joseph camp, edward knightly acm mobicom 2008
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Modulation Rate Adaptation in Urban and Vehicular Environments: Cross-Layer Implementation and Experimental Evaluation. Joseph Camp, Edward Knightly ACM MobiCom 2008. Background. Why need modulation rate adaptation? Time-varying link quality – Mobility of sender, receiver, or obstacles - PowerPoint PPT PresentationTRANSCRIPT
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Modulation Rate Adaptation in Urban and Vehicular Environments: Cross-Layer
Implementation and Experimental Evaluation
Joseph Camp, Edward KnightlyACM MobiCom 2008
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BackgroundWhy need modulation rate adaptation?
Time-varying link quality – Mobility of sender, receiver, or obstacles – Multiple paths existing
Ideal modulation rate for channel condition
Ideal rate
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For Protocol DesignerUse available information (loss, SNR, …) to
track ideal modulation rate
Rate Selection Protocol
SNR, loss, … Rate choice
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ProblemALL existing rate adaptation algorithms fail
to track the ideal rateUrban propagation environment ( ex: multi-
path )Even with non-mobile sender and receiver
Result becomes loss and under-utilization
Select rate
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ProposeUnderstand the origins of the failure to track
link variationCompare the result in in-door (lab) and out-
door (urban or downtown )
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Propose (cont)Unified Implementation Platform
Implement multiple algorithms on a common platform
Extract General Rate Adaptation PrinciplesEvaluate rate selection accuracy packet-by-packetCompare against ideal rate found via exhaustive
searchUse repeatable controlled channelsAccurately measured outdoor channels
Design core mechanisms to track real-world link variation
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WARP - Wireless Open-Access Research Platform Limits of Off-the-shelf platforms
Programmability and observability
WARP is clean-slate MAC and PHYNeeded to implement CSMA/CA (802.11-like MAC)
Cross-layer rate adaptation frameworkCore mechanisms for rate selection protocolsChannel measurementsEvaluation of selected rate versus ideal rate
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Core Mechanisms for Rate Adaptation Loss-triggered Rate Adaptation
Consecutive-Packet DecisionHistorical-Decision
Collision/Fading Differentiation
SNR-triggered Rate AdaptationEqual Air-time Assurance
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Rate Adaptation Accuracy MatricesIdeal rate found via exhaustive search of
channel conditionConsider case where at least one modulation
rate succeeds
Channel Condition
Idea
l Rat
e
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Rate Adaptation Accuracy Matrices (cont)Rate Selection Accuracy Categories
Over-selection (loss)Accurate (achieving optimal rate)Under-selection (under-utilization)
Channel Condition
Idea
l Rat
e
Over-selection
Accurate
Under-selection
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In-door (lab) experimentsImpact of Coherence Time
Increase fading of the channel to evaluate if rate adaptation can track
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In-door (lab) experiments (cont)Similar performance with long coherence of
channelSNR: high overhead penaltyEqual Air-time Assurance: overcomes overhead
penaltyDissimilar performance at short coherence of
channel
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Rate Choice InaccuraciesConsecutive low due to under-selectionSNR: extremely low throughput due to over-
selection
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SNR-based Coherence Time SensitivityFast to slow channel fading
Accurate at long coherenceOver-select at <1ms
Over-selection caused by coherence time sensitivity of SNR-rate relationship
1 ms
accurate
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Coherence Time Training for SNRConsider different SNR thresholds according
to coherence time
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Consideration of SNR and Coherence TimeConsider different SNR thresholds according
to coherence timeIdeal rate = f(SNR, CT)
Retrain SNR-based decision (for the same protocol)
Joint consideration of SNR and coherence time provides large gains
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Residential Urban and Downtown ScenariosResidential Urban (TFA)
Single-family residential, dense foliageCoherence Time: 100 ms on averageDriven to 15 ms with mobility of
scatterers (in static topology)Downtown Houston
Both sides of street lined with tall buildings (strong multipath)
Coherence Time: 80 ms on averageDriven to 300 us with mobility of scatterers (in
static topology)
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Rate adaptation accuracy in outdoorResidential urbanInaccurate in outdoor settings for loss-
triggeredConsecutive-decision due to the mobility of
scattersHistorical-decision due to different modulation
rate selection
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Rate adaptation accuracy in outdoor (cont)
Downtown - strong multipathForce loss-based to under-selectSNR: over and under-select with low coherence
time
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Static Sender to Mobile Receiver -UrbanSNR protocols are able to plateau for >4 sec
Per-packet decisionLoss-based protocols are only able to spike to
suboptimal rate choicesLoss sensitivity prevents protocol from
tracking
static
mobile
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SummaryLoss-based core mechanisms under-select with
Fast-fading, interference, competing links (even with collision/fading differentiation), and mobile environments
SNR-based mechanisms over-select with fast-fading but haveLarge gains from considering SNR and coherence time
jointlyRobustness to interference, competing links, and mobility
Despite use of 4-way handshake, SNR-based protocols outperform loss-based protocols in practical environments