Abstract:
A radio communication system includes: a plurality of cells having different scrambling sequences, respectively, wherein at least two cells communicate with at lease two user terminals connected to different serving cells; and a controller which controls the plurality of cells and provides a single scrambling sequence to said at least two cells and said at least two user terminals for control signal transmission and reception.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to a radio communication system and, more specifically, to techniques of control signal transmission in coordinated multi-point (CoMP) transmission/reception schemes. 
         [0002]    Recently, LTE (Long Term Evolution)-Advanced standard has been developed for 4th generation system (4G), where the fairly aggressive target in system performance requirements have been defined, particularly in terms of spectrum efficiency for both downlink (DL) and uplink (UL) as indicated in the Sect. 8 of 3GPP TR 36.913 v9.0.0, Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced), December 2009 (hereinafter referred to as “NPL 1”). Considering the target of the cell-edge user throughput and the average cell throughput, which is set to be roughly much higher than that of LTE Release 8 (Rel. 8), multiple techniques, such as carrier aggregation, downlink enhanced MIMO, coordinated multi-point transmission/reception (CoMP), have been included in LTE-Advanced. 
         [0003]    In Rel. 8/9/10, the downlink control channel (PDCCH) is defined to send control signal in Sect. 6.8 of 3GPP TS 36.211 v10.3.0, Physical Channels and Modulation for Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (hereinafter referred to as “NPL2”). Each UE&#39;s downlink control information (DCI) is aggregated into consecutive control channel elements (CCEs), where a control channel element corresponds to 9 RE groups as defined in Sect. 6.2.4 of NPL2. The DCI transports downlink or uplink scheduling information, requests for aperiodic CQI reports, notifications of uplink power control commands, etc. as described in the Sect. 5.3.3 of 3GPP TS 36.212 v10.3.0, Multiplexing and channel coding for Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (referred to as “NPL3”). The CCEs of multiple UEs connected to same serving cell are multiplexed and then scrambled by using a scrambling sequence initialized by a value c init  at the start of each subframe, which is a function of physical-layer cell identity (ID) of the serving cell as defined in the following equation in the Sect. 6.8.2 of NPL2 for interference randomization. In the following, the initialization value of scrambling sequence generation is called as the scrambling initialization value c init  for the sake of convenience. 
         [0000]        c   init   =└n   s /2┘2 9   +N   ID   ServCell   {Math. 1}
 
         [0000]    where n s  is the slot number within a radio frame. 
         [0004]    The scrambled bit sequence is QPSK (Quadrature Phase Shift Keying)-modulated and mapped to the resource elements of PDCCH. The serving cell reserves a radio resource region for PDCCH of its UEs, i.e., whole bandwidth of first several OFDM symbols (max. 4 OFDM symbols) in a subframe. With the assistance of blind detection at UE side, only the location of the reserved radio resource region is required to be known by UE. The information of the location of the reserved radio resource is dynamically indicated by using L1/L2 signal through such as physical control format indicator channel (PCFICH), defined in the Sect. 6.7 of NPL2. 
         [0005]    The present PDCCH, demodulated by cell-specific reference signal (CRS), is sent only by the serving cell and always occupies the entire system bandwidth of the first several OFDM symbols. It is not flexible to tailor the transmission characteristics of PDCCH to an individual UE and also impossible to coordinate transmissions in the frequency domain. This makes PDCCH ill-suited for new deployment, where the notion of a cell is less clear and where flexibility in how to transmit is needed to handle unexpected interference situations. Due to unexpected interferences, PDCCH capacity becomes a bottleneck when applying carrier aggregation, downlink enhanced MIMO and CoMP, etc. 
         [0006]    In order to eliminate such a bottleneck, enhanced PDCCH (ePDCCH) has been proposed by R1-113155, Nokia (referred to as “NPL4”) and R1-113356, Ericsson, ST-Ericsson (referred to as “NPL5”). As shown in  FIG. 1 , the ePDCCH is sent over allocated resource blocks (RBs) in physical downlink data channel (PDSCH) area to increase the capacity and coverage of the control signal. The employment of UE-specific RS (DM-RS) in ePDCCH transmission makes the transmission properties transparent to the UE. In principle, the enhanced single-point MIMO as well as multi-point MIMO (i.e., CoMP) schemes for improving the throughput of data transmission becomes also available for the DL control signal transmission, as stated in NPL5. For the blind detection of ePDCCH at UE side, the location of the reserved radio resource region may be informed semi-statically (e.g., 120 ms, 240 ms, etc.) as the information element of E-PDCCH-Config by RRC signaling, similar to the way to inform the configuration of the relay PDCCH (R-PDCCH) as introduced in the Sect. 6.3.2 of 3GPP TS 36.331 v10.3.0, Radio resource control (RRC) and Protocol specification of Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (hereinafter referred to as “NPL6”). 
         [0007]    For LTE-Advanced Rel. 11, CoMP has been agreed to be included as a tool to improve the coverage of high data rates, the cell-edge throughput, and also to increase system throughput as described in the Sect. 4 of 3GPP TR 36.819 v11.0.0, Coordinated multi-point operation for LTE physical layer aspects (Release 11) (hereinafter referred to as “NPL 7”). The CoMP schemes, joint transmission (JT), dynamic point selection (DPS), and coordinated scheduling/coordinated beamforming (CS/CB) are supposed to be supported as described in the Sect. 5.1.3 of NPL7. The CoMP cooperating set is defined in the Sect. 5.1.4 of NPL7 as a set of (geographically separated) points directly and/or indirectly participating in data transmission to a UE in time-frequency resource. In case of JT and DPS, UE&#39;s data, scrambled by a scrambling sequence with the serving cell&#39;s scrambling initialization value as defined in the Sect. 6.3.1 of NPL2, should be shared among more than one point in CoMP cooperating set; while, in case of CS/CB, data for a UE is only available at and transmitted from the one point (serving point) but user scheduling/beamforming decisions are made with coordinated among points corresponding to the CoMP cooperating set. It should be noted that the term “point” for coordinated multi-point transmission/reception can be used as a radio station, a transmission/reception unit, remote radio equipment (RRE) or distributed antenna of a base station, Node-B or eNB. Accordingly, hereinafter, a point, a radio station, a transmission/reception unit and a cell may be used synonymously. 
         [0008]    According to the performance evaluation results in Sect. 7 of NPL7, JT/DPS CoMP achieves better performance than CB/CS to improve the cell-edge user throughput of downlink data transmission. For a cell-edge UE, which suffers from poor channel condition of serving point and strong interference from CoMP point, JT/DPS CoMP can also be applied to improve the capacity of its control signal in a similar way as that of data, by sharing not only data but also control signal, scrambled by a scrambling sequence with the serving cell&#39;s scrambling initialization value c init  among the selected transmission points (TPs). 
         [0009]    A simple example of the above-described scheme is given in  FIGS. 2A and 2B . Assuming that UE1 and UE2 have Cell1 as serving cell and Cell2 as CoMP cell as shown in  FIG. 2A , ePDCCH can aggregate control information of the UE1 and UE2 using the same scrambling sequence for Cell1 and Cell2 as shown in  FIG. 2B . As described in Section 6.8.2 of the NPL2, the scrambling sequence generation is initialized with the following initialization value c init  determined by the ID of Cell1 (serving cell). 
         [0000]        c   init   =└n   s /2┘2 9   +N   ID   Cell1   {Math. 2}
 
         [0010]    In the case of the UE2 with a different serving cell, however, the aggregation of control signal with CoMP cannot be made because different scrambling initialization values and different radio resources are used for the control signals of the UE1 and UE2, respectively. As shown in  FIG. 3A , it is assumed that UE1 and UE2 are selected as CoMP UEs with multiple cooperating cells and the UE1 has Cell1 as serving cell and Cell2  102  as CoMP cell; while, the UE2 has Cell2 as the serving cell and Cell1 as the CoMP cell. For the employment of JT/DPS CoMP, the control signal of UE1, scrambled by using the Cell1&#39;s scrambling initialization value, is shared by Cell2. On the other hand, the control signal of UE2, scrambled by using the Cell2&#39;s scrambling initialization value, is shared by Cell1. Accordingly, the scrambling sequence generation is initialized with different initialization values c init1  and c init2  for Cell1 and Cell2, respectively: 
         [0000]        c   init1   =└n   s /2┘2 9   +N   ID   Cell1  
 
         [0000]        c   init2   =└n   s /2┘2 9   +N   ID   Cell2   {Math. 3}
 
         [0011]    Besides their different scrambling initialization values, different radio resource regions are reserved at Cell1 and Cell2 for sending UE1&#39;s and UE2&#39;s control signals, respectively as shown in  FIG. 3B . Within the previously reserved radio resource region, the occupied resource is dynamically allocated, resulting in remained resource. 
         [0012]    In  FIG. 3B , as an example, separate resources with max 3RBs for each one are reserved for each UE, but average 2RBs are used for each UE&#39;s control signal. As a consequence, an increasing number of CoMP UEs with different serving cells results in larger reserved radio resource regions in multiple cooperating cells. 
         [0013]    An object of the present invention is to provide a method and system which can efficiently send control signals with improved capacity and coverage of a control signal for UEs with different serving cells. 
       SUMMARY 
       [0014]    According to the present invention, a radio communication system includes: a plurality of cells having different scrambling sequences, respectively, wherein at least two cells communicate with at lease two user terminals connected to different serving cells; and a controller which controls the plurality of cells and provides a single scrambling sequence to said at least two cells and said at least two user terminals for control signal transmission and reception. 
         [0015]    According to the present invention, a method for controlling a plurality of cells having different scrambling sequences in a radio communication system, includes the steps of: setting at least two cells which communicate with at lease two user terminals connected to different serving cells; and providing a single scrambling sequence to said at least two cells and said at least two user terminals for control signal transmission and reception. 
         [0016]    According to the present invention, a control device for controlling a plurality of cells having different scrambling sequences in a radio communication system, includes: a setting section for setting at least two cells which communicate with at lease two user terminals connected to different serving cells; and a communication controller for providing a single scrambling sequence to said at least two cells and said at least two user terminals for control signal transmission and reception. 
       Advantageous Effects of Invention 
       [0017]    According to the present invention, the reserved radio resource region for control signals for UEs with different serving cells can be effectively reduced. In addition, the exchanging messages among cooperating cells for the control signal of UEs also become less for coordinating the distributed scheduling results of different cooperating cells. 
         [0018]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a schematic diagram illustrating an example of control signal configuration of PDCCH and enhanced PDCCH (ePDCCH). 
           [0020]      FIG. 2A  is a schematic diagram illustrating a radio communication system having two UEs with same serving cell. 
           [0021]      FIG. 2B  is a schematic diagram illustrating the control signal configuration for each UE in the radio communication system of  FIG. 2A . 
           [0022]      FIG. 3A  is a schematic diagram illustrating a radio communication system having two UEs with different serving cells. 
           [0023]      FIG. 3B  is a schematic diagram illustrating the control signal configuration for each UE in the radio communication system of  FIG. 3A . 
           [0024]      FIG. 4A  is a schematic diagram illustrating control signal configuration for a CoMP UE group for explaining an outline of the present invention. 
           [0025]      FIG. 4B  is a schematic diagram illustrating the function configuration of a control unit to implement the control signal configuration of  FIG. 4A . 
           [0026]      FIG. 5  is a diagram illustrating an example of a radio communication system according to a first illustrative embodiment. 
           [0027]      FIG. 6  is a diagram illustrating detailed functional configurations of the controller, TxRx units and UEs in the radio communication system of  FIG. 5 . 
           [0028]      FIG. 7  is a sequence diagram illustrating an example of operations of radio communication system of  FIG. 6 . 
           [0029]      FIG. 8A  is a schematic diagram illustrating a first example of the radio communication system employing JT CoMP to ePDCCH and PDSCH for UE1 and UE2 according to the first illustrative embodiment. 
           [0030]      FIG. 8B  is a schematic diagram illustrating the control signal configuration for each UE in the radio communication system of  FIG. 8A . 
           [0031]      FIG. 9A  is a schematic diagram illustrating a second example of the radio communication system employing DPS CoMP to ePDCCH and PDSCH for UE1 and UE2 according to the first illustrative embodiment. 
           [0032]      FIG. 9B  is a schematic diagram illustrating the control signal configuration for each UE in the radio communication system of  FIG. 9A . 
           [0033]      FIG. 10A  is a schematic diagram illustrating a third example of the radio communication system employing JT CoMP to PDCCH and PDSCH for UE1 and UE2 according to the first illustrative embodiment. 
           [0034]      FIG. 10B  is a schematic diagram illustrating the control signal configuration for each UE in the radio communication system of  FIG. 10A . 
           [0035]      FIG. 11A  is a schematic diagram illustrating a fourth example of the radio communication system employing DPS CoMP to PDCCH and PDSCH for UE1 and UE2 according to the first illustrative embodiment. 
           [0036]      FIG. 11B  is a schematic diagram illustrating the control signal configuration for each UE in the radio communication system of  FIG. 11A . 
           [0037]      FIG. 12  is a diagram illustrating an example of a radio communication system according to a second illustrative embodiment. 
           [0038]      FIG. 13  is a diagram illustrating detailed configurations of eNBs in the radio communication system of  FIG. 12 . 
           [0039]      FIG. 14  is a sequence diagram illustrating an example of operations of radio communication system of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    First, the general outlines of the present invention will be described with reference to  FIGS. 4A and 4B . 
         [0041]    As shown in  FIG. 4A , multiple UEs (UE1, . . . UEn) with the same CoMP cooperating set but different serving cells are aggregated as a CoMP UE group with a single scrambling initialization value which is shared among cooperating cells of the CoMP cooperating set. A reserved resource Rrsv is determined so as to accommodate a total amount of resources for control signals of the UE1-UEn in the CoMP UE group. The respective resources for control signals of the UE-UEn in the CoMP UE group are dynamically allocated within the reserved resource Rrsv and the control signals in the CoMP UE group are scrambled using the single scrambling initialization value. 
         [0042]    Referring to  FIG. 4B , it is assumed that a core control unit controls radio transmission and reception stations TxRx_1, . . . TxRx_n (hereinafter, referred to as TxRx units) which in turn control UE1-UEn with different serving cells corresponding to the TxRx units, respectively. The core control unit performs: grouping the UE1-UEn with different serving cells but the same CoMP cooperating set into a CoMP UE group; selecting the scrambling initialization value for the CoMP UE group; and reserving the shared resource Rrsv as shown in  FIG. 4A . Thereafter, the core control unit performs coordinated scheduling and informing control signal configuration to each TxRx unit. In this way, the information related to the scrambling initialization value and the reserved resource Rrsv is shared among the TxRx units and the UEs for transmitting and receiving control signals. 
         [0043]    As an example, considering that UE1 and UE2 are connected to different serving cells (Cell1 and Cell2) but having the same CoMP cooperating set, UE1 and UE2 can be grouped as a CoMP UE group. A common scrambling initialization value is used for initializing the scrambling sequence of their control signal. In addition, the reserved resource region Rrsv for control signal transmission can be set to 5RBs at Cell1 and Cell2, where each UE uses average 2RBs for sending DCI. In this case, the reserved resource region Rrsv is smaller than a total resource (6RBs) for separate control signal transmission of the UE1 and UE2. 
         [0044]    The illustrative embodiments will be explained by making references to the accompanied drawings. The illustrative embodiments used to describe the principles of the present invention are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless network. In the present technical field related to radio communication systems, the terms “point”, “cell”, “radio station” and “transmission/reception (TxRx) unit” of a base station (Node-B or eNB) may have same meaning, so serving point and cooperating point can be interpreted as serving cell and cooperating cell, serving TxRx unit and cooperating TxRx unit, or serving radio station and cooperating radio station, respectively. Accordingly, in this disclosure, the term “cell” or “TxRx unit” is used appropriately. 
       1. First Illustrative Embodiment 
       [0045]    According to the first illustrative embodiment, intra-eNB CoMP is applied to control signal transmission. Detailed configuration and operation will be described by referring to  FIGS. 5-7 . 
       1.1) System Configuration 
       [0046]    As shown in  FIG. 5 , it is assumed that a network is composed of a controller  10  and TxRx units  21  and  22  (or Cell1 and Cell2), to which a radio communication system according to the first illustrative embodiment is applied. The controller  10  controls the TxRx units  21  and  22  (or Cell1 and Cell2) through backhaul links BL1 and BL2, respectively. The UE1 and UE2 are communicating with the TxRx units  21  and  22  through radio channels under the control of the network. More detailed configuration of the radio communication system will be described below. 
         [0047]    Referring to  FIG. 6 , the controller  10  includes the function blocks of: CoMP cooperating set selection section  101 ; CoMP UE grouping section  102 ; scrambling initialization value selection section  103 ; resource reservation section  104 ; scheduler  105 ; backhaul link (BL) communication section  106 ; and a control section  107 . The TxRx units  21  and  22  have the same functional configuration as follows: BL communication section  211 ,  221 ; control section  212 ,  222 ; radio transmitter  213 ,  223 ; and radio receiver  214 ,  224 . The BL communication sections  211  and  221  are connected to the backhaul link (BL) communication section  106  through the backhaul links BL1 and BL2, respectively, so that data and control signal transmission/reception can be controlled by the controller  10 . The UE1 and UE2 have the same functional configuration as follows: radio transmitter  311 , 321 ; radio receiver  312 ,  322 ; DL signal detection section  313 ,  323 ; channel state information (CSI) estimation section  314 ,  324 ; and controller  315 ,  325 . Each cell (TxRx unit  21 ,  22 ) in CoMP cooperating set is communicating with the UE1 and UE2, which are also referred to as CoMP UEs. 
         [0048]    By using the above-mentioned function blocks, the CoMP cooperating set selection section  101  selects a CoMP cooperating set including more than one cell (here, TxRx units  21  and  22 ) for each UE (here, UE1, UE2). Thereafter, the CoMP UE grouping section  102  groups the CoMP UEs with the same CoMP cooperating set as a CoMP UE group. For sending the control signal of such a CoMP UE group, the scrambling initialization value selection section  103  chooses a single scrambling initialization value and the resource reservation section  104  reserves the shared radio resource region Rrsv. Next, the scheduler  105  performs the joint scheduling of multiple cells belonging to the CoMP cooperating set, where the network dynamically selects the transmission point(s), TP(s), of TxRx unit(s), and on selected TP(s) allocates the RBs as well as REs within the reserved resource region Rrsv for each UE in the CoMP UE group. In case of precoding at selected TP(s), the precoding matrix index (PMI) as well as rank indicator (RI) for each UE needs to be decided for each selected TP. The detailed process is described as follows. 
         [0049]    Referring to  FIG. 7 , at first, when the TxRx units  21  and  22  have received an uplink signal from the UE1 and UE2, respectively (operations  401  and  402 ), the control sections  212  and  222  transmits information indicating the received power of uplink sounding reference signal (SRS) or the UE feedback downlink reference signal received power (RSRP) to the controller  10  through the BL communication section  211  and  221  (operations  403  and  404 ). Based on the information indicating SRS power or the RSRP, the CoMP cooperating set selection section  101  selects the CoMP cooperating set for each UE (operation  405 ). For example, a cell, whose RSRP difference relative to that of the serving cell is within a threshold, will be regarded as a CoMP cell. The UE having more than one cooperating cell is regarded as a cooperating cell (CoMP cell). It is found that UE1 and UE2 are CoMP UEs, who have the same CoMP cooperating set consisting of Cell1 and Cell2, although UE1&#39;s serving cell is Cell1 and UE2&#39;s serving cell is Cell2. 
         [0050]    The CoMP UE grouping section  102  groups UE1 and UE2 into one CoMP UE group (operation  406 ). For this CoMP UE group, the scrambling initialization value selection section  103  selects a single scrambling initialization value for the scrambling sequence of control signal, e.g., PDCCH or ePDCCH (operation  407 ). The scrambling initialization value can be determined by the ID of one CoMP cooperating cell, i.e., Cell1&#39;s ID or Cell2&#39;s ID, or a different ID for the sake of interference randomization. For example, the scrambling sequence is initialized as a common initialization value c init  for Cell1-Celln as follows: 
         [0000]        c   init   =└n   s /2┘2 9   +N   ID   VIRTUAL   {Math. 4}
 
         [0000]    where N ID   VIRTUAL  is a specific virtual cell ID for the CoMP UE group. 
         [0000]        c   init   =└n   s /2┘2 9   +N   ID   ServCell   +N   offset   {Math. 5}
 
         [0000]    where N offset  is the ID offset for each UE belong to the CoMP UE group. N offset  is adjusted to obtain same c init  for each UE in CoMP UE group. 
         [0051]    The control section  107  sends the virtual cell ID or cell ID offset, parameter of scrambling initialization value c init , to the TxRx units  21  and  22  (operations  408  and  409 ) for generating the CoMP UE group&#39;s control signal, and the TxRx units  21  and  22  further send it to the UE1 and UE2 as the information element of PDCCH-Config or E-PDCCH-Config by RRC signaling for detecting the control signal, respectively (operations  410  and  411 ). 
         [0052]    Next, the resource reservation section  104  reserves the shared radio resource region Rrsv (see  FIG. 4A ) at both Cell1 and Cell2 for applying JT/DPS CoMP to control signal transmission (operation  412 ). The control section  107  notifies the TxRx units  21  and  22  of the location of the shared radio resource region Rrsv (operations  413  and  414 )), which further send it to the UE1 and UE2 (operations  415  and  416 ). 
         [0053]    According to the feedback CSI by UE, the scheduler  105  firstly carries out channel-dependent scheduling for data transmission and thereafter each UE&#39;s DCI including dynamic scheduling results can be aggregated into consecutive CCEs (operation  417 ). For each UE in the CoMP UE group, the control section  107  selects transmission points (TxRx units) and allocates RBs and REs within the reserved radio resource region Rrsv. In case of precoding, the PMI as well as RI for each selected TP of the CoMP UE are also decided, respectively. For control signal transmission, besides the virtual cell ID or cell ID offset for scrambling initialization value c init , the control section  107  also informs each selected TxRx unit, through a corresponding backhaul link, of dynamic scheduling results which includes the aggregated CCE number, the positions of allocated RBs and REs as well as PMI and RI for precoding (operations  418  and  419 ). 
         [0054]    The virtual cell ID or cell ID offset for generating the scrambling initialization value c init  of the CoMP UE group may be indicated semi-statically, e.g., 120 ms, 240 ms, etc.; while, the dynamic scheduling results need to be updated more frequently, e.g., with a period of 5 ms, 10 ms, etc. Accordingly, each of the control sections  212  and  222  generates the control signal of the CoMP UE group by multiplexing the CCEs of the UE1&#39;s DCI and UE2&#39;s DCI at first and then scrambling the bit sequence by using the scrambling initialization value c init  with the informed virtual cell ID or cell ID offset (operations  420  and  421 ). After that, the transmitter  213 ,  223  of a corresponding TxRx unit modulates the scrambled bit sequence and maps the modulated signal on the allocated REs within the allocated RBs to send the control signal of the CoMP UE group. 
         [0055]    As described above, for control signal detection at UE side, the control section  107  informs each UE in the CoMP UE group of the virtual cell ID or cell ID offset for generating the scrambling initialization value c init  as well as the location of the reserved radio resource region Rrsv. The signal related to the virtual cell ID or cell ID offset of the scrambling initialization value c init  and the signal related to the location of reserved radio resource region Rrsv may be sent simultaneously or independently. For example, the information of the scrambling initialization value c init  together with the location of reserved radio resource region Rrsv may be included in the information elements of PDCCH-Config or E-PDCCH-Config by RRC signaling and semi-statically indicated through PDSCH of serving cell with a period of 120 ms, 240 ms, etc. At the UE side, the blind detection within the informed reserved region Rrsv is carried out to detect the control signal. In another way, the location of radio resource region Rrsv may be dynamically sent to the UE by using L1/L2 signal with a period of 5 ms, 10 ms, etc., independently from that of the scrambling initialization value c init . For example, for PDCCH, the reserved region Rrsv is the first several OFDM symbols and the number of the OFDM symbols for PDCCH is dynamically informed to UE by using the L1/L2 signal through PCFICH, which includes the information of the length of Rrsv for PDCCH. For ePDCCH, the start position of ePDCCH can be semi-statically informed by using RRC signal and the length of Rrsv for ePDCCH can be dynamically informed to UE by using the L1/L2 signal though enhanced PCFICH at the beginning of ePDCCH, which carries the information of the length of the ePDCCH resource. Or, the dynamic signaling of the region Rrsv for ePDCCH is informed to UE through its serving cell&#39;s PDCCH. In this case, the UE firstly detects the PDCCH to obtain the location of the region Rrsv and then detects the ePDCCH within the region Rrsv. Thereafter, the blind detection may be avoided at the price of larger signaling overhead for the information in PDCCH. The detailed examples are given below. 
         [0056]    With the knowledge of the virtual cell ID or cell ID offset for scrambling initialization value c init  and the reserved resource region Rrsv, the DL signal detection section  313 ,  323  of each UE can detect the control signal, by demapping the received signal, demodulating the symbol sequence, and then descrambling the bit sequence (operations  422  and  423 ). Hereafter, the UE1&#39;s DCI and UE2&#39;s DCI are blindly detected in the informed reserved resource region Rrsv, respectively. 
         [0057]    According to each UE&#39;s DCI associated with the downlink transmission, the CSI estimation section  314 ,  324  can further detect its received downlink data in PDSCH as well as the downlink reference signal for CSI estimation. According to the UE&#39;s DCI associated with the uplink transmission, the control section  315 ,  325  generates the uplink data and sends over physical uplink shared channel (PUSCH) from each UE&#39;s transmitter  311 ,  321 . In addition, the control section  315 ,  325  generates the feedback CSI together with other uplink control information and sends over physical uplink control channel (PUCCH). 
       1.2) First Example 
       [0058]    A first example of the communication control method according to the first illustrative embodiment shows the case of ePDCCH with JT CoMP, which will be described by referring to  FIGS. 8A and 8B . 
         [0059]    As shown in  FIG. 8A , JT CoMP is applied to send ePDCCH of CoMP UE group from multiple selected TPs (TxRx units  21  and  22 ). Here, JT CoMP is also applied to data transmission over PDSCH for UE1 and UE2. The TxRx units  21  and  22  (Cell1 and Cell2) are the selected TPs, simultaneously transmitting both data and control signal to UE1 and UE2. For ePDCCH, a common scrambling initialization value c init  is needed and a common radio resource region Rrsv is reserved for UE1 and UE2. 
         [0060]    As shown in  FIG. 8B , over reserved resource region Rrsv, same RBs as well as REs are allocated for each UE&#39;s DCI at both Cell1 and Cell2 (TxRx units  21  and  22 ). In case of precoding of joint transmission, the PMI and RI at Cell1 and Cell2 need to be decided based on the UE feedback CSI. For ePDCCH generation, the information of the common scrambling initialization value c init  and the above dynamic scheduling results is indicated to each selected TxRx unit over a corresponding backhaul link BL. For ePDCCH detection, only the information related to the common scrambling initialization value c init  (i.e., virtual cell ID or cell ID offset for the CoMP UE group) together with the location of reserved resource region Rrsv is needed for the sake of blind detection at the UE side. 
       1.3) Second Example 
       [0061]    A second example of the communication control method according to the first illustrative embodiment shows the case of ePDCCH with DPS, which will be described by referring to  FIGS. 9A and 9B . 
         [0062]    As shown in  FIG. 9A , DPS CoMP is applied to send ePDCCH of the CoMP UE group from one dynamically selected TP (TxRx unit). The process is similar to that of ePDCCH with JT CoMP given in  FIGS. 8A and 8B , except that only one TP (TxRx unit) is dynamically selected for sending PDSCH and ePDCCH. Although the common radio resource region Rrsv is reserved at both Cell1 and Cell2 (TxRx units  21  and  22 ), the control section  107  only allocates RBs and REs within the reserved radio resource region Rrsv at each UE&#39;s selected TP (TxRx unit). 
         [0063]    As shown in  FIG. 9B , the UE1&#39;s data and DCI is sent from the TxRx unit  21  (Cell1); while the UE2&#39;s data and DCI is sent from the TxRx unit  22  (Cell2) at a current subframe. In another subframe, it is possible that the UE1&#39;s data and DCI is sent from the TxRx unit  22  (Cell2) but the UE2&#39;s data and DCI is sent from TxRx unit  21  (Cell1). The selected TP (TxRx unit) may be dynamically updated with a period of 5 ms, 10 ms, etc. For ePDCCH generation, the information related to the common scrambling initialization value c init  and the above dynamic scheduling results are indicated to the UE&#39;s selected TP (TxRx unit) over a corresponding backhaul link BL. For ePDCCH detection at the UE side, only the information of the common scrambling initialization value c init  and the location of reserved resource region Rrsv are needed. 
         [0064]    As illustrated in above example of ePDCCH with JT/DPS CoMP, only the location of reserved resource region Rrsv needs to be informed to UE semi-statically for blind detection of control signal. It is also possible to semi-statically inform the start position of ePDCCH but dynamically send the length of reserved resource region Rrsv, such as the number of RBs for Rrsv, in a L1/L2 signal through such as enhanced PCFICH (ePCFICH), which carries information about the number of RBs, used for transmission of ePDCCH in a subframe. To avoid blind detection, the aggregation level (i.e., number of aggregated CCEs) and the position of the allocated RBs and/or REs may be informed directly by using a L1/L2 signal over PDCCH, at the price of higher signaling overhead. 
       1.4) Third Example 
       [0065]    A third example of the communication control method according to the first illustrative embodiment shows the case of PDCCH with JT CoMP, which will be described by referring to  FIGS. 10A and 10B . 
         [0066]    As shown in  FIG. 10B , JT CoMP is applied to send PDCCH of the CoMP UE group from multiple selected TPs (TxRx units  21  and  22 ). The process is similar to that of ePDCCH with JT CoMP given in  FIGS. 8A and 8B , except that the allocated resources are restricted to the first several OFDM symbols in case of PDCCH. Since the CRS and PCFICH with cell-specific shift occupy the REs also in the first OFDM symbols, the UE1&#39;s DCI and UE2&#39;s DCI may be mapped to the REs without conflict with the CRS and PCFICH of Cell1 and Cell2. For PDCCH generation, the virtual cell ID or cell ID offset for common scrambling initialization value c init , the OFDM index as well as the aggregation level and the position of allocated RBs/REs for each UE needs to be known at each selected TP (TxRx unit). For PDCCH detection, the virtual cell ID or cell ID offset for common scrambling initialization value c init  is informed semi-statically to each UE of PDCCH-Config or E-PDCCH-Config by RRC signaling; while, the location of the reserved resource region Rrsv is indicated dynamically through PCFICH, which carries information about the number of OFDM symbols, used for transmission of PDCCH in a subframe. As shown in  FIG. 10B , the data and DCI of UE1 and UE2 are simultaneously transmitted by Cell1 and Cell2 (TxRx units  21  and  22 ) over allocated RBs and REs in the shared reserved OFDM symbols. The UE1 and UE2 can detect its own DCI by blind detection within the informed region Rrsv of PDCCH. 
       1.5) Fourth Example 
       [0067]    A fourth example of the communication control method according to the first illustrative embodiment shows the case of PDCCH with DPS, which will be described by referring to  FIGS. 11A and 11B . 
         [0068]    As shown in  FIG. 11A , DPS CoMP is applied to send PDCCH of the CoMP UE group from a dynamically selected TP (TxRx unit). The process is similar to that of PDCCH with JT CoMP given in  FIGS. 10A and 10B , except that only one TP (TxRx unit) is dynamically selected in a subframe for sending PDSCH and PDCCH. Although the common radio resource region Rrsv is reserved at both Cell1 and Cell2 (TxRx units  21  and  22 ), the control section  107  only allocates the RBs and REs within the reserved radio resource region Rrsv at each UE&#39;s selected TP (TxRx unit). 
         [0069]    As shown in  FIG. 11B , the UE1&#39;s data and DCI is sent from Cell1 (TxRx unit  21 ); while the UE2&#39;s data and DCI is sent from Cell2 (TxRx unit  22 ) at current subframe. In another subframe, it is possible that the UE1&#39;s data and DCI is sent from Cell2 (TxRx unit  22 ) but the UE2&#39;s data and DCI is sent from Cell1 (TxRx unit  21 ). The selected TP (TxRx unit) may be dynamically updated with a period of 5 ms, 10 ms, etc. For PDCCH generation, the information related to the common scrambling initialization value c init  and the above dynamic scheduling results is indicated to the UE&#39;s selected TP (TxRx unit). For PDCCH detection, the virtual cell ID or cell ID offset for common scrambling initialization value c init  for the CoMP UE group is informed semi-statically to each UE of PDCCH-Config or E-PDCCH-Config by RRC signalling; while, the location of the reserved resource region Rrsv, i.e. the number of OFDM symbols for PDCCH, is indicated dynamically as a L1/L2 signal through PCFICH. 
       1.6) Other Examples 
       [0070]    In the above-described examples as shown in  FIGS. 8-11 , the same CoMP scheme by using same selected TP(s) is used to send the downlink data over PDSCH and the downlink control signal over ePDCCH or PDCCH. However, the CoMP scheme as well as TP(s) can be independently decided for control signal and data transmission. For example, JT is used for data transmission but DPS is used for control signal transmission, considering the limited radio resources. 
       2. Second Illustrative Embodiment 
       [0071]    According to the second illustrative embodiment, inter-eNB CoMP is applied to control signal transmission. Detailed configuration and operation will be described by referring to  FIGS. 12-14 . 
         [0072]    As shown in  FIG. 12 , eNB1 and eNB2 are connected by X2 backhaul link. Each eNB includes the same functions as those of the controller  10  as shown in  FIG. 6 . More specifically, as shown in  FIG. 13 , Each eNB is provided with BL communication section ( 211 ,  221 ), radio transmitter ( 213 ,  223 ); radio receiver ( 214 ,  224 ); and control section ( 210 ,  220 ). The control section ( 210 ,  220 ) has not only the functions for eNB operations as described before but also the functions for inter-eNB CoMP applied to control signal transmission. The BL communication sections  211  and  221  are connected to each other through the X2 backhaul link, allowing the inter-eNB CoMP for control signal transmission. Other function blocks similar to those described with reference to  FIG. 6  are denoted by the same reference numerals and their detailed descriptions are omitted. 
         [0073]    By using the above-mentioned function blocks, the control section  210 ,  220  can find the CoMP UEs connected to eNB1 and eNB2, respectively. The UE1 has serving eNB1 and cooperating eNB2; while the UE2 has serving eNB2 and cooperating eNB1. By exchanging information over the X2 backhaul link, the CoMP UEs with the same CoMP cooperating set are grouped at each eNB. For control signal transmission of the UE1 and UE2, the common scrambling initialization value c init  is chosen and the shared radio resource region Rrsv is reserved. More specifically, the operations of the control sections  210  and  220  will be described by reference to  FIG. 14 . 
         [0074]    Referring to  FIG. 14 , at first, when the eNB1 and eNB2 have received an uplink signal from the UE1 and UE2, respectively (operations  501  and  502 ), the control sections  210  and  220  use information of the received power of uplink sounding reference signal (SRS) or the UE feedback downlink reference signal received power (RSRP) to select the CoMP cooperating set for each UE (operations  503 . 1 ,  503 . 2 ). After exchanging the information related to each UE&#39;s CoMP cooperating set through X2 backhaul between sections  211  and  221 , the control sections  210  and  220  group UE1 and UE2 into one CoMP UE group (operations  504 . 1 ,  504 . 2 ). For this CoMP UE group, the control sections  210  and  220  select a virtual cell ID or cell ID offset for determining the same scrambling initialization value c init  for ePDCCH of each UE in the CoMP UE group (operations  505 . 1 ,  505 . 2 ). The virtual cell ID or cell ID offset can be the same as the ID of one CoMP cooperating cell, i.e., Cell1&#39;s ID or Cell2&#39;s ID, or a different ID for the sake of interference randomization. The control sections  210  and  220  send the virtual cell ID or cell ID offset to the UE1 and the UE2, respectively (operations  506  and  507 ). The scrambling sequence is initialized by a common initialization value C init  for Cell1 and Cell2 as described before. 
         [0075]    Next, by exchanging the information over X2 backhaul, the control sections  210  and  220  reserve the shared radio resource region Rrsv (see  FIG. 4A ) at both Cell1 and Cell2 for control signal transmission (operations  508 . 1 ,  508 . 2 ). The control sections  210  and  220  notify the UE1 and UE2 of the location of the shared radio resource region Rrsv (operations  509  and  510 ). 
         [0076]    Next, the control sections  210  and  220  perform the distributed scheduling at eNB1 and eNB2, respectively (operations  511 . 1 ,  511 . 2 ). Each control section of the eNB1 and eNB2 dynamically assigns the resources for each UE connected to the corresponding eNB. In case of precoding, the PMI as well as RI for each UE needs to be decided. By coordinating the results of distributed scheduling through the X2 backhaul link, the control sections  210  and  220  corporate each other for the data transmission with JT/DPS CoMP. After that, each UE&#39;s DCI including the dynamic scheduling results can be aggregated into consecutive CCEs. 
         [0077]    For the UE in the CoMP UE group, each eNB allocates the RBs and REs within the reserved radio resource region Rrsv. By exchanging the information over the X2 backhaul link, the coordination among cooperating eNBs is needed for control signal transmission with JT/DPS CoMP. In case of JT CoMP, the same RBs as well as REs are allocated at eNB1 and eNB2 for UE1 and UE2, respectively. In case of DPS, the RBs and REs at one selected eNB is allocated to achieve largest data rate. For coordinating the distributed scheduling results of different cooperating cells, the exchanging messages for the aggregated control signal of a CoMP UE group is relatively smaller than that of separate control signal for different CoMP UEs. 
         [0078]    Accordingly, each of the control sections  210  and  220  generates the control signal of the CoMP UE group by multiplexing the CCEs of the UE1&#39;s DCI and UE2&#39;s DCI first and then scrambling the bit sequence by using the informed virtual cell ID or cell ID offset for generating same scrambling initialization value c init  for the CoMP UE group (operations  512  and  513 ). 
         [0079]    With the knowledge of the virtual cell ID or cell ID offset for scrambling initialization value c init  and the reserved resource region Rrsv, each UE can detect the control signal, by demapping the received signal, demodulating the symbol sequence, and then descrambling the bit sequence (operations  514  and  515 ). Hereafter, the UE1&#39;s DCI and UE2&#39;s DCI are blindly detected in the informed reserved resource region Rrsv, respectively. The detailed process of the employment of JT/DPS CoMP on ePDCCH and PDCCH is similar to that of the first to fourth examples, which is not redundantly described here. 
       3. Additional Statements 
       [0080]    The present invention can be applied to a mobile communications system employing coordinated transmission among multiple points to send control signal to multiple UEs. 
         [0081]    The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The above-described illustrative embodiment and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Part or all of the above-described illustrative embodiments can also be described as, but are not limited to, the following additional statements. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  controller 
           21 ,  22  transmission/reception (TxRx) unit 
         UE1, UE2 user equipment (user terminal) 
           101  CoMP cooperating set selection section 
           102  CoMP UE grouping section 
           103  scrambling initialization value selection section 
           104  resource reservation section 
           105  scheduler 
           106  backhaul link (BL) communication section 
           107  control section 
           210 ,  220  control section 
           211 ,  221  BL communication section 
           212 ,  222  control section 
           213 ,  223  transmitter 
           214 ,  224  receiver 
           311 ,  321  transmitter 
           312 ,  322  receiver 
           313 ,  323  DL signal detection section 
           314 ,  324  CSI estimation section