Traffic Model

The capacity for a CDMA cell site is driven by several issues. The first and
most obvious point for traffic modeling in a CDMA cell site involves how
many channel cards the cell site is configured with. A total of 55 possible
traffic channels are available for use at a CDMA cell site, but unless the
channel cards are installed, the full potential is not realizable utilizing IS-
95/J-STD-008 specifications.
Additionally, the other factor that fits into the traffic calculations for the
site involves system noise. A simple relationship exists 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,
with a maximum of 75 percent.

The third major element in determining the capacity of a CDMA cell is
the soft handoff factor. Since CDMA relies on soft 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 handoffs into capacity
is that if 33 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.
With CDMA, the capacity of the site is dynamic because as the system
noise floor is raised, the base station loading decreases. The specific capacity
for any CDMA base station is typically achieved through computer simulation
due to the dynamics of cell loading and interference levels, making
a pure traffic calculation on a spreadsheet rather impractical. However,
some rules of thumb should be followed for simple planning exercises that
do not require a computer simulation to run.
As stated earlier, a total of 64 Walsh codes are available. Typically, the
Walsh codes are allocated in the following manner:
Channel Type Number of Walsh Codes
Pilot 1
Synch 1
Paging 1—7
Traffic channels 55
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.
The effective traffic channels for a CDMA carrier are the number of
CDMA traffic channels needed to handle the expected traffic load. However,
since soft handoffs are an integral part of CDMA, they also need to be
included in the calculation for capacity. In addition to 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 are determined using the following
equation:
 soft handoff channels2
Actual traffic channels  1Effective traffic channels’

The maximum capacity for a CDMA cell site should be 75 percent of the
pole, but typical loading in IS-95 systems has found that the pole point is
really around 50 percent.
The physical limit for a CDMA system’s capacity is dictated by the
mutual interference driven by the forward channel. Therefore, the number
of users that can be placed onto an CDMA system at any time is limited by
mutual interference, which is directly related to power.
Looking at the pole point equation, it is obvious that it is unique for every
site since it is dependant upon the local situation at that site. Additionally,
due to the Eb/No factor, the cell can be allowed to degrade, allowing for the
soft capacity factor, which of course impacts the pole point, leading to more
dynamics and the need for computer simulation.
However, assuming the 50 percent pole point the following Erlangs of
offered traffic, using Erlang B, can be derived for an individual CDMA carrier
is shown in Figure 3-10.
The Channel Elements (CEs) are a pooled resource, and therefore equipping
a full compliment of CEs for all sectors to be used simultaneously is
not a practical approach. Instead it is typically recommended that only 95
percent of the CE’s estimate be installed for the cell.
When more than one carrier is in a sector, the capacity can be estimated.
In Table 3-10, it is assumed that the sector has two carriers; if more are in
that sector, then it is a matter of multiplication to arrive at the new traffic
levels since no trunking efficiency exists between CDMA carriers.