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Frequency-aware protocols for wideband networks

Frequency-aware protocols for wideband networks. Hariharan Rahul Farinaz Edalat , Dina Katabi , Charles Sodini. Wireless is moving to wider bands. UWB 100s of MHz to GHz Whitespaces 100 to 250 MHz from analog-digital TV transition 802.11n channel bonding 40 MHz

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Frequency-aware protocols for wideband networks

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  1. Frequency-aware protocols for wideband networks HariharanRahul FarinazEdalat, Dina Katabi, Charles Sodini

  2. Wireless is moving to wider bands • UWB • 100s of MHz to GHz • Whitespaces • 100 to 250 MHz from analog-digital TV transition • 802.11n channel bonding • 40 MHz • Unchannelized 802.11 a/b/g • Discard channels • Allow all nodes to use the entire 100 to 300 MHz

  3. So, what changes?

  4. So, what changes? 20 MHz

  5. So, what changes? 3.8 dB In 20MHz Wi-Fi channels, different frequencies have similar SNRs 20 MHz

  6. So, what changes? In Wideband channels, different frequencies have very different SNRs 19 dB 100 MHz Cannot treat all frequencies the same!

  7. Need to Change Rate Adaptation

  8. Need to Change Rate Adaptation High Bitrate Wideband needs frequency-aware rate adaptation! Low Bitrate Using one bitrate for all frequencies is either unsafe or inefficient

  9. Is that all?

  10. Need to Change MAC Different clients prefer different frequencies

  11. Need to Change MAC Wideband needs frequency-aware MAC! Different clients prefer different frequencies

  12. FARA: Frequency-Aware Architecture • Per-frequency SNR measurement with no overhead • Frequency-aware rate adaptation • Frequency-aware MAC • FPGA implementation and testbed evaluation

  13. FARA Architecture Each Receiver Computes Per-Frequency SNR Frequency-Aware Rate Adaptation Frequency-Aware MAC

  14. Leverage OFDM for Per-Frequency SNR OFDM divides the bandwidth into subbands Subband 1 Bandwidth

  15. Leverage OFDM for Per-Frequency SNR OFDM divides the bandwidth into subbands Subband 2 Bandwidth

  16. Leverage OFDM for Per-Frequency SNR OFDM divides the bandwidth into subbands Subband N Bandwidth

  17. Leverage OFDM for Per-Frequency SNR OFDM divides the bandwidth into subbands Bandwidth Per-Frequency SNR = Per-OFDM Subband SNR 0 1 1 1 0 1 1 1 0 1 0 0 0 0 1 0 0 1 OFDM Packet

  18. SNR in an OFDM Subband Signal Power in subbandi Si SNRi = But receiver can measure only Received Power, Ri Ni Noise Power in subbandi • Ri=Si +Ni • Challenge: How do we find Ni? • Solution: Exploit that channel noise, Ni, is white, i.e., the same across all subbands.

  19. Exploit OFDM Pilots for Noise Computation Pilots contain known bit pattern; used for synchronization 0 1 1 1 0 1 1 1 0 1 0 0 0 0 1 0 0 1 Received Signal, y = H x + n Noise Transmitted Signal Channel

  20. Exploit OFDM Pilots for Noise Computation Pilots contain known bit pattern; used for synchronization 0 1 1 1 0 1 1 1 0 1 0 0 0 0 1 0 0 1 Received Signal, y = H x + n Can Compute Noise Power, N, using pilot subbands

  21. Putting it all together Si SNRi = Ni

  22. Putting it all together Ri– Ni SNRi = Ni

  23. Putting it all together Ri– N SNRi = N Per-frequency SNR Measurement with no Overhead!

  24. FARA Architecture Each Receiver Computes Per-Frequency SNR Frequency-Aware Rate Adaptation Frequency-Aware MAC

  25. From SNR to Bitrate Bitrate is a choice of modulation and FEC code rate How do we get this table? Easy to obtain from offline measurements

  26. From SNR to Bitrate High Loss Rate 16-QAM, 1/2 Transition Region Low Loss Rate

  27. From SNR to Bitrate Threshold for (16-QAM,1/2) 16-QAM, 1/2 12.0 – 15.5 16-QAM, 1/2

  28. From SNR to Bitrate 16-QAM, 1/2 16-QAM, 3/4 12.0 – 15.5 16-QAM, 1/2

  29. From SNR to Bitrate 16-QAM, 1/2 16-QAM, 3/4 12.0 – 15.5 16-QAM, 1/2

  30. From SNR to Bitrate

  31. Per-Frequency Rate Adaptation • Upon packet reception, receiver • Measures per-subband SNR • Maps it to optimal subbandbitrate • Sends per-subband bitrates to transmitter in ACK • Upon ACK reception, transmitter • updates bitrates for next transmission Can directly map SNR to optimal bitrate  Quickly converges to correct rate

  32. FARA Architecture Each Receiver Computes Per-Frequency SNR Frequency-Aware Rate Adaptation Frequency-Aware MAC

  33. Frequency-Aware MAC Subbands Today, at any time, MAC assigns all subbands to one client

  34. Frequency-Aware MAC Today, at any time, MAC assigns all subbands to one client

  35. Frequency-Aware MAC But different clients prefer different subbands!

  36. Frequency-Aware MAC FARA MAC assigns subbands to clients based on performance

  37. Frequency-Aware MAC FARA MAC assigns subbands to clients based on performance

  38. Frequency-Aware MAC FARA MAC assigns subbands to clients based on performance

  39. Frequency-Aware MAC FARA MAC assigns subbands to clients based on performance

  40. How Does FARA Assign Subbands to Clients? • Requirements • Efficiency: Assign each receiver its preferred subbands • Fairness: All receivers should get the same throughput • Problem is NP Hard • We need a heuristic

  41. FARA MAC Allocation Algorithm RateCounter1 =0 SubbandAssignment RateCounter2 =0 Iterate to maximize the minimum RateCounter

  42. FARA MAC Allocation Algorithm RateCounter1 =0 SubbandAssignment RateCounter2 =0

  43. FARA MAC Allocation Algorithm RateCounter1 =90 SubbandAssignment RateCounter2 =70 Iterate: • For each subband, compute Δ: change in total rate if subband is moved from maximum to minimum client • Move subbands in decreasing order of Δs

  44. FARA MAC Allocation Algorithm RateCounter1 =90 SubbandAssignment RateCounter2 =70 Iterate: • For each subband, compute Δ: change in total rate if subband is moved from maximum to minimum client • Move subbands in decreasing order of Δs

  45. FARA MAC Allocation Algorithm RateCounter1 =80 SubbandAssignment RateCounter2 =85 Iterate: • For each subband, compute Δ: change in total rate if subband is moved from maximum to minimum client • Move subbands in decreasing order of Δs

  46. FARA MAC Allocation Algorithm RateCounter1 =80 SubbandAssignment RateCounter2 =85 Iterate: • For each subband, compute Δ: change in total rate if subband is moved from maximum to minimum client • Move subbands in decreasing order of Δs

  47. FARA MAC Allocation Algorithm RateCounter1 =83 SubbandAssignment RateCounter2 =82 Iterate: • For each subband, compute Δ: change in total rate if subband is moved from maximum to minimum client • Move subbands in decreasing order of Δs

  48. Performance

  49. Implementation • Carrier Freq: 5.247 GHz • Data Bandwidth: 100 MHz • OFDM sub-band width: 1 MHz • Implemented in FPGA of custom wideband radio

  50. Testbed tx

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