Traffic Model

The capacity for a CDMA2000 cell site is determined through the interaction
of several parameters and is driven by the radio access portion of the
system, provided the fixed network has the requite number of modules for
each platform. The parameters for determining the traffic load at
CDMA2000 site are similar to those used for an IS-95 system with the
exception that CDMA2000 introduces packet data and the inclusion of
128/256 Walsh code, to mention a few of the previously covered issues.
As with IS-95 base stations, the use of channel element cards is essential
for the handling of traffic whether it is for voice or data. The desired result
from traffic engineering for a CDMA2000 base station is to be able to determine
the amount of channel elements and cards required to support the
expected traffic. Another factor that fits into the traffic calculations for the site involves system noise. There is a simple relationship between system
noise and the capacity of the cell site. Typically the load of the cell site
design is somewhere in the vicinity of 40 to 50 percent of the pole capacity,
maximum 75 percent.
The next major element in determining the capacity for a CDMA cell is
the soft and softer handoff factor. Because CDMA2000, like IS-95, relies on
soft and softer handoffs as part of the fundamental design for the network,
this must also be factored into the usable capacity at the site. The reason for
factoring soft and softer handoffs into capacity is that if 35 percent of the
calls are in a soft handoff mode, then this will require more channel elements
to be installed at the neighboring cell sites to keep the capacity at the
desired levels.
The pole capacity for CDMA is the theoretical maximum number of
simultaneous users that can coexist on a single CDMA carrier. However at
the pole, the system will become unstable and therefore, operating at less
than 100 percent of the pole capacity is the desired method of operation.
Typically the design is for 50 percent of the pole capacity for the site.
However, because soft handoffs are an integral part of CDMA, they need
to be also included in the calculation for capacity. In addition for each traffic
channel that is assigned for the site, a corresponding piece of hardware
is needed at the cell site also.
The actual traffic channels for a cell site is determined using the following
equation:
Actual traffic channels
(effective traffic channels
 soft handoff channels)
The maximum capacity for a CDMA cell site should be 75 percent of the
theoretical limit. Unlike IS-95 systems that were power limited, the
CDMA2000 system is anticipated to be Walsh code limited.