ES2561484T3 - Method for performing a cell reselection procedure in a wireless communication system - Google Patents

Abstract

A method of performing a cell reselection procedure in a wireless communication system, the method comprising: evaluating (S210) the priorities of a service cell and the adjoining cells, where the service cell and adjoining cells use different frequency bands, where priorities are received using system information from a base station, where priorities are provided to determine a priority between different frequency bands; perform (S220) measurement between frequencies in an adjoining cell that has a higher priority than the service cell, where the measurement between frequencies is performed independently of the signal measurement of the service cell; perform (S220) the measurement between frequencies in an adjoining cell that has a lower or equal priority than the service cell, when a signal characteristic of the service cell is less than a threshold for the measurement between frequencies; and perform (S230) cell reselection according to priorities.

Description

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DESCRIPTION

Method for performing a cell reselection procedure in a wireless communication system Technical field

The present invention relates to wireless communications and, more particularly, to a method of carrying out a cell reselection procedure in a wireless communication system.

Prior art

The mobile communication systems of the third generation cooperation project (3GPP) based on a broadband code division multiple access radio (WCDMA) access technology are widely spread throughout the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the medium-term future. However, given that the requirements and expectations of users and service providers are continually increasing and the development of competing radio access techniques is continually in progress, new technical developments in 3GPP are required to ensure future competitiveness. The reduction of the cost per bit, the increase of the service availability, the flexible use of the frequency bands, the simple structure and open interface, the adequate power consumption of a user equipment (UE) and the like are defined as requirements .

A cell selection procedure is a procedure to select a cell in which a service is provided to the UE. In general, the UE selects a cell that has a more intense signal characteristic value by performing a signal measurement procedure with a base station (BS) in all search frequency bands. When the UE moves away from a service cell that currently provides a service to the UE, the UE reselects a desired cell capable of receiving a signal with a better characteristic. This is called a cell reselection procedure. The cell reselection procedure is to select a more desired cell according to a criterion for signal measurement and for cell reselection in an inactive state. The UE may request a network to provide a service or it may wait in the inactive state to receive the service from the network. When in an inactive state, the UE repeats the cell reselection procedure in which a cell having a better signal characteristic is re-selected by measuring the signals of the service cell and its adjoining cells.

The 3GPP Draft No. R12-072386, entitled "E-UTRA Measurements and Cell Reselection Considerations," describes cell EU measurements and cell reselection schemes for a UE in an E-UTRA wireless network.

European Patent No. EP 2 119 254 B1 describes methods for interworking between various RATs and devices for implementing the described methods.

If the cell reselection procedure is performed using only UE signal measurement, a system load (for example, a settlement load or a traffic load) can be generated when a large number of UEs are located in a cell. In this case, the BS cannot properly distribute the system load. The cell reselection procedure is important to provide a better quality of service to a UE that has mobility.

Therefore, there is a need for a method of effectively performing a cell reselection procedure of a UE.

Description of the invention

Technical problem

The present invention provides a method of carrying out a cell reselection procedure.

Technical solution

In one aspect, there is provided a method of carrying out a cell reselection procedure in a wireless communication system as set forth in the appended claims.

In another aspect, a user equipment is provided that performs a cell reselection procedure in a wireless communication system as set forth in the appended claims.

Advantageous effects

According to the present invention, in a cell reselection procedure of a user equipment, an unnecessary cell search operation according to priority can be avoided. In addition, a cell reselection indicator can be used to reduce a system load and power consumption of the user equipment by controlling

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a cell reselection criterion. Therefore, the cell reselection procedure can be performed effectively.

Brief description of the drawings

FIG. 1 shows a structure of a wireless communication system.

FIG. 2 is a block diagram showing a functional division between an evolved universal terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC).

FIG. 3 is a block diagram showing constituent elements of a user equipment (UE).

FIG. 4 is a diagram showing a radio protocol architecture for a user plane.

FIG. 5 is a diagram showing a radio protocol architecture for a control plane.

FIG. 6 shows a correlation between logical downlink channels and downlink transport channels.

FIG. 7 shows a correlation between uplink transport channels and physical uplink channels.

FIG. 8 shows a correlation between downlink transport channels and physical downlink channels.

FIG. 9 shows a correlation between uplink transport channels and physical uplink channels.

FIG. 10 is a flow chart showing a cell selection procedure performed by a UE in an inactive mode.

FIG. 11 shows a cell search criteria that is performed for cell reselection by a UE.

FIG. 12 is a flow chart showing a cell reselection procedure performed by a UE according to an embodiment of the present invention.

FIG. 13 is a flow chart showing a cell reselection procedure performed by a UE according to another embodiment of the present invention.

FIG. 14 shows a cell search criteria that is performed for cell reselection by a UE according to an embodiment of the present invention.

Mode for the invention

FIG. 1 shows a structure of a wireless communication system. The wireless communication system may have a network structure of an evolved universal mobile telecommunications system (E-UMTS). The E-UMTS can be known as a long-term evolutionary system (LTE). The Wireless communication system can be widely deployed to provide a variety of communication services, such as voice, data packets, etc.

With reference to FIG. 1, an evolved UMTS terrestrial radio access network (E-UTRAN) includes at least one base station (BS) 20 that provides a control plane and a user plane.

A user equipment (UE) 10 can be fixed or mobile and can be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, etc. The BS 20 is generally a fixed station that communicates with the UE 10 and can be referred to as another terminology, such as an evolved B node (eNB), a base transceiver system (BTS), an access point, etc. There is one or more cells within the coverage of the BS 20. The interfaces for transmitting user traffic or control traffic can be used between the BS 20. Thereafter, a downlink is defined as a communication link from the BS 20 to UE 10 and an uplink is defined as a communication link from UE 10 to BS 20.

The BS 20s are connected via an X2 interface. The BS 20s also connect via an S1 interface to an evolved packet core (EPC), more specifically, to a mobility management entity (MME) / service gateway (S-GW) 30. The S1 interface supports a many-to-many relationship between the BS 20 and the MME / S-GW 30.

FIG. 2 is a block diagram showing a functional division between the E-UTRAN and the EPC. The oblique striped boxes represent radio protocol layers and the white boxes represent the functional entities of the control plane.

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With reference to FIG. 2, the BS performs the following functions: (1) functions for radio resource management (RRM) such as radio bearer control, radio admission control, connection mobility control and dynamic allocation of resources to the UE; (2) Internet protocol (IP) header compression and encryption of user data flows; (3) routing user plane data to the S-GW; (4) programming and transmission of search messages; (5) programming and transmission of broadcast information; and (6) measurement and measurement notification configuration for mobility and programming.

The MME performs the following functions: (1) stratum signaling without access (ÑAS); (2) ÑAS signaling security; (3) UE accessibility inactively; (4) tracking area list management; (5) roaming; and (6) authentication.

The S-GW performs the following functions: (1) mobility anchor; and (2) lawful interception. The PDN gateway (P-GW) performs the following functions: (1) UE IP assignment; and (2) packet filtering.

FIG. 3 is a block diagram showing constituent elements of the UE. A UE 50 includes a processor 51, a memory 52, a radio frequency (RF) unit 53, a display unit 54 and a user interface unit 55. The layers of the radio interface protocol are implemented in the processor 51. The processor 51 provides the control plane and the user plane. The function of each layer can be implemented in processor 51. Processor 51 obtains information from the system to be described later.

Memory 52 is coupled to processor 51 and stores an operating system, applications and general files. The display unit 54 shows a variety of information of the UE 50 and can use a well known element such as a liquid crystal display (LCD), an organic light emitting diode (OLED), etc. The user interface unit 55 can be configured with a combination of well-known user interfaces such as a keyboard, a touch screen, etc. RF unit 53 is coupled to processor 51 and transmits and / or receives radio signals.

The layers of a radio interface protocol between the UE and the network can be classified into layer L1 (a first layer), layer L2 (a second layer) and layer L3 (a third layer) based on the three lowest layers of the Open Systems Interconnection (OSI) model that is well known in the communication system. A physical layer or simply a PHY layer belongs to the first layer and provides an information transfer service through a physical channel. A radio resource control layer (RRC) belongs to the third layer and serves to control radio resources between the UE and the network. The UE and the network exchange RRC messages through the RRC layer.

FIG. 4 is a diagram showing a radio protocol architecture for the user plane. FIG. 5 is a diagram showing a radio protocol architecture for the control plane. They illustrate an architecture of a radio interface protocol between the UE and the E-UTRAN. The user plane is a protocol stack for transmission of user data. The control plane is a protocol stack for control signal transmission.

With reference to FIG. 4 and 5, a PHY layer belongs to the first layer and provides an upper layer with an information transfer service through a physical channel. The PHY layer is coupled with a medium access control (MAC) layer, that is, an upper layer of the PHY layer, through a transport channel. The data is transferred between the MAC layer and the PHY layer through the transport channel. Between different PHY layers (that is, a PHY layer of a transmitter and a PHY layer of a receiver), data is transferred through the physical channel.

The MAC layer belongs to the second layer and provides services to a radio link control (RLC) layer, that is, an upper layer of the MAC layer, through a logical channel. The RLC layer in the second layer supports reliable data transfer. There are three modes of operation in the RLC layer, that is, a transparent mode (TM), an unrecognized mode (UM) and a recognized mode (AM) according to a data transfer method. An AM RLC provides bidirectional data transmission services and supports retransmission when the transfer of the RLC PDU fails.

A packet data convergence protocol (PDCP) layer belongs to the second layer and performs a header compression function to reduce an IP packet header size.

A radio resource control (RRC) layer belongs to the third layer and is defined only in the control plane. The RRC layer serves to control the logical channel, the transport channel and the physical channel in association with configuration, reconfiguration and release of radio bearers (RB). An RB is a service provided by the second layer for data transmission between the UE and the E-UTRAN. When an RRC connection is established between an RRC layer of the UE and an RRC layer of the network, the UE is said to be in an RRC connected mode. When the RRC connection is not yet established, the UE is said to be in an inactive RRC mode.

A layer of stratum without access (ÑAS) belongs to an upper layer of the RRC layer and serves to perform session management, mobility management or the like.

FIG. 6 shows a correlation between logical downlink channels and downlink transport channels. FIG. 7 shows a correlation between uplink transport channels and physical channels

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uplink. This is described in section 6.1.3.2 of TS 36.300 V8.3.0 of 3GPP (12-2007) Technical Group for Radio Access Network Specification; Universal Evolved Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); General description; Stage 2 (Publication 8).

With reference to FIG. 6 and 7, in the downlink, a search control channel (PCCH) is correlated with a search channel (PCH). In addition, a broadcast control channel (BCCH) is correlated with a broadcast channel (BCH) or a shared downlink channel (DL-SCH). A common control channel (CCCH), a dedicated control channel (DCCH), a dedicated traffic channel (DTCH), a multicast control channel (MCCH) and a multicast traffic channel (MTCH) are correlated with the DL-SCH The MCCH and MTCH are also correlated with a multicast channel (MCH). In the uplink, the CCCH, the DCCH and the DTCH are correlated with a shared uplink channel (UL-SCH).

A type of each logical channel is defined according to a type of information to be transmitted. A logical channel is classified into two groups, that is, a control channel and a traffic channel.

The control channel is used for transfer of control plane information. The BCCH is a downlink control channel for disseminating system control information. The PCCH is a downlink channel that transfers search information and is used when a network does not know the location of a UE. The CCCH is a channel for transmitting control information between the UE and the network and is used when there is no RRC connection established between the UE and the network. The MCCH is a point-to-multipoint downlink channel used to transmit multimedia broadcast multicast service control information (MBMS) from the network to the UE. The MCCH is used by UEs that receive an MBMS. The DCCH is a two-way point-to-point channel that transmits dedicated control information between the UE and the network and is used by UEs that have an RRC connection.

The traffic channel is used for the transfer of user plane information. The DTCH is a point-to-point channel dedicated to a UE, for the transfer of user information. The DTCH can exist in both the uplink and the downlink. The MTCH is a point-to-multipoint downlink channel for transmitting traffic data from the network to the UE and is used by UEs receiving the MBMS.

The transport channel is classified according to a type and characteristic of data transmission through a radio interface. The BCH is broadcast in the entire coverage area of the cell and has a fixed, predefined transport format. The DL-SCH is characterized by the hybrid automatic repeat request (HARQ) support, dynamic link adaptation support varying modulation, coding and transmission power (Tx), possibly to be broadcast in the entire cell, possibly to use Beam shaping, support for both dynamic and semi-static resource allocation, UE discontinuous reception support (DRX) to allow UE power savings and MBMS transmission support. The PCH is characterized by UE DRX support to allow UE power savings and requirement to be disseminated in the entire cell coverage area. The MCH is characterized by supporting a requirement to be disseminated in the entire cell coverage area and supporting a single frequency MBMS network (MBSFN).

An UL-SCH and a random access channel (RACH) are uplink transport channels. The UL-SCH is characterized by dynamic link adaptation support to change Tx power and HARQ modulation and coding and support and dynamic / semi-static resource allocation. RACH is characterized by limited control information and collision risk.

FIG. 8 shows a correlation between downlink transport channels and physical downlink channels. FIG. 9 shows a correlation between uplink transport channels and physical channels

uplink.

With reference to FIG. 8 and 9, in the downlink, a BCH is correlated with a physical broadcast channel (PBCH). In addition, an MCH is correlated with a physical multicast channel (PMCH). In addition, a PCH and a DL-SCH are correlated with a physical downlink shared channel (PDSCH). The PBCH carries a BCH transport block. The PMCH transports the MCH. The PDSCH transports the DL-SCH and the PCH. In the uplink, an UL-SCH is mapped to a shared physical uplink (PUSCH) channel. In addition, a RACH is correlated with a physical random access channel (PRACH). The PRACH carries a random access preamble.

There are several physical control channels used in a PHY layer. A physical downlink control (PDCCH) channel informs a UE of the resource allocation of the PCH and DL-SCH and HARQ information related to the DL-SCH. The PDCCH may carry an uplink programming concession that informs the UE of the allocation of resources for uplink transmission. A physical control format indicator (PCFICH) channel informs the UE of the number of orthogonal frequency division multiplexing (OFDM) symbols used for PDCCHs and is transmitted in each subframe. A physical hybrid ARQ indicator channel (PHICH) carries HARQ recognition (ACK) / negative recognition (NACK) signals in response to uplink transmission. A physical uplink control (PUCCH) channel carries HARQ ACK / NACK signals in response to downlink transmission, programming request and uplink control information (for example, a channel quality indicator (CQI) ).

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A BS informs the UE of a plurality of pieces of Basic Information for network access using System Information. The system information includes necessary information that needs to be known by the UE to access the BS. Therefore, the UE has to fully receive the system information before accessing the BS and always has to keep the latest system information. Since the system information has to be known to all UEs within a cell, the BS periodically transmits the system information.

Now, a cell selection and reselection procedure performed by a UE in an inactive mode will be described. Hereinafter, a physical signal characteristic in association with a signal strength or signal to interference ratio plus noise (SINR) will be known simply as a signal characteristic.

The cell selection is done so that the UE registers to a network. In this way, the UE can receive a service from a BS. Here, if the intensity or quality of a signal between the UE and the BS deteriorates due to the mobility of the UE, the UE reselects another cell in order to maintain the quality of data transmission.

Cell selection can be carried out according to a signal characteristic that depends on a wireless environment. The criteria used in the cell selection performed by the UE can be as follows: (1) UE capacity; (2) subscriber information; (3) settlement load balancing; and (4) traffic load balancing. The capacity of the UE indicates that a cell is selected according to a frequency band selectable by the UE. This is because a frequency band that can be used by the UE itself may be limited. The subscriber information indicates that the UE can be configured to enable or disable cell selection according to the subscriber information or a service provider policy. Settlement load balancing indicates that, in order to reduce a load resulting from data generated when a plurality of UE in an inactive state is activated in a cell, a cell used by a small number of UE is selected. Traffic load balancing is to change a cell to reduce a load that results from data generated in activated UEs. Using settlement load balancing and traffic load balancing, UEs within the same cell can allow communication by sharing radio resources. In addition, since load balancing is achieved between cells, radio resources can be used effectively.

FIG. 10 is a flow chart showing a cell selection procedure performed by a UE in an inactive mode.

With reference to FIG. 10, when the power is on, the UE selects an external network such as a public land mobile network (PLMN) through which the UE intends to receive a service (step S110). The UE can select the PLMN according to a priority list of the PLMN. The UE can obtain the list containing one or more PLMN using a system information provided through a broadcast channel.

The UE selects a radio access technology (RAT) to communicate with the selected PLMN (step S120). The PLMN and RAT can be selected by a user of the UE or can be previously stored in the UE.

The UE performs a cell selection operation (step S130). The UE searches for a cell of the selected PLMN and selects a desired cell to provide an available service. The cell selection operation includes an initial cell selection operation and a stored information cell selection operation. In the initial cell selection operation, the UE scans all RF channels without knowing an RF channel of the system. In the stored information cell selection operation, the UE selects a desired cell using cell information obtained from a previously measured cell or control information. If the desired cell cannot be selected in the stored information cell selection operation, the UE performs the initial cell selection operation.

Among the cells in which the intensity or quality of a signal transmitted from a BS is greater than a specific threshold, the UE selects a cell that has the highest value. The cell selection threshold indicates a value defined in the system to obtain guaranteed quality of a physical signal when data is transmitted or received. The threshold for cell selection may vary depending on the RAT selected. The UE tunes to a control channel of the selected cell. The UE receives system information transmitted periodically from the BS.

In the cell selection operation, the UE can be registered with the network. The UE registers its information with the network in order to receive a service such as search from the network. The UE is registered with the network when network information known to the UE is different from the network information obtained from the system information, instead of being registered with the network to be accessed whenever the UE performs the operation of cell selection

The UE performs a cell reselection operation evaluating the need for cell reselection (step S140). If cell reselection is not necessary, the UE does not perform the cell reselection operation but remains in a current service cell. If the intensity or quality of a signal measured from the service cell from which a service is currently received is less than the threshold for cell reselection, the UE selects one of the adjoining cells providing a better signal characteristic than the cell of service. Examples of the signal characteristic to be measured include received power of reference symbol (RSRP), received quality of reference symbol (RSRQ), received signal strength indicator (RSSI), etc.

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FIG. 11 shows a cell search criteria that is performed for cell reselection by a UE.

With reference to FIG. 11, the UE periodically measures a signal from a service cell and determines whether to perform cell reselection. The UE compares the intensity or quality of the measured signal of the service cell with a threshold and thus restricts a cell search operation for cell reselection.

Equation 1 shows a cell search criteria that is performed for cell reselection.

Mathematical Figure 1

[Mat. one]

Service discharge> Cintra search, do not perform intrafrequency measurements Service cell ^ Search, perform intrafrequency measurements

In Equation 1, Service drop indicates a signal characteristic (for example, RSRP) measured from the service cell and No search indicates a threshold for intra-frequency measurement. The intra-frequency measurement indicates that frequency bands are measured on the same E-UTRAN. In intra-frequency measurement, a signal characteristic of an adjacent cell that uses the same frequency band as the service cell is measured. If the signal characteristic measured from the service cell is greater than the threshold for intra-frequency measurement, the UE does not perform the intra-frequency measurement. That is, since the signal characteristic of the service cell is superior to that of the adjoining cell, cell reselection is not necessary. In this way, the UE does not perform the cell search for cell reselection. Otherwise, if the signal characteristic measured from the service cell is less than the threshold for intra-frequency measurement, the UE performs the intra-frequency measurement.

Equation 2 shows another cell selection criterion that is performed for cell reselection.

Mathematical Figure 2

[Mat. 2]

Service discharge ^ Center searches, do not perform measurements between frequencies ^ service termination ^ Center searches, perform measurements between frequencies

In Equation 2, Search Searches indicates a threshold for measurement between frequencies. Inter-frequency measurement indicates that a signal characteristic of an adjoining cell is measured using a different frequency band from a service cell. The measurement between frequencies indicates that frequency bands of different E-UTRAN are measured. If the signal characteristic measured from the service cell is greater than the threshold for measurement between frequencies, the UE does not perform the measurement between frequencies. Otherwise, if the signal characteristic measured from the service cell is less than the threshold for measurement between frequencies, the UE performs the measurement between frequencies.

If the signal quality of the service cell is lower than a first threshold for intra-frequency measurement, only the intra-frequency measurement is performed. Otherwise, if the signal quality of the service cell is less than a second threshold, intra-frequency measurement and measurement between frequencies are performed. A criterion to perform cell search through signal measurement is known as the S criterion.

When a criterion for performing the intra-frequency measurement or the measurement between frequencies is satisfied, the UE orders classifications of cells of the service cell and its adjoining cells. Equation 3 shows a criterion for ordering cell classifications. This criterion is known as criterion R.

Mathematical Figure 3

[Mat. 3]

Rs - Qrned s Qhysts

Rn = Qmed.n - Qcompensation

In Equation 3, Rs indicates a cell classification of the service cell, Rn indicates a cell classification of the adjoining cell, Qmed, s indicates an RSRP measurement value of the service cell and Qmed.n indicates a value of RSRP measurement of the adjoining cell. Qhysts are used to provide a weighting factor to the service cell. Compensation is used to provide a deviation between the service cell and a cell to be re-selected. Qhysts and Qcompensation can be provided to the UE using system information.

The UE performs a cell reselection operation in a cell that has a higher classification among adjoining cells that satisfy the condition of Rn> Rs for a period of reselect time. The reselection time period is notified by the BS to the UE using the system information. The period of time

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Reselection is used to impose a limitation in which a cell selection condition has to be satisfied for a specific period of time or more. A cell change may indicate a change in RAT. Examples of the type of RAT include a global system for mobile communications (GSM) / general packet radio service (GPRS), a UMTS, an E-UMTS, etc. The measurement Between RAT indicates that a RAT of the adjoining cell is measured for the change of RAT.

The UE may request the network to provide a service or may wait in an inactive mode in order to receive the service from the network. The UE in idle mode repeats the cell reselection operation in which a cell having a higher signal characteristic is re-selected by measuring signals from adjacent cells of a cell currently receiving a service.

In a method for performing cell search for cell selection on the basis of signal measurement, the power consumption of the UE can be reduced. However, the method based on signal measurement is dependent on the UE and thus does not consider necessary system factors such as settlement load or system traffic load. The BS needs to control the cell reselection of the UE if required.

FIG. 12 is a flow chart showing a cell reselection procedure performed by a UE according to an embodiment of the present invention.

With reference to FIG. 12, a BS can control the UE cell reselection procedure by providing a priority to the UE. Alternatively, the UE can perform the cell reselection procedure using a predetermined priority. Priority can be provided to determine a priority between a service cell and an adjoining cell, each of which uses a different frequency band in cell reselection. Priority can be provided to determine a priority between different E-UTRAN frequencies or different frequencies between RAT. The cell reselection procedure performed by the UE differs depending on whether or not priority is provided. When priority is provided, the UE performs measurement between frequencies or measurement between RATs in a cell that has a higher priority regardless of whether or not the S criterion is met.

The UE evaluates the priority (step S210). Priority can be provided from the service cell. The priority can be determined by the UE using information previously stored in the UE. The priority is a 1-bit information that can indicate whether the priority is applied. Alternatively, the priority may indicate whether the priority is applied for each of the measurement between frequencies or measurement between RAT. Alternatively, the priority can be provided to the UE in the form of a probability value in consideration of a settlement load or system traffic load. For example, if a system load of a currently used E-UTRAN is greater than that of another E-UTRAN, a decrease in priority can be provided to the UE and, otherwise, an increase in priority can be provided to the UE. For the loading of the system, the following can be taken into account, that is, the number of UEs in a cell that receives a service from the BS, an amount of traffic from a service provided in each cell by the BS, a quality of service (QoS) required, a QoS provided by the BS, a QoS of a subscriber, etc.

Priority can be provided to the UE using system information. Alternatively, priority can be provided to the UE using an RRC message. Examples of the RRC message include system information, a search message, a radio bearer establishment message, a radio bearer reconfiguration message, an RRC connection request message, an RRC connection establishment message, a system message.

RRC connection release, etc.

Priority can be provided to the UE using dedicated signaling. The UE ignores another priority provided using system information other than the priority provided using dedicated signaling. When the UE moves out of an assigned PLMN with the priority or when the UE enters an RRC connected state or until a valid priority time is terminated, the UE performs the cell reselection procedure using the priority provided using the dedicated signage.

When priority is provided, the UE performs the measurement between frequencies or measurement between RAT (step S220). The UE can perform the measurement between frequencies or measurement between RAT according to the priority. (1) The UE performs the measurement between frequencies or measurement between RAT in an E-UTRAN that has a higher priority than the priority of the E-UTRAN currently used. (2) The UE performs the measurement between frequencies or measurement between RAT in an E-UTRAN that has a lower priority than the E-UTRAN currently used using the criteria of Equation 2 above. In this case, Search Searches indicates a threshold for measurement between frequencies or measurement between RAT. That is, for an adjoining cell that has a high priority, the measurement between frequencies or measurement between RAT is performed independently of the S criterion and for an adjoining cell that has a low priority, the measurement between frequencies or measurement between RAT is performed only in an adjoining cell that satisfies criteria S.

The UE performs a cell reselection operation according to the priority as follows (step S230).

(1) Among cells in a RAT or E-UTRAN that have a higher priority than the currently used E-UTRAN, a cell that has a signal characteristic greater than a first priority threshold is re-selected. Between cells that have a high priority and that have a signal characteristic greater than the first threshold of

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priority, the UE reselects a cell that has a better signal characteristic. The first priority threshold is a criterion for reselecting cells that have a high priority. The first priority threshold may vary according to the E-UTRAN or RAT.

(2) For E-UTRANs that have the same priority, the UE can perform the cell reselection operation by ordering cell classifications according to Equation 3 above. Cell reselection of E-UTRANs that have the same priority can be considered as cell reselection that uses intra-frequency measurement for the same E-UTRAN.

(3) Between cells of the E-UTRAN or E-UTRAN / RAT currently used that have a high priority, if there is no cell that has a signal characteristic greater than the first priority threshold and if a signal characteristic of a Service cell is less than a value defined to perform cell reselection, the UE performs cell reselection in the E-UTRAN or RAT that has a lower priority. Between cells of the E-UTRAN or RAT that has a low priority, the UE performs cell reselection in a cell that has a signal characteristic greater than a second priority threshold. The second priority threshold is a criterion for reselecting cells that have a low priority. The second priority threshold may vary according to E-UTRAN or RAT.

FIG. 13 is a flow chart showing a cell reselection procedure performed by a UE according to another embodiment of the present invention. This is a case where a priority is not provided to the UE.

With reference to FIG. 13, when the priority is not provided to the UE, then the UE performs cell reselection based on the signal measurement, that is, based on the S criterion (step S310).

The UE measures the signals from a service cell and then performs intra-frequency measurement or measurement between frequencies according to Equation 1 or Equation 2 above (step S320).

If a criterion for performing the intra-frequency measurement or measurement between frequencies is satisfied, the UE orders the classifications of a service cell and its adjoining cells. A cell that has a higher classification is re-selected from adjoining cells that satisfy the condition of Rn> Rs (step S330).

The UE evaluates whether the priority for cell reselection is provided. If priority is provided, as shown in FIG. 12, the UE performs the cell reselection procedure based on priority. Otherwise, if priority is not provided, as shown in FIG. 13, the UE performs the cell reselection procedure based on criteria S.

If priority is not given to the UE, the cell reselection procedure is performed according to the channel quality of the UE. There is a need to control the cell reselection of the UE considering a system load even when priority is not provided.

FIG. 14 shows a cell search criteria that is performed for cell reselection by a UE according to an embodiment of the present invention.

With reference to FIG. 14, a BS endows the UE with a Cell Reselection Indicator to control cell reselection of the UE. The Cell Reselection Indicator can indicate a threshold of criterion S used in cell reselection or a parameter to adjust classifications of criteria R cells or a parameter increase / decrease value. The Cell Reselection Indicator can be broadcast to all UEs within a cell using system information. Alternatively, the Cell Reselection Indicator can be multicasted to a specific group of UEs or can be merged to a UE.

Equation 4 shows an Intrafrequency measurement criterion added with a parameter to adjust a threshold of criterion S.

Mathematical Figure 4 [Mat. 4]

Service weld 2 look at search + ASsearch, do not perform intra-frequency measurements

Sce / da service £ Your search + AS search, perform Intrafrequency measurements

In Equation 4, the search indicates a 1st parameter of criterion S to adjust an intra-frequency measurement threshold. The increase in the value of the 1st parameter causes the increase in the number of UEs that perform intra-frequency measurement. The decrease in the value of the 1st parameter causes the decrease in the number of UEs that perform intra-frequency measurement. That is, by adjusting the 1st parameter, the number of UEs that perform intra-frequency measurement can be adjusted.

Equation 5 shows a measurement criterion between frequencies added with a parameter to adjust a threshold of criterion S.

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Mathematical Figure 5 [Mat. 5]

Sce / c / a service> Feel searches + AS between searches, do not perform measurements between frequencies ^ cease service ^ Center searches AS between searches, perform measurements between frequencies

In Equation 5, AS between searches indicates a 2nd parameter of criterion S to adjust a measurement threshold between frequencies. The increase in the value of the 2nd parameter causes the increase in the number of UEs that measure between frequencies. The decrease in the value of the 2nd parameter causes the decrease in the number of UEs that measure between frequencies. That is, by adjusting the 2nd parameter, the number of UEs that measure between frequencies can be adjusted.

Equation 6 shows a criterion for ordering cell classifications added with a parameter for adjusting cell classifications of criterion R.

Mathematical Figure 6

[Mat. 6]

Rs - Qmeds Qhyst $

Rn, intra = Qmed.n - Qcompensation - ñQcompensation, intra

Rn, enter = Qmed.n - Qcompensation - AQcompensadónn, enter

In Equation 6, Rn, intra indicates a classification of cells of an adjacent cell in intra-frequency measurement, Rn, between indicates a classification of cells of an adjacent cell in measurement between frequencies, AQ compensator) n, ntra indicates a 3rd parameter to adjust The classification of cells of the adjacent cell in intra-frequency measurement, AQcompensadónn, between indicates a 4th parameter to adjust the classification of cells of the adjacent cell in measurement between frequencies. In intra-frequency measurement, the increase in the value of the 3rd parameter causes the decrease in a probability of intracellular reselection. In addition, the decrease in the value of the 3rd parameter causes the increase in a probability of intracellular reselection to the adjacent cell. In measurement between frequencies, the increase in the value of the 4th parameter causes the decrease in a probability of reselection between cells. In addition, the decrease in the value of the 4th parameter causes the increase in a probability of reselection between cells for the adjacent cell.

It will be assumed that the system load is high and in this way there is no need to control the cell reselection performed by the UEs. In order to reduce the system load, the BS allows UEs to perform cell reselection more frequently. The BS increases the 1st parameter or the 2nd parameter, thereby increasing the number of UEs that perform intra-frequency measurement or measurement between frequencies. In addition, the BS decreases the 3rd parameter or the 4th parameter, thereby increasing a probability that a UE will perform cell reselection for an adjoining cell. The BS can reduce the system load by selectively adjusting a parameter value between the 1st to 4th parameters. System load balancing can be controlled between BSs by sharing criteria S or criterion R.

Using the 3rd parameter and the 4th parameter, the BS can determine which operation will be performed first between intracell reselection and between cell reselection. For example, if the value of the 4th parameter decreases relatively more than the 3rd parameter, a probability of reselecting between cells increases. In this way, intracell reselection can be performed prior to reselection between cells. The 3rd parameter or the 4th parameter can be adjusted differently according to a cell or a layer. The 3rd parameter or the 4th parameter can have the same value in the same layer regardless of the cell.

The cell reselection indicator can be determined in consideration of an overload of system information. If the cell reselection indicator indicates actual values of the 1st to 4th parameters, the system information overload may be large. The cell reselection indicator may indicate increase / decrease values from the 1st to 4th parameters. Alternatively, the cell reselection indicator can indicate whether the 1st to 4th parameters are increased or decreased. For example, if it is previously determined that the 1st to 4th parameters are increased or decreased by a specific magnitude, the cell reselection indicator may indicate whether the 1st to 4th parameters are increased or decreased using a bit. The cell reselection indicator can selectively indicate a real value or an increase / decrease value of any of the 1st to 4th parameters.

Meanwhile, when considering system load, reselection between cells can be advantageous over intracell reselection in terms of effectiveness control. The 1st and 3rd parameters in association with the intra-frequency measurement can remain as fixed values and only the 2nd and 4th parameters in association with the measurement between frequencies can be adjusted as cell reselection indicators. The cell reselection indicator may indicate an increase / decrease value of each of the 2nd and 4th parameters or may indicate whether the parameters are increased or decreased. The 2nd and 4th parameters for system load balancing can be

increase or decrease by a specific predetermined magnitude between the BS and the UE. The cell reselection indicator can indicate whether the 2nd and 4th parameters are increased or decreased at once using a bit. For example, if the cell reselection indicator is ‘1’, it indicates the increase in a cell load and if the cell reselection indicator is ‘0’, it indicates the decrease in cell load. If the cell reselection indicator is not transmitted, 5 indicates no change in cell load. When the cell reselection indicator of 1 is transmitted, the UE performs reselection between cells by increasing the 2nd parameter to a predetermined value and decreasing the 4th parameter to a predetermined value. When the cell reselection indicator of 0 is transmitted, the UE performs reselection between cells by decreasing the 2nd parameter to a predetermined value and increasing the 4th parameter to a predetermined value.

10 Each function described above can be performed by a processor such as a microprocessor based on software encoded to perform such a function, a program code, etc., a controller, a microcontroller, an ASIC (Specific Applications Integrated Circuit) or Similar. The planning, development and implementation of such codes may be obvious to those skilled in the art based on the description of the present invention.

Although the embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope of the claims. Accordingly, the embodiments of the present invention are not limited to the embodiments described above but are defined by the following claims.

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Claims (3)

5 10 fifteen twenty 25 30 1. A method of performing a cell reselection procedure in a wireless communication system, the method comprising: evaluate (S210) the priorities of a service cell and adjoining cells, where the service cell and adjoining cells use different frequency bands, where priorities are received using system information from a base station, where Priorities are provided to determine a priority between the different frequency bands; perform (S220) measurement between frequencies in an adjoining cell that has a higher priority than the service cell, where the measurement between frequencies is performed independently of the signal measurement of the service cell; perform (S220) the measurement between frequencies in an adjoining cell that has a lower or equal priority than the service cell, when a signal characteristic of the service cell is less than a threshold for the measurement between frequencies; Y Perform (S230) cell reselection according to priorities. 2. The method of claim 1, wherein the measurement between frequencies measures frequencies of different evolved UMTS terrestrial radio access networks, E-UTRAN or different radio access technologies, RAT. 3. A user equipment that performs a cell reselection procedure in a wireless communication system, the user equipment comprising: a radio frequency unit; Y a processor coupled to the radio frequency unit and configured to: evaluate (S210) the priorities of a service cell and adjoining cells, where the service cell and adjoining cells use different frequency bands, where priorities are received using system information from a base station, where Priorities are provided to determine a priority between the different frequency bands; perform (S220) measurement between frequencies in an adjoining cell that has a higher priority than the service cell, where the measurement between frequencies is performed independently of the signal measurement of the service cell; perform (S220) the measurement between frequencies in an adjoining cell that has a lower or equal priority than the service cell, when a signal characteristic of the service cell is less than a threshold for the measurement between frequencies; Y Perform (S230) cell reselection according to priorities.