Reverse CDMA Channel

The cell site contiguously monitors the reverse access channel to receive
any message that the subscriber unit might send to the cell site during the
system access state. The reverse CDMA channel consists of an access channel
and the traffic channel. The access channel provides communication
from the mobile to the cell site when the subscriber unit is not utilizing a
traffic channel. One access channel is paired with a paging channel and
each access channel has its own PN code. The mobile responds to the cell
sites messages sent on the paging channel by utilizing the access channel.
The forward and reverse control channels utilize a similar control structure
that can vary from 9600, 4800, 2400, or 1200 bps, which enables the
cell or mobile to alter the channel rate dynamically to adjust for the
speaker. When a pause occurs in the speech, the channel rate decreases so to reduce the amount of energy received by the CDMA system, increasing
the overall system capacity.

Four basic types of control messages are used on the traffic channel. The
four messages involve messages that will control the call itself, handoff messages,
power control, security, and authentication.
CDMA power control is fundamentally different than that utilized for
AMPS or IS-54. The primary difference is that the proper control of total
power coming into the cell site, if limited properly, will increase the traffichandling
capability of that cell site. As more energy is received by the cell
site, its traffic-handling capabilities will be reduced unless it is able to
reduce the power coming into it.

The forward traffic power control is composed of two distinct parts. The
first part is the cell site, which will estimate the forward links transmission
loss, utilizing the mobile subscribers’ received power during the access
process. Based on he estimated forward link path loss, the cell site will
adjust the initial digital gain for each of the traffic channels. The second
part of the power control involves the cell site making periodic adjustments
to the digital gain, which is done in concert with the subscriber unit.
The reverse traffic channel signals arriving at the cell site vary significantly
and require a different algorithm to be used than that of the forward
traffic power control. The reverse channel also has two distinct elements
used for making power adjustments. The two elements are the open loop
estimate of the transmit power, which is performed solely by the subscriber
unit without any feedback from the cell site itself. The second element is the
closed loop correction for these errors in the estimation of the transmit
power. The power control subchannel is continuously transmitted on the
forward traffic channel every 1.25 ms, instructing the mobile to either
power up or power down, which affects the mean power output level. A total
of 16 different power control positions are available.

Table 3-6 illustrates the CDMA subscriber power levels available by station
class.