User's Manual

4.2.2 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. 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. 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 base station – will receive downstream signals from both adjacent Hub
Transceivers. This will cause unacceptable interference if both Hub Transceivers are
transmitting on the same frequency, even though the subscriber is receiving nominally the same
signal from both Hub Transceivers.
The solution for this is to ensure that adjacent Hub Transceivers are never transmitting on the
same frequency. Two downstream frequencies (A & B) are required for an omnidirectional
system employing 4x90-degree Hub Transceivers. Figure 4-2 shows the recommended ABAB
pattern.
A
A
B
B
Omnidirectional
Hub
4x90-sectors
Figure 4-2: Downstream Channel Example of ABAB.
4.2.3 Downstream Logical Channels
It is important to note the distinction between “downstream frequencies” and “downstream
logical channels”. The “downstream frequencies” (A & B) discussed above refer to the actual
RF carrier frequencies transmitted over the air. “Downstream logical channel” refers to the
downstream data stream at the output of a base station W-CMTS downstream module. In the
examples here, the frequencies employed may be driven from one to two downstream logical
channels. The difference lies in the traffic capacity of the Hub. For example, Figure 4-3, below,
June 2003 Page 4-4