In the LTE-Advanced (Long Term Evolution Advanced) study item of 3GPP (3rd Generation Partnership Project), introduction of a relay station (hereinafter RN: Relay Node) is under consideration. The RN is a technique for increasing the communication rate of a mobile station located at a cell edge (hereinafter UE: User Equipment) and for expanding the cell coverage of a base station (hereinafter eNB: Evolved Node B), for example.
In a network in which the RN is introduced, the base station (eNB: Evolved Node B) having a function of connecting with the RN is referred to as “Donor eNB (hereinafter DeNB)”.
The term “DeNB” is herein used to be distinguished from the normal eNB in a description of events unique to the DeNB related to connection with the RN.
The mobile station (hereinafter UE: User Equipment) which is directly connected to the DeNB is herein referred to as “eNB-UE”. In contrast, the mobile station which is directly connected to the RN is herein referred to as “RN-UE”. In a description of events common to both the eNB-UE and the RN-UE is referred to simply as “UE”.
In the discussion on the LTE-Advanced Study Item, there is a demand for supporting a multi-hop RN in the future. The multi-hop RN is a technique for connecting another RN in cascade with the RN connected to the DeNB. In the case of describing multi-hopping, the RN which is connected to a lower layer of the DeNB via a radio interface is herein referred to as “upper-level RN” and the RN which is connected to a lower layer of the upper-level RN via a radio interface is herein referred to as “lower-level RN” so as to make a distinction therebetween.
The radio interface between the DeNB and the RN and between the upper-level RN and the lower-level RN is referred to as “backhaul link”. On the other hand, the radio interface between the eNB and the eNB-UE and between the RN and the RN-UE is referred to as “access link”.
An outline of the RN under consideration in the LTE-Advanced study item of 3GPP will be described below. FIG. 1 is a diagram showing a network configuration example using the RN in LTE-Advanced. Base stations (eNBs) 8A and 8B are connected to a core network (CN) 5 of a mobile network operator. The CN 5 includes MME (Mobility Management Entity) and S-GW (Serving Gateway).
The base stations 8A and 8B generate cells 41A and 41B, respectively, and relay traffic between a mobile station (UE) 3 and the CN5. The base station 8B is connected to a relay station (RN) 9 via a backhaul link (BL1 in the figure). That is, the base station 8B corresponds to the above-mentioned DeNB. The relay station 9 is connected to the CN5 via the backhaul link (BL1) between the relay station 9 and the base station 8B. The relay station 9 generates a cell 42 and relays traffic between the mobile station 3 and the CN5.
In the LTE-Advanced study item, the following four alternatives for a network architecture using the RN are proposed (see 3GPP contribution R2-093972 (Non Patent Literature 1)).    1. Full-L3 relay, transparent for DeNB    2. Proxy S1/X2    3. RN bearers terminate in RN    4. S1 termination in DeNB
These four RN network architecture alternatives can be classified into the following two groups in terms of the termination point of S1 application protocol (S1AP) used for an interface (S1) between a control apparatus (hereinafter MME/S-GW) within the core network and each eNB. As for the details of the S1 interface and the S1AP, see 3GPP technical specifications TS36.413 (Non Patent Literature 2) and TS36.414 (Non Patent Literature 3).    Group I: S1 protocol is terminated at the DeNB (Alternatives 2 and 4)    Group II: S1 protocol is terminated at the RN (Alternatives 1 and 3)
Next, an outline of a handover procedure of a mobile station in LTE-Advanced will be described. A handover in LTE-Advanced includes two procedures: an X2 handover and an S1 handover.
In the X2 handover, a handover is performed using an X2 interface which is an inter-eNB interface. In other words, the X2 interface is used for signaling related to a handover between a source eNB (S-eNB) and a target eNB (T-eNB) and for transfer of user data. As for details of the X2 interface and X2AP, see 3GPP technical specifications TS36.423 (Non Patent Literature 4) and TS36.424 (Non Patent Literature 5).
On the other hand, in the S1 handover, an S1 interface is used for a handover from the S-eNB to the T-eNB. The S1 handover is executed in the case where the X2 interface cannot be used between the S-eNB and the T-eNB, or in the case of performing an inter-MME handover, for example.
FIG. 2A shows a successful example of the X2 handover procedure. The S-eNB (base station 8A shown in FIG. 1) receives a measurement report from the UE (mobile station 3 shown in FIG. 1) (step S101). The measurement report includes a measurement result of a reception quality of a radio signal from a neighboring cell of the cell 41A to which the UE (mobile station) belongs. The S-eNB (base station 8A) decides an optimum handover destination cell based on the measurement report. The S-eNB (base station 8A) transmits a handover request message (Handover Request) including information on the handover destination cell to the T-eNB (base station 8B) via the X2 interface (step S102).
When the handover is acceptable, the T-eNB (base station 8B) transmits, to the S-eNB (base station 8A) via the X2 interface, an acknowledge response message (Handover Request Acknowledge) including information necessary for the handover (e.g. configuration information on an access link) (step S103). The S-eNB (base station 8A) having received the acknowledge response transmits, to the UE (mobile station 3), a message (RRC Connection Reconfiguration) prompting to change an RRC connection destination. Then, the UE (mobile station 3) transmits a random access preamble to the T-eNB (base station 8B), thereby starting an RRC connection with the T-eNB (base station 8B) (step S105).
FIG. 2B shows a successful example of the S1 handover procedure. The S-eNB (base station 8A) receives a measurement report from the UE (mobile station 3) (step S111). The S-eNB (base station 8A) determines an optimum handover destination cell based on the measurement report. The S-eNB (base station 8A) transmits, to the MME via the S1 interface, a handover request message (Handover Required) including information on the handover destination cell (step S112). The MME having received the Handover Required message transmits a handover request message (Handover Request) to the T-eNB (base station 8B) (step S113). When the handover is acceptable, the T-eNB (base station 8B) transmits the acknowledge response message (Handover Request Acknowledge) to the MME via the S1 interface (step S114). In step S115, the MME transmits, to the S-eNB (base station 8A), a message (Handover Command) indicating acknowledgement of the handover. The subsequent steps S116 and S118 are similar to steps S104 and S105 of FIG. 2A.