If You Have An Interferer - WaveRider LMS4000 User Manual

12.3

If You Have an Interferer

The presence of an interferer can cause a variety of performance problems in the radio
network. These problems can be quite difficult to positively identify and track down. Typically,
the presence of an interferer is first identified by eliminating other potential causes of the
observed symptoms.
Interferer problems can be inconsistent, in both the significance of the effect, as well as the
duration, since interferers are frequently intermittent.
There are two general types of interferers that have to be addressed — those that share the
ISM band with the LMS4000 900MHz Radio Network, and those that operate adjacent to the
ISM band.
The interferers within the ISM band almost always affect individual EUMs since they are
generally associated with in-home devices such as cordless phones, baby monitors, and other
consumer-oriented ISM band devices. However, there are other outdoor 900 MHz ISM band
products. These are typically frequency-hopping spread-spectrum devices that can cover a
wide area; therefore, they could impact EUMs and/or the CCU directly. For these devices, it is
unlikely that all EUMs would be impacted unless the CCU is directly affected.
Interferers from outside the ISM band include paging signals, which are high power,
narrowband transmissions between 929 and 931MHz (above the 900MHz ISM band), and
cellular transmissions that extend up to 896MHz (just below the 900MHz ISM band). For CCU
radio networks working at the limits of the ISM band, these out-of-band signals can result in a
serious desensitization of the CCU receiver. The RF Planning section of this User Guide
explains how the frequency planning process must take these signals into account when
planning the CCU radio network and how filters are used at the CCU to provide enhanced
isolation. If, however, the transmitter sites for these signals are moved after the original CCU
design, then the impact can be immediate and significant, frequently requiring that the CCU
frequency plan be adjusted. Fortunately, once this problem is diagnosed, changing the
frequency plan is relatively straightforward.
In addition to ensuring that the CCU is not impacted by these out-of-band emissions, EUMs
can also be impacted if the antenna field of view of the EUMs looks directly at these
transmitters. Again, diligent planning is essential.
Interference troubleshooting is divided into two main categories:
• EUM-specific (only one EUM is affected, or a group of EUMs in a small area),
suggesting an interferer near these EUMs.
• Many, or all EUMS are affected, suggesting an interferer that is most likely impacting
the CCU directly.
As mentioned earlier, the typical impact of an interferer is to effectively increase the noise level
at the CCU or EUM, which causes an increased receive error rate. Modems in a CCU network
that operate at a lower signal level are more vulnerable to interference, and this provides a tool
that can be used to diagnose the problem. For example, if EUMs that normally operate at
lower signal levels are seeing higher error levels than those that operate at higher signal
levels, then you would suspect an interferer affecting the CCU.
Measuring the level of interference is difficult, unless you have access to a spectrum analyzer
and are prepared to shut down the system. In severe cases this may be required as a last
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