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

WiFi?

Inter-cell Interference WiFi? Live Video? SuperBowl 2015 900 WiFi APs Support 30000 users 6TB/s SuperBowl 2016 1300 WiFi APs. Support 70000 users 10TB/s Inter-cell Interference

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?

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

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.

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

Co-BCast Testbed

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

Challenges: Block Detection 10x STS 2.5x LTS 64 Samples 802.11 WiFi Packet Preamble

Challenges: Block Detection 32 Block Boundary LTS1 LTS2 64?

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

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

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

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

Co-BCast Applying Coding Forward Error Correction Codes Convolutional Codes (implemented in SDR & commercial chipsets) LDPC codes (part of 802.11 n/ac, not often seen in chipset yet)

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

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

Time to make it real: Co-BCast MAC

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)

Co-BCast Real Time Evaluation Real-Time Measurement Setup

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

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

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

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

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

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

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.

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.

Questions? Thank You!