CDMA Call Processing

The following is an example of a cellular call: set-up, progress and termination in a CDMA system. The example describes:

•Initial system access •Call initiation and setup, •Soft Handoff, and Call termination.

Basic CDMA Network Architecture

Messages in CDMA

Initial System Access

When the mobile is first turns on, it must find the best base station with which to communicate. The mobile unit tunes its receiver to a specified ``primary'' CDMA carrier frequency

The mobile then scans for available pilot signals, which are all on different time offsets of the same PN short (2 ^ 15 chips) code

The mobile selects the strongest pilot sequence and establishes frequency and time reference with this signal. If the mobile does not detect any pilot signals of adequate strength, the unit tunes its receive to another specified CDMA carrier frequency

The mobile then demodulates the sync channel which is always transmitted on Walsh 32. The Sync Channel provides master clock information by sending the state of the 42 bit shift register, which generates the long (2 ^ 42 chips) code, 320 ms in the future. The long code, generated in conjunction with a private user mask, is used for encryption and decryption

The mobile then starts listening to the paging channel and waits for a page directed to its phone number.

Find Strongest Pilot, Read Sync Channel

Pnone Operation on Access Channel

Registration

Call Initiation, Setup and Termination

The mobile user when decides to make a call, enters the desired phone number. This initiates an access probe. The mobile uses the access channel and attempts to contact the serving base station

Since no traffic channel has been established, the mobile uses open loop power control. Multiple tries are allowed at random times to avoid collisions that can occur on the access channel. Each successive attempt is made at a higher power level. After each attempt, the mobile listens to the paging channel for a response from the base stations.

Once the access request has been received by the base station, the base station responds with an assignment to a traffic channel (Walsh code). The base station initiates the land link, and conversation takes place.

Call termination can be initiated either from the mobile or the land side. In either case the transmissions are stopped, the Walsh code is freed, and the land line connection is broken. The mobile unit resumes monitoring the page channel of the current serving cell.

Outgoing Call

Receiving Incoming Call

Ending Call