AutoMAC : Rateless Wireless Concurrent Medium Access Aditya Gudipati, Stephanie Pereira, Sachin Katti Stanford University

Conventional Schemes : Avoid Interference at all costs ! AliceBob

Conventional Schemes : Avoid Interference at all costs ! Interference Avoidance – RTS/CTS – CSMA with exponential backoff Interference handling – ZigZag RTS CTS Alice RTS AliceBob CTS Alice CTS Alice

Our Approach : Encourage and Exploit Interference ! AliceBob Alice Decoder Bob Decoder Subtract

Our Approach : Encourage and Exploit Interference ! Alice Decoder Bob Decoder Subtract Received Power from Alice at AP = P Received Power from Bob at AP = P

Our Approach vs Conventional Scheme

Even on Downlink, Conventional Schemes avoid Interference Alice Bob P1P1 P2P2 P 2 > P 1

Our Approach on Downlink also Exploits Interference ! Alice Bob P1P1 P2P2 P 2 > P 1

Our Approach on Downlink also exploits Interference ! Received Power from AP at Alice = P 1 Received Power from AP at Bob = P 2 Alice Decoder ALICE P 2 > P 1 Alice Decoder Bob Decoder Subtract BOB

Our Approach on Downlink also exploits Interference ! Received Power from AP at Alice = P 1 Received Power from AP at Bob = P 2 BOB P 2 > P 1 ALICE Alice Decoder Bob Decoder Subtract

Our Approach vs Conventional Scheme

AutoMAC Contributions Practical system for exploiting interference Rateless encoding scheme – Upto 3 successful concurrent transmissions Novel MAC protocol – Leverage gains due to Interference Implemented on USRP2s and evaluated – 60% gain on Uplink – 35% gain on Downlink

Challenge 1 : Rate Adaptation Alice Decoder Bob Decoder Subtract

Challenge 1 : Rate Adaptation Increasing Rates

Challenge 1 : Rate Adaptation Increasing Rates

Challenge 1 : Rate Adaptation Alice needs to adapt its rate Alice needs to figure out – Own SNR at the AP – Who is interfering? – How strong is the Interference ? Can we avoid this ? Alice Decoder Bob Decoder Subtract

Solution 1 : Rateless Codes Orthogonal to choice of rateless code Strider [Sigcomm’11] – Encoder generates stream of transmissions – Receiver ACKs once decoded – SNR determines # transmissions – # transmissions determines rate

Solution 1 : Rateless Codes

Challenge 2 : Wasted Transmissions Weak Channel for Alice – More transmissions needed for Alice Decoder Strong Channel for Bob – Fewer transmissions needed for Bob Decoder Bob can’t be decoded before Alice ! – Wastage Alice Decoder Bob Decoder Subtract

Challenge 2 : Wasted Transmissions A 1 + B 1 A1A1 Noisy B 1 B1B1 3 transmissions wasted !! Alice Decoder Bob Decoder Subtract Alice Decoder needs 6 tx Bob Decoder needs 3 tx

Solution 2 : Speculative ACKing A 1 + B 1 A 1 + B 2 A1A1 Noisy B 1 Noisy B 2 B1B1 No wastage !! B2B2 ACKACK Alice Decoder Bob Decoder Subtract Alice Decoder needs 6 tx Bob Decoder needs 3 tx

MAC Protocol AP driven MAC – Frequency Domain Backoff [Sen et al Mobicom’11] Contention Ad Alice Charlie Bob David

MAC Protocol AP driven MAC – Frequency Domain Backoff [Sen et al Mobicom’11] 1-Alice 2 - Bob Alice Charlie Bob David Freq

MAC Protocol AP driven MAC – Frequency Domain Backoff [Sen et al Mobicom’11] Alice Charlie Bob David Pre Data PreData Pre

Evaluation Implement PHY layer on USRP2s – GNURadio platform with RawOFDM – OFDM with 64 subcarriers – External Clock to Synchronize USRP2s (Jackson) Upto 3 successful concurrent transmissions Compared to omniscient rate adaptation – 30% gain on Uplink, 35% gain on Downlink

Uplink CDF 30% median throughput gain

Downlink CDF 35% median throughput gain

Downlink gains depend on Relative SNRs

Interference Cancellation isn’t Perfect !

Simulations Simulate MAC layer – Dense Network (8 contending nodes) – Fully Loaded 60% uplink gain over MAC – Improved PHY layer – Efficient channel utilization at MAC layer

Conclusion Exploit interference instead of avoiding it Novel PHY & MAC protocol – Rateless encoding scheme enables SIC – AP driven MAC coordinates interferers Implemented on USRP2s and evaluated Future Work – Other applications of SIC, eg. MIMO systems