Abstract: A cellular system transfers a call from a source base station to the target base station. A mobile unit measures the strength of pilot signals emanating from surrounding base stations. In response to the result of the measurement, a different frequency hand off operation is initiated by the mobile unit when all pilot signals are lower than a threshold or is initiated by a system controller with consideration being given to the frequency band occupation state of surrounding base stations and the strength information reported from the mobile unit. In a handoff operation, the mobile unit reduces a maximum data rate to modulate user information with a current used frequency and to modulate a different frequency preposition code with a primary frequency so that neighboring base stations may measure the intensity of the preposition code and the system controller can determine how the different frequency hand-off should be operated.