CDMA extends the idea of diversity reception with the concept of soft handoff. In the slide, a mobile CDMA phone has established a call with base station one. As the mobile moves away from base station one and approaches base station two, a device in the phone known as the searcher identifies base station one as a good candidate for soft handoff.
The searcher identifies other base stations as good candidates for soft handoff when the received level exceeds the T_add (Threshold for adding a candidate cell for soft handoff) parameter of the system. Once a candidate exceeds the threshold, the phone sends the candidate information to the Mobile Telephone Switching Office (MTSO) via base station one. If the network has available capacity, the MTSO then directs the base stations and mobile to perform a soft handoff.
During soft handoff, the mobile listens to the two cells on different codes while the base stations each listen to the same transmission from the mobile. The signals from the base to mobile are treated as multipath signals and are coherently combined at the mobile unit.
Each base station sends its received signal via the network to the (MTSO), where a quality decision is made on a frame-by-frame basis, every 20 msec. The MTSO selects the better frame from the two signals returned from the base stations. Thus the two base stations act like a giant antenna diversity system. This helps to overcome the fading problem that occurs between cells where handoffs must take place.
As the mobile moves further away from base station one, the searcher in the phone will determine that its power has dropped below the system parameter T_drop. The T_drop information is sent to the MTSO, which then directs the soft handoff be terminated.
This allows for smooth handoffs between cells that the user is totally unaware of. Of course there is a price to pay for this clever design: the system uses more capacity for each soft handoff made and there is greatly increased network traffic between CDMA cellsites and the MTSO.