1

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

4

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