Here’s some theoretical calculations of FM iBiquity performance under Part 15 conditions.
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Reception
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2006 NPR testing indicated real-world reception certainties:
95% reception at 67 dBuV
85% reception at 60 dBuV
Concurrent operating rules provided for digital broadcasting at 1% ERP. The reception thresholds at elevated power levels are thus expected to be
10% ERP:
95% at 57 dBuV
85% at 50 dBuV
100% ERP:
95% at 47 dBuV
85% at 40 dBuV
dBm = dBuV – 90 – 10 * log (Z), [Z ~ 376.73031 Ω]
dBm ~ dBuV – 115.76
dBm table
( 1% 10% 100% ERP)
95%: -49 -59 -69
85%: -56 -66 -76
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Transmission
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Part 15 rules allow for transmission on the FM band at field strength levels up to 250 uV / m at a distance of 3 meters.
(250 uV / m) ^2
————— = 166 * 10 ^-12 W/m^2
376.73031 Ω
166 * 10^-12 W/m^2 from spherical radiator * 4*pi*(3m)^2 = 18.8 * 10^-9 W
Assuming an antenna aperture of roughly 1 m^2 (an approximation on the FM band, where Aeff for a short dipole ranges between 1.39 and 0.92 m^2), and a characteristic impedance of free space ~ 376.73031 ohms,
This gives a signal level of -67.8 dBm at 3 meters.
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Data theory
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Hypothesis: Original study recieved 100 kb/s channels.
Expected process gain: ~3 dB per halving. Signal modes available in iBiquity determined from Wikipedia
100 kb/s ( 0 dB)
50 kb/s (+3 dB)
25 kb/s (+6 dB) -standard all-digital fallback mode
12.5 kb/s (+9 dB)
5 kb/s (+13 dB)
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Ideal link budget
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Maximum transmission power at lowest frequency (88 MHz), power received by short dipole with 1.39 m^2 effective area.
(3m 10m 30m 100m)
dBm -66 -77 -86 -97
Maximum Process Gain +13 dB
(3m 10m 30m 100m)
dBm(eff) -53 -64 -73 -84
Compare with signal thresholds -69, -76 dBm: Process gain makes it possible to get an 85% receivable signal at 40-50 meters.
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Conclusion
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Without any additional process gain (increased redundancy, etc) performance of an iBiquity station is not expected to be substantially “better” than with analog FM. With the use of low-bitrate channels over all-digital modulation, it may be possible for a Part 15 station to have a reception radius of something like 40 meters.
Bear in mind that this is a theoretical inquiry based upon publicly available data. I should point out at least two potential sources of error. First, the signal thresholds are based on the single NPR study, which itself showed somewhat significant variation in signal detection depending on which station they tried to receive. A wider basis of signal reception data is probably available and I’d appreciate links to take a peek. Secondly, it’s been a while since I’ve done RF work so my math may be a bit off, I’d appreciate any corrections.
Finally, though it is used as a public broadcast standard in the United States, the HD Radio standards are not public information. This makes amateur experimentation difficult.