Embed Size (px)
Citation preview
AutoMAC : Rateless Wireless Concurrent Medium Access
Aditya Gudipati, Stephanie Pereira, Sachin KattiStanford University
Conventional Schemes : Avoid Interference at all costs !
Alice Bob
Conventional Schemes : Avoid Interference at all costs !
• Interference Avoidance– RTS/CTS– CSMA with exponential backoff
• Interference handling– ZigZag
RTSCTS
AliceRTSAlice Bob
CTSAlice
CTSAlice
Our Approach : Encourage and Exploit Interference !
Alice Bob
Alice Decoder
Bob DecoderSubtract
Our Approach : Encourage and Exploit Interference !
Alice Decoder
Bob DecoderSubtract
Alice Channel DecoderSignal = PNoise = Bob’s Power + N = P + NThroughput =
Bob Channel DecoderSignal = PNoise = NThroughput =
Received Power from Alice at AP = PReceived Power from Bob at AP = P
Our Approach vs Conventional Scheme
• Conventional Scheme
• Our Approach
• {Our Approach > Conventional Scheme}
Even on Downlink, Conventional Schemes avoid Interference
Alice Bob
P1P2
P2 > P1
Our Approach on Downlink alsoExploits Interference !
Alice Bob
P1P2
P2 > P1
Our Approach on Downlink also exploits Interference !
Received Power from AP at Alice = P1
Received Power from AP at Bob = P2
Alice Decoder
ALICE
P2 > P1
Alice Decoder
Bob DecoderSubtract
BOB
Our Approach on Downlink also exploits Interference !
Decode Bob’s PacketSignal = 0.5*P2
Noise = NThroughput =
Received Power from AP at Alice = P1
Received Power from AP at Bob = P2
BOB
P2 > P1
Decode Alice’s PacketSignal = 0.5*P1
Noise = Bob’s Power + N = 0.5*P1 + NThroughput =
ALICE
Alice Decoder
Alice Decoder
Bob DecoderSubtract
Our Approach vs Conventional Scheme
• Conventional Scheme
• Our Approach
• {Our Approach > Conventional Scheme} if P2 > P1
– Capacity function concave with power
AutoMAC Contributions
• Practical system for exploiting interference• Rateless encoding scheme– Upto 3 successful concurrent transmissions
• Novel MAC protocol – Leverage gains due to Interference
• Implemented on USRP2s and evaluated– 60% gain on Uplink– 35% gain on Downlink
Challenge 1 : Rate Adaptation
• Weak Channel for Alice– Alice SNR = – Bob SNR =
• More redundancy in Alice’s transmission– Need Rate adaptation
Alice Decoder
Bob DecoderSubtract
Challenge 1 : Rate Adaptation
5 10 15 200
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Thresholding behaviour of channel codes
16-QAM 1/2coderate
16-QAM 2/3coderate
64-QAM 1/2coderate
64-QAM 2/3coderate
SNR (in dB)
De
co
din
g P
rob
ab
ilit
y
Increasing Rates
Challenge 1 : Rate Adaptation
5 10 15 200
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Thresholding behaviour of channel codes
16-QAM 1/2coderate
16-QAM 2/3coderate
64-QAM 1/2coderate
64-QAM 2/3coderate
SNR (in dB)
De
co
din
g P
rob
ab
ilit
y
Increasing Rates
Challenge 1 : Rate Adaptation
• Alice needs to adapt its rate• Alice needs to figure out– Own SNR at the AP– Who is interfering?– How strong is the Interference ?
• Can we avoid this ?
Alice Decoder
Bob DecoderSubtract
Solution 1 : Rateless Codes
• Orthogonal to choice of rateless code• Strider [Sigcomm’11]– Encoder generates stream of transmissions– Receiver ACKs once decoded– SNR determines # transmissions– # transmissions determines rate
Solution 1 : Rateless Codes
5 7 9 11 13 15 17 190
1
2
3
4
5
6
7
8
9
Average Transmissions needed for Strider to decode
SNR (in dB)
Tra
nsm
issi
on
s
Challenge 2 : Wasted Transmissions
• Weak Channel for Alice– More transmissions needed for Alice Decoder
• Strong Channel for Bob– Fewer transmissions needed for Bob Decoder
• Bob can’t be decoded before Alice !– Wastage
Alice Decoder
Bob DecoderSubtract
Challenge 2 : Wasted Transmissions
A1 + B1 A1 + B1 A1 + B1 A1 + B1 A1 + B1 A1 + B1
A1
Noisy B1
Noisy B1
Noisy B1
Noisy B1
Noisy B1
Noisy B1
B1
3 transmissions wasted !!
Alice Decoder
Bob DecoderSubtract
Alice Decoder needs 6 txBob Decoder needs 3 tx
Solution 2 : Speculative ACKing
A1 + B1 A1 + B1 A1 + B1 A1 + B2 A1 + B2 A1 + B2
A1
Noisy B1
Noisy B1
Noisy B1
Noisy B2
Noisy B2
Noisy B2
B1 No wastage !! B2
ACK
Alice Decoder
Bob DecoderSubtract
Alice Decoder needs 6 txBob Decoder needs 3 tx
MAC Protocol
• AP driven MAC– Frequency Domain Backoff [Sen et al Mobicom’11]
Contention Ad
Alice
Charlie
Bob
David
MAC Protocol
• AP driven MAC– Frequency Domain Backoff [Sen et al Mobicom’11]
1-Alice 2 - Bob
Alice
Charlie
Bob
David
Freq
MAC Protocol
• AP driven MAC– Frequency Domain Backoff [Sen et al Mobicom’11]
Alice
Charlie
Bob
David
Pre
Data
Data
Pre Data
Pre Pre
Evaluation
• Implement PHY layer on USRP2s– GNURadio platform with RawOFDM– OFDM with 64 subcarriers – External Clock to Synchronize USRP2s (Jackson)
• Upto 3 successful concurrent transmissions• Compared to omniscient rate adaptation – 30% gain on Uplink , 35% gain on Downlink
Uplink CDF
1.5 2 2.5 3 3.5 4 4.5 5 5.5 60
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
CDF of throughput on uplink
Omniscient AutoMAC
Throughput (b/s/Hz)
Cu
mu
lati
ve
Fra
cti
on
30% median throughput gain
Downlink CDF
0 1 2 3 4 5 60
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
CDF of throughput on downlink
Omniscient AutoMAC
Throughput (b/s/Hz)
Cu
mu
lati
ve
Fra
cti
on
35% median throughput gain
Downlink gains depend on Relative SNRs
-1 1 3 5 7 9 11 13 151
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
Downlink gains from AutoMAC as a function of Relative SNR
Relative SNR (dB)
Re
lati
ve
do
wn
lin
k g
ain
s o
f A
uto
MA
C o
ve
r O
mn
isc
ien
t
Interference Cancellation isn’t Perfect !
10 12 14 16 18 20 22 240
1
2
3
4
5
6
Drop in weaker signal SNR after cancellation
Stronger signal SNR (dB)
Dro
p i
n w
ea
ke
r s
ign
al
SN
R (
dB
)
Simulations
• Simulate MAC layer– Dense Network (8 contending nodes)– Fully Loaded
• 60% uplink gain over 802.11 MAC– Improved PHY layer– Efficient channel utilization at MAC layer
Conclusion
• Exploit interference instead of avoiding it• Novel PHY & MAC protocol– Rateless encoding scheme enables SIC– AP driven MAC coordinates interferers
• Implemented on USRP2s and evaluated• Future Work– Other applications of SIC , eg. MIMO systems
