With neat diagram, explain reverse CDMA channel

The figure below shows the CDMA reverse link structure.

The reverse link is separated from the forward link by 45 MHz. It is used by all the mobiles in the cell coverage area to transmit to the base station. The reverse channel as it appears at the base station receiver is a composite of all the outputs from all the mobiles in the base station’s coverage area.

The reverse channel structure allows a maximum of 94 channels which include a maximum of

  • 32 different Access channels and
  • 62 different Traffic channels

Access Channels

  • It is used by the mobile to transmit control information(non traffic information) to the base station,
    1. When it needs to initiate an action (eg. Registration, call origination) or
    2. When it needs to respond to messages received on a paging channel.
  • Since multiple mobiles may attempt access at the same time, the access channel utilises some form of slotted random access protocol for contention management.
  • It uses a data rate of 4800 bps.
  • The number of access channels in a cell is configurable – generally one or two access channels for each paging channel.
  • Each access channel is identified by a distinct access channel long code sequence having an access number, a paging channels number associated with the access channel, and other system data.
  • The messages carried by the access channel include:
    1. Registration message
    2. Order message
    3. Data burst message
    4. Origination message
    5. Page response message
    6. Authentication challenge response message

Reverse Traffic Channels

  • Information on the traffic channel includes the primary traffic (voice), secondary traffic (data), and signalling in frames 20 ms in length.
  • Signalling information during a call is transmitted using blank and burst or dim and burst mode.
  • Each reverse traffic channel is paired with a corresponding traffic channel.
  • When the reverse traffic channel is used for signalling, the following are some of the typical messages that can be sent:
    1. Order messages
    2. Authentication challenge response message
    3. Data burst message
    4. Pilot strength measurement message
    5. Power measurement report message
    6. Send burst DTMF message
    7. Handoff completion message
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