Abstract:
Apparatus and method of generating a long term evolution (LTE) codebook and performing rank overriding are disclosed. Reordering rules are presented, whereby a second column vector of each rank-4 precoding matrix will not appear in column vectors of a rank-3 precoding matrix, and the first column vector of each rank-4 precodingmatrix is identical to the first column vector of the corresponding rank-3 precodingmatrix. Furthermore, precoder hopping between two precoding matrices corresponding to a particular precoding matrix index (PMI) is implemented, whereby a first one of the two precoding matrices comprises a first subset of column vectors of an original precoding matrix that corresponds to the particular PMI, and a second one of the two precoding matrices comprises a second subset of column vectors of the original precoding matrix. The precoder hopping is performed in time and/or frequency domain.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/986,651 filed Nov. 9, 2007, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION 
       [0002]    This application is related to wireless communications. 
       BACKGROUND 
       [0003]    Closed loop multiple-input multiple-output (MIMO) is an important operation mode in future long term evolution (LTE) networks. Under such a mode, a wireless transmit/receive unit (WTRU) feeds back a rank index (RI) and a precoding matrix index (PMI) to a base station, (i.e., an enhanced eNodeB (eNodeB)), along with channel quality indicator (CQI) information. In general, the base station responds to the WTRU feedback and sends downlink (DL) data accordingly. However, in certain circumstances, the base station may decide to override the RI feedback, and transmit DL data with a different rank than indicated by the WTRU feedback. Such an operation is referred to as rank overriding (RO). 
         [0004]    For RO to perform properly, two conditions must be met: 
         [0005]    1) The base station must derive a new precoding matrix for the newly selected rank; and 
         [0006]    2) The base station must derive new CQI values for the newly derived precoding matrix so that proper modulation and coding schemes (MCS) may be assigned to each layer of MIMO transmission. 
         [0007]    To meet the first condition, the LTE codebook forces a “nested property.” When the base station overrides the WTRU feedback rank with a lower rank, the “nested property” allows the base station to use a subset of the original precoding matrix as a new precoding matrix. According to the current LTE specification, it is difficult to derive an accurate CQI after the base station performs RO. Therefore, throughput after RO is reduced. 
         [0008]      FIG. 1  shows a conventional LTE codebook  100  for systems equipped with four (4) transmit antennas. The LTE codebook  100  includes four columns  105 ,  110 ,  115  and  120 , each having sixteen 4×4 precoding matrices W 0 -W 15 . Depending on the rank, all or a subset of column vectors of a 4×4 matrix is used as a precoding matrix. Column  105  is referred to as the rank-1 column of the codebook  100 , column  110  is referred to as the rank-2 column of the codebook  100 , column  115  is referred to as the rank-3 column of the codebook  100 , and column  120  is referred to as the rank-4 column of the codebook  100 . To feed back the information of the precoding matrix, a 2-bit RI and a 4-bit PMI are required. As shown in  FIG. 1 , the subscript of each 4×4 matrix represents the matrix index, and the superscript in brackets represents the column vectors. For example, W 0   {14}  is a rank-2 precoding matrix consisting of the first and fourth column vectors of matrix W 0 . 
         [0009]    The following is an example illustrating the problem of current LTE codebook with four (4) transmit antennas and overriding operation. According to current channel conditions, the WTRU determined rank-4 can be accommodated, and the best precoding matrix (out of 16) is W 0 . The WTRU then sends feedback PMI=0, and RI=3 (rank 4) to the base station. In the meantime, the WRTU also calculates CQI under the assumption RI=3 (rank 4), and PMI=0. According to the LTE specification, two codewords will be used for rank-4. Therefore, two CQI values must be calculated: CQI 1  and CQI 2 . CQI 1  is the channel quality indicator for the first codeword (CW 1 ), which is split into first and second layers. CQI 2  is the channel quality indicator for the second codeword (CW 2 ), which is split into third and forth layers. 
         [0010]    The channel matrix is H, and the effective channel vector is 
         [0000]      {tilde over (H)} n =HW 0   {n}   .   Equation (1) 
         [0000]    Although the exact formula to calculate CQI values may vary depending on the type of WTRU receivers, the channel quality of the first codeword (CQI 1 ) is proportional to the average strength of {tilde over (H)} 1  and {tilde over (H)} 2 , and the channel quality of the second codeword (CQI 2 ) is proportional to the average strength of {tilde over (H)} 3  and {tilde over (H)} 4 . 
         [0011]    In this example, if the base station decides to transmit DL data with rank-3, (which is different than the WTRU feedback), it would select rank-3 precoding matrix W o   {124}  as the new precoding matrix. According to the codeword to layer mapping rule, the first codeword (CW 1 ) is mapped to the layer corresponding to the effective channel {tilde over (H)} 1 , and the second codeword (CW 2 ) is mapped to the two layers corresponding to {tilde over (H)} 2  and {tilde over (H)} 4 . The base station would then require a pair of new CQIs corresponding to the new precoding matrix. The new CQI values should be such that CQI 1 _RO is proportional to the strength of the effective channel {tilde over (H)} 1 , and CQI 2 _RO is proportional to the average strength of the effective channels {tilde over (H)} 2  and {tilde over (H)} 4 . The CQI 1 _RO is different than the original WTRU feedback CQI 1 , and the CQI 2 _RO is different than the original WTRU feedback CQI 2 . Consequently, with the current LTE codebook and codeword to layer mapping rule, it would be difficult for the base station to calculate CQI 1 _RO and CQI 2 _RO according to CQI 1  and CQI 2 . Therefore, the base station will likely assign an improper MCS to each codeword, resulting inefficient transmission. 
       SUMMARY 
       [0012]    This application is related to an apparatus and method of generating an LTE codebook and performing rank overriding. Reordering rules are presented, whereby a second column vector of each rank-4 precoding matrix will not appear in column vectors of a rank-3 precoding matrix, and the first column vector of each rank-4 precoding matrix is identical to the first column vector of the corresponding rank-3 precoding matrix. Furthermore, precoder hopping between two precoding matrices corresponding to a particular PMI is implemented, whereby a first one of the two precoding matrices comprises a first subset of column vectors of an original precoding matrix that corresponds to the particular PMI, and a second one of the two precoding matrices comprises a second subset of column vectors of the original precoding matrix. The precoder hopping is performed in time and/or frequency domain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    A more detailed understanding may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
           [0014]      FIG. 1  shows a conventional LTE codebook with a rank-4 precoding matrix; 
           [0015]      FIG. 2  shows a new LTE codebook with a modified rank-4 precoding matrix; 
           [0016]      FIG. 3  shows rank overriding with precoder hopping in frequency domain; 
           [0017]      FIG. 4  shows rank overriding with precoder hopping in time domain; 
           [0018]      FIG. 5  shows rank overriding with precoder hopping in both time and frequency domain; 
           [0019]      FIG. 6  shows a precoder hopping rule for rank overriding; 
           [0020]      FIG. 7A  shows a conventional PMI independent rank-4 layer mapping; 
           [0021]      FIG. 7B  shows a proposed PMI dependent rank-4 layer mapping; 
           [0022]      FIG. 8  is a block diagram of a WTRU; and 
           [0023]      FIG. 9  is a block diagram of a base station. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. 
         [0025]    When referred to hereafter, the terminology “base station” includes but is not limited to an evolved or E-UTRAN Node-B (eNodeB), a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
         [0026]    One method of improving the data transmission after rank overriding is to change the order of the column vector in a rank-4 precoding matrix.  FIG. 2  shows an example of a new codebook with a modified rank-4 precoding matrix. The advantage of changing only the order of the column vector is that the performance of rank-4 precoding is not affected. 
         [0027]    New reordering rules are proposed herein, whereby the second column vector of each rank-4 precoding matrix will not appear in the column vectors of the rank-3 precoding matrix, and the first column vector of each rank-4 precoding matrix is identical to the first column vector of the corresponding rank-3 precoding matrix. 
         [0028]    Using the example above, the CQI 1  calculated by the WTRU is proportional to the average strength of {tilde over (H)} 1  and {tilde over (H)} 3 , and the CQI 2  calculated by the WTRU is proportional to the average strength of {tilde over (H)} 2  and {tilde over (H)} 4 . In this example, the CQI 2  is consistent with CQI 2 _RO. Therefore, the base station can use the WTRU feedback on CQI 2 , without modification, in assigning a MCS to the second codeword, without causing performance degradation to CW 2 . However, the CQI 1  definition differs from CQI 1 _RO, even after modification of rank-4 precoding matrices. 
         [0029]    Precoding hopping after rank overriding will now be described. Under the current LTE codebook definition and codeword to layer mapping, rank overriding is performed by arbitrarily removing one or more column vector(s) from the original precoding matrix fed back by WTRU. Therefore, it is then possible that column vectors corresponding to satisfactory channel quality are removed. This also causes CQI discrepancy between the WTRU and base station. In the proposed precoding hopping scheme, all column vectors of the original precoding matrix are used to precode DL data, even after rank overriding. Since the number of column vectors is larger than the rank, the base station switches the precoding matrix in either time and/or frequency domain. 
         [0030]    The following example describes rank 4 to rank 3 overriding to illustrate the concept of precoder hopping after rank overriding. The W 0   {1324}  is the original rank-4 precoding matrix fed back by the WTRU. To override the rank to 3, the current LTE specification would use W o   {124}  as the rank-3 precoding matrix in all orthogonal frequency division multiplexing (OFDM) symbols and all subcarriers. 
         [0031]      FIG. 3  shows an example of rank overriding with frequency domain precoder hopping, where the base station alternates the precoding matrix W 0   {124}  and W 0   {324}  in frequency. The precoding matrix W 0   {124}  is applied in odd subcarriers, and the precoding matrix W 0   {324}  is applied in even subcarriers. 
         [0032]    Similarly, the precoder hopping can be done in time domain, as shown in  FIG. 4 . Within the same precoding group (PCG), the precoding matrix W 0   {124}  is applied on all subcarriers of the odd OFDM symbols, and the precoding matrix W 0   {324}  is applied on all subcarriers of the even ODFM symbols. 
         [0033]    In addition, the precoder hopping can be performed in both time and frequency domain simultaneously as shown in  FIG. 5 , where the precoding matrix W 0   {124}  is applied on all of the odd subcarriers of odd OFDM symbols, and all of the even subcarriers of even OFDM symbols, and the precoding matrix W 0   {324}  is applied on all of the even subcarriers of odd OFDM symbols, and all of the odd subcarriers of even OFDM symbols. 
         [0034]    All of the precoder hopping patterns shown in  FIGS. 3-5  confirm that after rank overriding, the CQI for the first codeword CQI 1 _RO is the average strength of {tilde over (H)} 1  and {tilde over (H)} 3 , which is consistent with CQI 1  before rank overriding. 
         [0035]    Rank overriding is not limited to only rank-4 to rank-3 overriding. FIG.  6  shows a table that summarizes the precoder hopping pattern for other rank overriding scenarios. As shown in  FIG. 6 , two different precoding matrices may be used after rank overriding in some circumstances. In such cases, two precoders are used alternately in either frequency or/and time domain. For example, if the base station decides to override rank-4 with rank-2, two precoding matrices will be used alternate, (i.e., hopping), between matrices after the rank overriding, whereby the first matrix comprises the first and third column vectors of the original rank-4 matrix, and the second matrix comprises the second and fourth column vectors of the original rank-4 matrix. In another example, if the base station decides to override rank-3 with rank-2, two precoding matrices will alternate, (i.e., hop), between matrices after the rank overriding, whereby the first matrix comprises the first and second column vectors of the original rank-3 matrix, and the second matrix comprises the first and third column vectors of the original rank-3 matrix. In yet another example, if the base station decides to override rank-3 with rank-1, then no precoder hopping is necessary as only one precoding matrix exists in such a case. 
         [0036]    The order of the column vectors of rank-4 precoding matrices may be changed, which maintains the current codeword to layer mapping, or the rank-4 precoding matrices can remain unchanged, while changing the fixed rank-4 codeword to layer mapping to PMI dependent mapping, as shown in  FIG. 6 . 
         [0037]    In the original mapping shown in  FIG. 7A , the first codeword is mapped to the first and second layers ( 12 ), and the second codeword is mapped to the third and fourth layers ( 34 ), regardless of the PMI value 0-15.  FIG. 7B  shows an example of a modified mapping, whereby the first codeword is mapped to the first and third layers ( 13 ), and the second is mapped to the second and fourth layers ( 24 ) when the PMI value is 0. When the PMI value is 1, the first codeword is mapped to the first and fourth layers, and the second codeword is mapped to the second and third layers. It is noted that different PMI dependent layer mapping is also possible. However, in this case, the precoding vectors corresponding to the second codeword in rank-4 should also be applied to the second codeword in rank-3. 
         [0038]      FIG. 8  shows a WTRU  800  comprising a MIMO antenna  805 , a transmitter  810 , a processor  815  and a receiver  820 . The WTRU  800  may be configured to generate an LTE codebook having a rank-1 column, a rank-2 column, a rank-3 column and a rank-4 column. Each column includes a plurality of precoding matrices. Each precoding matrix corresponds to a respective PMI. The LTE codebook may have sixteen (16) different PMIs. Furthermore, the first column vector of each precoding matrix in the rank-4 column that corresponds to a particular PMI may be the same as the first column vector in a precoding matrix in the rank-3 column that also corresponds to the particular PMI. 
         [0039]    The processor  815  may be configured to assign a first column vector to each of the precoding matrices in the rank-1 column, assign a first column vector and a second column vector to each of the precoding matrices in the rank-2 column, assign a first column vector, a second column vector and a third column vector to each of the precoding matrices in the rank-3 column, and assign a first column vector, a second column vector, a third column vector and a fourth column vector to each of the precoding matrices in the rank-4 column. Either the second or third column vector of each precoding matrix in the rank-3 column that corresponds to a particular PMI is the same as the second column vector in a precoding matrix in the rank-2 column that also corresponds to the particular PMI. The last two column vectors of each precoding matrix in the rank-4 column that corresponds to a particular PMI are the same as the last two column vectors in the rank-3 column for the particular PMI. The second column vector of any precoding matrix in the rank-4 column that corresponds to a particular PMI is not included in a precoding matrix in the rank-3 column that also corresponds to the particular PMI. 
         [0040]    The WTRU  800  may also be configured to perform rank overriding using frequency domain precoder hopping in an LTE codebook having a rank-1 column, a rank-2 column, a rank-3 column and a rank-4 column. Each column includes a plurality of precoding matrices having column vectors assigned thereto. Each precoding matrix corresponds to a respective PMI. The processor  815  may be configured to alternate between the use of two precoding matrices corresponding to a particular PMI. A first one of the two precoding matrices comprises a first subset of column vectors of an original precoding matrix that corresponds to the particular PMI, and a second one of the two precoding matrices comprises a second subset of column vectors of the original precoding matrix. The alternation between the use of two precoding matrices is implemented by precoder hopping that is performed in time domain and/or frequency domain. 
         [0041]    In one scenario, the first one of two precoding matrices is applied on odd subcarriers of each OFDM symbol, and the second one of two precoding matrices is applied on even subcarriers of each OFDM symbol. 
         [0042]    In another scenario, the first one of the two precoding matrices may be applied on all subcarriers of odd orthogonal OFDM symbols, and the second one of the two precoding matrices is applied on all subcarriers of even OFDM symbols. 
         [0043]    In yet another scenario, the first one of the two precoding matrices may be applied on all odd subcarriers of odd OFDM symbols, and on all even subcarriers of even OFDM symbols. The second one of the two precoding matrices may be applied on all even subcarriers of odd OFDM symbols, and on all odd subcarriers of even OFDM symbols. 
         [0044]      FIG. 9  shows a base station  900  comprising a MIMO antenna  905 , a transmitter  910 , a processor  915  and a receiver  920 . The base station  900  may be configured to generate an LTE codebook having a rank-1 column, a rank-2 column, a rank-3 column and a rank-4 column. Each column includes a plurality of precoding matrices. Each precoding matrix corresponds to a respective PMI. The LTE codebook may have sixteen (16) different PMIs. Furthermore, the first column vector of each precoding matrix in the rank-4 column that corresponds to a particular PMI may be the same as the first column vector in a precoding matrix in the rank-3 column that also corresponds to the particular PMI. 
         [0045]    The processor  915  may be configured to assign a first column vector to each of the precoding matrices in the rank-1 column, assign a first column vector and a second column vector to each of the precoding matrices in the rank-2 column, assign a first column vector, a second column vector and a third column vector to each of the precoding matrices in the rank-3 column, and assign a first column vector, a second column vector, a third column vector and a fourth column vector to each of the precoding matrices in the rank-4 column. The second column vector of any precoding matrix in the rank-4 column that corresponds to a particular PMI is not included in a precoding matrix in the rank-3 column that also corresponds to the particular PMI. The base station  900  may also be configured to perform rank overriding using frequency domain precoder hopping in an LTE codebook having a rank-1 column, a rank-2 column, a rank-3 column and a rank-4 column. Each column includes a plurality of precoding matrices having column vectors assigned thereto. Each precoding matrix corresponds to a respective PMI. The processor  915  may be configured to alternate between the use of two precoding matrices corresponding to a particular PMI. A first one of the two precoding matrices comprises a first subset of column vectors of an original precoding matrix that corresponds to the particular PMI, and a second one of the two precoding matrices comprises a second subset of column vectors of the original precoding matrix. The alternation between the use of two precoding matrices is implemented by precoder hopping that is performed in time domain and/or frequency domain. 
         [0046]    In one scenario, the first one of two precoding matrices is applied on odd subcarriers of each OFDM symbol, and the second one of two precoding matrices is applied on even subcarriers of each OFDM symbol. 
         [0047]    In another scenario, the first one of the two precoding matrices may be applied on all subcarriers of odd orthogonal OFDM symbols, and the second one of the two precoding matrices is applied on all subcarriers of even OFDM symbols. 
         [0048]    In yet another scenario, the first one of the two precoding matrices may be applied on all odd subcarriers of odd OFDM symbols, and on all even subcarriers of even OFDM symbols. The second one of the two precoding matrices may be applied on all even subcarriers of odd OFDM symbols, and on all odd subcarriers of even OFDM symbols. 
         [0049]    Although the features and elements are described in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
         [0050]    Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. 
         [0051]    A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit/receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module.