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 (11a/g), 200 Mbps (11n)
– 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 (11n,
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
7000
MHz
5300
“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
Wireless Mic
TV
0 54-90 174-216 470
MHz
ISM (Wi-Fi)
2400 2500
700
Up to 3x of 802.11g
More
Spectrum
Longer
Range
}
5180
7000
MHz
5300
Potential Applications
Rural wireless broadband
City-wide mesh
at least 3 - 4x 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
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
2
3
4
5
6
# Contiguous Channels
>6
Each TV Channel is 6 MHz wide
Spectrum
is Fragmented
 Use
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
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
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
Scanner (SDR)
TV/MIC
detection
FFT
Net
Stack
FPGA
UHF RX
Daughterboard
Whitespace Radio
Connection Manager
Atheros Device Driver
Wi-Fi
Card
UHF
Translator
Variable Channel
Width Support*
*Case for Adapting Channel Widths, SIGCOMM 2008
16
WhiteFi System Challenges
Fragmentation
Spatial
Variation
Temporal
Variation
Impact
Discovery
Spectrum
Assignment
Disconnection
17
Discovering a Base Station
1 2 3 4 5
1 2 3 4 5
Discovery Time = (B x W)
Fragmentation
 the
Try different
center
channel
and widths
How
does
new
client
discover
BS and
Clients
must
use
same
channels
Can
we
optimize
this
discovery
time?
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
MHz
SIFT
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
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
Baseline =(B x W)
L-SIFT = (B/W)
J-SIFT = (B/W)
1
Discovery Time Ratio
(compared to baseline)
0.9
0.8
Linear-SIFT
Jump-SIFT
0.7
2X 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
WhiteFi System Challenges
Fragmentation
Spatial
Variation
Temporal
Variation
Impact
Discovery
Spectrum
Assignment
Disconnection
24
Channel Assignment in Wi-Fi
1
6
11
1
6
11
Fixed Width Channels  Optimize which channel to use
25
Spectrum Assignment in WhiteFi
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
1 2 3 4 5

1 2 3 4 5
1 2 3 4 5
=
1 2 3 4 5
27
Intuition
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
W
BS
MChamn (F, W) =
n (c)

5
Mhz c(30F ,W )
0
10
20
40
Background1traffic
4 delay
5 (ms)
2 - 3Packet
50
Pick (F, W) that maximizes
20 Mhz
10 MHz
(N5 MHz
* MChamBS + ΣnMChamn)
2
1
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
onContention
channel
2, 1/Contention)
get
to
transmit
on
channel
c
1
MCham-value
2.5
0.5
0
0
10
20
30
40
Background traffic - Packet delay (ms)
50
29
WhiteFi Prototype Performance
Throughput (Mbps)
25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
WhiteFi
0
25
50
75
100
125
OPT
150
175
200
225
250
Seconds
30
Conclusions and Future Work
• WhiteFi: 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