User's Manual

pulsAR radio Operator’s Manual
2-8
2.4 Radio co-location and interference
2.4.1 Radio co-location
As a network grows it often becomes necessary to deploy multiple radios at the same site. The
reasons to co-locate radios include the following:
1. In a Point-to-Multipoint or Tree network you want to achieve 360-degree coverage around a
central site, but would like to use sector antennas rather than one omni. Sector antennas have
higher gain than the omni and provide shielding from interfering signals originating at different
sectors. In this situation you might deploy a central site with six hub radios for example, each
one feeding a sector antenna covering 60-degree sectors.
2. The number of remote radios serviced by a single hub has grown to a point where the shared
bandwidth is no longer adequate. You may then add a second hub radio operating on a different
channel and split the remotes between two or more hubs.
3. You want to deploy two hubs to provide redundancy at the central site.
4. You want to deploy a repeater site with two “back to back” radios.
The problem is that when you co-locate two or more radios they can become the source of self-
interference, even if they are set to non-overlapping channels. The reason for this is explained in the
following section.
2.4.2 Co-located radios self-interference
The self-interference situation is illustrated in Figure 2.4, that shows radio A transmitting on channel
f1 while a co-located radio is trying to receive on channel f2. Because the antennas are in close
proximity antenna B will pick up a significant portion of the signal transmitted by radio A.
Figure 2.4 also shows a block diagram of the radio front end circuitry. It includes an RF filter to
reject out-of-band signals, followed by a Low Noise Amplifier (LNA), a second RF filter, Mixer and
finally the Intermediate Frequency (IF) filter. Channel selection occurs at the Intermediate Frequency
(IF), where the narrow band IF filter blocks out the other channels. This means that if the interferer
(radio A) is in close proximity, and is transmitting while radio B is trying to receive, it may saturate
the LNA or the Mixer of radio B. This results in radio B making errors even when it is set to a
different channel than radio A.