Abstract:
An apparatus and method for selecting at least two users to be serviced in SDMA in an SDMA wireless communication system are provided. All users are ordered according to a predetermined rule and all possible user combinations each having at least two users are created from the ordered users. An investigation limit number is set using the capacity of a j th  user combination. Or a total investigation number is reduced by comparing the interference-reflected SNR decrements of users with an investigation threshold. Therefore, the computation volume for selecting SDMA users is decreased.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “User Selection Apparatus And Method For SDMA In A MIMO System” filed in the Korean Intellectual Property Office on May 25, 2005 and assigned Serial No. 2005-43912, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a user selection apparatus and method for implementing Spatial Division Multiple Access (SDMA) in a Multiple Input Multiple Output (MIMO) system, and in particular, to an apparatus and method for selecting users with a reduced computation volume in a MIMO system supporting SDMA.  
         [0004]     2. Description of the Related Art  
         [0005]     With the recent advent of the wireless multimedia era, the rapid increasing demands for high-speed transmission of a large amount of data on radio channels have driven the worldwide development of wireless, high-speed data transmission systems to support Internet service on mobile channels and wireless channels.  
         [0006]     For mobile, wireless Internet services, MIMO has attracted a lot of attention as a prominent scheme of increasing data rate.  
         [0007]     The MIMO system, using a plurality of transmit and receive antennas, offers high data rates and outperforms a non-MIMO system in terms of the capacity of a radio link between a transmitter and a receiver. Under a multipath-rich environment, a plurality of orthogonal channels can be generated between the transmitter and the receiver. Hence, data for a single user can be sent by radio resources in parallel on the orthogonal channels with higher spectral efficiency, although using the same bandwidth as in the non-MIMO system.  
         [0008]     Furthermore, since the MIMO system supports SDMA, Signal-to-Noise Ratio (SNR) is improved and higher data rates are available.  
         [0009]      FIG. 1  is a flowchart illustrating a conventional operation for selecting SDMA users in a Base Station (BS) in a MIMO-SDMA system. The following description is based on the assumption that U SDMA users are selected from among K users using a single receive antenna (R=1), and the BS uses T antennas and has knowledge of channel information between the T antennas and the individual users. Notably, U is equal to or greater than 2 (U≧2).  
         [0010]     Referring to  FIG. 1 , the BS generates all possible combinations of U users from K users in step  101 .  
         [0011]     In step  103 , the BS calculates the capacity of a j th  user combination by Equation (1), Equation (2), Equation (3) and Equation (4) set forth below. j is the index of the total  K C U  combinations and its initial value is 1.  
         [0012]     A weight vector for the j th  user combination is calculated using the channel information H of the U users in the j th  user combination by 
 
 W   ZF   =H   H ( HH   H ) −1   =[W   1   , W   2   , . . . , W   U ]  (1) 
 
 where W ZF  denotes a zero-forcing weight vector and H denotes the total channel matrices [H 1 , H 2 , H 3 , . . . , H U ]T of the U users each having R receive antennas. Hu is the channel matrices between the T transmit antennas of the BS and the R receive antennas of a U th  user, [H 1   U , H 2   U , H 3   U , . . . , H R   U ] T , where H R   U  denotes the channel matrix between the T transmit antennas of the BS and an R th  receive antenna of the U th  user, [H R,1   U , H R,2   U , H  R,3   U , . . . , H  R,T   U ] T . H  R,T   U  denotes the channel between a T th  transmit antenna of the BS and the R th  receive antenna of the U th  user. 
 
         [0013]     Using the weight vector W ZF , a power scaling factor of each user is calculated by 
 
 P   u 1/| W   u | 2   (2) 
 
 where W u  denotes a weight column vector [W 1   u , W 2   u , . . . , W T   u ] T  applied to the T transmit antennas for a u th  user. 
 
         [0014]     The SNRs of the U users are calculated using their power scaling vectors. For the u th  user in the j th  user combination,  
               SNR   u     =           P   u     ⁢            S   u          2         σ   u   2       =                S   2          2     /            W   u          2         σ   u   2                 (   3   )             
 
 where P u  denotes the power scaling factor of the u th  user calculated by Equation (2), S u  denotes a transmission symbol transmitted in SDMA to the u th  user, and σ 2   u  denotes the noise power of the u th  user. 
 
         [0015]     Using the SNRs of the users, the total capacity of the j th  user combination, C j   total  is given by  
               C   j   total     =       ∑     u   =   1     U     ⁢       log   2     ⁡     (     1   +     SNR   u       )                 (   4   )             
 
         [0016]     After calculating the total capacity of the j th  user combination, the BS compares j with  k C u  to determine whether the capacity of every user combination has been calculated in step  105 . If j is less than  k C u  (j&lt; k C u ), which implies that there still remain user combinations whose capacities are to be calculated, the BS increases j by 1 (j=j+1) in step  107  and returns to step  103 .  
         [0017]     On the other hand, if j is equal to or greater than  k C u  (j≧ k C u ), which implies that the capacity of every user combination has been calculated, the BS selects a user combination that maximizes capacity and transmits data to the users of the selected user combination in SDMA in step  109 . Then the BS ends this algorithm.  
         [0018]     As described above, the conventional MIMO system generates all possible user combinations from total users to select SMDA users, calculates the capacity of every user combination, and selects a user combination offering the maximum capacity. Therefore, a large volume of computation is required.  
       SUMMARY OF THE INVENTION  
       [0019]     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 selecting SDMA users in a MIMO system.  
         [0020]     Another object of the present invention is to provide an apparatus and method for reducing the computation volume of SDMA user selection in a MIMO system.  
         [0021]     According to one aspect of the present invention, in an apparatus for selecting U users (U≧2) to be serviced in SDMA in an SDMA wireless communication system, a channel information storage stores channel information of total users. A user selector generates user combinations each having U users from the total users according to the channel information, calculates an investigation limit number according to the capacities of the user combinations, and selects users to be serviced in SDMA according to the investigation limit number.  
         [0022]     According to another aspect of the present invention, in an apparatus for selecting U users (U≧2) to be serviced in SDMA in an SDMA wireless communication system, a channel information storage stores channel information of total users. A user selector generates user combinations each having U users from the total users according to the channel information, and selects users to be serviced in SDMA by comparing interference-reflected SNR decrements of the user combinations with an investigation threshold.  
         [0023]     According to a further aspect of the present invention, in a method of selecting U users (U≧2) to be serviced in SDMA in an SDMA wireless communication system, total users are ordered according to channel status, user combinations each having U users are generated from the ordered users, and the capacity of a j th  user combination is calculated. An investigation limit number is determined by comparing interference-reflected SNRs of the users of the j th  user combination with maximum SNRs of the total users, if the capacity of the j th  user combination is greater than the capacities of previous user combinations. The investigation limit number is compared with j and, if j is equal to or greater than the investigation limit number, a user combination with the largest capacity is selected.  
         [0024]     According to still another aspect of the present invention, in a method of selecting U users (U≧2) to be serviced in SDMA in an SDMA wireless communication system, a maximum investigation number and an investigation threshold (SNR (Deg) ) are set. Total users are ordered according to channel status, user combinations each having U users are generated from the ordered users, and an SNR decrement (SNR u(Deg) ) of a j th  user combination is calculated. It is determined whether SNR decrements of all users of the j th  user combination have been compared with the investigation threshold, if SNR u(Deg)  is equal to or greater than SNR (Deg) . The j th  user combination is selected as a user combination having the largest capacity, if the SNR decrements of all users of the j th  user combination have been compared with the investigation threshold. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     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:  
         [0026]      FIG. 1  is a flowchart illustrating a conventional operation for selecting SDMA users in a BS in a MIMO system;  
         [0027]      FIG. 2  is a block diagram of a BS for implementing SDMA according to the present invention;  
         [0028]      FIG. 3  is a flowchart illustrating an operation for selecting SDMA users in the BS in the MIMO system according to the present invention; and  
         [0029]      FIG. 4  is a flowchart illustrating an operation for selecting SDMA users in the BS in the MIMO system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     Preferred 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 since they would obscure the invention in unnecessary detail.  
         [0031]     The present invention is directed to a method of selecting SMDA users in a BS in a MIMO communication system.  
         [0032]      FIG. 2  is a block diagram of a BS for implementing SDMA according to the present invention.  
         [0033]     Referring to  FIG. 2 , to implement SDMA by selecting U users from K users in a BS using T antennas, a user selector  200  selects data for U users from data destined for the K users based on the channel information of the K users received from a channel information storage  202  and calculates a weight vector for the selected U users, which will be described later in more detail with reference to  FIGS. 3 and 4 . Encoders  203  to  204  encode the data for the U users at a predetermined coding rate.  
         [0034]     Modulators  205  to  206  modulate the coded data of the U users in a predetermined modulation scheme. The modulation scheme can be one of Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), Pulse Amplitude Modulation (PAM), and Phase Shift Keying (PSK).  
         [0035]     A weight multiplier  207  multiplies the modulated data by the weight vector received form the user selector  200 . A channel divider  209  divides the weighted data by the number of the transmit antennas T in order to transmit the data separately through the T antennas.  
         [0036]     After passing through filters  211  to  212 , the divided signals are converted to analog signals by Digital to Analog (D/A) converters  213  to  214 , converted to Radio Frequency (RF) signals by RF processors  215  to  216 , and then transmitted through the antennas.  
         [0037]     Now a description will be made of a method of reducing the number of investigated user combinations in order to reduce the computation volume required for SDMA user selection in the MIMO system, and a method of using an SNR decrease caused by channel correlation. A user combination is a group of U users selected from K users, for SDMA.  
         [0038]     The method of reducing the computation volume required for SDMA user selection by decreasing the number of user combinations to be investigated is shown in  FIG. 3 .  
         [0039]      FIG. 3  is a flowchart illustrating an operation for selecting SDMA users in the BS in the MIMO system according to the present invention. It is assumed herein that U users are selected from K users each having a single antenna to implement SDMA, and the BS uses T antennas (THU) and has knowledge of the channel information between the T antennas and each user. U is equal to or greater than 2 (U≧2).  
         [0040]     Referring to  FIG. 3 , the BS performs initialization to select SDMA users in step  301 . For example, the K users are ordered in a descending order of SNR, SNR k   Pre  or channel power, |H l   k | 2  and a maximum investigation number N iter  is set to the total number of user combinations,  K C U . The index of a user combination with the maximum capacity, J and the maximum capacity C MAXx   Total  are each set to 0, and an index j indicating a user combination is set to an initial value of 0.  
         [0041]     After the initialization, the BS generates all possible user combinations each having U users from the K users in step  303 .  
         [0042]     In step  305 , the BS calculates the capacity of the j th  user combination by Equation (1), Equation (2), Equation (3) and Equation (4). Specifically, the weight vector W ZF  of the j th  user combination is calculated using the channel information H of the U users of the j th  user combination by Equation (1) and the power scaling factors P u  of the individual users in the j th  user combination are calculated by Equation (2). Then the SNRs of the users, SNR u  are calculated based on the power scaling factors P u , Finally, the total capacity of the j th  user combination, C j   total  is calculated using SNF u  by Equation (4).  
         [0043]     In step  307 , the BS compares C j   total  with C MAX   Total . If C j   total  is equal to or less than C MAX   Total  (C MAX   Total ≧C j   total ), the BS proceeds to step  309 . If C j   total  is greater than C MAX   Total  (C MAX   Total ≦C j   total ), the BS sets J to j (J=j) and C j   total  to C MAX   Total  (C MAX   Total =C j   total ) in step  315 .  
         [0044]     In step  317 , the BS selects an investigation limit number N j . Each of the SNRs of the users in the j th  user combination is compared with the SNRs of the K users ordered in a descending order, SNR Pre  and users having SNR Pre  equal to or greater than SNR u  (SNR Pre ≧SNR u ) are selected. SNR u  is the SNR of the u th  user of the j th  user combination when SDMA is provided to the users of the j th  user combination, and SNR Pre  is the SNR of the u th  user when a multiple-antenna service is provided only to the u th  user.  
         [0045]     For each user of the j th  user combination, a user having the highest user index, K u , is chosen from among the selected users. For example, given two users, user  1  and user  2 , in the j th  user combination, a user having the highest index, K 1 , is selected among the users having SNR Pre  equal to or greater than SNR u , for user  1  and a user having the highest index, K 2  is selected among the users having SNR Pre  equal to or greater than SNR u , for user  2 . The index of a user combination including the users K u  is set as the investigation limit number, N j . K u &lt;K u+1 (u=1, 2, . . . , U−1). If U=2, K 1  must be less than K 2 . If K 1  is greater than K 2 , N j  is set to a one level-higher user combination index. The user combination with this index includes k 1 .  
         [0046]     In step  319 , the BS compares N iter  with N j . If N j ≧N iter , the BS goes to step  309 . If N j &lt;N iter , the BS sets N iter  to N j  in step  321 .  
         [0047]     In step  309 , the BS compares j with N iter . If j&lt;N iter , the BS increases j by 1 (j=j+1) in step  313  and returns to step  305 . If j≧N iter , the BS selects a user combination with the maximum capacity in step  311 . The BS then sends data to the users of the selected user combination in SDMA and ends this algorithm.  
         [0048]     Now a description will be made of a method of reducing the computation volume required for selecting SDMA users by setting a threshold SNR (Deg)  for discontinuing investigation of users combinations based on a channel correlation-incurred SNR decrease. This method is described in  FIG. 4 .  
         [0049]      FIG. 4  is a flowchart illustrating an operation for selecting SDMA users in the BS in the MIMO system according to the present invention. It is assumed herein that U users are selected from K users each having a single antenna in order to implement SDMA, and the BS uses T antennas and has knowledge of the channel information between the T antennas and each user. U is equal to or greater than 2 (U≧2). A maximum investigation number N iter  is less than the total number of user combinations,  K C U  (N iter &lt; K C U ) and can be adjusted appropriately according to the computation ability of the BS.  
         [0050]     Referring to  FIG. 4 , the BS performs initialization to select SDMA users in step  401 . For example, the K users are ordered in a descending order of SNR, SNR k   Pre  or channel power, |H 1   k | 2  and the maximum investigation number N iter  is set. A threshold SNR (Deg)  for stopping investigation based on a channel correlation-incurred SNR decrease is set, the index of a user combination with the maximum capacity, J, and a maximum capacity C MAX   total  are each set to 0, and an index j indicating a user combination is set to an initial value of 0.  
         [0051]     After the initialization, the BS generates all possible user combinations each having U users from the K users in step  403 .  
         [0052]     In step  405 , the BS calculates the channel correlation-incurred SNR decrement of the j th  user combination by Equation (1), Equation (2), Equation (3) and Equation (5). Specifically, the weight vector W ZF  of the j th  user combination is calculated using the channel information H of the U users of the j th  user combination by Equation (1) and the power scaling factors P u  of the individual users in the j th  user combination are calculated by Equation (2). Then the SNRs of the users, SNR u  are calculated based on the power scaling factors P u . Finally, SN u  is expressed in dB by  
                       SNR   u     ⁡     (   dB   )       =       10   ⁢           ⁢     log   ⁡     (                H   1   u          2     ⁢            S   u          2         σ   u   2       )         +     10   ⁢     log   ⁡     (     1              W   1   u          2     ⁢            H   1   u          2         )                       =         SNR     u   ⁡     (   MAX   )         ⁡     (   dB   )       +       SNR     u   ⁡     (   Deg   )         ⁡     (   dB   )                       (   5   )             
 
 where SNR u (dB) denotes SNR u  in dB, H 1   u  denotes the channels between the T antennas and a first receive antenna of the u th  user, S u  denotes a transmission symbol transmitted to the u th  user in SDMA, σ u   2  denotes the noise power of the u th  user, W u  denotes a weight factor used for the u th  user by the BS, SNR u(MAX) (dB) denotes the maximum SNR of the u th  user, and SNR u(Deg) (dB) denotes the channel correlation-incurred SNR decrement of the u th  user. Here, u denotes a user index in the j th  user combination and its initial value is 1. 
 
         [0053]     After calculating SNR u(Deg) , the BS compares SNR (Deg)  with SNR u(Deg)  in step  407 . SNR u(Deg)  is a negative value. If SNR u(Deg) ≧SNR (Deg) , the BS increases u by 1 in step  419 .  
         [0054]     In step  421 , the BS compares the increased u with U. If u≦U, the BS returns to step  407  and calculates the SNR decrements of the remaining users of the j th  user combination. If u&gt;U, that is, if the SNR decrements of the users in the j th  user combination are less than or equal to the threshold, the BS sets J to j (J=1) in step  423  and goes to step  415 .  
         [0055]     If SNR u(Deg) &lt;SNR (Deg) , the BS calculates the capacity C j   total  of the j th  user combination by computing Equation (4) using the SNRs of the users of the j th  user combination calculated by Equation (3) in step  409 .  
         [0056]     In step  411 , the BS compares C j   total  with C MAX   Total . If C j   total  is equal to or less than C MAX   Total  (C MAX   Total ≧C j   total ), the BS proceeds to step  413 . If C j   total  is greater than C MAX   Total  (C MAX   Total &lt;C j   total ), the BS sets J to j (J=j) and C j   total  to C MAX   Total  (C MAX   Total =C j   total ) in step  425 .  
         [0057]     In step  413 , the BS compares j with N iter . If j is less than N iter (j&lt;N iter ), the BS increases j by 1 in step  417  and returns to step  405 . If j is equal to or greater than N iter  (j≧N iter ), the BS selects the user combination J with the largest capacity and sends data to the users of the selected user combination in SDMA in step  415 . Then the BS terminates the algorithm.  
         [0058]     In accordance with the present invention as described above, the BS in the MIMO system limits the number of user combinations to be investigated based on SNR or SNR decrement in selecting SDMA users. Therefore, the computation volume involved in the user selection can be reduced.  
         [0059]     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.