JP2013225944A - Base station and radio communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce interference affecting a surrounding cell by handing over a terminal, in a radio communication system including base stations each having a different cell radius.SOLUTION: A serving cell mitigates a handover decision criterion so that a terminal can easily hand over from a base station having a larger cell radius to a base station having a smaller cell radius, on the basis of radius information of the own cell and the surrounding cell acquired in advance. The serving cell obtains the value of an evaluation function based on a propagation condition (for example, reference signal reception power) between a terminal and a target cell and interference/load information exchanged between base stations, and compares the value of the evaluation function and the handover criterion value, so as to select a terminal to be handed over. As a handover destination, a target cell producing an interference amount reduction effect, such as a cell having a large reference signal reception power acquired from the terminal, is selected.

Description

  The present invention relates to a base station and a radio communication system, and more particularly, to a base station equipped with an interference-based handover control technique in a cellular radio communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The present invention relates to a communication system.

  A configuration example of a general cellular radio system will be described with reference to FIG. The cellular radio system covers an area called a cell by the radio base station 1201, and has a name such as a target cell, a micro cell, a serving cell, a femto cell, etc. depending on the size of the coverage area and the function classification. The cellular radio base station 1201 having the largest coverage area is also called a target cell base station, and accommodates a mobile station 1203 within a range of about several hundred to several thousand meters. The base station 1201 can communicate with the IP network or connect “calls” between the mobile stations 1203 existing at remote locations via a core network 1202 constituted by a host device such as a base station management device. I can do it.

  The third generation cellular radio communication system using a CDMA (Code Division Multiple Access) method enables communication of multimedia information via an IP network. As a wireless system adopting CDMA, for example, there are UMTS (Universal Mobile Telecommunications System) and EVDO (Evolution Data Optimized). There is a growing need for communication services with higher broadband speeds against the backdrop of rich mobile content. For example, it is expected that support for broadband communication will be common, such as best-effort data communication, voice communication using VoIP, and distribution of streaming information such as video.

  A wireless communication system using the OFDMA system has attracted attention as a next generation wireless communication system of CDMA with the keywords of wideband and high speed. The OFDMA system can take a hardware configuration more suitable for wideband than the CDMA system, and is said to be suitable for realizing large-capacity wireless communication.

  The OFDMA scheme is a technique for improving frequency use efficiency by orthogonally multiplexing a plurality of orthogonal carrier waves on the frequency axis. A cellular radio communication system employing the OFDMA scheme is positioned as a 3.9th generation cellular radio communication system, and E-UTRA (Evolved-UMTS Terrestrial Radio Access) is a representative standard. E-UTRA is also called LTE (Long Term Evolution) and has been internationally standardized by a standard organization called 3GPP (3rd Generation Partnership Project). E-UTRA is used as a radio access method, and E-UTRAN (Evolved-UMTS Radio Access Network) is used as a term indicating an access network.

  In a wireless communication system, it is known that radio interference from an adjacent base station and a terminal under its influence affects communication in wireless access for connecting a terminal and a base station. In particular, in the OFDMA wireless communication system, it is known that the influence of interference from an adjacent base station and its subordinate terminals is large, and the degree of difficulty in designing the arrangement of base stations is high.

  In radio access at the cell edge, which is the boundary of the coverage area of the base station (there is a sector, cell, etc.), the interference power level from the adjacent cell and the desired signal level from the own cell are in conflict with each other, and the channel quality It appears prominently in the form of degradation of SINR (Signal to Interference and Noise power Ratio), which is an index that expresses.

  In order to improve channel capacity in radio access as a whole system, it is important to reduce interference between adjacent cells. The interference reduction between adjacent cells is summarized in that all wireless stations transmit the minimum necessary power. However, if power is suppressed without darkness, problems such as a reduction in the coverage area and a reduction in communication speed occur. Therefore, it is necessary to specify excessive power and lower the power.

  In the uplink operation for radio access, the transmission power control of the terminal existing at the cell edge is also closely related to the interference control. The cell edge terminal needs to transmit with high power in order to overcome the propagation path loss occurring with the base station and ensure a certain communication quality. For this reason, for example, Patent Document 1 discusses a method in which a terminal preferentially selects a cell managed by a base station having a small propagation path loss.

  When suppressing the transmission power, in order to secure a certain communication quality, for example, it is necessary to allocate more radio resources than usual and lower the coding rate of the error correction code. Interference can be reduced by lowering the power, but the frequency resource usage rate increases, so the load on the radio access system increases.

  Further, in addition to considerations for reducing interference in the radio access part as described above, load distribution control between cells is also important in a place where terminal connections are concentrated. If the processing amount of the base station backbone is exceeded, it is likely that it is difficult to establish a call connection.

  As a means for carrying out load balancing, approaches that actively use handover to adjacent cells have been studied. The handover algorithm in the wireless system follows a standard Entity procedure.

JP 2009-159637 A

  Consider a situation where two base stations with different cell radii are adjacent. As shown in FIG. 4, the size of the cell radius varies depending on the location and use of the base station. For example, in the city center or the like, since terminals are densely populated, it is difficult to accommodate the terminals only with a base station having a large cell radius such as a macro cell from the viewpoint of load on the base station.

  In view of this, an operation to support a macro cell by interspersing small base stations such as a micro cell and a pico cell having a small cell radius is employed. Or, the introduction of a micro base station that can be installed in the home, such as a femto cell, has been specifically studied.

  As described above, when one or more base stations having different cell radii are adjacent to each other, the distance loss between the base station and the terminal is not constant. As shown in FIG. 5, a cell edge terminal belonging to a cell having a large cell radius performs uplink transmission with high power to satisfy the required power and compensate for the distance loss (503 in the figure). On the other hand, a cell edge terminal belonging to a cell having a small cell radius performs uplink transmission with relatively low power (501 in the figure).

  In this case, as shown in FIG. 4, the cell edge terminal belonging to the cell having a small cell radius is inhibited from communication by the large interference power due to the large power output from the cell belonging to the cell having a large cell radius. Therefore, when a cell edge terminal belonging to a cell having a large cell radius is handed over to a cell edge having a small cell radius, it is expected that interference given to neighboring cells can be reduced (the lower diagram in FIG. 4). From the above, it is desirable to make the terminal belong to the cell having the smallest cell radius as much as possible.

  However, if the handover is simply performed depending on the distance loss, the terminals concentrate on a specific cell and cannot withstand the load. Furthermore, since there is no room for allocated resources, the cell becomes vulnerable to interference from neighboring cells, and the system may fail.

  In view of the above points, the present invention provides a base station, a radio, and a radio that realizes terminal handover that reduces the amount of interference given to the system from interference information exchange between base stations and peripheral base station information from the terminal. An object is to provide a communication system. Another object of the present invention is to improve the channel capacity seen in the entire system and improve the overall cell throughput.

The present invention relates to a handover technique based on the amount of interference given to neighboring cells. Provided are a method for detecting a terminal having the largest amount of interference to neighboring cells and a method for determining a handover destination base station.
An object is to promote handover from a cell having a large terminal connection load to a cell having a small terminal connection load to realize load distribution and to minimize interference between cells. A criterion for handover and a scheduling method for reducing interference are provided.

  In the present invention, a system in which a serving cell (handover source) controls handover of a subordinate terminal will be described as an example. The serving cell determines a handover destination capable of balancing the load with a terminal that reduces the amount of interference of the entire system with one or more target cells (handover candidates).

  The serving cell has an evaluation function based on the propagation status between the terminal and the target cell and interference / load information exchanged between the target cell and the serving cell, and selects the terminal to be handed over using the function and the handover reference value. To do. A target cell having the greatest effect of reducing the amount of interference after handover is selected, and the terminal is handed over. The target cell side reflects its own cell load information in the interference information notified to the serving cell.

  The radio base station of the present invention is, for example, a radio base station connected to one or more base stations through a network, and the base station exchanges between one or more base stations having different cell IDs. Generating a handover criterion value and an interference criterion using the power information of the neighboring base station reported from the terminal, the cell radius of the neighboring cell, and the load information of the own cell and other cells, Has a handover parameter control unit.

The load information is, for example, the number of terminals connected to the cell. The load information may use a hardware usage rate exchanged between base stations.
The interference information is interfered information exchanged between base stations, for example. The interfered information may use Overload Indicator.

  The handover parameter control unit of the base station changes the interference determination criterion according to the cell radius compared with the adjacent cell. Further, the handover parameter control unit changes the handover determination reference value according to the cell radius compared with the adjacent cell.

  The handover parameter control unit of the base station changes the interference criterion according to the load information. Further, the handover parameter control unit changes the handover determination criterion according to the load information.

  The base station converts the interference cell power information reported from the terminal and interference information exchanged between the base stations into an evaluation function for each terminal, and the evaluation function and the handover Compared with the determination criterion, a handover determination unit is further provided that identifies a terminal that exceeds the criterion as a terminal to be handed over. The evaluation function for each terminal by the handover determination unit is used to reduce the distance between the terminal and the one or more interference information notified from the neighboring base station to the terminal reporting the power information of the neighboring base station. What multiplied the corresponding weight may be accumulated.

The handover destination base station to which the terminal to be handed over specified by the handover determination unit moves may be a cell in which the evaluation function value after movement is the smallest.
The handover parameter control unit may handle interference information for generating weights in units of resource blocks.

According to the first solution of the present invention,
A wireless communication system comprising a plurality of base stations,
Each base station
Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
Provided is a radio communication system that facilitates handover of a terminal in a cell of the base station to a cell of a neighboring base station when the cell radius of the base station is large.

According to the second solution of the present invention,
A wireless communication system comprising a plurality of base stations,
Each base station
Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
In order to make it easier to belong to a neighboring base station having a cell radius smaller than that of the own base station, an offset with respect to a determination reference power value when the terminal in the cell of the own base station returns from IDLE_MODE is selected to the terminal. A wireless communication system for notification is provided.

According to the third solution of the present invention,
Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
When the cell radius of the own base station is large, a base station that makes it easy to hand over a terminal in the cell of the own base station to a cell of a neighboring base station is provided.

According to the fourth solution of the present invention,
Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
In order to make it easier to belong to a neighboring base station having a cell radius smaller than that of the own base station, an offset with respect to a determination reference power value when the terminal in the cell of the own base station returns from IDLE_MODE is selected to the terminal. A reporting base station is provided.

  According to the present invention, it is possible to realize terminal handover that reduces the amount of interference given to the system from the interference information exchange between base stations and the peripheral base station information from the terminal. Since the channel capacity can be improved, an overall improvement in cell throughput can be expected.

The figure explaining the general system of a radio | wireless system. The figure explaining the apparatus connection of the handover of Intra-MME. The figure explaining the apparatus connection of the handover of Inter-MME. The figure explaining the system interference reduction by a hand-over. The figure explaining the difference of the cell edge terminal transmission power by the difference of a cell radius. The figure explaining the hardware constitutions of a base station management apparatus. The figure which shows the functional block of the base station of this invention. FIG. 3 is a handover sequence diagram according to the first embodiment of the present invention. The flowchart which adjusts a handover criteria value with a cell radius. The flowchart which adjusts an interference determination reference | standard with load information. The situation figure explaining a handover determination part operation | movement. The figure explaining the example (OI) of the interference information for every RB. The flowchart of the handover determination part which has the characteristics of this invention. Schematic of handover reference value adjustment according to cell radius difference. The figure explaining the example which adjusts a hand-over reference value by table lookup. 10 is a flowchart for adjusting a handover determination criterion with load information. The figure explaining the example which adjusts a handover criteria according to a load balance difference. The flowchart which adjusts interference determination conditions according to a cell radius difference. FIG. 10 is a handover sequence diagram in Embodiment 2 of the present invention. FIG. 10 is a handover sequence diagram in Embodiment 3 of the present invention. A table that manages cell radius information. A table that manages the power report value from the terminal. A table that manages the amount of interference from neighboring cells. It is explanatory drawing of a load information table. It is a block diagram of the radio | wireless communications system of this Embodiment.

The mode for carrying out the present invention will be described in several embodiments. These embodiments may be implemented individually or in combination. In the following description, those with the same reference numerals in the drawings perform the same operation, and thus the description thereof is omitted.
Example 1

  As a specific system configuration example, an entity configuration of E-UTRAN is shown in FIGS. FIG. 2 is a device connection diagram related to handover between base stations managed by one MME (Mobility Management Entity) 212. FIG. 3 is a device connection diagram regarding handover between base stations via different MMEs.

  Handover in E-UTRAN can be broadly divided into these two forms. Between the two cases, procedures such as bearer replacement are different, but the processing between the terminal 203 and the base station 201-a is approximately the same.

  The base station host device 204 used in the following description is a part of an OAM (Operation And Maintenance) device 210, MME 212, S-GW (Serving-Gateway), P-GW (Packet Data Network-Gateway), or Refers generically. The OAM device 210 also includes a base station control device (EMS: Element Management System).

FIG. 25 is a configuration diagram of the radio communication system according to the present embodiment.
The wireless communication system includes, for example, a plurality of base stations 201 and a base station host apparatus 204. The base station 201 includes ones having different cell radii. Each base station 201 communicates wirelessly with a terminal 203 in a cell covered by the base station. The base station may cover a plurality of cells (sometimes referred to as a sector).

  In the present embodiment, base stations having different cell radii will be described. However, base stations having different cell characteristics, for example, radio wave arrival distance, transmission power, radio wave arrival range parameters, and radio wave characteristics may be used. The cells are not limited to those specified by the concept of radius, but may be, for example, directional. In addition, the base station is not limited to these, and a base station that differs in the degree of interference caused by radio waves communicated with a certain terminal on other radio waves may be used. Corresponding information can also be used for cell radius information described later.

FIG. 6 is a hardware configuration diagram of the base station management apparatus.
The base station management device may be configured to be added to a normal base station, or may be configured integrally with the base station. The base station management apparatus includes, for example, a memory 401, a processing unit 402, an interface 403, and a logic circuit 404. The memory 401 has, for example, a cell radius information table (Coverage Information Table) 4011, a received power table (Received Power Report Table) 4012, an interference management table (Interference Management Table) 4013, and a load information table (Load Information Table) 40. Each table will be described later.

FIG. 7 shows functional blocks of the base station according to the present embodiment.
The network I / F unit 411 is a block that manages an interface for performing communication with the base station host apparatus 204 or another base station.
The transmission unit 412 performs signal processing in accordance with the wireless standard such as error correction coding and modulation processing, and transmits a wireless signal to the terminal 203.
The receiving unit 413 receives a radio signal from the terminal 203 and performs demodulation processing, error correction decoding, and the like.

The handover parameter control unit 414 performs handover parameter control (step 2103) in FIG. 8 to be described later based on information received from the network I / F unit 411, for example.
The handover determining unit 415 determines a terminal 203 to be handed over and a target cell to be handed over based on control shown in step 2104 described later.
The base station mounting form having these features may be realized by using either program processing using the DSP 402 or dedicated hardware combining the logic circuit 404.

FIG. 21 is an explanatory diagram of a cell radius information table for managing cell radius information.
The cell radius information table 4011 stores, for example, cell radius information corresponding to the cell IDs of the peripheral cells. In the example of FIG. 21, transmission power information from a base station and antenna tilt are stored as cell radius information. The transmission power information from the base station and the antenna tilt shown in FIG. 21 are examples of information indicating cell characteristics, and are examples of attribute information of base stations constituting the cell.

FIG. 22 is an explanatory diagram of a received power table for managing the power report value from the terminal.
The reception power table 4012 stores, for example, the reception power value of the serving cell and the reception power value of the target cell for each terminal ID. The received power of the target cell can be stored for cells in which the terminal can receive radio waves.

FIG. 23 is an explanatory diagram of an interference management table for managing the amount of interference from neighboring cells.
In the interference management table 4013, for example, interference information is stored in association with the cell ID and the resource block ID.

FIG. 24 is an explanatory diagram of a load information table.
The load information table 4014 stores the load information of the cell corresponding to the cell ID, for example.
For each ID, a number, a symbol, or the like assigned in advance can be used.

  FIG. 8 illustrates a handover preparation operation performed between the terminal 203, the serving cell 201-a serving as a base station in communication with the terminal 203, and the target cell 201-b serving as a handover candidate base station.

  In step 2101, the base station host apparatus 204 notifies the cell IDs of the serving cell 201-a and the target cell 201-b, and information related to a predetermined cell radius (cell radius information). As the cell radius information, the transmission power information itself from the base station may be notified. If the information about the sector gain or the antenna gain can be notified, the cell radius of the adjacent cell can be estimated with higher accuracy. Moreover, you may designate cell ID which can distinguish the kind of base station, such as a picocell base station and a femtocell base station, as cell radius information. With this information, the base station can know the approximate value of the transmission power of the neighboring base stations, and can know the cell radius of the adjacent cell. For example, each base station stores a base station type and a cell radius or transmission power information in association with each other. In addition to the power information, information about the tilt of the antenna may be notified because it is highly related to the cell radius. Hereinafter, the cell radius information includes one or more of these pieces of information.

  The cell radius information of the adjacent cell generated here may be stored in a table (FIG. 21: Coverage Information Table) 4011 on the memory 401. In the example of FIG. 21, the cell ID, transmission power information, and tilt are stored correspondingly. Whenever cell information is changed due to new installation of a base station, the information is updated and used. The method by which the base station detects the information change may be autonomous detection by measurement or the like, may be determined from a report from a terminal, or may be dependent on a command notification from the host device 204.

  In step 2001, the serving cell 201-a causes the terminal 203 to measure the received signal quality of the reference signal from the serving cell 201-a and one or more target cells 201-b. Received signal quality refers to received power intensity, signal to noise and interference power ratio (SINR: Signal to Interference and Noise Ratio), and propagation path loss (path loss) calculated from the difference between transmitted power information and received power intensity. May be.

In step 2002, uplink resource allocation (UL Allocation) for reporting the measurement value in step 2003 from the serving cell 201-a to the terminal 203 is performed.
In step 2003, the terminal 203 reports the measurement of the received signal quality from the serving cell 201-a and the target cell 201-b. For example, E-UTRA reports the received power strength of a reference signal called RSRP (Reference Signal Received Power).

  Step 2102-a exchanges load information and interference information between base stations. The means for exchanging information between base stations may be wired or wireless. For example, E-UTRAN, which is a cellular radio standard, provides a wired interface X2 I / F that defines information exchange between base stations.

  As the load information used below, for example, the hardware usage rate at the base station 201, the radio resource usage rate, the number of connected terminals, the line capacity of the backhaul line of the base station, and the usage rate thereof may be used. good. For example, if E-UTRAN (TR36.423) is assumed, a hardware load indicator or the like may be used.

  Similarly, the interference amount or the interference amount may be used as the interference information. For example, if E-UTRAN (TR36.423) is assumed, a load indicator or the like may be used.

The serving cell 201-a stores the received interference information and load information in the interference management table 4013 and the load information table 4014, respectively.
In the handover parameter control 2103, the serving cell 201-a includes interference information exchanged with one or more base stations having different cell IDs, and received signal quality information of the neighboring base station 201-b reported from the terminal 203. The handover criterion value is generated using the cell radius of the neighboring cell and the load information of the own cell and other cells.

  Detailed processing will be described with reference to FIGS. 9, 14, 15, and 16. When operating as a serving cell, the base station generates a handover criterion value according to the flow shown in FIG. The handover criterion value is a value based on the interference amount of the entire system. The handover determination reference value corresponds to a threshold value of a cost function for handover determination described later, and is a reference for determining whether or not to perform handover.

FIG. 9 is a flowchart for adjusting the handover determination reference value by the cell radius.
In step 3001, the serving cell 201-a determines whether the cell radius of the own cell is larger than that of the neighboring cells. If the cell radius of the own cell is larger than that of the surrounding cell, the process proceeds to step 3002. If the own cell has a smaller cell radius than the neighboring cells, the process proceeds to step 3003.

In step 3002, the serving cell 201-a relaxes the handover criterion in order to prompt a handover to a neighboring cell having a cell radius smaller than that of the own cell. For example, the handover determination reference value is lowered.
In step 3003, the serving cell 201-a strengthens the handover criterion in order to make it difficult to perform a handover to a neighboring cell whose cell radius is larger than that of the own cell. For example, the handover determination reference value is increased. Steps 3001 to 3003 described above are executed for each peripheral cell. An initial value is set in advance as a handover criterion.

  FIG. 14 is a table summarizing the results of the processing in steps 3002 and 3003. The handover determination reference value is adjusted based on the cell radius difference for each target cell ID. As shown in FIG. 15, the adjustment width can be increased as the cell radius difference from the own cell increases. The numerical values in FIG. 15 are merely examples, and may have different values depending on the evaluation method of the interference amount of the entire system. The correspondence relationship between the cell radius difference and the adjustment width as shown in FIG. 15 may be held in advance as a table by the base station. Further, the adjustment range may be autonomously adjusted based on the amount of system interference that has changed as a result of the handover.

FIG. 16 is a flowchart for adjusting the handover criterion based on the load information.
Similarly, a load distribution may be attempted by handover from a high-load cell to a low-load cell. Specifically, the serving cell 201-a adjusts the handover determination reference value according to the load information in the flow according to FIG. The process described in FIG. 16 may be executed for each peripheral cell. This processing is based on the following concept. A cell with low load and strong interference can provide high-speed communication by taking advantage of the load margin while maintaining low frequency utilization efficiency. That is, a certain amount of interference can be tolerated.

On the other hand, in order to provide high-speed communication, a cell with high load and strong interference requires both high frequency utilization efficiency. In order to obtain high frequency utilization efficiency in a state where there is no allowance for the allowable load, it is better to promote the handover of the terminal to a low-load cell.
However, it should be noted that a handover from a small cell radius to a large cell radius has a higher interference power transmitted by the terminal than a handover from a large cell radius to a small cell radius cell.

  In step 3011, the serving cell 201-a determines whether or not the load difference (load balance) between the neighboring cell being compared and its own cell exceeds a threshold value that is held in advance. If the load balance exceeds the threshold value, the process proceeds to step 3012. If the threshold is not exceeded, the process proceeds to step 3013. The handover criterion adjustment in step 2103 in FIG. 8 includes the purpose of balancing the load with the neighboring cell, and therefore, the adjustment is performed based on the load information relative to the neighboring cell.

  In step 3012, the serving cell 201-a may relax the handover criterion when the load of the own cell is heavier than that of other cells. In addition, the serving cell 201-a may reinforce the handover criterion when the load on the own cell is lighter than other cells. After executing this step, the process proceeds to step 3014.

  In step 3013, the load balance threshold used in step 3011 is updated, and the process proceeds to step 3014. In step 3014, when it is detected that the comparison between the load balance and the threshold value is completed, the flow is terminated. If the comparison has not ended, the process returns to step 3011. These processes can be realized, for example, by scanning a load balance threshold value and adjustment width table as shown in FIG. FIG. 17 shows a tendency that the adjustment range of the handover determination criterion increases as the load balance increases. However, the stated value is an example.

As described above, the serving cell 201-a adjusts the handover reference value used for the handover determination in Step 2104.
Further, the target cell 201-b adjusts the condition (interference determination condition) for determining that the target cell 201-b is receiving interference by using the cell radius of the neighboring cell and the load information of the own cell and other cells. One of the purposes of this process is to prevent a lightly loaded cell from unnecessarily complaining that the interference is large. In other words, it is tolerant to the interference of a heavily loaded cell.

FIG. 10 is a flowchart for adjusting the interference criterion based on the load information. According to the flow shown in FIG. 10, the interference determination method is changed according to the load information.
In step 3021, the target cell 201-b compares the load information of its own cell with the load information of other cells, and proceeds to step 3022 if the load difference exceeds a predetermined threshold. If the threshold is not exceeded, the process is terminated.

  In step 3022, the target cell 201-b adjusts the condition for determining interference information with a cell exceeding the threshold. If the target cell 201-b is more severe in load, the interference determination condition is strengthened. If the target cell 201-b has a lighter load, the interference determination condition is relaxed. For example, a high threshold is set for determining that interference has occurred.

  FIG. 18 is a flowchart for adjusting the interference determination condition according to the cell radius difference. The target cell 201-b adjusts the interference determination condition according to the cell radius of the neighboring cell according to the flow shown in FIG.

  In step 3031, the target cell 201-b compares the cell radius of its own cell with the cell radius of the surrounding cells. If the cell radius is larger than that of the surrounding cells, the process proceeds to step 3032. If the cell radius is smaller than that of the surrounding cells, the process proceeds to step 3033.

  In Step 3032, the target cell 201-b may tighten the interference determination condition. This is because a cell having a large cell radius has a large interference with neighboring cells. In these processes, for example, when the amount of interference is used as the interference information, the amount of interference may be set larger and reported.

  In step 3033, the target cell 201-b may perform processing for relaxing the interference determination condition. For example, when the amount of interference is used as interference information, the amount of interference may be set conservatively and reported. This is because a cell having a small cell radius has a relatively small interference with respect to neighboring cells. The serving cell 201-a and the target cell 201-b regularly exchange interference information (corresponding to step 2102), and the changed interference determination condition is reflected in, for example, the exchange of interference information at the next timing. Can be done.

  For example, taking E-UTRAN as an example, there is a load indicator as means for notifying the amount of interference using the inter-base station interface X2. The amount of interference may be notified using Overload Indication (OI) included in the Load Indicator. The OI designates the amount of interference with High, Middle, and Low for each resource block (Resource Block: RB). RB is a radio resource allocation unit based on OFDMA, which is divided in frequency and time directions. Although not OFDMA, the present invention may be applied to a scheme such as SC-FDMA (for example, used in E-UTRA Uplink) in which resource allocation is performed by dividing in time and frequency directions. When using OI, the judgment criteria of High, Middle, and Low may be changed based on the cell radius and load information.

  Since a certain serving cell is a target cell when viewed from the viewpoint of another cell, the operation of Step 2103 may be performed by all base stations. Further, when viewed from the serving cell, there are usually one or more target cells, and the processing in step 2103 is performed among a plurality of base stations.

  Returning to FIG. 8, in the handover determination in Step 2104, the serving cell 201-a determines the cost function for handover from the signal quality (for example, reference signal power) reported from the terminal 203 and the interference information / load information exchanged with the target cell. And the generated cost function for handover is compared with the handover reference value. When the value of the cost function exceeds the handover reference value, a process for handing over the terminal 203 is performed.

A handover determination process flow will be described with reference to FIG.
In step 4001, the serving cell 201-a collects interference information from neighboring cells. For example, information may be collected through an interface between base stations, X2 I / F in E-UTRAN, and the like. Note that the interference information acquired in step 2102-a may be used and step 4001 may be omitted.

  In Step 4002, the serving cell 201-a collects reference signal received power of neighboring cells reported from the cell edge terminal 203 belonging to the own cell. The terminal 203 may receive a reference signal of a plurality of neighboring cells to generate a handover event and report the reference signal received power of the neighboring cells to the serving cell. Note that the reference signal received power acquired in step 2003 may be used and step 4002 may be omitted.

  In step 4003, the serving cell 201-a obtains the correlation between the interference information of the neighboring cells exchanged between the base stations and the neighboring cell reference signal received power reported from the terminal 203. This correlation value is defined as a cost function.

  The method for obtaining the correlation in this step will be described with reference to FIGS. 11, 12, 22 and 23. FIG. As shown in the example of FIG. 11, it is assumed that there are a terminal (UE) # 1, UE # 2, and UE # 3 belonging to a cell (Cell) # 1. It is assumed that UE # 1 exists at the cell edge of Cell # 1 and can receive downlink reference signals of Cell # 2 and Cell # 3. Also, UE # 2 can receive downlink reference signals from Cell # 1 and Cell # 2, and UE # 3 can receive downlink reference signals from Cell # 1.

  At this time, UE # 1 reports the reference signal reception power of Cell # 2 and Cell # 3, which are the target cells, in addition to the reception power of the serving cell, to Cell # 1, which is the serving cell (step 2003 or 4002). . Similarly, UE # 2 reports the reference signal received power of Cell # 1 and Cell # 2 to Cell # 1. Thus, Cell # 1, which is a serving cell, receives a report of received power information from a neighboring cell from a subordinate terminal, and stores a correspondence relationship between the neighboring cell and the reported power (Received) as shown in FIG. (Power Report Table) 4012 is managed. Cell # 1, which is a serving cell, updates the value of the table according to the connection status of the terminal and the reported value of power.

  Serving cell Cell # 1 is the interface between base stations, and has received interfered information from target cell Cell # 2 and Cell # 3 (step 2102a or 4001). The interfered information may be in RB units as shown in FIG. The serving cell collects interference information from neighboring cells, has an interference amount management table (Interference Management Table) 4013 as shown in FIG. 23, and updates the information every time the interference information is notified.

  Under the above conditions, when the serving Cell # 1 calculates the cost function for handover of the UE # 1, the target Cell # 2 and Cell # 3 whose power information is stored in the Received Power Report Table 4012 of FIG. 23, the interference information of the Interface Management Table 4013 shown in FIG. 23 is referred to. Similarly, when calculating the cost function for handover of UE # 2, the interference information from Cell # 2 is referred.

  When calculating the cost function for handover from the interference information, the influence of the cell radius is taken into account. The distance between each Cell and UE # 1 is estimated from the reference signal received power information of each Cell reported from UE # 1 and the transmission power of neighboring cells acquired in Step 2101, and the distance attenuation (path loss) is calculated. May be. For example, the distance attenuation may be obtained based on the value obtained by subtracting the reference signal reception power of the terminal from the transmission power information of the cell. Note that when not the transmission power but the cell radius or cell ID is received in step 2101, the transmission power may be estimated from the cell radius and the distance attenuation may be obtained from the estimated transmission power. The same processing is performed for UE # 2.

  Using the distance attenuation value, the interference information exchanged between the base stations is weighted. For example, the path loss ratio between UE # 1 and Cell # 2, Cell # 3, and UE # 2 and Cell # 3 is calculated. In order to calculate a specific example, suppose that it was 10:20:15 respectively (refer FIG. 22). The magnitude of the path loss may be almost equivalent to the fact that the uplink transmission power needs to be set high.

  Further, the interfered information is also converted into a function. For simplicity, it is assumed that the interference information OI is scored as High = 2 points, Middle = 1 points, Low = 0 points, and the like. At this time, as shown in FIG. 12, when the interference information is fed back, the cost function of UE # 1 for Cell # 2 is calculated as (1 + 2 + 2 + 2 + 2) × 10 = 80. In this way, the value of the cost function is obtained by multiplying the sum of the scored interference information by the predetermined number of RBs by a value corresponding to the above-described distance attenuation ratio. Similarly, the cost function of UE # 2 for Cell # 2 is calculated as (1 + 2 + 2 + 2 + 2) × 15 = 150. When considering only Cell # 2, UE # 2 has a higher cost function, but UE # 1 also has a cost function for Cell # 3. The cost function of UE # 1 for Cell # 3 is calculated as (1 + 1 + 2 + 1 + 2) × 20 = 140, and the accumulated cost function for UE # 1 is 220, which is higher than 150 of UE # 2, and the amount of interference given to the system is It can be judged that it is large. In this way, when determining a terminal to be handed over to one of the target cells, an interference amount-based method that takes into account the influence on other target cells may be used.

  In step 4004, the serving cell determines whether or not the cost function of each handover candidate terminal exceeds a threshold value (the handover criterion adjusted in step 2103). Here, a cumulative value of cost functions for each terminal (220 in the above-described example of UE # 1) can be used. If the threshold is exceeded, the terminal is determined as a terminal to be handed over, and the process proceeds to Step 4005. If the threshold is not exceeded, the process may be terminated. Alternatively, the method may be switched to a method of selecting a terminal to be handed over based on a path loss focused on one target cell instead of the above-described interference amount base.

  In step 4005, for the terminal determined to be handed over in step 4004, a cell to be handed over (handover destination cell) is determined, and the process proceeds to a process for inquiring handover (step 2005). A method for determining a cell to be handed over may be a cell having the largest power value reported by the terminal to be handed over with reference to the Received Power Report Table 4012 in FIG. Alternatively, the cell radius information managed in FIG. 21 may be considered and a cell having a small cell radius may be selected with priority. Further, the handover destination base station to which the terminal moves may be a cell in which the value of the cost function after movement is the smallest.

For example, the necessity of handover and the selection of a handover destination cell may be as follows.
(1) As described above, whether a handover is necessary is determined by a cost threshold, and handover destination cell selection is determined by path loss.
(2) First, the necessity of handover is determined by path loss comparison, and handover destination cell selection is determined by path loss. Further, an AND condition is taken as to whether or not handover is necessary in the cost threshold determination. The previous two processes correspond to the method determined by the conventional path loss comparison, and a part considering the amount of system interference is added to this.

  In step 2005 of FIG. 8, the serving cell 201-a inquires of the target cell 201-b that is the handover destination whether the terminal 203 may be handed over. If there is an interface that can directly communicate with the serving cell 201-a and the target cell 201-b, the interface may be used. If there is not, an inquiry is made to the target cell 201-b via the base station host apparatus 204 and the core network 202.

  In step 2006, the target cell 201-b determines whether or not the terminal 203 may be newly accepted (Admission Control). The target cell 201-b may use the RB usage rate, the traffic type, the number of connected terminals, and the like as load information used for the determination criterion. If the load information reaches a certain value by newly accepting the terminal, the acceptance may be rejected.

  In step 2007, when the admission control in step 2006 is determined as acceptable (ACK), the target cell 201-b notifies the serving cell 201-a to that effect. As in step 2005, when an interface is prepared between the serving cell and the target cell, the interface may be used for notification. If not, the notification is sent via the host device 204 and the core network 202.

In step 2008, the serving cell 201-a performs downlink resource allocation for notifying the RRC Connection Reconfiguration when an ACK is returned from the target cell 201-b in response to the handover request.
In the RRC configuration in step 2009, the serving cell 201-a notifies the terminal 203 of information for performing a handover to the target cell 201-b.

With the above steps, preparation for handover is completed. Thereafter, handover is performed according to a predetermined handover procedure. For example, the terminal 203 performs a process of disconnecting from the serving cell 201-a (step 2010), and enters a synchronization process to the target cell 201-b (step 2012). In addition, the serving cell 201-a takes over the buffer data held for the terminal and the packet in communication with the target cell 201-b (step 2011).
The above flowchart performs a loop operation, and the load information and the interference information are processed while referring to each other.

(Example 2)
FIG. 19 is a handover sequence diagram according to the second embodiment.
The second embodiment will be described with reference to the sequence diagram shown in FIG. The difference from the first embodiment is that an interface for directly exchanging information between base stations may not be provided, and the other points are the same. For example, a femtocell base station is expected to correspond to this. This is because the femtocell base station is connected to a backhaul line provided by an Internet service provider (ISP).

  The processing performed in step 2102 is replaced with the following step 2102-b. In step 2102-b, the higher-level device 204 of the base station notifies the interference information and the load information that have been notified through the inter-base station interface (for example, the X2 I / F of E-UTRAN). For example, notification may be made via an interface connecting the base station and the MME called the S1 interface in E-UTRAN.

Alternatively, the base station host device 204 may have an EMS device or the like. That is, it can be realized by assigning a cell ID such as a macro cell base station to an EMS device (HeMS), which is a higher-level device 204 of the femto cell base station, and operating it as a macro cell base station. The hardware configuration is realized by the base station shown in FIG. 7 and the base station management apparatus shown in FIG.
As a means for collecting interference information and load information, a method in which the femtocell base station autonomously collects the information can be considered.

  The first is a method in which a femtocell base station receives reference signals of neighboring cells. Based on the received power and the transmission power of the femtocell base station itself, interference information that is predicted to occur in neighboring cells may be generated. Alternatively, the interference information amount may be generated based on quality information reported from the terminal.

  In the femtocell base station, the load information value may be adjusted to increase the capacity of the backhaul line of the base station and the specific gravity of the usage rate. This is because there is a high possibility that the performance of the backhaul line provided by the ISP is not sufficient.

(Example 3)
FIG. 20 is a handover sequence diagram according to the third embodiment.
In the flow of FIG. 8 described in the first embodiment, the processing in step 2101 may be performed as follows. The cell radius information acquisition of the peripheral cell may be notified from the terminal 203 instead of the notification from the host device 204 (step 2101-a). By dividing the cell ID allocation range between the femtocell and the macrocell base station, it is possible to identify the type of the neighboring cell from the reported cell ID. Other processes are the same as those in the first embodiment.
The hardware configuration is realized by the base station shown in FIG. 7 and the base station management apparatus shown in FIG.

Example 4
In the fourth embodiment, a method of indirectly adjusting the handover criterion in steps 3002, 3003, 3012 and 3013 in the first, second and third embodiments will be described.
In order to relax the handover criterion, in step 2001, an addition of an offset may be specified for the report power value from the base station to the terminal. That is, a process for reporting the received power from a cell having a small cell radius larger may be performed. The offset width at this time may be increased as the difference in cell radii increases.

  For example, in E-UTRAN, it can be realized by changing the selection method of Q_offset, which is an RRC parameter related to Cell Selection at the time of IDLE_MODE, according to the above criteria.

  The present invention is applicable to a radio communication system including a plurality of base stations having different cell characteristics, for example, a plurality of base stations having different cell radii.

201 Base Station of Wireless Communication System 202 Core Network of Wireless Communication System 203 Terminal 204 of Wireless Communication System Base Station Host Device 210 Operation and Maintenance Device 211 Packet Data Network Gateway Device 212 Mobility Management Element Device 213 Serving Gateway Management Device Base Device 213 Storage medium 402 Software processing unit 403 of base station management device Interface unit 404 connecting base station management device to base station and host device Logic circuit 501 supporting hardware of processing of base station management device of serving cell (large cell radius) Cell edge terminal transmission power 502 Propagation between serving cell (large cell radius) base station and cell edge terminal Path loss 503 Transmission power of cell edge terminal of target cell (small cell radius) 504 Propagation path loss between target cell (small cell radius) base station and cell edge terminal station

Claims (12)

A wireless communication system comprising a plurality of base stations,
Each base station
Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
A radio communication system that facilitates handover of a terminal in a cell of the base station to a cell of a neighboring base station when the cell radius of the base station is large. The wireless communication system according to claim 1,
Each of the base stations adjusts a handover determination reference value based on a difference in cell radius between the base station and the neighboring base station in order to facilitate handover of a terminal in the cell of the own base station to a cell of the neighboring base station. Wireless communication system. The wireless communication system according to claim 1,
Each base station adds an offset to the terminal for reporting the received power value from the neighboring base station in order to facilitate handover of the terminal in the cell of its own base station to the cell of the neighboring base station. The specified wireless communication system. The wireless communication system according to claim 1,
Each of the base stations
Holds handover criteria for determining whether to hand over a terminal in the cell of its own base station to a cell of a neighboring base station,
A wireless communication system that relaxes or strengthens the handover criterion according to the difference between the load information of its own base station and the load information of neighboring base stations. The wireless communication system according to claim 1,
The wireless communication system, wherein the load information is at least one of a hardware usage rate of a base station, a radio resource usage rate, the number of connected terminals, a line capacity of a backhaul line, and a usage rate thereof. A wireless communication system comprising a plurality of base stations,
Each base station
Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
In order to make it easier to belong to a neighboring base station having a cell radius smaller than that of the own base station, an offset with respect to a determination reference power value when the terminal in the cell of the own base station returns from IDLE_MODE is selected to the terminal. A wireless communication system to notify. The wireless communication system according to claim 6,
Each of the base stations is a radio communication system in which an offset with respect to a determination reference power value when selecting the cell is notified by Q_Offset, which is an RRC parameter to be notified by a radio control signal. Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
A base station that facilitates handover of a terminal in a cell of the base station to a cell of a neighboring base station when the cell radius of the base station is large. The base station according to claim 8, wherein
The base station adjusts a handover determination reference value based on a cell radius difference between the base station and the neighboring base station in order to easily hand over a terminal in the cell of the own base station to a cell of the neighboring base station. Bureau. The base station according to claim 8, wherein
In order to facilitate handover of a terminal in the cell of its own base station to a cell of a neighboring base station, the base station designates an addition of an offset to the terminal regarding a report of a received power value from the neighboring base station. Base station. Holds information on the cell radius defined by one or more of the transmission power information, antenna gain, antenna tilt, and base station type of its own base station and neighboring base stations,
Compare the cell radius of your base station with the cell radius of the neighboring base stations,
In order to make it easier to belong to a neighboring base station having a cell radius smaller than that of the own base station, an offset with respect to a determination reference power value when the terminal in the cell of the own base station returns from IDLE_MODE is selected to the terminal. The base station to notify. The base station according to claim 11, wherein
The base station notifies the offset with respect to the determination reference power value at the time of selecting the cell by Q_Offset, which is an RRC parameter that is notified by a radio control signal.

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