Isolation

Isolation is used to describe the amount of attenuation needed between the
source, transmitter, and the victim or receiver. All wireless communication
systems require some level of isolation between their own transmitters and
other transmitters, and their receivers at the base station. The fact that you
are using a pico versus a macro-cell site does not mean that more or less isolation
is required.
The amount of isolation needed for communication systems is dependent
upon a multitude of issues:
■ Location of potential offending transmitter to receiver
■ Technology platform utilized
■ Receiver sensitivity
The methods that follow are based on the simple fact that there is no
defective equipment, or there are not out-of-specification transmitters at
the location in question. Please keep in mind that the isolation requirements
may or may not be directly applicable to the communication facilities
that are collocated with you. As often is the case, the offending transmitter
is several buildings away.
Isolation can be achieved, once the offender(s) is identified, through
antenna placement using both horizontal and vertical separation. Another
method could be achieved through more selective filters. Just how much isolation
is needed?
An example of how to determine the amount of isolation needed for a
communication system is shown in Table 14.1.
Tx
852 MHz
Rx
849 MHz

Therefore 80 dB isolation is achieved with 10 feet of vertical separation,
which is sufficient to prevent compression. The previous example is for
cases where out-of-band emissions are the problem. When the problem is
intermodulation-related, it is possible to obtain the necessary isolation to
protect the receiver, if the mix is occurring at another location besides in the
receive path itself, through simple path loss alone.
Additionally, what is not discussed is the impact of spectral regrowth of
the transmitter into the receive band, which can only be resolved through
better transmit filtering at the expense of increased insertion loss or size for
the base station.