Spectral Occupancy at VHF: Implications for Cognitive Radios Steve Ellingson Mobile & Portable Radio Research Group (MPRG) Dept. of Electrical & Computer Engineering Virginia Polytechnic Institute & State University ellingson@vt.edu http://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.
Urban Measurement Setup Columbus, OH (Business/Residential Area) Antenna 14 m above ground Coax cable Agilent E4407B Spectrum Analyzer RS232 PC AOR Model DA3000 25-2000 MHz Measured Sensitivity -87 db(mw/[30 khz]), i.e. -132 db(mw/hz) Linear Power Detection 30 khz Channels 83 µs dwell ~ 400 sweeps
VHF Noise Backgrounds
Urban Measurement Setup Columbus, OH (Business/Residential Area) Antenna 14 m above ground Coax cable Agilent E4407B Spectrum Analyzer RS232 PC AOR Model DA3000 25-2000 MHz Linear Power Detection 30 khz Channels 83 µs dwell ~ 400 sweeps Measured Sensitivity -87 db(mw/[30 khz]), i.e. -132 db(mw/hz) Sensitivity Relative to ITU Business Model 30 MHz: +7 dbf am 300 MHz: +34 dbf am
Urban: HF to 90 MHz HF Comm TV4 TV6
Urban: HF to 90 MHz HF Comm TV4 TV6 30-60 MHz
FM Urban: 90-180 MHz 2-Way Radio Aeronautical/ Satellite
FM Urban: 90-180 MHz 2-Way Radio Aeronautical/ Satellite 140-180 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% BW Implied dynamic range < 30 db 140-180 67 411 khz 25% BW Implied dynamic range > 60 db -87 db(mw/[30 khz]) Detection Threshold
Rural Measurement Setup Rosman, NC Antenna 2 m above ground Coax cable R&S FSH3 Spectrum Analyzer RS232 PC Custom-Built Fat Dipole Linear power detection 300/30 khz channels
Rural FSH3 300 khz Spectrum analyzer ( ν=300 khz) at end of feedline Galaxy, VSWR=1 Galaxy, VSWR=12 Galaxy, VSWR=100 A4
Rural: 30-40 MHz @ 30 khz RBW FSH3 300 khz FSH3 30 khz Measurements taken @ PARI (Rosman, NC). Spectrum analyzer ( ν=300 khz) at end of feedline -15.5 dbm in [30,85] MHz A4
Rural: 66-75 MHz @ 30 khz RBW 30 khz 300 khz TV Ch 4 Spectrum analyzer ( ν=300 khz) at end of feedline A4
Rural Measurement Setup (Spectrometer) Rosman, NC Antenna 2 m above ground Coax cable Custom Spectrometer PCI PC Custom-Built Fat Dipole Linear Power Detection 4 MHz Swept BW 610 Hz Channels Measured Sensitivity -100 db(mw/[30 khz]), i.e. -145 db(mw/hz) Sensitivity ~F am relative to ITU Rural model ~13 db improvement over urban measurement
Rural: 25-90 MHz
Rural: Spectragram
Rural: Zoom Around 38 MHz Most of the spectrum starts to look pretty good at resolutions below 1 khz! FSH3 30 khz PLFM 610 Hz Max Hold Integration Measurements taken @ PARI (Rosman, NC). Spectrum analyzer ( ν=300 khz) at end of feedline -15.5 dbm in [30,85] MHz A4
Conclusions Most 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 ITUdefined 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