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

dbm Frequency “White spaces” 470 MHz 700 MHz What are White Spaces? 0 MHz 7000 MHz TV ISM (Wi-Fi) are Unoccupied TV Channels White Spaces Wireless Mic More Spectrum Longer Range 9 Orthogonal Channels (150Mbps per chan) at least 3 - 4x of Wi-Fi } Potential Applications Rural wireless broadband City-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: “[to] make a significant amount of spectrum available for new and innovative products and services” – FCC % of cities cannot support single channel in the white spaces Number of Analog TV Broadcasts Decreasing Over Time single channel 2 Reserved Channels Losing a Significant Amount of White Space to Mic Rules Reclaiming the White Spaces Goal: Rescue White Space by Enabling Mic Coexistence

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 5 Mic Receiver Wireless 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

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? 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 7 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) 9 2. Suppression Req. Varies by Mic 1. Avoid Disruption, Still Use 97% of Channel

Interference in Power Fix freq. and duration, vary power of WSD (2dB steps) 10 WSN m (dB) WSN s (dB) Record value of WSN m & WSN s when PESQ=1 No audio disruption when interference is just below the squelch tones! (WSN s = 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: )

Implications of Interference Study Coexistence in the same channel is possible! – Great! Just suppress bandwidth required at center frequency 11 Not that simple…

The Challenge of Suppression Weaker Mic  More Suppression 12 Stronger WSD  More Suppression `Perfect Suppression’ would have sharp edge Actual Suppression Leaks Power in to the Band

13 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 Power at the Mic Receiver

No feedback on either required components – Without feedback, the system is open-loop – Must suppress “worst-case” to be conservative 14 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.

Need a Closed-Loop to Avoid Vacation 15 White Space Device) Mic Receiver Mic Measurement Feedback Analysis Adaptation SEISMIC

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 17 White Space Device Mic Receiver Mic MicProtector Feedback

Measurement: MicProtector Key measurement needed: squelch & interference power – Squelch Measurement: estimated in the mic’s band – Interference Measurement: estimated in control bands 18 Frequency Amplitude Control Band Control Band 25KHz Interference Level Power in Mic Band Shifts

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., ) Strobe signals convey: – (1) impending disruption, (2) required bandwidth, (3) center frequency 19 Frequency Amplitude

Adaptation / Analysis: SEISMIC Protocol WSD: sends probe packets with increasing power (exploit capture) MicProtector: notifies of impending disruption, using strobes 20 Frequency Amplitude Protection Threshold Control Band Control Band 25KHz Interference Level

WSD: sends probe packets with increasing power (exploit capture) MicProtector: notifies of impending disruption, using strobes 21 Probe Strobe Pkts: Time WSD MicProt. Suppressed Frequency (KHz) Increase in Power MicProtector Strobes the WSD for interference near threshold Convergence To Coexistence Adaptation: SEISMIC Protocol

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  22 S E IS M I C

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 – SEISMIC WSD must never interfere despite mic signal fluctuations 25 Mobility creates quick fluctuations WSD continually and quickly adapts to avoid audio disruption! WSD vacates channel when mic power is low to be safe

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 26 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)

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? 27 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 28 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)

Summary 29 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

Deployment Challenge If a mic receiver does not have a MicProtector… 30 Time Power WSD View Mic System View (No MicProtector) Power Frequency No strobe is detected, WSD ramps up… … eventually begins disrupting

Fortunately, Can Partially Deploy Current ruling: all mics must report to DB – Augment database to include SEISMIC capability Only use channel if all mics have MicProtector – More SEISMIC receivers  better spectrum efficiency 31

RF Interference Impact Study First in-depth analysis on mic audio quality – Opposed to traditional “RF-interference” measurement Study Impact on 6 Mics: – Audio Technica, Sennheiser (3), Shure, and Electro-Voice Understand impact of 3 key components: 1.Time: can short transmissions avoid audio disruption? 2.Frequency: what if only part of the channel used? 3.Power: will low power transmissions avoid disruption? 32 Recorded Mic Audio Transmit 16μs long data packets (1/100 th of packet) spaced by 500ms in time

Effectiveness of SEISMIC’s Coexistence Challenge: Low-power & mobile microphone – SEISMIC WSD must never interfere despite mic signal fluctuations 33 Mobility creates quick fluctuations -98dBm USRP2 noise floor, 10dB Protection Threshold, MicProtector considers mic low -88dBm WSD continually and quickly adapts to avoid audio disruption! SEISMIC signals WSD to vacate channel when mic power is low to be safe

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 34 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)

Closing the Loop To avoid vacation we need a closed-loop system 35 Mic Receiver White Space Device Measurement Feedback Adaptation Analysis SEISMIC

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] 36 [1] Geo-location database: Unfortunately: 1. Availability is low 2. Mic coexistence rules lower availability (2 channels reserved) 39% of cities have zero white space available! “[to] make a significant amount of spectrum available for new and innovative products and services” – FCC Mics can still operate outside of reserved channels, driving availbility even lower 53% of cities cannot support single channel in the white spaces

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] 37 [1] Geo-location database: Key Points: 1. Availability is low 2. Efficiency of the spectrum use is critical “[to] make a significant amount of spectrum available for new and innovative products and services” – FCC % of cities cannot support single channel in the white spaces

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] 38 [1] Geo-location database: Unfortunately: 1. Availability is low 2. Mic coexistence rules lower availability (2 channels reserved) 39% of cities have zero white space available! “[to] make a significant amount of spectrum available for new and innovative products and services” – FCC Mics can still operate outside of reserved channels, driving availbility even lower 53% of cities cannot support single channel in the white spaces

FCC Ruling: Unable to use channel with an active mic – 2 channels reserved for wireless microphones Reclaiming the White Spaces 39 single channel Reclaim 95% Spectrum Around Mic Remove Need For 2 Reserved Channels Goal: Reclaim White Space by Enabling Mic Coexistence

Measurement: MicProtector Implements three components: – Interference Detection: estimated in control bands – Interference Protection: monitors squelch & interference – Impending Interference Notification: strobe signals 40 Frequency Amplitude Protection Threshold Strobe Signals Control Band Control Band 25KHz Interference Level