Method of locating a mobile station in a mobile telephone system having indoor and outdoor base stations

A method for handing off a mobile station from an outdoor base station to an indoor base station in a mobile telephone system with a plurality of cells intended for indoor and outdoor use, and comprising an external part-system for outdoor use which includes external base stations connected to a mobile switching center, and an internal part-system for indoor use and having internal base stations connected to the mobile switching center, said mobile telephone system being allocated a plurality of frequencies for radio channels, said method comprising the steps of:

• specifying a predetermined number of said allocated frequencies as identification frequencies for said indoor base stations;
• selecting unique frequency sets from among said identification frequencies, each set comprising at least two frequencies;
• assigning one of the unique frequency sets to each indoor base station each of said indoor base stations being assigned different frequency sets, wherein said indoor base stations transmit on their respective unique frequency set;
• measuring in said mobile station the signal strengths of said identification frequencies;
• reporting a number of said identification frequencies having a signal strength above a first threshold to the mobile switching center;
• comparing the reported frequencies with said unique frequency sets;
• selecting an indoor base station when an agreement is found between at least a subset of the reported frequencies and at least a subset of the indoor base stations unique frequency set;
• assigning said selected indoor base station a traffic channel from among the frequencies including said identification frequencies;
• and
• handing off said mobile station to said selected indoor base station, wherein said mobile station communicates with said indoor base station on said assigned traffic channel.