BTS

The BTS controls the interface between the CDMA2000 network and the
subscriber unit. The BTS controls many aspects of the system that are
directly related to the performance of the network. Some of the items the
BTS controls are the multiple carriers that operate from the site, the forward
power (allocated for traffic, overhead, and soft handoffs), and of course
the assignment of the Walsh codes.
With CDMA2000 systems, the use of multiple carriers per sector as with
IS-95 systems is possible. Therefore when a new voice or packet session is
initiated, the BTS must decide how to best assign the subscriber unit to
meet the services being delivered. The BTS in the decision process not only
examines the service requested, but it also must consider the radio configuration
and the subscriber type, and, of course, whether the service
requested is voice or packet. Therefore the resources the BTS has to draw
upon can be both physically- and logically-limited depending on the particular
situation involved.

The following is a brief summary of some of the physical and logical
resources the BTS must allocate when assigning resources to a subscriber:
■ FCH Number of physical resources available
■ FCH forward power Power already allocated and that which is
available
■ Walsh codes required and those available
■ Total FCHs used in that sector
The physical resources the BTS draws upon also involves the management
of the channel elements that are required for both voice- and packetdata
services. Although discussed in more detail, handoffs are accepted or
rejected on the basis of available power only.
Integral to the resource assignment scheme is the Walsh code management,
covered in another section in more detail. However, for 1XRTT,
whether 1x, 1xDO, or 1xDV, there are a total of 128 Walsh codes to draw
upon. However, with the introduction of 3X, the Walsh codes are expanded
to a total of 256.
For CDMA20001X, the voice and data distribution is handled by parameters
that are set by the operator that involve
■ Data resources Percent of available resources that includes FCH
and SCH
■ FCH resources Percent of data resources
■ Voice resources Percent of total available resources
These are best described by a brief example to help facilitate the issue of
resource allocation shown in Table 13-14.
Obviously the allocation of data/FCH resources directly controls the
amount of simultaneous data users on a particular sector or cell site.