Patent Publication Number: US-9414241-B2

Title: Method and apparatus for outer loop link adaptation for a wireless communication system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/CN2012/073118 filed Mar. 27, 2012, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention generally relates to a communication system, particularly to a method and apparatus for outer loop link adaptation (OLLA) in a wireless communication system. 
     DESCRIPTION OF THE RELATED ART 
     The Third Generation Partnership Project (3GPP) Standard for Long Term Evolution (LTE)/LTE-Advanced (LTE-A), also known as the evolution standard of the great success of 3 rd  Generation technology, is aiming at creating a new series of specifications for the new evolving radio-access technology, which is able to provide higher bandwidth, lower latency, and better quality of service (QoS) guarantees. 
     In the cellular communication systems such as LTE/LTE-A systems, adaptive modulation coding (AMC) technology is adopted to suppress radio channel time variance. It is known that in so-called inner loop link adaptation (as opposed to outer loop link adaptation) methods, a quantized indication of channel quality such as Channel Quality Indicator (CQI) is reported from a user equipment (UE) to a corresponding base station such as an eNode B (eNB). The eNB estimates signal to interference plus noise ratio (SINR) of the UE based on the reported indication of channel quality such as CQI. The eNB establishes a mapping relationship between the levels of SINR and modulation and coding schemes (MSCs) in advance. As such, the eNS can determine a proper MCS for the uplink/downlink transmission of the UE by comparing the estimated SINR with the predetermined thresholds of SINR. 
     However, the estimated SINR may not be accurate due to some factors of un-modeled interferences and error, such as CQI estimation errors, CQI report and process delay, etc. This results in improper MCS selection, which then lowers down system spectrum efficiency. 
     As an effective way to compensate for the above-mentioned inaccuracy which may occur in the procedure of inner loop link adaptation, so-called outer loop link adaptation (OLLA) is adopted to add an offset to the estimated SINR. The OLLA offset is adjusted based on information of UE-reported acknowledgement (ACK) or non-acknowledgement (NACK) of transmitted packets. Accordingly, it facilitates keeping accumulated frame error rate (FER) around a predefined value. The MCS selection with OLLA-adjusted SINR is more accurate and therefore the spectrum efficiency of the communication system is improved. 
     Some wireless communication scenarios for example the Transmission Mode 8 of LTE Releases 9 and 10 allow UEs to switch dynamically between single-user multiple input multiple output (SU-MIMO) transmission mode and multi-user multiple input multiple output (MU-MIMO) transmission mode. Due to interlaced transmission periods between SU-MIMO and MU-MIMO experienced by the UE, the existing OLLA scheme cannot track the SINR offsets effectively for both transmission modes. In addition, the varying inter-user interferences are introduced in the MU-MIMO transmission mode by dynamic UE pairing, which is, however, not considered in the existing OLLA scheme. 
     Considering the facts as mentioned above, the existing OLLA scheme is not effective in the scenarios which allow UEs to switch between SU-MIMO and MU-MIMO transmission modes. 
     SUMMARY OF THE INVENTION 
     To solve the problems in the prior art, one or more method and apparatus embodiments according to the present invention aim to provide an OLLA scheme of a wireless communication system for the application scenarios where hybrid SU-MIMO and MU-MIMO transmission modes are supported and to provide more reliable MCS estimation and improve the system throughout. 
     According to an aspect of the present invention, an embodiment of the present invention provides a method for link adaptation in a wireless communication system. The method comprises: determining whether a UE operates in a SU-MIMO transmission mode or in a MU-MIMO transmission mode; applying, in responsive to determination that the UE operates in the SU-MIMO transmission mode, a SU-MIMO OLLA offset to a signal to interference and noise ratio adjusted with inner loop link adaptation; applying, in responsive to determination that the UE operates in the MU-MIMO transmission mode, a MU-MIMO OLLA offset to a signal to interference and noise ratio adjusted with inner loop link adaptation, wherein the MU-MIMO OLLA offset is based on the SU-MIMO OLLA offset and an extra offset which is indicative of inter-user interference, wherein the SU-MIMO OLLA offset and the extra offset are updated in responsive to receipt of an ACK/NACK message for a packet reported from the UE. 
     According to another aspect of the present invention, an embodiment of the present invention provides an apparatus for link adaptation in a wireless communication system. The apparatus comprises: means for determining whether a UE operates in a SU-MIMO transmission mode or in a MU-MIMO transmission mode; means for applying, in responsive to determination that the UE operates in the SU-MIMO transmission mode, a SU-MIMO OLLA offset to a signal to interference and noise ratio adjusted with inner loop link adaptation; means for applying, in responsive to determination that the UE operates in the MU-MIMO transmission mode, a MU-MIMO OLLA offset to a signal to interference and noise ratio adjusted with inner loop link adaptation, wherein the MU-MIMO OLLA offset is based on the SU-MIMO OLLA offset and an extra offset which is indicative of inter-user interference, wherein the apparatus further comprises means for updating the SU-MIMO OLLA offset and the extra offset are in responsive to receipt of an ACK/NACK message for a packet reported from the UE. 
     According to further aspect of the present invention, an embodiment of the present invention provides a base station in a wireless communication system performing the method for link adaptation according to an embodiment of the present invention. 
     According to further aspect of the present invention, an embodiment of the present invention provides a wireless communication system comprising a base station according to an embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Inventive features regarded as the characteristics of the present invention are set forth in the appended claims. However, the present invention, its implementation mode, other objectives, features and advantages will be better understood through reading the following detailed description on the exemplary embodiments with reference to the accompanying drawings, where in the drawings: 
         FIG. 1  illustrates a flow chart of a procedure of link modulation and coding adaptation according to an embodiment of the present invention; 
         FIG. 2  illustrates a flow chart of a method of outer loop link adaptation according to an embodiment of the present invention; 
         FIG. 3  schematically illustrates a block diagram of OLLA offset generating module according to an embodiment of the present invention; 
         FIG. 4  illustrates a flow chart of a procedure of updating OLLA offsets according to an embodiment of the present invention; and 
         FIG. 5  schematically illustrates a block diagram of an eNB device according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, many specific details are illustrated so as to understand the present invention more comprehensively. However, it is apparent to the skilled in the art that implementation of the present invention may not have these details. Additionally, it should be understood that the present invention is not limited to the particular embodiments as introduced here. On the contrary, any arbitrary combination of the following features and elements may be considered to implement and practice the present invention, regardless of whether they involve different embodiments. Thus, the following aspects, features, embodiments and advantages are only for illustrative purposes, and should not be understood as elements or limitations of the appended claims, unless otherwise explicitly specified in the claims. 
       FIG. 1  illustrates a flow chart of a procedure of link modulation and coding adaptation according to an embodiment of the present invention. 
     At step S 100 , the processing at an eNB for link modulation and coding adaptation starts. 
     At step S 110 , a UE-specific SINR for each of RBs assigned to a UE is estimated based on a quantized indication of channel quality such as CQI reported from a UE. 
     In the cellular communication systems with MIMO technologies, multiple antennas are used at both the transmitter and receiver. Spatial multiplexing results in increased data rate in bandwidth limited scenarios by creating several parallel “channels” between the transmitting antennas and the receiving antennas. The term “layer” or “transmission layer” refers to each of channels between the transmitting antennas and the receiving antennas. And the term “stream” or “data stream” refers to data transmitted on a transmission layer. In particular, in the MU-MIMO transmission mode, multiple users can share the parallel transmission layers on the same time-frequency resource by combining the spatial properties with the appropriate interference suppressing and receiver processing in order to improve the overall cell capacity. 
     At step S 120 , the estimated SINR is adjusted with inner loop link adaptation. 
     There are various algorithms of inner loop link adaptation in the art, some of which have been prevailed in the field and some of which may be developed for future use, etc. It is appreciated that the OLLA scheme according to the embodiments of the present invention can be used in conjunction with any kind of inner loop link adaptation without any limitation. 
     At step S 130 , a procedure of the outer link loop adaptation according to an embodiment of the present invention is performed, to obtain OLLA-adjusted SINR for each of transmission layers. 
     As discussed previously, some wireless communication scenarios for example Transmission Mode 8 of LTE Releases 9 and 10 allow UEs to switch dynamically between the SU-MIMO transmission mode and the MU-MIMO transmission mode. In the outer link loop adaptation scheme according to an embodiment of the present invention, separate OLLA offsets for the SU-MIMO and MU-MIMO transmission modes are adopted to track the SINR offsets effectively for both transmission modes. In the outer link loop adaptation scheme according to an embodiment of the present invention, it is proposed that a MU-MIMO OLLA offset for a transmission layer is based on the SU-MIMO OLLA offset for the transmission layer and an extra offset which is indicative of inter-user interference; and ACK/NACK messages for SU and MU packets are used to trigger OLLA offset updating procedure. The detailed procedure of OLLA according to embodiments of the present invention will be set forth with reference to  FIGS. 2-4 . 
     At step S 140 , an effective SINR per layer is calculated based on the OLLA-adjusted SINRs. 
     In the cellular communication systems, the eNB assigns resource blocks (RB) to UEs. A RB refers to a group of subcarriers over one subframe. And each UE can be assigned with multiple RBs for transmission. The eNB needs to determine the number of transmission layers of the UE and an effective SINR per layer. 
     The OLLA offset of SINR is applied before getting the effective SINR according to an embodiment of the present invention. On different RBs assigned to a UE, the UE might have different paring UEs. According to the strength of the inter-user interference, a different OLLA offsets depending on different paring manners can be applied to the SINR of the RB and then these OLLA-adjusted SINRs of the assigned RBs can be mapped into an effective SINR. Those skilled in the art may appreciate that it will be advantageous that the effective SINR is calculated taken into UEs&#39; pairing matters in the MU-MIMO transmission mode. 
     At step S 150 , the calculated effective SINR is passed into the MCS look up table to find a suitable MCS for transmission. 
     At step S 160 , the processing ends. 
       FIG. 2  illustrates a flow chart of a method of outer loop link adaptation according to an embodiment of the present invention. 
     At step S 210 , it is determined whether the UE operates in a SU-MIMO transmission mode or in a MU-MIMO transmission mode. 
     At step S 220 , if it is determined that the UE operates in the SU-MIMO transmission mode, a SU-MIMO OLLA offset is applied to the SINR adjusted with inner loop link adaptation. 
     At step S 230 , if it is determined that UE operates in the MU-MIMO transmission mode, a MU-MIMO OLLA offset is applied to the SINR adjusted with inner loop link adaptation. 
     The SU-MIMO OLLA offset and MU-MIMO OLLA offset are determined by an OLLA offset generating module, wherein the MU-MIMO OLLA offset is determined based on the SU-MIMO OLLA offset and an extra offset which is indicative of inter-user interference and the SU-MIMO OLLA offset and the extra offset are updated in responsive to receipt of an ACK/NACK message for a packet reported from said UE. The functionality of the OLLA offset generating module will be discussed in details in conjunction with  FIG. 3 . 
       FIG. 3  schematically illustrates a block diagram of OLLA offset generating module according to an embodiment of the present invention. 
     As shown in  FIG. 3 , reference numeral  300  denotes an OLLA offset generating module, which comprises an OLLA update unit  310 , an OLLA memory unit  320  and an OLLA output unit  330 . 
     The OLLA memory unit  320  is used to maintain parameters and values used to generate OLLA offsets. According to an embodiment of the present invention, the OLLA memory unit  320  maintains, for each of transmission layers, a respective SU-MIMO OLLA offset, which can be referred to as β r , r=0, 1, 2, . . . , L, and a respective extra offset, which can be referred to as α r , r=0, 1, 2, . . . , L, wherein L is supported max number of transmission layers based on the UE capacity and the eNB configuration. Specifically, β 0  is the SU-MIMO OLLA offset for single-layer transmission, while β r , r=1, 2, . . . , L, is the SU-MIMO OLLA offset for the corresponding r th  transmission layer when there are multiple layers. Similarly, α 0  is the extra offset of MU-MIMO transmission mode for single-layer transmission, while α r , r=1, 2, . . . , L, is the extra offset for the corresponding r th  transmission layer when there are multiple layers. 
     According to an embodiment of the present invention, the OLLA memory unit  320  may maintain parameters which are utilized by the other units. In an exemplary implementation, the OLLA memory unit  320  may maintain the step sizes D step   _   up  and D step   _   down  for increasing and decrementing the OLLA offsets in the OLLA update unit  310 . D step   _   up  and D step   _   down  may be constant or adaptive in responsive to the accumulated FER. In another exemplary implementation, the OLLA memory unit  320  may maintain a constant scalar C scale , which is utilized by the OLLA output unit  330  to adjust the weight of the extra offset in the outputted MU-MIMO OLLA offset and will be discussed below. 
     The OLLA update unit  310  is used to update the SU-MIMO OLLA offset and the extra offset in responsive to receipt of an ACK/NACK message for a packet reported from a UE. Specifically, the OLLA update unit  310  updates a SU-MIMO OLLA offset β r , r=0, 1, 2, . . . , L, and a extra offset α r , r=0, 1, 2, . . . , L, in responsive to an ACK/NACK message. 
     Now, reference is made to  FIG. 4  to set forth an exemplary procedure performed by the OLLA update unit  310  according to a preferred embodiment of the present invention. 
     As shown in  FIG. 4 , at step S 400 , the processing starts. 
     At step S 410 , an ACK/NACK message for a packet from a UE is received at the eNB. 
     At step S 420 , it is determined whether the UE sending the ACK/NACK message operates in the SU-MIMO transmission mode or in the MU-MIMO transmission mode. 
     In an exemplary implementation, the determination can be performed in such a matter that the eNB keeps records of whether packets are in the SU-MIMO transmission mode or in the MU-MIMO transmission mode and when the ACK/NACK for a packet is received, the eNB can make the determination and then carry out corresponding OLLA offset update steps. 
     If it is determined at step S 420  that the UE operates in the SU-MIMO transmission mode, the processing proceeds with steps S 430 -S 434 . 
     At step  430 , it is determined whether the UE is configured with a single-layer or multiple layers. 
     If it is determined that the US is configured with a single transmission layer, at steps S 431  and S 432 , SU-MIMO OLLA offsets for all transmission layers with which said UE is configured are updated. Specifically, at step S 431 , in responsive to receipt of an ACK message of a packet from the UE, the SU-MIMO OLLA offsets for all transmission layers, i.e., β r , r=0, 1, 2, . . . , L, which are maintained in the OLLA memory unit  320 , are increased by D step   _   up . At step S 432 , in responsive to receipt of a NACK message of a packet from the UE, the SU-MIMO OLLA offsets for all transmission layers, i.e., β r , r=0, 1, 2, . . . , L, which are maintained in the OLLA memory unit  320 , are decreased by D step   _   down . 
     If it is determined that the UE is configured with multiple transmission layers, at steps S 433  and S 434 , only the SU-MIMO OLLA offset for the transmission layer on which the ACK/NACK message is reported is updated. Specifically, at step S 433 , in responsive to receipt of an ACK message of a packet from the UE, the SU-MIMO OLLA offset for the transmission layer on which the ACK message is reported, i.e., β r , rεR, R={r: r th -layer is ACKed}, which is maintained in the OLLA memory unit  320 , is increased by D step   _   up . At Step S 434 , in responsive to receipt of a NACK message of a packet from the UE, the SU-MIMO OLLA offset for the transmission layer on which the NACK message is reported, i.e., β r , rεS, S={r:r th -layer is NACKed}, which is maintained in the OLLA memory unit  320 , is decreased by D step   _   down . 
     If it is determined at step S 420  the UE operates in the MU-MIMO transmission mode, the processing proceeds with steps S 440 -S 449 . 
     At step  940 , it is determined whether the UE is configured with a single-layer or multiple layers. 
     If it is determined that the UE is configured with a single transmission layer, at steps S 441  and S 442 , SU-MIMO OLLA offsets and extra offsets for all transmission layers with which said UE is configured are updated. Specifically, at step S 441 , in responsive to receipt of an ACK message of a packet from the UE, the SU-MIMO OLLA offsets for all transmission layers, i.e., β r , r=0, 1, 2, . . . , L, and the extra offsets for all transmission layers, i.e., α r , r=0, 1, 2, . . . , L, which are maintained in the OLLA memory unit  320 , are all increased by D step   _   up . At step S 442 , in responsive to receipt of a NACK message of a packet from the UE, the SU-MIMO OLLA offsets for all transmission layers, i.e., β r , r=0, 1, 2, . . . , L, and the extra offsets for all transmission layers, i.e., α r , r=0, 1, 2, . . . , L, which are maintained in the OLLA memory unit  320 , are all decreased by D step   _   down . 
     If it is determined that the UE is configured with multiple transmission layers, at steps S 993  and S 5999 , only the extra offset for the transmission layer on which the ACK/NACK message is reported is updated. Specifically, at step S 443 , in responsive to receipt of an ACK message of a packet from the UE, the extra offset for the transmission layer on which the ACK message is reported, i.e., α r , rεR, R={r: r th -layer is ACKed}, which is maintained in the OLLA memory unit  320 , is increased by D step   _   up . At Step S 444 , in responsive to receipt of a NACK message of a packet from the UE, the extra offset for the transmission layer on which the NACK message is reported, i.e., α r , rεS, S={r: r th -layer is NACKed}, which is maintained in the OLLA memory unit  320 , is decreased by D step   _   down . 
     At step S 450 , the processing ends. 
     The update step sizes D step   _   up  and D step   _   down  can be predefined and maintained in the OLLA Memory unit  320 . According to an alternative implementation of the embodiment of the present invention, The update step sizes D step   _   up  and D step   _   down  is defined to meet following equation:
 
 D   step   _   down   =D   step   _   up /(1/FER−1),
 
     wherein FER denotes a predefined value of Frame Error Rate. 
     According to the update procedure as shown in  FIG. 4 , it can be noted that for transmission with multiple layers, only those offset corresponding to those specific layers are updated, while for single-layer transmission, offsets for all of the transmission layers are updated. The reason for doing so is that although only a single layer is configured of the concerned UE during single layer transmission, the fluctuations of the time varying channel experienced by the UE on this layer are also true to all of the layers if there are multiple layers configured. As such, the procedure helps the eNB to keep the OLLA offsets for multiple layers correctly tracking the channel condition during single layer transmissions, which is important for the OLLA effectiveness. 
     Furthermore, according to the update procedure as shown in  FIG. 4 , it can also be noted that in the single-layer transmission case, for an ACK/NACK message of the SU-MIMO packets, the SU-MIMO OLLA offsets for all transmission layers, i.e., β r , r=0, 1, 2, . . . , L, are updated, while for an ACK/NACK massage of the MU-MIMO packets, both the SU-MIMO OLLA offsets for all transmission layers, i.e., β r , r=0, 1, 2, . . . , L, and the extra offsets for all transmission layers, i.e., α r , r=0, 1, 2, . . . , L, are updated. As such, during the interlaced transmission periods between SU-MIMO and MU-MIMO transmission modes, even if the UE currently operates in MU-MIMO transmission mode, the eNB is able to update the SU-MIMO OLLA offsets based on ACK/NACK message of the MU-MIMO packets. Therefore the OLLA offsets for SU-MIMO can still track the channel fluctuations even during MU-MIMO transmission period. This is especially important for quick convergence of the OLLA offsets when a UE switches from MU-MIMO transmission mode to SU-MIMO transmission mode. 
     Returning now to  FIG. 3 , the OLLA output unit  330  generates OLLA offsets (dB) used in the SU-MIMO and MU-MIMO transmission modes based on the OLLA offset values and parameters maintained in the OLLA memory unit  320 , wherein the SU-MIMO OLLA offset δ 0   SU  for the single transmission layer can be denoted as:
 
δ 0   SU =β 0 ,  1)
 
     the SU-MIMO OLLA offset δ r   SU  for the corresponding r th  transmission layer, when there are multiple layers, can be denoted as:
 
δ r   SU =β r   , r= 1,2, . . . , L;   2)
 
     the MU-MIMO OLLA offset δ 0   MU  for the single transmission layer can be denoted as:
 
δ 0   MU =α 0 +β 0 ;  3)
 
     the MU-MIMO OLLA offset δ r   MU  for the corresponding r th  transmission layer, when there are multiple layers, can be denoted as:
 
δ r   MU =α r +β r   , r= 1,2, . . . , L,   4)
 
     wherein L is supported max number of transmission layers based on the UE capacity and the eNB configuration. 
     For the purpose of simplicity, equations 1) and 2) can be combined and denoted as:
 
δ r   SU =β r   , r= 1,2, . . . , L;   5)
 
     and 
     equations 3) and 4) can be combined and denoted as:
 
δ r   MU α r +β r   , r= 0,1,2, . . . , L,   6)
 
     In a preferred embodiment of the present invention, the OLLA output unit  330  can determine the MU-MIMO OLLA offset for corresponding transmission layer by modifying the extra offset with an index indicative of orthogonality of co-scheduled UEs on a RB. For example, an orthogonality index for beamforming vectors of the co-scheduled UEs γ r  can be introduced to modify the extra offset α r , so that the modified MU-MIMO OLLA offset may reflect the varying inter-user interference caused by dynamic UE pairing in the MU-MIMO transmission mode. 
     Preferably, the extra offset may be further modified with a constant scalar to ensure that the extra offset is not greater than the SU-MIMO OLLA offset. 
     According to the preferred embodiment, the equation 6) can be optimized as
 
δ r   MU   =C   scale ·γ r ·α r ·β r   , r= 0,1,2, . . . , L,   7)
 
     wherein L is supported max number of transmission layers based on the UE capacity and the eNB configuration; γ r  is an orthogonality index for the beamforming vectors of the co-scheduled UE, which can be used as a strength measurement of inter-user interference; C scale  is a constant scalar such that 0&lt;C scale ·γ r ≦1. 
     In an exemplary implementation of the preferred embodiment of the present invention, γ r  can be provided to the OLLA output unit  330  by a UE pairing module in the eNB, such as the scheduler of the eNB. 
     Suppose there are M UEs co-scheduled on the same RB, then for the concerned UE, 
     
       
         
           
             
               
                 
                   
                     
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     wherein V 0,r  and V m,r  are the normalized beamforming weights of the r th  layer of the concerned UE and the m th  co-scheduled UE on a RB, respectively. It is appreciated that RB index is omitted here, for the purpose of simplicity. 
     In an exemplary implantation of the preferred embodiment of the present invention, C scale  can be chosen in advance and maintained in the OLLA memory unit  320 . For example, C scale  can be chosen to meet following equation:
 
 C   scale   =M/T   threshold   9)
 
     where M is the number of co-scheduled UEs on the same resource, and T threshold  is the threshold on the beamforming weight correlation between two UEs. 
     Only if ∥V i,r V j,r ∥ 2 &lt;Threshold, then UE i and UE j will be co-scheduled on the same RB. Thus the max value of γ r  is T threshold . M is to account for the power split between the M co-scheduled UEs. 
     For example, in Transmission Mode 8 of LTE Release 9, it is specified that M=2, T threshold =0.3. Therefore, C scale =2/0.3=6.67. In this example, It can be seen that C scale  is chosen for following reasons: 
     1. V 1,r  and V 1,r  are first normalized and a BF correlation threshold 0.3 is used to rule out UEs whose BF weights are not orthogonal enough; and 
     2. then V 0,r  and V 1,r  are scaled by 1/sqrt(2) to account for power split between the two co-scheduled MU-MIMO UEs. 
     Those skilled in the art may appreciate that the preferred embodiments of the present invention as described above are only intended to provide non-limiting examples to illustrate how to optimize the MU-MIMO OLLA offset by modifying the extra offset. However, depending on practical use, those skilled in the art may choose one or more optimal scaling factors other than the above-described ones to optimize the extra offset, or even choose not to use any optimal scaling factors at all. Consequently, the features described in the preferred embodiments of the present invention should be regarded as being not indispensable but alternative and/or preferable to the overall solution. 
       FIG. 5  schematically illustrates a block diagram of an eNB device according to an embodiment of the present invention, 
     As shown in  FIG. 5 , reference numeral  500  denotes an eNB device according to an embodiment of the present invention. The eNB device comprises an inner loop link adaptation (ILLA) module  510 , an outer loop link adaptation (OLLA) module  520 , a modulation and coding module  530 , and a scheduling module  540 . 
     The ILLA module  510  is configured to perform estimation of UE-specific SINR for each of RBs assigned to a UE, based on a quantized indication of channel quality such as CQI reported from a UE. The ILLA module  510  performs inner loop link adaptation to adjust the estimated SINR. 
     The OLLA module  520  is configured to perform outer loop link adaptation according to embodiments of the present invention, to obtain OLLA-adjusted SINR for each of transmission layers. An exemplary operation of the OLLA module  520  is depicted in  FIG. 2 . The OLLA module  520  determines whether the UE operates in a SU-MIMO transmission mode or in a MU-MIMO transmission mode. Depending on the result of determination, the OLLA module  520  applies a SU-MIMO OLLA offset or a MU-MIMO OLLA offset generated by the OLLA offset generating module  300  to the SINR adjusted by the ILLA module  510 , respectively. 
     The OLLA offset generating module  300  of exemplary embodiments of the present invention has been described above in connection with  FIG. 3 . In the OLLA offset generating module  300 , the MU-MIMO OLLA offset δ r   MU  is determined based on the SU-MIMO OLLA offset δ r   SU =β r  and an extra offset α r  which is indicative of inter-user interference and the SU-MIMO OLLA offset β r  and the extra offset α r  are updated in responsive to receipt of an ACK/NACK message for a packet reported from said UE, for example, according to the processing illustrated in  FIG. 4 . According to a preferred embodiment of the present invention, the extra offset α r  can be modified with an index indicative of orthogonality of co-scheduled UEs on a RB. As an example, an orthogonality index for beamforming vectors of the co-scheduled UE γ r  can be provided from the scheduling module  540  to the OLLA offset generating module  300  of the OLLA module  520  to modify the extra offset α r . Preferably, the extra offset α r , may further modified with a constant scalar to ensure that the extra offset is not greater than the SU-MIMO OLLA offset. Therefore, according to the preferred embodiment, the MU-MIMO OLLA offset δ r   MU  for corresponding transmission layer can be denoted as
 
δ r   MU   =C   scale ·γ r ·α r +β r   , r= 0,1,2, . . . , L,  
 
     wherein L is supported max number of transmission layers based on the UE capacity and the eNB configuration; γ r  is an orthogonality index for the beamforming vectors of the co-scheduled UE, which can be used as a strength measurement of inter-user interference; C scale  is a constant scalar such that 0&lt;C scale ·γ r ≦1. 
     The ILLA module  510  is configured to calculate an effective SINR based on OLLA-adjusted SINRs from the OLLA module  520 . Then, a suitable MCS for transmission can be determined from a MCS lookup table and used in the modulation and coding module  530 . 
     Here, the eNB device  500  is described with the modules or components which are most relevant to the embodiments of the present invention. However, those skilled in the art can appreciate that the eNB device  500  also comprises other modules and components for performing the functionality of cellular communication, including antennas; transceiver (having a transmitter (TX) and a receiver (RX)); processors such as one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi-core processor architecture; memory module be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology; etc. Those modules or components are well known in the art and the description thereof is omitted for the purpose of conciseness. 
     A link adaptation processing in an eNB according to an embodiment of the present invention has been depicted in detail with reference to  FIGS. 1, 2 and 4 . It should be noted that the above depiction is only exemplary, not intended for limiting the present invention. In other embodiments of the present invention, this method may have more, or less, or different steps, and numbering the steps is only for making the depiction more concise and much clearer, but not for stringently limiting the sequence between each steps, while the sequence of steps may be different from the depiction. For example, in some embodiments, the above one or more optional steps may be omitted. Specific embodiment of each step may be different from the depiction. All these variations fall within the spirit and scope of the present invention. 
     In general, the various exemplary embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the exemplary embodiments of this invention may be illustrated and described as block and signaling diagrams, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. 
     As such, it should be appreciated that at least some aspects of the exemplary embodiments of the inventions may be practiced in various components such as integrated circuit chips and modules. As well known in the art, the design of integrated circuits is by and large a highly automated process. 
     The present invention may also be embodied in the computer program product which comprises all features capable of implementing the method as depicted herein and may implement the method when loaded to the computer system. 
     The present invention has been specifically illustrated and explained with reference to the preferred embodiments. The skilled in the art should understand various changes thereto in form and details may be made without departing from the spirit and scope of the present invention.