Inter-frequency handover (intra-system handover)
The need for inter-frequency handover occurs in high capacity areas where multiple 5 MHz WCDMA carriers are deployed. Inter-frequency handover, which is handover between WCDMA carriers on different frequencies,has many similarities with GSM handover,for example the compressed mode functionality.
(i) Channel type switching
In WCDMA there are different types of channels that can be used to carry data in order to maximize the total traffic throughput. The two most basic ones are common channels and dedicated
channels. Channel type switching functionality is used to move subscribers between the common and the dedicated channel, depending on how much information the subscriber needs to transmit. The dedicated channel is used when there is much information to transmit, such as a voice conversation or downloading a web page. It utilizes the radio resources efficiently as it supports both power control
and soft handover.The common channel, on the other hand, is less spectrum efficient. One benefit is that the common channel reduces delays as many subscribers can share the same resource. Hence it is the preferred channel for the transfer of very limited information.
(ii) Admission control
As there is a very clear trade-off between coverage and capacity in WCDMA systems, the admission control functionality is used to avoid system overload and to provide the planned coverage. When a new subscriber seeks access to the network, admission control estimates the network load and based on the new expected load, the subscriber is either admitted or
blocked out. By this the operator can maximize the network usage within a set of network quality levels,i.e. levels depending on what kind of service/information the subscriber wants to use.
Even though an efficient admission control is used,overload may still occur, which is mainly caused by subscribers moving from one area to another area.If overload occurs, four different actions can be taken.First, congestion control is activated and reduces the bit rate of non real-time applications, to resolve the overload. Second, if the reduced bit rate activity is not sufficient, the congestion control triggers the inter- or intra-frequency handover, which moves some sub-scribers to less loaded frequencies. Third, handover of some subscribers to GSM and forth action is to discontinue connections, and thus protect the quality of the remaining connections.
One of the basic requirements when WCDMA was standardized was to avoid dependence on external systems for accurate synchronization of base stations.This has been achieved by a mechanism, where the handset, when needed, measures the synchronization offset between the cells and reports this to the network.In addition, there is also an option to use an external source, such as GPS, for synchronizing the nodes, i.e. to always provide the best solution both asynchronous and synchronous nodes are supported.