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RF carriers separation calculation (gsm)

S

Steve Sousa

Jan 1, 1970
0
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
 
J

JosephKK

Jan 1, 1970
0
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
You might start here:

http://www.3gpp.org/
 
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