Networking with Wi Fi like Connec3vity

Transcription

1 Networking with Wi Fi like Connec3vity Victor Bahl, Ranveer Chandra, Thomas Moscibroda, Microso- Research Rohan Murty*, MaC Welsh Harvard University Higher Frequency Analog TV Digital TV USA (2009) Spain (2010) Japan (2011) Canada (2011) Broadcast TV UK (2012) China (2015) Wi Fi (ISM)

2 What are White Spaces? TV Wireless Mic ISM (Wi-Fi) MHz 50 TV Channels 60 White spaces 7000 MHz Each channel is 6 MHz wide dbm FCC RegulaOons* Sense TV sta1ons and Mics MHz Frequency 700 MHz Portable devices on channels TV StaOons in America White Spaces are Unoccupied TV Channels 3 Why should we care about White Spaces? 4 2

3 The Promise of White Spaces TV Wireless Mic ISM (Wi-Fi) MHz More Spectrum Longer Range Up to 3x of g at least 3 4x of Wi Fi 7000 MHz }Poten3al Applica3ons Rural wireless broadband City wide mesh Goal: Deploy Infrastructure Wireless Base StaOon (BS) Good throughput for all nodes Avoid interfering with incumbents 6 3

4 Why not reuse Wi Fi based soluoons, as is? 7 Frac3on of Spectrum Segments White Spaces Spectrum Availability Urban >6 # Con3guous Channels Suburban Rural Differences from ISM(Wi Fi) Fragmenta3on Variable channel widths Each TV Channel is 6 MHz wide Spectrum is Fragmented Use mul3ple channels for more bandwidth 8 4

5 White Spaces Spectrum Availability Differences from ISM(Wi Fi) Fragmenta3on Variable channel widths Spa3al Varia3on Cannot assume same channel free everywhere TV Tower Loca3on impacts spectrum availability Spectrum exhibits spa3al varia3on 9 White Spaces Spectrum Availability Differences from ISM(Wi Fi) Fragmenta3on Variable channel widths Spa3al Varia3on Cannot assume same channel free everywhere Temporal Varia3on Same Channel will not always be free Any connec3on can be disrupted any 3me Incumbents appear/disappear over 3me Must reconfigure aher disconnec3on 10 5

6 WhiteFi System Prototype Hardware Plaiorm Base StaOons and Clients Algorithms and Implementa3on Discovery Spectrum Assignment Handling DisconnecOons Evalua3on Deployment of prototype nodes SimulaOons 11 KNOWS White Spaces Plaiorm Windows PC TV/MIC detecoon FFT FPGA Scanner (SDR) UHF RX Daughterboard Net Stack ConnecOon Manager Whitespace Radio Atheros Device Driver Variable Channel Width Support* Wi Fi Card UHF Translator *Case for Adap1ng Channel Widths, SIGCOMM

7 WhiteFi System Challenges Fragmenta3on Spa3al Varia3on Temporal Varia3on Impact Discovery Spectrum Assignment Disconnec3on 13 Discovering a Base Sta3on Discovery Time = Ο(B x W) Can we opomize this discovery Ome? How does the new client discover BS and Clients must use same channels channels used by the BS? FragmentaOon Try different center channel and widths 14 7

8 Whitespaces Plaiorm: Adding SIFT PC TV/MIC detecoon FFT FPGA Scanner (SDR) UHF RX Daughterboard Net Stack Temporal Analysis (SIFT) ConnecOon Manager Whitespace Radios Atheros Device Driver Wi Fi Card UHF Translator SIFT: Signal Interpreta3on before Fourier Transform 15 SIFT, by example 10 MHz 5 MHz ADC SIFT SIFT Does not decode packets PaBern match in Dme domain Amplitude Data SIFS ACK Time 16 8

9 BS Discovery: Op3mizing with SIFT 18 MHz Amplitude Matched against 18 MHz packet signature Time SIFT enables faster discovery algorithms 17 BS Discovery: Op3mizing with SIFT Linear SIFT (L SIFT) Jump SIFT (J SIFT)

10 Discovery: Comparison to Baseline Discovery Time Ra3o (compared to baseline) Baseline =Ο(B x W) L SIFT = Ο(B/W) J SIFT = Ο(B/W) 2X reduc3on White Space Con3guous Width (MHz) Linear SIFT Jump SIFT 19 WhiteFi System Challenges Discovery Spectrum Assignment Disconnec3on 20 10

11 Channel Assignment in Wi Fi Fixed Width Channels OpOmize which channel to use 21 Spectrum Assignment in WhiteFi Spectrum Assignment Problem Goal Maximize Throughput Include Assign Spectrum at clients Center Channel & Width FragmentaOon OpOmize for both, center channel and width SpaOal VariaOon BS must use channel iff free at client 22 11

12 Accoun3ng for Spa3al Varia3on = 23 Intui3on Intui3on Use widest possible channel But Limited by most busy channel BS Carrier Sense Across All Channels All channels must be free ρ BS (2 and 3 are free) = ρ BS (2 is free) x ρ BS (3 is free) Tradeoff between wider channel widths and opportunity to transmit on each channel 24 12

13 Throughput (Mbps) Mul3 Channel Air3me Metric (MCham) Mhz 5 MHz 10 MHz MCham n (F, W) = Background traffic Packet delay (ms) 2.5 Pick (F, W) that maximizes 20 Mhz 10 MHz 2 (N * MCham ρ n (c) = Approx. opportunity node n will BS + Σ n MCham n ) 5 MHz ρ BS (2) Free Air Time on Channel 2 ρ BS 1.5 ρ BS (2) = Max (Free Air Time on channel 2, 1/Conten3on) get to transmit on channel c MCham value BS Background traffic Packet delay (ms) 25 WhiteFi Prototype Performance Throughput (Mbps) WhiteFi Seconds OPT 26 13

14 Conclusions and Future Work WhiteFi: White Spaces based wireless network Go beyond consideraoons of a single link Change in spectrum access paradigm SIFT for quick BS discovery MCham to assign spectrum Handling DisconnecOons On going work: Campus wide deployment 27 QuesOons? rohan@eecs.harvard.edu 28 14