A wireless activation system, such as a keyless car entry system, uses repeated code sequences for communication between a transmitter device and a receiver device. By varying the sequence duration in a synchronism at the transmitter device and receiver device, either by varying the number of symbols or chips in the sequence, or by varying the symbol or chip rate, it is made more difficult for an unauthorised party to detect the code sequences and relay the code sequences to achieve unauthorised activation at the receive device.