Forward CDMA Channel

The forward CDMA channel, shown in Figure 3-25, consists of the pilot
channel, one sync channel, up to seven paging channels, and potentially 64
traffic channels. The cell site transmits the pilot and sync channels for the
mobile to use when acquiring and synchronizing with the CDMA system.
When this occurs, the mobile is in the mobile station initiation state. The
paging channel also transmitted by the cell site is used by the subscriber
unit to monitor and receive messages that might be sent to it during the
mobile station idle state or system access state.

The pilot channel is continuously transmitted by the cell site. Each cell
site utilizes a time offset for the pilot channel to uniquely identify the forward CDMA channel to the mobile unit. The cell site can utilize a possible
512 different time offset values. If multiple CDMA channels are assigned to
a cell site, the cell will still utilize only one time offset value, which is utilized
during the handoff process.

The sync channel is a forward channel that is used during the system
acquisition phase. Once the mobile acquires the system, it will not normally
reuse the synch channel until it powers on again. The sync channel provides
the mobile with the timing and system configuration information. The sync
channel utilizes the same spreading code, time offset, as the pilot channel
for the same cell site. The sync channel frame is the same length as the pilot
PN sequence. The information sent on the sync channel is the paging channel
rate and the time of the base station’s pilot PN sequence with respect to
the system time.

The cell site utilizes the paging channel to send overhead information
and subscriber-specific information. The cellsite will transmit at the minimum
one paging channel for each supported CDMA channel that has a
synch channel.

Once the mobile has obtained the paging information from the sync
channel, the mobile will adjust its timing and begin monitoring the paging
channel; each mobile, however, only monitors a single paging channel. The
paging channel conveys four basic types of information. The first set of
information conveyed by the paging channel is the overhead information.
The overhead information conveys the system’s configuration by sending
the system and access parameter messages, the neighbor lists, and CDMA
channel list messages.

Paging is another message type sent when a mobile unit is paged by the
cell site for a land-to-mobile or mobile-to-mobile call. The channel assignment
messages allow the base stations to assign a mobile to the traffic
channel, alter the paging channel assignment, or redirect the mobile to utilize
the analog FM system.

The forward traffic channel is used for the transmission of primary or
signaling traffic to a specific subscriber unit during the duration of the call.
The forward traffic channel also transmits the power control information on
a subchannel continuously as part of the closed loop system. The forward
traffic channel will also support the transmission of information at 9600,
4800, or 1200 bps, utilizing a variable rate that is selected on a frame-byframe
basis, but the modulation symbol rate remains constant.