An apparatus comprising: a control unit of a first communication system of a first radio access technology, the control unit being configured to receive a request in a first signaling message from a second communication system of a second radio access technology, the request including information on availability of spectrum blocks shared between the first and second communication systems, characterized in that the control unit is configured to perform carrier aggregation mechanisms based on the information with respect to the spectrum blocks, wherein in case the information indicates availability of at least one of the spectrum blocks, the control unit is configured to activate at least one additional carrier corresponding to the at least one spectrum block and aggregate the at least one additional carrier using the carrier aggregation mechanisms, and in case the information indicates unavailability of at least one of the spectrum blocks, the control unit is configured to deactivate at least one additional carrier corresponding to the at least one spectrum block and deactivate the corresponding carrier aggregation using the carrier aggregation mechanisms, and wherein the control unit is configured to transmit a response to the request in a second signaling message to the second communication system, the response including an acknowledgment of an activation/deactivation of the at least one additional carrier and/or the corresponding carrier aggregation, and the first signaling message is a virtual inter-system handover request message and the second signaling message is a virtual inter-system handover response message. a control unit of a first communication system of a first radio access technology, the control unit being configured to receive a request in a first signaling message from a second communication system of a second radio access technology, the request including information on availability of spectrum blocks shared between the first and second communication systems, characterized in that the control unit is configured to perform carrier aggregation mechanisms based on the information with respect to the spectrum blocks, wherein in case the information indicates availability of at least one of the spectrum blocks, the control unit is configured to activate at least one additional carrier corresponding to the at least one spectrum block and aggregate the at least one additional carrier using the carrier aggregation mechanisms, and in case the information indicates unavailability of at least one of the spectrum blocks, the control unit is configured to deactivate at least one additional carrier corresponding to the at least one spectrum block and deactivate the corresponding carrier aggregation using the carrier aggregation mechanisms, and wherein the control unit is configured to transmit a response to the request in a second signaling message to the second communication system, the response including an acknowledgment of an activation/deactivation of the at least one additional carrier and/or the corresponding carrier aggregation, and the first signaling message is a virtual inter-system handover request message and the second signaling message is a virtual inter-system handover response message.