This presentation summarizes the AutoMAC protocol, which aims to increase wireless network throughput by exploiting interference rather than avoiding it. It allows multiple clients to transmit concurrently to an access point using rateless coding and successive interference cancellation at the receiver. This approach allows the access point to decode all constituent packets and achieves throughput gains of 50-60% over conventional WiFi. The key techniques used are rateless coding, speculative ACKs based on estimated SNR, and decoding packets in order of strongest signal. Evaluation shows it can decode packets with interference from up to 3 other packets and provides significant throughput improvements over 802.11 MAC protocols.

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