CDMA Channel Organization

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CDMA Channel organization

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Code channels used in CDMA

A code channel is a stream of data designated for a specific use or person. This channel may be voice data or overhead control data. Channels are separated by codes.

The forward and reverse links use different types of channels.

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Forward link channels The forward link uses four types of

channels to transmit voice and control data to the mobile. The types of forward link channels are:

Pilot Sync Paging Traffic

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Pilot channel

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Pilot Channel The pilot channel is constantly transmitted. The mobile uses the pilot signal to acquire the

system. After the mobile has acquired the system the

pilot is used for signal strength measurement. The strength of the pilot is used to determine

the power required for mobile transmit. The pilot contains no information but the it is

the strongest signal on the forward link.

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Sync channel The BTS constantly transmits over the sync

channel so the mobile can synchronize with the BTS.

It provides the mobile with the system time and the identification number of the cell site.

The mobile ignores the sync channel after it is synchronized.

Used to acquire initial time synchronization.

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Sync Channel contd.., Sync message includes 15-bit system id (SID),

16-bit network id (NID), offset of PN short code and paging channel data rate (4.8/9.6 kbps).

Operates at 1200bps. Once the MS has recovered the information

from the sync channel and fully synchronised with the network, it enters the idle mode and awaits paging messages from the network.

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Paging channel CDMA uses up to seven paging

channels. The paging channel transmits overhead

information such as commands and pages to the mobile.

The paging channel also sends commands and traffic channel assignment during call set-up.

The mobile ignores the paging channel after a traffic channel is established

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Paging Channel

Used to page MS in case of incoming call

Also carries control messages for call setup

The rate is 4.8kbps or 9.6kbps

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Some of the control messages contained in paging channel are

System parameters: This contains general information including no of paging channels available at BTS, the handover thresholds and a number of registration parameters which allow MS to identify a particular location area

List of neighboring BTS, CDMA channel list etc.,

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Traffic Channels

Carry user information Two possible data rates

RS1 = {9.6, 4.8, 2.4, 1.2 kbps} RS2 = {14.4, 7.2, 3.6, 1.8 kbps}

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Forward link traffic channel

CDMA uses between fifty-five and sixty-one forward traffic channels to send both voice and overhead control data during a call.

Once the call is completed, the mobile tunes back in to the paging channel for commands and pages.

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Reverse link channels

The reverse link uses two types of channels to transmit voice and control data to the BTS. The types of reverse link channels are: Access Traffic

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Access channel The mobile uses the access channel

when not assigned to a traffic channel. The mobile uses the access channel to:

Register with the network Originate calls Respond to pages and commands from

the base station Transmit overhead messages to the

base station

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Reverse link traffic channel

The reverse traffic channel is only used when there is a call.

The reverse traffic channel transmits voice data to the BTS.

It also transmits the overhead control information during the call.

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Reverse Link traffic channel

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Call Processing

There are four modes of operation in call processing in CDMA technology as

Initialization mode Idle mode Access mode Traffic mode

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Initialization mode The subscriber unit performs initialization

when the phone is turned on. During initialization, the mobile:

searches for a usable pilot signal acquires the system via the Pilot code channel

synchronizes with the system by reading the Sync code channel broadcast from that site

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Idle mode The mobile is not involved in a call

during idle mode, but it must stay in communication with the base station.

The mobile and the base station communicate over the access and paging code channels

So in the Idle State the mobile is mainly listening to the Paging Channel for incoming messages. (Also monitors the strength of pilot channel in neighbouring cells)

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Access mode

The mobile accesses the network via the Access code channel during call origination.

The Access channel and Paging channel carry the required call set-up communication between the mobile phone and the BTS until a traffic channel is established.

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Traffic mode Once assigned to a Traffic Channel, the mobile

is no longer monitoring the Paging Channel. During a land to mobile (LTM) call:

The mobile receives a page on the paging channel. The mobile responds on the access channel. The traffic channel is established and maintained

throughout the call. During a mobile to land call (MTL):

The call is placed using the Access channel. The base station responds on the paging channel. The traffic channel is established and maintained

throughout the call.