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RARA: Rate Adaptation Using RateRARA: Rate Adaptation Using Rate--adaptive adaptive Acknowledgment for IEEE 802.11 WLANsAcknowledgment for IEEE 802.11 WLANs
정하경
IEEE Consumer Communications & Networking Conference (CCNC) 2008, Las Vegas, USA
Hakyung Jung
School of Computer Science and Engineering
Seoul National University, Korea
IEEE 802.11 Rate AdaptationIEEE 802.11 Rate Adaptation
§ The 802.11 a/b/g/n standards allow the use of multiple transmission rates§ 802.11b, 4 rate options (1,2,5.5,11Mbps)
§ 802.11a, 8 rate options (6,9,12,18,24,36,48,54 Mbps)
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§ 802.11g, 12 rate options (11a set + 11b set)
§ The method to select the transmission rate in real time is called “Rate Adaptation”
§ Rate adaptation is important yetunspecified by the 802.11 standards
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Sender Receiver
54MbpsSignal is good
Rate Adaptation ExampleRate Adaptation Example
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§ Ideally, the transmission rate should be adjusted according to the channel condition
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Sender Receiver
Signal becomes weaker
12Mbps
Importance of Rate Importance of Rate AdaptationAdaptation
§ Rate adaptation plays a critical role to the throughput performance
§ Rate too high → loss ratio increases → throughput
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§ Rate too high → loss ratio increases → throughput decreases
§ Rate too low → under-utilize the capacity →throughput decreases
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OutlineOutline
§ Related work
§ Motivation
§ Proposed algorithm
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§ Proposed algorithm
§ Simulation Results
§ Conclusion
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Related WorkRelated Work
§ Open-loop approach (ARF, AARF, CARA, RRAA, ...)
§ pros
• simple to implement
• standard compliant in general
§ cons
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§ cons
• may misinterpret the causes of frame losses
§ Closed-loop approach (RBAR, OAR, …)
§ pros
• accurate channel estimation by receivers
§ cons
• RTS/CTS exchange overhead
• not conform to the 802.11 standard
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MotivationMotivation
§ 802.11 requires ACK frames be transmitted at a rate, constrained by:§ Tx rate of ACK frame ≤ Tx rate of preceding DATA frame
§ ACK frame should be selected within basic rate set
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§ ACK frame should be selected within basic rate set
§ What happens if we use the next lower(or higher) rate than the legacy ACK rate ?§ Still interoperable and possible to be used to inform the
transmitter of the channel condition information for the next data frame
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Proposed rate adaptation Proposed rate adaptation scheme: RARAscheme: RARA
§ When to increase the Tx rate
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§ When to decrease the Tx rate
§ Not the scope of this work
§ Any schemes can be combined (ARF, CARA, ….)
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Operation ExampleOperation Example
1 2 3 4
11 12
AP 5 6 7 8 9 10
ARF operation
Data Transmission at 11Mbps
Data Transmission at 5.5Mbps
ACK Transmission at 2Mbps
ACK Transmission at 1Mbps
13 14 15 16
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rSNR of MS
11Mbps
5.5Mbps
2Mbps
1Mbps
MS
RARA operation
1 2 3
13 14
AP
MS
4 5 6 7 8 9 10 11 12
15
16 17 18 19
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Simulation SetupSimulation Setup
§ ns-2 simulator§ 802.11b PHY & indoor environment
• Empirical BER vs. SNR curves (by Intersil)
• Transmission power: 15 dBm
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• Background noise level: -94 dBm
• Large scale model: Shadowing model (path-loss exponent of 4)
• Small scale model: Ricean fading model (RiceanK 3 dBm)
• Basic rate set: 1 and 2 Mbps
• LLC/IP/UDP, MSDU length: 1500bytes, Saturated traffic
§ Testing schemes
• ARF, Adaptive-ARF(AARF), CARA
• RARA, RARA+CARA
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Simulation Results (1/3)Simulation Results (1/3)
§ One-to-one topology with various distance§ From 5 to 60 meters
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§ From 5 to 60 meters
§ Throughput curve of RA schemes follows the outer envelope of those of single-rate schemes
§ RARA achieves better throughput over the entire range
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Simulation Results (2/3)Simulation Results (2/3)
§ Star topology with varying number of contending stations§ ARF and AARF shows § ARF and AARF shows
poor performance• Not able to differentiate
collisions from channel errors
§ RARA performs better than CARA• CARA acts the same with
ARF to decide when to increase Tx rate
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Simulation Results (3/3)Simulation Results (3/3)
§ One-to-one topology with varying Ricean parameter Kparameter K§ Every scheme performs
better as K increases
§ RARA achieves better than others for the entire range of K
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ConclusionConclusion
§ We proposed a novel rate adaptation scheme,§ Receiver controls the ACK transmission rate as a means to
dictate the sender to adjust data transmission rate
§ Responsive to time-varying wireless channel owing to the accurate and instant feedback
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§accurate and instant feedback
§ Compared with previous closed-loop schemes, adjusting ACK transmission rate is not costly
§ Future Works§ Analytical study
§ Comparison with closed-loop scheme e.g. RBAR
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