Soft/Softer Handoff in CDMA
Multi-path combination in the BSC during soft handoff
Multi-path combination in the BTS during softer handoffs
Soft handoff means that during the handoff of a mobile station at the coverage edge areas of two or multiple base stations, the mobile station receives the signals from multiple base stations (two in most cases) at the same time and the base stations receive the signals from this mobile station at the same time. The mobile station will not disconnect from the original base station until some conditions are satisfied.
The reasons for soft handoff are as follows:
1. CDMA system can implement the frequency multiplexing of adjacent cells with the same frequency.
2. For each channel, the mobile station and base station adopt multiple RAKE receivers to receive multi-path signals at the same time.
In the course of soft handoff, the signals from various base stations, for a mobile station, are equal to multi-path signals and the receiving of these signals by the mobile station is equal to a kind of space diversity.
This soft handoff or softer handoff can improve the quality of service during the handoff in a real sense. That is to say, the conversation quality at the coverage edge is improved and the call dropping rate greatly reduced. This is not common commercial propagation, but a fundamental solution because call dropping in a GSM system mostly takes place during the handoff.