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Interference Avoidance and Control Ramki Gummadi (MIT) Joint work with Rabin Patra (UCB) Hari Balakrishnan (MIT) Eric Brewer (UCB)

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HotNets Interference-limited networks Interference: Fundamental consequence of resource sharing Wireless LANs 3G, WiMax Mesh networks Increasingly interference-limited, not noise-limited

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HotNets 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 isnt random If strong enough, understand and cancel it Avoid or control internal interference So, treating interference as noise is inefficient

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HotNets Goal: Improve aggregate throughput Concurrent transmissions improve throughput More total received power But they also increase interference Eliminate interference, maintaining concurrency?

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HotNets VWID: Variable WIDth channels Interferers in orthogonal channels Variable widths for heterogeneous SINRs and bursty demands

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HotNets Key questions (and talk outline) How does VWID compare analytically to: TDMA? CSMA? How much improvement in practice?

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HotNets Capacity of variable-width channels Multiple transmitters, one receiver Radios have a power limit Single antenna at a node Channel doesnt vary in frequency or time Restriction removed in implementation Additive White Gaussian Noise (AWGN)

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HotNets Two-transmitter capacity region R1R1 R2R2 (bits/s/Hz) R 1 < l o g 2 ( 1 + P 1 N ) b i t s / s / H z ; R 2 < l o g 2 ( 1 + P 2 N ) b i t s / s / H z ; R 1 + R 2 < l o g 2 ( 1 + P 1 + P 2 N ) b i t s / s / H z : l o g 2 ( 1 + P 1 N ) l o g 2 ( 1 + P 2 N ) Optimum sum-capacity Transmitter 1s Rate

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HotNets VWID throughput R1R1 R2R2 (bits/s/Hz) A B Optimum throughput at ® = P 1 P 1 + P 2 l o g 2 ( 1 + P 2 N ) l o g 2 ( 1 + P 1 N ) R 1 < ® l o g 2 ( 1 + P 1 ® N ) b i t s / s / H z ; R 2 < ( 1 ¡ ® ) l o g 2 ( 1 + P 2 ( 1 ¡ ® ) N ) b i t s / s / H z : l o g 2 ( 1 + P 2 N ) l o g 2 ( 1 + P 1 N ) ® = 0 ® = 1

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HotNets TDMA throughput: VWID throughput: Improvement higher for smaller allocations, due to additional in vs. VWID vs. TDMA: Two-node case l o g 2 ( 1 + P ® N ) l o g 2 ( 1 + P N ) C 1 + C 2 2 ; C 1 = l o g 2 ( 1 + P 1 N ) ; C 2 = l o g 2 ( 1 + P 2 N ) ® > C 1 + C 2 2 l o g 2 ( 2 C C 2 ¡ 1 ) VWID TDMA R1R1 R2R2 (bits/s/Hz) A B l o g 2 ( 1 + P 1 N ) VWID l o g 2 ( 1 + P 2 N )

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HotNets 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 Relative throughput 6 th node SINR (dB) 5 transmitters at 10 dB SINR l o g 2 ( 1 + n P N ) l o g 2 ( 1 + P N ) µ ( l o g 2 ( n )) ! l o g 2 ( 1 + P wea k N ) ! l o g 2 ( 1 + P s t ron g + n P wea k N ) VWID improves throughput linearly with power (dB) of stronger node VWID improves throughput linearly with power (dB) of stronger node

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HotNets 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 R 1, R 2 : 1 R R 2 Relative throughput 2 nd node SINR (dB) Two transmitters, one at 10 dB SINR 1 1 R R 2 = R 1 R 2 R 1 + R 2 · m i n f R 1 ; R 2 g VWID improves aggregate throughput linearly with total received power (dB) VWID improves aggregate throughput linearly with total received power (dB)

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HotNets Key questions (and talk outline) How does VWID compare analytically to: TDMA? CSMA? How much improvement in practice?

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HotNets VWID design Channel assignment algorithm 5,10 or 20 MHz variable-width sub-channels Maximize measured aggregate throughput Fairness: Dont degrade link throughput Exhaustive search for sub-channels Accounts for frequency-selective fading Worst-case exponential in interferers

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HotNets 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 E2 1,

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HotNets Point-point throughput improvement VWID Point-Point No VWID, Point-Point Median link throughput improves by 50%

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HotNets Point-Multipoint throughput improvement VWID Point-Point No VWID, Point-Point Worst link throughput improves by 2x

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HotNets 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 1 st timeslot 2 nd timeslot

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HotNets Conclusions Increase concurrency, total received power Throughput improvements ~ % over TDMA and CSMA Weakness: Inter-AP coordination (tomorrow) Future work: Practical implementation

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