Hi Bob
On Thursday, February 26, 2015 12:40 PM, bob wole <[email protected]> wrote:
> Hi, Before I start working on Op25 I want to know what kind of demodulator
> detector is op25> using? Correlation filters or frequency discriminator
> (quadrature demod) ? Which algos> for symbol timing and frequency offset,
> Equalization?
Any given P25 system might use one of two different, quasi-compatible
modulation families: so-called "C4FM" and "CQPSK". In practice we've yet to
find a single receiver algo that optimally receives both these two different
modulation types; accordingly, there are two different user-selectable
demodulator chains in OP25.
C4FM is conventional 4-level FSK at 4800 baud. The OP25 demodulator for this
flavor consists of an FM demodulator [quadrature_demod_cf(), or often just a
piece of hardware tapped after the FM discriminator stage] feeding into Frank's
FSK4 demod block for GNU Radio which is detailed here:
https://sites.google.com/site/radiorausch/GnuradioFourLevelFSK.html
CQPSK is a variation of PI/4 DQPSK in which the transmitter power is not
constant (as it would be with C4FM). In CQPSK the carrier amplitude may be
reduced to virtually zero halfway between each successive data symbol. OP25
uses an analog Costas/Gardner loop followed by a differential demodulator for
this modulation format. Here is a page I put together some time back with some
more technical info about the waveform
http://www.lightlink.com/mhp/demod/
As to your question about equalization, that's a very interesting question and
it's one of the things that would be very nice to add (in a world where
infinite time existed, it might already be). One of the complications with
this is the use of P25 "LSM" simulcast, in which the same signal is sent over
multiple transmitters at the same time. This makes it very challenging to come
up with a plausible-looking model for the channel. In addition, we suspect
that some flavors of LSM contain a slow signalling side channel that appears to
be used for dynamic equalization training, but it's undocumented and
proprietary...
Max
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