Patent Publication Number: US-2013250876-A1

Title: Method And Apparatus Providing Inter-Transmission Point Phase Relationship Feedback For Joint Transmission CoMP

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority under 35 U.S.C. §119(e) from Provisional Patent Application No. 61/610,560 filed Mar. 14, 2012, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The exemplary and non-limiting embodiments of this invention relate generally to wireless communication systems. methods, devices and computer programs and, more specifically, relate to coordinated multipoint (CoMP) transmission and reception techniques, such as those proposed for long term evolution (LTE) advanced (LTE-A), including coherent joint transmission (JT) CoMP techniques, and to resource allocation such as channel state information (CSI) and reference signal (RS) allocation and related feedback from a user equipment (UE) to a network transmission point, such as an evolved NodeB or more simply an eNB. 
     BACKGROUND 
     This section is intended to provide a background or context to the invention that is recited in the claims. 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 indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section. 
     In 3GPP LTE the eNB allocates physical layer resources for uplink (UL, to the eNB) and downlink (DL, from the eNB) shared channels. The physical layer resources include physical resource blocks (PRB) and a modulation coding scheme (MCS). The MCS determines the bit rate. and thus the capacity, of the PRBs. Allocations may be valid for one or more transmission time intervals (TTIs). 
     Coordinated MultiPoint (CoMP) transmission and reception is being investigated for 3GPP LTE-A to enhance cell-edge data rates in order to create a more uniform data rate experience for the end-user over the entire cell area. CoMP techniques involve increased collaboration between different network nodes (eNBs or more generically ‘transmission points’) in downlink (DE) transmission to the UE and in uplink (UL) reception from the UE. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the invention, an apparatus comprises at least one processor and memory storing computer program code. The memory storing the computer program code is configured to, with the at least one processor, cause the apparatus to at least configure a set of n channel state information reference signal resources to be measured by a user device, where the set of n channel state information reference signal resources includes at least one channel state information reference signal resource that spans over at least two transmission points, request from a user device channel state information feedback corresponding to each channel state information reference signal resource, and, upon receiving channel state information from the user device, select at least one precoder for accomplishing a coherent joint cooperative multipoint transmission based at least in part on inter-transmission point phase relationship information contained in the channel state information feedback received from the user device. 
     In another embodiment of the invention, a method comprises configuring a set of n channel state information reference signal resources to be measured by a user device, where the set of n channel state information reference signal resources includes at least one channel state information reference signal resource that spans over at least two transmission points, requesting from a user device channel state information feedback corresponding to each channel state information reference signal resource, and, upon receiving channel state information from the user device, selecting at least one precoder for accomplishing a coherent joint cooperative multipoint based at least in part on inter-transmission point phase relationship information contained in the channel state information feedback received from the user device. 
     In another embodiment of the invention, a computer-readable medium stores a program of instructions. Execution of the program of instructions by a processor configures an apparatus to at least configure a set of n channel state information reference signal resources to be measured by a user device, where the set of n channel state information reference signal resources includes at least one channel state information reference signal resource that spans over at least two transmission points, request from a user device channel state information feedback corresponding to each channel state information reference signal resource and, upon receiving channel state information from the user device, select at least one precoder for accomplishing a coherent joint cooperative multipoint based at least in part on inter-transmission point phase relationship information contained in the channel state information feedback received from the user device. 
     In another embodiment of the invention, an apparatus comprises at least one processor and memory storing computer program code. The memory storing the computer program code is configured to, with the at least one processor, cause the apparatus to at least direct measurement by a user device of n channel state information reference signal resources configured for measurement by the user device, where the n channel state information reference signal resources include at least one channel state information reference signal resource that spans over at least two transmission points, and configure channel state information feedback for transmission by the user device corresponding to each of the n configured channel state information reference signal resources, wherein the channel state information feedback comprises inter-transmission point phase relationship information to be used by a base station for selection of at least one precoder for accomplishing a coherent joint cooperative multipoint transmission based at least in part on inter-transmission point phase relationship information contained in the channel state information feedback based at least in part on inter-transmission point phase relationship information contained in the channel state information feedback. 
     In another embodiment of the invention, a method comprises directing measurement by a user device of n channel state information reference signal resources configured for measurement by the user device, where the n channel state information reference signal resources include at least one channel state information reference signal resource that spans over at least two transmission points and configuring channel state information feedback for transmission by the user device corresponding to each of the n configured channel state information reference signal resources, wherein the channel state information feedback comprises inter-transmission point phase relationship information to be used by a base station for selection of at least one precoder for accomplishing a coherent joint cooperative multipoint transmission. 
     In another embodiment of the invention, a computer readable medium stores a program of instructions. Execution of the program of instructions by a processor configures an apparatus to at least direct measurement by a user device of n channel state information reference signal resources configured for measurement by the user device, where the n channel state information reference signal resources include at least one channel state information reference signal resource that spans over at least two transmission points and configure channel state information feedback for transmission by the user device corresponding to each of the n configured channel state information reference signal resources, wherein the channel state information feedback comprises inter-transmission point phase relationship information to be used by a base station for selection of at least one precoder for accomplishing a coherent joint cooperative multipoint transmission based at least in part on inter-transmission point phase relationship information contained in the channel state information feedback. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a wireless network that may be configured according to an embodiment of the present invention. 
         FIG. 2A  illustrates an uplink resource grid showing a resource configuration that may be used in systems according to embodiments of the present invention, and shows the relationship of single carrier frequency-division multiple access (SC-FDMA) symbols, subcarriers, resource blocks and resource elements. 
         FIG. 2B  illustrates an uplink resource grid showing a resource configuration that may be used in systems according to embodiments of the present invention, and shows the relationship of orthogonal frequency-division multiplexing (OFDM) symbols, subcarriers, resource blocks and resource elements. 
         FIG. 3  illustrates joint transmission cooperative multi-point operation by elements that may be used in an embodiment of the present invention. 
         FIG. 4  shows a simplified block diagram of various electronic devices that are suitable for use in practicing embodiments of this invention. 
         FIG. 5  presents a table showing mapping from a channel state information (CSI) reference signal configuration to (k′, l′) for a normal cyclic prefix. 
         FIG. 6  illustrates a prior-art channel state information reference signal (CSI-RS) configuration for two transmission points (TP1 and TP2). 
         FIG. 7  illustrates a channel state information reference signal (CSI-RS) configuration Table illustrating configuration of an additional two transmission port CSI-RS resource shared by two transmission points as compared to the approach illustrated in  FIG. 5 . 
         FIG. 8  presents an exemplary codebook for transmission on antenna ports {0,1} and for channel state information (CSI) reporting based on antenna ports {0,1} or {15,16}. 
         FIG. 9  illustrates an exemplary use of precoding matrix indicator (PMI) feedback for indication of a phase difference between two antennas each associated with one transmission point of a pair of transmission points. 
         FIG. 10  illustrates a process according to one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     One modern communication system is known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA). In this system the downlink (DL) access technique is OFDMA, and the uplink (UL) access technique is SC-FDMA. 
     One specification of interest is 3GPP TS 36.300 V11.0.0 (2011-12)  Technical Specification  3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN): Overall description; Stage 2 (Release 11). 
       FIG. 1  illustrates the overall architecture a system  100 , such as a E-UTRAN system, in which one or more embodiments of the present invention may be used. The system  100  includes network access nodes or base stations which may be implemented in the form of eNodeBs (eNBs)  102 A,  102 B, and  102 C, and which may provide an E-UTRAN user plane and control plane (radio resource control (RRC)) protocol terminations towards user devices, which in an embodiment may be implemented as user equipments (UEs), here UEs  104 A and  104 B. The eNBs  102 A- 102 C may be interconnected with one another by means of an X2 interface  106 . In addition or as an alternative to using the logical X2 interface, for cooperative multi-point (CoMP) operation, other inter-transmission point connections such as optical or other non-standardized low-latency, high capacity interfaces may be used. The eNBs  102 A- 102 C may also be connected by means of an S1 interface to an evolved packet core (EPC). In an embodiment, the connection may take the form of S1 mobility management entity (S1 MME) interface  108  to MME serving gateways (MME/S-GWs)  110 A and  110 B. The S1 interface  108  supports a many-to-many relationship between MMEs. S-GWs and eNBs. 
     Uplink and downlink frames (of 10 msec duration) are defined in 3GPP TS 36.211 V10.4.0 (2011-12)  Technical Specification  3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA): Physical Channels and Modulation (Release 10).  FIG. 2A  illustrates an uplink resource grid  200  presenting an exemplar) definition of resources that may be used in a system such as the system  100  of  FIG. 1 , and showing the relationship of SC-FDMA symbols, subcarriers, resource blocks and resource elements. The grid comprises a plurality of uplink slots, exemplified by the slot  202 , which is a time slot. One or more of the uplink slots may carry single carrier frequency-division multiple access (SC-FDMA) symbols, and the grid may be arranged horizontally from l=0 to some maximum, which in the present example may be l=(N symb   UL )−1, and from k=0 to some maximum, which in the present example may be k=(N RB   UL )(B sc   RB )−1. The grid may comprise a resource block comprising, for example, a number (N symb   UL )(N sc   RB ) of resource elements. The overall grid may comprise (N RB   UL (N sc   RB ) subcarriers, while the resource block may comprise N sc   RB ) subcarriers. 
       FIG. 2B  illustrates a downlink resource grid, showing the relationship of OFDM symbols, subcarriers, resource blocks and resource elements. The resource blocks can be referred to as physical resource blocks (PRBs). The grid comprises a plurality of downlink slots, exemplified by the slot  252 , which is a time slot. One or more of the downlink slots may carry orthogonal frequency-division multiplexing (OFDM) symbols, and the grid may be arranged horizontally from l=0 to some maximum, which in the present example may be l=(N symb   RDL )−1, and from k=0 to some maximum, which in the present example may be k=(N RB   DL )(N sc   RB )−1. The grid may comprise a resource block  204  comprising, for example, a number (N symb   DL )(N sc   RB ) of resource elements such as the resource element  206 . The overall grid may comprise (N RB   DL )(N sc   RB ) subcarriers, while the resource block may comprise (N sc   RB ) subcarriers. 
     One CoMP scheme is referred to as coherent joint transmission (JT) CoMP. In coherent JT CoMP, data is jointly transmitted to a UE from multiple transmission points. Based on CSI feedback from the UE a base station such as an eNB is able to precode the data so that the received signal quality (for example, signal to interference plus noise ratio (SINR), throughput, or both, or some alternative or additional characteristic or combination of characteristics) is maximized. To provide the information needed to accomplish this precoding, the eNB needs to be able to obtain information not only about a preferred precoder for each participating transmission point, but also information descriptive of a phase difference between the cooperating transmission points in order to optimize the JT CoMP performance. 
     One general example of the operation of JT CoMP is illustrated in  FIG. 3 . Multiple (two in this non-limiting example) transmission points  302 A and  302 B send data to a UE  304  using the same time-frequency resources, which may, for example, be physical resource blocks defined in formats similar to those illustrated in  FIGS. 2A and 2B . The two transmission points  302 A and  302 B may each be equipped with multiple transmit antennas (which in a non-limiting example may be four). The transmission points  302 A and  302 B may or may not belong to the same cell,—that is, they may have the same or a different physical cell identifier (ID). The transmission point  302 A transmits signals  306 A- 306 D from its four antennas, and the transmission point  302 B transmits signals  308 A- 308 D from its four antennas. 
     One or more embodiments of the invention provide mechanisms for aligning the phases of the signals transmitted from each of the transmission points. Transmissions are referred to as “coherent” when their phases are aligned. Such alignment of phases aids in achieving the best possible performance for communication with the UE. If the transmissions are not phase aligned (coherent) then there is a possibility that the signals with opposite phase can cancel each other (destructively interfere) at the UE, resulting in a reduced SINR/throughput and thus reducing any gain that could be achieved by the use of JT CoMP. 
     One or more embodiments of the invention provide for a simple, efficient technique to provide inter-transmission point phase information to the eNB to enable coherent joint processing for downlink cooperative multipoint (DL CoMP). In one or more embodiments of the invention, providing such phase information is standards-transparent—that is, mechanisms for providing such information can be implemented while following existing standards. 
     Before describing in further detail the exemplary embodiments of this invention, reference is made to  FIG. 4  for illustrating a simplified block diagram of various electronic devices and apparatus that are suitable for use in practicing the exemplary embodiments of this invention. In  FIG. 4  a wireless network  400  is adapted for communication over a wireless link with an apparatus, such as a mobile communication device or node which may be referred to as a UE  402 , via a network access node, such as a transmission point or Node B (base station), and more specifically an eNB  404 . The network  400  may include a network control element (NCE)  406  that may include MME/S-GW functionality similar to that shown in  FIG. 1 , and which may provide connectivity with a further network, such as a telephone network, a data communications network such as the Internet, both such networks, or additional or alternative networks. 
     The UE  402  includes a controller, such as at least one computer or a data processor (DP)  410 A, at least one non-transitory computer-readable memory medium embodied as a memory (MEM)  410 B that stores a program of computer instructions (PROG)  410 C, and at least one suitable radio frequency (RF) transmitter and receiver pair (transceiver)  410 D for bidirectional wireless communications with the eNB  404  via one or more antennas  410 E. 
     The eNB  404  also includes a controller, such as at least one computer or a data processor (DP)  420 A, at least one computer-readable memory medium embodied as a memory (MEM)  420 B that stores a program of computer instructions (PROG)  420 C, and at least one suitable RF transceiver  420 D for communication with the UE  402  via one or more antennas  420 E (typically several such as when multiple input/multiple output (MIMO) operation is in use and/or JT CoMP is in use). The NCE includes a controller, such as at least one computer or a data processor (DP)  421 A, at least one computer-readable memory medium embodied as a memory (MEM)  421 B that stores a program of computer instructions (FROG)  421 C. The eNB  404  is coupled via a data/control path  422  to the NCE  406 . The path  422  may be implemented as an S1 interface similar to that illustrated in  FIG. 1 . The eNB  404  may also be coupled to another eNB via data/control path  424 , which may be implemented using an X2 interface similar to that illustrated in  FIG. 1 , by some high-capacity, low latency connection which may be proprietary and which may use, for example, optical fiber, or through the use of some combination of interfaces such as an X2 interface and a high-capacity low latency connection. 
     Also shown in  FIG. 4  is a second transmission point or network access node, such as a remote radio head (RRH) or a second eNB  425  that establishes a connection with the UE  402 . The second eNB  425  may include a DP  430 A, memory  430 B, and transceiver  430 D. The second eNB  425  may establish the connection with the UE  402  via antennas  430 E. The second eNB  425  may be connected with the NCE  406  via the path  422 , and may be connected to the first eNB  404  via the data/control path  424 . The radio access node (e.g., second eNB  425 ) is shown so as to represent at least one second transmission point that can operate with the first eNB  404  in order to perform JT CoMP with the UE  402  in accordance with certain examples of one or more embodiments of this invention, as described in detail below. The second transmission point (eNB  425 ) may have the same or a different cell ID than the first transmission point (eNB  404 ). 
     For the purposes of describing exemplary embodiments of this invention the UE  402  may be assumed to also include a channel state information-reference signal (CSI-RS) measurement and reporting function (MRF)  410 F that is operable with, for example, DL RRC signaling from the eNB  404 . RRC signaling is described in, for example, 3GPP TS 36.331 V10.4.0 (2011-12)  Technical Specification  3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 10). 
     The eNB  404  and the eNB  425  may include CSI-RS resource schedulers (RS)  420 F, operating in accordance with embodiments of this invention, such as with a non-limiting resource configuration shown in  FIG. 5  and discussed in greater detail below in connection with  FIG. 5 . It will be noted that embodiments of the invention are not limited to use with just one assemblage and arrangement of data. Further, and by example, in data configurations such as that illustrated in  FIG. 5 , there can be a different number of transmission points represented, and a different number of antennas and ports per transmission point. 
     It will be noted that a second CSI-RS resource scheduler (RS)  430 F could also be present at the second transmission point (eNB  425 ). Alternatively a joint scheduler can be used. Note that a single baseband (BB) unit could also be used for both the first and second transmission points, or each transmission point could operate with its own associated BB unit. In other words, it should be appreciated that the embodiment shown in  FIG. 4  is not be construed in a limiting sense on the how the exemplary embodiments of this invention can be implemented and instantiated. 
     At least the programs  410 C,  420 C,  421 C, and  430 C may be assumed to include program instructions that, when executed by the associated data processors  410 A,  420 A,  421 A and  430 A, enable the device to operate in accordance with the embodiments of this invention, as will be discussed below in greater detail. In general, the embodiments of this invention may be implemented at least in part by computer software executable by the DP  400 A of the UE  402  and/or by the DP  420 A of the eNB  404 , and/or by the DP  421 A of the NCE  406 , and/or by the DP  430 A of the eNB  425 , or by hardware, or by a combination of software and hardware (and firmware). 
     The various data processors, memories, programs, transceivers and interfaces depicted in  FIG. 4  can all be considered to represent means for performing operations and functions that implement the several non-limiting aspects and embodiments of this invention. 
     In general, the various embodiments of the UE  402  can include, but are not limited to, cellular mobile devices, smartphones, personal digital assistants (PDAs) having wireless communication capabilities, portable computers having wireless communication capabilities, image capture devices such as digital cameras having wireless communication capabilities, gaming devices having wireless communication capabilities, music storage and playback appliances having wireless communication capabilities, Internet appliances permitting wireless Internet access and browsing, as well as portable units or terminals that incorporate combinations of such functions. 
     The computer-readable memories  410 B,  420 B,  421 B, and  430 B may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, random access memory, read only memory, programmable read only memory, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors  410 A,  420 A.  421 A, and  430 A may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi-core processor architectures, as non-limiting examples. 
     Discussing now in greater detail examples of embodiments of this invention, in 3GPP terminology a transmission point (e.g. the eNB  404 ) is visible to the UE  402  as a channel state information-reference signal (CSI-RS) resource, assuming that the network is configuring different CSI-RS resources for each of the transmission points involved in the CoMP operation. However, the UE  402  is not aware of whether and how the different CSI-RS resources belong to different transmission points. The eNB  404  may configure the MRF  410 E of the UE  402  to perform CSI measurements and to report measurement results based on one or more CSI-RS resources. This provides at least the RS  420 F of the eNB  404  with information necessary to schedule data transmission (i.e., to select the CoMP transmission mode such as JT as well as to determine the appropriate physical resources and/or precoding weights) from any of the transmission points. 
     One or more embodiments of the invention provide for mechanisms for obtaining inter-transmission point phase information. 
     Consider the case of the two transmission points TP1 and TP2 (for example, eNB  404  and eNB  430  in  FIG. 4 ) each having 4 transmit (TX) antennas  420 E as was shown in  FIG. 4 . The network would configure the UE  402  to measure CSI from two different CSI-RS resource configurations, denoted here with CSI-RS_TP1 and CSI-RS_TP2 for TP1 and TP2, respectively. As each TP has 4 TX antennas  420 E the individual CSI-RS resource configurations need to contain 4 CSI-RS signals and should be selected in such a manner that they do not use the same resource elements. This can be achieved by selecting a different CSI-RS resource configuration from a table such as the table  500  illustrated in  FIG. 5 , which presents definitions of cross-reference signal configurations, and may be used as a cross-reference table between configuration identifiers and their definitions. Such definitions may follow, for example, Section 6.10.5, CSI reference signals, of 3GPP TS 36.211 
     For example, suppose that CSI-RS resource configurations 1 and 2 of the table  500  of  FIG. 5 , are selected. These CSI-RS configurations indicate different resource elements. Each of the CSI-RS resource configurations for the two TPs contains 4 CSI reference signals according to a 4 TX configuration. The CSI reference signals are denoted here as
         CSI-RS_TP1={port15 — 1, port16 — 1, port17 — 1, port18 — 1} for the antennas Ant1_TP1 to Ant4_TP1   CSI-RS_TP2=(port15 — 2, port16 — 2, port17 — 2, port18 — 2) for the antennas Ant1_TP2 to Ant4_TP2.       

     This can be logically denoted in the exemplary Table  600  shown in  FIG. 6  wherein the entry  602  follows CSI-RS configuration 1 presented in  FIG. 5  and wherein the entry  604  follows CSI-RS configuration 2 presented in  FIG. 5   
     It is currently considered in 3GPP (for example, as in R1-114094 and in other proposals), that a UE such as the UE  402  would report the following:
         1. CSI1 based on CSI-RS_TP1 (CSI for the TP1)   2. CSI2 based on CSI-RS_TP2 (CSI for TP2)   3. Inter-CSI-RS phase between CSI-RS_TP1 and CSI-RS_TP2—namely the phase relationship between one of the antenna ports of the two CSI-RS configurations, i.e., the phase offset of port15 — 1/ant1_TP1 and port15 — 2/ant1_TP2 quantized, e.g., to a 4 phase state/QPSK alphabet.       

     One significant issue with such an approach is that such a measurement is fundamentally quite different from those defined in the current 3GPP specification from at least a RAN4 test specification point of view. For example, it is unclear how the reliability of this proposed type of phase-only measurement could be verified with the current RAN4 test methodologies. Currently, the performance requirement—that is, the definition for the CSI accuracy—is such that when scheduled according to the recommended CSI (including CQI, pre-coding matrix indicator (PMI) and rank indicator (RI)), the physical uplink shared channel (PDSCH) block error rate shall not exceed 10%. This is given only for the full CSI of a single CSI-RS resource containing—for example, the RI. PMI and the corresponding CQI. That is, the feedback from the UE  402  needs to be sufficient to achieve a certain overall data throughput, but need not be sufficient to enable a separate evaluation of the accuracy of the RI, PMI and CQI feedback. Standardization of such a measurement would call for a significant amount of effort, while also complicating the implementation of the UE  402 . 
     One or more embodiments of the invention, therefore, provide a network implementation-based method that allows for the network to perform robust coherent joint transmission DL CoMP by providing an eNB such as the eNB  404  of  FIG. 4  with information that is needed regarding the phase relationship between different transmission points, and to accomplish this task without a need to standardize a completely new measurement and test methodology in 3GPP, or a need to standardize new required configuration signaling. One or more embodiments of the invention also provide for one or more UE designs or configurations providing directed to enabling the network to perform DL CoMP. 
     One or more exemplary embodiments of the present invention use CSI-RS resource configuration in a novel manner, so as to cause a UE such as the  402  to feedback inter-transmission point phase information to an eNB such as the eNB  404  bile using existing codebooks. The goal is to make the inter-transmission point feedback transparent or substantially transparent to a UE such as the UE  402  without requiring any changes to specifications beyond any changes needed to allow CoMP in general. Measures for CoMP include, for example, the possibility to configure more than a single CSI-RS resource and or configure CSI feedback from more than a single CSI-RS resource. Embodiments do not present a need for new “phase measurement” specification effort. 
     Exemplary embodiments of this invention in one aspect thereof configure one additional CSI-RS resource that spans one of the antennas (e.g., CSI-RS_TP12) in each of the participating (e.g., two) transmission points in order to obtain from the UE  402  the phase feedback in the same manner as the proposed inter-CSI-RS phase feedback. 
     In contrast to the example discussed above with reference to  FIG. 6  three different CSI-RS resources are configured in the following way for a non-limiting example of using the first antenna of each transmission point (for example, the first antenna of eNB  404  and the first antenna of eNB  425 ) as a phase reference:
         CSI-RS_TP1={port15 — 1, port16 — 1, port17 — 1, port18 — 1} for the antennas Ant1_TP1 to Ant4_TP1;   CSI-RS_TP2={port15 — 2, port16 — 2, port17 — 2, port18 — 2} for the antennas Ant1_TP2 to Ant4_TP2; and   CSI-RS_TP1 — 2={port15 — 3, port16 — 3} for the antennas Ant1_TP1 and Ant1_TP2.       

     The foregoing technique is shown logically in the Table  700  depicted in  FIG. 7 , wherein the table entries  702 ,  704 , and  706  correspond to the first, second, and third configurations, respectively, defined by the table  500  of  FIG. 5 . 
     It can be noted that instead of using the first antenna pair (Ant1_TP1 and Ant1_TP2) from two different transmission points, the CSI-RS_TP1 — 2 may be configured to comprise some other pair of antennas (for example the second antenna pair, or the third antenna pair) from two different transmission points. Note also that, for example, Ant1_TP1 and Ant2_TP2, etc., could be selected. That is, the antennas selected need not be a complementary pair of antennas from two transmission points. More generally, the antennas selected need not be a complementary set of k antennas from m transmission points, where k is equal to or greater than 2. 
       FIG. 8  presents a codebook table  800  taking the form of a precoding matrix indicator associating codebook indices with phase information. In the case of Rank=1 reporting for CSI-RS_TP1 — 2, the phase of the second antenna port in the PMI of the table  800  of  FIG. 8  corresponds to the phase to be applied for transmission between the transmission points. The PMI provides phase information between the transmission points, and this phase information may be used for pre-equalization of phase differences by the network because such equalization is needed for coherent transmission. It will be recognized that the QPSK/4 phase state information is exactly contained in the Rank=1 PMI, because the phase difference is given by: 
     argument{PMI(port15 — 3)/PMI(port16 — 3)}. 
     Therefore, for the Rank=1 restriction (that is, for the case in which the number of layers=1) the PMI fed back from the UE  402  contains the inter-transmission point phase feedback information with 2 bits. Reference can also be made to the Table  900  of  FIG. 9 , which shows one non-limiting example of how the fed back PMI can indicate to the eNB  404  (to the resource scheduler  404 E) the phase difference between two antennas, each of which is associated with one transmission point of a pair of transmission points. 
     The PMI that is fed back basically indicates to the eNB  404  how it should modify the phase of the second antenna so that the transmitted signals become coherently aligned at the UE  402 . Thus, and by example, if the phase offset of the second antenna is 90° (=pi/2=j) compared the first antenna (and the UE  402  thus feeds back codebook/PMI index then the eNB  404  is enabled to rotate the phase of the second antenna in the opposite direction (by −90°=pi/2=−j). The phase difference between the antennas can be derived based on the precoding weight from the first and second antenna by calculating the argument/phase of w1/w2, as shown in the table  900  depicted in  FIG. 9 . It has been noted that a 2-bit, quadrature phase shift keying (QPSK) alphabet seems to be sufficient to achieve most of the gains from inter-CSI-RS phase reporting. 
     Examples of the embodiments of this invention may be extended to accommodate the case when, for example, four transmission points are cooperating. In such a case, one antenna port would be again configured for each of the four transmission points and a 4-TX codebook (or a subset) with Rank=1 restriction could be used. Alternatively, and further in accordance with exemplary embodiments of this invention, one 2-antenna port inter-transmission point CSI-RS configuration could be used for each pair of transmission points. 
     One non-limiting implementation of one or more embodiments of this invention can be summarized with the following steps. Reference is also made to  FIG. 10  for showing a logic flow diagram presenting a process that may, for example, illustrate the operation of a method, and a result of execution of computer program instructions, in accordance with the exemplary embodiments of this invention. 
     At step  1002 , a network represented by, for example, the RS  420 F of the eNB  404 ) configures (for example, by RRC signaling) a set of n CSI-RS resources for the UE  402  to measure. The set of n CSI-RS resources includes at least one CSI-RS resource that spans over at least two transmission points, where n is a number greater than the number of cooperating transmission points. 
     At step  1004 , the network also configures CSI feedback corresponding to each CSI-RS resource. This CSI feedback configuration may involve either periodic or aperiodic feedback. The reporting mode and other related parameter (periodicity) may be configured separately for each CSI-RS resource. For the configured inter-transmission point CSI-RS resource or resources, one advantageous configuration is to restrict the reporting to Rank=1 in order for the PMI to reflect the intended phase relationship between the cooperating transmission points. 
     At step  1006 , the UE  402  performs CSI measurements based on the configured CSI-RS resources and transmits (reports) the CSI feedback to the network. The CSI feedback contains, for example, at least the PMI and may contain one or both of the RI and the corresponding CQI. The fed back PMI is received by the network and, in accordance with exemplary embodiments of this invention, indicates to the eNB  404  the inter-transmission point phase relationship. 
     At step  1008 , the network may utilize the CSI feedback information (including the indicated inter-transmission point phase relationship) in the selection of preferred precoders for coherent JT CoMP transmission. 
     A further aspect of the exemplary embodiments of this invention related to a non-transitory computer-readable medium that contains software program instructions, where execution of the software program instructions by at least one data processor results in performance of operations that comprise the execution of the process shown in  FIG. 10  and described above. 
     The exemplary embodiments of this invention can in one aspect be viewed as being primarily (if not totally) related to the eNB/network implementation, and largely transparent to the UE  10 . The use of the exemplary embodiments of this invention enables configuration of the antenna ports in the transmission points to enable the network to perform coherent JT CoMP. In one or more embodiments, of this invention needs only one antenna port per transmission point to be jointly configured to obtain inter-transmission point phase information at the eNB  404 , whereas the prior proposals assume all antenna ports need to be taken into account in the calculation. 
     Exemplary embodiments of this invention provide that the eNB  404  configures the inter-transmission point CSI-RS resource to include one TX antenna in each transmission point. This is significantly different than the prior art, where an aggregated feedback model assumes that all TX antennas in the cooperating transmission points are included in the CSI-RS resource, and where the inter-CSI-RS phase measurement may take all antennas in each transmission point into account. 
     The TX antennas between which the inter-TP feedback is configured may be of the same orientation—for example, the “first” antenna of each transmission point, and where the ‘paired’ antennas have, for example, the same polarization (vertical or horizontal). 
     In operation, the eNB  404  performs transmission by knowing the CSI for transmission point 1 (TP1) and the CSI for transmission point 2 (TP2), and the inter-transmission point PMI and the antennas for which the CSI for each transmission point has been calculated. The eNB  404  can then calculate the precoders for coherent JT CoMP from TP1 and TP2 as: 
     TP1: PMI(CSI-RS_TP1) for TP1; and 
     TP2: PMI(CSI-RS_TP2)*PMI(port16 — 3)/PMI3(port15 — 3) for TP2. 
     Through the use of exemplary embodiments of this invention it becomes possible for an eNB designer to provide for coherent JT CoMP by applying an intelligent configuration of CSI-RS resources as well as transmitter algorithms without requiring explicit specification support. 
     The benefits, advantages and technical effects made possible by the use of these exemplary embodiments may include at least the following. For example, the invention provides in one aspect thereof a standards-transparent solution to enable coherent JT CoMP with no added implementation complexity for the UE  402 , and with no additional standardization effort. In practice the UE  402  does not need to know that it should actually perform a specific phase measurement for coherent joint transmission, as exactly the same CSI measurement can be applied as with conventional multi-input-multi-output (MIMO) operation. 
     Furthermore, the use of the exemplary embodiments of this invention enables different reporting granularities (in the frequency/PRB domain) or periodicities (in the time domain) to be configured for the intra-transmission point feedback and for the inter-transmission point feedback, allowing the signaling overhead to be optimized. 
     Note that while an additional CSI-RS resource needs to be configured to the UE  402 , this incurs but a small additional amount of signaling overhead. 
     The various blocks shown in  FIG. 10  and discussed above may be viewed, for example, as method steps, as operations that result from operation of computer program code, as a plurality of logic circuit elements constructed to carry out the associated function or functions, or of some combination of such steps or operations. 
     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 diagrams, flow charts, or using some other pictorial representation, 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. 
     It should thus be appreciated that a further aspect of the exemplary embodiments of this invention is an apparatus that comprises at least one data processor and at least one memory that includes computer program code. The at least one memory and computer program code are configured, with the at least one data processor, to cause the apparatus at least to configure a set of n CSI-RS resources for a UE to measure, where the set of n CSI-RS resources includes at least one CSI-RS resource that spans over at least two transmission points. The at least one memory and computer program code are further configured, with the at least one data processor, to also cause the apparatus to configure CSI feedback corresponding to each CSI-RS resource, to receive CSI feedback from the UE, the CSI feedback comprising at least PMI and possibly RI and corresponding CQI for each configured CSI-RS resource. The PMI value indicates an inter-transmission point phase relationship. The at least one memory and computer program code are further configured, with the at least one data processor, to utilize CSI feedback information, including the indicated inter-transmission point phase relationship, for selecting precoders for accomplishing a coherent joint CoMP transmission. 
     In this apparatus the configured inter-transmission point CS RS resource(s) restrict the reporting to Rank=1. 
     A still further aspect of the exemplary embodiments of this invention is an apparatus that comprise means for configuring a set of n CSI-RS resources for a UE to measure, where the set of n CSI-RS resources includes at least one CSI-RS resource that spans over at least two transmission points. The apparatus further comprises means for configuring the UE for CSI feedback corresponding to each CSI-RS resource and means for receiving CSI feedback from the UE, where the CSI feedback comprises at least PMI and possibly RI and corresponding CQI for each configured CSI-RS resource. The PMI value indicates an inter-transmission point phase relationship. The apparatus further comprises means for utilizing the CSI feedback information, including the inter-transmission point phase relationship that is indicated by the PMI value, for selecting precoders for accomplishing a coherent joint CoMP transmission. 
     The exemplary embodiments of this invention further encompass a method, an apparatus and a computer program product configured to enable a user equipment to receive a measurement configuration for a set of n CSI-RS resources, where the set of n CSI-RS resources includes at least one CSI-RS resource that spans over at least two transmission points, and to measure and report CSI feedback corresponding to each CSI-RS resource, where the CSI feedback comprises at least PMI and possibly RI and corresponding CQI for each configured CSI-RS resource. 
     In this method, apparatus and computer program product at least one PMI value that is fed back indicates an inter-transmission point phase relationship for enabling a network resource scheduler function to at least select precoders for accomplishing a coherent joint COMP transmission to the user equipment. 
     In this method, apparatus and computer program product the user equipment is restricted to reporting Rank=1. 
     It should thus 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, and that the exemplary embodiments of this invention may be realized in an apparatus that is embodied as an integrated circuit. The integrated circuit, or circuits, may comprise circuitry (as well as possibly firmware) for embodying at least one or more of a data processor or data processors, a digital signal processor or processors, baseband circuitry and radio frequency circuitry that are configurable so as to operate in accordance with the exemplary embodiments of this invention. 
     Various modifications and adaptations to the foregoing exemplary embodiments of this invention may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this invention. 
     For example, while the exemplary embodiments have been described above in the context of the E-UTRAN (LTE-A) system, it should be appreciated that the exemplary embodiments of this invention are not limited for use with only this one particular type of wireless communication system, and that they may be used to advantage in other wireless communication systems. 
     It should be noted that the terms “connected,” “coupled,” or any variant thereof, mean any connection or coupling, either direct or indirect, between two or more elements, and may encompass the presence of one or more intermediate elements between two elements that are “connected” or “coupled” together. The coupling or connection between the elements can be physical, logical, or a combination thereof. As employed herein two elements may be considered to be “connected” or “coupled” together by the use of one or more wires, cables and/or printed electrical connections, as well as by the use of electromagnetic energy, such as electromagnetic energy having wavelengths in the radio frequency region, the microwave region and the optical (both visible and invisible) region, as several non-limiting and non-exhaustive examples. 
     Further, the various names used for the described parameters (e.g., CSI-RS_TP1={port15 — 1, port16 — 1, port17 — 1, port18 — 1}, CQI, PMI, RI, etc.) are not intended to be limiting in any respect, as these parameters may be identified by any suitable names. Further, any formulas and expressions that use these various parameters may differ from those expressly disclosed herein. Further, the various names assigned to different channels (e.g., PDSCH) and information elements are not intended to be limiting in any respect, as these various channels and information elements may be identified by any suitable names. 
     Furthermore, some of the features of the various non-limiting and exemplary embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles, teachings and exemplary embodiments of this invention, and not in limitation thereof.