1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-072393, filed on Mar. 19, 2008, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a wireless communication system including a plurality of base stations and at least one mobile station and, more particularly, to a method for handover between different radio access schemes as well as to a base station and mobile station for implementing the method.
2. Description of the Related Art
Currently, various radio transmission schemes, such the third generation mobile systems (3G), standard specification for broadband wireless access (WiMAX: Worldwide Interoperability for Microwave Access), and standard specification for wireless LAN (WiFi: Wireless Fidelity), have been proposed and are in practical use. However, these different schemes provide different transmission rates and different coverages. Since the transmission rate and coverage are in a tradeoff relationship in general, it is impossible to achieve a high transmission rate and wide-area coverage at the same time with a single radio transmission system. Therefore, there are some cases where appropriate radio transmission schemes are set according to various circumstances, with the result that cells using different radio transmission schemes are next to each other. For such a wireless communication system in which a plurality of radio transmission schemes coexist, several handover techniques are proposed that can be used when a wireless terminal capable of operating with each of these radio transmission schemes moves from a cell using one of the radio transmission schemes to a cell using another one of the radio transmission schemes.
For example, in a mobile communication system disclosed in Japanese Patent Application Unexamined Publication No. 2001-54168, a mobile terminal receives radio signals of all communication systems (radio transmission schemes) that the mobile terminal can use, identifies the type of each communication system and also calculates the quality of communication QoS from the respective signals' frequencies, and then reports these results to a base station (source base station) that the mobile terminal is currently communicating with. When the mobile terminal is notified of a target base station to switch to from the network side in response to the report, the mobile terminal secures a radio link with each of the source base station and the target base station and then switches to the target-side radio transmission scheme.
Moreover, according to the Inter Radio Access Technology (RAT) handover described in 3GPP TS 36.300 V8.3.0 (2007-12), a target system provides information regarding its radio transmission scheme (including the radio resource structure, information on the target cell system, and the like) to a mobile terminal via a currently communicating source system, thus enabling the mobile terminal to switch to the target system's radio transmission scheme.
However, according to the method in which a mobile station identifies the type of a signal source's radio transmission scheme based on the frequency of a received downlink radio signal, the type of a radio transmission scheme cannot be identified from a frequency in use, in a system, such as a cognitive radio system, where a plurality of radio transmission schemes may use available frequencies.
Moreover, in a case where a plurality of radio access schemes coexist in a single wireless communication system, such as a wireless LAN (IEEE 802.11) in which IEEE 802.11a/IEEE 802.11g using multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) and IEEE 802.11b using a single carrier coexist, a radio access scheme cannot be identified from a frequency band in use, because IEEE 802.11g and IEEE 802.11b use the same frequency.
Another possible method for identifying a radio access scheme is that a mobile station receives a common control signal periodically transmitted by a base station and, based on the result of demodulating the signal, identifies a radio transmission scheme used by this base station. However, according to this method, since the mobile station does not identify a target base station's radio transmission scheme until the mobile station receives a common control signal, a delay of about twice an interval between common control signal transmissions, or longer, may occur, causing a delay in handover.