[op25-dev] Re: P25 Transmission

ikj1234i ikj1234i at yahoo.com
Fri Dec 14 14:28:45 UTC 2012



--- In op25-dev at yahoogroups.com, "rosin.f at ..." <rosin.f at ...> wrote:

> - Why is the channel spacing arbitrarily 25 kHz (just a SW limitation ?)

No reason for this, it's just behavior inherited from the previous iteration of the p25 transmitter.  Would be simple to add code to transmit on specific, discrete frequencies (the selected frequencies would have to fall within the USRP limits, of course).

Please note that the transmitter also supports the use of external hardware to do the actual transmission (that is, standard NBFM transmitter hardware), although this is a difficult problem and we don't have a lot of experience with it.   Nonetheless in that case the frequencies would be determined by the external hardware, not the software.

> - Are you transmitting the P25 air interface for voice ?
> 

Nominally yes, the transmissions should be compliant with the P25 Common Air Interface (CAI).  Early testers found a few nitpicks though, most notably the bit settings for the MI fields weren't being set properly to indicate the proper alg ID.  In addition we don't currently prepend HDU's to each transmission.  

All of the above are minor omissions and would not take a lot of effort to add.

In addition although we might say that nominally it's transmitting the CAI it has never been certified or even tested for any regulatory compliance, etc...

Best

Max

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