auto mac rateless wireless concurrent medium access
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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
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