Download - What are White Spaces?
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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
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dbm
Frequency
-60
-100
“White spaces”
470 MHz 700 MHz
What are White Spaces?
0 MHz
7000 MHz
TV ISM (Wi-Fi)
700470 2400 51802500 5300
are Unoccupied TV ChannelsWhite Spaces
54-90 170-216
Wireless Mic
More Spectrum
Longer Range
9 Orthogonal Channels (150Mbps per chan)
at least 3 - 4x of Wi-Fi
} Potential ApplicationsRural wireless broadbandCity-wide mesh
……..
……..
Proliferation of Wireless & Mobile:ISM Band is Insufficient to Meet Demand
White Space Availability• Spectrum availability is critical to adoption and goals
• Spectrum is most critical in populated areas– Measure spectrum availability in top 30 U.S. cities [1]
3[1] Geo-location database: http://whitespaces.msresearch.us/
“[to] make a significant amount of spectrum available for new and innovative products and services” – FCC 08-260
53% of cities cannot support single 802.11
channel in the white spaces
Number ofAnalog TVBroadcastsDecreasingOver Time
single channel
2 Reserved Channels
Losing aSignificantAmount of
White Spaceto Mic Rules
Reclaiming the White Spaces
Goal: Rescue White Space by Enabling Mic Coexistence
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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
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Analog Microphone Background
Mic ReceiverWireless Mic
• One-way communication: receiver never transmits– Mic always transmitting (even idle)
• Signal Components: FM Modulated Audio Signal
Used By Mic Receiver to Detect Low Signal and Mute
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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
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How Do You Coexist to Reclaim Spectrum?
• First in-depth analysis RF interference on mic audio quality– 3 Dimensions: time, frequency, and power
• Study Impact on 6 Mics: – Audio Technica, Sennheiser (3), Shure, and Electro-Voice
• First in-depth analysis RF interference on mic audio quality– 3 Dimensions: time, frequency, and power
Experimental Setup
2. MIC Recording to Computer
1. PC Output to Speakers
PESQ worldwide audio evaluation standard• Compare recording to original• 0 (total disruption) 1 (perfect)
White Space Device
Mic Receiver
Mic
Interference in Frequency• Fix power and duration, vary frequency (25KHz steps)
92. Suppression Req. Varies by Mic1. Avoid Disruption, Still Use 97% of Channel
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Interference in Power• Fix freq. and duration, vary power of WSD (2dB steps)
WSNm (dB)
WSNs (dB)
•Record value of WSNm & WSNs when PESQ=1
•No audio disruption when interference is just below the squelch tones! (WSNs = 1dB)
Despite 25dB of noise!
“FM receivers exhibit a `capture effect’ in which they respond to only the strongest signal received on a frequency and
reject any weaker interfering signals.”
- FCC (First Order: 04-113)
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Implications of Interference Study• Coexistence in the same channel is possible!– Great! Just suppress bandwidth required at center frequency
Not that simple…
The Challenge of Suppression
Weaker Mic More Suppression
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Stronger WSD More Suppression`Perfect Suppression’
would have sharp edgeActual Suppression
Leaks Power in to the Band
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• Two components needed at WSD to suppress properly:
Information Needed to Suppress
White Space Device(WSD)
Mic Receiver Wireless Mic
2. Mic Signal Power at Mic Receiver
1. WSD Interference Powerat the Mic Receiver
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• No feedback on either required components– Without feedback, the system is open-loop– Must suppress “worst-case” to be conservative
Lack of Information at WSD
Unfortunately, worst-case is
vacation! (6MHz)
Given the open-loop state of the system (info available)…
… the FCC made the right decision by requiring vacation.
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Need a Closed-Loop to Avoid Vacation
White Space Device)Mic Receiver
Mic
Measurement
Feedback
Analysis
Adaptation
SEISMIC
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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
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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
White Space Device Mic Receiver Mic
MicProtector
Feedback
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Measurement: MicProtector• Key measurement needed: squelch & interference power
– Squelch Measurement: estimated in the mic’s band– Interference Measurement: estimated in control bands
Frequency
Ampl
itude
ControlBand
ControlBand
25KHz25KHz
Interference Level
Power in Mic Band Shifts
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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: – (1) impending disruption, (2) required bandwidth, (3) center frequency
Frequency
Ampl
itude
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Adaptation / Analysis: SEISMIC Protocol
• WSD: sends probe packets with increasing power (exploit capture)• MicProtector: notifies of impending disruption, using strobes
Frequency
Ampl
itude
Protection Threshold
ControlBand
ControlBand
25KHz25KHz
Interference Level
• WSD: sends probe packets with increasing power (exploit capture)• MicProtector: notifies of impending disruption, using strobes
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Probe StrobePkts:
Time
WSD
Mic
Prot
.
Suppressed Frequency (KHz)
Increase in Power
MicProtector Strobes the WSDfor interference near threshold
50 100
100
150
200
250 250 25
0
ConvergenceTo Coexistence
Adaptation: SEISMIC Protocol
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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
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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
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
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Effectiveness of SEISMIC’s Coexistence
• Challenge: Low-power & mobile microphone– SEISMIC WSD must never interfere despite mic signal fluctuations
Mobility createsquick fluctuations
WSD continuallyand quickly
adapts to avoidaudio disruption!
WSD vacateschannel whenmic power is
low to be safe
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Efficiency of SEISMIC’s Coexistence• Evaluate efficiency under two mobile mic scenarios
1. Far mic (low/moderate signal) .. close WSD (high interference)2. Mod. mic (moderate signal) .. nearby WSD (moderate interference
Mic Distance (ft) WSD Distance (ft)
1. Far (50-70) Close (5)
2. Moderate (10-30) Moderate (15)
Common scenario:95% of time >5.7MHz
Challenging scenario:70% of time >5.2MHz
Low (-95-75)
Moderate (-65)
Mic Signal (dBm)
High (-50-30)
Moderate (-70)
WSD Signal (dBm)
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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 Mics2008 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
SEISMIC Spectrum Efficiency
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• On average, SEISMIC-enabled WSDs:– SEISMIC has 22x, 3.6x, and 1.6x availability compared to channel vacation– SEISMIC near perfect suppression, suppresses only when necessary (closed-loop)
– With 150MHz: can support 7 150Mbps channels! (2.5x 2.4GHz, with 4x range)
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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