User's Manual

5 Radio Network Planning
APCD-LM043-4.0 69
As an example, consider the frequency plan shown in Table 13.
Table 13 Sample Frequency Plan — Multi-CAP Design
In Table 13, Frequency Set A uses the minimum frequency spacing that should be considered
for a single CAP site, 6.6MHz. Frequency Set A’ represents a set of channels which are
interstitial to those in Frequency Set A. The channels in Frequency Set A’ fall midway between
the channels in Frequency Set A yet still adhere to the minimum recommended spacing
between any two colocated channels, 6.6MHz.
From Table 12, if two sites have a frequency separation of 3.4 MHz (Frequency Set A to
Frequency Set A', for example), a C/I signal margin of 11dB is required.
CAUTION: The concept of frequency reuse patterns, commonly
used in the design of cellular radio systems, cannot be directly
applied in the design of LMS4000 900MHz radio networks.
Instead, due to the nature of the Polling MAC, you should never
reuse frequencies in networks where a CCU or EUM can receive a
signal from a unit in another sector or coverage area. The
minimum channel separation cannot be less than 0.2MHz s a
minimum. When Polling MAC is applied in a multi-CAP
environment, it is possible for an EUM to inadvertently lock onto
the signal from a remote CCU if that CCU is operating on the
same frequency. This situation does not occur if the remote CCU
is offset by 0.2MHz or more from the local CCU, and the required
C/I ratio is maintained. In summary, no two CCUs in a single
network can be assigned exactly the same frequency .
5.3.4 Dealing with External Interference
Up to this point, the discussion has been concentrating on the effect of self-generated
interference—that is, interference between CAPs or EUMs in the same network.
As indicated in Basic System Design on page 60, you must also account for the effect of
external interferers such as cellular and paging systems. The RF system design engineer
needs to make sure external interference sources do not affect system operation. You can use
a similar treatment to the one developed above for self-generated interference to assess the
effects of external interference sources.
5.3.5 Verifying the Design
No matter how carefully the system has been designed, you must verify the system in the field
before turning it up to ensure network operation is consistent with the design standards set out
by the system design engineer. With this in mind, your system implementation plan must
Frequency Set A 905.0 - 911.6 - 918.4 - 925.0
Frequency Set A’ - 908.4 - 915.0 - 921.6 -