interference avoidance and control ramki gummadi (mit) joint work with rabin patra (ucb) hari...
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Interference Avoidance and Control
Ramki Gummadi (MIT)
Joint work with Rabin Patra (UCB)
Hari Balakrishnan (MIT)
Eric Brewer (UCB)
HotNets 2008 2
Interference-limited networks
Interference: Fundamental consequence of resource sharing• Wireless LANs
• 3G, WiMax
• Mesh networks
Increasingly interference-limited, not noise-limited
HotNets 2008 3
Interference: Friend or foe?
Challenges: Interference is time-varying• Bursty data traffic, not predictable voice traffic
• Radio propagation hard to model or predict
Opportunity: Unlike noise, interference isn’t random• If strong enough, understand and cancel it
• Avoid or control internal interference
• So, treating interference as noise is inefficient
HotNets 2008 4
Goal: Improve aggregate throughput
Concurrent transmissions improve throughput• More total received power
But they also increase interference• Eliminate interference, maintaining concurrency?
HotNets 2008 5
VWID: Variable WIDth channels
Interferers in orthogonal channels
Variable widths for heterogeneous SINRs and bursty demands
HotNets 2008 6
Key questions (and talk outline)
How does VWID compare analytically to:• TDMA?
• CSMA?
How much improvement in practice?
HotNets 2008 7
Capacity of variable-width channels
Multiple transmitters, one receiver Radios have a power limit Single antenna at a node Channel doesn’t vary in frequency or time
• Restriction removed in implementation
Additive White Gaussian Noise (AWGN)
HotNets 2008 8
Two-transmitter capacity region
R1
R2
(bits/s/Hz)
(bits/s/Hz)
R1 < log2(1+P1
N) bits/ s/Hz;
R2 < log2(1+P2
N) bits/ s/Hz;
R1 + R2 < log2(1+P1 + P2
N) bits/ s/ Hz:
log2(1+ P1N )
log2(1+ P2N )
Optim
um sum
-capacity
Transmitter 1’s Rate
HotNets 2008 9
VWID throughput
R1
R2
(bits/s/Hz)
(bits/s/Hz)
A
B
Optimum throughput at ®=
P1
P1+P
2
log2(1+ P2N )
log2(1+ P1N )
R1 < ®log2(1+P1
®N) bits/ s/ Hz;
R2 < (1¡ ®) log2(1+P2
(1¡ ®)N) bits/ s/ Hz:
log2(1+ P2N )
log2(1+ P1N )
®=0
®=1
HotNets 2008 10
TDMA throughput: VWID throughput:
Improvement higher for smaller allocations, due to additional in vs.
VWID vs. TDMA: Two-node case
log2(1+P®N ) log2(1+
PN )
C1+C22 ;C1= log2(1+
P1N );C2= log2(1+
P2N )
®
> C1+C22
log2(2C1 +2C2 ¡ 1)
VWID
TDMA
R1
R2(bits/s/Hz)
A
B
log2(1+P1N )
VWIDlog2(1+P2N )
HotNets 2008 11
VWID vs. TDMA: n-node case
VWID improves throughput by bits/s/Hz with n transmitters• vs.
SINRs show large variation With n weak nodes and one
strong node, aggregate TDMA throughput
VWID throughput
Rel
ativ
e th
roug
hput
6th node SINR (dB)
5 transmitters at 10 dB SINR
log2(1+nPN ) log2(1+
PN )
µ(log2(n))
! log2(1+PweakN )
! log2(1+Pstrong+nPweak
N )
VWID improves throughputlinearly with power (dB) of stronger node
VWID improves throughputlinearly with power (dB) of stronger node
HotNets 2008 12
Time to send two bits at rates
CSMA node throughput:
• Hurts stronger node
VWID aggregate throughput improves with the total received power
VWID vs. CSMA: Two-node case
R1, R2:1R1+ 1R2
Rel
ativ
e th
roug
hput
2nd node SINR (dB)
Two transmitters, one at 10 dB SINR
11
R 1+ 1
R 2
= R1R2R1+R2 · minfR1;R2gVWID improves aggregate throughput
linearly with total received power (dB)VWID improves aggregate throughput linearly with total received power (dB)
HotNets 2008 13
Key questions (and talk outline)
How does VWID compare analytically to:• TDMA?
• CSMA?
How much improvement in practice?
HotNets 2008 14
VWID design
Channel assignment algorithm• 5,10 or 20 MHz variable-width sub-channels
• Maximize measured aggregate throughput
• Fairness: Don’t degrade link throughput
• Exhaustive search for sub-channels Accounts for frequency-selective fading Worst-case exponential in interferers
HotNets 2008 15
Evaluation testbed
Outdoor testbed• Worst-case scenario (unequal
SINRs)
10 links (2-4 km), 25 dBi antennas, 5.3 GHz, Atheros
Point-point and point-multipoint topologies
CSMA MAC• Higher throughput than TDMA if
traffic is bursty
Unidirectional UDP traffic
E21,
HotNets 2008 16
Point-point throughput improvement
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12
Throughput (Mbps)
CD
F
VWID Point-PointVWID Point-Point
No VWID, Point-Point
Median link throughput improves by 50%Median link throughput improves by 50%
HotNets 2008 17
Point-Multipoint throughput improvement
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12
Throughput (Mbps)
CD
F
VWID Point-Point
No VWID, Point-Point
Worst link throughput improves by 2xWorst link throughput improves by 2x
HotNets 2008 18
Related Work
Interference cancellation• Decode colliding transmissions jointly• Signals typically differ by large SINR or coding rates
ZigZag decoding
• No coordination, but no net concurrency increase
1st timeslot2nd timeslot
HotNets 2008 19
Conclusions
Increase concurrency, total received power Throughput improvements ~ 50-100% over
TDMA and CSMA Weakness: Inter-AP coordination (tomorrow) Future work: Practical implementation