Networking Over TV White Spaces Bahl et al
Networking Over TV White Spaces Bahl et al. Sigcomm 2009 (Best paper award winner)
Wi-Fi’s Success Story • Wi-Fi is extremely popular (billion $$ business) – Enterprise/campus LANs, Home networks, Hotspots • Why is Wi-Fi successful – Wireless connectivity: no wires, increased reach – Broadband speeds: 54 Mbps (11 a/g), 200 Mbps (11 n) – Free: operates in unlicensed bands, in contrast to cellular
Problems with Wi-Fi • Poor performance: – Contention with Wi-Fi devices – Interference from other devices in 2. 4 GHz, such as Bluetooth, Zigbee, microwave ovens, … • Low range: – Can only get to a few 100 meters in 2. 4 GHz – Range decreases with transmission rate
Overcoming Wi-Fi’s Problems • Poor performance: – Fix Wi-Fi protocol – several research efforts (11 n, MIMO, interference cancellation, …) – Obtain new spectrum? • Low range: – Operate at lower frequencies?
Higher Frequency Analog TV Digital TV USA (2009) Spain (2010) Japan (2011) Broadcast TV Canada (2011) UK (2012) China (2015) Wi-Fi (ISM) …. …. …. .
What are White Spaces? Wireless Mic TV 0 54 -90 170 -216 470 700 MHz • 50 TV Channels -60 ISM (Wi. Fi) 2400 2500 5180 5300 7000 MHz “White spaces” • Each channel is 6 MHz wide dbm • FCC Regulations TV Stations in America • Sense TV-100 stations and Mics 470 MHz Frequency 700 MHz White Spaces are Unoccupied TV Channels 6
Why should we care about White Spaces? 7
The Promise of White Spaces TV 0 54 -90 174 -216 470 MHz Wireless Mic ISM (Wi. Fi) 2400 2500 700 Up to 3 x of 802. 11 g More Spectrum Longer Range } 5180 5300 7000 MHz Potential Applications Rural wireless broadband City-wide mesh at least 3 - 4 x of Wi-Fi ……. . 8
Goal: Deploy Infrastructure Wireless Base Station (BS) Good throughput for all nodes Avoid interfering with incumbents 9
Cognitive Radios Frequency Signal Strength 1. Dynamically identify currently unused portions of spectrum 2. Configure radio to operate in available spectrum band take smart decisions how to share the spectrum Frequency
Cognitive Radio Challenges How should nodes connect? Which spectrum-band should two cognitive radios use for transmission? 1. Frequency…? 2. Channel Width…? 3. Duration…? How should they discover one another? Need analysis tools to reason about capacity & overall spectrum utilization Which protocols should we use?
Why not reuse Wi-Fi based solutions, as is? 12
Fraction of Spectrum Segments White Spaces Spectrum Availability 0. 8 Urban 0. 7 Differences from ISM(Wi-Fi) 0. 6 Suburban 0. 5 Rural Fragmentation Variable channel widths 0. 4 0. 3 0. 2 1 20. 13 4 5 0 1 1 2 3 4 5 6 # Contiguous Channels >6 Each TV Channel is 6 MHz wide Spectrum Use is Fragmented multiple channels for more bandwidth 13
White Spaces Spectrum Availability Differences from ISM(Wi-Fi) Fragmentation Variable channel widths Spatial Variation Cannot assume same channel free everywhere 1 2 3 4 5 TV Tower Location impacts spectrum availability Spectrum exhibits spatial variation 14
White Spaces Spectrum Availability Differences from ISM(Wi-Fi) Fragmentation Variable channel widths Spatial Variation Cannot assume same channel free everywhere 1 2 3 4 5 Temporal Variation Same Channel will not always be free Any connection can be disrupted any time Incumbents appear/disappear over time Must reconfigure after disconnection 15
KNOWS White Spaces Platform PC TV/MIC detection Scanner (SDR) FFT Net Stack FPGA UHF RX Daughterboard Whitespace Radio Connection Manager Atheros Device Driver Variable Channel Width Support* Wi-Fi Card UHF Translator *Case for Adapting Channel Widths, SIGCOMM 2008 16
White. Fi System Challenges Fragmentation Spatial Variation Temporal Variation Impact Discovery Spectrum Assignment Disconnection 17
Discovering a Base Station 1 2 3 4 5 Discovery Time = (B x W) Try different center channel and Fragmentation How does themust new client discover BS and Clients use same channels Can we optimize this discovery time? widths channels used by the BS? 18
Whitespaces Platform: Adding SIFT PC TV/MIC detection Net Stack Scanner (SDR) FFT Temporal Analysis (SIFT) FPGA UHF RX Daughterboard Whitespace Radios Connection Manager Atheros Device Driver Wi-Fi Card UHF Translator SIFT: Signal Interpretation before Fourier Transform 19
SIFT, by example 10 5 MHz SIFT Does not decode packets Pattern match in time domain Amplitude ADC Data ACK SIFS Time 20
BS Discovery: Optimizing with SIFT 1 2 3 4 5 Amplitude 18 MHz Matched against 18 MHz packet signature Time SIFT enables faster discovery algorithms 21
BS Discovery: Optimizing with SIFT Linear SIFT (L-SIFT) 1 2 3 4 5 Jump SIFT (J-SIFT) 1 2 3 4 5 6 7 8 22
Discovery: Comparison to Baseline 1 Baseline = (B x W) L-SIFT = (B/W) J-SIFT = (B/W) Discovery Time Ratio (compared to baseline) 0. 9 0. 8 Linear-SIFT Jump-SIFT 0. 7 2 X reduction 0. 6 0. 5 0. 4 0. 3 0. 2 0. 1 0 0 30 60 90 120 White Space - Contiguous Width (MHz) 150 180 23
White. Fi System Challenges Fragmentation Spatial Variation Temporal Variation Impact Discovery Spectrum Assignment Disconnection 24
Channel Assignment in Wi-Fi 1 6 11 Fixed Width Channels Optimize which channel to use 25
Spectrum Assignment in White. Fi Spectrum Assignment Problem Goal Maximize Throughput Include Spectrum at clients 1 2 3 4 5 Assign 1 2 3 4 5 Center Channel & Width Fragmentation Optimize for both, center channel and width Spatial Variation BS must use channel iff free at client 26
Accounting for Spatial Variation 1 2 3 4 5 1 2 3 4 5 = 1 2 3 4 5 27
Intuition Use widest possible channel BS But Limited by most busy channel 1 2 3 4 5 § Carrier Sense Across All Channels § All channels must be free §ρBS(2 and 3 are free) = ρBS(2 is free) x ρBS(3 is free) Tradeoff between wider channel widths and opportunity to transmit on each channel 28
Throughput (Mbps) Multi Channel Airtime Metric (MCham) 3. 5 3 2. 5 2 1. 5 1 0. 5 0 20 Mhz 5 MHz 10 MHz MChamn (F, W) = 0 10 BS 20 30 40 Background 1 traffic 4 delay 5 (ms) 2 - 3 Packet 50 20 30 40 Background traffic - Packet delay (ms) 50 Pick (F, W) that maximizes 20 Mhz 10 MHz (N 5 MHz * MCham. BS + Σn. MChamn) 2 ρn(c) =(2)Approx. opportunity node n will ρ (2) ρ Free Air Time on Channel 2 BS 1. 5 BS ρBS(2) = Max (Free Air Time on channel 2, 1/Contention) get to transmit on channel c 1 MCham-value 2. 5 0 0 10 29
White. Fi Prototype Performance Throughput (Mbps) 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 5 4 3. 5 3 2. 5 2 1. 5 1 0. 5 0 White. Fi 0 25 50 75 100 125 Seconds OPT 150 175 200 225 250 30
Conclusions and Future Work • White. Fi: White Spaces based wireless network – Go beyond considerations of a single link – Change in spectrum access paradigm • SIFT for quick BS discovery • MCham to assign spectrum • Handling Disconnections 31
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