User's Manual

3.3.2 Downstream Adjacent 90-degree Sectors
The 90-degree-Sector Transceiver has a half-power horizontal beam width of 90
degrees, i.e., 45 degrees each side of center, call the “bore sight” of the antenna.
But, the energy of the antenna does not simply cut off at 45 degrees in horizontal
pattern from the centerline of the antenna. Rather, the energy falls off as the angle
from the centerline increases. A typical antenna pattern is shown in Section on
Basic RF.
This means that a subscriber in the vicinity of 45 degrees clockwise from antenna A
will also be in the vicinity of 45 degrees counterclockwise from adjacent antenna B.
Subscribers in the overlap zone – especially if they are relatively close to the Access
Point – will receive downstream signals from both adjacent Access Point
Transceivers. This will cause unacceptable interference if both Access Point
Transceivers are transmitting on the same frequency, even though the subscriber is
receiving nominally the same signal from both Access Point Transceivers.
The solution for this is to ensure that adjacent Access Point Transceivers are never
transmitting on the same frequency. Two downstream frequencies (A & B) are
required for an omnidirectional system employing 4x90-degree Access Point
Transceivers. Figure 3-2 shows the recommended ABAB pattern.
A
A
B
B
Omnidirectional
Hub
4x90-sectors
Figure 3-2: Downstream Channel Example of ABAB.
ARCXtend manual, August 2003 3-6