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The authors have been studying an IP-based microcellular network named MM-MAN (mobile multimedia metropolitan area network) which is assumed to become a component network of the 4G mobile systems, and will be deployed along major roads in metropolitan areas. One of the most advanced services of MM-MAN is LAN extension to microcellular network providing moving office to business users during fast driving. In order to satisfy such a special service in the cellular environment, mobility management in MM-MAN should be considered to allow upkeep of the identical IP address during quick cell-to-cell move. To achieve this, the LMC (logical macrocell) which consists of several adjacent cells and is virtually regarded as a single cell is designed. A downstream packet to a targeted MT (mobile terminal) is cast in every base station in an LMC through a multicast tunnel which corresponds to the LMC. The MT can be identified with an identical IP address if it stays in the LMC through the multicast tunnel. To follow the terminal moves, a polling set is emitted to every microcell in the LMC, which is switched over to the next LMC which central microcell the polling response comes to when the target MT moves to a different microcell. A PON (passive optical network) which provides broadcasting capability is used as the terrestrial network.