Handover method and apparatus based on history information in wireless networks

A handover method in a plurality of wireless networks includes selecting one or more candidate networks from the plurality of wireless networks available to a mobile terminal of a user; calculating the latest connection time with a serving network currently serving the mobile terminal; deriving a history utility function based on the latest connection time; deriving system utility functions of the candidate networks in consideration of user and system environments; deriving selection functions for the candidate networks using the history utility function and the system utility functions; and selecting one of the candidate networks as a target network for handover using the selection functions. A target network for handover is selected using not only a system utility function reflecting user and system parameters but also a history utility function reflecting history information, thereby reducing the number of handovers and enhancing service quality.

FIELD OF THE INVENTION

The present invention relates to a handover method and apparatus in multiple wireless networks and, more particularly, to a handover method and apparatus which are capable of selecting an optimal network service and providing it to a mobile terminal located in an area where two or more overlapping wireless networks are co-existed.

BACKGROUND OF THE INVENTION

In recent years, wireless networks take a form of an overlay network in which a plurality of wireless networks, e.g., a radio access network for high-speed data transmission, a broadband cellular network, and a local area network (LAN) are co-existed and thus their service areas overlap.

In such an overlay network environment, to provide a seamless service to a mobile terminal from an optimal network regardless of the location of the mobile terminal, it is necessary to provide a vertical handover technique that enables the mobile terminal to select an optimal one of the multiple networks and connect to the optimal network.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a handover method and apparatus that select an optimal network for a vertical handover satisfying requirements of user environment and required service quality.

In accordance with an aspect of the present invention, there is provided a handover method in a plurality of wireless networks, including: selecting one or more candidate networks from the plurality of wireless networks available to a mobile terminal of a user; calculating the latest connection time with a serving network currently serving the mobile terminal; deriving a history utility function based on the latest connection time; deriving system utility functions of the candidate networks in consideration of user and system environments; deriving selection functions for the candidate networks using the history utility function and the system utility functions; and selecting one of the candidate networks as a target network for handover using the selection functions.

In accordance with another aspect of the present invention, there is provided a handover apparatus in a plurality of wireless networks, including: a first selector selecting one or more candidate networks from the plurality of wireless networks available to a mobile terminal of a user; a connection time calculator calculating the latest connection time with a serving network currently serving the mobile terminal; a history utility function composer deriving a history utility function based on the latest connection time; a system utility function composer deriving system utility functions for the candidate networks in consideration of user and system environments; a selection function composer deriving selection functions for the candidate networks using the history utility function, the system utility functions, and the latest connection time; and a second selector selecting one of the candidate networks as a target network for handover using the selection functions.

In accordance with further another aspect of the present invention, there is provided a computer-readable medium storing a computer-executable program that directs a computer to execute the above method.

In the handover method of the present invention, a history utility function is defined in consideration of a user environment and a latest connection time with the serving network, and a target network for handover is selected using the history utility function. Thus, the number of unnecessary handovers and system load are reduced and performance for the user and system is enhanced.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that they can be readily implemented by those skilled in the art.

FIG. 1is a diagram illustrating overlapping of multiple wireless networks. As illustrated inFIG. 1, a plurality of wireless networks may overlap with each other depending upon locations of their base stations. That is, a mobile terminal10of a user may be located on multiple wireless networks employing different wireless technologies, or be located in multiple overlapping cells of a single wireless network.

The multiple wireless networks may include a wireless network for high-speed data transmission such as WiBro (Wireless Broadband), WiFi (Wireless Fidelity) and WiMAX; a broadband cellular network such as WCDMA (Wideband Code Division Multiple Access) and GSM (Global System for Mobile communication); and a wireless local area network (WLAN).

InFIG. 1, two base stations managing wireless cells A1and A2provide network services of a wireless network A. Three base stations managing wireless cells B1, B2and B3provide network services of a wireless network B. A base station managing a wireless cell C1provides network services of a wireless network C.

The mobile terminal10is located in an area where the wireless cells A2and C1overlap with each other, and thus may access the wireless networks A and C.

Therefore, the mobile terminal10may be a wireless telephone, a cellular telephone, a personal digital assistant, a personal computer, or any other device that is capable of sending and receiving communication signals on wireless networks, all of which have a multi-mode capability that has an interface for supporting two or more wireless technologies used for communication in the overlay network.

FIG. 2is a block diagram illustrating a handover apparatus in accordance with an embodiment of the present invention. The handover apparatus may be installed at base stations of individual wireless networks. The handover apparatus includes a first selector100, a connection time calculator110, a history utility function composer120, a system utility function composer130, a selection function composer140, and a second selector150.

The first selector100selects candidate networks meeting minimum conditions as a handover target network from the wireless networks available to the mobile terminal10of the user. More specifically, the first selector100searches for the wireless networks to create a list of candidate networks available to the mobile terminal10for handover. At this time, those wireless networks not meeting the minimum conditions are excluded, and thus the number of candidate networks and computation tasks required for handover are reduced accordingly.

The connection time calculator110calculates the latest connection time between the mobile terminal10and a serving network currently providing wireless services to the mobile terminal10. In this regard, the latest connection time refers to the time elapsed to the present time from the beginning of the wireless connection made with the serving network. For, example, if the wireless connection at present has been made through a vertical handover, the latest connection time becomes the time elapsed from the completion of the handover to the present time.

The history utility function composer120derives a history utility function based on the latest connection time, and computes the value of the history utility function by substituting the latest connection time calculated by the connection time calculator110with the history utility function. In addition, the system utility function composer130derives system utility functions of the individual candidate networks, and computes the value of the system utility function of each candidate network.

The system utility function of a candidate network indicates a satisfaction level of the user to the candidate network in terms of user and network environments. The history utility function based on the latest connection time means a function based on the latest connection time to the serving network, and the value thereof decreases as the latest connection time increases.

When the latest connection time of the mobile terminal10with the serving network is short, a corresponding history utility function gives a relatively large value, which results in reducing the number of unnecessary handovers to other networks.

Equation 1 is an example of a history utility function based on the latest connection time.
UiH(n,t,v,s,w)=e−at·U(Tmin−t)  [Equation 1]

Equation 1 denotes a history utility function associated with the i-th network, ‘n’ is the number of handovers performed so far, ‘t’ is the latest connection time, ‘v’ is the moving velocity of the mobile terminal10, ‘s’ is the type of a service by which the user is being served, and ‘w’ is the type of a serving network currently serving the mobile terminal. The history utility function is evaluated using these parameters. The type of a service may be a real-time service such as a voice service or streaming service, or be a non real-time service such as an FTP service.

In Equation 1, the term U(Tmin−t) is a function of the is latest connection time ‘t’, and Tminis a minimum criteria value for the time needed to surely receive a service from a network, which may be given by the user or the network. When the user has received a service from the serving network for duration longer than or equal to Tmin, this function U(Tmin−t) makes the value of the history utility function zero, which means the selection of candidate network is not affected by the history utility function. That is, if the mobile terminal10has received a service from the serving network for a duration shorter than Tmin, the calculated value of the history utility function has influence on a selection function for network (NSF); and if the terminal has received a service from the serving network for a duration longer than Tmin, the calculated value of the history utility function has no influence on the network selection function.

Equation 1 has a nonzero value in relation only to the serving network currently serving the mobile terminal10, and has a value of zero in relation to other networks.

The value of Equation 1 decreases with the time elapsed, and the decrement differs depending on the value of α(alpha). The handover sensitivity to the latest connection time incurred by a vertical handover can be adjusted by varying the value of α as illustrated inFIG. 4, which illustrates varying values of history utility functions having different values of a with the passage of time.

The selection function composer140derives the selection function (NSF) for network using the latest connection time calculated by the connection time calculator110, the history utility function composer120, and system utility functions of the candidate networks.

An example of the selection function for network is represented as Equation 2.
NSFi=Ui(•)+η·UiH(•),∀i[Equation 2]

The selection function for network is used to select one of the candidate networks as a target network for a vertical handover.

In Equation 2, Ui(·) is a system utility function reflecting system parameters such as capacity, delay, path loss and monetary cost, and user preferences for the system parameters. UiH(·) is a history utility function represented by Equation 1. η acts as a weighting factor to adjust an weight between the system utility function and the history utility function. For example, if it is desired to increase the influence of the history information such as the number of handovers performed, η is set to a relatively large value; and to decrease the influence of the history information, η is set to a value close to zero.

The second selector150selects one of the candidate networks as a target network for handover using the selection functions. The selected candidate network has the greatest selection function value computed from the selection function composer140.

The operation of the handover apparatus ofFIG. 2is described in connection with the flow chart ofFIG. 3illustrating a handover method according to another embodiment of the present invention.

At step200, first of all, one or more candidate networks are selected as a handover target network from the wireless networks available to the mobile terminal10by the first selector100.

Next, at step210, a latest connection time between the mobile terminal10and a serving network currently providing a wireless service to the mobile terminal10is calculated by the connection time calculator110.

Thereafter, at step220, a history utility function is derived by the history utility function composer120based on the latest connection time, and the value of the history utility function is calculated by the connection time calculator110.

Subsequently, at step220, system utility functions of the individual candidate networks are derived, and the value of the system utility function of each candidate network are calculated by the system utility function composer130.

After that, at step230, the selection function (NSF) for network is derived by the selection function composer140using the latest connection time calculated at step210, the history utility function derived at step220, and the system utility functions of the candidate networks.

And then, at step240, one of the candidate networks is selected as a target network for a vertical handover using the selection functions. The selected candidate networks have the greatest selection function value computed from the selection function represented by Equation 2.

Therefore, the mobile terminal10can continuously receive seamless services from the selected target network.

In addition, the present invention may be implemented as computer-executable codes stored in a computer-readable storage medium. The computer-readable storage medium may be any of storage media that can store data readable by a computer system. Examples of the computer-readable storage medium include a ROM, RAM, CD-ROM, magnetic tape, floppy disk, optical data storage, and carrier wave (for transmission through the Internet). The computer-executable codes or the computer-executable program may be distributed among and executed by computer systems connected through a network to carry out the functions of the present invention in a distributed manner.