Patent Publication Number: US-2021175951-A1

Title: Methods, Apparatuses, and Computer Software Products for Processing in MIMO System

Description:
TECHNICAL FIELD 
     Various example embodiments relate generally to wireless communication and, more specifically, relates to massive Multiple Input Multiple Output (MIMO) antenna systems. 
     BACKGROUND 
     This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued but are not necessarily ones that have been previously conceived, implemented or described. Therefore; unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted being prior art by inclusion in this section. Abbreviations that may be found in the specification and/or the drawing figures are defined below, after the main part of the detailed description section. 
     In future wireless system, non-orthogonal multiple access (NOMA) is regarded as an important radio access technology to achieve multi-user capacity. In some scenarios, such as massive machine type connections (mMTC), this technique is especially critical as it supports overloaded transmission. On the other hand, massive multiple input multiple output (MIMO) is the key technique in 5G. The combined application of both MIMO and NOMA would be crucially important for 5G scenarios. However, in massive MIMO system, full channel information for massive users at transmitter side is highly difficult to achieve or can cause high feedback overhead. Additionally, the traditional user selection for concurrent transmission may be no longer applicable when precoding at transmitter and receiver sides exist. 
     BRIEF SUMMARY 
     This section is intended to include examples and is not intended to be limiting. 
     According to one example embodiment, a method comprises: sending codebook based training information to at least one UE; receiving Precoding Index reports from said at least one UE; grouping said at least one UE into at least one group, wherein the UEs in the same group are covered by the same transmitting precoding; sending beamformed CSI-RS in accordance with said at least one groups; receiving CSI reports from UEs of said at least one group, so as to transfer data to said at least one UE accordingly. 
     According to one example embodiment, a method comprises: receiving codebook-based training information from BS; selecting the best precoder in accordance with said codebook-based training information; sending a precoding index reports to said BS, said precoding index reports including index of said best precoder; receiving a beamformed CSI-RS from said BS; sending CSI in accordance with said beamformed CSI-RS to said BS. 
     According to another example embodiment, a first apparatus, comprises: at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the first apparatus to perform at least the following: send codebook based training information to at least one UE; receive precoding index reports from said at least one UE; group said at least one UE into at least one group, wherein the UEs in the same group are covered by the same transmitting precoding; send beamformed CSI-RS in accordance with said at least one group; receive CSI reports from UEs of said at least one group, so as to transfer data to said at least one UE accordingly. 
     According to another example embodiment, a second apparatus, comprises: at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the second apparatus to perform at least the following: receive codebook-based training information from BS; select the best precoder in accordance with said codebook-based training information; send precoding index reports to said BS, said precoding index reports including index of said best precoder; receive beamformed CSI-RS from said BS; send CSI in accordance with said beamformed CSI-RS to said BS. 
     According to another example embodiment, a computer program product comprising a non-transitory computer-readable medium storing computer program code thereon which when executed by a device causes the device to perform at least: send codebook based training information to at least one UE; receive Precoding Index reports from said at least one UE; group said at least one UE into at least one group, wherein the UEs in the same group are covered by the same transmitting precoding; send beamformed CSI-RS in accordance with said at least one group; receive CSI reports from UEs of said at least one group, so as to transfer data to said at least one UE accordingly. 
     According to another example embodiment, a computer program product comprising a non-transitory computer-readable medium storing computer program code thereon which when executed by a device causes the device to perform at least: receive codebook-based training information from BS; select the best precoder in accordance with said codebook-based training information; send a precoding index reports to said BS, said precoding index reports including index of said best precoder; receive beamformed CSI-RS from said BS; send CSI in accordance with said beamformed CSI-RS to said BS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For proper understanding of the invention, reference should be made to the accompanying drawings, wherein: 
         FIG. 1  illustrates the operation of an exemplary method or methods. 
         FIG. 2  illustrates a one possible and non-limiting exemplary system in which the exemplary embodiments may be practiced. 
         FIG. 3  illustrates a block diagram of an exemplary downlink system with hybrid NOMA and OMA processing according to certain embodiments. 
         FIG. 4  illustrates a diagram of spectral efficiency comparison according to certain embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” 
     Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. All of the embodiments described in this Detailed Description are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention, which is defined by the claims. 
     In some embodiment, the method according to this disclosure is implemented by a first apparatus; in some embodiments, the method according to this disclosure is implemented by a first apparatus and a second apparatus. 
     Herein, the first apparatus could be a Base Station (BS), or could be a component or device that is able to implement all the steps of the according method, which could be included in a Base Station or other equipment with equivalent or similar functions. The second apparatus could be a User Equipment (UE), or could be a component or device that is able to implement all the steps of the according method, which could be included in a User Equipment or other equipment with equivalent or similar functions. 
     The BS serves a wireless network with a massive Multiple Input Multiple Output (MIMO) antenna system. Especially, a system that is able to implement both non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) scheme. 
     The exemplary embodiments herein describe techniques for hybrid of NOMA and OMA scheme in a massive MIMO antenna system. Additional description of these techniques is presented after a system into which the exemplary embodiments may be used is described. 
     Turning to  FIG. 1 , this figure illustrates the operation of an exemplary method or methods. 
     The method according to an exemplary including steps S 101 , S 102 , S 103 , S 104  and S 105  implemented by a first apparatus, and steps S 201 , S 202 , S 203 , S 204  and S 205  implemented by a second apparatus. 
     In step S 101 , the first apparatus sends codebook-based training information to at least one UE. 
     Next, in step S 201 , the second apparatus receive codebook-based training information. 
     Then, in step S 202 , the second apparatus select the best precoder in accordance with said codebook-based training information. 
     The codebook-based training information herein is applied by the UEs to determine the precoding pairs of antennas at both BS side and UE side. 
     In some examples, the training process can be conducted based on frequency divided sweeping to save the resources, e.g., different training precoders are transmitted on different sub-bands. The UE selects the strongest transmitting precoders among all training ones and reports the index of the selected precoder. 
     Also, in the case that UE has multiple antennas (or antenna ports), the UE will sweep a codebook of itself and select a precoder to form a best pair. 
     Then, in step S 203 , the second apparatus sends precoding index reports to said BS, said precoding index reports including the index of said best precoder. 
     And in step S 102 , the first apparatus receives precoding index reports from the at least one UE. 
     The precoding index reports contain the index of the codebook based precoder that is chosen by the UE. 
     Then, in step S 103 , the first apparatus groups the at least one UE into at least one groups, wherein the UEs in the same group are covered by the same transmitting precoding. 
     The number of the at least one UE could be indicated by K, the number of the at least one group could be indicated by M, wherein M≤K. 
     Also, indicating the number of antennas of the BS with N t , those skilled in the art could easily find an initial M that is smaller than N t . 
     In one embodiment, the first apparatus firstly obtains a target number M, and groups the UEs with same precoding index into one group; then the first apparatus obtains a set Ω of precoding index of the groups after grouping the UEs with same precoding index, wherein |Ω| is the total number of said set Ω; if |Ω|&gt;M, the first apparatus selects at least two groups to be aggregated; aggregates the at least two groups and updates the set Ω accordingly. The first apparatus repeat aggregates the groups until the total number |Ω|&lt;M. 
     Also, if |Ω|&lt;M, the first apparatus updates the value of M with the value of |Ω|. 
     Wherein, the first apparatus selects at least two groups to be aggregated based on at least one of the bellow: 
     (1) the distance of precoding index between two groups; 
     (2) the amount of UEs of the two groups. 
     For example, the first apparatus could aggregate two groups into one group if the distance of precoding index between the two groups are less than a predetermined threshold; or the first apparatus could aggregate two groups into one group if the amount of UEs of these two groups is the smallest, comparing that with other pairs of groups; or, the first apparatus could only aggregate two groups into one only when the distance between them less than the threshold of distance and the amount of UE of these two groups is the smaller than another threshold. 
     In a more specific example, Ω i , i=1, 2, . . . , |Ω| indicates the element in the set Ω, which clearly is the precoding index selected from the set [ 1 ,  2 , . . . , |W BS |]. Then the distance of precoding index between two groups could be given by: 
     
       
         
           
             
               
                 
                   
                     
                       v 
                       
                         Ω 
                         i 
                       
                     
                     = 
                     
                       mod 
                        
                       
                         ( 
                         
                           
                              
                             
                               
                                 Ω 
                                 i 
                               
                               - 
                               
                                 Ω 
                                 i 
                               
                             
                              
                           
                           , 
                           
                             ⌈ 
                             
                               
                                 
                                    
                                   
                                     W 
                                     BS 
                                   
                                    
                                 
                                 + 
                                 1 
                               
                               2 
                             
                             ⌉ 
                           
                         
                         ) 
                       
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In which i′ could be given by: 
         i ′=mod( i ,|Ω|)+1   (2);
 
     wherein v Ω     i    is the distance between the i-th group and i′-th group,  v  is an adjustable threshold of distance, Ω i  is the index of the i-th group in the set Ω, and Ω i′  is the index of the i′-th group in the set Ω, and |W BS | is the number of precoders of said codebook. 
     And the first apparatus selects two groups to be aggregated if the two has a distance small than the threshold, and an amount of UEs no larger than current record of the smallest total UE number of two adjacent groups. For example, an possible code corresponding to the aggregation progress could be presented as below: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 Input: Ω, M, K,  v   
               
               
                 Initialize: S 1  = S 2  = . . . = S |Ω|  = [ ]; 
               
               
                 For k = 1, 2, . . . , K 
               
               
                  Let S i  = S i  ∪ {k} if the k-th user selected the transmit precoding with 
               
               
                  index Ω i . 
               
               
                 End For 
               
               
                 If |Ω| &lt; M 
               
               
                  M = |Ω|; 
               
               
                 Else 
               
               
                  While |Ω| &gt; M 
               
               
                   υ min  = K; i min  = 0; 
               
               
                   For i = 1, 2, . . . , |Ω| 
               
               
                    Calculate i′ and v Ω     i    using equation (2) and (1) respectively; 
               
               
                    υ Ω     i    = |S i | + |S i′ |; 
               
               
                    If v Ω     i    &lt;  v  &amp;&amp; υ Ω     i    &lt; υ min   
               
               
                     υ min  = υ Ω     i   ; i min  = i; 
               
               
                    End if 
               
               
                   End for 
               
               
                   If i min  = 0 
               
               
                     v  =  v  + 1; 
               
               
                   Else 
               
               
                   
               
               
                    
         S     i   min       =         S     i   min       ⋃       S     i   min   ′            ;                   Ω       =       Ω        /          {       Ω     i   min       ,     Ω     i   min   ′         }          ;                   Ω     =     Ω   ⋃       {         Ω     i   min       +     Ω     i   min   ′         2     }          ;                 
 
               
               
                   
               
               
                   End if 
               
               
                  End while 
               
               
                 End if 
               
               
                 Return: S 1 , S 2 , . . . , S |Ω| ; 
               
               
                   
               
            
           
         
       
     
     Wherein S i  indicates the i-th group, and S i′  indicates the i′-th group, i min  and i′ min  indicate the serial numbers of two group that currently meet the condition, ϑ Ω     i    indicates the total amount of UEs of the i-th group and the i′-th group; ϑ min  indicates temporal recording of the smallest total UE number of two adjacent groups; and since the meanings of other characters have been described above in this description, we will not repeat here. 
     According to this specific but non-restrictive example, in case the i min -th group and i′ min -th group meet the condition, the corresponding elements Ω i     min    and Ω i′     min    are replaced by a intermedia value 
     
       
         
           
             
               
                 Ω 
                 
                   i 
                   min 
                 
               
               + 
               
                 Ω 
                 
                   i 
                   min 
                   ′ 
                 
               
             
             2 
           
         
       
     
     in the set Ω. 
     However, it is easy for those skilled in the art, based on the disclosure of current disclosure, to understand that there might be different ways to update the element of set Ω. And all those equivalent ways to update the set Ω should be seen as covered by the claims of this application. 
     In some examples, NOMA is applied to the group(s) with multiple UEs, to cancel the intra-group interference among UEs. 
     And in some examples, OMA is also applied to the group(s) with single UE. 
     Then, in step S 104 , the first apparatus sends beamformed CSI-RS in accordance with said at least one group. 
     Correspondingly, in step S 204 , the second apparatus receives beamformed CSI-RS from said BS. 
     And the second apparatus estimates the effective channels based on the beamformed CSI-RI. Since the UEs has been grouped into M groups, and the UEs in the same group are covered by the same transmitting precoding, the channel with N t  antenna ports are reduced to that with M (M&lt;N t ) beam ports. Therefore, the second apparatus only needs to report the effective channel of M dimensions. 
     And next, in step  205 , the second apparatus send CSI in accordance with said beamformed CSI-RS to said BS. 
     As being mentioned above, the CSI only needs to report the effective channel of M dimensions. 
     Then in step S 105 , the first apparatus receives CSI reports from UEs of said at least one groups, so as to transfer data to said at least one UE accordingly. 
     Specifically, the first apparatus receives CSI reports from UEs of said at least one group, conducts multi-group precoding to cancel the inter-group interference to obtains a final precoding matrix, so as to transfer data to said at least one UE accordingly. And those skilled in the art could understand that various algorithms, such as block diagonal (BD), can be used to determine the multi-group precoding. 
     Herein indicating FP as the final precoding matrix used by the first apparatus, and the FP is normalized to satisfy ∥FP∥ F   2 =M. In which F denotes the first stage matrix of precoders chosen by UEs received in step S 102 , and P is the matrix used for inter-group interference cancelation. 
     The first apparatus can obtain the matrix P through various methods. As with transmitting the first stage matrix F, the first apparatus can then send precoded reference signals (RS) for UEs to estimate the effective channel  H , which has the reduced dimension of only K×M. The reduced-rank matrix P can then be determined based on the effective channel information  H . The effective channel with the matrix F can be written as: 
     
       
         
           
             
               
                 
                   
                     H 
                     _ 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 v 
                                 
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                                 f 
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                                 H 
                                 
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                             … 
                           
                           
                             
                               
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                                   , 
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                                 H 
                                 
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                                     K 
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                                 H 
                               
                                
                               
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                                     1 
                                   
                                 
                               
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                             ⋮ 
                           
                           
                             
                                 
                             
                           
                           
                             
                                 
                             
                           
                           
                             ⋮ 
                           
                         
                         
                           
                             
                               
                                 v 
                                 
                                   M 
                                   , 
                                   
                                     K 
                                     M 
                                   
                                 
                                 H 
                               
                                
                               
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                                   , 
                                   
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                       ] 
                     
                     = 
                     
                         
                       
                         [ 
                         
                           
                             
                               
                                 
                                   
                                     H 
                                     _ 
                                   
                                    
                                   
                                       
                                   
                                 
                                 1 
                               
                             
                           
                           
                             
                               
                                 
                                   
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                                     _ 
                                   
                                    
                                   
                                       
                                   
                                 
                                 2 
                               
                             
                           
                           
                             
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                                   H 
                                   _ 
                                 
                                 M 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Herein the matrix  H  has K 1 +K 2 + . . . +K m =K rows in total, therefore,  H  is a matrix with a dimension of K×M, f m  is the m-th column vector of the first stage precoding matrix F in corresponding to the UEs in the m-th group. And to each  H   m , m=1, 2, . . . , M in the equation (3),  H   m  contains K m ×M elements. 
     In order to cancel the inter-group interference, at the m-th group, it is required that 
     
       
         
           
             
               
                 
                   
                     H 
                     ¯ 
                   
                   m 
                 
                  
                 
                   
                     ∑ 
                     
                       
                         i 
                         = 
                         1 
                       
                       , 
                       
                         i 
                         ≠ 
                         m 
                       
                     
                     M 
                   
                    
                   
                     
                       P 
                       i 
                     
                      
                     
                       
                         s 
                         ¯ 
                       
                       i 
                     
                   
                 
               
               = 
               0 
             
             . 
           
         
       
     
     In other words, the precoding for the i-th group needs to be orthogonal with channels of other groups, i.e.,  H   m P i =0 for all m≠i. To obtain P m , we perform singular vector decomposition (SVD) to the following matrix: 
     
       
         
           
             
               
                 
                   
                     
                       H 
                       ^ 
                     
                     m 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 H 
                                 ¯ 
                               
                               1 
                             
                           
                         
                         
                           
                             ⋮ 
                           
                         
                         
                           
                             
                               
                                 H 
                                 ¯ 
                               
                               
                                 m 
                                  
                                 ­ 
                                  
                                 1 
                               
                               : 
                             
                           
                         
                         
                           
                             
                               
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                                 ¯ 
                               
                               
                                 m 
                                 + 
                                 1 
                               
                             
                           
                         
                         
                           
                             ⋮ 
                           
                         
                         
                           
                             
                               
                                 H 
                                 ¯ 
                               
                               M 
                             
                           
                         
                       
                       ] 
                     
                     = 
                     
                       
                         U 
                         m 
                       
                        
                       
                         
                           ∑ 
                           m 
                         
                          
                         
                           
                             [ 
                             
                               
                                 
                                   
                                     
                                       V 
                                       m 
                                       
                                         ( 
                                         1 
                                         ) 
                                       
                                     
                                      
                                     
                                         
                                     
                                   
                                 
                                 
                                   
                                     V 
                                     m 
                                     
                                       ( 
                                       0 
                                       ) 
                                     
                                   
                                 
                               
                             
                             ] 
                           
                           H 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where V m   (1)  denotes the first r right singular vectors, where r is the rank of matrix Ĥ m  and V m   (0)  is the last M−r singular vector. It is notable that r&lt;M only when K−K m &lt;M, which may not hold. When r=M, V m   (0)  is set as the last singular vector. In this case, interference to other groups cannot be totally removed if K−K m &gt;M. Let Ū m   Σ   n   V   m   H =H m V m   (0)  the precoding vector P m  is then determined as the first singular vector in  V   m . 
     And after BD precoding process, the first apparatus conducts a process of data transmitting. 
     Firstly, the signal vector to be transmitted, could be given by: 
     
       
         
           
             
               
                 
                   
                     
                       s 
                       ~ 
                     
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 
                                   
                                     α 
                                     
                                       1 
                                       , 
                                       1 
                                     
                                   
                                    
                                   
                                     s 
                                     
                                       1 
                                       , 
                                       1 
                                     
                                   
                                 
                                 + 
                                 … 
                                 + 
                                 
                                   
                                     α 
                                     
                                       1 
                                       , 
                                       
                                         K 
                                         1 
                                       
                                     
                                   
                                    
                                   
                                     s 
                                     
                                       1 
                                       , 
                                       
                                         K 
                                         1 
                                       
                                     
                                   
                                 
                               
                             
                           
                           
                             
                               ⋮ 
                             
                           
                           
                             
                               
                                 
                                   
                                     α 
                                     
                                       M 
                                       , 
                                       1 
                                     
                                   
                                    
                                   
                                     s 
                                     
                                       M 
                                       , 
                                       1 
                                     
                                   
                                 
                                 + 
                                 … 
                                 + 
                                 
                                   
                                     α 
                                     
                                       M 
                                       , 
                                       
                                         K 
                                         1 
                                       
                                     
                                   
                                    
                                   
                                     s 
                                     
                                       M 
                                       , 
                                       
                                         K 
                                         1 
                                       
                                     
                                   
                                 
                               
                             
                           
                         
                         ] 
                       
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 
                                   s 
                                   ~ 
                                 
                                 1 
                               
                             
                           
                           
                             
                               ⋮ 
                             
                           
                           
                             
                               
                                 
                                   s 
                                   ~ 
                                 
                                 M 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     wherein the number of UEs is K, the number of said at least one groups is M, each group in the M groups has K m  users, m=1, 2, . . . , M; s m,k  is the signal and the α m,k  is the NOMA power allocation coefficient respectively for the k-th user in the m-th group, and the received signal is given by: 
         y   m,k   =v   m,k   H   H   m,k   FP{tilde over (s)}+v   m,k   H   n    (6);
 
     where v m,k  is the receiving beamforming vector for the k-th user in the m-th group, and H m,k  is the channel information for the k-th user in the m-th group, 
     Let P i  denote the i-th column of the matrix P, equation (6) can also be written as: 
     
       
         
           
             
               
                 
                   
                     
                       y 
                       
                         m 
                         , 
                         k 
                       
                     
                     = 
                     
                       
                         
                           v 
                           
                             m 
                             , 
                             k 
                           
                           H 
                         
                          
                         
                           H 
                           
                             m 
                             , 
                             k 
                           
                         
                          
                         
                           FP 
                           m 
                         
                          
                         
                           α 
                           
                             m 
                             , 
                             k 
                           
                         
                          
                         
                           s 
                           
                             m 
                             , 
                             k 
                           
                         
                       
                       + 
                       
                         
                           v 
                           
                             m 
                             , 
                             k 
                           
                           H 
                         
                          
                         
                           H 
                           
                             m 
                             , 
                             k 
                           
                         
                          
                         
                           FP 
                           m 
                         
                          
                         
                           
                             ∑ 
                             
                               i 
                               ≠ 
                               k 
                             
                           
                            
                           
                             
                               α 
                               
                                 m 
                                 , 
                                 i 
                               
                             
                              
                             
                               s 
                               
                                 m 
                                 , 
                                 i 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           v 
                           
                             m 
                             , 
                             k 
                           
                           H 
                         
                          
                         
                           H 
                           
                             m 
                             , 
                             k 
                           
                         
                          
                         F 
                          
                         
                           
                             ∑ 
                             
                               j 
                               ≠ 
                               m 
                             
                           
                            
                           
                             
                               P 
                               j 
                             
                              
                             
                               
                                 s 
                                 ~ 
                               
                               j 
                             
                           
                         
                       
                       + 
                       
                         
                           v 
                           
                             m 
                             , 
                             k 
                           
                           H 
                         
                          
                         n 
                       
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     With equations (3) to (5), the first apparatus could obtain the signal to interference and noise ratio (SINR) at UE, which is the k-th UE in the m-th group, and the SINR is given by: 
     
       
         
           
             
               
                 
                   
                     
                       SINR 
                       
                         m 
                         , 
                         k 
                       
                     
                     = 
                     
                       
                         
                           
                              
                             
                               
                                 v 
                                 
                                   m 
                                   , 
                                   k 
                                 
                                 H 
                               
                                
                               
                                 H 
                                 
                                   m 
                                   , 
                                   k 
                                 
                               
                                
                               
                                 FP 
                                 m 
                               
                             
                              
                           
                           2 
                         
                          
                         
                           α 
                           
                             m 
                             , 
                             k 
                           
                           2 
                         
                       
                       
                         
                           
                             ∑ 
                             
                               i 
                               ≠ 
                               k 
                             
                           
                            
                           
                             
                               
                                  
                                 
                                   
                                     v 
                                     
                                       m 
                                       , 
                                       k 
                                     
                                     H 
                                   
                                    
                                   
                                     H 
                                     
                                       m 
                                       , 
                                       k 
                                     
                                   
                                    
                                   
                                     FP 
                                     m 
                                   
                                 
                                  
                               
                               2 
                             
                              
                             
                               α 
                               
                                 m 
                                 , 
                                 k 
                               
                               2 
                             
                           
                         
                         + 
                         
                           
                             ∑ 
                             
                               j 
                               ≠ 
                               m 
                             
                           
                            
                           
                             
                                
                               
                                 
                                   v 
                                   
                                     m 
                                     , 
                                     k 
                                   
                                   H 
                                 
                                  
                                 
                                   H 
                                   
                                     m 
                                     , 
                                     k 
                                   
                                 
                                  
                                 
                                   FP 
                                   j 
                                 
                               
                                
                             
                             2 
                           
                         
                         + 
                         
                           
                             
                                
                               
                                 v 
                                 
                                   m 
                                   , 
                                   k 
                                 
                               
                                
                             
                             2 
                           
                           / 
                           ρ 
                         
                       
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     wherein ρ represents the transmit signal to noise ratio (SNR). 
     According to the embodiments of the present disclosure, with the hybrid of NOMA and OMA technical, the channel feedback overhead can be significantly reduced. For each of the K UEs, this hybrid scheme only needs to report M dimensional channel information plus one precoding index. Since the full channel feedback has to report the N t  dimensional channel information for K users, which is daunting in practice since the number N and K are both large. 
     By applying hybrid MIMO-NOMA and MIMO-OMA scheme, the channel feedback overhead is significantly reduced so that it is feasible in practice, and such scheme can achieve the high performance while reduce the implementation complexity. Also, the combination of inter-group interference cancelation by linear precoding and intra-group interference cancelation with NOMA can effectively deal with the interference at UE side and guarantee the performance. The combination of OMA and NOMA scheme as well as the proposed user grouping can simultaneously properly serve both correlated UEs (for which linear precoding cannot guarantee the performance), and the uncorrelated (orthogonal) UEs. 
     Considering a simple scenario to evaluate the proposed hybrid reduced-rank precoding for simplicity. Referring to  FIG. 3  and  FIG. 4 ,  FIG. 3  illustrates a downlink system with a BS and 4 single-antenna UEs (UE1-UE4). The BS is equipped with 4 antennas and serves 4 UEs simultaneously. The 4×4 DFT matrix is applied as the codebook of first stage precoding, that is, the matrix F in this case. In this scenario, 2 UEs (UE2 and UE3) are at the same direction, so that the channels are correlated, and the same precoding index is selected by these 2 UEs. That is these 2 UEs (UE2 and UE3) are in the same group. And also referring to  FIG. 3 , the rest 2 UEs (UE2 and UE3) are in the other 2 groups respectively. 
     And the block diagonal (BD) based precoding algorithm, which is explained in accordance with equations (5) to (8), is then applied to cancel the inter-group interference. The 2 UEs (UE2 and UE3) in separate groups are served by OMA processing and the 2 UEs (UE2 and UE3) in the same group are served by NOMA processing. While the power allocation is applied with NOMA processing is given by: 
     
       
         
           
             
               
                 
                   
                     α 
                     
                       m 
                       , 
                       k 
                     
                   
                   = 
                   
                     
                       
                         
                           ∏ 
                           
                             
                               i 
                               = 
                               1 
                             
                             , 
                             
                               i 
                               ≠ 
                               k 
                             
                           
                           
                             K 
                             m 
                           
                         
                          
                         
                           ( 
                           
                             
                               v 
                               
                                 m 
                                 . 
                                 i 
                               
                               H 
                             
                              
                             
                               H 
                               
                                 m 
                                 , 
                                 i 
                               
                             
                              
                             
                               f 
                               m 
                             
                           
                           ) 
                         
                       
                       
                         
                           ∑ 
                           
                             j 
                             = 
                             1 
                           
                           
                             K 
                             m 
                           
                         
                          
                         
                           ( 
                           
                             
                               ∏ 
                               
                                 
                                   i 
                                   = 
                                   1 
                                 
                                 , 
                                 
                                   i 
                                   ≠ 
                                   j 
                                 
                               
                               
                                 K 
                                 m 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   v 
                                   
                                     m 
                                     , 
                                     i 
                                   
                                   H 
                                 
                                  
                                 
                                   H 
                                   
                                     m 
                                     , 
                                     i 
                                   
                                 
                                  
                                 
                                   f 
                                   m 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                     
                      
                     
                       E 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Wherein E m  is the total transmission power for the m-th group. Successive interference cancelation receiver is applied in group with multiple UEs. 
     Further referring to  FIG. 4 ,  FIG. 4  illustrates a diagram of spectral efficiency comparison according to certain embodiments. Specifically,  FIG. 4  shows a comparison of spectral efficiency with certain SNRs between the 4 UEs in the hybrid scenario as shown in  FIG. 3 , and such 4 UEs in a pure OMA scenario. 
     The line  401  depicts the spectral efficiency (SE) of total 4 UEs in a hybrid scenario as shown in  FIG. 3 , line  402  depicts the SE of total 4 UEs in a pure OMA scenario, line  403  depicts the average SE of the two groups with a single UE in the hybrid scenario, line  404  depicts the SE of a stronger UE in the group with 2 UEs in the hybrid scenario, line  405  depicts the average SE of the two groups with a single UE in the pure OMA scenario, line  406  depicts the SE of a stronger UE (e.g. UE2, as it is closer to BS) in the group with 2 UEs in the pure OMA scenario, line  407  depicts the SE of the weaker UE (e.g. UE3, as it is less close to BS) in the group with 2 UEs in the hybrid scenario, line  408  depicts the SE of the weaker UE in the group with 2 UEs in the pure OMA scenario. 
     And it is obvious that applying hybrid NOMA and OMA can provides obvious benefits over pure OMA when some of the UEs are correlated. Also, the results validate the performance of the proposed reduced-rank precoding. And it also can be seen that the mean SE of the UEs in single UE group can achieve high performance in the proposed hybrid scenario. In addition to correlated users, applying hybrid NOMA and OMA can achieve higher performance than pure OMA scheme for those in single-user groups. 
     Referring to  FIG. 2 ,  FIG. 2  illustrates block diagram of a one possible and non-limiting exemplary system, in which the exemplary embodiments may be practiced. 
     Specifically,  FIG. 2  shows a block diagram of a simple system with both a first apparatus and a second apparatus. 
     Wherein the first apparatus comprises at least one processor  11 ; and at least one memory  12  including computer program code. The at least one memory  12  and the computer program code configured to, with the at least one processor  201 , cause the first apparatus to perform operation of at least the following: send codebook based sweeping information to at least one UE; receive Precoding Index reports from said at least one UE; group said at least one UE into at least one group, wherein the UEs in the same group are covered by the same transmitting precoding; send beamformed CSI-RS in accordance with said at least one group; receive CSI reports from UEs of said at least one group, so as to transfer data to said at least one UE accordingly. 
     The second apparatus, comprises at least one processor  21 ; and at least one memory  22  including computer program code, the at least one memory  22  and the computer program code configured to, with the at least one processor  21 , cause the second apparatus to perform operation of at least the following: receive codebook-based training information; select the best precoder in accordance with said codebook-based training information; send a precoding index reports to said BS, said precoding index reports including index of said best precoder; receive a beamformed CSI-RS from said BS; send a CSI in accordance with said beamformed CSI-RS to said BS. 
     The operations of the first apparatus and the second apparatus are similar with the steps that have been described above and will not repeated herein. 
     Also, a computer program product is disclosed. The computer program product comprising a non-transitory computer-readable medium storing computer program code thereon which when executed by a device causes the device to perform at least: send codebook based sweeping information to at least one UE; receive Precoding Index reports from said at least one UE; group said at least one UE into at least one group, wherein the UEs in the same group are covered by the same transmitting precoding; send beamformed CSI-RS in accordance with said at least one group; receive CSI reports from UEs of said at least one group, so as to transfer data to said at least one UE accordingly. 
     And a computer program product comprising a non-transitory computer-readable medium storing computer program code thereon which when executed by a device causes the device to perform at least: receive codebook-based training information; select the best precoder in accordance with said codebook-based training information; send precoding index reports to said BS, said precoding index reports including index of said best precoder; receive beamformed CSI-RS from said BS; send CSI in accordance with said beamformed CSI-RS to said BS. 
     For a person skilled in the art, it is apparent that the present disclosure is not limited to the details of the above exemplary embodiments, and the present disclosure can be implemented in other specific forms without departing from the spirit or essential characteristics of the present disclosure. Therefore, the embodiments should be regarded as exemplarily and not restrictive, and the scope of the present disclosure is defined by the appended claims rather than the above description, and therefore it is intended that the claims All changes that come within the meaning and range of equivalency of the invention are encompassed by the invention. Any reference signs in the claims should not be regarded as limiting the involved claims. In addition, it is clear that the word “comprising” does not exclude other units or steps, and the singular does not exclude the plural. The multiple units or devices recited in the system claims may also be implemented by one unit or device through software or hardware. Words such as first, second, etc., are used to indicate a name and do not indicate any specific order. 
     The following abbreviations that may he found in the specification and/or the drawing figures are defined as follows: 
     BD block diagonal 
     BS base station 
     CSI channel state information 
     MIMO massive multiple input multiple output 
     mMTC massive machine type connections 
     NOMA non-orthogonal multiple access 
     OMA orthogonal multiple access 
     RS reference signals 
     SE spectral efficiency 
     SINR signal to interference and noise ratio 
     SNR signal to noise ratio 
     SVD singular vector decomposition 
     UE user equipment