Base Transceiver Station (BTS)

The BTS is the official name of the cell site. It is responsible for allocating
resources and both power and Walsh codes for consumption by the subscribers.
The BTS also has the physical radio equipment that is used for
transmitting and receiving the CDMA2000 signals.
The BTS controls the interface between the CDMA2000 network and the
subscriber unit. The BTS also 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 also must consider the radio configuration,
the subscriber type, and, of course, whether the service requested is
voice or packet. Thus, the resources the BTS has to draw upon can be both
physically and logically limited, depending on the particular situation
involved.
BTS can perform a downgrade from a higher RC or spreading rate to a
lower RC or spreading rate if
■ The resource request is not a handoff
■ The resource request is not available
■ Alternative resources are available
The following are some of the physical and logical resources the BTS
must allocate when assigning resources to a subscriber:
■ The Fundamental Channels (FCHs) (the number of physical resources
available)
■ The FCH forward power (the power already allocated and that which is
available)
■ The Walsh codes required (and those available)
The physical resources the BTS draws upon also involve the management
of the channel elements that are required for both voice and packet
data 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 Walsh code management,
covered in another section in more detail. However, for CDMA2000, phase
1, whether 1x, 1xEV-DO, or 1xEV-DV is used, a total of 128 Walsh codes can
be drawn upon.With the introduction of 3X, the Walsh codes are expanded
to a total of 256.
For CDMA2000 1x, the voice and data distribution is handled by parameters
set by the operator that involve
■ Data resources (percent of available resources, which includes FCH
and supplemental channel (SCH))
■ FCH resources (percent of data resources)
■ Voice resources (percent of total available resources)
This is best described by a brief example to help facilitate the issue of
resource allocation, as shown in Table 7-2.
Obviously, the allocation of data/FCH resources directly controls the
amount of simultaneous data users on a particular sector or cell site.