KR20100089026A - Apparatus and method of performing cell selection in wireless communication system - Google Patents

Abstract

PURPOSE: A method and a device for performing cell selection in a wireless communication system are provided to provide better quality service for a terminal. CONSTITUTION: An RF(Radio Frequency) unit(1030) transmits or receives a RF signal. A memory(1020) stores basic priority about a plurality of frequencies. A processor(1010) embodies an wireless Internet protocol. The processor measures signal strength of at least one cell in at least one frequency among the plurality of frequencies. If measurement result of a CSG(Closed Subscriber Group) cell is bigger than CSG threshold value, the processor performs cell selection according to override priority.

Description

Method and apparatus for performing cell selection in a wireless communication system {APPARATUS AND METHOD OF PERFORMING CELL SELECTION IN WIRELESS COMMUNICATION SYSTEM}

The present invention relates to wireless communication, and more particularly, to a method and apparatus for performing cell selection in a wireless communication system.

The 3rd Generation Partnership Project (3GPP) long term evolution (LTE), an improvement of the Universal Mobile Telecommunications System (UMTS), is introduced as a 3GPP release 8. 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in downlink and single carrier-frequency division multiple access (SC-FDMA) in uplink. A multiple input multiple output (MIMO) with up to four antennas is employed. Recently, a discussion on 3GPP LTE-Advanced (LTE-A), an evolution of 3GPP LTE, is underway.

The cell selection process is a process in which a terminal selects a cell for receiving a service. Cell reselection is a process in which the terminal selects a cell again while the terminal has already selected the cell and selected the cell.

Closed Subscriber Groups (CSGs) were introduced to provide higher quality services by allowing limited access to specific subscribers. A base station capable of providing a CSG service is called a home eNodeB (HNB), and a cell providing an authorized service to subscribers of the CSG is called a CSG cell. The basic requirements of CSG in 3GPP are described in 3GPP TS 22.220 V1.0.1 (2008-12) "Service requirements for Home NodeBs and Home eNodeBs (Release 9)".

Even in a cell where the CSG service is supported, the channel environment may be deteriorated. If the CSG service continues to be maintained in a cell in which the channel environment is deteriorated, the quality of service may be worse than that in the normal cell.

In a system supporting CSG, channel environment needs to be considered for cell selection.

The present invention provides a method and apparatus for performing cell selection based on a plurality of priorities in a wireless communication system.

The present invention also provides a method and apparatus for performing cell selection in a wireless communication system supporting CSG.

In one aspect, a wireless device for performing cell selection in a wireless communication system is provided. The wireless device includes a radio frequency (RF) unit for transmitting and receiving radio signals, a memory for storing basic priorities for a plurality of frequencies, and a processor connected to the RF unit and the memory to implement a radio interface protocol. do. The processor measures the signal strength of at least one cell at at least one of the plurality of frequencies, and the cell having the best measurement result is a Closed Subscriber Group (CSG) cell, and the measurement result of the CSG cell is a CSG threshold. If greater, cell selection is performed according to override priority.

If the cell having the best measurement result is a CSG cell and the measurement result of the CSG cell is smaller than a CSG threshold, the processor may perform cell selection according to the basic priority.

The override priority may give the highest priority to a frequency including the CSG cell among the plurality of frequencies.

The at least one cell may include a neighbor cell and a serving cell.

The serving cell may be a CSG cell.

The basic priority may be received from a base station.

In another aspect, a method of performing cell selection in a wireless communication system is provided. The method sets a basic priority for a plurality of frequencies, measures the signal strength of at least one cell at at least one of the plurality of frequencies, and the cell with the best measurement result is a closed subscriber group (CSG). Cell, and if the measurement result of the CSG cell is greater than the CSG threshold, performing cell selection according to an override priority.

If the quality of the currently staying CSG cell is deteriorated, the terminal may reselect a cell at a different frequency, thereby providing a better quality of service. In addition, the CSG cell can be selected only when the UE can be provided with a sufficiently good quality from the CSG cell, thereby maintaining the quality of service of the UE.

1 shows a wireless communication system to which the present invention is applied.
2 is a block diagram illustrating a radio protocol structure for a user plane.
3 is a block diagram illustrating a radio protocol architecture for a control plane.
4 is an exemplary diagram illustrating a procedure for UE operation of selecting a cell in an idle mode.
5 is an exemplary diagram illustrating a network structure for operating an HNB using an HNB gateway.
6 is a flowchart illustrating a method in which a terminal checks a connection mode of a base station.
7 shows an example of cell reselection when staying in a CSG cell.
8 illustrates a situation due to a problem of the existing technology.
9 is a flowchart illustrating cell selection according to an embodiment of the present invention.
10 shows an example to which the present invention is applied.
11 shows another example to which the present invention is applied.
12 shows another example to which the present invention is applied.
13 shows another example to which the present invention is applied.
14 shows another example to which the present invention is applied.
15 illustrates a wireless device in which an embodiment of the present invention is implemented.

1 shows a wireless communication system to which the present invention is applied. This may also be called an Evolved-UMTS Terrestrial Radio Access Network (E-UTRAN), or Long Term Evolution (LTE) / LTE-A system.

The E-UTRAN includes a base station (BS) 20 that provides a control plane and a user plane to a user equipment (UE). The terminal 10 may be fixed or mobile and may be called by other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a mobile terminal (MT), a wireless device (Wireless Device), and the like. . The base station 20 refers to a fixed station communicating with the terminal 10, and may be referred to by other terms such as an evolved-NodeB (eNB), a base transceiver system (BTS), an access point, and the like.

The base stations 20 may be connected to each other through an X2 interface. The base station 20 is connected to a Serving Gateway (S-GW) through an MME (Mobility Management Entity) and an S1-U through an Evolved Packet Core (EPC) 30, more specifically, an S1-MME through an S1 interface.

EPC 30 is composed of MME, S-GW and P-GW (Packet Data Network-Gateway). The MME has information about the access information of the terminal or the capability of the terminal, and this information is mainly used for mobility management of the terminal. S-GW is a gateway having an E-UTRAN as an endpoint, and P-GW is a gateway having a PDN as an endpoint.

Layers of the Radio Interface Protocol between the terminal and the network are based on the lower three layers of the Open System Interconnection (OSI) reference model, which is widely known in communication systems. L2 (second layer), L3 (third layer) can be divided into, the physical layer belonging to the first layer of which provides an information transfer service (Information Transfer Service) using the physical channel (Physical Channel), The RRC (Radio Resource Control) layer located in the third layer plays a role of controlling radio resources between the terminal and the network. To this end, the RRC layer exchanges an RRC message between the terminal and the base station.

FIG. 2 is a block diagram illustrating a radio protocol architecture for a user plane. 3 is a block diagram illustrating a radio protocol structure for a control plane. The data plane is a protocol stack for user data transmission, and the control plane is a protocol stack for control signal transmission.

2 and 3, a physical layer (PHY) layer provides an information transfer service to a higher layer using a physical channel. The physical layer is connected to a medium access control (MAC) layer, which is an upper layer, through a transport channel. Data is moved between the MAC layer and the physical layer through the transport channel. Transport channels are classified according to how and with what characteristics data is transmitted over the air interface.

Data moves between physical layers between physical layers, that is, between physical layers of a transmitter and a receiver. The physical channel is modulated by an orthogonal frequency division multiplexing (OFDM) scheme and utilizes time and frequency as radio resources.

The functions of the MAC layer include mapping between logical channels and transport channels and multiplexing / demultiplexing into transport blocks provided as physical channels on transport channels of MAC service data units (SDUs) belonging to the logical channels. The MAC layer provides a service to a Radio Link Control (RLC) layer through a logical channel.

Functions of the RLC layer include concatenation, segmentation, and reassembly of RLC SDUs. In order to guarantee the various Quality of Service (QoS) required by the radio bearer (RB), the RLC layer has a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (Acknowledged Mode). Three modes of operation (AM). AM RLC provides error correction through an automatic repeat request (ARQ).

Functions of the Packet Data Convergence Protocol (PDCP) layer in the user plane include delivery of user data, header compression, and ciphering. The functionality of the Packet Data Convergence Protocol (PDCP) layer in the user plane includes the transfer of control plane data and encryption / integrity protection.

 The RRC (Radio Resource Control) layer is defined only in the control plane. The RRC layer is responsible for the control of logical channels, transport channels, and physical channels in connection with configuration, re-configuration, and release of radio bearers. RB means a logical path provided by the first layer (PHY layer) and the second layer (MAC layer, RLC layer, PDCP layer) for data transmission between the terminal and the network. The establishment of the RB means a process of defining characteristics of a radio protocol layer and a channel to provide a specific service, and setting each specific parameter and operation method. RB can be divided into SRB (Signaling RB) and DRB (Data RB). The SRB is used as a path for transmitting RRC messages in the control plane, and the DRB is used as a path for transmitting user data in the user plane.

If there is an RRC connection between the RRC layer of the terminal and the RRC layer of the E-UTRAN, the terminal is in the RRC connected state (RRC_CONNECTED), otherwise it is in the RRC idle state (RRC_IDLE).

The downlink transmission channel for transmitting data from the network to the UE includes a BCH (Broadcast Channel) for transmitting system information and a downlink shared channel (SCH) for transmitting user traffic or control messages. Traffic or control messages of a downlink multicast or broadcast service may be transmitted through a downlink SCH or may be transmitted through a separate downlink multicast channel (MCH). Meanwhile, the uplink transport channel for transmitting data from the terminal to the network includes a random access channel (RACH) for transmitting an initial control message and an uplink shared channel (SCH) for transmitting user traffic or control messages.

It is located above the transport channel, and the logical channel mapped to the transport channel is a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic (MTCH). Channel).

The physical channel is composed of several symbols in the time domain and several sub-carriers in the frequency domain. One sub-frame consists of a plurality of symbols in the time domain. One subframe consists of a plurality of resource blocks, and one resource block consists of a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use specific subcarriers of specific symbols (eg, the first symbol) of the corresponding subframe for the physical downlink control channel (PDCCH), that is, the L1 / L2 control channel. The transmission time interval (TTI), which is a unit time for transmitting data, is 1 ms corresponding to one subframe.

Hereinafter, the RRC state and the RRC connection method of the UE will be described in detail.

The RRC state refers to whether or not the RRC layer of the UE is in a logical connection with the RRC layer of the E-UTRAN. It is called (RRC_IDLE state). Since the UE in the RRC_CONNECTED state has an RRC connection, the E-UTRAN can grasp the existence of the UE in units of cells, and thus can effectively control the UE. On the other hand, the UE in the RRC_IDLE state cannot be grasped by the E-UTRAN, and the core network manages the tracking area unit, which is a larger area unit than the cell. That is, the UE in the RRC_IDLE state is only detected in a large area unit and must move to the RRC_CONNECTED state in order to receive a normal mobile communication service such as voice or data.

When the user first powers on the terminal, the terminal first searches for an appropriate cell and then stays in the RRC_IDLE state in the cell. When it is necessary to establish an RRC connection, the UE staying in the RRC_IDLE state makes an RRC connection with the E-UTRAN through an RRC connection procedure and transitions to the RRC_CONNECTED state. There are several cases in which a UE in RRC_IDLE state needs to establish an RRC connection. For example, an uplink data transmission is necessary due to a user's call attempt, or a paging message is received from E-UTRAN. In one case, the response message may be transmitted.

The non-access stratum (NAS) layer located above the RRC layer performs functions such as session management and mobility management.

In order to manage mobility of the UE in the NAS layer, two states of EMM-REGISTERED (EPS Mobility Management-REGISTERED) and EMM-DEREGISTERED are defined, and these two states are applied to the UE and the MME. The initial terminal is in the EMM-DEREGISTERED state, and the terminal performs a process of registering with the corresponding network through an initial attach procedure to access the network. If the attach procedure is successfully performed, the UE and the MME are in an EMM REGISTERED state.

In order to manage a signaling connection between the UE and the EPC, two states are defined, an EPS Connection Management (ECM) -IDLE state and an ECM-CONNECTED state, and these two states are applied to the UE and the MME. When the UE in the ECM-IDLE state establishes an RRC connection with the E-UTRAN, the UE is in the ECM-CONNECTED state. The MME in the ECM-IDLE state becomes the ECM-CONNECTED state when it establishes an S1 connection with the E-UTRAN. When the terminal is in the ECM-IDLE state, the E-UTRAN does not have context information of the terminal. Accordingly, the UE in the ECM-IDLE state performs a terminal-based mobility related procedure such as cell selection or cell reselection without receiving a command from the network. On the other hand, when the terminal is in the ECM-CONNECTED state, the mobility of the terminal is managed by the command of the network. In the ECM-IDLE state, if the position of the terminal is different from the position known by the network, the terminal informs the network of the corresponding position of the terminal through a tracking area update procedure.

The following is a description of system information.

The system information includes essential information that the terminal needs to know in order to access the base station. Therefore, the terminal must receive all system information before accessing the base station, and must always have the latest system information. In addition, since the system information is information that all terminals in a cell should know, the base station periodically transmits the system information.

According to section 5.2.2 of 3GPP TS 36.331 V8.4.0 (2008-12) "Radio Resource Control (RRC); Protocol specification (Release 8)", the system information includes a master information block (MIB) and a scheduling block (SB). , SIB System Information Block). The MIB enables the UE to know the physical configuration of the cell, for example, bandwidth. SB informs transmission information of SIBs, for example, a transmission period. SIB is a collection of related system information. For example, some SIBs contain only information of neighboring cells, and some SIBs contain only information of an uplink radio channel used by the terminal.

In general, services provided by a network to a terminal can be classified into three types as follows. In addition, the terminal also recognizes the cell type differently according to which service can be provided. The following describes the service type first, followed by the cell type.

1) Limited service: This service provides Emergency Call and Tsunami Warning System (ETWS), which can be provided in an acceptable cell.

2) Normal service: This service means a public use for general use, and can be provided in a suitable or normal cell.

3) Operator service: This service means service for network operator. This cell can be used only by network operator and not by general users.

In relation to the service type provided by the cell, the cell types may be classified as follows.

1) Acceptable cell: A cell in which the terminal can receive limited service. This cell is a cell that is not barred from the viewpoint of the terminal and satisfies the cell selection criteria of the terminal.

2) Normal cell (Suitable cell): The cell that the terminal can receive a regular service. This cell satisfies the conditions of an acceptable cell and at the same time satisfies additional conditions. As an additional condition, this cell must belong to a PLMN to which the terminal can access and must be a cell which is not prohibited from performing a tracking area update procedure of the terminal. If the cell is a CSG cell, the terminal should be a cell that can be connected to the cell as a CSG member.

3) Barred cell: A cell that broadcasts information that a cell is a prohibited cell through system information.

4) Reserved cell: A cell that broadcasts information that a cell is a reserved cell through system information.

4 is an exemplary diagram illustrating a procedure for UE operation of selecting a cell in an idle mode.

The terminal selects a PLMN (Public Land Mobile Network) and a RAT (Radio Access Technology) to be serviced by the terminal. The PLMN and the RAT may be selected by the user of the terminal, or may use the one stored in the USIM (S410).

The terminal selects a cell having the largest value among the measured base station and a cell whose signal strength or quality is greater than a specific value (S420). The base station periodically receives system information. The above specific value refers to a value defined in the system in order to ensure the quality of the physical signal in data transmission / reception. Therefore, the value may vary depending on the RAT applied.

If the network needs to be registered, the terminal registers its information (eg, IMSI) in order to receive a service (eg, paging) from the network (S430, S440). The terminal does not register with the network to which it connects every time the cell is selected. For example, if the system information of the network to be registered (for example, Tracking Area Identity (TAI)) is different from the information of the network known to the user, the network is registered in the network.

If the terminal is a value of the strength or quality of the signal measured from the base station being service is lower than the value measured from the base station of the adjacent cell, another cell that provides better signal characteristics than the cell of the base station to which the terminal is connected One of them is selected (S450). This process is referred to as cell reselection, distinguished from initial cell selection in step S420. In this case, in order to prevent the cell from being frequently reselected according to the change of the signal characteristic, a time constraint may be provided.

Next, a procedure of selecting a cell by the terminal will be described in detail.

When the power is turned on or staying in the cell, the terminal selects / reselects a cell of appropriate quality and performs procedures for receiving service.

A UE in an RRC idle (RRC_IDLE) state should always be prepared to select a cell of appropriate quality and to receive service through this cell. For example, a terminal that has just been powered on must select a cell of appropriate quality to register with the network. When the terminal in the RRC_CONNECTED state enters the RRC_IDLE, the terminal should select a cell to stay in the RRC idle state. As such, the process of selecting a cell satisfying a certain condition in order for the terminal to stay in a service standby state such as an RRC idle state is called cell selection. Importantly, since the cell selection is performed in a state in which the UE does not currently determine a cell to stay in the RRC_IDLE state, it is most important to select a cell as soon as possible. Therefore, if the cell provides a radio signal quality of a predetermined criterion or more, even if this cell is not the cell providing the best radio signal quality to the terminal, it may be selected during the cell selection process of the terminal.

Now, referring to 3GPP TS 36.304 V8.3.0 (2008-09) "User Equipment (UE) procedures in idle mode (Release 8)", a method and procedure for the UE to select a cell will be described in detail.

When the terminal is initially powered on, it searches for available PLMNs and selects an appropriate PLMN to receive services. Subsequently, the UE selects a cell having a signal quality and characteristics capable of receiving an appropriate service from among cells provided by the selected PLMN.

There are two main cell selection processes.

First, an initial cell selection process, in which the terminal does not have prior information on the radio channel. Accordingly, the terminal searches all radio channels to find an appropriate cell. The terminal finds the strongest cell in each channel. Thereafter, the terminal selects a corresponding cell if it finds a suitable cell that satisfies the cell selection criteria.

The other is a cell selection process using stored information. In this process, cell selection is performed by using information stored in the terminal for a wireless channel or by using information broadcast in a cell. Therefore, the cell selection may be faster than the initial cell selection process. If the terminal only finds a cell that satisfies a cell selection criterion, the terminal selects the cell. If a suitable cell that satisfies the cell selection criteria is not found through this process, the UE performs an initial cell selection process.

The cell selection criterion used by the terminal in the cell selection process is shown in Equation 1 below.

Where Srxlev = Qrxlevmeas-(Qrxlevmin + Qrxlevminoffset) + Pcompensation, Qrxlevmeas is the reception level (RSRP) of the measured cell, Qrxlevmin is the minimum required reception level (dBm) in the cell, Qrxlevminoffset is the offset to Qrxlevmin, Pcompensation = max (PEMAX-PUMAX, 0) (dB), PEMAX is the maximum transmit power (dBm) that the terminal can transmit in the cell, and PUMAX is the maximum transmit power (dBm) of the terminal radio transmitter (RF) according to the performance of the terminal. )to be.

In Equation 1, the UE may know that the strength and quality of the measured signal select a cell larger than a specific value determined by a cell providing a service. In addition, parameters used in Equation 1 are broadcast through system information, and the terminal receives these parameter values and uses them for cell selection criteria.

When the terminal selects a cell that satisfies a cell selection criterion, the terminal receives information necessary for the RRC_IDLE mode operation of the terminal in the cell from the system information of the cell. After the UE receives all the information necessary for the RRC_IDLE mode operation, the UE waits in the idle mode to request a service (eg, an originating call) or to receive a service (eg, a terminating call) from the network.

After the terminal selects a cell through a cell selection process, the strength or quality of a signal between the terminal and the base station may change due to a change in mobility or a wireless environment of the terminal. Therefore, if the quality of the selected cell is degraded, the terminal may select another cell that provides better quality. When reselecting a cell in this way, a cell that generally provides better signal quality than the currently selected cell is selected. This process is called cell reselection. The cell reselection process has a basic purpose in selecting a cell that generally provides the best quality to a terminal in view of the quality of a radio signal.

In addition to the quality of the wireless signal, the network may determine the priority for each frequency and notify the terminal. Upon receiving this priority, the UE considers this priority prior to the radio signal quality criteria in the cell reselection process.

As described above, there is a method of selecting or reselecting a cell according to a signal characteristic of a wireless environment.In selecting a cell for reselection when reselecting a cell, the following cell reselection is performed according to a cell's RAT and frequency characteristics There may be a method of selection.

Intra-frequency cell reselection: Reselects a cell with the same center-frequency as the cell used by the UE

Inter-frequency cell reselection: Reselects a cell with a center-frequency different from the same RAT as the cell being used by the UE

-Reselection of Inter-RAT cell: Reselection of a cell using a RAT different from that of the UE

The principle of the cell reselection process is as follows.

First, the UE measures the quality of a serving cell and a neighboring cell for cell reselection.

Second, cell reselection is performed based on cell reselection criteria. The cell reselection criteria have the following characteristics with respect to serving cell and neighbor cell measurements.

Intra-frequency cell reselection is basically based on ranking. Ranking is an operation of defining index values for cell reselection evaluation and using the index values to order the cells in the order of the index values. The cell with the best indicator is often called the best ranked cell. The cell index value is a value obtained by applying a frequency offset or a cell offset as necessary based on the value measured by the terminal for the corresponding cell.

Inter-frequency cell reselection is based on the frequency priority provided by the network. The terminal attempts to camp on the frequency with the highest frequency priority. The network may provide the priorities to be commonly applied to the terminals in the cell or provide the frequency priority through broadcast signing, or may provide the priority for each frequency for each terminal through dedicated signaling.

For inter-frequency cell reselection, the network may provide the UE with parameters (for example, frequency-specific offset) used for cell reselection for each frequency.

For intra-frequency cell reselection or inter-frequency cell reselection, the network may provide the UE with a neighboring cell list (NCL) used for cell reselection. This NCL contains cell-specific parameters (eg cell-specific offsets) used for cell reselection.

For intra-frequency or inter-frequency cell reselection, the network may provide the UE with a cell reselection prohibition list (black list) used for cell reselection to the UE. The UE does not perform cell reselection for a cell included in the prohibition list.

Next, the ranking performed in the cell reselection evaluation process will be described.

The ranking criterion used to prioritize the cells is defined as in Equation 2.

Here, Rs is a ranking indicator of the serving cell, Rn is a ranking indicator of the neighboring cell, Qmeas, s is a quality value measured by the UE for the serving cell, Qmeas, n is a quality value measured by the UE for the neighboring cell, and Qhyst is The hysteresis value, Qoffset, for the ranking is an offset between two cells.

In intra-frequency, Qffoset = Qoffsets, n when the UE receives an offset (Qoffsets, n) between the serving cell and neighbor cells, and Qoffset = 0 when the UE does not receive Qoffsets, n.

In the inter-frequency, Qoffset = Qoffsets, n + Qfrequency when the UE receives an offset (Qoffsets, n) for the cell, and Qoffset = Qfrequency when the UE does not receive Qoffsets, n.

If the ranking index Rs of the serving cell and the ranking index Rn of the neighboring cell change in a state similar to each other, the ranking ranking is constantly changed as a result of the change, so that the terminal may alternately select two cells. Qhyst is a parameter for giving hysteresis in cell reselection to prevent the UE from reselecting two cells alternately.

The UE measures the Rs of the serving cell and the Rn of the neighboring cell according to the above equation, regards the cell having the highest ranking indicator value as the best ranked cell, and reselects the cell.

According to the criteria, it can be seen that the quality of the cell serves as the most important criterion in cell reselection. If the reselected cell is not a normal cell, the terminal excludes the frequency or the corresponding cell from the cell reselection target.

We now describe a Closed Subscriber Group (CSG).

A base station providing a CSG service is called a home node b (HNB) or a home enode b (henb) in 3GPP. Hereinafter, both the HNB and HeNB are collectively referred to as NNB. The HNB basically aims to provide specialized services only to members belonging to the CSG. However, the service may be provided to other users besides the CSG according to the operation mode setting of the HNB.

5 is an exemplary diagram illustrating a network structure for operating an HNB using an HNB gateway (GW).

The HNBs are connected to the EPC or directly to the EPC via the HNB GW. Here, the HNB GW looks like a typical BS to the MME. The HNB GW also looks like the MME to the HNB. Therefore, the HNB and the HNB GW are connected to the S1 interface, and the HNB GW and the EPC are also connected to the S1 interface. In addition, when the HNB and the EPC are directly connected, they are connected to the S1 interface. The function of the HNB is mostly the same as that of the general BS.

In general, the HNB has a lower wireless transmission power than the BS owned by the mobile network operator. Therefore, the coverage provided by the HNB is generally smaller than the coverage provided by the BS. Due to this characteristic, a cell provided by an HNB is often classified as a femto cell in comparison with a macro cell provided by a BS in view of a service area.

In terms of the service provided, when the HNB provides service only to the CSG group, the cell provided by this HNB is called a CSG cell.

Each CSG has its own unique identifier, which is called a CSG identity (CSG identity). The terminal may have a list of CSGs belonging to the member, and the CSG list may be changed by a request of the terminal or a command of the network. According to the current specification of 3GPP, one HNB can support one CSG.

The terminal has a list of CSG registered as a member, which is called a CSG white list.

The HNB delivers the CSG ID of the CSG supported by the HNB through the system information so that only the member terminals of the corresponding CSG are connected. When the UE finds a CSG cell, it can check which CSG the CSG cell supports by reading the CSG ID included in the system information. The terminal reading the CSG ID is regarded as a cell that can access the cell only when the UE is a member of the CSG cell, that is, when the CSG corresponding to the CSG ID is included in its CSG whitelist.

HNB does not always need to allow access to the CSG terminal. Depending on the configuration setting of the HNB, it is also possible to allow access of the terminal other than the CSG member. Which UE is allowed to access depends on the configuration setting of the HNB, where the configuration setting refers to the setting of the operation mode of the HNB. The operation mode of the HNB is classified into three types according to which UE provides a service.

1) Closed access mode: A mode that provides services only to specific CSG members. The HNB provides a CSG cell.

2) Open access mode: A mode that provides a service without the restriction of a specific CSG member like a general BS. The HNB provides a generic cell that is not a CSG cell.

3) Hybrid access mode: A mode in which a CSG service can be provided to a specific CSG member and a service is provided to a non-CSG member like a normal cell. CSG member UEs are recognized as CSG cells, and non-CSG member UEs are recognized as normal cells. Such a cell is called a hybrid cell.

The HNB informs the UE whether the cell it serves is a general cell, which is a CSG cell, so as to know whether the UE can access the cell. The HNB operating in the closed access mode broadcasts through the system information that it is a CSG cell. The HNB operating in open access mode broadcasts via system information that it is not a CSG cell. As such, the HNB includes a CSG indicator in the system information indicating whether the cell it serves is a CSG cell or not.

For example, the CSG cell broadcasts by setting the CSG indicator to 'TRUE'. If the serving cell is not a CSG cell, the CSG indicator may be set to 'FALSE' or a method of omitting CSG indicator transmission may be used. Since the UE should be able to distinguish the normal cell from the CSG cell, the general BS may also transmit a CSG indicator (eg, the CSG indicator set to 'FALSE') to let the UE know that the cell type provided by the UE is the general cell. In addition, the general BS may not let the UE know that the cell type provided by the UE is a general cell by not transmitting the CSG indicator.

Table 1 shows CSG related parameters transmitted by a corresponding cell for each cell type. The CSG related parameters may be transmitted through system information.

CSG cell Normal cell CSG indicator Referred to as 'CSG Cell' Pointed to 'non-CSG cell' or not transmitted CSG ID Support CSG ID Transfer Do not send

Table 2 shows the types of terminals that allow connection by cell type.

CSG cell Normal cell Endpoints that do not support CSG Not accessible Accessible Non-CSG Member Terminal Not accessible Accessible Member CSG Terminal Accessible Accessible

6 is a flowchart illustrating a method in which a terminal checks an access mode of a base station.

The terminal first checks the CSG indicator in the system information of the target cell in order to identify what type of cell the target cell is (S510).

After checking the CSG indicator, if the CSG indicator indicates that the target cell is a CSG cell, the terminal recognizes the corresponding cell as a CSG cell (S520, S530). Thereafter, the terminal checks the CSG ID in the system information to check whether the UE is a CSG member of the target cell (S540).

When the terminal determines that the UE is a CSG member of the target cell from the CSG ID, the terminal recognizes the corresponding cell as an accessible CSG cell (S550 and S560). If the terminal confirms that the UE is not a CSG member of the target cell from the CSG identifier, the terminal regards the cell as an inaccessible cell (S550 and S570).

In the first step, if the CSG indicator indicates that the target cell is not a CSG cell, the terminal recognizes the target cell as a general cell (S520, S580). In addition, if the CSG indicator is not being transmitted in step S510, the terminal recognizes the target cell as a general cell.

In general, a CSG cell and a macro cell may be operated simultaneously at a specific frequency. A frequency in which only a CSG cell exists is called a CSG dedicated frequency. The frequency at which the CSG cell and the macro cell exist simultaneously is called a mixed carrier frequency. The network may reserve a particular physical layer cell identifier for the CSG cell separately at the mixed carrier frequency. The physical layer cell identifier is called PCI (Physical Cell Identity) in the E-UTRAN system and PSC (Physical Scrambling Code) in the UTRAN. For convenience of description, the physical layer cell identifier is expressed in PCI.

The CSG cell informs the system about the PCI reserved for the CSG at the current frequency. Upon receiving this information, the UE can determine whether a cell is a CSG cell or not a CSG cell from PCI of this cell when it discovers a cell at a corresponding frequency. The following describes how two terminals use this information.

First, in case of a UE that does not support CSG-related functions or does not have a CSG list as a member thereof, the UE does not need to consider the CSG cell as a selectable cell during cell selection / reselection. In this case, the terminal checks only the PCI of the cell, and if the PCI is a PCI reserved by the CSG, the cell can be immediately excluded from the cell selection / reselection process. In general, the PCI of a cell can be known immediately at the terminal when the physical layer confirms the existence of the cell.

Second, in case of a UE having a CSG list of which the UE is a member, when the UE wants to know a list of neighboring CSG cells at the mixed carrier frequency, the CSG identifier of the system information of all cells found in the entire PCI range. Instead of verifying, we only know that the cell with the PCI reserved for CSG can know that the cell is a CSG cell.

Now, the cell reselection process related to the CSG cell will be described.

The CSG cell is a cell for supporting a better CSG service to the CSG member UE. Therefore, when the UE is camping on the CSG cell, reselecting a cell of the inter-frequency because the UE finds an inter-frequency having a frequency priority higher than the frequency priority of the serving frequency is a quality of service. It may not be desirable in view.

When the terminal stays in the CSG, in order to prevent the cell reselection with inter-frequency having a higher frequency priority than the serving frequency, the terminal may determine that the CSG cell of any frequency is based on the cell reselection evaluation criteria at that frequency. If it is determined to be the best ranked cell, it is assumed that the frequency priority of the frequency is higher than that of other frequencies.

When the terminal designates a frequency priority higher than the frequency priority that the network can specify for a specific frequency, such frequency priority is called an 'implicit highest priority'. This may help the UE stay in the CSG cell, while obeying the existing rules of cell selection that the UE considers frequency priority first when cell reselection.

7 shows an example of cell reselection when staying in a CSG cell.

Initially, the terminal is staying in the CSG cell (S710).

Since the serving cell is a CSG cell, an implied highest priority is given to the serving frequency (S720).

Subsequently, the quality of the serving cell and the neighboring cell is measured (S730).

The normal reselection rule is applied based on the measurement result (S740). The terminal first finds a cell at a frequency having a higher priority than the serving frequency. If no suitable cell is found, the UE searches for the best ranked cell at a frequency having the same priority as the serving frequency. If no suitable cell is found, the terminal finds an optimal cell at a frequency having a lower priority than the service frequency.

If a new cell for reselection is found, the cell is reselected to the cell (S750, S760).

If the UE in the CSG cell reselects the non-CSG cell of the corresponding frequency, the UE revokes the implicit highest priority assumption of the CSG cell and uses the frequency priority value delivered by the network in the cell reselection evaluation.

If the UE finds another CSG cell that is best ranked at a frequency having the same frequency priority when the UE stays in the CSG cell, whether the UE reselects the CSG cell or currently stays and remains in the CSG cell Follow.

According to the prior art as described above, if the CSG cell is a serving cell, the UE reconfigures the cell at the frequency as long as the CSG cell is evaluated as the best ranked cell by the cell reselection rule at the frequency in which the CSG cell exists, that is, the serving frequency. Assume the selection priority is higher than the cell reselection priority of other frequencies. This is to ensure that the UE can receive the CSG service while staying in the CSG cell of the corresponding frequency.

If the quality of the CSG cell in which the UE stays is evaluated as the best ranked cell at the serving frequency as a result of the UE measurement, the quality may actually be worse than cells in other frequencies. For example, if the serving frequency is generally degraded due to interference, although the quality of the CSG cell in which the UE currently resides is still ranked best at the serving frequency, there is a high possibility that there is a better quality cell at another frequency.

8 illustrates a situation due to a problem of the existing technology.

There are three cells (cells 1a, 1b, 1c) at the first frequency F1, four cells (cells 2a, 2b, 2c, 2d) at the second frequency F2, and 4 at the third frequency F3. Suppose there are three cells (cells 3a, 3b, 3c, 3d). The UE currently stays in cell 1a, which is a CSG cell, and cell 1a is a serving cell.

The serving cell of the current terminal is the best ranked cell at the first frequency F1 which is the serving frequency. Therefore, due to the implicit highest priority assumption, the UE does not consider cells of other frequencies in cell reselection at all. Due to this, although the terminal stays in the CSG cell, the actual quality of service may be worse than reselecting to another cell.

As a result, according to the conventional cell reselection rule, which assumes the highest priority unconditionally when the CSG cell is the best ranked cell at the serving frequency, when the overall quality of the serving frequency deteriorates, the cell reselection to another frequency is prevented. It can lead to deterioration of quality.

9 is a flowchart illustrating cell selection according to an embodiment of the present invention.

The terminal sets a basic priority based on the information received from the base station (S900). The basic priority refers to a priority for each of a plurality of frequencies used for cell selection, and is also called a signaled frequency priority.

The terminal measures signals for the serving cell and the neighboring cells (S910). The serving cell may be a CSG cell, but may also be a normal cell. The measurement result may include Reference Signal Received Power (RSRP) and / or Reference Signal Received Quality (RSRQ). The measurement result may mean any numerical value specific to the radio channel physical characteristics of the mobile communication technology in which the terminal is operated, in addition to the RSRP and the RSRQ.

As a result of the measurement, if the cell having the best measurement result at least one frequency among the plurality of frequencies is the CSG cell, the measurement result Scsg of the corresponding CSG cell is compared with the CSG threshold Tcsg (S920 and S930).

If the cell having the best measurement result is not the CSG cell, the basic priority is applied (S950).

If Scsg is larger than Tcsg, an override priority is applied (S940). The override priority means giving the highest priority, ie, implied highest priority, to the frequency in which the CSG cell is included regardless of the preset basic priority. If the best ranked cell is a CSG cell and the measurement result is larger than Tcsg, cell reselection is performed to the corresponding CSG cell (S960).

If Scsg is smaller than Tcsg, the basic priority is applied (S950), and cell reselection is performed (S960).

The CSG threshold Tcsg is a value for applying an override priority that overrides the default priority. That is, even if the CSG cell of a specific frequency is the best cell, override priority is applied when the measurement result exceeds the CSG threshold.

The CSG threshold Tcsg may be transmitted through a broadcast channel like the system information, or may be transmitted through a dedicated channel.

If the measurement result for a CSG cell at a frequency is lower than the CSG threshold, even if the corresponding CSG cell is evaluated as the best ranked cell by the cell reselection rule at that frequency, the UE gives an implied priority to the priority of the frequency. It does not count and keeps the default priority that has been set.

Now, various embodiments are described below to more specifically understand the present invention.

The following assumptions are made below.

(1) As the basic priority, frequency priorities for each of three frequencies F1, F2, and F3 are known to both the base station and the terminal. Therefore, when the UE performs a cell reselection procedure, it selects one cell among the cells at these three frequencies.

(2) All cells are suitable cells.

(3) The terminal currently uses the CSG cell at the first frequency F1 as the serving cell. The measurement result of the staying CSG cell is called Scsg.

(4) The CSG cell of F1 where the UE stays is the best ranked cell in F1.

(5) Measured quality of each cell is expressed as Smeas, and the higher the value of Smeas, the better the cell.

(6) The smaller the frequency priority value, the higher the priority. Therefore, when applying the basic priority, a frequency having a priority value of '1' becomes the highest priority, and a frequency having a priority value of '0' has an implied highest priority.

(7) Assumed frequency priority refers to actual frequency priority that the UE uses for cell reselection evaluation.

10 shows an example to which the present invention is applied. It is assumed that the second frequency F2 has the highest priority as the basic priority, and the first frequency F1 and the third frequency F3 have the same priority.

The CSG cell (cell 1a) at the first frequency F1 which is the current serving frequency is the best ranked cell. However, the quality of this cell is below the CSG threshold, thus giving up the implicit first priority assumption.

Accordingly, the terminal applies the basic priority to the first, second, and third frequencies, respectively, and performs cell reselection. Accordingly, the terminal may select a cell (cell 2a) at the second frequency F2 having a higher priority than the first frequency F1. The selected cell 2a may or may not be a CSG cell. If the selected cell 2a is not a CSG cell, the UE does not receive CSG service.

The following table compares the home frequency priorities according to the present invention and the prior art.

Frequency number Default priority Home frequency priority Conventional technology Invention F1 3 0 3 F2 2 2 2 F3 3 3 3

11 shows another example to which the present invention is applied. Unlike the example of FIG. 10, it is assumed that the first frequency F1 and the third frequency F3 have the same priority as the basic priority, and the second frequency F2 has the lowest priority.

The CSG cell (cell 1a) at the first frequency F1 which is the current serving frequency is the best ranked cell. However, the quality of this cell is below the CSG threshold, thus giving up the implicit first priority assumption.

Accordingly, the terminal applies the basic priority to the first, second, and third frequencies, respectively, and performs cell reselection. Accordingly, the terminal may select cell 3a which is the best ranked cell of the third frequency F3. The following table compares the home frequency priorities according to the present invention and the prior art.

Frequency number Default priority Home frequency priority Conventional technology Invention F1 3 0 3 F2 4 4 4 F3 3 3 3

12 shows another example to which the present invention is applied. Unlike the example of FIG. 10, the best ranked cell of the third frequency F3 is a CSG cell.

The CSG cell (cell 1a) at the first frequency F1 which is the current serving frequency is the best ranked cell. However, the quality of this cell is below the CSG threshold, thus giving up the implicit first priority assumption.

In addition, although the best ranked cell of the third frequency F3 is a CSG cell, the measurement result of this cell is also below the CSG threshold. Therefore, no implied highest priority is given to the third frequency F3.

As a result, the terminal applies the basic priority to the first, second, and third frequencies to perform cell reselection. Accordingly, the terminal may select a cell (cell 2a) at the second frequency F2 that is higher than the priority of the first frequency F1 that is currently connected. The selected cell 2a may or may not be a CSG cell. If the selected cell 2a is not a CSG cell, the UE does not receive CSG service.

The following table compares the home frequency priorities according to the present invention and the prior art.

Frequency number Default priority Home frequency priority Conventional technology Invention F1 3 0 3 F2 2 2 2 F3 3 0 3

This scenario is intended to show that frequency prioritization using CSG thresholds can be applied to other frequencies as well as the serving frequency.

13 shows another example to which the present invention is applied. Unlike the example of FIG. 12, the quality of the CSG cell that is best ranked at the third frequency F3 is higher than the CSG threshold Tcsg.

As in Fig. 12, the CSG cell (cell 1a) at the first frequency F1 which is the current serving frequency is the best ranked cell. However, the quality of this cell is below the CSG threshold, thus giving up the implicit first priority assumption.

Since the best ranked cell of the third frequency F3 is the CSG cell and the measurement result of this cell is higher than the CSG threshold, the implied highest priority is given to the third frequency F3.

As a result, the terminal performs cell reselection by applying a basic priority to the first and second frequencies and an implicit highest priority for frequency 3. Accordingly, the terminal selects cell 3a at the third frequency F3. The UE may be provided with a good CSG service in the selected cell 3a.

The following table compares the home frequency priorities according to the present invention and the prior art.

Frequency number Default priority Home frequency priority Conventional technology Invention F1 3 0 3 F2 2 2 2 F3 3 0 0

14 shows another example to which the present invention is applied. The measurement result of the CSG cell of the serving cell exceeds the CSG threshold.

The CSG cell (cell 1a) at the first frequency F1 which is the current serving frequency is the best ranked cell. Since the quality of this cell is greater than or equal to the CSG threshold, the terminal continues to give an implied highest priority to the first frequency.

Therefore, even though a cell having a better quality than the current serving cell exists at another frequency, the terminal stays in the current serving cell and is provided with a high quality CSG service.

The following table compares the home frequency priorities according to the present invention and the prior art.

Frequency number Default priority Home frequency priority Conventional technology Invention F1 3 0 0 F2 2 2 2 F3 3 3 3

This scenario is an embodiment for showing that when the measurement result of the CSG cell exceeds the CSG threshold, the operation of the terminal is the same as the existing.

The UE does not give the highest priority in cell reselection because the CSG cell is the best ranked cell. If the UE accesses the CSG cell, if the quality of the serving CSG cell deteriorates, the UE revokes the implied highest priority for the serving frequency. Accordingly, the UE can additionally consider other frequencies that have been excluded from the cell reselection target as the cell reselection target. The UE may leave the current CSG cell of poor quality and reselect a cell at a different frequency, thereby providing a better quality of service.

If the UE is not connected to the CSG cell, even if any CSG cell is the best ranked cell in the neighboring frequency does not satisfy a specific quality condition, the UE does not give an implied highest priority to the frequency. The UE assigns an implicit first priority to a corresponding frequency only when the actual quality of the CSG cell is sufficiently good at a certain frequency. The CSG cell is selected only when the UE can be provided with substantially good quality from the CSG cell, thereby maintaining the quality of service of the UE.

15 illustrates a wireless device in which an embodiment of the present invention is implemented. This may be part of the terminal. The wireless device 1000 includes a processor 1010, a memory 1020, and an RF unit 1030.

The processor 1010 implements the proposed functions, processes and / or methods. Layers of the air interface protocol may be implemented by the processor 1010. The processor 1010 measures signal strength for a plurality of frequencies and performs cell selection / reselection based on the measured result.

The memory 1020 is connected to the processor 1010 and stores a priority for cell selection.

The RF unit 1030 is connected to the processor 1010 and transmits and / or receives a radio signal.

The processor 1010 may include an application-specific integrated circuit (ASIC), another chipset, a logic circuit, and / or a data processing device. The memory 1020 may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium, and / or other storage device. The RF unit 1030 may include a baseband circuit for processing a radio signal. When the embodiment is implemented in software, the above-described techniques may be implemented with modules (processes, functions, and so on) that perform the functions described above. The module may be stored in the memory 1020 and executed by the processor 1010. The memory 1020 may be inside or outside the processor 1010 and may be connected to the processor 1010 by various well-known means.

In the exemplary system described above, the methods are described based on a flowchart as a series of steps or blocks, but the invention is not limited to the order of steps, and certain steps may occur in a different order or concurrently with other steps than those described above. Can be. In addition, those skilled in the art will appreciate that the steps shown in the flowcharts are not exclusive and that other steps may be included or one or more steps in the flowcharts may be deleted without affecting the scope of the present invention.

The above-described embodiments include examples of various aspects. While it is not possible to describe every possible combination for expressing various aspects, one of ordinary skill in the art will recognize that other combinations are possible. Accordingly, it is intended that the invention include all alternatives, modifications and variations that fall within the scope of the following claims.

Claims (12)

A wireless device for performing cell selection in a wireless communication system,
RF (Radio Frequency) unit for transmitting and receiving a radio signal;
A memory for storing basic priorities for the plurality of frequencies; And
A processor coupled to the RF unit and a memory to implement a radio interface protocol;
Measuring signal strength of at least one cell at at least one of the plurality of frequencies, and
The cell having the best measurement result is a closed subscriber group (CSG) cell, and if the measurement result of the CSG cell is greater than the CSG threshold, the wireless device performs cell selection according to an override priority. 2. The wireless device of claim 1, wherein the processor performs cell selection according to the basic priority if the cell having the best measurement result is a CSG cell and the measurement result of the CSG cell is less than a CSG threshold. The wireless device of claim 1, wherein the override priority gives a highest priority to a frequency in which the CSG cell is included among the plurality of frequencies. The wireless device of claim 1, wherein the at least one cell comprises a peripheral cell and a serving cell. 5. The wireless device of claim 4 wherein the serving cell is a CSG cell. 2. The wireless device of claim 1 wherein the basic priority is received from a base station. A method of performing cell selection in a wireless communication system,
Set the default priority for multiple frequencies,
Measuring signal strength of at least one cell at at least one of the plurality of frequencies, and
If the cell with the best measurement result is a Closed Subscriber Group (CSG) cell, and the measurement result of the CSG cell is greater than a CSG threshold, performing cell selection according to an override priority. 8. The method of claim 7, further comprising performing cell selection according to the base priority if the cell with the best measurement result is a CSG cell and the measurement result of the CSG cell is less than a CSG threshold. 8. The method of claim 7, wherein the override priority gives the highest priority to the frequency at which the CSG cell is included among the plurality of frequencies. 8. The method of claim 7, wherein the at least one cell comprises a peripheral cell and a serving cell. The method of claim 10, wherein the serving cell is a CSG cell. 8. The method of claim 7, wherein the basic priority is received from a base station.

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