Patent ID: 8023948

Claim:
A vertical handover method for an overlay communication system including a plurality of base stations each providing at least one communication network technology to a mobile terminal, comprising: determining if a mobile terminal is located in a measurement region with reference to a vertical handover map in relation to a serving base station; detecting reachable base stations by activating all network interfaces when the mobile terminal is located in a measurement region; selecting one of the reachable base stations as a handover target base station; determining if the mobile terminal is located in a handover region with reference to the vertical handover map; and performing a handover if the mobile terminal is located in a handover region, wherein the vertical handover map is a map configured by a graph in which an X axis denotes a Distance (D) between the serving base station and the mobile terminal, and a Y axis denotes a traffic Load (L) of the serving base station, wherein the measurement region is a region determined by the vertical handover map, wherein the handover region is a region determined by the vertical handover map, and wherein detecting reachable base stations comprises determining if there is a base station satisfying {└D n <D n — MT ┘ and └L n <L n — MT ┘}, where D n is a distance between the mobile terminal and n th base station, L n is a traffic load of n th base station, D n — MT is a reachable threshold distance, and L n — MT is a reachable threshold traffic load of the n th base station.