User's Manual
3.3.3 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 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 Access Point.
For example, Figure 3-3, below, illustrates a 4-sector system that utilizes the ABAB
downstream frequency pattern for one
downstream logical channel, which is split
ultimately to all four sectors. This system has an aggregate raw downstream
capacity of 31 Mbps downstream using 64QAM modulation.
The same Figure 3-3 has two Upstream air frequencies in an XYXY pattern which
can be mapped into one or two upstream cable channels discussed in the next
section.
4-port cable tap
Trans/Ant 1
Trans/Ant 2
Trans/Ant 3
Trans/Ant 4
Fa, Fx
Fb, Fy
Fa, Fx
Fb, Fy
Cable system
Figure 3-3: Four-Sector ABAB System with 1 downstream data stream.
3.4 Frequency Planning – Upstream
This Section discusses several issues related to planning the use of the frequency
band for Upstream communications.
3.4.1 Upstream Logical Channels
It is important to recall that any given downstream logical channel may have a large
number of upstream logical channels associated with it, and that any given upstream
ARCXtend manual, August 2003 3-7