1. Wireless Network Presentation
AutoMAC
(Rateless Wireless Concurrent Medium Access)
Aditya Gudipati, Stephanie Pereira, Sachin Katti
Standford University

2. Presented on :
Mobile Computing and Networking 2012 (MobiCom'12), August 22-26, 2012, Istanbul,Turkey.

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. 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. Exploiting Interference (Uplink)
y = h1x1+ h2x2 + N
AP
SIC
(Successive Interference
Cancelation)
h1x1
x1
y
A
x2
B
h2x2 + N
h2x2

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. 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. 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. Exploiting Interference (Downlink)
Throughput A (Alice) and B (Bob) can be achieved by exploiting
interference compare to interference avoidance scheme

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. 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. 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. 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. 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. 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. Evaluation
The Gains will increase as relative with SNR increase, and
AutoMAC gain 50 - 60 % over Conventional or Time Fair Wifi.

17. Evaluation
Time Spent and throughput fairness comparison

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. Conslusion and References