There is provided a method comprising: determining an information on an inter-symbol interference between signals on a same frequency resource and a same time resource from one or more first cells and one or more second cells, wherein an active set of the apparatus comprises at least the one or more second cells, the inter-symbol interference is an estimated inter-symbol interference if at least one of the one or more first cells is not in the active set, and the inter-symbol interference is a measured inter-symbol interference if the active set consists of the one or more first cells and the one or more second cells; and deciding whether or not to inform a network comprising the one or more first cells and the one or more second cells on the information on the inter-symbol interference.