Patent Publication Number: US-2005128993-A1

Title: Apparatus and method for transmitting/receiving channel quality information of subcarriers in an orthogonal frequency division multiplexing system

Description:
PRIORITY  
      This application claims priority under 35 U.S.C. § 119(a) to an application entitled “Apparatus and Method for Transmitting/Receiving Channel Quality Information of Subcarriers in an Orthogonal Frequency Division Multiplexing System” filed in the Korean Intellectual Property Office on Nov. 20, 2003 and assigned Serial No. 2003-82600, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to an orthogonal frequency division multiplexing (OFDM) mobile communication system. More particularly, the present invention relates to a method and apparatus for transmitting and receiving the channel quality information of subcarriers used for data transmission/reception between a Node B (or base station) and a user equipment (UE).  
      2. Description of the Related Art  
      OFDM is defined as a two-dimensional access scheme that combines time division access (TDA) and frequency division access (FDA). Therefore, each OFDM symbol is transmitted on a predetermined sub-channel composed of distributed subcarriers.  
      The orthogonal nature of OFDM allows the spectrums of sub-channels to overlap, having a positive effect on spectral efficiency. Since OFDM modulation/demodulation is implemented by inverse fast fourier transform (IFFT) and fast fourier transform (FFT), a modulator/demodulator can be realized digitally with efficiency. Also, the robustness of OFDM against frequency selective fading and narrow band interference renders OFDM effective for existing European digital broadcasting and high-speed data transmission schemes, standardized as IEEE 802.11a, IEEE 802.16a, and IEEE 802.16b, which are generally used in large-volume radio communication systems.  
      OFDM is a special case of multi carrier modulation (MCM) in which a serial symbol sequence is converted to parallel symbol sequences and modulated to mutually orthogonal subcarriers (sub-channels) prior to transmission.  
      The first MCM systems appeared in the late 1950&#39;s for military high frequency radio communication, and OFDM, with overlapping orthogonal subcarriers, was initially developed in the 1970&#39;s. In view of orthogonal modulation between multiple carriers, OFDM has limitations in actual implementation for systems. In 1971, Weinstein et. al. proposed an OFDM scheme that applies discrete fourier transform (DFT) to parallel data transmission as an efficient modulation/demodulation process, which was a driving force for the development of OFDM. The introduction of a guard interval and a cyclic prefix as the guard interval further mitigates adverse effects of multi-path propagation and delay spread on systems. That is why OFDM has widely been exploited for digital data communications such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (W-ATM). Although hardware complexity was originally an obstacle to widespread use of OFDM, recent advances in digital signal processing technology, including FFT and IFFT, enable OFDM to be implemented today much more easily than before.  
      OFDM, which is similar to frequency division multiplexing (FDM), boasts of optimum transmission efficiency in high-speed data transmission because it transmits data on subcarriers, maintaining orthogonality among them. The optimum transmission efficiency is further attributed to good frequency use efficiency and robustness against multi-path fading in OFDM. Overlapping frequency spectrums leads to an efficient use of frequency and robustness against frequency selective fading and multi-path fading. OFDM reduces effects of the ISI by use of guard intervals and enables design of a simple equalizer hardware structure. Furthermore, since OFDM is robust against impulse noise, it is increasingly popular in communication systems.  
       FIG. 1  is a block diagram of a conventional OFDM mobile communication system. Its structure will be described in detail with reference to  FIG. 1 . With the input of a binary signal, a channel encoder  100  outputs code symbols. A serial-to-parallel (S/P) converter  105  converts the serial code symbol sequence received from the channel encoder  100  to parallel symbol sequences. A modulator  110  maps the code symbol to a signal constellation by quadrature phase shift keying (QPSK), 8-ary phase shift keying (8PSK), 16-ary quadrature amplitude modulation (16QAM), or 64QAM. An IFFT  115  inverse-fast-fourier-transforms modulation symbols received from the modulator  110 . A parallel-to-serial (P/S) converter  120  converts parallel symbols received from the IFFT  115  to a serial symbol sequence. The serial symbols are transmitted through a transmit antenna  125 .  
      A receive antenna  130  receives the transmitted series symbols from the transmit antenna  125 . An S/P converter  135  converts the received serial symbol sequence to parallel symbols. An FFT  140  fast-fourier-transforms the parallel symbols. A demodulator  145 , having the same signal constellation as used in the modulator  110 , demodulates the FFT symbols to binary symbols by the signal constellation. A channel estimator  150  channel-estimates the demodulated binary symbols. The channel estimation estimates situations involved in transmission of data from the transmit antenna, thereby enabling efficient data transmission. A P/S converter  155  converts the channel-estimated binary symbols to a serial symbol sequence from a parallel symbol sequence. A decoder  160  decodes the serial binary symbols and outputs decoded binary bits.  
       FIG. 2  illustrates an operation in a Node B for allocating subcarriers to a UE in an OFDM mobile communication system. With reference to  FIG. 2 , subcarrier allocation to the UE will be described below. Specific components such as an IFFT, a P/S converter, an S/P converter, and an FFT are not illustrated here.  
      An IFFT  200  transmits transmission data through an antenna  202 . As stated earlier, the transmission data is transmitted on a plurality of subcarriers. The Node B uses all of the subcarriers or some of them, for the data transmission. A feedback information generator  206  estimates the channel status of data received through a receive antenna  204 . The feedback information generator  206  measures the SIR (Signal-to-Interference power Ratio) or CNR (Channel-to-Noise Ratio) of the received signal. The feedback information generator  206  measures the channel status of an input signal transmitted on a particular channel (or on a particular subcarrier, from a particular transmit antenna, or to a particular combination of receive antennas) and transmits the measurement to a subcarrier allocator  208 . Table 1 below illustrates an example of feedback information that the feedback information generator  206  generates considering only the channel characteristics of subcarriers and transmits to the subcarrier allocator  208 .  
                           TABLE 1                                   Subcarrier   Feedback information                          Subcarrier #0   a           Subcarrier #1   b           Subcarrier #2   d           Subcarrier #3   c           Subcarrier #4   e           Subcarrier #5   g           Subcarrier #6   d           Subcarrier #7   e           .   .           .   .           .   .           Subcarrier #N − 1   f                      
 
      In the case illustrated in Table 1, data is transmitted on N subcarriers. Feedback information a to g is SIRs or CNRs generated from the feedback information generator  206 . The feedback information is generally represented in several bits. The subcarrier allocator  208  determines a subcarrier on which data is delivered based on the feedback information. The subcarrier allocator  208  selects a subcarrier having the highest SIR or CNR. If two or more subcarriers are used between the Node B and the UE, as many subcarriers having the highest SIRs or CNRs as required are selected sequentially.  
      If the SIR or CNR is higher in the order of a&gt;b&gt;c&gt;d&gt;e&gt;f&gt;g, the subcarrier allocator  208  allocates subcarriers in the order of subcarrier #0, subcarrier #1, subcarrier #3, subcarrier #2, . . . . If one subcarrier is needed, subcarrier #0 is selected. If two subcarriers are used, subcarrier #0 and subcarrier #1 are allocated. If three subcarriers are used, subcarrier #0, subcarrier #1, and subcarrier #3 are allocated. If four subcarriers are used, subcarrier #0, subcarrier #1, subcarrier #3 and subcarrier #2 are allocated.  
      In the subcarrier allocation, only one UE is considered. If a plurality of UEs transmit the feedback information of subcarriers to the subcarrier allocator  208 , the subcarrier allocator  208  allocates subcarriers to the UEs, comprehensively taking the feedback information into account.  
      The above-described subcarrier allocation is carried out in two steps: the feedback information is arranged according to channel statuses and then as many subcarriers as needed are allocated to a UE based on the arranged feedback information. The feedback information generator  203  measures the channel status on a per-subcarrier basis and transmits the channel status measurement to the subcarrier allocator  208 .  
      Existing mobile communication systems, however, face many limitations in transmitting data on the uplink. Therefore, transmission of the feedback information of all subcarriers on the uplink is slower than the downlink, and causes serious waste of radio resources. Moreover, when the channel environment varies with time as in a mobile communication system, the subcarrier allocation must be periodic and shorter than the coherence time. Transmission of the feedback information of all individual subcarriers takes a long time, however, which makes it impossible to allocate sub-carries to the UE within the coherence time.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for reducing uplink feedback information about the channel qualities of subcarriers.  
      Another object of the present invention is to provide an apparatus and method for allocating subcarriers to UEs according to a varying channel status.  
      The above objects are achieved by providing a method of transmitting/receiving CQIs of a plurality of subcarriers in an OFDM system where data is transmitted on the plurality of subcarriers via one or more transmit antennas.  
      According to one aspect of the present invention, in a method of transmitting channel quality indicators (CQIs) of a plurality of subcarriers in an OFDM system where data is transmitted on the plurality of subcarriers via one or more transmit antennas, a UE groups the subcarriers into subcarrier groups each having at least one subcarrier, generates CQIs for one or more allocated subcarrier groups and the transmit antennas, and transmits the CQIs to a Node B in one or more physical channel frames.  
      According to another aspect of the present invention, in a method of transmitting CQIs of a plurality of subcarriers in an OFDM system where data is transmitted on the plurality of subcarriers via one or more transmit antennas, a UE groups the subcarriers into subcarrier groups each having at least one subcarrier, and divides each of the subcarrier groups into subgroups each having one or more subcarriers. The UE generates group CQIs for one or more allocated subcarrier groups and the transmit antennas and transmits the group CQIs to a Node B in one or more physical channel frames. The UE generates subgroup CQIs for the allocated subcarrier groups, subgroups of the allocated subcarrier groups, and the transmit antennas, and transmits the subgroup CQIs to the Node B in one or more physical channel frames.  
      According to a further aspect of the present invention, in a method of receiving CQIs of a plurality of subcarriers in an OFDM system where data is transmitted on the plurality of subcarriers via one or more transmit antennas, a Node B groups the subcarriers into subcarrier groups each having at least one subcarrier, receives CQIs for one or more allocated subcarrier groups via the one or more transmit antennas in one or more physical channel frames, allocates the subcarrier groups to UEs based on the received CQIs, and transmits user data to the UEs on subcarriers of the allocated subcarrier groups.  
      According to still another aspect of the present invention, in a method of receiving CQIs of a plurality of subcarriers in an OFDM system where data is transmitted on the plurality of subcarriers via one or more transmit antennas, a Node B groups the subcarriers into subcarrier groups each having at least one subcarrier, and divides each of the subcarrier groups into subgroups each having one or more subcarriers. The Node B receives group CQIs for one or more allocated subcarrier groups via the one or more transmit antennas in one or more physical channel frames, and receives subgroup CQIs for the allocated subcarrier groups, and also receives subgroups of the allocated subcarrier groups via the one or more transmit antennas in one or more physical channel frames. The Node B allocates subcarriers to UEs based on the group CQIs or subgroup CQIs and transmits user data to the UEs to the allocated subcarriers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a block diagram of a conventional OFDM mobile communication system;  
       FIG. 2  is a block diagram of a conventional configuration for allocating subcarriers to a UE by a subcarrier allocator in a Node B in a conventional method;  
       FIG. 3  is a block diagram of a configuration for allocating subcarriers to a UE by a subcarrier allocator in a Node B according to an embodiment of the present invention;  
       FIG. 4  is a detailed block diagram of a feedback information generator illustrated in  FIG. 3 ;  
       FIG. 5  is a flowchart illustrating a Node B operation according to an embodiment of the present invention;  
       FIG. 6  is a flowchart illustrating a UE operation according to an embodiment of the present invention;  
       FIG. 7  is a block diagram illustrating a system configuration for allocating subcarriers to the UE in the Node B in a multiantenna system according to an embodiment of the present invention;  
       FIG. 8  illustrates the format of feedback information directed from the UE to the Node B according to an embodiment of the present invention;  
       FIG. 9  illustrates the format of feedback information that the UE generates in a system using two transmit antennas according to an embodiment of the present invention;  
       FIG. 10  illustrates the structure of subcarrier groups according to an embodiment of the present invention;  
       FIG. 11  illustrates the format of feedback information for a subgroup that the UE generates according to an embodiment of the present invention;  
       FIG. 12  illustrates the format of feedback information that the UE generates in a system using two transmit antennas according to an embodiment of the present invention;  
       FIG. 13  illustrates transmission of feedback information from a plurality of UEs according to an embodiment of the present invention;  
       FIG. 14  illustrates transmission of feedback information from a UE that has been assigned a plurality of subcarrier groups according to an embodiment of the present invention;  
       FIG. 15  is a flowchart illustrating an operation in the UE that operates in mode 1 and mode 2 according to an embodiment of the present invention; and  
       FIG. 16  is a flowchart illustrating an operation in the Node B that operates in mode 1 and mode 2 according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
      Exemplary embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail for purposes of conciseness.  
       FIG. 3  is a block diagram of a configuration for allocating subcarriers to a UE by a subcarrier allocator in a Node B according to an embodiment of the present invention. In  FIG. 3 , the Node B groups a plurality of subcarriers and allocates subcarriers to the UE by groups. The UE then transmits feedback information about the individual subcarrier groups. Hereinbelow, a description will be made of subcarrier allocation from the Node B to the UE according to an embodiment of the present invention.  
      It is assumed that N subcarriers are available and they are grouped into G subcarrier groups in an OFDM mobile communication system. Grouping the N subcarriers into G subcarrier groups will first be addressed. G varies with the channel status. For example, for high frequency selectivity, one subcarrier group includes less subcarriers. When the channel shows a flat-frequency response, more subcarriers are allocated to each subcarrier group. Besides the frequency selectivity, low rate of the slow uplink can be considered in determining the number of subcarriers in each group. Hence, G depends on the number of subcarriers in each group.  
      The subcarriers are grouped by ASA (Alternative Subcarrier Allocation) or SSA (Subband Subcarrier Allocation). The ASA and SSA will be described, taking an example where subcarriers. #0 to #(N−1) are available and they are grouped into two subcarrier groups. The ASA allocates subcarriers #0, #2, . . . , #(N−2) to the first subcarrier group, and subcarriers #1, #3, . . . , #(N−1) to the second subcarrier group. The SSA allocates subcarriers #0, #1, . . . , #(N/2−1) to the first subcarrier group, and subcarriers #N/2, #(N/2+1), . . . , #(N−1) to the second subcarrier group. This method for allocating subcarriers, however, should not be construed as a limiting factor in regard to the embodiments of the present invention, as it is possible that subcarriers can be allocated to the subcarrier groups by user selection.  
      The Node B determines the subcarrier grouping method and the number of subcarrier groups according to whether the UE wants packet data communication or circuit data communication and according to a QoS (Quality of Service) level that the UE requests. Adjacent subcarriers bring similar results in view of the nature of coherent bandwidth. Hence, grouping adjacent subcarriers in one group does not lead to significant performance degradation. Hereinafter, it is presumed that adjacent subcarriers are allocated to the same group. Yet, it should be apparent to those skilled in the art of the present invention that subcarriers spaced by predetermined intervals are allocated to one group to achieve diversity gain, or subcarriers are cyclically allocated to one subcarrier group in a predetermined period, or any other way can be used to form subcarrier groups. The Node B notifies the UE of whatever changes in the grouping by physical layer signaling or upper layer signaling. For example, the physical layer signaling is done on an HS-SCCH (High Speed Shared Control CHannel) used for HSDPA (High Speed Downlink Packet Access). The signaling used in relation to grouping changes is beyond the scope of the present invention and thus will not be described in detail.  
      Referring to  FIG. 3 , the system is configured to have a modulator  300 , a plurality of partial IFFTs  310  to  312 , a plurality of adders  320  to  322 , a transmit antenna  330 , a receive antenna  340 , a feedback information generator  350 , and a subcarrier allocator  360 .  
      The modulator  300  modulates input data. The number G of the partial IFFTs  310  to  312  is determined according to the number of available subcarriers and coherent bandwidth. The partial IFFTs  310  and  312  load modulated signals received from the modulator  300  on the subcarriers of predetermined groups under the control of the subcarrier allocator  360 . The subcarriers of a group can be successive, as described above.  
      The first partial IFFT  310  allocates the received modulated signals to the subcarriers of a first group. The Gth partial IFFT  312  allocates the received modulated signals to the subcarriers of a Gth group. The adder  320  adds the IFFT signals received from the first partial IFFT  310  and the adder  322  adds the IFFT signals received from the Gth partial IFFT  312 . The adders  320  to  322  transmit the sum signals through the transmit antenna  330  on a radio channel.  
      The receive antenna  340  provides a signal received on the subcarriers from the transmit antenna  330  to the feedback information generator  350 . The feedback information generator  350  measures the channel statuses of the subcarriers, generates feedback information for the individual subcarrier groups, and transmits the feedback information to the subcarrier allocator  360 . Operation of the feedback information generator  350  will be described in greater detail below. The subcarrier allocator  360  selects a subcarrier group for the UE based on the per-group feedback information and tells the partial IFFTs  310  to  312  the selected subcarrier group. The Node B transmits data to the UE on the selected subcarrier group.  
       FIG. 4  is a block diagram of the feedback information generator  350  illustrated in  FIG. 3 . The feedback information generator  350  includes a channel estimator  400 , an averager  402 , and a channel information generator  404 .  
      The channel estimator  400  calculates various channel estimation values for each subcarrier, inclusive of SIR, SINR (Signal-to-Noise and Interference Ratio), BRE (Bit Error Rate), FER (Frame Error Rate), and CNR. The channel status is estimated by calculating SIR herein. While the SIRs of the subcarriers of one group are measured in  FIG. 4 , the channel estimator  400  performs channel estimation on all received subcarriers.  
      The averager  402  calculates the average of the SIRs of subcarriers for each group by  
               SIR   g     =       1   L     ⁢       ∑     f   =     L   ⁡     (     g   -   1     )           LG   -   1       ⁢     SIR   j                 (   1   )             
 
      where SIR g  denotes the average of the channel estimation values of the subcarriers in a gth group, SIR j  denotes the channel estimation value of a jth subcarrier, L denotes the number of the subcarriers in the gth group, G denotes the maximum number of g, that is, the total number of subcarrier groups, and f denotes the index of a subcarrier. When receiving the channel estimation values of all subcarriers, the average 402 computes Eq. (1). Table 2 below lists the average channel estimation values of the individual subcarrier groups output from the averager  402 .  
                           TABLE 2                                   Subcarrier group   Average channel estimation value                          First group   B           Second group   A           Third group   E           Fourth group   C           .   .           .   .           .   .           Gth group   G                      
 
      The channel information generator  404  maps the average channel estimation values to predetermined values according to a predetermined rule, as illustrated in Table 3.  
                           TABLE 3                                   Average channel estimation value   Mapping value                          A to B   00           C to D   01           E   10           F to G   11                      
 
      If the mapping value is 2 bits, the average channel estimation values are classified into four levels as in Table 3. The range of the average channel estimation value at each level can be controlled by user selection. While the average channel estimation values are classified into four levels in Table 3, the number of levels can be set to 2 or more by user selection. Yet, when the average channel estimation values are sorted into too many levels, the number of bits required to represent mapping values increases, thereby increasing the amount of data to be transmitted on the uplink. Therefore, the number of mapping levels to be used must take into account the amount of data on the uplink and radio resources.  
      In an embodiment of the present invention, since high and low average channel estimation values are produced with a relatively low probability, the ranges of average channel estimation values mapped to 00 and 11 are set to be wide, A to B and F to G, respectively. On the other hand, since the probability of average channel estimation values is relatively high, an average channel estimation value range mapped to 10 is set to be narrow, E. Thus, the probabilities of generating the mapping values can be maintained almost the same.  
      In an embodiment of the present invention, the mapping values can be set by comparing the average channel estimation values rather than considering their generation probabilities. Given four groups, 00 is allocated to a group with the highest-average channel estimation value and 01, 10 and 11 are allocated sequentially to the other groups in a descending order of average channel estimation value. These embodiments are mere exemplary applications and thus setting of the mapping values varies depending on configuration.  
      Table 4 illustrates an example of feedback information that the channel information generator  404  provides to the subcarrier allocator  360  on the transmitting side.  
                           TABLE 4                                   Subcarrier group   Feedback information                          First group   00           Second group   11           Third group   10           Fourth group   01           .   .           .   .           .   .           Gth group   11                      
 
      It is assumed that the feedback information has a higher priority in the order of 00, 01, 10 and 11 in Table 4. The subcarrier allocator  360  selects a subcarrier group for the UE based on the feedback information. Upon receipt of the feedback information illustrated in Table 4, the subcarrier allocator  360 , if it is to allocate one subcarrier group to the UE, selects group #0. For two subcarrier groups, the subcarrier allocator  360  selects group #0 and group #4 for the UE.  
       FIG. 5  is a flowchart illustrating an operation in the Node B according to the preferred embodiment of the present invention.  
      Referring to  FIG. 5 , the Node B groups all available subcarriers in step  500 . The number of subcarrier groups is determined according to the number of the subcarriers and a coherent bandwidth. Each subcarrier group includes adjacent subcarriers. The subcarriers of the subcarrier groups can be changed in an every predetermined time period to prevent continuous allocation of the same subcarriers (i.e. the same bandwidth) to a particular user.  
      For example, if subcarriers #0 to #5 belong to a first group, the first group can replace them with subcarriers #2 to #7 a predetermined time later. Another predetermined time later, the first group may have subcarriers #4 to #9. Hence, the other subcarrier groups have different subcarriers. Aside from the periodic re-allocation of subcarriers to the subcarrier groups, the subcarrier groups can be reset when the channel environment faces a rapid change or an upper layer requests a subcarrier group resetting.  
      The Node B allocates transmission data to the subcarrier groups in step  502  and the transmits the transmission data on the subcarriers of the groups in step  504 .  
      In step  506 , the Node B awaits receipt of feedback information. The Node B selects a subcarrier group to be assigned to the UE based on received feedback information in step  508 . The Node B arranges the feedback information of the respective subcarrier groups in the order of better channel status and selects the subcarrier group for the UE. In step  510 , the Node B transmits data to the UE on the subcarriers of the selected subcarrier group.  
       FIG. 6  is a flowchart illustrating an operation in the UE according to an embodiment of the present invention.  
      Referring to  FIG. 6 , the UE groups all available subcarriers in step  600 . The resulting subcarrier groups are identical to those set in the Node B. The UE can receive information about the setting of the subcarrier groups from the Node B on a radio channel different from or identical to the radio channel on which it receives data.  
      The UE measures the channel statuses, particularly SIRs or CNRs of the subcarriers, in step  602 . In step  604 , the UE sorts the channel status measurements by subcarrier groups and calculates the average of the channel status measurements for each of the subcarrier groups. Instead of calculating the average channel status measurement, the Node B can calculate the sum of channel status measurements for each subcarrier group. The average or sum becomes the channel quality information of the subcarrier group.  
      The UE generates feedback information based on the channel quality information of each subcarrier group in step  606 , as illustrated in Table 3. In the case where the channel status measurements of the subcarriers of each subcarrier group are summed, the UE uses the channel estimation sum rather than the average channel estimation value shown in Table 3. Irrespective of the average channel estimation value or the channel estimation sum, mapping is done in the same manner.  
      Up to this point, subcarrier allocation has been described in the context of a mobile communication system using one transmit antenna and one receive antenna. Now, a description will be made of subcarrier allocation in a mobile communication system having a plurality of transmit antennas and a plurality of receive antennas.  FIG. 7  is a block diagram of a configuration of an OFDM mobile communication system using a plurality of transmit antennas for data transmission. As illustrated in  FIG. 7 , the transmit antennas transmit data on a plurality of subcarriers at a predetermined frequency.  
      Referring to  FIG. 7 , the OFDM mobile communication system is comprised of a user data processor  700 , a group buffer  710 , a plurality of partial IFFTs  720  to  722 , an antenna mapper  730 , a plurality of transmit antennas  740  to  742 , a subcarrier allocator  770 , and UE receivers  760  and  762  having receive antennas  750  and  752 , respectively. In the illustrated case, two transmit antennas  740  to  742  and one receive antenna  750  or  752  for one UE receiver  760  or  762  are used.  
      The user data processor  700  processes an input signal and converts the processed signal to as many parallel symbol sequences as the number of the subcarriers used. The group mapper  710  maps the parallel symbol sequences to the plurality of partial IFFTs  720  to  722  under the control of the subcarrier allocator  770 . The number of the partial IFFTs  720  to  722  is determined according to the number of the subcarriers, the coherent bandwidth, and the number of the transmit/receive antennas.  
      The partial IFFTs  720  to  722  allocate the received symbols to the subcarriers of the subcarrier groups corresponding to them. Each subcarrier group can include adjacent subcarriers. The symbols input to the first partial IFFT  720  are allocated to the subcarrier of a first group, added, and then provided to the antenna mapper  730 . The symbols input to the Gth partial IFFT  722  are allocated to the subcarriers of a Gth group, added and then provided to the antenna mapper  730 .  
      The antenna mapper  730  maps the outputs of the partial IFFTs  720  to  722  to the transmit antennas  740  to  742  under the control of the subcarrier allocator  770 . The antenna mapper  730  can map the subcarrier of one group to one or more antennas. The subcarrier of the first group is transmitted through at least one of the transmit antennas  740  to  742 .  
      The receive antennas  750  and  752  receive the signals from the transmit antennas  740  and  742 . The receive antenna  740  provides the received signals to the first UE receiver  760  and the receive antenna  742  provides the received signals to the second UE receiver  762 .  
      The UE receivers  760  and  762  generate feedback information about the subcarrier groups and transmit it to the subcarrier allocator  770  of the Node B on uplink channels. The subcarrier allocator  770  controls the group mapper  710  and the antenna mapper  730  based on the feedback information.  
      The plurality of transmit and receive antennas are considered in generating the feedback information in the UE receivers  760  and  760 . Hence, this feedback information is larger in amount than that generated in the feedback information generator  350  illustrated in  FIG. 3 . The UE receivers  760  and  762  construct feedback information for the respective transmit antennas as illustrated in Table 5a. Table 5a tabulates feedback information generated in the UE receivers  760  and  762  having the single receive antennas  750  and  752 , respectively. In the case of a Node B having a plurality of transmit antennas providing an OFDM service to a UE having a plurality of receive antennas, the UE receiver generates CQIs for the respective receiver antennas as well as the transmit antennas, as illustrated in Table 5b.  
                                   TABLE 5a                                   First group   Second group   . . .    Gth group                                                        Transmit   00   01   . . .    11       antenna 740       Transmit   01   10   . . .    01       antenna 742       .   .   .   .   .       .   .   .   .   .       .   .   .   .   .       Transmit   01   10   . . .    00       antenna 744                  
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 5b 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Gth 
               
               
                   
                 First group 
                 Second group 
                 . . .  
                 group 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 First transmit antenna, 
                 00 
                 01 
                 . . .  
                 11 
               
               
                 First receive antenna 
               
               
                 First transmit antenna, 
                 01 
                 00 
                 . . .  
                 01 
               
               
                 Second receive antenna 
               
               
                 Second transmit antenna, 
                 11 
                 11 
                 . . .  
                 10 
               
               
                 First receive antenna 
               
               
                 Second transmit antenna, 
                 01 
                 10 
                 . . .  
                 00 
               
               
                 Second receive antenna 
               
               
                   
               
            
           
         
       
     
      The UE receivers  760  and  762  each generate feedback information by subcarrier groups and transmit antennas, or by subcarrier groups, transmit antennas and receive antennas, as illustrated in Table 5a and Table 5b, and provide it to the subcarrier allocator  770  of the Node B. The subcarrier allocator  770  selects a subcarrier group and a transmit antenna to be allocated to each receive antenna and controls the partial IFFTs  720  to  722  and the antenna mapper  730  based on the feedback information.  
      Table 6a below lists feedback information received in the subcarrier allocator  770  with respect to transmit antennas and UEs. Table 6a e is concerned with the situation in which a Node B having two transmit antennas services two UEs in an OFDM mobile communication system. The subcarrier allocator  770 , receiving feedback information illustrated in Table 5a from each of two UEs, sorts the feedback information as in Table 6a.  
      Table 6b lists feedback information received in the subcarrier allocator  770  with respect to transmit antennas and UEs. Table 6b is concerned with the situation in which a Node B having two transmit antennas services two UEs each having two receive antennas in an OFDM mobile communication system. The subcarrier allocator  770 , receiving feedback information illustrated in Table 5b from each of two UEs, sorts the feedback information as in Table 6b.  
                                   TABLE 6a                                       Second                   First group   group   . . .    Gth group                                                        First transmit antenna, UE 1   00   01   . . .    11       First transmit antenna, UE 2   01   00   . . .    01       Second transmit antenna,   11   11   . . .    10       UE 1       Second transmit antenna,   01   10   . . .    00       UE 2                  
 
      In Table 6a, it is noted that UE 1 is placed in the best channel status when the Node B transmits data on the subcarriers of the first group via the first transmit antenna, and UE 2 is placed in the best channel status when the Node B transmits data on the subcarriers of the Gth group via the second transmit antenna. Therefore, the subcarrier allocator  770  decides to transmit data on the subcarriers of the first group via the first transmit antenna for UE 1, and on the subcarriers of the Gth group via the second transmit antenna for UE 2.  
      If there are a plurality of transmit antennas and a plurality of subcarrier groups that lead to a good channel status for a UE, the Node B prioritizes them according to a QoS level and service type requested by the UE. If UE 1 requests packet data, UE 2 requests circuit data, and the same transmit antenna and subcarrier group bring the best channel status for both UE 1 and UE 2. The subcarrier allocator  770  serves UE 1 over UE 2 with priority. This, however, is a mere exemplary application and thus the criterion to allocate subcarriers is set depending on system implementation.  
                                       TABLE 6b                                   First   Second   Third       Gth           group   group   group   . . .   group                                                            First transmit antenna, UE   00   01   10   . . .   11       1,       first receive antenna       First transmit antenna, UE   01   00   11   . . .   01       1,       second receive antenna       Second transmit antenna,   11   11   01   . . .   10       UE 1,       first receive antenna       Second transmit antenna,   01   10   11   . . .   00       UE 1, second receive       antenna       First transmit antenna, UE   01   01   00   . . .   11       2,       first receive antenna       First transmit antenna, UE   10   10   01   . . .   01       2,       second receive antenna       Second transmit antenna,   11   11   01   . . .   10       UE 2,       first receive antenna       Second transmit antenna,   01   00   10   . . .   11       UE 2, second receive       antenna                  
 
      From Table 6b, it is noted that UE 1 is placed in the best channel status when the Node B transmits data to the first receive antenna on the subcarriers of the first group via the first transmit antenna, or when the Node B transmits data to the second receive antenna on the subcarriers of the Gth group via the second transmit antenna. UE 2 is placed in the best channel status when the Node B transmits data to the second receive antenna on the subcarriers of the second group via the second transmit antenna, or when the Node B transmits data to the first receive antenna on the subcarriers of the third group via the first transmit antenna.  
      Therefore, the subcarrier allocator  770  decides to transmit data to UE 1 on the subcarriers of the first group using the first transmit antenna and the first receive antenna, or on the subcarriers of the Gth group using the second transmit antenna and the second receive antenna. The subcarrier allocator  770  decides to transmit data to UE 2 on the subcarriers of the second group using the second transmit antenna and the second receive antenna, or on the subcarriers of the third group using the first transmit antenna and the first receive antenna.  
       FIG. 8  illustrates the structure of an uplink channel that delivers the channel quality indicator (CQI) of each subcarrier group to a Node B according to an embodiment of the present invention. An existing WCDMA system estimates an SIR using a CPICH (Common Pilot CHannel) and determines a CQI such that the total throughput is maximized. The CQI is transmitted in a 2-ms subframe on an HS-DPCCH (High Speed Dedicated Physical Control Channel). The HS-DPCCH delivers control information related to an HS-DSCH (High Speed Downlink Shared Channel) that transmits downlink packet data for HSDPA service.  
      20 bits are actually transmitted in the subframe. 5 of the bits are information bits and the other 15 bits are redundancy bits. Therefore, a UE represents 31 CQIs in the 5 bits. A CQI is used in deciding a modulation/demodulation scheme and a transport block size.  
      As described above, each UE estimates the SIR of a total carrier band using the CPICH, decides a CQI according to the SIR so as to maximize the total throughput, and transmits the CQI together with an HARQ (Hybrid Automatic Retransmission reQuest), ACK/NACK (Acknowledgement/Negative Acknowledgement) signal in a 2-ms subframe on the HS-DPCCH in the existing WCDMA system. In an embodiment of the present invention, however, the UE estimates the SIR of each subcarrier group rather than the SIR of the total carrier band, decides a CQI for the subcarrier group based on the SIR, and transmits the per-group CQIs together with the HARQ ACK/NACK signal in a 2-ms HS-DPCCH subframe. The HS-DPCCH subframe is divided into three slots. The first of them delivers the HARQ ACK/NACK information and the other two slots deliver the CQIs measured by the UE.  
      In the illustrated case of  FIG. 8 , a kth UE transmits feedback information about subcarrier signals received from an mth transmit antenna. The subcarriers are grouped into F groups, #g to #(g+F−1). The UE transmits the CQIs of the subcarrier groups in CQI areas of the subframe, sequentially starting with group g. The Node B determines a subcarrier group to be allocated to the UE based on the CQI information of the F subcarrier groups. A CQI feedback period by which the position of a subframe for delivering the CQI information to the Node B is determined by signaling from an upper layer.  
       FIG. 9  illustrates the transmission format of feedback information to a Node B having two transmit antennas according to an embodiment of the present invention.  
      Referring to  FIG. 9 , the UE transmits the CQIs of subcarrier groups received from a first transmit antenna of the Node B and then the CQIs of the subcarrier groups received from a second transmit antenna. The subcarriers are divided into G subcarrier groups and each CQI is for a particular subcarrier group from a particular transmit antenna. The UE transmits the CQI information in 20 bits available for the CQI delivery in the HS-DPCCH subframe. As more bits are required to represent one CQI, the number of subcarrier groups representable by one subframe is decreased.  
      In the illustrated case of  FIG. 9 , one subframe delivers the CQIs of all subcarrier groups transmitted by one transmit antenna. In the case where it is impossible to transmit the CQIs of all subcarrier groups transmitted by one transmit antenna in one subframe, the next subframe is used. In  FIG. 9 , after transmitting the CAQI s of the subcarrier groups transmitted by the first transmit antenna, the UE transmits the CQIs of the subcarrier groups transmitted by the second transmit antenna. It can be further contemplated, as another embodiment of the present invention, that after transmitting the CQIs of the first group, the UE transmits the CQIs of the following groups, sequentially. While two subframes are shown in parallel in  FIG. 9 , it should be apparent to those skilled in the art of the present invention that the two subframes are transmitted serially at a predetermined time interval in real implementation.  
      The Node B decides a subcarrier group for the UE based on the CQI information and transmits data to the UE on the subcarrier group. Since the CQI of a subcarrier group is the average of the CQIs of the subcarriers in the subcarrier group, it is impossible to achieve the accurate CQI information of a particular subcarrier. In this context, the present invention proposes a method of further dividing each of the subcarrier groups into a plurality of subgroups and transmitting the CQIs of the subgroups in another embodiment.  
       FIG. 10  illustrates the structure of subgroups according to an embodiment of the present invention.  FIG. 10  illustrates one subcarrier group that includes L subcarriers, and is divided into Z subgroups, subgroups #1 to #Z. Each subgroup has P subcarriers. Thus, L=P×Z.  
       FIG. 11  illustrates the transmission format of the CQIs of the F subgroups according to an embodiment of the present invention.  
      Referring to  FIG. 11 , the F subgroups are subgroups #z to #(z+F−1). The index of a UE is denoted by a k, m denotes the index of a transmit antenna, and g denotes the index of a subcarrier group. In the illustrated case, a kth UE transmits the CQI information of the subgroups of a gth subcarrier group allocated to an mth transmit antenna. Compared to the transmission format illustrated in  FIG. 8 , this transmission format contains an indicator indicating that this transmission format is about subgroups. The indicator is one or more bits according to user selection or the number of the CQI bits transmitted.  
       FIG. 12  illustrates the transmission format of feedback information to a Node B having two transmit antennas according to an embodiment of the present invention.  
      Referring to  FIG. 12 , the UE transmits the CQIs of subgroups #1 to #Z in a particular subcarrier group received from a first transmit antenna of the Node B in one subframe and then the CQIs of subgroups #1 to #Z of the subcarrier group received from a second transmit antenna in the next subframe. The UE transmits the CQIs in some of 20 bits available for the CQI delivery in one subframe and an indicator indicating a subgroup transmission format in the remaining bits. As more bits are required to represent one CQI, the number of subgroups representable by one subframe is decreased.  
      In the illustrated case of  FIG. 12 , one subframe delivers the CQIs of all subgroups in a subcarrier groups transmitted by one transmit antenna. In the case where it is impossible to transmit the CQIs of all subgroups of a subcarrier group transmitted by one transmit antenna in one subframe, the next subframe is used. In  FIG. 12 , after transmitting the CQI s of the subgroups of a subcarrier group transmitted by the first transmit antenna, the UE transmits the CQIs of the subgroups of the subcarrier groups transmitted by the second transmit antenna. In another embodiment of the present invention, after transmitting the CQIs of the first subgroup, the UE transmits the CQIs of the following subgroups, sequentially. While two subframes are shown in parallel in  FIG. 12 , it should be apparent to those skilled in the art of the present invention that the two subframes are transmitted serially at a predetermined time interval in real implementation.  
      According to the embodiments of the present invention as described above, subcarriers are grouped into a plurality of groups and the CQIs of the respective subcarrier groups are transmitted. Since each subcarrier group includes two or more subcarriers, the subcarrier group is further divided into subgroups to thereby acquire more accurate channel quality information of the subcarriers of the subcarrier group. When the channel status varies significantly, however, only the channel status information of each subcarrier group can be transmitted. Depending on the channel status change and available radio resources, it is determined whether to transmit the channel status information of the subcarrier groups or the subgroups of the subcarrier groups.  
       FIG. 13  is a timing diagram illustrating CQI timings of subcarrier groups and subgroups in a mobile communication system having one Node B and three UEs according to a third embodiment of the present invention.  
      Referring to  FIG. 13 , after transmitting the CQI of an allocated subcarrier group in one subframe, UE 1 transmits the CQIs of subgroups of the subcarrier group in the following three subframes marked with empty rectangles in  FIG. 13 . In the case where the subgroup CQIs are not completely transmitted in one subframe, additional subframes can be used as illustrated in  FIG. 13 .  
      UE 2, after transmitting the CQI of an allocated subcarrier group in one subframe, transmits the CQIs of the subgroups of the subcarrier group in the following two subframes.  
      As can be seen from  FIG. 13 , the CQI period of the subcarrier groups (three subframes) for UE 2 is shorter than that (four subframes) for UE 1. This is because UE 2 is placed in an unstable channel status, relative to UE 1.  
      Meanwhile, UE 3 transmits the CQI of an allocated subcarrier group in one subframe and then the CQIs of the subgroups of the subcarrier group in two subframes. The CQI period for UE 3 is longer than the CQI transmission periods of UE 1 and UE 2. This implies that UE 3 is placed in the most stable channel status.  
       FIG. 14  illustrates transmission of the CQIs of the subgroups of G allocated subcarrier groups from a UE according to an embodiment of the present invention. Subcarrier group 1 to subcarrier group G are transmitted via first and second transmit antennas Ant 1 and Ant 2 from a Node B. The UE transmits the CQIs of the subgroups of the subcarrier groups.  
      The UE transmits the CQIs of subgroup 1 to subgroup Z in subcarrier group 1 transmitted from Ant 1 in a first subframe, and the CQIs of subgroup 1 to subgroup Z in subcarrier group 1 transmitted from Ant 2 in a second subframe. In the same manner, the UE transmits the CQIs of subgroup 1 to subgroup Z in subcarrier group G transmitted from Ant 1 in a (2G−1)th subframe and the CQIs of subgroup 1 to subgroup Z in subcarrier group G transmitted from Ant 2 in a 2Gth subframe.  
      Each subframe has CQI information in part of the 20 bits and an indicator in the remaining bits. If the remaining bits are sufficient, the indicator is filled in them through bit repetition. The indicator includes a frame format indicator and a subcarrier group indicator. The frame format indicator indicates a subframe transmission format and the subcarrier group indicator indicates a subcarrier group the subgroup CQIs of which are transmitted in the subframe. While the subframes are shown in parallel, as would be apparent to one skilled in the art of the present invention, the subframes are transmitted serially at predetermined intervals in real implementation.  
      The operation described above can be summarized as follows. The Node B allocates appropriate subcarrier groups to UEs referring to a resource map by an allocation algorithm. Antennas can be chosen on a subcarrier group basis. After being allocated to different subcarrier groups, the UEs are notified of antennas to which the subgroups of the subcarrier groups are mapped. The subcarrier group/subgroup allocation is periodically performed in the subcarrier allocator  770 . The allocation period is several to tens of TTIs (Transmit Time Intervals).  
      The UEs generates the CQI of each subcarrier group or each subgroup of the subcarrier groups. Transmission of CQI information on a per-subcarrier group basis is called mode 1, whereas transmission of CQI information on a per-subgroup basis is called mode 2.  
      In Mode 1, CQIs are calculated for all cases of a kth user, that is, for m antennas and g subcarrier groups (m=1, 2, . . . , M, g=1, 2, . . . , G), as follows. The SIR of an nth subcarrier (n=1, 2, . . . , N) transmitted is determined by 
 
SIR=P k,n,m   (2) 
 
      The SIRs of each subcarrier group computed by Eq. (2) are arithmetically averaged. In computing the SIR, the UE maps CPICH power to HS-PDSCH power according to a WCDMA standard, 3GPP (3 rd  Generation Partnership Project) TS25.214 by 
 
 P   HSPDSCH   =P   CPICH +Γ+Δ where Γ is the measurement power offset signalled by higher layer and Δ is given by CQI mapping table in TS25.124.  (3) 
 
      The average SIR of each subcarrier group is computed by  
                         ρ   _       k   ,   m       (   g   )       =       ∑     n   =     L   ⁡     (     g   -   1     )           Lg   -   1       ⁢     ρ     k   ,   n   ,   m           ,             g   =   1     ,   2   ,   …   ⁢           ,   G                 (   4   )             
 
      Using Eq. (4), CQI bits can be generated by Eq. (5). A CQI mapping function roughly expresses a channel status by an average SIR. Depending on configuration, a variety of CQI mapping functions are available. For example, the CQI mapping function produces CQI(k, m, g) by linearly mapping the average SIRs or achieving the lognormals of the average SIRs and mapping them in terms of decibel.  
                     CQI   ⁡     (     k   ,   m   ,   g     )       =     f   ⁡     (       ρ   _       k   ,   m       (   g   )       )                   where   ⁢           ⁢     f   ⁡     (   •   )       ⁢           ⁢   is   ⁢           ⁢   CQI   ⁢           ⁢   mapping   ⁢           ⁢   function                 (   5   )             
 
      If CQIs are expressed in two bits, CQI(k, m, g) can be mapped according to channel status as follows. 
          CQI (k, m, g)=11 (high quality)     CQI (k, m, g)=10 (medium quality)     CQI (k, m, g)=01 (medium quality)     CQI (k, m, g)=00 (low quality)        

      The kth UE transmits CQI (k, m, g) calculated in mode 1 to the Node B in an uplink HS-DPCCH subframe. The UE attempts to transmit F CQIs in subframes for one TTI. If the F CQIs are not completely transmitted, the remaining CQIs are transmitted for the next TTI. In mode 1, the Node B updates the resource map based on the reported CQIs and periodically allocates subcarriers referring to the resource map.  
      In mode 2, CQIs can be transmitted on a subgroup basis. The Node B gets the average SIR of a smaller unit (i.e. subgroup). L subcarriers of a gth subcarrier group are divided into Z subgroups, each having P subcarriers. Thus, L=Z×P. Then, the lognormal mean SIR of each subgroup is computed by  
                         ρ   _       k   ,   m       (     g   ,   z     )       =       ∑     n   =     gL   +     P   ⁡     (     z   -   1     )             gL   +   Pz   -   1       ⁢     ρ     k   ,   n   ,   m           ,             z   =   1     ,   2   ,   …   ⁢           ,   Z                 (   6   )             
 
      The lognormal mean SIR is mapped to a CQI by  
                     CQI   ⁡     (     k   ,   m   ,   g   ,   z     )       =     f   ⁡     (       ρ   _       k   ,   m       (     g   ,   z     )       )                   where   ⁢           ⁢     f   ⁡     (   •   )       ⁢           ⁢   is   ⁢           ⁢   CQI   ⁢           ⁢   mapping   ⁢           ⁢   function                 (   7   )             
 
      The UE then transmits the CQI to the Node B in the same manner as in mode 1.  
      In mode 2, the Node B can select antennas on a subgroup basis. F CQIs, CQI(k, m, g)&#39;s or CQI (k, m, g, z)&#39;s are transmitted per TTI. Mode 2 leads a diversity gain by transmission of CQI (k, m, g, z), and allows the Node B to update the resource map by transmission of CQI(k, m, g).  
      Hereinbelow, operations in the UE and the Node B will be described with reference to  FIGS. 15 and 16 .  FIG. 15  is a flowchart illustrating the UE operation according to an embodiment of the present invention.  
      Referring to  FIG. 15 , the UE determines whether it is in an OFDM service in decision step  1500 . The determination is made by checking whether data is received on an OFDM channel or a subcarrier allocation control signal is received from the network, or based on any other criterion. If the OFDM service is supported (“Yes” path from decision step  1500 ), the UE goes to step  1502 . If the OFDM service is not supported (“No” path from decision step  1500 ), the UE terminates the procedure in step  1504 .  
      In step  1502 , the UE channel-estimates its allocated subcarrier groups. The channel estimation is the process of measuring the channel statuses of the subcarrier groups and generating CQIs (G′CQIs) for the subcarrier groups based on the channel statuses. An OFDM pilot or any other predetermined signal can be used in the channel estimation. The UE transmits the G′CQIs to the Node B in step  1506  and determines again whether it receives the OFDM service in decision step  1508 . If it does (“Yes” path from decision step  1508 ), the UE moves to decision step  1510  and if not, the UE terminates the procedure in step  1504  (“No” path from decision step  1508 ).  
      In step decision  1510 , the UE determines whether it is to operate in mode 2 according to its channel status or a system indication from upper layer signaling. If the UE is to operate in mode 2 (“Yes” path from decision step  1510 ), it goes to step  1512 . If the UE is not to operate in mode 2 (“No” path from decision step  1500 ), it goes to step  1514 . In step  1514 , the UE waits until the next subcarrier group CQI period (G′period).  
      In step  1512 , the UE channel-estimates the subgroups of the allocated subcarrier groups and generates CQIs for the subgroups (SG′CQIs). An OFDM pilot signal or any other predetermined signal can be used in the channel estimation, The UE transmits the SG′CQIs to the Node B in step  1516  and checks the G′CQI period in decision step  1518 . Upon expiration of the G′CQI period (“Yes” path from decision step  1518 ), the UE returns to step  1500 . If the G′CQI period has not elapsed (“No” path from decision step  1518 ), the UE waits until the next SG′ period in step  1512 .  
       FIG. 16  is a flowchart illustrating the Node B operation according to an embodiment of the present invention.  
      Referring to  FIG. 16 , the Node B determines whether G′CQIs have been received in decision step  1600 . If they have (“Yes” path from decision step  1600 ), the Node B goes to step  1602 . If they have not, the Node B stays in step  1600  (“No” path from decision step  1600 ). In step  1602 , the Node B allocates transmit antennas and subcarrier groups to UEs based on the G′CQIs. The Node B transmits data to the UEs using the allocated subcarrier groups and antennas in step  1604  and determines whether data still remains for a particular UE in decision step  1606 . If data remains (“Yes” path from decision step  1606 ), the Node B goes to decision step  1608  and otherwise, it terminates the procedure in step  1610  (“No” path from decision step  1608 ).  
      In decision step  1608 , the Node B determines whether to perform mode 2 according to the channel status of the UE or a system indication. If mode 2 is not to be performed (“No” path from decision step  1608 ), the Node B awaits reception of G′CQIs in the next CQI period in step  1614 . If mode 2 is to be performed (“Yes” path from decision step  1608 ), the Node B determines whether SG′CQIs have been received from the UE in decision step  1612 . Upon receipt of the SG′CQIs (“Yes” path from decision step  1612 ), the Node B goes to step  1616 . If the SG′CQIs have not been received (“No” path from decision step  1612 ), the Node B returns to step  1612 .  
      The Node B allocates subgroups to transmit antennas based on the SG′CQIs in step  1616  and transmits data to the UE on the subcarriers of the allocated subgroups via the allocated transmit antennas in step  1618 . In decision step  1620 , the Node B determines whether a G′CQI period has expired. If the G′CQI period has not expired (“No” path from decision step  1620 ), the Node B awaits reception of SG′CQIs in the next SG′CQI period in step  1622 . If the G′CQI period has expired (“Yes” path from decision step  1620 ), the Node B returns to step  1622 .  
      In another embodiment of the present invention, Node B determines whether CQIs in a subframe are for subcarrier groups or the subgroups of a subcarrier group by checking an included indicator, without the need for determining mode 2.  
      As described above, the present invention groups into a plurality of subcarrier groups and further into a plurality of subgroups in an OFDM system, thereby achieving multiple antenna select diversity. Also, uplink transmission of feedback information on a subcarrier group basis leads to efficient use of radio resources.  
      While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.