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RF carriers separation calculation (gsm)

Hello:

The problem is what is the necessary channel separation to transmit several gsm signals on the same medium (direct intensity modulation on an optic fiber). I have searched all over books, google, I've read the etsi docs but i can't find it. I've even asked teachers and got nowhere! I'm looking for some sort of "guard-band" spec i can use, imagine the usual BW plot, a rectangle:

----------- ----------- | | | | | |

--------------------- how close? --------------------- -bw/2 fc bw/2 -bw/2 fc bw/2

Say i have a gsm signal with a BW of 24 MHz and carrier at 890-914. How close can i set another carrier? 866-890 seems too close, but i don't want to pull a number from the air and say "MHz between them is fine", there has to be some sort of calculation/rule/norm that applies, my understanding from ITU-R is that from 470 MHz to 2450 MHz transmitter tolerance for mobile stations is 500 KHz, but I'm not sure this is it. I know that the first channel is not used, so that would give 200 KHz. Or do i have to estimate some sort of filter crossing bw/2 at -3db and some maximum attenuation at some harmonic beyond that, like radio-amateurs use (something around -50db?) What if one is gsm and the other a different modulation?

Suggestions for a more appropriate group are welcome.

Thank you.

I know you'll ask, this is not homework but it is part of my studies, an extra-curricular activity that i now see might be way above my head.

Best Regards

Steve Sousa

Reply to
Steve Sousa
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Strange idea.

One GSM trunk is ~270kbit/s. Why can't you take it from the baseband and go digital?

The main question here is what is the maximum difference in the strength between the different channels. For the normal operation of the GSM, the SNR in a channel must be > 9dB. So the separation must be sufficient for the adjacent channel spillage (see GSM spectral mask) and the 3rd order IMD (due to the nonlinearity) from the stronger channels would not trash the weaker channels. This depends on the number of channels, the dynamic range, and the difference in the strength between the channels.

866-890 seems too close, but i don't

???????

Required rejection of spillage and IMD = difference between the strongest and the weakest channel + minimum SNR for normal operation.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

several=20

can't=20

usual=20

want=20

has=20

from=20

estimate=20

at=20

-50db?)

=20

You might start here:

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Reply to
JosephKK

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