Spectral Occupancy at VHF: Implications for Cognitive Radios

Steve EllingsonMobile & Portable Radio Research Group (MPRG)Dept. of Electrical & Computer EngineeringVirginia Polytechnic Institute & State Universityellingson@vt.eduhttp://www.ece.vt.edu/swe/

Fall 2005 IEEE Vehicular Technology Conference (Dallas)

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)

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.

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

VHF Noise Backgrounds

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

HF Comm TV4 TV6

Urban: HF to 90 MHz

HF Comm TV4 TV6

30-60 MHz

Urban: HF to 90 MHz

FM 2-Way Radio

Aeronautical/Satellite

Urban: 90-180 MHz

FM 2-Way Radio

Aeronautical/Satellite 140-180 MHz

Urban: 90-180 MHz

TV10

Urban: 180-270 MHz

Spectral Occupancy in 30-60 MHz

Spectral Occupancy in 140-180 MHz

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

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

A4

Galaxy, VSWR=1Galaxy, VSWR=12Galaxy, VSWR=100

Spectrum analyzer (∆ν=300 kHz) at end of feedline

FSH3300 kHz

Rural

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

A4

300 kHz

30 kHz

TV Ch 4 Spectrum analyzer (∆ν=300 kHz) at end of feedline

Rural: 66-75 MHz @ 30 kHz RBW

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

Rural: 25-90 MHz

Rural: Spectragram

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

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