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Post subject: Relation between # of RF carriers and composite power
Posted: Tue Apr 10, 2007 11:51 am
Joined: Sat Sep 18, 2004 7:27 pm
Is there a rule of thumb for calculating the composite power as the number of RF carriers is doubled? I have heard that it is approximately 2 dB increase when the # of carriers is doubled. Say, equally powered carriers at 4 dBm each. When the # of carriers is 2 is the composite power = 4+2=6 dBm and when # of carriers is 4, then is the composite power = 6+2=8 dBm and like that
Also, is it protocol dependent? For example, is the 2 dB rule true for CDMA carriers only and not the GSM/EDGE based carriers?
The thing is that I can measure this in the field but I would have to take down a live system just for the measurements. If someone has already come across this, that would greatly help.
Posted: Mon Apr 16, 2007 5:19 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
If the system were to be perfectly linear, then doubling the number of carriers would add 3dB, so 2 x 4dBm carriers would produce 7dBm total power. I guess the rule of thumb is warning of 2 mechanisms in practice that reduce the power:
1) the combining component will itself add some loss into the system (say 0.5dB)
2)that due to intermodulation the power at f1 and at f2 (the frequencies of the two carriers) is reduced a tad (say another 0.5dB) because some power from each carrier contributes to the 2*f1+f2, 2*f1-f2, 2*f2+f1 and 2*f2-f1 (among others) unwanted products of adding the carriers in a non-linear system.