A communication system that includes mobile user equipment and a communication network is provided. The communication network includes a first base station communicating with the user equipment according to a first defined radio frame format with a first sequence of radio frames each having the same length and each being allocated a respective first System Frame Number (SFN) to sequentially number the radio frames of the first sequence. A second base station is provided to communicate with the user equipment according to a second defined radio frame format including a second sequence of radio frames each having the same length and each being allocated a respective second SFN such that the radio frames of the second sequence are sequentially numbered. A radio signal includes first information indicative of a difference between the first and second SFNs at a particular time and second information usable with the first information.