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Auto mac rateless wireless concurrent medium access


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Auto mac rateless wireless concurrent medium access

  1. 1. Wireless Network Presentation AutoMAC (Rateless Wireless Concurrent Medium Access) Aditya Gudipati, Stephanie Pereira, Sachin Katti Standford University
  2. 2. Presented on : Mobile Computing and Networking 2012 (MobiCom'12), August 22-26, 2012, Istanbul,Turkey.
  3. 3. Current Wireless Design • Current Design : one Tx, one frequency, one at time (uplink and downlink) >> Avoid the interference • RTS/CTS/Backoff Mechanism (protocols overhead) >> Client need to sense the carrier, to avoid collision Limiting network throughput
  4. 4. Proposed Wireless Design • Opposite approach : Exploit the interference • Multiple clients concurently Tx to AP (uplink) • AP concurently Tx to multiple clients (downlink) • Decode all constituent packets from interference Increasing network throughput
  5. 5. Exploiting Interference (Uplink) y = h1x1+ h2x2 + N AP SIC (Successive Interference Cancelation) h1x1 x1 y A x2 B h2x2 + N h2x2
  6. 6. Exploiting Interference (Uplink) • No Interference Troughput • Throughput for Client A Rsic > Rnint • Throughput for Client B • Throughput system gained extra throughput, without hurting ongoing Tx
  7. 7. Exploiting Interference (Downlink) (x1 + x2) /√2 SNR High AP (Stronger Node) B y = h1x1 /√2 + h2x2 /√2 + N SIC (Successive Interference Cancelation) SNR Low (Weaker Node) A h1x1 /√2 y y = h1x1 /√2 + h2x2 /√2 + N h1x1 /√2 h2x2/√2+ N h2x2/√2
  8. 8. Exploiting Interference (Downlink) • number Tx required by stronger client < weaker client to decode • dividing Tx power, instead of increasing • higher throughput when SNR different • throughput at the equal SNR ∼ no interference schemes
  9. 9. Exploiting Interference (Downlink) Throughput A (Alice) and B (Bob) can be achieved by exploiting interference compare to interference avoidance scheme
  10. 10. AutoMAC • Sender didn't know actual SINR at receiver for decoding ->Rateless Property >> Rateless Codes (send rateless Tx, until receiver can decode) • Wastefull Tx because packet can be decoded less than rateless encoding requirements >> Speculative ACKing (number rateless Tx depends on the SINR value)
  11. 11. AutoMAC : MAC Design • Short Contention advertisement • announce the number of users send concurrently Tx in the next slot • not sent, if AP want to transmit or concurrent Tx in previous slot continue • Frequency domain backoff technique • client pickup random subcarrier
  12. 12. AutoMAC : Exploit Uplink Interference • Packet Encoding • Orthogonal choice of rateless code (Strider or Spinal codes) • Decoding Constituent Packet with SIC • Per-frame channel estimation • Decoding Packet (Strider) • Substracting Node'1 collission contribution
  13. 13. AutoMAC : Speculative ACKing • Diferrent encoding time -> encoding order and relative SNR between AP-Client • Number of Tx is less for last decoded packet than the first packet -> better SNR • "Key" of ACKing technique : • Estimate number of Tx for encoding packet -> estimate effective SNR value • SINR vs number Tx graph -> after M Tx, receiver start ACK packet p
  14. 14. AutoMAC : Exploiting Broadcast on Downlink • same encoding technique for all client – rateless encoding (strider's) -> rateless frame – n frames add in complex domain -> one frame – prepend header, pass frame to OFDM PHY for Tx • decoding the own packet at last – every client assumed best channel – ensure SINR to decode and send the ACK -> AP receive know the received signal strength – apply the SIC in the order Tx from AP
  15. 15. Evaluation outperform by 35% (include downlink) decode packet in interference upto 3 packets eleminates hidden terminal multiplex upto 3 packets (downlink) accurately estimates channel, frequency, and sampling offsets • 60% throughput gain over 802.11 MAC (uplink) and 50% over an 802.11 style MAC • • • • •
  16. 16. Evaluation The Gains will increase as relative with SNR increase, and AutoMAC gain 50 - 60 % over Conventional or Time Fair Wifi.
  17. 17. Evaluation Time Spent and throughput fairness comparison
  18. 18. Gain Analysis • Collision minimum, because no lack of coordination and inability to decode -> Centralized MAC • Lower contention time -> frequency domain backoff technique • No overheads time for ACK waiting (SIFS,DIFS) -> ACK one for every batch • fraction time spent is higher 20% -> efficient MAC protocol
  19. 19. Conslusion and References