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CSpy : Finding the Best Quality Channel without Probing. Souvik Sen , Bozidar Radunovic , Jeongkeun Lee, Kyn -Han Kim MobiCom’13. Wireless Everywhere. Wireless Display. Wireless Input. Performance really matters. Mobile Wireless. Wireless Performance in Theory. Rx. Tx.
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CSpy: Finding the Best Quality Channel without Probing SouvikSen, BozidarRadunovic, Jeongkeun Lee, Kyn-Han Kim MobiCom’13
Wireless Everywhere Wireless Display Wireless Input Performance really matters Mobile Wireless
Wireless Performance in Theory Rx Tx Channel Measurement Textbook Algorithms Channel Quality Limits Wireless Performance Question: How to SelecttheBestChannel? Adaptions Scheduling …
Premier: Propagations and Channel Rx Tx Single Path Rx Tx Q1: How to evaluate different channels? Multipath
How to Determine Channel Quality • Goal: High Throughput • Metric: Effective SNR (SIGCOMM’10) Effective SNR Measurements of A Single Channel
Which Channel to Select? • Goal: High Throughput • Metric: Channel with Strongest Effective SNR Measurements of Multiple Channels Q2: How to find the best channel?
How to Find the Best Quality Channel • Method: Probing • Measure every channel and compare their qualities • Problem: • Time consuming • Large overhead if frequent probing is needed Goal: Can we find the strongest channel Without Probing?
Predicting an Unseen Channel • Insight: • Probing does not leverage information about the underlying propagation • Channel State Information (CSI) contains information about the multipath propagation structure Determine the strongest from a set of channels while probing only one channel
Review: Channels, CSI, CIR # channels << # paths: underdeterminedsystem CSI • Explore correlation between • CIR from one channel • Index of the strongest channel IDFT
Hypotheses and Measurements • Hypothesis 1: Links with the same strongest channel shares similarity in their multipath structure
Hypotheses and Measurements • Hypothesis 2: Links with different strongest channel index exhibits different multipath characteristics
Why does Prediction Work • A Simple Theoretical Analysis (proof in the paper) • If: phase of 1sttap < phase of 2ndtap in channel c • Then :amplitude of 1st tap in c+1 > amplitude of 1st tap in c-1 • A High-level Intuition • The phases of different paths are uniformly random and independent • When constructing an estimator based on relative phases, we can recover a bias correlated with channel quality Intractable for multiple channels Solution?
Learning a Predictor • Feature Representation: • The relative phases of all taps w.r.t. the 1st tap • Training and Classification: • Multi-class support-vector machine (SVM)
Experiment Setup • 70 links in 2 buildings, and 1 parking lot • CSI Tool enabled laptops operating at 5GHz • 9 channels (#36-48, #149-165)
Performance • SCI (Strongest Channel Index) Estimation Mean Accuracy: 83%
Performance • Accuracy/FP across different observing channels Performance degrades on edge channels
Performance • Accuracy/FP w.r.t. increasing frequency gap Reliably estimate the strongest channel within 4-5 adjacent channels
Performance • Throughput Gain 250%, 100% and 85% gain w.r.t. weakest, average, and an optimal algorithm
Performance • Comparison with Probing-based Scheme