1. CSpy: Finding the Best Quality Channel without Probing Souvik Sen, Bozidar Radunovic, Jeongkeun Lee, Kyn-Han Kim MobiCom’13 1

2. Wireless Everywhere Wireless Display Mobile Wireless Wireless Input Performance really matters 2

3. Wireless Performance in Theory 3 Rx Tx Channel Measurement Textbook Algorithms Adaptions Scheduling … Channel Quality Limits Wireless Performance Question: How to Select the Best Channel?

4. Premier: Propagations and Channel 4 Rx Tx Single Path Rx Tx Multipath Q1: How to evaluate different channels?

5. How to Determine Channel Quality Goal: High Throughput Metric: Effective SNR (SIGCOMM’10) 5 Measurements of A Single Channel Effective SNR

6. Which Channel to Select? Goal: High Throughput Metric: Channel with Strongest Effective SNR 6 Measurements of Multiple Channels Q2: How to find the best channel?

7. 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? 7

8. 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 8

9. Review: Channels, CSI, CIR 9

10. 10 IDFT CSI Explore correlation between CIR from one channel Index of the strongest channel Explore correlation between CIR from one channel Index of the strongest channel # channels << # paths: underdetermined system

11. Hypotheses and Measurements Hypothesis 1: Links with the same strongest channel shares similarity in their multipath structure 11

12. Hypotheses and Measurements Hypothesis 2: Links with different strongest channel index exhibits different multipath characteristics 12

13. Why does Prediction Work A Simple Theoretical Analysis (proof in the paper) If: phase of 1 st tap < phase of 2 nd tap in channel c Then :amplitude of 1 st tap in c+1 > amplitude of 1 st 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? Intractable for multiple channels Solution? 13

14. Learning a Predictor Feature Representation: −The relative phases of all taps w.r.t. the 1 st tap Training and Classification: −Multi-class support-vector machine (SVM) 14

15. Experiment Setup 70 links in 2 buildings, and 1 parking lot CSI Tool enabled laptops operating at 5GHz 9 channels (#36-48, #149-165) 15

16. Performance SCI (Strongest Channel Index) Estimation 16 Mean Accuracy: 83%

17. Performance Accuracy/FP across different observing channels 17 Performance degrades on edge channels

18. Performance Accuracy/FP w.r.t. increasing frequency gap 18 Reliably estimate the strongest channel within 4-5 adjacent channels

19. Performance Throughput Gain 19 250%, 100% and 85% gain w.r.t. weakest, average, and an optimal algorithm

20. Performance Comparison with Probing-based Scheme 20

21. Thank You! 21