A radio station system comprising: one or more radio stations configured to: provide a first radio protocol stack to communicate with a radio terminal on one or more first cells in accordance with a first radio access technology, a second radio protocol stack to communicate with the radio terminal on one or more second cells in accordance with a second radio access technology, and a common Packet Data Convergence Protocol, PDCP, layer associated with both the first and second radio protocol stacks; select from the one or more first cells and the one or more second cells, on a cell-by-cell basis, at least one specific cell on which the radio terminal is allowed to perform at least one of data transmission and data reception on a radio bearer used for uplink transmission or downlink transmission or both via the common PDCP layer; and transmit configuration information indicating the at least one specific cell to the radio terminal, characterized in that the common PDCP layer is configured to: provide a first radio bearer that uses the first radio protocol stack, and provide a second radio bearer that uses the second radio protocol stack; and derive a temporary key for ciphering or deciphering of data of the first radio bearer from a first key, and derive a temporary key for ciphering or deciphering of data of the second radio bearer from a second key that differs from the first key. one or more radio stations configured to: provide a first radio protocol stack to communicate with a radio terminal on one or more first cells in accordance with a first radio access technology, a second radio protocol stack to communicate with the radio terminal on one or more second cells in accordance with a second radio access technology, and a common Packet Data Convergence Protocol, PDCP, layer associated with both the first and second radio protocol stacks; select from the one or more first cells and the one or more second cells, on a cell-by-cell basis, at least one specific cell on which the radio terminal is allowed to perform at least one of data transmission and data reception on a radio bearer used for uplink transmission or downlink transmission or both via the common PDCP layer; and transmit configuration information indicating the at least one specific cell to the radio terminal, provide a first radio protocol stack to communicate with a radio terminal on one or more first cells in accordance with a first radio access technology, a second radio protocol stack to communicate with the radio terminal on one or more second cells in accordance with a second radio access technology, and a common Packet Data Convergence Protocol, PDCP, layer associated with both the first and second radio protocol stacks; select from the one or more first cells and the one or more second cells, on a cell-by-cell basis, at least one specific cell on which the radio terminal is allowed to perform at least one of data transmission and data reception on a radio bearer used for uplink transmission or downlink transmission or both via the common PDCP layer; and transmit configuration information indicating the at least one specific cell to the radio terminal, characterized in that the common PDCP layer is configured to: provide a first radio bearer that uses the first radio protocol stack, and provide a second radio bearer that uses the second radio protocol stack; and derive a temporary key for ciphering or deciphering of data of the first radio bearer from a first key, and derive a temporary key for ciphering or deciphering of data of the second radio bearer from a second key that differs from the first key. provide a first radio bearer that uses the first radio protocol stack, and provide a second radio bearer that uses the second radio protocol stack; and derive a temporary key for ciphering or deciphering of data of the first radio bearer from a first key, and derive a temporary key for ciphering or deciphering of data of the second radio bearer from a second key that differs from the first key.