User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–44
Original 11/97
CDMA Channel List Message: The CDMA channel list reports the number of
CDMA frequencies supported by the cell station in use as well as surrounding cell
site frequencies and configurations.
Slotted Page Message: The Slotted and Non–slotted page messages allow the
cell site to page CDMA phones for incoming calls. CDMA mobiles operating in the
slotted mode must first register with the cell site before they can be paged. This
registration is required to establish which slot will be used by the cell site to transmit
the page to the mobile.
Channel Assignment Message: The channel assignment message is used to
communicate the information needed to get the mobile onto a traffic channel.
CDMA Call Initiation
When a user keys in a phone number and hits the send key the mobile sends out an
“Access Probe”. The “Access Channel” is one of the two channels used by a
mobile, it is used by the mobile to initiate calls, the other channel is the Traffic
channel. The difference between the two channels is in the coding. The Access
Channel applies a mask to the Long Code that is derived from information received
from the Sync and Paging channels: the information is; paging channel number,
access channel number, base station ID, and the Pilot PN offset used by the base
station. A new sub–subject comes up at this point, Power Control, which will be
discussed in the next sub–topics. Since a two–way link has not been established
yet, open loop power control will be used by the mobile to set its transmitter output
power. Multiple tries are allowed with random times between tries to prevent two
mobiles from consistently transmitting access probes at the same time.
Reverse Link Open Loop Power Control
The key to maximizing CDMA capacity is power control. The limiting factor for
CDMA system capacity is total interference. Total interference can best be
described as all of the unwanted signals a base station receives. These signals
include other CDMA signals, natural back ground noise, and man made interference
such as noisy power lines. Ideally a base station would receive all the mobiles
signals at the same level. If the mobiles transmit a stronger signal than necessary
then more interference would be created and capacity would drop. When a CDMA
mobile first tries to contact a base station, open loop power control is used. Open
loop power control sets the sum of transmit and receive power to a constant, –73
dBm. The formula is; Transmit Power = (–73) – (Receive Power): all units are in
dBm. If a mobile received a base stations signal at –85 dBm the mobiles transmit
power would be (–73 dBm) – (–85 dBm) = +12 dBm transmit power.
The mobile’s Open Loop power control slew rate is limited to match the slew rate of
closed loop power control. If not limited the mobiles power output could swing
wildly during sudden reverse link signal strength changes.
A third point must be made before leaving Reverse Link Open Loop Power Control.
Suppose the CDMA mobile is traveling between two base stations, one has a large
area to cover and transmits signals at high output power.