Reclaiming the White Spaces:
Spectrum Efficient Coexistence
with Primary Users
George Nychis†, Ranveer Chandra§, Thomas Moscibroda★,
Ivan Tashev§, Peter Steenkiste†
†Carnegie Mellon University, §Microsoft Research, ★Microsoft Research Asia
1
What are White Spaces?
Wireless Mic
TV
0 54-90 170-216 470
MHz
700
-60
ISM (Wi-Fi)
2400 2500
5180
7000
MHz
5300
“White spaces”
9 Orthogonal Channels (150Mbps per chan)
}
More
Potential Applications
Proliferation
of Wireless & Mobile:
dbm
Rural wireless broadband
ISM
Band is Insufficient to Meet City-wide
Demand
Spectrum
mesh
-100
Longer
Range
White
Spaces
470 MHz
Frequency 700 MHz
……..
……..
are Unoccupied
TV Channels
at least
3 - 4x of Wi-Fi
2
White Space
Availability
Reclaiming
the White
Spaces
• Spectrum availability is critical to adoption and goals
Goal:
“[to] make a significant amount of spectrum available for new
innovative
and services”
– FCC 08-260
Rescue WhiteandSpace
by products
Enabling
Mic Coexistence
• Spectrum is most critical in populated areas
– Measure spectrum availability in top 30 U.S. cities [1]
single channel
1
70
CDF
Amplitude (dB)
Number of
Analog TV
Broadcasts
Decreasing
Over Time
60
0.8
50
0.6
40
30
0.4
20
0.2
10
00
0
-3000
53% of cities cannot
support
single 802.11
2 Reserved
Channels
channel in the white spaces
Considering Active TV Broadcasts
20-200040 -1000
60
80 0100 120
160
1000 140 2000
White Space
Availability
Frequency
(KHz)(MHz)
[1] Geo-location database: http://whitespaces.msresearch.us/
Losing a
Significant
Amount of
White Space
to Mic Rules
180
3000
3
Outline
• Background on mic signals
• Data transmission impact on mic audio
– Critical insight on coexistence
• SEISMIC: Spectrum Efficient Interference-Free
System for MICs
– Regains spectrum with zero audible interference
– Evaluation of effectiveness and efficiency
4
Analog Microphone Background
• One-way communication: receiver never transmits
– Mic always transmitting (even idle)
• Signal Components:
70
Mic Receiver
Wireless Mic
Main Carrier Tone
60
Amplitude (dB)
FM Modulated Audio Signal
50
40
30
Squelch Tone
Squelch Tone
20
10
0
-60
-40
-20
0
20
40
60
Frequency (KHz)
Used By Mic Receiver to Detect Low Signal and Mute
5
Outline
• Background on mic signals
• Data transmission impact on mic audio
– Critical insight on coexistence
• SEISMIC: Spectrum Efficient Interference-Free
System for MICs
– Regains spectrum with zero audible interference
– Evaluation of effectiveness and efficiency
6
How Do You Coexist to Reclaim Spectrum?
70
Amplitude (dB)
60
50
40
30
20
10
0
-3000
-2000
-1000
0
1000
2000
3000
Frequency (KHz)
• First in-depth analysis RF interference on mic audio quality
– 3 Dimensions: time, frequency,
frequency, and
and power
power
• Study Impact on 6 Mics:
– Audio Technica, Sennheiser (3), Shure, and Electro-Voice
7
Experimental Setup
PESQ  worldwide audio evaluation standard
• Compare recording to original
• 0 (total disruption)  1 (perfect)
1. PC Output
to Speakers
Mic
2. MIC Recording
to Computer
Mic Receiver
White Space Device
Interference in Frequency
• Fix power and duration, vary frequency (25KHz steps)
70
Amplitude (dB)
60
50
40
30
20
10
0
-3000
-2000
-1000
0
1000
2000
3000
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Sennheiser EW100
0
50
100
150
200
Frequency Suppressed (KHz)
1. Avoid Disruption, Still Use 97% of Channel
Frequency Requirement (KHz)
Normalize PESQ Score
Frequency (KHz)
350
300
250
200
150
100
50
0
ATW
BPU
E935 EW100 SK2000 UR-2
2. Suppression Req. Varies by Mic
9
Interference in Power
• Fix freq. and duration, vary power of WSD (2dB steps)
70
Main Carrier Tone
WSNm (dB)
Amplitude (dB)
60
50
40
30
Squelch Tone
Squelch Tone
WSNs (dB)
20
10
0
-60
-40
-20
0
20
Frequency (KHz)
40
60
Despite 25dB of noise!
• Record in
value
of WSN
m & WSN
s when PESQ=1
“FM receivers exhibit a `capture effect’
which
they
respond
to only the strongest signal•received
on a frequency
and is
No audio disruption
when interference
reject any weaker
signals.”
just belowinterfering
the squelch tones!
(WSNs = 1dB)
- FCC (First Order: 04-113)
10
Implications of Interference Study
• Coexistence in the same channel is possible!
– Great! Just suppress bandwidth required at center frequency
Frequency Requirement (KHz)
Not that simple…
350
300
250
200
150
100
50
0
ATW
BPU
E935 EW100 SK2000 UR-2
11
The Challenge of Suppression
70
Amplitude (dB)
60
50
Stronger WSD 
40
30
More Suppression
20
10
`Perfect
Suppression’
Actual Suppression 0-200
would
haveinsharp
edge
Leaks
Power
to the
Band
-150
-100
-50
0
50
100
150
200
Frequency (KHz)
70
Amplitude (dB)
60
Weaker Mic 
More Suppression
50
40
30
20
10
0
-200
-150
-100
-50
0
50
Frequency (KHz)
100
150
200
12
Information Needed to Suppress
• Two components needed at WSD to suppress properly:
1. WSD Interference Power
at the Mic Receiver
White Space Device
(WSD)
Mic Receiver
Wireless Mic
2. Mic Signal Power
at Mic Receiver
13
Lack of Information at WSD
• No feedback on either required components
Suppression Needed (KHz)
– Without feedback, the system is open-loop
– Must suppress “worst-case” to be conservative
6000
Mic at -70dBm
Mic at -60dBm
Mic at -50dBm
5000
Given
the open-loop state of
4000
the
system (info available)…
3000
2000
1000
0
-90
… the FCC made the right
by
-80 decision
-70
-60 requiring
-50
-40
WSD Interference Power at Mic Receiver (dBm)
vacation.
Unfortunately,
worst-case is
vacation! (6MHz)
14
Need a Closed-Loop to Avoid Vacation
Feedback
SEISMIC
Analysis
Measurement
Mic Receiver
White Space Device)
Mic
Adaptation
15
Outline
• Background on mic signals
• Data transmission impact on mic audio
– Critical insight on coexistence
• SEISMIC: Spectrum Efficient Interference-Free
System for MICs
– Regains spectrum with zero audible interference
– Evaluation of effectiveness and efficiency
16
SEISMIC System Overview
• Implements closed-loop design to avoid vacation:
– Measurement: MicProtector measures key components at receiver
– Feedback: “Strobe” signal to notify WSD of impending disruption
– Analysis / Adaptation: SEISMIC protocol to adapt frequency
Feedback
MicProtector
White Space Device
Mic Receiver
Mic
17
Measurement: MicProtector
• Key measurement needed: squelch & interference power
– Squelch Measurement: estimated in the mic’s band
– Interference Measurement: estimated in control bands
Power in Mic Band Shifts
Control
Band
Amplitude
Control
Band
Interference
Level
25KHz
Frequency
25KHz
18
Feedback: Strobes
• MicProtector needs to convey information to WSD
• Strobe is similar to Morse-codes and on/off-keying (OOK)
– Allows us to convey necessary information without complex protocol (e.g., 802.11)
• Strobe signals convey:
Amplitude
– (1) impending disruption, (2) required bandwidth, (3) center frequency
Frequency
19
Adaptation / Analysis: SEISMIC Protocol
• WSD: sends probe packets with increasing power (exploit capture)
• MicProtector: notifies of impending disruption, using strobes
Control
Band
Amplitude
Control
Band
Protection
Threshold
Interference
Level
25KHz
Frequency
25KHz
20
Adaptation: SEISMIC Protocol
• WSD: sends probe packets with increasing power (exploit capture)
• MicProtector: notifies of impending disruption, using strobes
Suppressed Frequency (KHz)
Strobe
250
250
250
200
150
100
100
Increase in Power
Probe
Convergence
To Coexistence
MicProtector Strobes the WSD
for interference near threshold
50
WSD
MicProt.
Pkts:
Time
21
Summary of SEISMIC Design
• MicProtector components:
– detection of impending audio disruptions
– feedback using strobes to WSD enables closed-loop
– Note: can be built directly in to future receivers
• WSD and MicProtector engage in SEISMIC protocol
– converge to optimal suppression around mic
• Formalization shows correctness with multiple WSD
– WSDs will converge and never interfere, in paper 
SEISMIC
22
Outline
• Background on mic signals
• Data transmission impact on mic audio
– Critical insight on coexistence
• SEISMIC: Spectrum Efficient Interference-Free
System for MICs
– Regains spectrum with zero audible interference
– Evaluation of effectiveness and efficiency
23
Evaluating SEISMIC
• Full MicProtector and SEISMIC WSD prototypes
– custom USRP2 builds with UHF front ends
• Evaluation on several coexistence points:
– Effectiveness: ability to avoid audio disruption
– Efficiency: ability to enable high spectrum re-use
• Single microphone scenario
• Many microphone scenario
24
Effectiveness of SEISMIC’s Coexistence
• Challenge: Low-power & mobile microphone
Squelch Tone Power (dBm)
– SEISMIC WSD must never interfere despite mic signal fluctuations
-50
-60
-70
-80
-90
-100
0
5
10
15
20
25
30
20
25
30
Time (seconds)
WSD Spectrum (KHz)
6000
5000
WSD continually
Mobility creates
4000
and
quickly
3000
quick
fluctuations
2000
adapts to avoid
audio disruption! 10000
0
5
10
15
Time (s)
WSD vacates
channel when
mic power is
low to be safe
25
Efficiency of SEISMIC’s Coexistence
• Evaluate efficiency under two mobile mic scenarios
1.
Airtime With Spectrum > X
2.
1.MicFar
mic (low/moderate
signal)
(high
Mic Signal
(dBm) .. close
WSD Signal (dBm)
Distance
(ft)
WSDWSD
Distance
(ft)interference)
2. Far
Mod.
mic (moderate
signal) .. nearby WSD
interference
(50-70)
Close(moderate
(5)
Low (-95-75)
High
(-50-30)
Moderate (10-30)
Moderate (-65)
Moderate (15)
Moderate (-70)
Common scenario:
95% of time >5.7MHz
1
0.8
0.6
0.4
Challenging scenario:
70% of time >5.2MHz
Far Mic / High Interference
Moderate Mic / Moderate Interference
0.2
0
0
1000
2000
3000
4000
WSD Spectrum (KHz)
5000
6000
26
SEISMIC Efficiency with Many Mics
• Obtain real mic freq placement from coordinators
– Model components: 1) Mic signals, 2) WSDs & interference
– Given components, how much spectrum does WSD X have?
Event
Mics
2008 NBA All-Star-Game
191
2010 BCS Championship
108
2010 WPC Conference
77
SEISMIC-enabled Mic System
SEISMIC-enabled WSD
• Mic signals generated using mobile
mic measurements
• Adaptrum WSD freq suppression
w/ leakage used for WSDs 27
SEISMIC Spectrum Efficiency
• On average, SEISMIC-enabled WSDs:
Spectrum Availability (MHz)
– SEISMIC has 22x, 3.6x, and 1.6x availability compared to channel vacation
– SEISMIC near perfect suppression, suppresses only when necessary (closed-loop)
175
150
125
100
75
50
25
Channel Vacation
Average SEISMIC Client
‘Perfect Suppression’ of Mic Operational Bands
NBA
BCS
WPC
– With 150MHz: can support 7 150Mbps channels! (2.5x 2.4GHz, with 4x range)
28
Summary
• Spectrum availability is critical to white spaces
• Coexistence possible between WSD and mics
– Closed-loop solution required to avoid vacation
• SEISMIC enables disruption-free coexistence
– Up to 95% of the spectrum can be regained
• Demoed to FCC chairman, mic manufacturers,
mic coordinators, audio community
29