RESOURCE MANAGEMENT PROCEDURES

Description

Technical Field

[0001] The present invention relates to the field of communications, and in particular to a resource management method for a cell cluster.Background

[0002] Ultra-Dense Networks (UDNs) have powerful capabilities of spectral reuse, which may satisfy a requirement of a future radio communications system for large-capacity communications, hence, they are deemed as a key technology in the future of radio communications systems. In an ultra-dense network, a communication capacity is usually improved by deploying multiple Small Cells (SCs), each having a short range but deployed in such a way as to have overlapping ranges. Short-haul signal transmission is performed between Small Base Stations (SBSs) providing communication services and user equipment within coverage ranges of the small cells. However, as the small cells are densely deployed, harmful mutual interference is produced between small cells using the same radio resources, which is more severe to those small cells using the same resources at edges of a macro cell. Several methods are known for reducing interference between cells (such as the small cells in the above UDN). For example improving performance of cell-edge user equipment via a coordinated MultiPoint (COMP), which is carried out by a joint processing technology. Another method is to process interference via Inter-Cell Interference Coordination (ICIC), in which uplink and downlink usable resources are reported between neighbouring cells, and related messages need to be transmitted via an X2 interface.

[0003] However, when CoOrdinated MultiPoint (COMP) is used, a large quantity of information regarding user equipment needs to be shared between multiple base stations in a very short time, and in an ultra-dense network, it is possible that some user equipment has no ideal backhaul, or there is more interference to be coordinated immediately. In this case, a capacity of interference coordination is inadequate.

[0004] When the existing ICIC method is used, all usable resource information needs to be transmitted via an X2 interface, and a scalability of the network is inadequate.

[0005] US2015173011A1 discloses a cluster head (or other small cell) serves as a local mobility anchor for small cells in cluster, thus reducing the signaling to network entity (e.g., core network or evolved packet core). OAM entity may consider the cluster to be essentially a macro network (e.g., macro e / NodeB). CH is configured for performing spectrum coordination (in time, frequency, geography) of small cells within the cluster. CH shares spectrum coordination info from the operator OAM to the small cells of the cluster.Statements of invention

[0006] According to a first aspect, there is provided a resource management method as defined in Claim 1. An advantage of the embodiments is that by transmitting resource allocation information by a base station taken as a cluster head of a cell cluster to other base stations in the cell cluster, resource allocation for the cells may be performed efficiently, capabilities of coordinating inter-cell interference may be improved, and as the cell cluster may be expanded to cell clusters of a lower level or more levels, scalability of the network may be improved.Brief Description of the Drawings

[0007] Embodiments will now be described, aided by the accompanying Figures in which: Figure 1 is a schematic diagram of an ultra-dense network to which the resource management method and apparatus for a cell cluster in a first embodiment; Figure 2 is a schematic diagram of a cell cluster shown in Figure 1; Figure 3 is a flowchart of the resource management method of the claimed invention; Figure 4 is a flowchart of a method for updating resource allocation information; Figure 5 is a schematic diagram of interference coordination within the cell cluster; Figure 6 is a schematic diagram of the resource management apparatus for a cell cluster; and Figure 7 is a schematic diagram of a structure of a base station. Detailed Description

[0008] In the embodiments of the present invention, as used herein, a base station may include: an access point; a broadcast transmitter; a nodeB or an evolved nodeB (eNB), etc., and may include a device having some or all functions of the aforementioned device. As used herein, the term "base station" includes a device that provides communication coverage for a specific geographical region. As used herein, term "cell" includes a base station and / or a coverage region thereof, depending on context. A mobile station or equipment may be referred to as user equipment (UE). The UE may be fixed or mobile, and may also be referred to as a mobile station, a terminal, an access terminal, a user unit, or a station, etc. The UE may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handhold device, a lap-top computer, and a cordless telephone, etc. The first embodiment provides a resource management method for a cell cluster, which is applicable to a base station taken as a cluster head of a cell cluster. In this embodiment, an application scenario of the method shall be illustrated taking an Ultra-Dense Network (UDN) as an example.

[0009] The communications system is divided into at least three levels, which are, for example, in a descending order of the levels, a macro base station (a macro cell), a cell cluster (such as a Small Cell Cluster (SCC)) and cells in the cell cluster. However, the present invention is not limited thereto, and the communications system may include a structure of four or more levels, for example, the cell cluster may further include cell clusters of a lower level or more levels.

[0010] The cell cluster may include at least one cell and in some embodiments, one of the cells is designated as a cluster head.

[0011] Figure 1 is a schematic diagram of an UDN 100. The ultra-dense network 100 includes three macro base stations (MBSs) 101, which are MBS1 101a, MBS2 101b and MBS3 101c, respectively, and respectively serve three macro cells 107a, 107b, 107c.

[0012] MCU1 105a and MCU2 105b are user equipment within a coverage range of a macro cell 107a served by MBS1 101a; MCU3 105c and MCU5 105e are user equipment within a coverage range of a macro cell 107b served by MBS2 101b, and MCU4105d and MCU6 105f are user equipment within a coverage range of a macro cell served by MBS3 101c.

[0013] At an edge of the coverage range of the macro cell served by MBS1, there are two small cell clusters (SCCs), SCC1103a and SCC2 103b, both of which are constituted by multiple small cells SCU 108 within a similar coverage range of the macro cell MBS 1. Furthermore, one of the SCUs is designated as a cluster head 110 for the small cell cluster 103a and a user equipment 111 is shown in the range of the small cell cluster SCC1 103a. Taking SCC1 103a as an example, when two or more small cells in SCC1 use same resources for communication, interference will occur between these small cells.

[0014] Figure 2 is a schematic diagram of the small cell cluster SSC1 103a shown in Figure 1. The small cell cluster 103a includes five small cells, SC1-SC5 109a-109e, and small base stations served for them are SBS1-SBS5 108a-108e, respectively. In the small cells 109, the small base stations SBS1-SBS5 108a-108e communicate with user equipment SCU1-SCUS 111a-111e within a coverage range of the small cells, respectively. Furthermore, in Figure 2, one of the small base stations SBS1 108a is acting as a cluster head for the small cell cluster and so also has the reference 110.

[0015] The small cells SC1-SC5 in Figures 1 and 2 constitute "the cell cluster", the small cells SC1-SC5 may be equivalent to "cells", and the small base stations SBS1-SBS5 may be equivalent to "base stations. Of course, the network structure of the embodiments of the present invention may further be expanded, and the above correspondence relationships are illustrative only.

[0016] Figure 3 is a flowchart of the resource management method for a cell cluster as performed by the cluster head 110 according to the claimed invention.

[0017] In step s302 resource allocation information is transmitted by the cluster head 110 to other base stations in the cell cluster.

[0018] In this embodiment, the method further includes the following preferable steps: In step s301 information is exchanged between the cluster head and a macro base station or a cluster head of a cell cluster of an upper level.

[0019] In step s303 information is exchanged between the cluster head and a cluster head of another cell cluster of the same level as the cell cluster.

[0020] In step s304 information is exchanged between the cluster head and a cluster head of a cell cluster of a lower level; and

[0021] In step s305: further information other than resource allocation information is exchanged between the cluster head and the other base stations in the cell cluster.

[0022] There is no determined temporal order relationship between steps s301-s305, and an order of executing the steps is not limited.

[0023] It can be seen from the above embodiment that by transmitting the resource allocation information by the base station taken as the cluster head 110 of the cell cluster to the other base stations in the cell cluster, resource allocation for the cells may be performed efficiently, capabilities of coordinating inter-cell interference may be improved, and as the cell cluster 103 may be expanded to cell clusters of a lower level or more levels, scalability of the network may be improved.

[0024] The cell cluster 103 may be established by a macro base station 101 of a macro cell 107 to which the cell cluster 103 belongs, and may also be established by another macro base station 101. The cluster head 110 of the cell cluster may be designated by the macro base station 101, and may also be changed by the macro base station. Furthermore, cell clusters 103 within a coverage range of a macro cell 107 served by the macro base station 101 may also be reorganized by the macro base station.

[0025] Therefore, when the cluster head of the cell cluster is a base station serving for a cell in the cell cluster, the base station taken as the cluster head transmits the resource allocation information to base stations of other cells than the cell where the cluster head is located, and does not transmit the resource allocation information to itself.

[0026] For example, as shown in Figure 2, SBS1108a in SBS1-SBS5 may be taken as the cluster head 110, and in this case, SBS1 110 transmits the resource allocation information to SBS2 108b, SBS3 108c, SBS4 108d and SBSS 108e. If an independent base station other than SBS1-SBS5 is provided and taken as the cluster head of the cell cluster, and in this case, the independent base station transmits the resource allocation information to SBS1-SBS5.

[0027] The base station taken as the cluster head transmits the resource allocation information to the other base stations in multiple ways. For example, it may transmit via an X2 interface, or may transmit via broadcast or multicast.

[0028] Resources allocated in the resource allocation information include at least one of a spectrum resource, a time resource, and a space resource, and this embodiment is described taking the spectrum resource as an example.

[0029] In this embodiment, the resource allocation information may be pre-obtained, or may be generated by receiving some information.

[0030] In Step s301, the base station taken as the cluster head of the cell cluster receives the information on the usable resources of the cell cluster from the macro base station or the cluster head of the cell cluster of an upper level.

[0031] In this case, the base station taken as the cluster head generates the resource allocation information according to the information on the usable resources of the cell cluster, and transmits the resource allocation information to the other base stations in the cell cluster.

[0032] In this embodiment, the macro base station may be a macro base station adjacent to the cell cluster, and may also be another macro base station.

[0033] As shown in Figure 1, the base station taken as the cluster head 110 of the cell cluster SCC1 103a receives the information on the usable resources of the cell cluster SCC1 from the macro base station MBS1 101a, generates the resource allocation information according to the information on the usable resources, and transmits the resource allocation information to another base station in SCC1, thereby achieving resource allocation of the cells in SCC1.

[0034] The information on the usable resources of the cell cluster received by the base station taken as the cluster head from the macro base station or the cluster head of the cell cluster of an upper level is, for example, reusable spectrum resource information (RSRI).

[0035] The reusable spectrum resource information may be determined by the macro base station or the cluster head of the cell cluster of an upper level. For example, the macro base station or the cluster head of the cell cluster of an upper level determines the reusable spectrum resource information of the cell cluster according to at least one of a policy of an operator, a position of the cell cluster, feedback of the cell cluster, information exchange with a neighbouring macro base station or a cluster head of a cell cluster of the same level as the cell cluster of an upper level, and scheduling information.

[0036] The macro base station or the cluster head of the cell cluster of an upper level may determine different reusable spectrum resource information for different cell clusters within its coverage range.

[0037] In this embodiment, the cluster head 110 is further operable to: update the resource allocation information; and transmit the updated resource allocation information to the other base stations in the cell cluster.

[0038] The base station taken as the cluster head of the cell cluster may update the resource allocation information in a predetermined period, and transmit the updated resource allocation information to the other base stations; and it may also be that the macro base station or the cluster head of the cell cluster of an upper level updates the reusable spectrum resource information of the cell cluster and transmits the information to the base station taken as the cluster head, and the base station taken as the cluster head updates the resource allocation information according to the updated reusable spectrum resource information.

[0039] Furthermore, it may also be that the base station taken as the cluster head updates the resource allocation information after receiving some requests from the other base stations, which shall be illustrated below.

[0040] Figure 4 is a flowchart of a method for updating the resource allocation information.

[0041] In step s401: a request for extra resource allocation is received from at least one other base station by the base station taken as the cluster head of the cell cluster; and

[0042] In step s402: the resource allocation information is updated by the base station taken as the cluster head of the cell cluster according to the request for extra resource allocation.

[0043] Hence, when resource competition occurs between the cells in the cell cluster, the base station taken as the cluster head updates the resource allocation information according to the request from the at least one other base station, that is, performing resource allocation again, which may efficiently deal with the resource competition between the cells, and further reducing interference.

[0044] In performing the resource allocation in the cell cluster, the base station taken as the cluster head may, for example, orthogonal resources may be allocated to the cells.

[0045] In this embodiment, "orthogonal" may not be completely orthogonal in the strict sense, i.e., there completely exists no interference between the cells. Taking a frequency domain resource as an example, the "orthogonal" may be soft fractional frequency reuse (soft FFR).

[0046] As shown in Figure 2, if reusable spectrum resource of the cell cluster has three different bands. Then SBS1108a in the cell SC1109a and SBS5 108e in the cell SC5 109e are respectively allocated to use the same first band, SBS2 108b in the cell SC2 109b and SBS4 108d in the cell SC4 109d are respectively allocated with using the same second band, and SBS3 108c in the cell SC3 109c is allocated with using a third band.

[0047] In this embodiment, the base station taken as the cluster head may update the resource allocation information according to the priority of the at least one other base station transmitting the request for extra resource allocation, the priority of the service provided by the at least one other base station or the priority of the user equipment.

[0048] For example, after receiving a request for occupying extra resource allocation from the other base stations, the base station taken as the cluster head first attempts to allocate resources that are not currently used, and when there exists no idle resource currently or idle resources cannot satisfy demands of the other base stations and what is demanded by the other base stations is occupying an extra resource in a short time, the base station taken as the cluster head may update the resource allocation information according to the priority of the other base stations, such as transmitting instructions for suspending communications to base stations of relatively low priorities, or lowering transmission power of the base stations of relatively low priorities.

[0049] In this embodiment, the priorities of the base stations may be predetermined. For example, the priorities of the base stations are determined according to a policy of an operator.

[0050] Figure 5 is a schematic diagram of interference coordination within the cell cluster. As shown in Figure 5, the base station taken as the cluster head 110 is a base station independent of SBS1-SBS5, and transmits the resource allocation information to SBS1-SBS5 for performing resource allocation within the cell cluster, and SBS1, SBS3 and SBS5 use orthogonal resources to communicate with user equipment in the respective cells.

[0051] As shown in FIGs. 2 and 5, for example, SBS2 and SBS4 are allocated with using the same resource (the second band), and SBS2 has a priority higher than that of SBS4. When both SBS2 and SBS4 transmit requests (REQs) for using an extra resource to the base station taken as the cluster head, as SBS2 has a higher priority, the base station taken as the cluster head transmits an acknowledgement message (ASK) to SBS2 and allocates an extra resource for SBS2, and furthermore, requires SBS4 to suspend the communication or lower the transmission power.

[0052] In this embodiment, when some events occur, the base station taken as the cluster head transmits a request for reorganizing the cell cluster to the macro base station or the cluster head of the cell cluster of an upper level. For example, when the base station taken as the cluster head cannot satisfy requests of the other base stations for using resources, it transmits a request for reorganizing to the macro base station or the cluster head of the cell cluster of an upper level.

[0053] The base station taken as the cluster head of the cell cluster may transmit the information on usable resources of the cell cluster of a lower level to the cluster head of the cell cluster of a lower level, and the base station taken as the cluster head of the cell cluster of a lower level may transmit the resource allocation information to the other base stations in the cell cluster of a lower level.

[0054] In the claimed invention a refining level of allocating resources of the cluster head of the cell cluster of a lower level is higher than a refining level of allocating resources of a cluster head of a cell cluster of a previous level of the cell cluster of a lower level, and usable resources for resource allocation of the cluster head of the cell cluster of a lower level is less than usable resources for resource allocation of the cluster head of the cell cluster of a previous level of the cell cluster of a lower level.

[0055] It can be seen from the above embodiment that by transmitting the resource allocation information by the base station taken as the cluster head of the cell cluster to the other base stations in the cell cluster, resource allocation for the cells may be performed efficiently, capabilities of coordinating inter-cell interference may be improved, and as the cell cluster may be expanded to cell clusters of a lower level or more levels, scalability of the network may be improved.

[0056] Figure 6 is a schematic diagram of the resource management apparatus (not encompassed in the wording of the claims, and present for illustration purposes only) for a cell cluster as shown in Figure 2, the apparatus 600 includes: a first exchanging unit 601 configured to transmit resource allocation information to other base stations in the cell cluster. a second exchanging unit 602 configured to exchange information with a macro base station or a cluster head of a cell cluster of an upper level; a third exchanging unit 603 configured to exchange information with a cluster head of another cell cluster of the same level as the cell cluster; and a fourth exchanging unit 604 configured to exchange information with a cluster head of a cell cluster of a lower level; and the first exchanging unit 601 further exchanges other information than the resource allocation information with the other base stations in the cell cluster.

[0057] The second exchanging unit 602 receives information on usable resources of the cell cluster from the macro base station or the cluster head of the cell cluster of an upper level; and in this case, the first exchanging unit 601 generates the resource allocation information according to the information on usable resources of the cell cluster, and transmits the resource allocation information to the other base stations in the cell cluster.

[0058] In this embodiment, the first exchanging unit 601, the second exchanging unit 602, the third exchanging unit 603 and the fourth exchanging unit 604 may mutually perform data transmission.

[0059] In this embodiment, the apparatus 600 further includes: an updating unit 605 configured to update the resource allocation information.

[0060] And in this case, the first exchanging unit 601 transmits the updated resource allocation information to the other base stations in the cell cluster.

[0061] For example, the first exchanging unit 601 may receive a request for extra resource allocation from at least one other base station, and the updating unit 605 updates the resource allocation information according to the request for extra resource allocation.

[0062] The second exchanging unit 602 may be configured to transmit a request for reorganizing the cell cluster to the macro base station or the cluster head of the cell cluster of an upper level.

[0063] The fourth exchanging unit 604 may exchange information with the cluster head of the cell cluster of a lower level, such as transmitting the information on usable resources of the cell cluster of a lower level to the cluster head of the cell cluster of a lower level; and

[0064] In this embodiment, a refining level of allocating resources of the cluster head of the cell cluster of a lower level is higher than or equal to a refining level of allocating resources of a cluster head of a cell cluster of a previous level of the cell cluster of a lower level, and usable resources for resource allocation of the cluster head of the cell cluster of a lower level is less than or equal to usable resources for resource allocation of the cluster head of the cell cluster of a previous level of the cell cluster of a lower level.

[0065] It can be seen from the above embodiment that by transmitting the resource allocation information by the base station taken as the cluster head of the cell cluster to the other base stations in the cell cluster, resource allocation for the cells may be performed efficiently, capabilities of coordinating inter-cell interference may be improved, and as the cell cluster may be expanded to cell clusters of a lower level or more levels, scalability of the network may be improved.

[0066] Figure 7 is a schematic diagram of the base station of the embodiment of the present invention. As shown in Figure 7, the base station 700 may include a central processing unit (CPU) 701 and a memory 702, the memory 702 being coupled to the central processing unit 701. For example, the memory 702 may store various data, and furthermore, it may store a program for information processing, and execute the program under control of the central processing unit 701.

[0067] For example, the central processing unit 701 may be configured to perform the following control: transmitting resource allocation information by a base station taken as a cluster head of the cell cluster to other base stations in the cell cluster.

[0068] Furthermore, as shown in Figure 7, the base station 700 may include a transceiver 703, and an antenna 704, etc.; wherein, functions of the above components are similar to those in the related art, and shall not be described herein any further. It should be noted that the base station 700 does not necessarily include all the parts shown in Figure 7, and furthermore, the base station 700 may include parts not shown in Figure 7, and the related art may be referred to.

[0069] An embodiment further provides a computer-readable program, wherein when the program is executed in a resource management apparatus for a cell cluster or a base station, the program enables the resource management apparatus for a cell cluster or the base station to carry out the resource management method for a cell cluster as described above.

[0070] An embodiment provides a storage medium in which a computer-readable program is stored, wherein the computer-readable program enables a resource management apparatus for a cell cluster or a base station to carry out the resource management method for a cell cluster as described in Embodiment 1.

[0071] The above apparatuses and methods of the present invention may be implemented by hardware, or by hardware in combination with software. The present invention relates to such a computer-readable program that when the program is executed by a logic device, the logic device is enabled to carry out the apparatus or components as described above, or to carry out the methods or steps as described above. The present invention also relates to a storage medium for storing the above program, such as a hard disk, a floppy disk, a CD, a DVD, and a flash memory, etc.

[0072] The method / apparatus described with reference to the embodiments of the present invention may be directly embodied as hardware, a software module executed by a processor, or a combination thereof. For example, one or more of the block diagrams and / or one or more combinations of the block diagrams shown in Figure 6 may correspond to soft modules of a process of a computer program, and may also correspond to hardware modules.

[0073] The soft modules may be located in an RAM, a flash memory, an ROM, an EPROM, and EEPROM, a register, a hard disc, a floppy disc, a CD-ROM, or any memory medium in other forms known in the art. A memory medium may be coupled to a processor, so that the processor may be able to read information from the memory medium, and write information into the memory medium; or the memory medium may be a component of the processor. The processor and the memory medium may be located in an ASIC. The soft modules may be stored in a memory of a mobile terminal, and may also be stored in a memory card of a pluggable mobile terminal. For example, if equipment (such as a mobile terminal) employs a MEGA-SIM card of a relatively large capacity or a flash memory device of a large capacity, the soft modules may be stored in the MEGA-SIM card or the flash memory device of a large capacity.

[0074] One or more functional blocks and / or one or more combinations of the functional blocks in figures may be realized as a universal processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware component or any appropriate combinations thereof. And they may also be realized as a combination of computing equipment, such as a combination of a DSP and a microprocessor, multiple processors, one or more microprocessors in communication combination with a DSP, or any other such configuration.Alternatives and Modifications

[0075] In the embodiment, the cluster head of the cell cluster is a base station serving for a cell in the cell cluster, i.e., a serving base station of a cell. In an alternative, the cluster head is another base station other than one of the base stations serving for cells in the cell cluster.

[0076] When the cluster head of the cell cluster is another base station than the base stations serving for the cells in the cell cluster, the small base station taken as the cluster head transmits the resource allocation information to base stations of all cells in the cell cluster.

[0077] Furthermore, when the cell cluster includes only one cell. Then, the base station taken as the cluster head does not transmit the resource allocation information to the other base stations. When another cell joins in the cell cluster, then base station taken as the cluster head transmits the resource allocation information to a base station of the other cell.

Claims

1. A resource management method for a cell cluster (103), the cell cluster comprising at least one cell, wherein the method comprises: receiving, at a base station taken as a cluster head (110) of the cell cluster, information on usable resources (s301) of the cell cluster from a macro base station (101); generating, by said base station taken as the cluster head of the cell cluster, resource allocation information including spectrum resource, and by using the received information on usable resources of the cell cluster; transmitting said resource allocation information by the base station taken as the cluster head of the cell cluster to other base stations in the cell cluster (s302) and to a cluster head of a cell cluster of a lower level than the cluster head of the cell cluster (s304); and characterised in that: a refining level of allocating resources of a cluster head of the cell cluster of the lower level is higher than a refining level of allocating resources of a cluster head of a cell cluster of an upper level therefrom, and usable resources for resource allocation of the cluster head of the cell cluster of the lower level are less than usable resources for resource allocation of the upper level.

2. The method according to Claim 1, wherein the information on the usable resources of the cell cluster (103) is information on reusable spectral resources.

3. The method according to any preceding claim, wherein the base station taken as the cluster head (110) of the cell cluster is: a base station serving for a cell in the cell cluster (103); or another base station than base stations serving cells in the cell cluster.

4. The method according to any preceding claim, wherein the method further comprises: updating the resource allocation information by the base station taken as the cluster head (110) of the cell cluster (103); and transmitting the updated resource allocation information by the base station taken as the cluster head of the cell cluster to the other base stations in the cell cluster.

5. The method according to claim 4, wherein updating the resource allocation information by the base station taken as the cluster head (110) of the cell cluster (103) comprises: receiving (s401) a request for extra resource allocation from at least one other base station by the base station taken as the cluster head of the cell cluster (s401); and updating (s402) the resource allocation information by the base station taken as the cluster head of the cell cluster according to the request for the extra resource allocation (s402).

6. The method according to claim 5, wherein updating the resource allocation information by the base station taken as the cluster head (110) of the cell cluster (103) according to the request for the extra resource allocation comprises: updating the resource allocation information by the base station taken as the cluster head of the cell cluster according to a: priority of the at least one other base station transmitting the request for the extra resource allocation; or priority of a service provided by the at least one other base station.

7. The method according to any preceding claim, wherein the method further comprises the step of transmitting a request for reorganizing the cell cluster (103) by the base station taken as the cluster head (110) of the cell cluster to the macro base station (107).

8. The method according to any preceding claim, wherein the method further comprises the steps of: transmitting the information on usable resources of the cell cluster (103) of the lower level by the base station taken as the cluster head (110) of the cell cluster to the cluster head of the cell cluster of the lower level; and transmitting the resource allocation information by the base station taken as the cluster head of the cell cluster of the lower level to the other base stations in the cell cluster of the lower level.