Abstract:
A method of handover in earner aggregation scenario is proposed in the present invention. In an embodiment of the present invention, the following steps are comprised: a base station sends a handover command to a user equipment, wherein the handover command includes information for indicating a Pcell and a Scell requiring a timing advance different from said Pcell; the user equipment receives the handover command, and performs a random access on said Pcell and the Scell requiring a timing advance different from said Pcell; the base station sends information for indicating corresponding timing advance to said user equipment, according to a successful random access of said user equipment on said Pcell and the Scell requiring a timing advance different from said Pcell By means of the technical solution provided in the present invention, the Pcell and the Scell requiring different timing advance in carrier aggregation scenarios can fee supported. In addition, in some embodiments, by supporting changing Pcell, the success rate of handover in carrier aggregation, scenarios is effectively improved, so an to avoid unnecessary connection reestablishment procedures.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to mobile communication technologies, more specifically, relates to user handover technologies for carrier aggregation scenarios. 
       BACKGROUND OF THE INVENTION 
       [0002]    In LTE-A, a higher bandwidth is used to meet the requirement of a higher data transfer rate, and the maximum bandwidth can reach 100 MHz. In order to be backward compatible, the 100 MHz bandwidth is divided into multiple carrier components, and each carrier component can have a maximum of 20 MHz bandwidth. Therefore, each user can support a maximum of 5 carrier components. For each user equipment, there is a primary carrier component (PCC) and optionally a plurality of secondary carrier components (SCC), where the primary carrier component always remains active. Another related concept is Pcell and Scell that are already adopted by 3GPP, where a Pcell means a downlink/uplink primary carrier component pair, and a Scell means a downlink/uplink secondary carrier component pair or a single downlink carrier. A Pcell is established by a R8 radio resource control connection program, while a Scell is created by a new R10 Scell adding message. 
         [0003]    In Release 10 (R10) of LTE-A protocol released, a user equipment performs a random access procedure only on a Pcell. For a handover procedure of R10, a target eNB indicates both a Pcell and a Scell in a handover command (HO command), but a user equipment performs a random access procedure on the indicated Pcell only. If the random access on the Pcell is successful, the handover is regarded as succeeding; if the random access on the Pcell fails, the handover is regarded as failing. 
       SUMMARY OF THE INVENTION 
       [0004]    Due to the existence of remote radio frequency head (RRH) and frequency selective repeater, signals of Pcell and each Scells may pass through different transmission paths, and therefore, different timing advances (TA) are required. It has been commonly agreed in RAN51 that it is necessary to support multiple timing advances. 
         [0005]    For R10, random access procedures of user equipments are limited onto Pcell. For handover procedures of R10, user equipments perform random accesses on Pcell only. In order to realize timing alignment in carrier aggregation (CA) scenarios, it is necessary to support multiple timing advances, and the scheme in R10 is insufficient to meet such requirement. 
         [0006]    As a result, an object of the present invention is to provide a method for handover in carrier aggregation scenarios, so as to meet the aforementioned requirement. 
         [0007]    In an embodiment of the present invention, a method for handover in a base station is provided. It comprises the following steps of: a. sending a handover command to a user equipment, said handover command including information for indicating a Pcell and a Scell requiring a different timing advance different from said Pcell; b. sending information for indicating a corresponding timing advance to the user equipment, according to a successful random access of said user equipment on said Pcell and the Scell requiring a timing advance different from said Pcell. 
         [0008]    In a further embodiment of the present invention, a method for handover in a user equipment is provided. It comprises the steps of: A. receiving a handover command from a target base station, said handover command including information for indicating a Pcell and a Scell requiring a timing advance different from said Pcell; B. performing random accesses on said Pcell and the Scell requiring a timing advance different from said Pcell. 
         [0009]    In yet another embodiment of the present invention, a first handover apparatus for handover in base stations is provided. It comprises: a first sending unit, for sending a handover command to a user equipment, the handover command including information for indicating a Pcell and a Scell requiring a timing advance different from said Pcell; a second sending unit, for sending information for indicating a corresponding timing advance to said user equipment, according to a successful random access of said user equipment on said Pcell and the Scell requiring a timing advance different from said Pcell. 
         [0010]    In yet another embodiment of the present invention, a second handover apparatus for handover in user equipments is provided. It comprises: a handover command receiving unit, for receiving a handover command from a target base station, said handover command including information indicating a Pcell and a Scell requiring a timing advance different from said Pcell; a random access unit, for performing random accesses on said Pcell and the Scell requiring a timing advance different from said Pcell. 
         [0011]    By means of the methods and apparatuses provided in the present invention, Pcell and Scells requiring different timing advances in carrier aggregation scenarios can be supported. Furthermore, in some embodiments of the present invention, by means of supporting changing the Pcell in the base station, the success rate of handovers is effectively improved in carrier aggregation scenarios, so as to avoid unnecessary connection reestablishment procedures. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    Other features, objectives and advantages of the present invention will become more apparent from the following detailed description of the non-limiting embodiments taken in conjunction with the accompanying drawings: 
           [0013]      FIG. 1  illustrates a schematic diagram of an exemplary handover scenario in a mobile communication system; 
           [0014]      FIG. 2  illustrates a flowchart of a method for handover in a mobile communication system according to an embodiment of the present invention; 
           [0015]      FIG. 3  illustrates a block diagram of a first handover apparatus for handover in base stations according to an embodiment of the present invention; 
           [0016]      FIG. 4  illustrates a block diagram of a second handover apparatus for handover in user equipments according to an embodiment of the present invention; 
       
    
    
       [0017]    In the figures, throughout different views, the identical or similar reference marks indicate the corresponding characteristics. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]      FIG. 1  illustrates a schematic diagram of an exemplary handover scenario in a mobile communication system. As shown in the figure, user equipment  3  will perform a handover operation, and base station  1  is the original serving base station of user equipment  3 , while base station  2  is the target base station for user equipment  3  to perform the handover. Base station can be also called site, and those skilled in the art should understand that, base station has different special names corresponding to different protocol standards. For example, in a LTE system or a LTE-A system, base station is also called Node B or evolved node B (eNB). Base station or site called in the present application is for example the evolved node B in LTE-A system but is not limited thereto. 
         [0019]      FIG. 2  illustrates a flowchart of a method for a handover in a mobile communication system according to an embodiment of the present invention. The method is usually performed between a user equipment and its target base station, and comprises three steps  11 ,  13  and  15 . In the following, the method is described in detail according to  FIG. 2  and in conjunction with user equipment  3 , its base station  2  shown in  FIG. 1 . 
         [0020]    In step  11 , base station  2  sends a handover command to user equipment  3 , wherein the handover command includes information for indicating a Pcell and a Scell requiring a timing advance different from said Pcell. 
         [0021]    Generally, a handover command includes information indicating a Pcell and Scells. Because the Pcell and each Scell may be mapped to different remote radio frequency heads, frequency selective repeaters or other devices, modules, the signals of the Pcell and each Scell may experience different transmission paths, and therefore, different timing advances are required to achieve timing adjustment. Base station  2  can determine which Scell(s) requires a timing advance different from the Pcell according to its configuration. Thus, in the handover command, besides the Pcell and each Scell, it is also indicated which Scell(s) requires a timing advance different from the Pcell. 
         [0022]    User equipment  3  will receive a handover command from base station  2 , and then, in step  13 , user equipment  3  performs random accesses on the Pcell and the Scell(s) requiring a timing advance different from the Pcell indicated by the handover command. 
         [0023]    Optionally, in step  13 , user equipment  3  performs random accesses in parallel on the Pcell and the Scell(s) requiring a timing advance different from the Pcell. In this way, it can speed up the completion of the random access procedure. 
         [0024]    Or, optionally, in step  13 , user equipment  3  performs random accesses on a Pcell and Scell(s) requiring a timing advance different from the Pcell successively, wherein the random access is firstly performed on the Pcell. 
         [0025]    The advantage of performing step  13  in two ways lies in that, Scell initialization uplink synchronization is achieved in the handover procedure. Thus, after the handover, all Scells on which the random access is successfully performed are in a state of being well configured but inactive, which is consistent with the corresponding rules in LTE-A R10. Therefore, it is not necessary to initiate an uplink synchronization procedure after the handover, unless there exists a Pcell or a Scell that fails in the random access of the handover procedure. For the Pcell or Scell that fails in the random access, user equipment  3  can perform a random access again after receiving a physical downlink control channel (PDCCH) order signaling from base station  2 , or perform a random access again automatically. 
         [0026]    After a random access is performed successfully on any carrier component (pair), base station  2  determines the corresponding timing advance; then, in step  15 , base station  2  will send information for indicating the corresponding timing advance to user equipment  3 , according to the successful random accesses of user equipment  3  on the Pcell and the Scell requiring a timing advance different from the Pcell indicated by the handover command. In the other words, base station  2  will indicate the corresponding timing advance of every Pcell or Scell with a successful random access to user equipment  3 . 
         [0027]    In LTE-A R10, because a random access is performed on a Pcell only, the end of the random access procedure on the Pcell also means the end of a handover. However, in the technical solution proposed by the present invention, the end of a handover can be defined as by the following two definitions: 
         [0028]    The first definition (of the end of a handover): random accesses on both Pcell and Scell end and fail; 
         [0029]    The second definition (of the end of a handover): a random access on the Pcell ends, and a random access procedure on at least one Scell succeeds. 
         [0030]    In an embodiment of the present invention, said handover method further comprises the step: if a random access of user equipment  3  on the Pcell fails, and a random access on at least one Scell succeeds, base station  2  determines whether to change the Pcell or not. 
         [0031]    Furthermore, said handover method further comprises the step: if changing Pcell is determined, base station  2  sends a radio resource control layer signaling to user equipment  3  for indicating a new Pcell. In general, the new Pcell should be one of the Scells on which the random access is performed successfully. At the same time, base station  2  will configure dedicated resources related to the new Pcell, including physical uplink control channel (PUCCH), semi persistent schedule (SPS) resource etc. Optionally, configuration information of these resources is also contained in the radio resource control layer signaling for indicating the new Pcell. 
         [0032]    Thus, the above solution allows the target base station to change the Pcell in the case that a random access fails to be performed on the Pcell, so as to increase the probability of a successful handover, and to avoid unnecessary connection reestablishment procedure. 
         [0033]    In an embodiment of the present invention, the above handover method further comprises the step: when the random access on the Pcell is successful, user equipment  3  sends a handover complete command through the Pcell. 
         [0034]    In an embodiment of the present invention, said handover method further comprises: when at least one random access on said Pcell and the Scell is successful, user equipment  3  sends a handover (HO) complete command through the Pcell or Scell on which the random access is successfully performed. Specifically, when the random access on the Pcell is successful, user equipment  3  preferably sends the handover complete command through the Pcell; when the random access on the Pcell fails, but the random access on at least one Scell is successful, user equipment  3  sends the handover complete command through the Scell on which the random access is successfully performed. 
         [0035]    For a handover procedure in LTE-A R10, a failure of random access on the Pcell means a handover failure. However, in the technical solution proposed by the present invention, a handover failure can be defined by the following two definitions: 
         [0036]    The first definition (of a handover failure:) both random accesses on the Pcell and the Scell fail; 
         [0037]    The second definition (of a handover failure_: the random access on the Pcell fails, and the random access on at least one Scell succeeds, but base station determines not to change the Pcell. 
         [0038]    In an embodiment of present invention, corresponding to the first definition of said handover failure, said handover method further comprises the step: when random accesses on both Pcell and Scell requiring a timing advance different from the Pcell that are indicated by the handover command fail, user equipment  3  initiates a reestablishment procedure of a radio resource control layer connection. 
         [0039]    In another embodiment of the present invention, corresponding to the second definition of said handover failure, said handover method further comprises: 
         [0040]    When the random access on the Pcell indicated by the handover command fails, base station  2  determines not to change the Pcell, and sends a radio resource control layer signaling to user equipment  3  for indicating user equipment  3  to reestablish a radio resource control layer connection; 
         [0041]    User equipment  3  initiates the reestablishment procedure of the radio resource control layer connection after receiving the signaling for indicating the reestablishment of the radio resource control layer connection from base station  2 . 
         [0042]    The aforementioned two solutions for the two handover failures and the successive operations are also helpful to improve the success rate of handovers, so as to avoid unnecessary connection reestablishment procedures, as well as to enable the base station to keep the power to decide changing Pcell. The two solutions also cover non-contention based random access procedures. For example, if a preamble is indicated in a handover command, user equipment should firstly perform a non-contention based random access procedure. 
         [0043]      FIG. 3  illustrates a block diagram of a first handover apparatus for handover in base stations according to an embodiment of the present invention. As shown in the figure, first handover apparatus  20  in the embodiment comprises a first sending unit  21  and a second sending unit  22 . First handover apparatus  20  is typically installed in a base station, such as in base station  2  shown in  FIG. 1 . 
         [0044]    First sending unit  21  and second sending unit  22  are used to realize steps  11  and  15  of the aforementioned handover method, respectively. 
         [0045]    First sending unit  21  is for sending a handover command to a user equipment, wherein said handover command includes information for indicating a Pcell and a Scell requiring a timing advance different from said Pcell; 
         [0046]    Second sending unit  22  is for sending information for indicating the corresponding timing advance to said user equipment, according to a successful random access of the user equipment on said Pcell and the Scell requiring a timing advance different from said Pcell. 
         [0047]      FIG. 4  illustrates a block diagram of a second handover apparatus for handover in a user equipment according to an embodiment of the present invention. As shown in the figure, second handover apparatus  30  in the embodiment comprises a handover command receiving unit  31  and a random access unit  32 . Second handover apparatus  30  is typically installed in a user equipment, such as in user equipment  3  shown in  FIG. 1 . 
         [0048]    Handover command receiving unit  31  corresponds to step  11  of the aforementioned handover method, and random access unit is used to realize step  13  of the aforementioned handover method. 
         [0049]    Handover command receiving unit  31  is for receiving a handover command from a target base station, wherein said handover command includes information indicating a Pcell and a Scell requiring a timing advance different from said Pcell. 
         [0050]    Random access unit  32  is for performing random accesses on said Pcell and Scell requiring a timing advance different from said Pcell. 
         [0051]    In other embodiments of the present invention, said first handover apparatus  20  and second handover apparatus  30  further comprise corresponding unit for realizing the other steps in the aforementioned handover method. 
         [0052]    Those skilled in the art should understand that, each unit in the present invention can be realized by a hardware module, or realized by a functional module in software, and also can be realized by a hardware module integrated with software functional modules. 
         [0053]    Those skilled in the art should understand that, the above embodiments are only illustrative rather than restrictive. The different technical features in the different embodiments can be combined, so as to achieve beneficial effects. Based on the drawings, the descriptions and the claims, Those skilled in the art can understand and realize the other variants of the disclosed embodiments. In the claims, the term “comprising” does not exclude other unit or steps; the indefinite article “a/an” does not exclude plurality; terms “first”, “second” are used to represent names other than any specific order. Any reference sign in the claims should not be understood as a restriction to the protection scope. The functions of multiple parts appearing in the claims can be realized by a single hardware or software module, and the function of a certain part can be also realized by multiple different hardware or software modules. Some technical features appearing in different dependent claims does not mean that these technical features cannot be combined to gain beneficial effects.