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IMBE flavors - INMARSAT vs. P25

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Feb 24, 2001
Someone asked me this ... thought I'd share the response here.
casey said:
I was doing some reading about satellite communcations and I read that Inmarsat use IMBE for their digtal transmissions. One website I found said:
INMARSAT-IMBE Improved Multi-Band Excitation Coding (IMBE) 4.15 kbit/s

I can't tell if that number is the bare IMBE bits - which requires a perfect noise-free line or the larger FEC (forward error correction) encoded version.

How does that compare to the IMBE coding we are familiar with in P25 systems?
Off the top of my head ...

One P25 IMBE voice data cell is 88 bits and carries 20ms (that's 0.02 seconds) of speech. You need 50 cells to fill a whole second of audio. That works out to 4400 bits per second. If you add the Golay and Reed-Solomon encoding for error correction - the cell size grows to 144 bits. P25 groups 9 cells into a single frame. The frames require 48 bits for syncronization and 64 bits to provide some protection from break-through (skip). FWIW, ProVoice groups 4 cells into a single frame w/ 64 bits for syncronization.

While P25 uses a fixed bit-rate - the chips and/or DSP code licensed from DVSI can provide variable compression rates - allows finding the right trade-off between bit-rate and speech quality. They don't have to be limited to the format described above.

Do you think a downconverter that allowed the P25 scanner to "receive" in that band would decode anything?
No, (putting forth my best guess) in all likelihood ... (a) the actual analog signal is modulated differently and (b) the recovered bits are arranged differently.
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