Once a call is established, the mobile is constantly searching for other possible cell sites that might be good candidate for soft handoffs. Most of the search time is limited to looking for those PN offsets specified in the neighbor list found on the paging channel.

The rake receiver’s searcher is the device that scans for other possible cell sites. If the mobile identifies a pilot that exceeds the T_add threshold defined by the cell site, it alerts the base station by sending a pilot strength message.

The pilot strength message is sent on the traffic channel using either dim and burst or blank and burst signaling. This is the action that initiates a soft handoff. When the pilot strength message is received, the base station passes this request to the MTSO (Mobile Telephone Switching Office).

If the MTSO has an available channel card, it passes the request to the second base station to see if a traffic channel is available for the soft handoff request. A second channel card is required at the switch for a soft handoff to allow the MTSO to select the better of the two signals being returned from the two cell sites involved in the soft handoff.

In Short,

  • Mobile Finds Second Pilot of Sufficient Power (exceeds T_add Threshold)
  • Mobile Sends Pilot Strength Message to First Base Station
  • Base Station Notifies MTSO
  • MTSO Requests New Walsh Assignment from Second Base Station
  • If Available, New Walsh Channel Info is Relayed to First Base Station