As in 2G/GSM or 3G/UTRAN, the purpose of cell reselection is to ensure that the terminal in IDLE mode (meaning not active and not engaged in an on-going service) is camped on the best cell in terms of signal strength and quality. In wireless networks, cell reselection is a necessary process, mainly because of terminal mobility, but also because of the fluctuation in the radio environment, which implies variations in the signal strength and interference level, even for a still or slow-moving terminal.
As a basic principle (still being used from 2G/GSM networks), the cell reselection criteria are evaluated by each terminal, using radio measurements performed by the terminal itself (like received beacon channel levels) and parameters (such as threshold values) provided by the network and part of the System Information. For a multi-mode terminal in an environment which provides multiple access types, the terminal needs to evaluate the reselection criteria using measurements from the different available frequencies and access technologies.
The fact that the network controls the value of parameters used in the reselection criteria allows the network to drive terminals in IDLE mode towards the most relevant network layer when appropriate. Some examples are provided below: