Abstract:
Orthogonality in cyclic shift (CS) and orthogonal cover code (OCC) selection for DMRS in MIMO is improved by new n DMRS  to n DMRS   (2)  mapping patterns. Values in the mapping tables are arranged in sets, with minimum CS separation between the values in each set. Additionally, the semi-static n DMRS  is independently configurable for each UL component carrier (CC) in the case of cross-CC scheduling in carrier aggregation, and the PHICH allocation formula that defines the allocation of the PHICH process relative to the k th  codeword (CW) on the c th  UL CC is a function of both the CS index n DMRS,k,c   (2)  that is dynamically assigned to a certain layer of the considered CW and the semi-static CS offset n DMRS,c   (1)  for the c th  CC.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/957,957, filed Aug. 2, 2013, which is a continuation of U.S. patent application Ser. No. 13/169,733, filed Jun. 27, 2011, which claims the benefit of U.S. Provisional Application No. 61/358,985, filed Jun. 28, 2010, the disclosures of which are fully incorporated herein by reference 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to wireless communication networks, and in particular to the selection of orthogonal transmission parameters for reference signals in MIMO and carrier aggregation systems. 
       BACKGROUND 
       [0003]    Wireless communication networks are a ubiquitous part of modern life in many areas. The inexorable trend in wireless communication development is a demand for higher data rates, to deliver a broader array of services and a richer user experience. One recent development with the promise to improve data rates and reliability is the use of multiple antennas in a transmitter and/or receiver. The use of multiple antennas on both the transmitter and receiver results in a multiple-input multiple-output (MIMO) communication channel, having the greatest performance gains over single-antenna or hybrid systems. 
         [0004]    Wireless communication networks operate under one or more industry standards, such as WCDMA, WiMax, GMS/EDGE, UTMS/HSPA, and the like. One such standard is the Long Term Evolution (LTE), developed and promulgated by the 3rd Generation Partnership Project (3GPP). Release 10 of the LTE standard, also known as LTE Rel-10, or LTE-Advanced, supports MIMO antenna deployments and MIMO related techniques. A current working assumption in the uplink (UL) of LTE Rel-10 is the support of a spatial multiplexing mode (SU-MIMO) in the communication from a single User Equipment (UE) to the base station, or enhanced Node B (eNodeB or eNB). SU-MIMO targets high data rates in favorable channel conditions. SU-MIMO consists of the simultaneous transmission of multiple data streams on the same bandwidth, where each data stream is referred to as a layer. Multi-antenna techniques such as linear precoding are employed at the transmitter in order to differentiate the layers in the spatial domain and allow the recovery of the transmitted data at a receiver. 
         [0005]    Another MIMO technique supported by LTE Rel-10 is MU-MIMO, where multiple UEs belonging to the same cell are completely or partly co-scheduled on the same bandwidth and time slots. Each UE in the MU-MIMO configuration may possibly transmit multiple layers, thus operating in SU-MIMO mode. 
         [0006]    It is necessary to allow the receiver to estimate the equivalent channel associated with each transmitted layer in the cell, in order to allow detection of all the data streams. Therefore, each UE must transmit a unique reference signal (RS, or pilot signal) at least for each transmitted layer. Different types of RS are defined—those relevant to the present invention are the DeModulation RS, or DMRS. The receiver is aware of which DMRS is associated with each layer, and performs estimation of the associated channel by executing a channel estimation algorithm, as known in the art. The estimated channel is then employed by the receiver in the detection process to recover the transmitted data from the received data stream. 
         [0007]    According to the LTE Rel-10 standard, in its current status, a set of potential RS is defined, where each DMRS is uniquely defined by a cyclic shift (CS) value, with 12 CS values supported, and an orthogonal cover code (OCC), with 2 OCC values defined. In LTE Rel-8, downlink control information (DCI) format 0 for the Physical Uplink Shared Channel (PUSCH) scheduling includes a 3-bit field (n DMRS ) for signaling of the CS for DMRS. To support SU-MIMO in the uplink of LTE Rel-10, multiple cyclic shifts and/or orthogonal cover codes must be signaled to the UE for DMRS multiplexing. However, it is not practical to signal multiple cyclic shift indices explicitly for all layers due to the large overhead that would be incurred. Accordingly, the working assumption for CS signaling, as proposed in 3GPP document R1-102764, “Conveying OCC for PUSCH Transmissions,” by Qualcomm Inc., is as follows: 
         [0008]    Only one cyclic shift index is signaled in the corresponding DCI as in Rel-8. The mapped cyclic shift value n DMRS   (2)  from the signaled cyclic shift index n DMRS  is used for DMRS of layer-0; the cyclic shift values for other layers are derived from n DMRS   (2)  according to a pre-defined rule. The table of  FIG. 1  provides the working assumption for such pre-defined rule. 
         [0009]    There are two possible OCC over the two DMRS symbols within one sub-frame (see  FIG. 1 ). In addition to separating multiple DMRS by different CS, OCC can be signaled to the UE to provide better orthogonality among the multiplexed DMRS from different layers. The working assumption for OCC signaling in RAN1 is implicit signaling of OCC: 
         [0010]    The implicitly assigned OCC can be derived from the signaled cyclic shift value: n DMRS   (1) +n DMRS   (2) , where n DMRS   (1)  is provided by higher layers as a semi-static CS and n DMRS   (2)  is the signaled (dynamic) CS value in the most recent DCI for the corresponding PUSCH transmission, according to a pre-defined rule. The table of  FIG. 1  provides the working assumption for such pre-defined rule. No additional bit is needed in the corresponding DCI for OCC signaling. 
         [0011]    The working assumption for mapping from CS value to OCC is illustrated in the table of  FIG. 1 , where different OCC are mapped to adjacent CS values. Note that n DMRS   (2)  itself will only be able to signal 8 CS values: 0, 2, 3, 4, 6, 8, 9, and 10. However, n DMRS   (1) +n DMRS   (2)  will be able to address all possible CS values. 
         [0012]    The DMRS for each layer (also known as each virtual antenna) is constructed according to the following procedure. 
         [0013]    First, after receiving the dynamic CS value n DMRS   (2)  from the corresponding Physical Downlink Control Channel (PDCCH) and the semi-static CS value n DMRS   (1)  from higher layers, according to the pre-defined rule depicted in Table 1, the mapped orthogonal cover code index is determined as: I OCC =f(n DMRS   (1) +n DMRS   (2) ). 
         [0014]    Second, the DMRS for each layer/virtual antenna can be constructed according to the rules depicted in Table 1 for each rank: 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Layer-specific Rules for CS and OCC Calculation 
               
             
          
           
               
                   
                 Layer 
                   
               
               
                   
                 (Virtual Antenna) 
                 DMRS in Slot 0 &amp; 1 
               
               
                   
                   
               
             
          
           
               
                 Rank-1 
                 0 
                 CS: n DMRS   (2) , OCC Index: I OCC   
               
               
                 Transmission 
               
               
                 Rank-2 
                 0 
                 CS: n DMRS   (2) , OCC Index: I OCC   
               
               
                 Transmission 
                 1 
                 CS: n DMRS   (2)  + 6, OCC Index: 1-I OCC   
               
               
                 Rank-3 
                 0 
                 CS: n DMRS   (2) , OCC Index: I OCC   
               
               
                 Transmission 
                 1 
                 CS: n DMRS   (2)  + 3, OCC Index: 1-I OCC   
               
               
                   
                 2 
                 CS: n DMRS   (2)  + 6, OCC Index: I OCC   
               
               
                 Rank-4 
                 0 
                 CS: n DMRS   (2) , OCC Index: I OCC   
               
               
                 Transmission 
                 1 
                 CS: n DMRS   (2)  + 3, OCC Index: 1-I OCC   
               
               
                   
                 2 
                 CS: n DMRS   (2)  + 6, OCC Index: I OCC   
               
               
                   
                 3 
                 CS: n DMRS   (2)  + 9, OCC Index: 1-I OCC   
               
               
                   
               
             
          
         
       
     
         [0015]    Note that, in Table 1, the CS values for each layer comprise the mapped dynamic CS value for layer 0, n DMRS   (2) , offset by a predetermined amount for each successive layer. Of these offsets, the minimum value is three (i.e., for rank-3 and rank-4 transmissions). Also, note that the OCC index is the value determined from the table in  FIG. 1  and the semi-static CS value n DMRS   (2)  for layer 0, and then alternating to the other defined OCC value for each successive layer. Ideally, the combination maximally separates DMRS in successive layers, by a CS separation of three, and alternating OCC values. 
         [0016]    Schemes for constructing the DMRS for multi-layer transmission, other than those in Table 1 above, are equivalently supported. For example, alternative rules for assigning the CS and OCC values for successive Layers/Virtual Antennas based on n DMRS   (2)  are possible. See, e.g., 3GPP document R1-102964, “OCC Configuration and Sequence Group Hopping,” by Nokia and Nokia Siemens Networks. 
         [0017]    In addition to MIMO support, 3GPP LTE Rel-10 additionally supports multi-carrier operations, also known as carrier aggregation, in order to improve spectrum allocation size and flexibility. In case of multi-carrier operation, independent data channels are modulated onto and transmitted on each of two or more carrier frequencies, known as component carriers (CC), or simply “carriers.” The allocation of uplink (UL) and downlink (DL) carriers is flexible, so it is possible to allocate a different set and number of DL and UL carriers for a certain UE. 
         [0018]    Cross-CC scheduling is a new Rel-10 resource allocation modality where a single DL CC controls multiple UL CCs. Therefore, control information for all the controlled UL CCs is conveyed on the same DL CC. For example, the collected ACK/NACK control messages (PHICH) referred to UL transmissions for all the UL CCs are collected on the same DL CC. In order to allow multiplexing of different PHICH messages on the same CC, each PHICH message is defined by unique n PHICH   group  and n PHICH   seq  parameters, which are in turn functions of several allocation parameters including n DMRS  for a given CC. Therefore, the working assumption in RAN1 is that cyclic shifts of UL DMRS are available as mechanism to avoid PHICH collisions. See the 3GPP document R1-103501, “On PHICH for Carrier Aggregation,” by Ericsson and ST Ericsson. In particular, the working assumption on the PHICH formulas is: 
         [0000]        n   PHICH   group =( I   PRB     —     RA   lowest     —     index   +n   DMRS )mod  N   PHICH   group   +I   PHICH   N   PHICH   group    
         [0000]        n   PHICH   seq =(└ I   PRB     —     RA   lowest     —     index   /N   PHICH   group   ┘+n   DMRS )mod 2 N   SF   PHICH   (1)
 
         [0000]    where the parameters I PRB     —     RA   lowest     —     index , N PHICH   group , I PHICH  and N SF   PHICH  have the meanings defined in 3GPP TS 36.212 V.9.0.0. According to Rel-8 assumptions, n DMRS  in equation (1) is given by the latest DCI format 0. 
         [0019]    In case of multi-codeword (CW) transmission on the same UL CC (as in the case of multi-layer transmission), an individual PHICH should be generated for each UL CW on each UL CC in the Cross-CC scheduling group. 
         [0020]    The proposed working solution has several deficiencies. The scheduling flexibility appears to be limited in some cases of major practical interest, such as Cross-CC scheduling. Collision avoidance in PHICH signaling imposes constraints in the UL-DMRS allocation that reduce scheduling flexibility. Constraints on the allocation of UL-DMRS may lead to unnecessarily suboptimal performance in channel estimation due to poor orthogonality between DMRS of different UEs or layers. Reduced flexibility in the DMRS allocation due to PHICH signaling constraints leads to complex allocation procedures for DMRS. Finally, reduced flexibility in the scheduling due to DMRS constraints leads to complex resource allocation. 
       SUMMARY 
       [0021]    According to embodiments described and claimed herein, various n DMRS  to n DMRS   (2)  mapping patterns are proposed, which allow CS and OCC selection for DMRS in MIMO operation to observe minimum effective orthogonality. Values in the mapping tables are arranged in sets, with minimum CS separation between the values in each set. Additionally, the semi-static n DMRS  is independently configurable for each UL CC in the case of cross-CC scheduling, and the PHICH allocation formula that defines the allocation of the PHICH process relative to the k th  CW on the c th  UL CC is a function of both the CS index n DMRS,k,c   (2)  that is dynamically assigned to a certain layer of the considered CW and the semi-static CS offset n DMRS,c   (1)  for the c th  CC. 
         [0022]    One embodiment relates to method of determining CS and OCC values associated with DMRS for each transmission layer, by a transceiver in a wireless communication system employing MIMO operation. A semi-static CS value n DMRS   (1)  and a dynamic CS value n DMRS  are received. A predetermined table is indexed with n DMRS  to obtain a first CS value n DMRS   (2)  and an OCC value n DMRS   OCC  associated with DMRS for layer 0. Within the predetermined table, the CS values n DMRS   (2)  are arranged in two or more sets of CS values n DMRS   (2) , the CS values n DMRS   (2)  within each set being separated by a minimum predetermined, layer-specific offset. A first CS value associated with DMRS for other layers is derived by adding a predetermined, layer-specific offset to n DMRS   (2) . A second CS value for each layer is calculated by adding n DMRS   (1)  and n DMRS   (2) . 
         [0023]    Another embodiment relates to a method of determining CS and OCC values associated with DMRS for each transmission layer and component carrier, and PHICH allocation, by a transceiver in a wireless communication system employing MIMO operation and carrier aggregation. A semi-static CS value n DMRS,c   (1)  associated with each component carrier (CC) in the case of cross-CC scheduling is received. The allocation of the PHICH process relative to the k th  codeword on the c th  CC is a function of both a first CS value n DMRS,k,c   (2)  associated with DMRS for a certain layer of a codeword and the semi-static CS value n DMRS,c   (1)  associated with the c th  CC. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a mapping table for DMRS cyclic shift values according to the prior art. 
           [0025]      FIG. 2  is a diagram of the prior art DMRS cyclic shift mapping for multi-carrier operation according to the table of  FIG. 1 . 
           [0026]      FIG. 3  is a functional block diagram of a wireless communication network. 
           [0027]      FIG. 4  is a mapping table for DMRS cyclic shift values according to one embodiment of the present invention. 
           [0028]      FIG. 5  is a mapping table for DMRS cyclic shift values according to another embodiment of the present invention. 
           [0029]      FIG. 6  is a mapping table for DMRS cyclic shift values according to yet another embodiment of the present invention. 
           [0030]      FIG. 7  is a diagram of the DMRS cyclic shift mapping for multi-carrier operation according to the table of  FIG. 4 . 
           [0031]      FIG. 8  is a flow diagram of a method of determining cyclic shift values for DMRS. 
           [0032]      FIG. 9  is a diagram of the DMRS cyclic shift mapping for multi-carrier operation according to the table of  FIG. 1  but using semi-static cyclic shift values according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 3  depicts a representative wireless communication network  10 , such as an LTE-Advanced network  10  (although embodiments of the invention are not limited to this Radio Access Technology). A UE  12  communicates with a NodeB or eNodeB  14 , which provides radio communication services to a plurality of UE  12  in a geographic area, or cell  16 . The eNodeB  14  is controlled by a Radio Network Controller (RNC)  18 , which connects through a Core Network (CN)  20  to one more other packet data or telecommunication networks, such as the Public Switched Telephone Network (PSTN)  22 . 
         [0034]    The UE  12  includes a Radio Frequency (RF) transceiver  30 , which receives and transmits wireless communication (e.g., data and control) signals from and to the eNodeB  14  on one or more antennas  31 A,  31 B. The transceiver  30  is controlled by a controller  32 , which may comprise a general purpose processor, Digital Signal Processor (DSP), or other processing circuit, as known in the art. Functionality comprising embodiments of the present invention may be implemented as software modules stored in memory  34  and executed by the controller  32 . 
         [0035]    Similarly, the eNodeB  14  includes an RF transceiver  40 , which receives and transmits wireless communication signals from and to one or more UE  12  in the cell  16 , on one or more antennas  41 A,  41 B. The transceiver  40  is controlled by a controller  42 , which may comprise a general purpose processor, Digital Signal Processor (DSP), or other processing circuit, as known in the art. Functionality comprising embodiments of the present invention may be implemented as software modules stored in memory  44  and executed by the controller  42 . Additionally, a table mapping a CS index n DMRS  to a dynamic CS value for layer 0 n DMRS   (2) , as discussed further herein, may reside in memory  44 . The dual antennas  31 A,  31 B and  41   a ,  41   b  on the UE  10  and eNodeB  14 , respectively, indicate that the network  10  supports SU- and MU-MIMO. Furthermore, the dual wireless communication indicators mean that the network  10  supports carrier aggregation. 
         [0036]    When employing multi-layer transmission it is important to achieve maximum orthogonality between the DMRS of the different layers by combining CS and OCC separation and by maximizing the distance between adjacent DMRS. The minimum inter-DMRS distance becomes particularly important when four layers are co-scheduled on the same CC. These layers may all belong to the same UE or to different UEs co-scheduled in MU-MIMO configuration. 
         [0037]    In order to maximize the distance between layers, the working assumption in case of four layers per CC is to divide adjacent DMRS with a combination of three CS and possibly OCC. Simulation results (see document R1-102764 cited above) show that the performance achieved with smaller inter-DMRS distance is not sufficient to achieve acceptable link performance in case of four-layer transmission. 
         [0038]    In case of two layers per UE, the working assumption is to separate the 2 DMRS of the UE by six CS values, while in case of three layers per UE the working assumption is to divide the adjacent DMRS of the UE by three CS values and OCC. Thus, according to the working assumption in Rel-10, DMRS should be allocated to positions that are multiple of three CS positions in order to maximize spacing between DMRS belonging to the same UE or to different UEs in MU-MIMO modality. 
         [0039]    As previously observed, the n DMRS  field is used in Rel-8 also for PHICH allocation according to equation (1). In case of Cross-CC scheduling and multi-CW transmission, as for Rel-10, the PHICH allocation shall be different for each CW and each CC. A natural extension of equation (1) is to substitute n DMRS  with n DMRS,k,c   (2) , thus obtaining: 
         [0000]        n   PHICH,k,c   group =( I   PRB     —     RA   lowest     —     index   +n   DMRS,k,c   (2) )mod  N   PHICH   group   +I   PHICH   N   PHICH   group    
         [0000]        n   PHICH,k,c   seq =(└ I   PRB     —     RA   lowest     —     index   /N   PHICH   group   ┘+n   DMRS,k,c   (2) )mod 2 N   SF   PHICH   (2)
 
         [0040]    In equation (2), the field n DMRS,k,c   (2)  represents the CS index for one of the layers associated to the k th  CW on the c th  UL CC. In case the considered CW is mapped to multiple layers (and so multiple CS values) n DMRS,k,c   (2)  is chosen according to a rule. For example, n DMRS,k,c   (2)  could be the CS associate to the DMRS corresponding to the 1 st  layer of the considered CW. 
         [0041]    It is observed that the current mapping of n DMRS  to n DMRS,k,c   (2)  values according, e.g., to the table of  FIG. 1 , does not respect the desired regularity property, making it inefficient to schedule users, especially in MU-MIMO configuration. An example of this is demonstrated in  FIG. 2 , which depicts the CS/OCC spacing using the mapping table of  FIG. 1 . In this example, two UL carriers are controlled by one DL carrier. Two UE  12  are co-scheduled in MU-MIMO modality on each CC and two layers per UE  12  are assumed. According to Rel-10 working assumptions, in case of rank-2 transmission (two layers per UE  12 ) a different CW is associated with each layer. Therefore, a PHICH instance is generated according to the index n DMRS,k,c   (2)  for each k th  allocated codeword on each c th  UL CC. 
         [0042]    Note that, according to the working assumption in the prior art (e.g.,  FIG. 1 ), the allocation of the CS on the second CC is suboptimal, as the spacing of three CS and OCC between adjacent layers is not respected. Following the mapping in  FIG. 1  and the rules listed above in Table 1, the first two DMRS, transmitted on the first carrier, are assigned cyclic shifts of 0, 3, 6, and 9, with alternating OCC. However, this is not possible for the DMRS transmitted on the second carrier. A CS of one is not supported in the table of  FIG. 1 , so DMRS for layer 2 is mapped to a CS of 2. The rules of Table 1 require a minimum CS spacing of three; however a CS of five is not supported in the table of  FIG. 1 , so DMRS for layer 2 is mapped to a CS of 4. 
         [0043]    According to one embodiment of the present invention, a table mapping n DMR  to n DMRS,k,c   (2)  values comprises a plurality of sets, wherein the CS values in each set have a minimum CS spacing corresponding to the minimum among the layer-specific offsets specified in the rules of Table 1. In particular, of the twelve potential CS values, the table mapping n DMRS  to n DMRS,k,c   (2)  values comprises two sets, and the minimum CS spacing in each set is three.  FIG. 4  depicts one embodiment wherein the table conforms to this restriction. In the table of  FIG. 4 , the mapping is constructed according to the principle of mapping 8 CS out of the grid of available 12 CS in a regular way, so that it is possible to allocate DMRS that are spaced by three CS values in MU-MIMO settings.  FIGS. 5 and 6  depict alternative mappings that conform to the same restriction. 
         [0044]      FIG. 7  depicts the allocation of resources in the same example as for  FIG. 2 , but considering the allocation rule according to the embodiment of the present invention depicted in  FIG. 4 . It is now possible to achieve optimal inter-DMRS spacing for the considered configuration, thus overcoming the technical deficiency in the prior art mapping (e.g.,  FIG. 1 ). In particular, the DMRS on carrier 0 are mapped to CS 0, 3, 6, and 9, with alternating OCC, as in the prior art. However, according to the mapping of the table of  FIG. 4 , the DMRS on carrier 1 are able to be mapped to CS 1, 4, 7, and 10, also achieving a CS separation of three. 
         [0045]      FIG. 8  depicts a method  100  of determining CS and OCC values associated with DMRS, for each transmission layer, by a transceiver, such as a UE  12 , in a wireless communication system  10  employing MIMO operation. A semi-static CS value n DMRS   (1)  is received (block  102 ), such as from higher layer signaling via an eNodeB  14 . A dynamic CS index value n DMRS  is received (block  104 ), such as in a DCI transmission from the eNodeB  14 . A predetermined table is indexed with n DMRS  (block  106 ) to obtain one of, e.g., twelve first CS values n DMRS   (2)  and an OCC value n DMRS   OCC  associated with DMRS for layer 0. The CS values n DMRS   (2)  in the table are arranged into, e.g., two sets of CS values n DMRS   (2) , the CS values n DMRS   (2)  within each set being separated by a minimum predetermined offset (e.g., three). For layers other than layer 0, a first CS value associated with DMRS for that layer is derived by adding an integer multiple of the minimum predetermined offset to n DMRS   (2)  (block  108 ). A second CS value for each layer (the one used to encode DMRS) is calculated by adding n DMRS   (1)  and (n DMRS   (2) +offset) (block  110 ). An OCC value is calculated by adding n DMRS   OCC  (the OCC value for layer 0, obtained by indexing the table with n DMRS ) to a layer-specific offset (block  112 ). DMRS are then encoded using the final CS and OCC values for each layer, and transmitted (block  114 ). The DMRS are received and decoded, such as by the eNodeB  14 , and are used to characterize the channel, as an aid in interpreting received data streams on each layer. The process then repeats with the reception of a new dynamic CS value n DMRS  (block  104 ). The semi-static CS value n DMRS   (1)  is updated (block  102 ) on an infrequent basis by higher level signaling, as indicated by the dashed line in  FIG. 8 . 
         [0046]    One embodiment of the present invention is based on a modification of the PHICH mapping rule. According to the prior art (e.g., Rel-8), the PHICH allocation is a function of the dynamically signalled DMRS allocation index n DMRS  on PDCCH. 
         [0047]    However, flexibility in CS allocation is enhanced by the combined use of the dynamically signalled index n DMRS   (2)  and a semi-statically signalled index n DMRS   (1) . According to one embodiment, n DMRS   (1)  is employed in order to improve flexibility in PHICH resource allocation in case of cross-CC scheduling. In particular, the n DMRS   (1)  index is signalled per UL CC in case of cross-CC scheduling, and is re-labelled as n DMRS,c   (1)  where c denotes the CC index. 
         [0048]    Additionally, the PHICH allocation formula of equation (1) is modified. Dependency of the PHICH allocation on the DMRS index n DMRS,k,c   (2)  for CW k and carrier c is introduced, optionally in conjunction with the semi-static DMRS allocation offset per carrier n DMRS,c   (1) . 
         [0049]    The modified PHICH allocation rule according to one embodiment is: 
         [0000]        n   PHICH,k,c   group =( I   PRB     —     RA   lowest     —     index   +n   DMRS,k,c   (2)   +n   DMRS,c   (1) )mod  N   PHICH   group   +I   PHICH   N   PHICH   group    
         [0000]        n   PHICH,k,c   seq =(└ I   PRB     —     RA   lowest     —     index   /N   PHICH   group   ┘+n   DMRS,k,c   (2)   +n   DMRS,c   (1) )mod 2 N   SF   PHICH   (3)
 
         [0050]    The modified PHICH allocation rule according to another embodiment is: 
         [0000]        n   PHICH,k,c   group =( I   PRB     —     RA   lowest     —     index   +n   DMRS,k,c   (2)   +n   DMRS,c   (1) )mod  N   PHICH   group   +I   PHICH   N   PHICH   group    
         [0000]        n   PHICH,k,c   seq =(└ I   PRB     —     RA   lowest     —     index   /N   PHICH   group   ┘+n   DMRS,k,c   (2) )mod 2 N   SF   PHICH   (4)
 
         [0051]    The modified PHICH allocation rule according to another embodiment is: 
         [0000]        n   PHICH,k,c   group =( I   PRB     —     RA   lowest     —     index   +n   DMRS,k,c   (2) )mod  N   PHICH   group   +I   PHICH   N   PHICH   group    
         [0000]        n   PHICH,k,c   seq =(└ I   PRB     —     RA   lowest     —     index   /N   PHICH   group   ┘+n   DMRS,k,c   (2)   +n   DMRS,c   (1) )mod 2 N   SF   PHICH   (5)
 
         [0052]    These embodiments achieve two objectives. First, multiplexing of different PHICH messages referring to different CW in cross-CC scheduling modality is enabled. Second, enhancement of DMRS allocation flexibility is achieved by exploiting the different n DMRS,c   (1)  on different CC. 
         [0053]      FIG. 9  depicts an example, considering the same settings as those described with respect to the example of  FIG. 2 , and n DMRS   (1) =0 and n DMRS   (1) =1 is considered. The n DMRS  to n DMRS   (2)  mapping rule according to the prior art (i.e., the table of  FIG. 1 ) is considered. The modified PHICH allocation rules according to any of equations (3), (4) or (5) are considered. It is now possible to preserve optimal DMRS allocation even in this setting, because none of the allocated DMRS are associated with identical (n PHICH,k,c   group , n PHICH,k,c   seq ) parameters. 
         [0054]    Embodiments of the present invention present numerous advantages over the prior art. Embodiments enable greater orthogonally of DMRS by allowing for the minimum recommended DMRS separation in CS and OCC for each transmission layer. Embodiments also allow for additional efficiency in DMRS assignment compared to the prior art. Improved scheduling flexibility is enabled for multi-carrier operation, and PHICH constraints are reduced for practical scheduling configurations. 
         [0055]    Although embodiments of the present invention have been described herein as being performed in a UE  12 , based on CS parameters received from an eNodeB  14 , the invention is not limited to this configuration. Rather, embodiments may be advantageously performed in any transceiver node of a wireless communication network  10  that transmits reference signals to assist a receiver in channel characterization. Furthermore, although embodiments have been described herein with respect to an LTE-Advanced network  10 , the present invention is not limited to this protocol or Radio Access Technology, and may be advantageously applied in a wide variety of wireless communication systems. 
         [0056]    The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.