Abstract:
The present invention relates to a method and apparatus for determining radio link failure between the UE and the eNB in a wireless communication system. Particularly, the present invention provides a method and apparatus for determining radio link failure efficiently in such a way that the UE measures the radio link quality of the anchor carrier, the radio link qualities of the aggregated carriers, and the radio link quality of neighbor cells in order, and determines whether the radio link quality fulfills a predetermined condition.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cellular radio communication system and, in particular, to a method and apparatus for controlling radio link based on whether radio link failure is detected. 
     2. Description of the Related Art 
     Recently, many researches are being conducted on the Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) as useful schemes for high speed data transmission over a radio channel. In such multiple access schemes, the user-specific data and/or control information are mapped to time-frequency resources without overlapped from each other, i.e. maintaining orthogonality, to identify the user-specific data and/or control information. 
     In a cellular communication system, one of the significant factors to provide high-speed wireless data service is bandwidth scalability for dynamic resource allocation. For example, Long Term Evolution (LTE) system can support the bandwidths of 20/15/10/5/3/1.4 MHz. The carriers can provide services with at least one of the bandwidths, and the user equipments can have different capabilities such that some supports only 1.4 MHz bandwidth and others up to 20 MHz bandwidth. The LTE-Advanced (LTE-A) system, aiming at achieving the requirements of the IMT-Advanced service, can provide broadband service by aggregating carries up to 100 MHz. 
     The LTE-A system needs the bandwidth wider than that of LTE system for high-speed data transmission. Simultaneously, the LTE-A system needs to be backward compatible with the LTE system such that the LTE UEs can access the services of the LTE-Advanced system. For this purpose, the entire system bandwidth of the LTE-A system is divided into sub-bands or component carriers that have a bandwidth supporting transmission or reception of the LTE UE and can be aggregated for supporting the high speed data transmission of the LTE-A system in the transmission/reception process of the legacy LTE system per component carrier. 
     Meanwhile, if the radio rank quality between a transmitter and a receiver is degraded below a predetermined level, it is difficult to expect the normal data communication. Accordingly, the UE or the eNB monitors the radio rank quality to determine the Radio Link Failure (hereinafter, referred to as RLF) and performs the operation corresponding to the RLF. A description is made of a procedure for a UE to determine whether RLF occurs in the legacy system not supporting carrier aggregation with reference to  FIG. 1 . 
     The UE connects to an eNB to transmit and receive data ( 108 ), recognizes, if the radio link quality with the base station degrades continuously for a predetermined observation period, the problem of the radio link ( 110 ), and determines, if the radio link quality does not recovered for a predetermined time T 1  ( 112 ), occurrence of RLF ( 104 ). If the RLF is detected, the UE attempts to connect to a cell having the best radio link quality among the neighbor cells for a predetermined time duration T 2  ( 114 ) to continue, if the attempt is successful, data transmission/reception and end, if the attempt fails, end all the transmission/reception operations and transition to idle mode ( 116 ). 
     The procedure related to the RLF is defined for the conventional system without consideration of carrier aggregation and thus there is a need to define a procedure for controlling the radio link depending on whether RLF occurs or not in the wireless communication system supporting carrier aggregation. 
     SUMMARY OF THE INVENTION 
     Problem to be Solved 
     In order to solve the above problems, the present invention provides a method and apparatus for controlling radio links by determining whether the radio link failure occurs in the wireless communication implementing wide bandwidth by aggregating carriers. 
     Means for Solving the Problem 
     In order to achieve the above objective, a radio link control method of a terminal for a cellular radio communication system supporting carrier aggregation according to the present invention includes measuring radio link qualities of more than one component carrier, determining whether at least one radio link quality is equal to or greater than a threshold value, determining, when at least one radio link quality is equal to or greater than the threshold, that the radio link quality of a set of component carriers is good, and maintaining, when the radio link quality of the component carriers set is good, connection state with a current base station. Preferably, the radio link control method further includes determining, when the radio link qualities of all the component carriers is less than the threshold value, that the radio link quality of the component carriers set is bad and releasing, when the radio link quality of the component carriers set is bad, the connection state with the current base station. 
     Also, a radio link control apparatus of a terminal for a cellular radio communication system supporting carrier aggregation according to the present invention includes an RS subcarrier symbol receiver which receives reference signals for at least one component carrier and a radio link failure controller which measures radio link qualities of the at least one component carriers using the reference signals provided by the RS subcarrier symbol receiver, determines, when at least one radio link quality is equal to or greater than the threshold, that the radio link quality of a set of component carriers is good, and controls to maintain, when the radio link quality of the component carriers set is good, connection state with a current base station. Preferably, the radio link control apparatus controls to determine, when the radio link qualities of all the component carriers is less than the threshold value, that the radio link quality of the component carriers set is bad and release, when the radio link quality of the component carriers set is bad, the connection state with the current base station. 
     Advantageous Effects 
     As described above, in the system implementing wide transmission by aggregating carriers such as LTE-A, the present invention determines the radio link failure efficiently by reducing unnecessary radio link failure determinations. According to the present invention, it is possible to maintain the continuity of data transmission and reduce transmission delay caused by neighbor cell connection attempts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is diagram illustrating a procedure for determining radio link failure in a conventional LTE system. 
         FIG. 2  is a diagram illustrating the concept for determining a radio link failure in a LTE-A system. 
         FIG. 3  is a flowchart illustrating a procedure for determining radio link failure in a wireless communication system according to an embodiment of the present invention. 
         FIG. 4  is a diagram illustrating a method for calculating radio link quality of CCC in the LTE-A system according to an embodiment of the present invention. 
         FIG. 5  is a diagram illustrating a method for calculating radio link quality of CCC in the LTE-A system according to another embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a method for determining radio link quality recovery of CCC in the LTE-A system according to an embodiment of the present invention. 
         FIG. 7  is a diagram illustrating a method for determining radio link quality recovery of CCC in the LTE-A system according to another embodiment of the present invention. 
         FIG. 8  is a block diagram illustrating an apparatus for determining radio link failure of the UE according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Detailed description of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. In addition, terms used in the following description of the present invention are prepared in view of functions thereof, so they will be changed depending on the intention of users, operators, or custom. Thus, definition of the terms must be determined based on the whole content of the specification. 
     Although the description is directed to the Advanced E-UTRA (or LTE-A) supporting carrier aggregation in the following embodiments of the present invention, it will be understood by those skilled in the art that the present invention can be applied to other communication systems supporting the similar technical background and channel format with a slight modification without departing from the spirit and scope of the invention. 
     The present invention proposed a method and apparatus for determining radio link failure in the wireless communication system securing wideband through carrier aggregation proposed as the subject matter of the present invention. Particularly, when the LTE-A system implements the wide transmission bandwidth by aggregating the LTE carriers, it is necessary to minimize the occurrence probability of the radio link failure so as to improve reliability of data communication and reduce the transmission delay. 
     The subject matter of the present invention is described in detail hereinafter with reference to  FIG. 2 .  FIG. 2  is a diagram illustrating the concept for determining a radio link failure in a LTE-A system according to an embodiment of the present invention. 
     The UE measures the radio link quality of each of the component carriers. The configuration information on the component carriers aggregated is notified to the UE by the eNB via signaling. For simplifying the explanation, a group of the aggregated component carriers is called Configured Component Carriers (CCC) hereinafter. The CCC includes at least one anchor carrier, and the anchor carrier works as a reference point for providing control information to the UE or the mobility control of the UE. 
     The UE measures the received signal strength of the reference signal (hereinafter, ‘RS’) of each component carrier receiving from the eNB for a predetermined duration and compares the received signal strength with the threshold Qout predetermined per component carrier to determine whether the radio link quality of the component carrier is good or bad (strong or weak). The threshold value Qout defined per component carrier is the received signal strength of RS corresponding to a Block Error Rate (BLER) of Physical Downlink Control Channel (PDCCH) and determined according to the bandwidth of the component carrier and a number of transmit antennas. 
     In more detail, the UE measures the radio link quality between the UE and the anchor carrier based on the received signal strength of the RS of the anchor carrier for a predetermined time duration ( 201 ). If the radio link quality measured to the anchor carrier is worse than the Qout defined for the anchor carrier over a predetermined time duration, the UE measures the radio link qualities of the rest component carriers ( 202 ) except for the anchor carrier rather than premature determination of radio link failure and compares the radio link qualities with the corresponding Qout values ( 204 ). If at least one of the component carriers has the radio link quality better than the corresponding threshold Qout, the UE configures a new CCC with the component carriers having the good radio link qualities. 
     Otherwise, if all the component carriers have the radio link qualities worse than the corresponding threshold Qout values, the UE determines radio link failure in the current cell ( 203 ) and attempt access to neighbor cells ( 205 ). At this time, if failing access to the neighbor cells, the UE ends the communication with the base station and enters idle state. 
     The radio link failure determination method of the present invention can be applied regardless of whether the aggregated component carriers are consecutive or non-consecutive in frequency domain and without limitation on the number of component carriers. 
     The radio link failure determination method proposed in the present invention is described in detail through following embodiments. 
       FIG. 3  is a flowchart illustrating a procedure for a UE to determine radio link failure in a wireless communication system implementing wide bandwidth through carrier aggregation. 
     At step  302 , the UE measures radio link quality of each component carrier. The UE measures the radio link quality for each component carrier with the received signal strength of RS received on the corresponding component carrier and calculates total radio link quality of the CCC. 
     At step  304 , the UE determines whether at least one component carrier has the measurement result equal to or greater than the corresponding threshold value in the CCC. If there is at least one component carrier having the measurement result equal to or greater than the threshold value, the UE compares the radio link quality of the anchor carrier with the threshold value to determine whether the radio link quality is good or bad at step  314 . If it is determined that the radio link quality of the anchor carrier is good, i.e., equal to or greater than the threshold value, the UE maintains current anchor carrier and CCE as they are and keeps the connection state (CONNECTED_STATE) with the base station at step  316 . Otherwise if it is determined that the radio link quality of the anchor carrier is bad, i.e., less than the threshold value, the UE reconfigures the CCC with the component carriers, except for the anchor carrier, within the current CCE and keeps the connection state (CONNECTED_STATE). 
     At this time, the UE requests the eNB to set a new CCE with the component carriers of which radio link qualities are equal to greater than the threshold values and sets the component carrier having the best radio link quality as a new anchor carrier via signaling. The eNB can accept the request of the UE entirely or send the control information for adjusting the request to the UE and thus completes the reconfiguration. 
     Meanwhile, if all the component carriers have the measurement results less than the threshold value at step  304 , the UE determines whether the radio link quality of the CCC is recovered before a predetermined time (timer T 1 ) at step  306 . As a result of step  306 , if the radio link quality of the CCC is recovered, the UE branches the procedure to step  314 . 
     If the radio link quality of the CCC is not recovered before the time T 1  expires at step  306 , the UE branches the procedure to step  308  to determine the occurrence of radio link failure in the CCE and measure the radio link qualities of the neighbor cells. Next, the UE attempts connection to the neighbor cell having the greatest (superior) radio link quality at step  310  and, if the connection is established successfully before a predetermined timer T 2  expires, resets the corresponding neighbor cell as the anchor carrier while maintaining the connection state (CONNECTED_STATE) at step  320 . 
     At this time, the UE requests the eNB to resets the neighbor cell having the best radio link quality as the anchor carrier via signaling. The eNB can accept the request of the UE entirely or send the control information adjusting the request to the UE and thus completes the reconfiguration. If the connection to the neighbor cell fails at step  310 , the UE ends the connection with the eNB and enters the idle state. 
     T 0 , T 1 , and T 2  representing time values are predetermined, and the eNB notifies the UE of these values via signaling or shared as the fixed values between the UE and the eNB. 
     How to determine the radio link quality of the CCC at step  304  of  FIG. 3  is described hereinafter in more detail with reference to  FIG. 4 . 
       FIG. 4  is a diagram illustrating step  304  for calculating the radio link quality of the CCC in more detail according to an embodiment of the present invention. 
     Defining the function for calculating the radio link quality of CCC as f(.), the function f(.) calculates the radio link quality  404  of the CCC with the input values of radio link qualities  1 ,  2 , . . . , k  401  of individual component carriers constituting the CCC and Qout 1 , Qout 2 , . . . , Qoutk  403  of individual component carriers. The function f(.) compares the radio link qualities of the individual component carriers with the component carrier-specific threshold values and, if at least one component carrier maintains the radio link quality equal to or greater than the corresponding threshold value before the expiry of T 0 , determines that the radio link quality of the CCC is excellent (or good, used in the same meaning herein) and, otherwise if all the component carriers maintain the radio link qualities less than the corresponding threshold values before the expiry of T 0 , determines that the radio link quality of CCC is poor (or bad, used in the same meaning herein). 
     In the present invention, when at least one component carrier maintains the radio link quality equal to or greater than the threshold value, it is restricted that the UE makes a premature determination of radio link failure due to some component carriers having bad radio link qualities even though there is the component carrier maintaining good radio link quality in the CCC, thereby maintaining the continuity of data transmission and reducing transmission delay caused by unnecessary neighbor cell connection attempts. 
     Here, the threshold values Qout 1 , Qout 2 , and Qoutk  403  defined for individual component carriers correspond to the received signal strengths of RSs represented by Block Error Rate (BLER) of Physical Downlink Control Channel (PDCCH) and are determined depending on the bandwidths of individual component carriers and the number of transmit antennas. 
     A description is made of the definition of another operation of function f(.) for calculating the radio link quality of the CCC hereinafter with reference to  FIG. 5 . FIG. is a diagram illustrating step  304  for calculating the radio link quality of the CCC in more detail according to another embodiment of the present invention. 
     In  FIG. 5 , the radio link quality of the CCC is calculated by applying weights to the component carriers constituting the CCC according to the importance of the component carriers. The importance of the component carrier is determined whether the corresponding component carrier is the anchor carrier or carrying the physical control channel for the UE. 
     In  FIG. 5 , the radio link quality of the CCC is determined with the additional input values of α 1 , α 2 , . . . , αk  504  as compared to  FIG. 4 . α 1 , α 2 , . . . , αk  504  are the weights applied to the individual component carriers and each is set to 1 for the anchor carrier and 0 for non-anchor carrier. In this case, the radio link quality of the CCC can be calculated directly from the radio link quality of the anchor carrier. That is, if the radio link quality of the anchor carrier to which the weight is applied is worse than the threshold value Qout of the anchor carrier for the time duration T 0 , it is determined that the radio link quality of the CCE is bad even though there is any component carrier having good radio link quality among the rest component carriers. This means that the component carriers having the weight of 0 do not influence the calculation of radio link quality of the CCC, but the radio link quality of the component carrier having the weight of 1 is input to the function f(.) of FIG. to influence the result. The weights α 1 , α 2 , . . . , αk  504  are notified to the UE by the eNB via signaling in CCE configuration or reconfiguration process. 
     However to determine the recovery of the radio link quality of CCC over a predetermined value before the expiry of the timer T 1  at step  306  of  FIG. 3  is described hereinafter in more detail with reference to  FIG. 6 . 
       FIG. 6  is a diagram illustrating step  306  for determining whether the radio link quality of the CCC is recovered according to an embodiment of the present invention. 
     Here, if the function for determining whether the radio link quality of the CCE is recovered is defined as g(.), the function g(.) calculates the radio link quality recovery  604  of the CCC with the input values of the radio link qualities  1 ,  2 , . . . , k  601  and the component carriers threshold values Qin 1 , Qin 2 , . . . , Qink  603 . 
     The function g(.) compares the radio link quality of each of the component carriers with the threshold value of the corresponding component carrier and, if at least one of the component carriers maintains the good quality equal to or greater than the threshold value for the time duration T 1 , determines the recovery of the radio link quality of the CCC and, otherwise if all the component carrier maintain the bad qualities less than the corresponding threshold values, determines recovery failure of radio link quality of the CCC. 
     Through the above described operations, the present invention prevent the UE from making a premature determination of radio link failure due to some component carriers having bad radio link qualities even though there is the component carrier maintaining good radio link quality in the CCC, thereby maintaining the continuity of data transmission and reducing transmission delay caused by unnecessary neighbor cell connection attempts. 
     The threshold values Qin 1 , Qin 2 , . . . , Qink  603  defined for individual component carriers correspond to the received signal strengths of the RSs represented by Block Error Rate (BLER) of Physical Downlink Control Channel (PDCCH) and are determined based on the bandwidths of the component carriers and the number of transmit antennas. 
     A description is made of the definition of another operation of function g(.) for determining the radio link quality recovery of the CCC herein after with reference to  FIG. 7 .  FIG. 7  is a diagram illustrating step  306  for determining the radio link quality recovery of the CCC according to another embodiment of the present invention. 
     In  FIG. 7 , whether the radio link quality of the CCE is recovered is calculated with the weight applied according to the importance of the component carrier in the CCE. The importance of the component carrier is determined whether the corresponding component carrier is the anchor carrier or carrying the physical control channel for the UE. In  FIG. 7 , the radio link quality of the CCC is determined with the additional input values of β 1 , β 2 , . . . , βk  704  as compared to  FIG. 6 . β 1 , β 2 , . . . , βk  704  are the weights applied to the individual component carriers and each is set to 1 for the anchor carrier and 0 for non-anchor carrier. 
     In this case, the radio link quality of the CCC can be calculated directly from the radio link quality of the anchor carrier. That is, if the radio link quality of the anchor carrier to which the weight is applied is worse than the threshold value Qin of the anchor carrier for the time duration T 1 , it is determined that the radio link quality of the CCE is not recovered even though there is any component carrier having good radio link quality among the rest component carriers. This means that the component carriers having the weight of 0 do not influence the calculation of radio link quality recovery of the CCC, but only the radio link quality of the component carrier having the weight of 1 is input to the function g(.) of  FIG. 6  to influence the result. The weights β 1 , β 2 , . . . , βk  704  are notified to the UE by the eNB via signaling in CCE configuration or reconfiguration process. 
       FIG. 8  is a block diagram illustrating a configuration a receiver of the UE for determining radio link failure according to an embodiment of the present invention. 
     The RF/IF receiver  810  configures the bandwidth and reception center frequency to receiving the downlink signal on the CCE set for the UE under the control of the RF/IF controller  811 . 
     The FFT  800  performs Fourier Transform on the received downlink OFDM signal and outputs individual subcarrier reception symbols. The reception symbols are input to the decoder of the corresponding channel by means of the carrier symbol de-mapper  801 . 
     The Physical Downlink Shared Channel (PDSCH) and Physical Downlink Control Channel (PDCCH) symbol decoders  802  and  803  performs decoding on the subcarrier reception symbols of the corresponding channels to acquire data and control information. 
     The RS symbol receiver  804  extracts RS from the signal received by the UE and use the RS for channel estimating and compensating on the PDSCH and PDCCH and determining whether the radio link failure occurred. 
     The radio link failure controller (RLF controller)  805  measures the radio link qualities of the component carriers based on the RSs of the respective component carriers and determine the radio link failure based on the measurement result. 
     In more detail, the RLF controller measures the radio link quality of each component carrier using the RS on one or more component carriers. According to an embodiment of the present invention, the radio link failure controller can measure the radio link quality by applying weight to each component carrier. 
     If at least one of the radio link qualities measured on the component carriers is equal to or greater than a predetermined threshold value, the RLF controller determines that the radio link quality of a set of the component carriers is good and thus maintains the connection state with the current eNB. 
     In this case, the radio link failure controller measures the radio link quality of the anchor carrier and, if the measured radio link quality of the anchor carrier is equal to or greater than the threshold value of the anchor carrier, controls to maintains the current anchor carrier. 
     However, if the measured radio link quality of the anchor carrier is less than the threshold value of the anchor carrier, the radio link failure controller can control to reconfigure the set of the component carrier with the component carrier except for the anchor carrier in the component carrier set. In this case, the radio link failure controller can select the component carrier having the best radio link quality as a new anchor carrier. 
     Meanwhile, if the radio link qualities of all the component carriers are less than the threshold values, the RLF controller determines that the radio link quality of the component carrier set is bad so as to end the connection state with the current eNB. 
     In this case, the radio link failure controller determines whether the radio link quality of the component carrier set is recovered before the expiry of a predetermined time duration. According to an embodiment of the present invention, the radio link failure controller can determine the radio link quality recovery by applying weight to each component carrier. 
     If the radio link quality is not recovered to a good state, the radio link failure controller determines the radio link failure of the current radio link in the current component carrier set and measures the radio link quality of neighbor cells. Afterward, the radio link failure controller can control to attempt connection to the neighbor cell having the best radio link quality. 
     If it is failed to connect to the neighbor cells, the radio link failure controller ends the connection with the eNB and enters idle state. 
     In order to control the operations as above, the radio link failure controller controls the RF/IF controller  811  to search, when the radio link failure is determined, for the neighbor cell having the best radio link quality and controls the transmitter  806  to connect to the neighbor cell having the best radio link quality. If it is failed to connect to the neighbor cell before the expiry of T 2 , the radio link failure controller  805  controls the transmitter  806  to stop transmission and controls the PDSCH and PDCCH symbol decoders  802  and  803  to stop the normal data reception operations and then enter idle state. 
     The specification and drawings are to be regarded in an illustrative rather than a restrictive sense in order to help understand the present invention. It is obvious to those skilled in the art that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention.