A wireless metropolitan area network standard for the fixed broadband wireless access (BWA) is defined by the IEEE 802.16 protocol. The standard defines different physical layer techniques for different frequency bands. The standard is mainly applied in home, small office/home office (SOHO), remote worker, and small enterprise (SME).
The IEEE 802.16 MAC is capable of supporting a network environment with hundreds of subscriber terminals on each channel, and supporting continuous traffic and burst traffic.
However, when the number of the network terminals to be served simultaneously is large, it would easily cause an uneven load to the network side equipment or an uneven resource distribution, and the communication quality of the terminals would be influenced if serious. Therefore, the terminals should be handed over between different equipment on the network providing the same service, so as to balance the load or resource of the network side equipment, thereby improving the service quality of the network.
As shown in FIG. 1, the common terminal handover process with a centralized control manner requires the following three steps (the base station serving node (BSSN) in the Figure is the access service network gateway (ASN-GW) below. The process can be described by ASN-GW.).
(1) A mobile subscribe station (MS) communicates with a current serving base station (BS), i.e. the BS in the middle on the left side of FIG. 1, via an air interface, and the current serving BS communicates with a current serving ASN-GW1, i.e. the BSSN1, as shown by line (1).
(2) After the MS is handed over, the MS communicates with a target BS, i.e. the BS at the lower part on the left side of FIG. 1. Assuming that the target BS and the current serving BS communicate with the same ASN-GW1, then only the interface R6 should be migrated, and the interface R3 remains unchanged during the handover process, as shown by line (2).
(3) After the MS is handed over, the MS communicates with the target BS. If the target BS communicates with a target ASN-GW2, the original interfaces R6 and R3 should be migrated. The signaling interfaces from the core network to the terminal must be migrated to the target ASN-GW2 after the step (2), and the interfaces involved include R3, R4, and R6;
(4) After the MS handover is performed, the signaling interfaces with the core network are entirely migrated to the target BS and the target ASN-GW2. The path is shown by line (3) in the Figure.
In the above network structure, the BS generally includes logic function units such as a media access control (MAC) and physical layer (PHY) unit, a service flow management (SFM) unit, and an HO function unit.
The ASN-GW generally includes function units such as an authenticator, an MIP client, a foreign agent (FA), a paging controller (PC), a service flow authorization, and a relaying HO function unit.
Referring to FIG. 2, a conventional terminal handover control process is shown. Handover interactions occur between a Serving HO function, the Relaying HO function, and a Target HO Function, and the HO Function is used to control the whole handover operation and the signaling process related to the handover.
During the handover, a terminal initiates a terminal handover request to the current serving BS at first. Upon receiving the air interface message, and an HO Function (i.e. the Serving HO Function) unit in the serving BS is triggered to initiate a handover request. The handover request is forwarded to the Target HO Function unit via a BSSN (having the Relay HO Function unit therein) between the serving BS and the target BS, so as to implement the handover interactions.
The HO Request message is used for the Serving HO Function unit to initiate a handover request to the Target HO Function unit, and the HO Response message is used for the Target HO Function unit to return a handover response to the Serving HO Function unit.
The interaction of the HO Request message and the HO Response message enables the network to prepare a corresponding target BS list for the terminal handover, and send it to the terminal.
The HO Confirm message is used for the serving BS to inform the target BS to get ready for the handover via a backbone network, after the terminal makes a handover instruction formally and sends it to the serving BS.
The above scheme can not support the handover initiated by the BSSN or by the network, and can only support the handover initiated by the terminal, and therefore, the BSSN triggering a terminal handover based on the requirement for the resource or load balance cannot be realized.
An improved scheme is that, the BSSN determines directly the terminal to be handed over, and makes the terminal send directly a handover request to a target BSSN or a target BS. However, it assumes that the BSSN know the wireless channel condition of all terminals thereof, and know the load condition between every two BSs thereof. Therefore, since the BSSN does not know the real operation conditions of the terminal and the network side equipment, the terminal handover may not be performed effectively. Or, it should cost much to get knowledge of the real operation conditions of the terminal and the network side equipment, which causes high resource consumption of the terminal handover, and causes handover delay.