Behavior of different protocol signals in a single system - RF Cafe Forums
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Post subject: Behavior of different protocol signals in a single system
Posted: Sun Apr 15, 2007 12:48 pm
Joined: Sat Sep 18, 2004 7:27 pm
I have a in-building antenna system where I have threee different Wireless Providers. Two of the providers are GSM based and the third is a CDMA provider. The system is designed to provide a ubiquitous -85 dBm or better coverage in the coverage area. The power per carrier levels for each of the three providers is +4 dBm for each RF carrier. When the two GSM Providers came on the system at the said power levels for eac of their RF carriers, the RSSI measurements matched the design predictions. However, for the CDMA Provider the RSSI levels in the coverage area are atleast 10 dB or more lower than the GSM levels. And note that all are coming into my antenna system at the same power level per RF carrier. Also, all operate in the same vicinity of the spectrum (1900 MHz). With this strange behavior, while the two GSM Providers are satisfied with the coverage provided, I am unable to provide the same level of coverage to the third provider.
Posted: Mon Apr 16, 2007 5:33 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
I guess you have done all the obvious things to eliminate strange component behaviour, like frequency sweep the combining/splitting system, the feeders and the antenna with a VNA for S11 and S21 over the entire band?
Also you need to check the vertical pattern of the antenna and see if, at the CDMA frequency, there is an excess up-tilt or down-tilt due to element phasing. On the other hand, perhaps you are using multiple distributed antennas around the building?
If nothing unusual comes from those investigations, then my first thought is this problem is due to unfortunate vector cancellation. If the average level across many parts of the building is consistently 10dB below expectation, however, this is unlikely to be the explanation. It would only present itself in certain areas but not in others.