Title of Invention

MOBILE COMMUNICATION SYSTEM, RELAY STATION, BASE STATION, AND CONTROL METHOD THEREOF

Technical Field

[0001]

The present application relates to a method of selecting a cell to be used for connection between a relay station and a base station from a plurality of candidate cells in a mobile communication system including a base station and a relay station that belongs to the base station.

Background Art

[0002]

In LTE- Advanced (Long Term Evolution Advanced) of 3 GPP (3rd Generation

Partnership Project), the introduction of relay stations (hereinafter "RNs (Relay Nodes)") has been examined (see NPL 1 and 2). The RN is one of techniques for increasing the

communication speed of mobile stations (hereinafter "UEs (User Equipments)") located at cell edges and/or for increasing cell areas of base stations (hereinafter "eNBs (Evolved Node Bs)"). The details of the RN architecture examined in 3GPP are shown in NPL 2.

[0003]

The outline of a mobile communication system based on the RN architecture disclosed in NPL 2 is explained hereinafter. Fig. 1 shows a network configuration example when the RN examined in 3 GPP is used. A base station (eNB) 91 belongs to a core network (hereinafter "CN") 4 of a mobile telecommunications carrier. The base station (eNB) 91 creates an eNB cell 10 and relays traffic between a mobile station (UE) 3 and the core network (CN) 4. A relay station (RN) 92 belongs to the base station (eNB) 91 by means of a backhaul link (BL1 in the figure) and also belongs to the core network (CN) 4 via the backhaul link (BL1). The base station (eNB) 91 and the relay station (RN) 92 can connect to a management apparatus 5 via the core network (CN) 4. The management apparatus 5 is an OAM (Operation Administration and Maintenance) system, and administers information set by the mobile telecommunications carrier. The relay station (RN) 92 is able to acquire information from the management apparatus 5. The mobile station (UE) 3 belongs to the base station (eNB) 91 or the relay station (RN) 92 by means of an access link (ALl in the figure). The relay station (RN) 92 creates an RN cell 20 and relays traffic between the mobile station (UE) 3 and the core network (CN) 4. Details of the backhaul link and the access link are explained later.

[0004]

Fig. 2 is a sequence diagram showing the outline of an RN start-up procedure described in Section 4.7.6 of NPL 3. The start-up procedure includes a phase 1 and a phase 2 explained below. In the phase 1, the relay station (RN) 92 connects to a network (E-UTRAN/EPC) as a mobile station (UE). Then, the relay station (RN) 92 acquires initialization parameters from the management apparatus 5 (i.e., OAM system). The initialization parameters include a list (i.e., a donor cell list) of eNB cells to which the relay station (RN) 92 can belong as an RN by using a backhaul link (BL1). After acquiring the initialization parameters, the relay station (RN) 92 releases the network connection as the UE and thereby finishes the phase 1. Note that the base station (eNB) 91-1 to which the relay station (RN) 92 belongs in the phase 1 does not necessarily have the function of allowing a relay station (RN) to belong thereto.

[0005]

In the phase 2, the relay station (RN) 92 selects one of at least one eNB cell (i.e., candidate for donor cell) indicated by the donor cell list acquired from the management apparatus 5, and belongs to a base station (eNB) 91-2 operating the selected cell as a relay station (RN). Then, the relay station (RN) 92 acquires configuration information of the backhaul link (BL1) from the base station (eNB) 91-2 to which the relay station (RN) 92 belongs, and configures the backhaul link (BL1). After finishing the procedure of the phase 2, the relay station (RN) 92 starts to operate the relay station cell (RN cell) 20.

[0006]

Further, as disclosed in NPL 4, there are three types, i.e., type 1, type la, and type lb in the RN examined in 3GPP. The RN may support only one of these three types, or may change the operating mode between plural types. Alternatively, different operating modes may be used for different UEs. The type-1 RN uses the same carrier (i.e., the same frequency) for the backhaul link and the access link (in-band), and time-divides the radio resources for the backhaul link and the radio resources for the access link. The main purpose of this scheme is to avoid the interference from the access-link transmission to the backhaul-link reception in the RN.

[0007]

The type-la RN uses different carriers (i.e., different frequencies) for the backhaul link and the access link (out-band). Therefore, the type- la RN does not require the time-division for radio resources unlike the type-1 RN, and performs mutually independent communications between the backhaul link and the access link. [0008]

Similarly to the type-1 RN, the type- lb RN uses the same frequency for the backhaul link and the access link. However, the type- lb RN does not time-divide the radio resources. This type is used on condition that the interference from the access-link transmission to the backhaul-link reception is sufficiently suppressed.

[0009]

In the phase 2 of the above-described start-up procedure, the RN transmits RN identification information including an RN type to the eNB. Then, the eNB determines the control method of the backhaul link based on the RN type information included in the RN identification information. More specifically, the eNB determines whether the resources of the backhaul link should be time-divided or not based on the RN type information.

[0010]

In the specification of the present application, among the eNBs, an eNB that can allow an RN to belong thereto is called "Donor eNB" (hereinafter "DeNB"). A mobile station (UE) that directly belongs to a relay station (RN) is called "RN-UE". Further, in the discussion on 3 GPP, a demand for supporting multihop RNs in the future is arising. The multihop RN is a technique that makes it possible to connect an additional relay station (RN) to a relay station (RN) that already belongs to an eNB in a cascade configuration. In this specification, in the explanation relating to the multihop technique, a relay station (RN) belonging to an eNB through a radio interface is called "upper RN" and a relay station (RN) belonging to the upper RN through a radio interface is called "lower RN" in order to distinguish them from each other. Further, in this specification, a radio interface between an eNB and an RN and between an upper RN and a lower RN is called "backhaul link". Meanwhile, a radio interface between an eNB and an eNB-UE and between an RN and an RN-UE is called "access link".

Citation List

Non Patent Literature

[0011]

NPL 1: 3 GPP TR 36.912 V9.2.0 (2010-03), "Feasibility study for Further

Advancements for E-UTRA (LTE-Advanced)", 3GPP (3rd Generation Partnership Project), March 2010

NPL 2: 3 GPP TR 36.806 V9.0.0 (2010-03), "Relay architectures for E-UTRA (LTE- Advanced)", 3 GPP, March 2010

NPL 3: 3 GPP TS 36.300 V10.2.0 (2010-12), "Overall description; Stage 2 (Release 10)", 3 GPP, December 2010

NPL 4: 3 GPP TR 36.814 V9.0.0 (2010-03), "Further advancements for E-UTRA physical layer aspects", 3 GPP, March 2010 Summary of Invention

Technical Problem

[0012]

The inventors of the present application have made detailed examination on the donor cell selection method that is used when plural candidate donor cells are indicated by the donor cell list. In the above-described procedure disclosed in NPL 3, the relay station (RN) acquires a donor cell list from a management apparatus such as an OAM system that is different from the base station (eNB). Further, the RN selects a donor cell from the donor cell list based solely on information administered by the RN (e.g., received power measured by the RN) and requests connection with the selected cell. However, the candidate donor cells indicated by the donor cell list, which is supplied from the management apparatus (i.e., OAM system) to the RN, are controlled by eNB(s) based on information administered by the eNB(s) (e.g., the load of the cells and the backhaul link configuration of other RN). Therefore, there is a possibility that a donor cell that is selected based solely on the information administered by the RN (such as received power measured by the RN) is not a desirable cell for the eNB(s) included in the mobile communication system.

[0013]

For example, assume a case where an eNB generates two cells (first and second cells). When the first and second cells are both indicated by a donor cell list acquired from the management apparatus, the RN selects one of the first and second cells as a donor cell based on the reception quality of the first and second cells measured by the RN itself. In this example, assume that the first cell is selected. Then, the RN requests the eNB that the RN connects to the first cell as an RN. However, there is a possibility that the second cell is more desirable than the first cell as the cell to which the RN newly connects in consideration of the status of the first and second cells at the time when the connection is requested from the RN, i.e., in consideration of the load of the first and second cells, the connection status of other RN, and the like. However, the eNB cannot effectively control the decision on the donor cell that is made by the RN.

[0014]

As a way of solving this problem, it is conceivable that the eNB (s) waits until the RN requests connection with the most suitable donor cell. Specifically, the eNB (s) may respond with the rejection to the connection request by the RN when the donor cell requested by the RN is not a suitable cell. The RN that receives the rejection response requests connection with a newly- selected donor cell. The eNB (s) repeats the rejection response as long as the donor cell requested by the RN is not a suitable cell, and waits for a connection request for the most suitable donor cell. However, in this method, the rejection response is possibly repeated for many times and therefore a long time is wasted until finding the optimal donor cell. In particular, when the donor cell is switched (backhaul link is switched) after the operation of the RN cell is started, i.e., when the donor cell is switched in a state where an RN-UE already exists, the operation of the RN is temporarily suspended. Therefore, taking a long time for the switching of the donor cell may cause a significant impact on the RN-UE.

[0015]

The present invention has been made in consideration of the above-described problems. Certain embodiments of the present invention provide a mobile communication system, a relay station, a base station, a control method thereof, and a program, capable of determining a donor cell based on both donor cell selection criteria on a relay station side and donor cell selection criteria on a base station side.

Solution to Problem

[0016]

According to one embodiment, a mobile communication system includes at least one base station and a relay station. The relay station is configured to acquire a donor cell list indicating at least one candidate cell. Further, one of a first base station, included in the at least one base station, and the relay station is configured to determine a donor cell, to be connected to the relay station by the backhaul link, from the donor cell list based on both first cell

management information and second cell management information. The first cell management information relates to the at least one candidate cell and originates from the at least one base station. The second cell management information originates from the relay station.

[0017]

According to another embodiment, a relay station apparatus includes a radio

communication unit and a control unit. The radio communication unit is configured to relay transmission and reception data to and from a mobile station by using a backhaul link connected to one of at least one base station and an access link connected to the mobile station. The control unit acquires a donor cell list indicating at least one candidate cell through the radio

communication unit. Further, the control unit maintains second cell management information. Further, the control unit determines a donor cell, to be connected to the relay station apparatus by the backhaul link, from the donor cell list in response to receiving first cell management information relating to the at least one candidate cell and originating from the at least one base station.

[0018]

According to still another embodiment, a base station apparatus includes a radio communication unit and a control unit. The radio communication unit is configured to perform data transfer, through a backhaul link connected to a relay station, with a mobile station connected to the relay station by means of an access link. The control unit maintains or receives, from another base station apparatus, first cell management information. Further, the control unit determines a donor cell, to be connected to the relay station by the backhaul link, from the donor cell list in response to receiving second cell management information from the relay station. The first cell management information relates to the at least one candidate cell.

[0019]

According to yet another embodiment, a control method of a relay station apparatus includes:

(a) acquiring a donor cell list indicating at least one candidate cell;

(b) maintaining second cell management information; and

(c) determining a donor cell, to be connected to the relay station apparatus by the backhaul link, from the donor cell list in response to receiving first cell management information relating to the at least one candidate cell from the at least one base station.

[0020]

According to further embodiment, a control method of a base station includes:

(a) maintaining or receives, from another base station apparatus, first cell management information; and

(b) determining a donor cell, to be connected to the relay station by the backhaul link, from the donor cell list in response to receiving second cell management information from the relay station, the first cell management information relating to the at least one candidate cell and originating from at least one base station including the base station apparatus.

[0021]

According to still further embodiment, a program for causing a computer to carry out the method according to the yet another embodiment is provided.

[0022]

According to yet further embodiment, a program for causing a computer to carry out the method according to the further embodiment is provided.

Advantageous Effects of Invention

[0023]

According to the above-mentioned embodiments of the present invention, it is possible to provide a relay station, a base station, a control method thereof, and a program which are capable of determining a donor cell based on both donor cell selection criteria on a relay station side and donor cell selection criteria on a base station side. Brief Description of Drawings

[0024]

[Fig- 1]

Fig. 1 is a block diagram showing a configuration example of a mobile communication system according to related art;

[Fig. 2]

Fig. 2 is a sequence diagram showing a start-up procedure of a relay station according to related art;

[Fig- 3]

Fig. 3 is a block diagram showing a configuration example of a mobile communication system according to a first illustrative embodiment of the present invention;

[Fig. 4]

Fig. 4 is a block diagram showing a configuration example of a base station according to the first illustrative embodiment of the present invention;

[Fig- 5]

Fig. 5 is a block diagram showing a configuration example of a base station according to the first illustrative embodiment of the present invention;

[Fig. 6]

Fig. 6 is a block diagram showing a configuration example of a base station according to the first illustrative embodiment of the present invention;

[Fig. 7]

Fig. 7 is a block diagram showing a configuration example of a base station control apparatus according to the first illustrative embodiment of the present invention;

[Fig. 8]

Fig. 8 is a sequence diagram showing an example of a donor cell selection procedure of a relay station according to the first illustrative embodiment of the present invention;

[Fig- 9]

Fig. 9 is a sequence diagram showing another example of a donor cell selection procedure of a relay station according to the first illustrative embodiment of the present invention;

[Fig. 10]

Fig. 10 is a flowchart showing an operation example of a relay station according to the first illustrative embodiment of the present invention;

[Fig. 11]

Fig. 11 is a flowchart showing an operation example of a base station according to the first illustrative embodiment of the present invention;

[Fig. 12]

Fig. 12 is a sequence diagram showing an example of a donor cell selection procedure of a relay station according to a second illustrative embodiment of the present invention;

[Fig. 13]

Fig. 13 is a flowchart showing an operation example of a relay station according to the second illustrative embodiment of the present invention;

[Fig. 14]

Fig. 14 is a flowchart showing an operation example of a base station according to the second illustrative embodiment of the present invention;

[Fig. 15]

Fig. 15 is a sequence diagram showing an example of a donor cell selection procedure of a relay station according to a third illustrative embodiment of the present invention; and [Fig. 16]

Fig. 16 is a flowchart showing an operation example of a base station according to the third illustrative embodiment of the present invention.

Description of Embodiments

[0025]

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings. Identical or corresponding elements are designated by identical reference numerals throughout the drawings, and redundant description thereof will be omitted except when necessary.

[0026] <First illustrative embodiment

This illustrative embodiment shows an example in which a relay station 2 determines a donor cell from at least one candidate cell indicated by a donor cell list based on first cell management information received from a base station 1 and second cell management

information administered by the relay station 2. The first cell management information relates to at least one candidate cell and originates from the base station 1 (or a plurality of base stations 1). In other words, the first cell management information relates to at least one candidate cell and is administered or held by the base station(s) 1. The first cell management information relates to a first parameter(s) that should be taken into account for at least one base station when the donor cell is determined. For example, the first cell management information includes at least one of the following items: the operation status of at least one candidate cell that is taken by the base station(s) 1 ; and the performance condition of the donor cell requested by the base station(s) 1. More specifically, the first cell management information includes at least one of (1) the load of the at least one candidate cell, (2) the connection status to the at least one candidate cell from another relay station, (3) cell configuration information indicating the time-division

implementing status of radio resources of the at least one candidate cell, (4) the priority among the at least one candidate cell held by the base station(s) 1, and (5) a list of cells selected by the base station(s) 1 based on at least one of the aforementioned items (1) to (4). The first cell management information may include information of a cell(s) that is operated by a base station(s) located near or adjacent to the base station(s) 1.

[0027]

Meanwhile, the second cell management information originates from the relay station 2. In other words, the second cell management information is administered or held by the relay station 2. For example, the second cell management information includes at least one of the following items: the radio quality of at least one candidate cell that is taken by the relay station 2; and the performance condition of the donor cell requested by the relay station 2. More specifically, the second cell management information includes at least one of (a) the reception quality of a down link signal (e.g., downlink received power) from the at least one candidate cell, (b) the type information (e.g., RN-type and Category class) of the relay station 2, (c) the performance of the relay station 2, (d) the priority among the at least one candidate cell held by the relay station 2, and (e) a list of cells selected by the relay station 2 based on at least one of the aforementioned items (a) to (d).

[0028]

A mobile communication system according to this illustrative embodiment is explained on the assumption that the mobile communication system is an FDD (Frequency division Duplex)-OFDMA mobile communication system, more specifically an LTE- Advanced type mobile communication system. Fig. 3 is a block diagram showing a configuration example of a mobile communication system according to this illustrative embodiment. In Fig. 3, the base station (eNB) 1 belongs to a core network (CN) 4 of a mobile telecommunications carrier and relays traffic between a mobile station (UE) 3 and the core network (CN) 4. The base station (eNB) 1 is capable of allowing the relay station 2 to belong thereto (i.e., base station 1 capable of functioning as a DeNB), and is also capable of allowing the mobile station 3 to belong thereto at the same time. The relay station 2 is capable of connecting to the management apparatus 5 through the core network (CN) 4.

[0029]

A configuration and an operation of a mobile communication system according to this illustrative embodiment are explained hereinafter in detail. Fig. 4 is a block diagram showing a configuration example of the base station 1. In Fig. 4, a radio communication unit 11 generates a downlink signal by performing various processes including mapping onto resource elements, OFDM signal generation (e.g., IDFT (Inverse Discrete Fourier Transform)), frequency conversion, and signal amplification for a transmission symbol sequence of a physical channel supplied from a transmission data processing unit 12. The generated downlink signal is wirelessly transmitted from an antenna. Further, the radio communication unit 11 receives an uplink signal transmitted from the mobile station 3 or the relay station 2, and restores a reception symbol sequence.

[0030]

A transmission data processing unit 12 stores data that is obtained from a

communication unit 14 and is to be transmitted to the mobile station or the relay station in a buffer that is arranged for each mobile station and each bearer, and generates a transport channel by performing error correction encoding, rate matching, interleaving, and the like. Further, the transmission data processing unit 12 generates a radio frame by adding control information to the data series of the transport channel. Furthermore, the transmission data processing unit 12 generates a transmission symbol sequence for each physical channel by performing scrambling and modulation symbol mapping for the data series of the radio frame.

[0031]

A reception data processing unit 13 restores received data for each logical channel from a reception symbol sequence supplied from the radio communication unit 11. User traffic data and part of control data included in the obtained reception data are transferred to the core network 4 through the communication unit 14.

[0032]

A backhaul link control unit 15 controls transmission timing and radio resource allocation relating to communication with the relay station 2 through the backhaul link, and information regarding the backhaul link. Further, the backhaul link control unit 15 transmits first cell management information to the relay station 2 through the transmission data processing unit 12 and the radio communication unit 11.

[0033]

Fig. 5 is a block diagram showing a configuration example of the relay station 2. The relay station has equivalent functions to those of the base station 1 unless specified otherwise. In Fig. 4, a lower radio link communication unit 21 receives an uplink signal transmitted from a mobile station 3, through an antenna. A reception data processing unit 23 has similar functions to those of the reception data unit 13 of the base station 1. Reception data obtained by the reception data processing unit 23 is transmitted to the base station 1 through an upper radio link communication unit 24.

[0034]

A transmission data processing unit 22 has similar functions to those of the transmission data processing unit 12 of the base station 1, and generates a transmission symbol sequence from transmission data that is obtained from the upper radio link commumcation unit 24 and is to be transmitted to the mobile station 3. The lower radio link communication unit 21 generates a downlink signal from the transmission symbol sequence and transmits the generated downlink signal to the mobile station 3.

[0035]

A backhaul link control unit 25 controls information regarding communication with the base station 1 through the backhaul link. For example, the backhaul link control unit 25 selects a cell used for the backhaul link. Specifically, the backhaul link control unit 25 determines a donor cell based on both the donor cell selection criteria of the base station 1 and the donor cell selection criteria of the relay station 2 by using both the first cell management information received from the base station 1 and the second cell management information administered by the relay station 2 itself.

[0036]

Fig. 6 is a block diagram showing a configuration example of the mobile station 3. A radio communication unit 31 receives a downlink signal through an antenna. A reception data processing unit 32 sends reception data restored from the received downlink signal to a buffer unit 35. Reception data stored in the buffer unit 35 is read out and used according to the purpose. Further, a transmission data control unit 33, a transmission data processing unit 34, and the radio communication unit 31 generate an uplink signal by using transmission data stored in the buffer unit 35, and transmit the generated uplink signal to the base station 1 or the relay station 2.

[0037]

Fig. 7 is a block diagram showing a configuration example of the management apparatus 5. A communication unit 51 transmits/receives a data packet to/from the relay station 2 through the core network 4. A reception data processing unit 53 restores information included in a received upward data packet. When the restored information is a signal requesting a cell list, the reception data processing unit 53 sends this information to a candidate cell management unit 54. The candidate cell management unit 54 administers at least one candidate cell (candidate donor cell) that the relay station 2 can use for the backhaul link. When a signal requesting a donor cell list is received from the relay station 2, the candidate cell management unit 54 transmits a donor cell list indicating at least one candidate cell to the relay station 2 through a transmission data processing unit 52.

[0038]

Next, a specific example of a donor cell selection procedure according to this illustrative embodiment is explained hereinafter with reference to Figs. 8 to 11. Fig. 8 is a sequence diagram showing an example of a donor cell selection procedure according to this illustrative embodiment. Fig. 8 shows interactions between the base station 1 , the relay station 2, and mobile station 3. Further, the symbols "eNB 1", "RN 2", and "RN-UE 3" in Fig. 8 correspond to the base station 1 , the relay station 2, and the mobile station 3 respectively. Note that in the example shown in Fig. 8, the base station (eNB) 1 functions as a donor base station (DeNB) that connects a backhaul link with the relay station (RN) 2.

[0039]

A step SI 01 indicates that the RN 2 holds a donor cell list including information of at least one candidate cell. For example, the donor cell list includes the frequency, the Cell ID, and the like of each candidate cell.

[0040]

In a step SI 02, the eNB 1 transmits first cell management information to the RN. As described previously, the first cell management information relates to the candidate cell for the donor cell and is administered or held by the eNB 1. The first cell management information includes, for example, the load status of the candidate cell, the connection status of other RN(s) and UE(s), and the like. Further, the first cell management information may include, in addition to the information regarding the cell operated by the eNB 1 itself, information regarding a cell(s) that is operated by a base station(s) located near or adjacent to the eNB 1.

[0041]

The transmission/reception of the first cell management information in the step SI 02 may be performed in the phase 1 (i.e., the phase in which the RN 2 connects to the eNB 1 as a UE) of the RN start-up procedure, or may be performed in the phase 2 (i.e., the phase in which the RN 2 connects to the eNB 1 as an RN) of the RN start-up procedure. Alternatively, the transmission/reception of the first cell management information may be performed in an independent connection phase that is different from both the phase 1 and phase 2. Further, the first cell management information may be transmitted by using a broadcast channel from the eNB 1. In this case, the RN2 can receive the first cell management information even when the RN 2 has no RRC (Radio Resource Control) connection with the eNB 1 , that is, even when the RN 2 is in an RRC IDLE state where RRC layer is not connected.

[0042]

In a step SI 03, the RN 2 determines a cell used for the backhaul link from at least one candidate cell indicated by the donor cell list based on the first cell management information received from the eNB and the second cell management information possessed by the RN 2 itself. As described previously, the second cell management information is administered or held by the RN. The second cell management information includes, for example, the radio quality (e.g., downlink received power) of the candidate cell, the RN type to be defined and the RN cell configuration to be configured for each candidate cell, and the performance of the RN 2.

Specific examples of the donor cell determination method using the first and second cell management information are explained later.

[0043]

In a step S 104, the RN 2 performs a process corresponding to the phase 2 of the RN start-up procedure. That is, the RN 2 belongs to the eNB 1 that operates the donor cell determined in the step SI 03, and connects a backhaul link with the eNB 1. After finishing the backhaul link connection process, the RN 2 starts to operate an RN cell (step SI 05). After that, the RN 2 allows the RN-UE 3 to connect thereto (step SI 06).

[0044]

Note that in the step S101 of Fig. 8, the donor cell list held in the RN 2 may be supplied from the management apparatus 5 to the RN 2. Specifically, the RN 2 may acquire the donor cell list from the management apparatus 5 in the phase 1 of the RN start-up procedure. Fig. 9 shows a procedure through which the RN acquires the donor cell list from the management apparatus 5 before the step S101 of Fig. 8 (step S201 and S202). The symbol "OAM 5" in Fig. 9 corresponds to the management apparatus 5. Further, the base station (eNB) 1-2 in Fig. 9 corresponds to the base station (eNB) 1 in Fig. 8. Meanwhile, the base station (eNB) 1-1 in Fig. 9 is a base station to which the RN 2 belongs in the phase 1 of the RN start-up procedure in order to acquire the donor cell list. The base stations (eNBs) 1-1 and 1-2 in Fig. 9 may be different base stations or may be the same base station.

[0045]

In a step S201, the RN 2 belongs to the eNB 1-1 as a mobile station (UE) and thereby connects to the core network 4 through the eNB 1-1. In a step S202, the RN 2 acquires the donor cell list by accessing the OAM 5 through the eNB 1 - 1.

[0046]

The example shown in Figs. 8 and 9 shows a case where the base station 1 (base station 1 -2) to which the relay station 2 accesses to acquire the first cell management information in the step SI 02 is the same as the base station 1 (base station 1-2) to which the backhaul link is connected in the subsequent step SI 04. However, as described above, the first cell management information may include information of a base station(s) located near the base station 1 (e.g., adjacent base station) that is different from the base station 1 that has transmitted the first cell management information. Therefore, the base station 1 with which the relay station 2 connects the backhaul link in the step SI 04 may be different from the base station 1 to which the relay station 2 accesses to acquire the first cell management information in the step SI 02. The base station 1 in the step SI 04 is a base station located near or adjacent to the base station 1 in the step SI 02, for example.

[0047]

Fig. 10 shows a flowchart showing an operation example of the relay station 2 performed when a donor cell is determined. In a step S301, the relay station 2 determines whether or not the relay station 2 holds a donor cell list. When the relay station 2 holds a donor cell list (Yes at step S301), the relay station 2 proceeds to a step S302. When the relay station 2 holds no donor cell list (No at step S301), the relay station 2 returns to the step S301 again.

[0048]

In the step S302, the relay station 2 determines whether or not the relay station 2 has received first cell management information from the base station 1. When the RN has received first cell management information (Yes at step S302), the relay station 2 determines a donor cell used for the backhaul link from the donor cell list based on the first cell management information received from the base station 1 and second cell management information administered or held by the relay station 2 itself (step S303). Then, the relay station 2 starts communication to the determined donor cell through the backhaul link and the operation of an RN cell, and finishes the process shown in Fig. 10. On the other hand, when the RN has not received first cell management information (No at step S302), the relay station 2 returns to the step S202 where the relay station 2 waits for the reception of first cell management information again.

[0049]

Fig. 11 shows a flowchart showing an operation example of the base station 1 performed when a backhaul link is determined. The base station 1 sends the first cell management information to the relay station 2 (step S301), and finishes the process.

[0050]

The operation of the mobile station 3 is not different from ordinary operations, and therefore its explanation is omitted.

[0051]

Next, specific examples of the donor cell determination method using first and second cell management information are explained hereinafter. Various examples can be conceivable according to the contents of the first and second cell management information. Several specific examples are explained hereinafter.

[0052]

(1) Example of first cell management information

( 1 - 1 ) Load information of eNB cell 10 (Cell load)

The load of at least one eNB cell 10 operated by at least one base station 1 is used. The load of the eNB cell 10 may be expressed by, for example, a band usage ratio or the total number of UEs connecting to the cell. When this information is used for the determination of the donor cell, for example, it is conceivable to prioritize a cell having the lowest load among the at least one eNB cell 10 as the donor cell.

[0053]

(1-2) Connection status of other relay station(s)

When other RN(s) has already connected a backhaul link to the eNB cell 10, whether the time-division of radio resources is implemented between the backhaul link and the access link or not is taken into consideration.

[0054]

(1-3) Priority among eNB cells held by base station 1

A priority among eNB cells that are hold by the base station 1 is used. For example, it is conceivable to lower the priority of "non-backward compatible carrier" below that of "backward compatible carrier". The "backward compatible carrier" means a cell that transmits control channels similar to those in the conventional cell when a carrier aggregation in which a plurality of carriers (carrier frequency bands) are bundled is implemented. In contrast to this, the "non-backward compatible carrier" is a carrier that is used in a dependent manner when a carrier aggregation is implemented, in which at least some of the control channels are not transmitted. The control of the "non-backward compatible carrier" is carried out by using the control channels of "backward compatible carrier". The "backward compatible carrier" and "non-backward compatible carrier" can be also called "primary cell" and "secondary cell" respectively.

[0055]

(2) Example of second cell management information

(2-1) Radio quality of candidate cell

The radio quality of at least one candidate cell indicated by a donor cell list is used. Examples of the radio quality of a candidate cell include the reception quality (e.g., received power or an SINR (Signal to Interference plus Noise power Ratio) of a downlink signal from the candidate cell). When the radio quality of a candidate cell is used for the determination of the donor cell, it is conceivable to determine the donor cell from, for example, candidate cells having a downlink reception quality equal to or higher than a predetermined level.

[0056]

(2-2) Type information of relay station 2

For example, the above-described RN type is used. When the RN type of the relay station 2 is Type- la, different carries (different frequencies) are used for the backhaul link and the access link. Therefore, communication is carried out independently between the backhaul link and the access link without requiring the time-division of radio resources. Accordingly, when the RN type of the relay station 2 is Type- la, a candidate cell in which the time-division of radio resources is not implemented may be selected and determined as the donor cell based on the first cell management information with higher priority than the one in which the time- division of resources is implemented (connection status of other relay stations in this example). Meanwhile, when the RN type of the relay station 2 is Type-1, the same carrier is used for the backhaul link and the access link. Therefore, it is necessary to implement the time-division of radio resources between the backhaul link and the access link. Accordingly, when the RN type of the relay station 2 is Type-1, a candidate cell in which the time-division of radio resources is already implemented may be selected and determined as the donor cell based on the first cell management information with higher priority than the one in which the time-division of resources is not yet implemented (connection status of other relay station(s) in this example). [0057]

(2-3) Priority among candidate cells held by relay station 2

A priority among candidate cells hold by the relay station 2 is used. For example, it is conceivable to lower the priority of "non-backward compatible carrier" below that of "backward compatible carrier".

[0058]

(2-4) Performance of relay station 2

The performance of the relay station 2 such as the capacity of a buffer memory in which transfer data is held or the maximum communication speed is used. Further, similarly to UE category specified in 3GPP TS 36.306, when category classes are defined according to the performance of the relay station 2, the category classes of RN may be used. For example, when the relay station 2 is a high-performance relay station, i.e., when the capacity of the buffer memory is large and/or the maximum communication speed is high, a candidate cell in which the time-division of radio resources is not yet implemented may be selected and determined as the donor cell based on the first cell management information with higher priority than the one in which the time-division of resources is already implemented (connection status of other relay station(s) in this example). On the other hand, when the relay station 2 is a low-performance relay station, a candidate cell in which the time-division of radio resources is already

implemented may be selected and determined as the donor cell based on the first cell

management information with higher priority than the one in which the time-division of resources is not yet implemented (connection status of other relay station(s) in this example).

[0059]

In addition to the above-described combination of the first and second cell management information, it is also possible to combine the above-listed first cell management information items and second cell management information items as desired and to use the combined information items for the determination of the donor cell. For example, when "load information of eNB cell 10 (Cell load)" and "radio quality of candidate cell" are used as the first and second cell management information respectively, a candidate cell of which the radio quality is equal to or higher than a predetermined reference level and the cell load is equal to or lower than a predetermined reference level may be determined as the donor cell.

[0060]

As described above, in this embodiment, when a donor cell is selected from at least one candidate cell indicated by a donor cell list supplied from the management apparatus 5, the first and second cell management information are used. The first cell management information relates to at least one cell operated by the base station 1 and is administered or held by the base station 1. The second cell management information is administered or held by the relay station 2. In other words, the first cell management information originates from the base station 1 , but the second cell management information originates from the relay node 2. That is, in this

embodiment, the first cell management information as well as the second cell management information is referred to when the donor cell is determined. Therefore, in this embodiment, it is possible to determine the donor cell based on not only the donor cell selection criteria on the relay station side but also based on the donor cell selection criteria on the base station side.

[0061]

Further, in this embodiment, the relay station 2 determines the donor cell. Therefore, the relay station 2 can select the donor cell with consideration given to the current status of the base station 1 such as the cell load and the connection status of other RN(s) based on the details of the first cell management information.

[0062]

<Second illustrative embodiment

In this embodiment, a modified example in which the base station 1 determines the donor cell by using the first and second cell management information is explained. Therefore, the relay station 2 transmits, to the base station 1, the second cell management information regarding at least one candidate cell indicated by the donor cell list received from the

management apparatus 5. The base station 1 determines the donor cell by referring to the second cell management information received from the relay station 2 and first cell management information administered or held by the base station 1 itself. A configuration example of a mobile communication system according to this illustrative embodiment is similar to that of the first illustrative embodiment shown in Fig. 3. A specific example of a donor cell selection procedure according to this illustrative embodiment is explained hereinafter with reference to Figs. 12 to 14.

[0063]

Fig. 12 is a sequence diagram showing an example of a donor cell selection procedure according to this illustrative embodiment. The symbols "eNB 1", "RN 2", and "RN-UE 3" in Fig. 12 correspond to the base station 1, the relay station 2, and the mobile station 3 respectively. Steps SI 01 and SI 04 to SI 06 in Fig. 12 are similar to the steps assigned with the same numeral symbols in Figs. 8 and 9, and therefore their explanation is omitted. In a step S501, the relay station 2 transmits second cell management information to the base station 1. Similarly to the first embodiment, the second cell management information transmitted in the step S501 is information originating from the relay station 2. The second cell management information includes at least one of the following items: the radio quality of at least one candidate cell that is taken by the relay station 2; and the performance condition of the donor cell requested by the relay station 2. The relay station 2 may transmit second cell management information regarding all the candidate cells indicated by the donor cell list to the base station 1, or may transmit second cell management information regarding some of candidate cells indicated by the donor cell list to the base station 1.

[0064]

In a step S502, the base station 1 determines the donor cell by referring to first cell management information administered by the base station 1 itself and the second cell management information received from the relay station 2. In a step S503, the base station 1 transmits information regarding the determined donor cell to the relay station 2 (step S503).

[0065]

The transmission/reception of the second cell management information performed in the step S501 and transmission/reception of the donor cell information performed in the step S503 may be performed in the phase 1 (i.e., the phase in which the RN 2 connects to the eNB 1 as a UE) of the RN start-up procedure explained in the related art section, or may be performed in the phase 2 (i.e., the phase in which the RN 2 connects to the eNB 1 as an RN) of the RN start-up procedure. Alternatively, the transmission/reception of the second cell management information may be performed in an independent connection phase that is different from both the phase 1 and phase 2.

[0066]

Note that, the example shown in Fig. 12 shows a case where the base station 1 that determines the donor cell through the procedure from the steps S501 to S503 is the same as the base station 1 with which the relay station 2 connects the backhaul link in the subsequent step SI 04. However, as described previously in the first embodiment, the first cell management information may include information of a base station(s) located near the base station 1 (e.g., adjacent base station). Therefore, the donor cell determined in the step S502 may not be a cell operated by the base station 1 itself that has made the determination, but may be a cell operated by a nearby base station. In this case, the base station 1 with which the relay station 2 connects the backhaul link in the step SI 04 may be a base station different from the base station 1 that determines the donor cell through the procedure from the steps S501 to S503 (e.g., adjacent base station).

[0067] Fig. 13 shows a flowchart showing an operation example of the relay station 2 performed when a donor cell is determined. A step S301 is similar to the step assigned with the same numeral symbol in Fig. 10, and therefore its explanation is omitted. When the relay station 2 holds a donor cell list (Yes at step S301), the relay station 2 transmits second cell management information regarding candidate cells to the base station 1 (step S601).

[0068]

In a step S602, the relay station 2 determines whether or not the relay station 2 has received donor cell information from the base station 1. When the relay station 2 has received donor cell information (Yes at step S602), the relay station 2 starts communication to the specified donor cell through the backhaul link and the operation of an R cell, and finishes the process shown in Fig. 13. When the relay station 2 has not received donor cell information (No at step S502), the relay station 2 returns to the step S602 where the relay station 2 waits for the reception of donor cell information again.

[0069]

Fig. 14 shows a flowchart showing an operation example of the base station 1

performed when a donor cell is determined. The procedure of Fig. 14 is started when the relay station 2 receives second cell management information from the base station 1.

[0070]

When the base station 1 has received second cell management information (Yes at step S701), the base station 1 determines a donor cell by referring to the second cell management information notified from the relay station 2 and the first cell management information

administered by the base station 1 itself (step S602). Specific examples of the donor cell determination method using the first and second cell management information are explained later. In a step S703, the base station 1 transmits information regarding the determined donor cell to the relay station 2 (step S703), and finishes the process shown in Fig. 14.

[0071]

The operation of the mobile station 3 is not different from ordinary operations, and therefore its explanation is omitted.

[0072]

The specific example of the donor cell determination method using first and second cell management information is similar to that described in the first embodiment.

[0073]

In this embodiment, similarly to the first embodiment, when a donor cell is selected from at least one candidate cell indicated by the donor cell list supplied from the management apparatus 5, the first and second cell management information are used. Therefore, in this embodiment, it is possible to determine the donor cell based on not only the donor cell selection criteria on the relay station side but also based on the donor cell selection criteria on the base station side.

[0074]

Further, in this embodiment, the base station 1 determines the donor cell. Therefore, the base station 1 can select the donor cell with consideration given to the current status of the base station 1 such as the cell load and the connection status of other RN(s) based on the details of the first cell management information.

[0075]

<Third illustrative embodiment

In this embodiment, a modified example of the above-described second embodiment is explained. In this embodiment, the base station 1 can transfer second cell management information received from the relay station 2, to other base stations 1. Specifically, when the second cell management information, which is transmitted from the relay station 2 and received by the base station 1-2, includes information regarding a cell operated by a base station 1-3 other than the base station 1-2, the base station 1-2 transfers the second cell management information to the corresponding base station 1-3. Then, the base station 1-3 determines a donor cell by referring to first cell management information regarding the cell operated by the base station 1-3 itself and the transferred second cell management information. A configuration example of a mobile communication system according to this illustrative embodiment is similar to that of the first illustrative embodiment shown in Fig. 3. A specific example of a donor cell selection procedure according to this illustrative embodiment is explained hereinafter with reference to Figs. 15 and 16.

[0076]

Fig. 15 is a sequence diagram showing an example of a donor cell selection procedure according to this illustrative embodiment. Each of the symbols "eNB 1-2" and "eNB 1-3" in Fig. 15 correspond to the base station 1. Further, the symbols "RN 2" and "RN-UE 3" in Fig. 15 correspond to the relay station 2 and the mobile station 3 respectively. Steps S101, S501, SI 05 and SI 06 in Fig. 15 are similar to the steps assigned with the same numeral symbols in Fig. 12, and therefore their explanation is omitted.

[0077]

In a step S 801, the eNB 1-2 checks candidate cells indicated by second cell management information received from the RN 2. When the second cell management information includes information regarding a cell operated by an eNB 1-3 different from the eNB 1-2, the eNB 1-2 transfers this second cell management information to the eNB 1-3 (step S802).

[0078]

In a step S803, the eNB 1-3 determines a donor cell by referring to the transferred second cell management information and first cell management information regarding the cell operated by the eNB 1-3 itself. In a step S804, the eNB 1-3 transmits information regarding the determined donor cell to the eNB 1-2. In a step S805, the eNB 1-2 transfers the information regarding the donor cell received from the eNB 1-3 to the RN 2. In a step S806, the RN 2 performs a process, based on the donor cell information received from the eNB 1 -2, the RN 2 belongs to the eNB 1-3 operating that donor cell as an RN and connects the backhaul link.

[0079]

Fig. 16 shows a flowchart showing an operation example of the base station 1 performed when a donor cell is determined. The flowchart shown in Fig. 16 relates to an operation of the base station 1 (eNB 1-2 in Fig. 15) that receives second cell management information from the relay station 2. Steps S601, S602 and S603 in Fig. 16 are similar to the steps assigned with the same numeral symbols in Fig. 14, and therefore their explanation is omitted.

[0080]

In a step S901, the base station 1 (eNB 1-2) determines whether the second cell management information received from the relay station 2 indicates a cell operated by a base station 1 (eNB 1-3) other than the base station 1 (eNB 1-2) itself. When the second cell management information indicates a cell operated by another base station 1 (Yes at step S901), the base station 1 (eNB 1 -2) transfers the second cell management information to the

corresponding base station 1 (eNB 1-3) operating the indicated cell (step S902). When the second cell management information does not indicate any cell operated by other base stations 1 (No at step S901), the base station 1 (eNB 1-2) determines the donor cell in a similar manner to that of the second embodiment (step S702). In a step S903, the base station 1 (eNB 1-2) determines whether the base station 1 (eNB 1-2) has received donor cell information from the another base station 1 (eNB 1-3) or not. When the base station 1 (eNB 1-2) has received donor cell information (Yes at step S903), the base station 1 (eNB 1-2) transfers the donor cell information to the relay station 2 and finishes the process shown in Fig. 16. When the base station 1 (eNB 1-2) has not received donor cell information (No at step S903), the base station 1 (eNB 1-2) returns to the step SI 103 where the base station 1 (eNB 1-2) waits for the reception of donor cell information again. [0081]

The operation of the RN 2 is similar to that of the second embodiment, and therefore its explanation is omitted. Further, the operation of the mobile station 3 is not different from ordinary operations, and therefore its explanation is omitted.

[0082]

As described above, in this illustrative embodiment, when the second cell management information, which is transmitted from the relay station 2 and received by the base station 1, indicates a candidate cell operated by other base station, the base station 1 transfers the second cell management information to the other base station operating the indicated candidate cell. Then, the other base station operating the indicated candidate cell determines a donor cell by referring to the first cell management information regarding the indicated candidate cell and the transferred second cell management information. This embodiment may be suitable for cases where the donor cell is switched while the relay station is operating an RN cell, for example, for handovers between relay stations 2 (i.e. Mobile Relay) that can be moved such as relay stations installed in buses, trains, and the like.

[0083]

<Other illustrative embodiments>

In the above-described first to third illustrative embodiments, examples of an LTE- Advanced-type mobile communication system are explained. However, the application of the present invention is not limited to LTE- Advanced-type mobile communication systems. That is, the present invention can be widely applied to mobile communication systems including a relay station.

[0084]

Any of the processes of the base station 1 and the relay station 2 that are performed for determining a donor cell as described in the above-described first to third illustrative

embodiments may be implemented by using a semiconductor processing device such as an ASIC (Application Specific Integrated Circuit) or a DSP (Digital Signal Processor). Alternatively, these processes may be implemented by causing a computer such as a microprocessor to execute a program. Specifically, a program including instructions to cause a computer to execute an algorithm shown in at least one of Figs. 10, 1 1, 13, 14 and 16 may be created, and the created program may be supplied to a computer.

[0085]

This program can be stored in various types of non-transitory computer readable media and thereby supplied to computers. The non-transitory computer readable media includes various types of tangible storage media. Examples of the non-transitory computer readable media include a magnetic recording medium (such as a flexible disk, a magnetic tape, and a hard disk drive), a magneto-optic recording medium (such as a magneto-optic disk), a CD-ROM (Read Only Memory), a CD-R, and a CD-R/W, and a semiconductor memory (such as a mask ROM, a PROM (Programmable ROM), an EPROM (Erasable PROM), a flash ROM, and a RAM (Random Access Memory)). Further, the program can be supplied to computers by using various types of transitory computer readable media. Examples of the transitory computer readable media include an electrical signal, an optical signal, and an electromagnetic wave. The transitory computer readable media can be used to supply programs to computer through a wire communication path such as an electrical wire and an optical fiber, or wireless communication path.

[0086]

Further, the first to third illustrative embodiments according to the invention can be combined as desired.

[0087]

For the setting-up and handover of the relay station, it is desirable that determination of the donor cell is performed in a shorter time in comparison to the setting-up and handover of the mobile station. This is because the faster determination can prevent the communication disconnection of the RN-UE. In the handover of mobile stations, the target cell is typically determined by selecting a cell of which the quality of the downlink reception at the mobile station is the best from a neighbor cell list supplied from the source base station. If this target cell determination method used in the handover of mobile stations is applied to relay stations, only the circumstances on the relay station side and the requirement conditions from the relay station to the donor cell are taken into consideration. As a result, as stated previously as the problem to be solved, if the base station cannot allow the relay station to belong to the base station due to the circumstances on the base station side, the relay station is rejected from belonging to the base station. In contrast to this, in each of the above-described embodiments, the donor cell is determined with consideration given to not only the circumstances/requirement conditions on the relay station side but also to the circumstances/requirement conditions on the base station side. Therefore, in each of the above-described embodiments, it is possible to select an appropriate candidate cell as a donor cell in a short time and thereby to prevent the frequent occurrences of the repetition of the initial start-up procedure and the handover procedure, which would otherwise occur due to the unsuccessful donor cell selection.

[0088] Further, the present invention is not limited to the above-described illustrative embodiments, and needless to say, various modifications can be made without departing from the spirit and scope of the present invention described above.

[0089]

This application is based upon and claims the benefit of priority from Japanese patent application No. 2011-089287, filed on April 13, 2011, the disclosure of which is incorporated herein in its entirety by reference.

Reference Signs List

[0090]

1 BASE STATION

2 RELAY STATION

3 MOBILE STATION

4 CORE NETWORK

11 RADIO COMMUNICATION UNIT

12 TRANSMISSION DATA PROCESSING UNIT

13 RECEPTION DATA PROCESSING UNIT

14 COMMUNICATION UNIT

15 BACKHAUL LINK CONTROL UNIT

21 LOWER RADIO LINK COMMUNICATION UNIT

22 TRANSMISSION DATA PROCESSING UNIT

23 RECEPTION DATA PROCESSING UNIT

24 UPPER RADIO LINK COMMUNICATION UNIT

25 BACKHAUL LINK CONTROL UNIT

31 RADIO COMMUNICATION UNIT

32 RECEPTION DATA PROCESSING UNIT

33 TRANSMISSION DATA CONTROL UNIT

34 TRANSMISSION DATA PROCESSING UNIT

35 BUFFER UNIT

51 COMMUNICATION UNIT

52 TRANSMISSION DATA PROCESSING UNIT

53 RECEPTION DATA PROCESSING UNIT

54 CANDIDATE CELL MANAGEMENT UNIT