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

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

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

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

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

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

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)

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

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

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 )

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

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)

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

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

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,

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

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

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

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