In general, Passive Intermodulation
(PIM) in any device is present as a result of the use of wrong materials
, physical design and/or assembly of the device. High Levels of PIM are
present in the BTS RF interconnection generally as the result of poor
quality system techniques.Until now it has been
difficult to test for PIM on-site and therefore it has not been widely
considered as a practical QC (quality certification) parameter. It is
however absolutely necessary.
RF interconnection component suppliers are required to
meet high levels of PIM during manufacture (generally close to, or
better than 110 dBm) to remove the possibility of significant internally
generated, receiver channel, in band interference. System PIM
performance is not an average of all the interconnected components.
Rather it is the direct result of the worst physical connection in the
overall system. (In this case in the RF interconnection.)
At Triasx, our experience supports the value of
quality certifying the RF interconnection. In the past, PIM interference
has been managed by selecting a combination of frequency bands that
ensure any likely PIM interference will fall outside the receiver bands.
Where this has not been possible, channels affected by this internal
interference were not used.
The demand for more cellular bandwidth, more services
and greater capacity, combined with the increasing difficulty and cost
of introducing new sites, is driving a trend to collocate technologies.
This can be a very cost effective way of meeting subscriber demands,
while at the same time lowering the average cost of service. However a
poor quality RF interconnection will make this option impossible to
implement, particularly in the spread spectrum CDMA environment where
the presence of PIM can mean much more than the loss of a channel or
two.
The consequence of accepting a poor quality RF
interconnection installation is likely to be a higher than necessary
cost of service due to one or more of the following:
