Patent Publication Number: US-2016233938-A1

Title: Multiple Restrictions For CSI Reporting

Description:
TECHNICAL FIELD 
     This invention relates generally to wireless communications and, more specifically, relates to channel state information (CSI) feedback for communication systems using many antennas. 
     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 or pursued. 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. Abbreviations that may be found in the specification and/or the drawing figures are defined below, prior to the claims. 
     Multiple-antenna (e.g., MIMO) technology is becoming mature for wireless communications and has been incorporated into wireless broadband standards like LTE and Wi-Fi. Basically, the more antennas the transmitter/receiver is equipped with, the more the possible signal paths and the better the performance in terms of data rate and link reliability. The price to pay is increased complexity of the hardware (e.g., the number of RF amplifier frontends) and the complexity and energy consumption of the signal processing at both ends. 
     Massive MIMO uses a very large number of service antennas (e.g., hundreds or thousands) that are operated fully coherently and adaptively. Extra antennas help by focusing the transmission and reception of signal energy into ever-smaller regions of space. This brings improvements in throughput and energy efficiency, in particularly when combined with simultaneous scheduling of a large number of user equipment (e.g., tens or hundreds). 
     While massive MIMO has benefits, it also has drawbacks, particularly for CSI measurement and reporting. 
     BRIEF SUMMARY 
     This section is intended to include examples and is not intended to be limiting. 
     An exemplary method includes configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information. The restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources. The method includes transmitting the reference signals and interference measurement resources to the user equipment, and receiving from the user equipment the channel state information determined based on the reference signals, interference measurement resources, and the restrictions. 
     An additional exemplary embodiment includes a computer program, comprising code for performing the method of the previous paragraph, when the computer program is run on a processor. The computer program according to this paragraph, wherein the computer program is a computer program product comprising a computer-readable medium bearing computer program code embodied therein for use with a computer. 
     An apparatus comprises: means for configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information, wherein the restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources; means for transmitting the reference signals and interference measurement resources to the user equipment; and means for receiving from the user equipment the channel state information determined based on the reference signals, interference measurement resources, and the restrictions. 
     An exemplary apparatus includes one or more processors and one or more memories including computer program code. The one or more memories and the computer program code are configured to, with the one or more processors, cause the apparatus to perform at least the following: configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information, wherein the restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources; transmitting the reference signals and interference measurement resources to the user equipment; and receiving from the user equipment the channel state information determined based on the reference signals, interference measurement resources, and the restrictions. 
     An exemplary computer program product includes a computer-readable storage medium bearing computer program code embodied therein for use with a computer. The computer program code includes: code for configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information, wherein the restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources; code for transmitting the reference signals and interference measurement resources to the user equipment; and code for receiving from the user equipment the channel state information determined based on the reference signals, interference measurement resources, and the restrictions. 
     In another exemplary embodiment, a method includes configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information. The restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources. The method includes receiving from a base station the reference signals and interference measurement resources, determining the channel state information based on the specific resources for the reference signals and interference measurement resources, and transmitting the channel state information to the base station. 
     An additional exemplary embodiment includes a computer program, comprising code for performing the method of the previous paragraph, when the computer program is run on a processor. The computer program according to this paragraph, wherein the computer program is a computer program product comprising a computer-readable medium bearing computer program code embodied therein for use with a computer. 
     In a further exemplary embodiment, an apparatus comprises: means for configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information, wherein the restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources; means for receiving from a base station the reference signals and interference measurement resources; means for determining the channel state information based on the specific resources for the reference signals and interference measurement resources; and means for transmitting the channel state information to the base station. 
     An exemplary apparatus includes one or more processors and one or more memories including computer program code. The one or more memories and the computer program code are configured to, with the one or more processors, cause the apparatus to perform at least the following: configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information, wherein the restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources; receiving from a base station the reference signals and interference measurement resources; determining the channel state information based on the specific resources for the reference signals and interference measurement resources; and transmitting the channel state information to the base station. 
     An exemplary computer program product includes a computer-readable storage medium bearing computer program code embodied therein for use with a computer. The computer program code includes: code for configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information, wherein the restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources; code for receiving from a base station the reference signals and interference measurement resources; code for determining the channel state information based on the specific resources for the reference signals and interference measurement resources; and code for transmitting the channel state information to the base station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached Drawing Figures: 
         FIG. 1  is a block diagram of an exemplary system in which the exemplary embodiments may be practiced; 
         FIG. 2  is an example of scheduling details in an exemplary embodiment; 
         FIG. 3  is a logic flow diagram performed by a base station and  FIG. 4  is a logic flow diagram performed by a user equipment for multiple restrictions for CSI reporting, and these figures illustrate the operation of an exemplary method, a result of execution of computer program instructions embodied on a computer readable memory, functions performed by logic implemented in hardware, and/or interconnected means for performing functions in accordance with exemplary embodiments; and 
         FIG. 5  is an example of IMR restriction; 
         FIG. 6  is an example of how multiple CSI processes, each with restricted measurements for CSI-RS and IMR, could co-exist in a subframe and then switch positions in the next sub-frame; 
         FIG. 7  is an example of an information element in one exemplary. embodiment; and 
         FIG. 8  is a logic flow diagram performed by a base station and  FIG. 9  is a logic flow diagram performed by a user equipment for multiple restrictions for CSI reporting, and these figures illustrate the operation of an exemplary method, a result of execution of computer program instructions embodied on a computer readable memory, functions performed by logic implemented in hardware, and/or interconnected means for performing functions in accordance with exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The exemplary embodiments herein describe techniques for multiple measurement restrictions for CSI reporting, such as CQI and rank reporting. Additional description of these techniques is presented after a system in which the exemplary embodiments may be used is described. 
     Turning to  FIG. 1 , this figure shows a block diagram of an exemplary system in which the exemplary embodiments may be practiced. In  FIG. 1 , N UEs  110 - 1  through  110 -N are in wireless communication with a wireless network  100 . It is assumed the UEs  110  are similar and only UE  110 - 1  will be discussed herein. The user equipment  110  (e.g., UE  110 - 1 ) includes one or more processors  120 , one or more memories  125 , and one or more transceivers  130  interconnected through one or more buses  127 . Each of the one or more transceivers  130  includes a receiver, Rx,  132  and a transmitter, Tx,  133 . The one or more buses  127  may be address, data, or control buses, and may include any interconnection mechanism, such as a series of lines on a motherboard or integrated circuit, fiber optics or other optical communication equipment, and the like. The one or more transceivers  130  are connected to one or more antennas  128 . The one or more memories  125  include computer program code  123 . The UE  110  includes a CSI F/B (feedback) module  140 , comprising one of or both parts  140 - 1  and/or  140 - 2 , which may be implemented in a number of ways. The CSI F/B module  140  may be implemented in hardware as CSI F/B module  140 - 1 , such as being implemented as part of the one or more processors  120 . The CSI F/B (feedback) module  140 - 1  may be implemented also as an integrated circuit or through other hardware such as a programmable gate array. In another example, the CSI F/B module  140  may be implemented as CSI F/B module  140 - 2 , which is implemented as computer program code  123  and is executed by the one or more processors  120 . For instance, the one or more memories  125  and the computer program code  123  may be configured to, with the one or more processors  120 , cause the user equipment  110  to perform one or more of the operations as described herein. Each UE  110  communicates with eNB  170  via a wireless link  111 , and there are N wireless links shown. 
     The eNB  170  is a base station that provides access by wireless devices such as the UE  110  to the wireless network  100 . The eNB  170  includes one or more processors  152 , one or more memories  155 , one or more network interfaces (N/W I/F(s))  161 , and one or more transceivers  160  interconnected through one or more buses  157 . Each of the one or more transceivers  160  includes a receiver, Rx,  162  and a transmitter, Tx,  163 . The one or more transceivers  160  are connected to multiple (e.g., many) antennas  158 . The antennas may be a 3D planar antenna structure, where each column is a cross-polarized array, for instance. The one or more memories  155  include computer program code  153 . The eNB  170  includes a MIMO module  150 , comprising one of or both parts  150 - 1  and/or  150 - 2 , and the scheduler  151 , both of which may be implemented in a number of ways. The MIMO module  150  and/or the scheduler  151  may be implemented in hardware as MIMO module  150 - 1  or as the scheduler  151 - 1 , respectively, such as being implemented as part of the one or more processors  152 . The MIMO module  150  and/or the scheduler  150  may be implemented also as an integrated circuit or through other hardware such as a programmable gate array. In another example, the MIMO module  150  or the scheduler  151  may be implemented as MIMO module  150 - 2  or scheduler  151 - 2 , respectively, which are implemented as computer program code  153  and are executed by the one or more processors  152 . For instance, the one or more memories  155  and the computer program code  153  are configured to, with the one or more processors  152 , cause the eNB  170  to perform one or more of the operations as described herein. The one or more network interfaces  161  communicate over a network such as via the links  176  and  131 . The scheduler  151  performs operations such as scheduling communications between the eNB  170  and the UEs  110 . The MIMO module  150  performs operations such as communicating between the eNB  170  and the UEs  110  using (e.g., massive) MIMO, which uses many antennas, such as SU-MIMO or MU-MIMO. Two or more eNBs  170  communicate using, e.g., link  176 . The link  176  may be wired or wireless or both and may implement, e.g., an X2 interface. 
     The one or more buses  157  may be address, data, or control buses, and may include any interconnection mechanism, such as a series of lines on a motherboard or integrated circuit, fiber optics or other optical communication equipment, wireless channels, and the like. For example, the one or more transceivers  160  may be implemented as a remote radio head (RRH)  195 , with the other elements of the eNB  170  being physically in a different location from the RRH, and the one or more buses  157  could be implemented in part as fiber optic cable to connect the other elements of the eNB  170  to the RRH  195 . 
     The wireless network  100  may include a network control element (NCE)  190  that may include MME/SGW functionality, and which provides connectivity with a further network, such as a telephone network and/or a data communications network (e.g., the Internet). The eNB  170  is coupled via a link  131  to the NCE  190 . The link  131  may be implemented as, e.g., an S1 interface. The NCE  190  includes one or more processors  175 , one or more memories  171 , and one or more network interfaces (N/W I/F(s))  180 , interconnected through one or more buses  185 . The one or more memories  171  include computer program code  173 . The one or more memories  171  and the computer program code  173  are configured to, with the one or more processors  175 , cause the NCE  190  to perform one or more operations. 
     The wireless network  100  may implement network virtualization, which is the process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Network virtualization involves platform virtualization, often combined with resource virtualization. Network virtualization is categorized as either external, combining many networks, or parts of networks, into a virtual unit, or internal, providing network-like functionality to software containers on a single system. Note that the virtualized entities that result from the network virtualization are still implemented, at some level, using hardware such as processors  152  or  175  and memories  155  and  171 , and also such virtualized entities create technical effects. 
     The computer readable memories  125 ,  155 , and  171  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, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The processors  120 ,  152 , and  175  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 a multi-core processor architecture, as non-limiting examples. 
     In general, the various embodiments of the user equipment  110  can include, but are not limited to, cellular telephones such as smart phones, 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, tablets with wireless communication capabilities, as well as portable units or terminals that incorporate combinations of such functions. 
     Now that one possible system has been discussed, problems with massive MIMO are discussed. It is well recognized that with massive MIMO, in the context of reciprocity-based operation, the prediction of accurate MCS and rank is the bottleneck for performance. Note that the determination of a precoder can be performed in certain cases with good accuracy (for example, with multiple transmit antennas at the UE) and is not a bottleneck for performance. It is also well recognized that in the case of massive MIMO, MU-MIMO transmission is critical and the challenge here is to estimate the interference due to co-scheduled UEs. That is, UE1 co-scheduled with UE2 means that both UEs are sharing a common time-frequency resource as well as Tx power when receiving data from the eNB. They receive transmission from the same eNB on the same time-frequency resource but utilizing two different precoders. These transmissions interfere with each other depending on the precoders and the channel. 
     Furthermore, CATT contribution R1-144948 and Intel contribution R1-144670 proposed to use beamformed CSI-RS transmission for massive MIMO. These contributions were introduced at 3GPP TSG-RAN WGI #79, November 2014. While these contributions have possible benefits, they still have the possibility of having precoding weights that can change rather quickly (e.g., dynamically) both in time and in frequency. 
     Rel-10 eICIC introduced a subframe subset concept, which can be considered as a type of measurement restriction. An introduction to this concept is provided in Pedersen, et al., “eICIC functionality and performance for LTE HetNet co-channel deployments”, Vehicular Technology Conference (VTC Fall), 2012 IEEE. IEEE Press, 2012. The Pedersen article states the following: “It is therefore necessary for the network to configure restricted CSI measurements for Rel-10 UEs, so that the eNB receives such reports corresponding to normal subframes and ABS, respectively.” That is, CSI measurements may be restricted to either normal subframes or ABSs. By contrast, in the instant embodiments, signal and interference for one CQI report (as an example) may follow different restrictions for time and/or frequency, as described below. 
     Exemplary embodiments herein relate to massive MIMO systems, e.g., to be deployed in 5G as well as future FD-MIMO LTE-A systems in Rel-13 and beyond. Focus is placed on the design aspects on 3D-MIMO, especially channel state information (CSI) feedback. 
     Channel reciprocity is one key feature of a TDD system, where an estimated channel from uplink could be used to form the beamforming precoder for a downlink transmission. It is especially interesting in a massive MIMO environment, with a large number of antenna ports, since codebook-based PMI feedback amount is too high. 
     In this document, exemplary solutions are proposed that are applicable to the problems noted above. Specifically, in an example, a proposal is to use measurement restrictions for CSI measurement that will enable the eNB to use UE-specific precoded CSI-RS (e.g., CSI measurement resources) for accurate MCS and rank selection for data transmission. In addition, as another example, it is proposed to use UE-specific IMRs (interference measurement resources) that are resource restricted to enable estimation of interference due to co-scheduled MU-MIMO UEs for enhancing MU-MIMO transmission. Furthermore, measurement restrictions are proposed in embodiments to be defined for CQI and RI feedback to allow for CSI-based beamforming without requiring PMI feedback. 
     A motivation for the exemplary embodiments herein is a need for CQI and RI feedback using precoded CSI-RS (e.g., CSI measurement resource) and in conjunction with IMRs for MU-MIMO purposes. That is, since an appropriate precoding weight can change rather quickly (e.g., dynamically) both in time and in frequency, it is necessary that a UE does not average the measurements obtained from CSI-RS or IMR instances in an unrestricted fashion in time or frequency or both. Therefore it is beneficial to have restrictions on how much a UE can average in time, frequency, or both while measuring multiple instances of precoded CSI-RS and IMR. 
     In this instance, since there is no need for PMI feedback, the accuracy requirement of channel estimation and interference estimation is reduced. In other words, the accuracy requirement of channel and interference estimation can be relaxed to a certain extent because the UE is not required to feedback a PMI in this case. Rank in this case is defined in an open loop sense of comparing single-port transmission with two-port transmission (with no PMI). On the other hand, due to the UE-specific nature of CSI-RS and IMR needed in this case, there is a need for more physical resources to be dedicated to CSI-RS and IMR within a serving cell, relative to without using UE-specific CSI-RS and UE-specific IMR. This is because UE-specific CSI-RS and IMR will be used, e.g., for multiple UEs, and this UE-specific CSI-RS and IMR is not used in a conventional system. 
     This invention, in an exemplary embodiment, allows one to configure separately measurement restrictions in time and/or frequency for CSI-RS resources and IMR. The accuracy of CQI, RI is expected to be not affected significantly, especially as more measurement samples become available to the UE as time progresses. Measurement restriction can be configured by the network (e.g., via the eNB  170 ) and the UE  110  shall separately measure the signal and interference part following each measurement restriction. 
     Exemplary scheduling details for MU-MIMO is detailed in  FIG. 2 , where it is shown how UE-specific precoded CSI-RS and UE-specific IMR can be utilized for accurate link adaption for MU-MIMO by incorporating some additional packet delay at the scheduler. A precoded CSI-RS along with a measurement restriction is considered to be UE-specific if the physical resources for the precoded CSI-RS with such a restriction are dedicated for a particular UE—this is a provisioning issue at the eNB  170 . The same rule applies to a UE-specific IMR. The UE does not know if some other UE is also measuring on the same resource. Other than this provisioning aspect, there is no unique property of a CSI-RS that makes it UE specific. 
       FIG. 2  shows a CSI-RS, IMR timeline  210  at the eNB  170 , such that UE-specific CSI-RS and IMR precoding are transmitted by eNB  170  at times  215 - 1 ,  215 - 2 , and  215 - 3 .  FIG. 2  also shows a non-UE-specific CSI-RS, IMR timeline  220 , illustrating times  225 - 1 ,  225 - 2 ,  225 - 3 , and  225 - 4  when the eNB  170  transmits the non-UE-specific CSI-RS, IMR. The eNB scheduler timeline  230  shows a time  260  at which a SU-MIMO CSI is received in response to the non-UE-specific CSI-RS, IMR transmitted by the eNB  170  at time  225 - 1 . For ease of reference, the other receptions by the eNB  170  in response to the times  225 - 2  through  225 - 4  are not shown. MU-MIMO prescheduling occurs at time  235 - 1  and the MU-MIMO scheduling occurs at time  235 - 2 . The additional packet delay  240  is also shown. Furthermore, at time  250 , the eNB transmits UE-specific CSI-RS that is precoded using a precoder intended for UE  110  and UE-specific IMR intended for UE110. At time  250 , the eNB also transmits precoded signals using a precoder not intended for UE110 on a resource that coincides with the UE specific IMR intended for UE110. It is noted that CSI-RS is a signal that is measured at the UE  110 . IMR, on the other hand, is not a signal but a time-frequency-resource, and the UE  110  measures the power on the designated IMR and assumes that this is the interference power. The UE  110  responds at time  215 - 2  with transmission of MU-MIMO CSI, which is received by the eNB  170  at time  270 . The MU-MIMO CSI reflects the signal to interference plus noise ratio (SINR) corresponding to a MU-MIMO transmission to UE  110 . It may be noted that UE  110  may assume a SU-MIMO hypothesis for determining CSI transmitted at  215 - 2 . 
     A typical, exemplary process is now described. A UE  110  is configured with a CSI-process-1 that is comprised of a CSI-RS and an IMR. This CSI-RS and IMR is not precoded, is not UE specific, and has no measurement restrictions associated with the CSI-RS or the IMR (or the process may have a measurement restriction on IMR). This is represented by the timeline  220 . The UE provides CSI feedback (e.g., CQI/RI/PMI feedback) according to this CSI-process every 10 ms, as illustrated by times  225 . 
     The same UE  110  is also configured with another CSI-process-2 that is comprised of a UE-specific precoded CSI-RS with measurement restrictions and a UE-specific IMR with measurement restrictions. This is represented by the timeline  210 . The UE provides CQI feedback according to this CSI-process every 10 ms, as illustrated by the times  215 . 
     The scheduling timeline at the eNB is represented by the timeline  230 . The eNB  170  considers the CSI (e.g., CQI/PMI/RI reports) received due to CSI-process-1 (SU-MIMO CSI in the figure) and determines the best MU-MIMO pairing for the UE (MU-MIMO pre-scheduling  290  in the figure). At the same time, the eNB  170  determines the precoding weight for the UE to be used for MU transmission as well as the precoding weight for a paired UE. Once this is done, the eNB  170  is able to precode a CSI-RS and an IMR with the determined precoding weights as needed by CSI-process-2 and transmits the same at time  250 . The eNB uses the precoding weight determined for the UE for precoding the CSI-RS and uses the precoding weight determined for the paired UE for precoding the IMR. The eNB  170  then receives a CQI associated with CSI-process-2 from the UE (at time  270 ) and proceeds to data transmission (MU-MIMO scheduling  295  in the figure). The additional packet delay  240  in scheduling at least for certain scheduling instances (may not be all) is unique to embodiments herein. 
     Note that there is an improvement of link adaptation for MU-MIMO by enabling accurate estimation of interference from co-scheduled UEs. That is, if the CSI-process-2 does not exist, which can be considered as the conventional way of scheduling, then the PMI fed back by the UE is modified by the eNB due to MU transmission and the interference observed by the UE in CSI-process-1 does not include the interference due to the paired UE. Both of these reasons result in poor link adaptation performance. Due to the additional CSI-process-2, the eNB can actually create a somewhat dummy MU transmission via CSI-process-2 and expect an accurate CQI that can be used for the actual data transmission with exactly the same precoding weights used for precoding the CSI-process-2. In more detail, when a UE receives data due to a MU (multi-user) transmission, another UE is also receiving data on the same resources as well as taking up one-half the power (assuming a pairing of two UEs). When the UE estimates CSI using CSI-process-1, the UE does not assume a MU transmission but instead assumes a SU (single-user) transmission where there are no co-scheduled UEs and transmission happens with full power. Therefore, the estimated CSI using CSI-process-1 does not help the eNB  170  enough to perform an efficient MU transmission (e.g., as the eNB cannot select proper MCS). The CSI-process-2 assigns one-half power to the UE (for a pairing of two UEs) and the process also emulates the interference due to the co-scheduled UE in the IMR resources—this is exactly how the UE would perceive a MU transmission and the CSI determined from CSI-process-2 then helps the eNB to perform an efficient MU transmission (e.g., as the eNB can select proper MCS). 
     Now that an example of scheduling has been described, more detailed flows for the eNB  170  and UE  110  are described in relation to  FIGS. 2-4 .  FIG. 3  is a logic flow diagram performed by a base station and  FIG. 4  is a logic flow diagram performed by a user equipment for multiple measurement restrictions for CSI reporting. These figures illustrate the operation of an exemplary method, a result of execution of computer program instructions embodied on a computer readable memory, functions performed by logic implemented in hardware, and/or interconnected means for performing functions in accordance with exemplary embodiments. The operations performed by the eNB  170  may be performed under the control in part of the scheduler  151 , and the operations performed by the eNB  170  for MIMO transmissions and receptions may be performed under the control in party of the MIMO module  150 . The UE  110  may be considered to perform the blocks in  FIG. 4 , e.g., under control in part by the CSI F/B module  140 . 
     In blocks  305  and  405 , the eNB  170  configures the UE  110  (and the UE configures itself in block  405 ) with the first CSI process for non-UE-specific measurement signals such as CSI-RS and IMR. As stated above, there are no measurement restrictions associated with the CSI-RS or the IMR (or the process may have a measurement restriction on IMR) for blocks  305  and  405 . In blocks  310  and  410 , the eNB  170  configures the UE  110  (and the UE  110  configures itself for block  410 ) with a second CSI process for UE-specific measurement signals (such as CSI-RS and IMR) and with measurement restrictions in time, frequency, or both time and frequency. The measurement restrictions configure the UE  110  with one or more restrictions that restrict resources carrying reference signals (e.g., CSI-RS) and IMRs to specific resources to be used by the UE  110  for determining channel state information. The one or more restrictions are one or both of the following: restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and/or restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources. 
     In block  315 , the eNB schedules and transmits non-UE-specific measurement signals (e.g., CSI-RS and IMR) to the UE  110  (and other UEs  110 ) for the first CSI process using SU-MIMO. This corresponds to blocks  413  and  415 , where the UE  110  receives scheduling for the non-UE-specific measurement signals (block  413 ) and then receives corresponding non-UE-specific measurement signals (e.g., CSI-RS and IMR) from the base station for the first CSI process (block  415 ). This is illustrated in  FIG. 2  at the time  225 , where the eNB transmits the non-UE-specific CSI-RS and IMR. 
     In block  418 , the UE  110  determines CSI (such as CQI/RI/PMI) for the first CSI process and in block  420  transmits the CSI to the base station for the first CSI process. This CSI is based on the received non-UE-specific measurement signals. Note that the non-UE-specific IMR (for CSI-process-1) does not reflect the interference condition for MU. The eNB  170  receives the CSI from the UE (and other UEs) for the first CSI process in block  320 . This is illustrated in  FIG. 2  by the time  260 , where the SU-MIMO CSI is received. 
     In block  325 , the eNB considers the CSI (e.g., CQI/PMI/RI reports) received due to the first CSI process and determines best MU-MIMO pairing(s) for the UE, where another UE is (or other UEs are) paired with the original UE. The pairing means that the original UE (e.g., UE  110 - 1 ) and the other UE(s) (e.g., UEs  110 - 2  . . . ) will be part of a MU transmission. The MU-MIMO pre-scheduling  290  in  FIG. 2  includes at least the determining the best MU-MIMO pairing(s) for the UE. In block  330 , the eNB  170  determines the precoding weight for the UE to be used for MU transmission as well as the precoding weight for the paired UE(s). These determinations are based on the CSI from the UE and the paired UE(s). 
     The eNB  170  then, in block  335 , precodes UE-specific CSI-RS using the precoding weight for the UE and precodes UE-specific IMR using the precoding weight for the paired UE(s) for the second CSI process. In terms of precoding UE-specific IMR, for the original UE, this UE is configured to measure the UE-specific resource(s) carrying the IMR, based on the configured measurement restrictions. From the perspective of the original UE, the UE-specific IMR is simply a resource (or resources) with restrictions on which resources will be used to determine CSI. From the perspective of the eNB, a precoding transmission coincides on the same resource as the IMR, the precoder(s) being designed for the paired UE(s). The transmission could be a “dummy” one, intended simply for the purposes of emulating the interference. The transmission also could also be a valid transmission to the paired UE(s), including a reference signal. In block  340 , the eNB  170  schedules and transmits the UE-specific measurement signals, the precoded CSI-RS and IMR, to the UE and may also transmit to the paired UE(s) at the same time. This is illustrated in  FIG. 2  at time  250 , where UE-specific CSI-RS and IMR are transmitted by the eNB  170 . Block  340  corresponds to blocks  437  and  440 . The UE  110 , in block  437 , receives scheduling for the UE-specific measurement signals for second CSI process, and in block  440  receives the UE-specific CSI-RS and IMR for the second CSI process from the base station. As described above, from the perspective of the original UE, the UE-specific IMR is simply a resource (or resources) with restrictions on which resources will be used to determine CSI. From the perspective(s) of the paired UE(s), a precoded transmission coincides on the same resource as the IMR, the precoder(s) designed for the paired UE(s). 
     The UE  110  in block  443  determines CSI for the second CSI process based on the measurement restrictions in time, frequency or both time and frequency. That is, the UE uses the configured one or more restrictions that restrict resources carrying reference signals (e.g., CSI-RS) and IMRs to specific resources to be used by the UE for determining the CSI. This CSI is typically CQI and/or RI. Note that the reference signals and IMRs can be independently configured. Measurement restrictions could be a set of {subcarriers or PRBs or subbands, sub-frames}, e.g., that occur every frame (10 ms), e.g., {subband 0, sub-frames 4/10, 5/10} occurring every frame. Measurement restrictions could also be a function of sub-frame number and thereby change with time (e.g., by cycling through a set of restrictions). Measurement restrictions should be consistent with the signaled configurations for CSI-RS and IMR. That is, the UE should not be forced to measure at a particular resource where the UE does not expect to receive the CSI-RS signal or IMR. 
     In block  445 , the UE  110  transmits the determined CSI to the base station for the second CSI process. In  FIG. 2 , this is illustrated at times  215  and specifically  215 - 2 . The eNB  170  in block  345  receives the CSI from UE (and from the paired UE(s)) for the second CSI process, and this is illustrated in  FIG. 2  at time  270 , where MU-MIMO CSI is received. 
     In block  350 , the eNB  170  determines precoding to apply to information based on the CSI from UE for the second CSI process. This operation is similarly performed for the paired UE(s). In block  355 , the eNB  170  applies the determined precoding to the information and schedules and transmits the precoded information to the UE (and to the paired UE(s)) using (e.g., massive) MU-MIMO in block  360 . The scheduling and transmitting is illustrated in  FIG. 2  at time  235 - 2 , by the MU-MIMO scheduling  295 . Block  360  corresponds to blocks  457  and  460 , where the UE  110  receives scheduling for the precoded information to be transmitted from base station (block  457 ) and receives precoded information from the base station using (e.g., massive) MU-MIMO. 
     To clarify how the restriction of the UE-specific precoded CSI-RS and IMR may look in practice,  FIG. 5  presents an example of IMR measurement restriction. A similar example could be used for CSI-RS. In a given sub-frame  500 , the IMR resources are partitioned in frequency. In different sub-bands, different interference is composed by the eNB that corresponds to different MU-MIMO pairings. The UE is not expected to average interference across sub-bands but is allowed to average interference across different sub-frames  500 . In this example, the entire bandwidth (shown as “frequency”) is divided into four sub-bands, of which sub-bands  520 - 1  and  530 - 1  correspond to interferences for a first UE pairing (pairing- 1 ), sub-band  520 - 2  corresponds to interferences for a second UE pairing (pairing- 2 ), and sub-band  520 - 3  corresponds to interferences for a third UE pairing (pairing- 3 ). The measurement restriction CSI-RS may be similarly designed for frequency and time. In  FIG. 5 , the UE is using both sub-bands  520 - 1  and  530 - 1  for a single CSI calculation for the purposes of an exemplary embodiment-meaning at least one of the CSI components (e.g., rank) is determined based on both the sub-bands. 
       FIG. 6  shows an example of how multiple CSI processes, each with restricted measurements for CSI-RS and IMR, could co-exist in a subframe and then switch positions in the next sub-frame. This illustrates multiplexing of CSI processes in an FDM fashion for both periodic and aperiodic feedback reporting cases. As can be seen, sub-frame  600 - k  has CSI process-2 610-2 in sub-bands  1 ,  2 ,  5 , and  6  and CSI process-3 610-3 in sub-bands  3  and  4 . In a later (e.g., subsequent) subframe  600 -( k +λ), the CSI process-2 610-2 is in sub-bands  3  and  4  and CSI process-3 610-3 is in sub-bands  1 ,  2 ,  5 , and  6 . A similar situation as in  FIG. 5  applies to  FIG. 6 . For instance, the UE is using both sub-bands  610 - 2  for sub-frame  600 - k  for a single CSI calculation for the purposes of an exemplary embodiment—meaning at least one of the CSI components (e.g., rank) is determined based on both the sub-bands for this particular sub-frame. In  FIG. 6 , at least one of the restrictions will not span multiple sub-frames—e.g., the IMR may not span multiple subframes but the CSI-RS can. 
     Communication of measurement restrictions from the eNB  170  to the UE  110  might be as follows. Resource restrictions could be communicated to a UE  110  in an explicit manner, e.g., exclusively via RRC signaling or a combination of RRC and dynamic signaling (dynamic selection of one resource restriction or rank restriction could be dynamic for example). An example information element  700  for RRC signaling of CSI process with resources restriction is shown in  FIG. 7 .  FIG. 7  illustrates components of higher layer signaling. 
     Concerning UE behavior considering measurement restrictions, the following are examples of such. For CSI feedback, the UE  110  will recognize the resource restrictions when measuring CSI measurement REs or interference from IMRs that are configured by higher layer signaling. A UE, for purposes of determining CSI feedback, will not average across the measurement restrictions for the purposes of determining CQI and RI feedback. If a 1-port CSI-RS is configured then rank determination is not applicable. If a 2-port CS-RS is configured, then the UE may determine and feedback rank. It is also possible for the eNB to restrict the rank of an UE to 1 (one). 
       FIG. 8  is a logic flow diagram performed by a base station for multiple restrictions for CSI reporting. This figure illustrates the operation of an exemplary method, a result of execution of computer program instructions embodied on a computer readable memory, functions performed by logic implemented in hardware, and/or interconnected means for performing functions in accordance with exemplary embodiments. It is assumed the blocks in  FIG. 8  are performed by a base station such as eNB  170 , e.g., under control in part of the MIMO module  150  and scheduler  151 . 
     For ease of reference, assume that the flow in  FIG. 8  is a method  800 . The eNB  170 , as part of method  800 , in block  810  performs the operation of configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information. The restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals (block  815 ); and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources (block  820 ). The first and second restrictions may be different. In block  825 , the eNB  170  performs the operation of transmitting the reference signals and interference measurement resources to the user equipment. The eNB  170 , in block  830 , performs the operation of receiving from the user equipment the channel state information determined based on the reference signals, interference measurement resources, and the restrictions. 
     Additional exemplary embodiments are as follows. A method as in method  800 , wherein the restrictions restrict specific resources to one of a set of subcarriers, a set of physical resource blocks, a set of subbands, or a set of sub-frames for one or both of the reference signals or the interference measurement resources. A method as in this paragraph, wherein the specific resources are further restricted to certain resources that occur during a frame. 
     A method as in method  800  and the previous paragraph, wherein the configuring further comprises configuring the user equipment to restricting use by the user equipment to the specific resources as a function of sub-frame number and changing the sub-frame number with time. 
     A method as in method  800  and paragraphs referencing method  800 , wherein: 
     the method further comprises: 
     determining a pairing between the user equipment and one or more paired user equipment based on channel state information from the user equipment that was determined from non-user-equipment-specific reference signals and interference measurement resources and from a plurality of other user equipment including the one or more paired user equipment; and 
     precoding, using the determined pairing, user equipment-specific channel state information reference signals based on a precoding weight determined for the user equipment and precoded information to be transmitted on resources corresponding to the user equipment-specific interference measurement resources for the user equipment; and 
     transmitting comprises transmitting the precoded user equipment-specific channel state information reference signals to the user equipment and transmitting precoded information on the resources corresponding to the user equipment-specific interference measurement resources. 
     A method as in method  800  and paragraphs referencing method  800 , further comprising: coding information based on a precoder selected using the received channel state information; and transmitting the coded information to the user equipment. This is typically a MU-MIMO transmission. 
     A method as in method  800  and paragraphs referencing method  800 , wherein the channel state information comprises one or more of a channel quality indicator or a rank indictor. 
     A method as in method  800  and paragraphs referencing method  800 , wherein configuring further comprises transmitting an information element indicating the restrictions to the user equipment using radio resource control signaling. 
     A method as in method  800  and paragraphs referencing method  800 , where an entire bandwidth is divided into a number of sub-bands, and the restrictions limit use by the user equipment to particular ones of the sub-bands for one or both of the reference signals or the interference measurement resources. The method of this paragraph, wherein configuring further comprises changing the restrictions from one set of sub-bands in a first subframe to a different set of sub-bands in a second subframe. 
     Another example is an apparatus comprising: means for configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information. The restrictions are both of the following: first restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and second restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources. The first and second restrictions may be different. The apparatus comprises means for transmitting the reference signals and interference measurement resources to the user equipment and means for receiving from the user equipment the channel state information determined based on the reference signals, interference measurement resources, and the restrictions. An apparatus as in this paragraph, with means for performing any of the methods in the paragraphs referencing method  800 . 
     Additional exemplary embodiments are as follows. An apparatus as in any apparatus above, wherein the restrictions restrict specific resources to one of a set of subcarriers, a set of physical resource blocks, a set of subbands, or a set of sub-frames for one or both of the reference signals or the interference measurement resources. An apparatus as in this paragraph, wherein the specific resources are further restricted to certain resources that occur during a frame. 
     An apparatus as in any apparatus above, wherein the means for configuring further comprises means for configuring the user equipment to restricting use by the user equipment to the specific resources as a function of sub-frame number and changing the sub-frame number with time. 
     An apparatus as in any apparatus above, wherein: 
     the apparatus further comprises: 
     means for determining a pairing between the user equipment and one or more paired user equipment based on channel state information from the user equipment that was determined from non-user-equipment-specific reference signals and interference measurement resources and from a plurality of other user equipment including the one or more paired user equipment; and 
     means for precoding, using the determined pairing, user equipment-specific channel state information reference signals based on a precoding weight determined for the user equipment and precoded information to be transmitted on resources corresponding to the user equipment-specific interference measurement resources for the user equipment; and 
     the means for transmitting comprises means for transmitting the precoded user equipment-specific channel state information reference signals to the user equipment and transmitting precoded information on the resources corresponding to the user equipment-specific interference measurement resources. 
     An apparatus as in any apparatus above, further comprising: means for coding information based on a precoder selected using the received channel state information; and means for transmitting the coded information to the user equipment. This is typically a MU-MIMO transmission. 
     An apparatus as in any apparatus above, wherein the channel state information comprises one or more of a channel quality indicator or a rank indictor. 
     An apparatus as in any apparatus above, wherein the means for configuring further comprises means for transmitting an information element indicating the restrictions to the user equipment using radio resource control signaling. 
     An apparatus as in any apparatus above, where an entire bandwidth is divided into a number of sub-bands, and the restrictions limit use by the user equipment to particular ones of the sub-bands for one or both of the reference signals or the interference measurement resources. The apparatus of this paragraph, wherein the means for configuring further comprises means for changing the restrictions from one set of sub-bands in a first subframe to a different set of sub-bands in a second sub frame. 
     Another exemplary embodiment an apparatus that includes one or more processors and one or more memories including computer program code. The one or more memories and the computer program code are configured to, with the one or more processors, cause the apparatus to perform the method  800  or any of the methods in the paragraphs referencing method  800 . 
       FIG. 9  is a logic flow diagram performed by a user equipment for multiple restrictions for CSI reporting. Further, this figure illustrates the operation of an exemplary method, a result of execution of computer program instructions embodied on a computer readable memory, functions performed by logic implemented in hardware, and/or interconnected means for performing functions in accordance with exemplary embodiments. It is assumed the blocks in  FIG. 9  are performed by a UE  110 , e.g., under control in part of the CSI F/B module  140 . 
     For ease of reference, assume that the flow in  FIG. 9  is a method  900 . The UE  110  in block  910  performs the operation of configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information. The restrictions are both of the following: restrictions in time, frequency, or both time and frequency of resources that carry the reference signals (block  915 ); and restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources (block  920 ). In block  925 , the UE  110  performs the operation of receiving from a base station the reference signals and interference measurement resources. The UE  110 , in block  930 , performs the operation of determining the channel state information based on the specific resources for the reference signals and interference measurement resources, and in block  930 , the UE  110  performs the operation of transmitting the channel state information to the base station. 
     Additional examples are as follows. A method as in method  900 , wherein the restrictions restrict specific resources to one of a set of subcarriers, a set of physical resource blocks, a set of subbands, or a set of sub-frames for one or both of the reference signals or the interference measurement resources. A method as in this paragraph, wherein the specific resources are further restricted to certain resources that occur during a frame. 
     A method as in method  900  and the previous paragraph, wherein: 
     the method further comprises measuring the received reference signals and measuring power on the received interference measurement resources; and 
     determining the channel state information further comprises determining the channel state information based on: 
     the measured received reference signals and the restrictions in time, frequency, or both time and frequency of the resources that carried the reference signals; and 
     the measured power and the restrictions in time, frequency, or both time and frequency of the resources that carried the measured interference measurement resources. 
     A method as in method  900  and paragraphs referencing method  900 , further comprising: receiving from the base station previously coded information, wherein the previously coded information was coded by the base station based on a precoder selected using the channel state information. 
     A method as in method  900  and paragraphs referencing method  900 , wherein the channel state information comprises one or more of a channel quality indicator or a rank indictor. 
     A method as in method  900  and paragraphs referencing method  900 , wherein configuring further comprises receiving from the base station an information element indicating the restrictions using radio resource control signaling. 
     A method as in method  900  and paragraphs referencing method  900 , where an entire bandwidth is divided into a number of sub-bands, and the restrictions limit use by the user equipment to particular ones of the sub-bands for one or both of the reference signals or the interference measurement resources. A method as in this paragraph, wherein configuring further comprises changing the restrictions from one set of sub-bands in a first subframe to a different set of sub-bands in a second sub frame. 
     Another example is an apparatus comprising: means for configuring a user equipment with restrictions that restrict resources carrying reference signals and interference measurement resources to specific resources to be used by the user equipment for determining channel state information. The restrictions are both of the following: restrictions in time, frequency, or both time and frequency of resources that carry the reference signals; and restrictions in time, frequency, or both time and frequency of resources that carry the interference measurement resources. The apparatus comprises means for receiving from a base station the reference signals and interference measurement resources, means for determining the channel state information based on the specific resources for the reference signals and interference measurement resources, and means for transmitting the channel state information to the base station. An apparatus as in this paragraph, with means for performing any of the methods in the paragraphs referencing method  900 . 
     Additional examples are as follows. An apparatus as in any of the apparatus above, wherein the restrictions restrict specific resources to one of a set of subcarriers, a set of physical resource blocks, a set of subbands, or a set of sub-frames for one or both of the reference signals or the interference measurement resources. An apparatus as in this paragraph, wherein the specific resources are further restricted to certain resources that occur during a frame. 
     An apparatus as in any of the apparatus above, wherein: 
     the apparatus further comprises means for measuring the received reference signals and measuring power on the received interference measurement resources; and 
     the means for determining the channel state information further comprises means for determining the channel state information based on: 
     the measured received reference signals and the restrictions in time, frequency, or both time and frequency of the resources that carried the reference signals; and 
     the measured power and the restrictions in time, frequency, or both time and frequency of the resources that carried the measured interference measurement resources. 
     An apparatus as in any of the apparatus above, further comprising: means for receiving from the base station previously coded information, wherein the previously coded information was coded by the base station based on a precoder selected using the channel state information. 
     An apparatus as in any of the apparatus above, wherein the channel state information comprises one or more of a channel quality indicator or a rank indictor. 
     An apparatus as in any of the apparatus above, wherein the means for configuring further comprises means for receiving from the base station an information element indicating the restrictions using radio resource control signaling. 
     An apparatus as in any of the apparatus above, where an entire bandwidth is divided into a number of sub-bands, and the restrictions limit use by the user equipment to particular ones of the sub-bands for one or both of the reference signals or the interference measurement resources. An apparatus as in this paragraph, wherein the means for configuring further comprises means for changing the restrictions from one set of sub-bands in a first subframe to a different set of sub-bands in a second subframe. 
     Another exemplary embodiment an apparatus that includes one or more processors and one or more memories including computer program code. The one or more memories and the computer program code are configured to, with the one or more processors, cause the apparatus to perform the method  900  or any of the methods in the paragraphs referencing method  900 . 
     A system comprises any of the apparatus referring to method  800  and any of the apparatus referring to method  900 . 
     An additional exemplary embodiment includes a computer program, comprising code for performing the methods  800  or  900  or any methods referring to methods  800  or  900 , when the computer program is run on a processor. The computer program according to this paragraph, wherein the computer program is a computer program product comprising a computer-readable medium bearing computer program code embodied therein for use with a computer. 
     Exemplary advantages and technical effects of various embodiments include the following non-limiting and non-exhaustive examples: 
     1. There is improvement of link adaption for reciprocity-based operation by enabling feedback of post beamforming CQI. 
     2. There is improvement of link adaption for MU-MIMO by enabling accurate estimation of interference from co-scheduled UEs. 
     3. Existing methods of CSI feedback from LTE-A are built on with legacy support. 
     4. There is overhead reduction for the resources needed for CSI-RS and IMR. 
     The various controllers/data processors, memories, programs, transceivers and antenna arrays depicted in  FIG. 1  may all be considered to represent means for performing operations and functions that implement the several non-limiting aspects and embodiments of this invention. 
     At least some embodiments herein may be implemented in software (executed by one or more processors), hardware (e.g., an application specific integrated circuit), or a combination of software and hardware. In an example of an embodiment, the software (e.g., application logic, an instruction set) is maintained on any one of various conventional computer-readable media. In the context of this document, a “computer-readable medium” may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer, with one example of a computer described and depicted, e.g., in  FIG. 1 . A computer-readable medium may comprise a computer-readable storage medium (e.g., memories  125 ,  155 ,  171  or other device) that may be any media or means that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer. A computer-readable storage medium does not comprise propagating signals. 
     If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined. 
     Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. 
     It is also noted herein that while the above describes example embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims. 
     The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows: 
     3D-MIMO three-dimensional-MIMO 
     3GPP third-generation partnership project 
     5G fifth generation 
     ABS almost-blank subframe 
     CSI channel state information 
     CSI-RS channel state information-reference signal 
     CQI channel quality indicator 
     eICIC enhanced inter-cell interference coordination 
     eNB evolved NodeB (e.g., an LTE base station) 
     FD-MIMO full dimension-MIMO 
     FDM frequency-division multiplexing 
     HetNet heterogeneous network 
     IMR interference measurement resource 
     LTE long term evolution 
     LTE-A LTE-advanced 
     MCS modulation and coding scheme 
     MIMO multiple input, multiple output 
     ms milliseconds 
     MU-MIMO multi-user MIMO 
     PMI precoding matrix indicator 
     PRB physical resource block 
     RAN radio access network 
     RE resource element 
     Rel release 
     RI rank indicator 
     RRC radio resource control 
     RS reference signal 
     Rx reception or receiver 
     TDD time-division duplex 
     TSG technical specification group 
     Tx transmission or transmitter 
     UE user equipment (e.g., a wireless device) 
     WG working group