If it is available, the second cell site returns the Walsh Code that will be assigned for the soft handoff. At this point, the original base station orders the soft handoff by using a handoff direction message on the traffic channel using one of the two types of signaling: dim and burst or blank and burst.
- First Base Station Orders Soft Handoff with new Walsh Assignment
- MTSO Sends Land Link to Second Base Station
- Mobile Receives Power from Two Base Stations
- MTSO Chooses Better Quality Frame Every 20 Milliseconds
Once the handoff direction message is acknowledged, the MTSO sends the land link to the second base station who then begins to send the information on the assigned Walsh code traffic channel.
The mobile then receives both signals from the two cell sites, each operating of different PN offsets and Walsh coded traffic channels and combines the signals from both base stations by using the two Pilot signals as coherent phase references.
In a two way soft handoff, two of the mobile’s rakes are used: one for each of the received base station’s signals. This provides greatly improved fading performance. At the same time, each of the cell sites independently receives the signal transmitted by the mobile station.
Each cell site demodulates the signal and returns the decoded data to the MTSO. The MTSO then compares the two signals on a frame-by-frame basis and selects the better of the two to send to the codec to pass along to the public telephone network.