Soft Handovers

Soft handovers with WCDMA are very different to the handovers used in the GSM system and this basic principle stems from the US system CDMA. Basically the MS is connected to more than one BTS (Base Station Transceiver) at a time. The reason for this is to reduce interference into other cells (BTSs). It also has the advantage of improving performance through macro diversity.

Figure 34 illustrates a soft handover principle. The uplink signal fro the MS is received by both BTSs, which, after demodulation and combining, pass the signal forward to the combining point, typically this is the BSC. From a downlink point of view the same information is transmitted via both BTSs, and the MS receives the information from both of the BTSs as a separate multipath signals and can therefore combine them.

Infrequent handovers are needed for utilizations of a hierarchical call structures; macro, micro and indoor cells. Several carriers and inter-frequency handovers may also be used for taking care of high capacity needs in hot spots. Infrequent Handovers will be needed also for handovers to the 2G systems (See UMTS). There are two methods that are being considered for WCDMA, (1) Dual Receiver and, (2) Slotted Mode.