
~ 16us (SIFS)
~ 16us (SIFS)

21. Co-BCast Real Time Evaluation
Real-Time Measurement Setup

22. Co-BCast Real Time Evaluation
ACK
SENDNOW
Data Packet
Coordinated Transmission is correctly received and ACKed!

23. Co-BCast Real Time Evaluation
PAP
SAP1
SAP2
Measuring CTO
CDF of CTO

24. Co-BCast Large Scale Simulation
PAP
Relocated PAP
Not Practical: User Re-association
Not Possible: PAP Relocation

25. Co-BCast Large Scale Simulation
PAP
Relocated PAP
Co-BCast

26. Co-BCast Large Scale Simulation
Relocated PAP
BCast
Co-Bcast rate
=6x BCast
= 1.8x PAP
= 1.7x Relocated PAP

27. Co-BCast Large Scale Simulation
Relocated PAP
BCast
More gains in large scale:
>120Mbps
15x Bcast
8x PAP
6x Relocated PAP
AP Cluster Size: 4
AP Cluster Size: 36

28. Conclusion Co-BCast: a coordinated high rate WiFi broadcasting system
Explore and address challenges in WiFi PHY layer.
WiFi compatible MAC coordination protocol.
Demonstrate an order of magnitude improvement in broadcast rates.
Achieve >100Mbps which can support >10 HD video channels.

29. Co-BCast Future Work Replace the receiver with real mobile devices.
Show the gains with a real video streaming application.
Prototype and deployment in real stadiums.

30. Questions?
Thank You!