User's Manual

5 Radio Network Planning
APCD-LM043-4.0 61
the spectrum. Keep in mind that one of the major attractions of the ISM band is the fact that it
is license free; as such, it is shared spectrum. To regulate the band, regulatory bodies, such
as FCC and Industry Canada, require that new operators in the band take responsibility for
resolving interference issues when their newly installed system interferes with systems that
are already in operation. The spectral survey identifies systems that are operating in the ISM
band and establishes a documented baseline, which may provide you some protection from
future ISM-band installations that interfere with the operation of your system.
It cannot be overemphasized that radio communications is, by nature, a non-static
environment. As a wireless ISP, the more you know about the RF environment in which you
are operating, the better prepared you will be to address future service-affecting changes in
this environment. Given that the RF environment is dynamic, WaveRider recommends
performing spectral surveys on a regular basis, perhaps every 3-6 months.
5.2.3 In-band Interference
In-band interference occurs when other wireless systems are operating in the same band and
in the same geographical area as your system. The impact of in-band interference may be
limited—that is, the unwanted signal level may be so low as to have no impact at all, or it may
only affect service to a single end user or a small number of end users. In-band interference
may, however, be system wide, particularly if it is geographically dispersed around your
serving area or it is in close proximity to the CAP. System-wide interference obviously causes
the most impact to system operation since it affects all end-users in the serving area.
A primary purpose of the spectral survey is to identify in-band interference so that, if it is
present, the RF network design can address the interference sources through careful location
of the CAP, equipment configuration, and frequency selection, with the goal of maximizing the
ratio of the desired to the interfering signals throughout the serving area. If these measures
are not adequate, channel filters can in many cases reduce the interference to levels within
the operating tolerance of the LMS4000 radio equipment. Channel filters are discussed in
Using Bandpass Filters at CAP Sites on page 63.
5.2.4 Out-of-band Interference
The radio spectrum is a finite commodity, which in the growing world of wireless
communications, means that all users must compete for this limited resource. The implication
is that throughout the service life of your LMS4000 system, you need to be aware of your “RF
neighbors” and the impact they may have on your system operation and performance. As
described in Physical Layer (DSSS Radio) on page 28, the LMS4000 900MHz product
operates in the 902–928MHz ISM band. In many areas of the world, including North and
South America, the 900MHz ISM band is sandwiched between the top end of the cellular radio
band and the bottom end of the commercial paging band.
Cellular radio and paging systems are common in many regions, so you must take precautions
when planning your LMS4000 900MHz radio network. Specifically, you need to know the
location of all cellular and paging transmitters that are in, close by, or planned for, your serving
area, so that you can limit the impact of these potential interferers through proper site location,
equipment configuration, and frequency selection.