A mobile station is adapted to communicate with a core network portion of a mobile communications network via an unlicensed radio access network. The mobile station has a SIM card adapted to generate a unique response word using at least a key unique to the mobile station and a fixed length random number. The mobile station includes processing circuitry and unlicensed radio interface circuitry coupled to the processing circuitry. This circuitry is adapted to generate a fixed-length random number, calculate a first response word with the SIM card on the basis of the generated random number, formulate and transmit an authentication request to the unlicensed radio access network containing the fixed-length random number, receive an authentication response from the unlicensed radio access network containing a second response word, and compare the calculated first response word with the received second word to authenticate said core network. In this manner, mobile station is able to authenticate the network with an existing second-generation SIM card and with minimum modification of its operation.