Handover to GSM (inter-system handover)
When WCDMA was standardized a key aspect was to ensure that existing investments could be re-used as much as possible. One example is handover between the new (WCDMA) network and the existing (GSM) network, which can be triggered by coverage, capacity or service requirements.
Handover from WCDMA to GSM, for coverage reasons,is initially expected to be very important since
operators are expected to deploy WCDMA gradually within their existing GSM network. When a subscriber moves out of the WCDMA coverage area, a handover to GSM has to be conducted in order to keep the connection.Handover between GSM and WCDMA can also have a positive effect on capacity through the possibility of load sharing. If for example the numbers of subscribers in the
GSM network is close to the capacity limit in one area, handover of some subscribers to the WCDMA network can be performed. Another function that is related to inter-system handover is the compressed mode. When performing handover to GSM, measurements have to be made in order to identify the GSM cell to which the handover will be made. The compressed mode is used to create the measurement periods for the handset to make the required measurements. This is typically achieved by transmitting all the information during the first 5 milliseconds of the frame with the remaining 5 milliseconds being used for measurements on the other systems.