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Spectral Occupancy at VHF: Implications for Cognitive Radios
Steve EllingsonMobile & Portable Radio Research Group (MPRG)Dept. of Electrical & Computer EngineeringVirginia Polytechnic Institute & State [email protected]://www.ece.vt.edu/swe/
Fall 2005 IEEE Vehicular Technology Conference (Dallas)
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Frequency/Bandwidth-Agile Cognitive Radio
…An emerging paradigm for the operation of radio networks in which individual radios will be able to monitor the available spectrum and select combinations of frequencies and bandwidths which are mutually optimum in some sense.
Possible solution to present-day rigid, inefficient use of spectrum– The VHF (30-300 MHz) band is particularly bad off in this
respect: Badly fragmented, with much spectrum left unused or used with only very low duty cycle
Preliminary questions:– How much spectrum might actually be available for this? – How difficult is it to access (esp. receiver dynamic range)
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Receiver Sensitivity / Dynamic Range Trade-Off
Tradeoff can be made less painful by reducing bandwidth, but 10’s of MHz will probably be required in any event.
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Coax cableAgilent E4407B
Spectrum Analyzer
Columbus, OH (Business/Residential Area)Antenna 14 m above ground
PC
AOR Model DA300025-2000 MHz
Linear Power Detection30 kHz Channels83 µs dwell
RS232
~ 400 sweeps
Measured Sensitivity-87 dB(mW/[30 kHz]), i.e.
-132 dB(mW/Hz)
Urban Measurement Setup
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VHF Noise Backgrounds
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Coax cableAgilent E4407B
Spectrum Analyzer
Columbus, OH (Business/Residential Area)Antenna 14 m above ground
PC
AOR Model DA300025-2000 MHz
Linear Power Detection30 kHz Channels83 µs dwell
RS232
~ 400 sweeps
Measured Sensitivity-87 dB(mW/[30 kHz]), i.e.
-132 dB(mW/Hz)
Sensitivity Relative to ITU “Business” Model30 MHz: +7 dBFam
300 MHz: +34 dBFam
Urban Measurement Setup
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HF Comm TV4 TV6
Urban: HF to 90 MHz
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HF Comm TV4 TV6
30-60 MHz
Urban: HF to 90 MHz
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FM 2-Way Radio
Aeronautical/Satellite
Urban: 90-180 MHz
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FM 2-Way Radio
Aeronautical/Satellite 140-180 MHz
Urban: 90-180 MHz
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TV10
Urban: 180-270 MHz
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Spectral Occupancy in 30-60 MHz
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Spectral Occupancy in 140-180 MHz
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Statistics of Spectral Occupancy
Band # openings Mean(MHz) >= 30 kHz Opening
30-60 40 651 kHz
67% BWImplied dynamic range < 30 dB
140-180 67 411 kHz
25% BWImplied dynamic range > 60 dB
-87 dB(mW/[30 kHz]) Detection Threshold
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Coax cable R&S FSH3
Spectrum Analyzer
Rosman, NC Antenna 2 m above ground
PC
Custom-Built“Fat Dipole”
RS232
Rural Measurement Setup
Linear power detection300/30 kHz channels
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A4
Galaxy, VSWR=1Galaxy, VSWR=12Galaxy, VSWR=100
Spectrum analyzer (∆ν=300 kHz) at end of feedline
FSH3300 kHz
Rural
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A4
-15.5 dBm in [30,85] MHz
Measurements taken@ PARI (Rosman, NC).Spectrum analyzer (∆ν=300 kHz) at end of feedline
FSH3300 kHz
FSH330 kHz
Rural: 30-40 MHz @ 30 kHz RBW
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A4
300 kHz
30 kHz
TV Ch 4 Spectrum analyzer (∆ν=300 kHz) at end of feedline
Rural: 66-75 MHz @ 30 kHz RBW
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Coax cable Custom
Spectrometer
Rosman, NC Antenna 2 m above ground
PC
Custom-Built“Fat Dipole”
PCI
Rural Measurement Setup (Spectrometer)
Linear Power Detection4 MHz Swept BW610 Hz Channels
Measured Sensitivity-100 dB(mW/[30 kHz]), i.e.-145 dB(mW/Hz)
Sensitivity~Fam relative to ITU “Rural” model~13 dB improvement over urban measurement
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Rural: 25-90 MHz
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Rural: Spectragram
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A4
-15.5 dBm in [30,85] MHz
Measurements taken@ PARI (Rosman, NC).Spectrum analyzer (∆ν=300 kHz) at end of feedline
FSH330 kHz
PLFM610 Hz
Most of the spectrum starts to look pretty good at resolutions below 1 kHz!
Max HoldIntegration
Rural: Zoom Around 38 MHz
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ConclusionsMost of the VHF spectrum away from persistent broadcast signals appears to be useful for frequency-agile cognitive radio
– E.g., 80% of the 30-60 MHz band was clear to within 7 dB of ITU-defined noise floor in an urban area
– Tens of empty gaps found in the 30-60 MHz and 140-180 MHz bands, with mean bandwidths of 100’s of kHz (WRT -87 dB(mW/[30 kHz]) threshold)
– Very little activity above -93 dB(mW/[30 kHz]) in the rural setting
Much more thorough measurement campaign needed to completely understand this– 1 ms x 1 kHz time-frequency resolution– Continuous observation over many day/week cycles of human activity