Trace-based Evaluation of Rate Adaptation Schemes in Vehicular

EnvironmentsKevin C. Lee

WiVeC 2010, 5/17/10

Rate Adaptation Overview

• 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)– 802.11g, 12 rate options (11a set + 11b set)

• The method to select the transmission rate in real time is called “Rate Adaptation”

Why Rate Adaptation?

• Ideally, the transmission rate should be adjusted according to the channel condition

Sender Receiver

54MbpsSignal is good

12MbpsSignal is weaker

Motivation

• MANY rate adaptation algorithms yet no fair comparison– Unrealistic propagation (unrealistic)– Dynamic changing conditions (non-repeatable)– Long system setup and device driver

implementation (impractical)

Framework & Goal

Physical Layer

MAC Layer

Use field-collected SNR to replace synthetic value

Implementation of various rate adaptations schemes

Realistic SNR to reflect the environment

Rapid deployment independent of hardware spec

Implementation of different application Application Layer

…

Repeatable evaluation of rate adaptation schemes

Physical Layer

• Collect SNR traces from moving cars– Server broadcasts @ 6Mbps– 2 Clients receive and record SNR

• Increase range and power of signal with an external 7dBM antenna

• Replace SNR logic with SNR from the field

• Derive BER and then bit error probability

Trace Collection

• Traces from 3 different areas: City, Residential, and Highway

Trace Collection Map

Static Traffic Route

• Car A centered at the mid point, stationary• Car B and C move back and forth toward and

away from A

Rate Adaptation Schemes Implementation

• RRAA-DYN adjusts rates before the current estimation windowm

Static Traffic Route Result

• SNR from 440s to 540s

• 40 seconds to complete one loop

• Signal strengthdirectly proportional to the distance between them

Instantaneous Throughput for All Algorithms

• Packet-based rate adaption schemes react similarly to the SNR-based scheme (RAM)

• Sample rate plateau from 460-470s and 500-530s

Throughput in Different Transmission Rates

• Throughput increases with transmission rate• ARF, RRAA-DYN, and RAM top 3• AMRR and Sample bottom 2

Rate Distribution for All Schmes• 6Mbps occupies the largest fraction for top 3

schemes but there are other rates => short-term lossy channel

• Sample & AMRR can’t adapt to short-term fluctuation

Success of ARF

• Comes from the fact that rate increases conservatively and decreases drastically

• Not too good if the channel condition does not change frequently

• Conclusion: Packet-based scheme does a subpart job because of fixed parameters of packet statistics; adaptive parameters to improve

Impact of Environments

• Throughput degradation from residential, highway, city; speed & traffic density play a factor

• RRAA-DYN beats RRAA & RRAA-BASIC=> changing trans-mission wind. helpsimprove responsive-ness

Impact of Propagation Model• Rayleigh has higher throughput b/c it considers

fading where there is no dominant propagation along a line of sight between transmitter and receiver

• A more accurate prop. model to useb/c lead car and trailing car are oftenseparated by cars in between

Conclusion

• An integrated framework that utilizes empirical data collected from the testbed to objectively compare different rate adaption schemes– Repeatable– Rapid– Realistic