AutoMAC (Rateless Wireless Concurrent Medium Access) Aditya Gudipati, Stephanie Pereira, Sachin Katti Standford University Wireless Network Presentation
Jun 25, 2015
AutoMAC(Rateless Wireless Concurrent Medium Access)
Aditya Gudipati, Stephanie Pereira, Sachin KattiStandford University
Wireless Network Presentation
Presented on :
Mobile Computing and Networking 2012 (MobiCom'12), August 22-26, 2012, Istanbul,Turkey.
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
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 packetsfrom interference
Increasing network throughput
-y
h2x2 + N
h2x2
Exploiting Interference (Uplink)
A
B
AP
x1
x2
y = h1x1+ h2x2 + N SIC(Successive Interference
Cancelation)
h1x1
Exploiting Interference (Uplink)• No Interference Troughput
• Throughput for Client A
• Throughput for Client B
• Throughput system
Rsic > Rnint
gained extra throughput, without hurting ongoing Tx
Exploiting Interference (Downlink)
y = h1x1 /√2 + h2x2 /√2 + N
A
B
AP
y = h1x1 /√2 + h2x2 /√2 + N
(x1 + x2) /√2
SNR Low(Weaker Node)
SNR High(Stronger Node)
SIC(Successive Interference
Cancelation)
h1x1 /√2
-y
h2x2/√2+ N
h2x2/√2
h1x1 /√2
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
Exploiting Interference (Downlink)
Throughput A (Alice) and B (Bob) can be achieved by exploiting interference compare to interference avoidance scheme
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)
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
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
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
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
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
Evaluation
The Gains will increase as relative with SNR increase, and AutoMAC gain 50 - 60 % over Conventional or Time Fair Wifi.
Evaluation
Time Spent and throughput fairness comparison
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
Conslusion and References