Untitled record

ES 2 561 484 T3

DESCRIPTION

Method of 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 performing a cell reselection procedure in a wireless communication system.

Background of the technique

The third generation cooperative project (3GPP) mobile communication systems based on a broadband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA), which 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, since the requirements and expectations of users and service providers are continually increasing and the developments of competing radio access techniques are continually in progress, further technical evolutions in 3GPP are required to ensure future competitiveness. Reduced cost per bit, increased service availability, flexible use of frequency bands, simple structure and open interface, adequate power consumption of a user equipment (UE) and the like are defined as requirements. .

A cell selection procedure is a procedure for selecting a cell in which a service is provided to the UE. In general, the UE selects a cell having a stronger 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 serving 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 based on criteria for signal measurement and for cell reselection in an idle state. The UE may request a network to provide a service or it may wait in the idle state to receive the service from the network. When in the idle state, the UE repeats the cell reselection procedure in which a cell having a better signal characteristic is reselected by measuring the signals from the serving cell and its neighboring cells.

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

European patent No. EP 2 119 254 B1 describes methods for interworking between various RATs and devices to implement the described methods.

If the cell reselection procedure is performed using only UE signal measurement, a system load (eg, a settlement load or a traffic load) may 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 is mobile.

Accordingly, there is a need for a method of efficiently performing a UE cell reselection procedure.

Description of the invention

Technical problem

The present invention provides a method of performing a cell reselection procedure.

Technical solution

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

In another aspect, 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 selection indicator can be used to reduce a system load and user equipment power consumption by monitoring

ES 2 561 484 T3 a cell reselection criterion. Therefore, the cell reselection procedure can be effectively performed.

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 downlink logical channels and downlink transport channels.

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

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

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

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

FIG. 1 shows a structure of a wireless communication system. The wireless communication system may have an evolved universal mobile telecommunications system (E-UMTS) network structure. The E-UMTS can be known as a Long Term Evolution System (LTE). The wireless communication system can be widely deployed to provide a variety of communication services, such as voice, packet data, 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 other 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 may be referred to as other terminology, such as an evolved Node B (eNB), a base transceiver system (BTS), an access point, etc. There are one or more cells within the BS 20 coverage. Interfaces for transmitting user traffic or control traffic can be used between BS 20. Hereinafter, a downlink is defined as a communication link from BS 20 to UE 10 and an uplink is defined as a communication link. communication from UE 10 to BS 20.

The BS 20s are interconnected via an X2 interface. The BSs 20 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 BS 20 and MME / S-GW 30.

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

ES 2 561 484 T3

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 streams; (3) routing user plane data to the S-GW; (4) scheduling and transmitting paging messages; (5) programming and transmission of broadcast information; and (6) measurement and measurement notification settings for mobility and scheduling.

The MME performs the following functions: (1) no-access stratum signaling (NAS); (2) NAS signaling security; (3) Idle Mode UE accessibility; (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 (PGW) 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 radio interface protocol layers are implemented in processor 51. 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 displays a variety of information from the UE 50 and may use a well-known item 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 FIGS. 4 and 5, a PHY layer belongs to the first layer and provides an upper layer with an information transfer service over 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. 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 RLC PDU transfer 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 radio bearer (RB) configuration, reconfiguration, and release. 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 RRC idle mode.

A non-access stratum layer (NAS) belongs to a higher layer of the RRC layer and serves to perform session management, mobility management or the like.

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

Uplink ES 2 561 484 T3. This is described in section 6.1.3.2 of 3GPP TS 36.300 V8.3.0 (12-2007) Radio Access Network Technical Specification Group; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); General description; Stage 2 (Post 8).

With reference to FIGS. 6 and 7, on the downlink, a search control channel (PCCH) is correlated with a search channel (PCH). Furthermore, a broadcast control channel (BCCH) is mapped to a broadcast channel (BCH) or a downlink shared 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. MCCH and MTCH are also mapped to a multicast channel (MCH). On the uplink, the CCCH, DCCH, and DTCH are mapped to an uplink shared 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 control plane information transfer. The BCCH is a downlink control channel for broadcasting system control information. The PCCH is a downlink channel that transfers paging 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 (MBMS) control information from the network to the UE. The MCCH is used by UEs that receive an MBMS. The DCCH is a bi-directional 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 over the entire coverage area of ​​the cell and has a fixed, predefined transport format. DL-SCH is characterized by hybrid automatic repeat request (HARQ) support, dynamic link adaptation support varying modulation, encoding and transmit power (Tx), possibly to be broadcast over the entire cell, possibly to use beamforming, support for both dynamic and semi-static resource allocation, UE discontinuous reception (DRX) support to enable UE power saving, and MBMS transmission support. The PCH is characterized by UE DRX support to allow UE power saving and requirement to be broadcast over the entire coverage area of ​​the cell. The MCH is characterized by support of a requirement to be broadcast over the entire coverage area of ​​the cell and support of an MBMS single frequency network (MBSFN).

A UL-SCH and a Random Access Channel (RACH) are uplink transport channels. The UL-SCH is characterized by dynamic link adaptation support for changing Tx power and modulation and encoding and HARQ support and dynamic / semi-static resource allocation. The RACH is characterized by limited collision risk and control information.

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

With reference to FIGS. 8 and 9, on the downlink, a BCH is mapped to a physical broadcast channel (PBCH). Also, an MCH is mapped to a physical multicast channel (PMCH). Furthermore, a PCH and a DL-SCH are mapped to a physical downlink shared channel (PDSCH). The PBCH carries a BCH transport block. The PMCH carries the MCH. The PDSCH carries the dL-SCH and the PCH. On the uplink, a UL-SCH is mapped to a physical uplink shared channel (PUSCH). Furthermore, a RACH is mapped to 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 channel (PDCCH) informs a UE of the PCH and DL-SCH resource allocation and HARQ information related to the DL-SCH. The PDCCH may carry an uplink scheduling grant informing the UE of the resource allocation for uplink transmission. A physical control format indicator channel (PCFICH) 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 acknowledge (ACK) / negative acknowledge (NACK) signals in response to uplink transmission. A physical uplink control channel (PUCCH) carries HARQ ACK / NACK signals in response to downlink transmission, scheduling request, and uplink control information (for example, a channel quality indicator (CQI) ).

ES 2 561 484 T3

A BS informs the UE of a plurality of pieces of basic information for network access using information from the system. 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 by 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 idle mode will be described. Hereinafter, a physical signal characteristic in association with a signal strength or signal-to-interference plus noise ratio (SINR) will simply be referred to as a signal characteristic.

Cell selection is done so that the UE registers to a network. In this way, the UE can receive a service from a BS. Herein, if the strength or quality of a signal between the UE and the BS deteriorates due to the mobility of the UE, the UE reselects another cell for the purpose of maintaining the data transmission quality.

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 based on subscriber information or a service provider policy. Settling load balancing indicates that, in order to reduce a load resulting from data generated when a plurality of UEs are activated in a cell in an idle state, a cell used by a small number of UEs is selected. Traffic load balancing is to change a cell in order to reduce a load that results from data generated on activated UEs. Using settlement load balancing and traffic load balancing, UEs within the same cell can allow communication sharing radio resources. Furthermore, since load balancing is achieved between cells, radio resources can be used efficiently.

FIG. 10 is a flow chart showing a cell selection procedure performed by a UE in an idle 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 may select the PLMN according to a priority list of the PLMN. The UE can obtain the list containing one or more PLMNs 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 the RAT can be selected by a user of the UE or they can be previously stored in the UE.

The UE performs a cell selection operation (step S130). The UE searches for a cell in 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 a system RF channel. In the stored information cell selection operation, the UE selects a desired cell using cell information obtained from a previously measured cell or from 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 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 threshold for cell selection 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 periodically transmitted system information from the BS.

In the cell selection operation, the UE can register 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 registers 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 serving cell. If the strength or quality of a signal measured from the serving cell from which a service is currently received is less than the threshold for cell reselection, the UE selects one of the neighboring cells providing a better signal performance than the cell from service. Examples of the signal characteristic to be measured include Reference Symbol Received Power (RSRP), Reference Symbol Received Quality (RSRQ), Received Signal Strength Indicator (RSSI), etc.

ES 2 561 484 T3

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 serving cell and determines whether to perform cell reselection. The UE compares the measured signal strength or quality of the serving cell with a threshold and thus restricts a cell search operation to cell reselection.

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

Mathematical Figure 1

[Mat. 1]

Service sceida> No search, do not perform intrafrequency measurements

Service sceida <Sntrabearch, perform intrafrequency measurements

In Equation 1, Serving Sceida indicates a signal characteristic (eg, RSRP) measured from the serving cell and Sintrapaque indicates a threshold for intrafrequency measurement. Intrafrequency measurement indicates that frequency bands are measured in the same E-UTRAN. In intrafrequency measurement, a signal characteristic of a neighboring cell using the same frequency band as the serving cell is measured. If the signal characteristic measured from the serving 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 serving cell is higher than that of the neighboring cell, no cell reselection is necessary. In this way, the UE does not perform the cell search for cell reselection. Otherwise, if the signal characteristic measured from the serving cell is less than the threshold for intra-frequency measurement, the UE performs the intra-frequency measurement.

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

Mathematical Figure 2

[Mat. two]

Sceida service> Sentre searches, do not perform measurements between frequencies

Sceida service <Sentre searches, perform measurements between frequencies

In Equation 2, Sentre searches indicate a threshold for measurement between frequencies. Inter-frequency measurement indicates that a signal characteristic of a neighboring cell is measured using a different frequency band from a serving cell. Measurement between frequencies indicates that frequency bands of different EUTRANs are measured. If the signal characteristic measured from the serving cell is greater than the threshold for inter-frequency measurement, the UE does not perform the inter-frequency measurement. Otherwise, if the signal characteristic measured from the serving cell is less than the threshold for inter-frequency measurement, the UE performs the inter-frequency measurement.

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

When a criterion is satisfied to perform the intra-frequency measurement or the inter-frequency measurement, the UE commands cell classifications of the serving cell and its neighboring cells. Equation 3 shows a criterion for ordering the cell classifications. This criterion is known as criterion R.

Mathematical Figure 3

[Mat. 3]

R s = Qmed, s + Qhysts

R n = Qmed, n - Qcompensation

In Equation 3, Rs indicates a cell classification of the serving cell, Rn indicates a cell classification of the neighboring cell, Qmed, s indicates a RSRP measurement value of the serving cell, and Qmed, n indicates a value of RSRP measurement of the neighboring cell. Qhysts is used to provide a weighting factor to the serving cell. Qcompensation is used to provide an offset between the serving cell and a cell to be reselected. Qhysts and Qcompensation can be provided to the UE using system information.

The UE performs a cell reselection operation on a cell having a higher rank among neighboring cells satisfying the condition of Rn> Rs during a reselection time period Treselection. The reselection time period is notified by the BS to the UE using the system information. The period of time

ES 2 561 484 T3 reselection is used to enforce a limitation that a cell selection condition has to be satisfied for a specific period of time or more. A cell change may indicate a RAT change. 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 Between RAT measurement indicates that a neighboring cell RAT is measured for the RAT change.

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

In a method of 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 system factors such as settlement load or system traffic load to be necessary. 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 may control the cell reselection procedure of the UE by providing a priority to the UE. Alternatively, the UE can perform the cell reselection procedure using a predetermined priority. The priority can be provided to determine a priority between a serving cell and a neighboring cell, each of which uses a different frequency band in cell reselection. The priority can be provided to determine a priority between different E-UTRAN frequencies or different frequencies between RATs. The cell reselection procedure performed by the UE differs depending on whether the priority is provided or not. When the priority is provided, the UE performs inter-frequency measurement or inter-RAT measurement on a cell that has a higher priority regardless of whether or not criterion S is satisfied.

The UE evaluates the priority (step S210). The priority can be provided from the serving cell. The priority can be determined by the UE using information previously stored in the UE. The priority is a 1-bit piece of information that can indicate whether the priority is applied. Alternatively, the priority may indicate whether the priority applies for each of the inter-frequency measurement or inter-RAT measurement. Alternatively, the priority may 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 priority downgrade may be provided to the UE, and otherwise a priority upgrade may be provided to the UE. For the system load, the following can be taken into consideration, that is, the number of UEs in a cell receiving a service from the BS, an amount of traffic of a service provided in each cell by the BS, a quality of required service (QoS), a QoS provided by the BS, a QoS from a subscriber, etc.

The priority can be provided to the UE using information from the system. Alternatively, priority can be given to the UE using an RRC message. Examples of the RRC message include the system information, a page message, a radio bearer establishment message, a radio bearer reconfiguration message, an RRC connection request message, an RRC connection establishment message, an RRC connection establishment 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 a PLMN assigned with the priority or when the UE enters a RRC connected state or until a valid time of the priority expires, the UE performs the cell reselection procedure using the priority provided using the dedicated signage.

When the priority is provided, the UE performs the inter-frequency measurement or the inter-RAT measurement (step S220). The UE can perform the measurement between frequencies or measurement between RAT according to the priority. (1) The UE performs inter-frequency measurement or inter-RAT measurement in an E-UTRAN that has a higher priority than the currently used E-UTRAN priority. (2) The UE performs the inter-frequency measurement or inter-RAT measurement in an EUTRAN that has a lower priority than the E-UTRAN currently used using the criteria of Equation 2 above. In this case, Sentre searches indicate 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 criterion S 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 criterion S.

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

(1) Among cells of 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 reselected. Between cells that have a high priority and that have a signal characteristic greater than the first threshold of

ES 2 561 484 T3 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 depending on the E-UTRAN or RAT.

(2) For E-UTRANs having 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 having the same priority can be considered as cell reselection using intra-frequency measurement for the same E-UTRAN.

(3) Between cells of the currently used E-UTRAN or E-UTRAN / RAT 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 serving cell is less than a defined value to perform cell reselection, the UE performs cell reselection in the E-UTRAN or RAT which has a lower priority. Between cells of the E-UTRAN or RAT that have a low priority, the UE performs cell reselection on 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 depending on the E-UTRAN or the 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 given to the UE.

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

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

If a criterion is satisfied to perform intrafrequency measurement or inter-frequency measurement, the UE sorts the classifications of a serving cell and its neighboring cells. A cell having a higher rank is reselected from neighboring 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 criterion S.

If the 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 cell reselection of the UE by 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 provides the UE with a cell reselection indicator to control cell reselection of the UE. The cell reselection indicator may indicate a threshold of criterion S used in cell reselection or a parameter to adjust cell rankings of criterion R or an increase / decrease value of the parameter. The cell reselection indicator can be broadcast to all UEs within a cell using system information. Alternatively, the cell reselection indicator may be multicast to a group of specific UEs or it may be unicast to a UE.

Equation 4 shows an intrafrequency measure added with a parameter to set a threshold of criterion S.

Mathematical Figure 4

[Mat. 4]^Sservice cell> no search + k no search, do not perform intrafrequency measurements ^Sservice cell <Notrasearch + ^ Notrasearch, perform intrafrequency measurements

In Equation 4, ^ Sintrasearch indicates a 1^er S criterion parameter to set an intrafrequency measurement threshold. The increase in the value of 1^er parameter causes the increase in the number of UEs that perform intrafrequency measurement. The decrease in the value of 1^er parameter causes the decrease in the number of UEs that perform intrafrequency measurement. That is, adjusting the 1^er parameter, the number of UEs that perform intrafrequency measurement can be set.

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

ES 2 561 484 T3

Mathematical Figure 5

[Mat. 5]

Sceida de servicio> Sentre searches + ESbetween searches, do not perform measurements between frequencies

Sceida de servicio &lt;Sentre searches + ESbetween searches, perform measurements between frequencies

In Equation 5, ESbetween searches indicates a 2nd parameter of the S criterion 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 perform measurement 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 perform measurement between frequencies can be adjusted.

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

Mathematical Figure 6

[Mat. 6]

R s = Qmed, s + Qhysts

Rn, intra = Qmed, n - Qcompensation - AQcompensationn, intra

R n, between = Qmed, n - Qcompensation - AQcompensationn, between

In Equation 6, R_n, intra indicates a cell classification of a neighboring cell in intrafrequency measurement, R_n, between indicates a cell classification of a neighboring cell in measurement between frequencies, AQcompensation_n, intra indicates a 3^er parameter to adjust the cell sorting of the neighboring cell in intrafrequency measurement, AQcompensation_n, between indicates a 4th parameter to adjust the cell classification of the neighboring cell in measurement between frequencies. In intrafrequency measurement, the increase in the value of 3^er parameter causes the decrease in a probability of intracell reselection. Furthermore, the decrease in the value of 3^er parameter causes the increase in a probability of intracell reselection to the neighboring cell. In measurement between frequencies, the increase in the value of the 4th parameter causes a decrease in a probability of reselection between cells. Furthermore, the decrease in the value of the 4th parameter causes an increase in a probability of reselection between cells for the neighboring cell.

The system load will be assumed to be high and thus there is no need to control the cell reselection performed by the UEs. In order to reduce the load on the system, the BS allows UEs to perform cell reselection more frequently. BS increases 1^er 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 by 3^er parameter or the 4th parameter, thereby increasing a probability that a UE will perform cell reselection for a neighboring cell. The BS can reduce the load on the system by selectively setting a parameter value between 1^er to 4th parameters. The load balancing of the system can be controlled between the BSs by sharing the S criterion or the R criterion.

Using the 3^er parameter and the 4th parameter, the BS can determine which operation will be performed first between intracell reselection and intercell reselection. For example, if the value of the 4th parameter is decreased relatively more than the 3rd^er parameter, increases a probability of reselecting between cells. In this way, intracell reselection can be performed prior to intercell reselection. Number 3^er parameter or 4th parameter can be set differently depending on a cell or a layer. Number 3^er parameter or 4th parameter can have the same value in the same layer regardless of the cell.

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

Meanwhile, when system load is considered, inter-cell reselection may be advantageous over intracell reselection in terms of control effectiveness. The 1^er and 3^er Parameters in association with the intra-frequency measurement can remain fixed values ​​and only the 2nd and 4th parameters in association with the inter-frequency measurement can be set as cell reselection indicators. The cell reselection indicator can indicate an increase / decrease value for each of the 2nd and 4th parameters or it can indicate whether the parameters are increased or decreased. The 2nd and 4th parameters for system load balancing can be

ES 2 561 484 T3 increase or decrease by a specific predetermined amount between the BS and the UE. The cell reselection flag can indicate whether the 2nd and 4th parameters are increased or decreased at once using one 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 flag is not transmitted, it indicates no change in cell load. When the cell reselection flag 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 flag 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.

Each function described above can be performed by a processor such as a software-based microprocessor encoded to perform such a function, a program code, etc., a controller, a microcontroller, an ASIC (Application Specific Integrated Circuit) or the like. . 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 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 claims that follow.