Abstract:
According to the teachings herein, a wireless device enhances uplink channel estimation at a node in a supporting wireless communication network by beamforming its uplink reference signal transmission towards the node, and correspondingly compensates for the effect of that beamforming when receiving a downlink transmission that was adapted in dependence on the uplink channel transmission. Such processing provides significant advantages in Multiple-Input-Multiple-Output, MIMO, systems that use a potentially large number of antennas for downlink MIMO transmissions and assume reciprocity between the uplink and downlink channels. In particular, uplink beamforming increases the received signal quality of the uplink reference signal used for estimating the uplink channel, while “automatic” compensation by the wireless device of the corresponding downlink transmission obviates the need for the network to know which precoder was used for uplink beamforming, or even that uplink beamforming is in use.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to wireless communication networks, and particularly relates to accounting for the effect of beamforming uplink reference signals. 
       BACKGROUND 
       [0002]    Ongoing technology and standardization developments make the use of large antenna arrays at cellular base stations and other wireless access points a viable option to boost the air interface capacities and maximum data rates of wireless communication networks. Consider a base station or an access point equipped with a large number of antennas. The node can simultaneously schedule multiple wireless devices in the same time/frequency band, using simple linear processing such as maximum-ratio transmission or zero-forcing in the downlink and maximum-ratio combining or zero-forcing in the uplink. Current literature often refers to these multi-antenna arrangements as very large multiple-input-multiple-output, VL-MIMO, or as “massive” MIMO. VL-MIMO systems are also sometimes referred to as “full dimension” or FD systems. FD-MIMO provides throughput gains without consuming additional spectrum and further offers substantial improvements in radiated energy efficiency. Reflecting the burgeoning interest in FD-MIMO technology, the Third Generation Partnership Project, 3GPP, has an active work item focused on the use of FD-MIMO. 
         [0003]    Narrow beam forming in the downlink represents a key aspect of FD-MIMO. Base stations use narrow beam forming to focus transmitted energy towards desired users—i.e., towards the wireless devices being served at any given time. Focusing the radiated energy boosts coverage and raises the maximum data rates achievable on the downlink under real-world channel conditions. 
         [0004]    Accurate channel state information, CSI, is a requisite for effective beamforming and acquiring accurate CSI in a scalable fashion for FD-MIMO systems is non-trivial. In conventional systems, radio network nodes transmit per-antenna pilot signals, and wireless devices estimate downlink channel gain based on measurements of the pilot signals. These per-antenna approaches are not feasible for a base station that uses a large number of downlink transmit antennas. 
         [0005]    Where reciprocity exists between the uplink and downlink channels, such as in Time Division Duplex, TDD, operation, a wireless device transmits a Sounding Reference Signal, SRS, or other type of reference signal on the uplink. The receiving network base station uses the received reference signal to estimate both the uplink and downlink channels between it and the wireless device. For the channel estimation to be of sufficiently high quality, the base station must receive the uplink reference signal(s) with a sufficiently high Signal-to-Noise Ratio or SNR. This requirement poses challenges for the typical wireless device, which generally is battery operated or otherwise power-limited. Because the uplink should be sounded over the entire frequency band of interest, potentially significant energy radiation by the wireless device is required to achieve sufficient signal quality at the base station. Operation by a wireless device within a cell edge region exacerbates the problem of providing the network base station with reference signals of sufficiently high reception quality. 
         [0006]    Beamforming represents one trick available to multi-antenna wireless devices for ensuring that the network base station receives its uplink reference signals at a sufficient received-signal strength. With uplink beamforming, more of the radiated signal energy is steered towards the network base station, thereby improved received signal quality at the network base station for the uplink reference signal. 
         [0007]    However, this disclosure recognizes that several disadvantages or problems attend the use of beamforming for uplink reference signal transmission. For example, the network base station uses the received reference signal to estimate the downlink channel to the wireless device, e.g., for link adaptation of its downlink transmissions to the wireless device. Taking the exact same propagation channel conditions, the uplink channel as perceived by the network base station will look different in depending on whether or not the wireless device uses beamforming for its uplink reference signal transmission. Further, beamforming characteristics depend on antenna weights used by the wireless device for beamforming. Consequently, the perceived or “effective” channel seen by the network base station depends on the particular precoder—antenna weight matrix—used by the wireless device. 
         [0008]    Providing information to the network base station regarding uplink beamforming solves these problems. However, that approach requires additional uplink signaling from the wireless device and, therefore, represents an added signaling burden. 
       SUMMARY 
       [0009]    According to the teachings herein, a wireless device enhances uplink channel estimation at a node in a supporting wireless communication network by beamforming its uplink reference signal transmission towards the node, and correspondingly compensates for the effect of that beamforming when receiving a downlink transmission that was adapted in dependence on the uplink channel transmission. Such processing provides significant advantages in Multiple-Input-Multiple-Output, MIMO, systems that use a potentially large number of antennas for downlink MIMO transmissions and assume reciprocity between the uplink and downlink channels. In particular, uplink beamforming increases the received signal quality of the uplink reference signal used for estimating the uplink channel, while “automatic” compensation by the wireless device of the corresponding downlink transmission obviates the need for the network to know which precoder was used for uplink beamforming, or even that uplink beamforming is in use. 
         [0010]    One embodiment comprises a method at a wireless device configured for operation in a wireless communication network. The example method includes transmitting a reference signal to the wireless communication network using uplink beamforming. The method includes correspondingly receiving a downlink transmission from the wireless communication network that was beamformed in dependence on an effective channel arising from the use of uplink beamforming on the reference signal. Still further, the method includes accounting for the effective channel by applying a linear transform to the received downlink transmission to obtain a compensated received transmission, wherein the linear transform is based on a precoder matrix used for the uplink beamforming. 
         [0011]    Another embodiment comprises a wireless device configured for operation in a wireless communication network. The example device includes a communication interface configured to transmit signals to the wireless communication network and to receive signals from the wireless communication network. Further, the device includes processing circuitry that is operatively associated with the communication interface and configured to receive a beamformed downlink transmission from the wireless communication network. Here, the network beamformed the downlink transmission in dependence on an effective channel arising from the use of uplink beamforming on a reference signal transmitted by the device. The processing circuitry is configured to account for the effective channel by applying a linear transform to the received downlink transmission, to obtain a compensated received transmission. The linear transform is based on a precoder matrix used for the uplink beamforming. 
         [0012]    Another embodiment comprises a non-transitory computer-readable medium storing a computer program comprising program instructions. When executed by a processor of a wireless device that receives a downlink transmission that is beamformed by a network node in a wireless communication network according to a channel estimate determined from a reference signal transmitted by the device, the program instructions configure the device to obtain a compensated received transmission. The compensated received transmission is obtained by the device linearly transforming the received downlink transmission according to a precoder matrix used by the device for beamforming the reference signal. 
         [0013]    In yet another embodiment, a wireless device includes a communication module configured to receive a downlink transmission that is beamformed by a network node in a wireless communication network according to a channel estimate determined from a reference signal transmitted by the device. The device further includes a compensation module that is configured to obtain a compensated received transmission by linearly transforming the received downlink transmission according to a precoder matrix used by the wireless device for beamforming the reference signal. 
         [0014]    Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a block diagram of one embodiment of a network node and a wireless device. 
           [0016]      FIG. 2  is a logic flow diagram of one embodiment of a method of processing at a wireless device. 
           [0017]      FIG. 3  is a logic flow diagram of another embodiment of a method of processing at a wireless device. 
           [0018]      FIG. 4  is a logic flow diagram of yet another embodiment of a method of processing at a wireless device. 
           [0019]      FIG. 5  is block diagram of one embodiment of a wireless communication network. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates an example network node  10  and an example wireless device  12 . While not shown in  FIG. 1 , it shall be understood that the network node  10  forms part of a wireless communication network—the “network”—and the wireless device  12  is configured for operation in the network. The network comprises, for example, a Third Generation Partnership Project, 3GPP, cellular communication network such as a Long Term Evolution, LTE, network. Correspondingly, the network node  10  may be a cellular radio base station, such as an LTE eNB, and the wireless device  12  comprises a User Equipment or UE in 3GPP parlance. No limitations should be inferred by this example, however, and the wireless device  12  may comprise essentially any type of wireless communication apparatus, including a mobile communication device, a computer or other device with wireless networking capabilities, a Machine-Type Communication, MTC, device, etc. 
         [0021]    The wireless device  12  includes two or more antennas  14  for transmitting and/or receiving signals, and the network node  10  also may include multiple antennas or antenna elements  16 . Uplink beam forming used at least selectively by the wireless device  12  enables the wireless device  12  to improve received signal strength at the network node  10 , with respect to uplink transmissions by the wireless device  12 . Because these beamformed transmissions on the uplink include reference signals used for downlink channel adaptation by the network node  10 , the effective channel on the downlink between the network node  10  and the wireless device  12  thus reflects the effects of the uplink beamforming. Advantageously, the wireless device  12  compensates for those effects in its received signal processing. 
         [0022]    To better appreciate an example implementation of such compensation, the wireless device  12  includes a communication interface  20  that includes one or more radiofrequency receivers  22  and transmitters  24  and associated protocol processing circuitry that are adapted to support the uplink and downlink air interfaces implemented within the wireless communication network—not shown—in which the network node  10  operates. Processing circuitry  26  is operatively associated with the communication interface  20  and comprises fixed circuitry, programmed circuitry, or a combination of fixed and programmed circuitry. 
         [0023]    In an example embodiment, the processing circuitry  26  is at least partly implemented using programmed circuitry and comprises, for example, one or more processor circuits, such as one or more microprocessors, Digital Signal Processors or DSPs, Application Specific Integrated Circuits or ASICs, Field Programmable Gate Arrays or FPGAs, or other digital processing circuitry. Correspondingly, the processing circuitry  26  includes or is associated with one or more types of computer-readable media—“STORAGE  28 ” in the figure—such as one or more types of memory circuits such as FLASH, EEPROM, SRAM, DRAM, etc. Additionally, or alternatively, the storage  28  comprises hard disk storage, Solid State Disk, SSD, storage, etc. 
         [0024]    In general, the storage  28  provides both working memory and longer-term storage. In at least one embodiment, the storage  28  provides non-transitory storage for a computer program  30  and one or more items of configuration data  32 . Here, non-transitory does not necessarily mean permanent or unchanging storage but does means storage of at least some persistence—i.e., holding information for subsequent retrieval. The computer program  30 , which may comprise a number of related or supporting programs, comprises program instructions that, when executed by the processing circuitry  26 , configure the wireless device  12  to operate according to the examples described herein. 
         [0025]    In other words, in some embodiments, one or more processing circuits within the wireless device  12  are specially adapted to carry out the teachings herein, based on their execution of the computer program instructions comprising the computer program  30 . Such execution configures the processing circuitry  26  to include, for example, a communication module  40  configured to receive a downlink signal that is beamformed by a network node  10  in a wireless communication network according to a channel estimate determined from a reference signal transmitted by the wireless device  12  and a compensation module  42  configured to obtain a compensated received signal by linearly transforming the received downlink signal according to a precoder matrix used by the wireless device  12  for beamforming the reference signal. In at least one such embodiment, the processing circuitry  26  implements a demodulation module  44  that is configured to demodulate the compensated received signal, for information recovery at the wireless device  12 . 
         [0026]    More broadly, the wireless device  12  in one or more embodiments is configured for operation in a wireless communication network—not shown—and the aforementioned communication interface  20  is configured to transmit signals to the wireless communication network and to receive signals from the wireless communication network. Correspondingly, the processing circuitry  26  is operatively associated with the communication interface  20  and configured to receive a beamformed downlink transmission from the network that was beamformed in dependence on an effective channel arising from the use of uplink beamforming on a reference signal transmitted by the wireless device  12 , and account for the effective channel by applying a linear transform to the received downlink transmission, to obtain a compensated received transmission, wherein the linear transform is based on a precoder matrix used for the uplink beamforming. 
         [0027]    The processing circuitry  26  uses the compensated received transmission for information recovery, for example, based on being configured to demodulate the compensated received downlink transmission, for recovering information conveyed in the received downlink transmission. Here, rather than performing demodulation processing on the received downlink transmission—held as signal samples in buffer memory in the processing circuitry  26 —the processing circuitry  26  performs demodulation processing on compensated signal samples resulting from application of the linear transform to the signal samples comprising the received downlink transmission. 
         [0028]    In some embodiments, the processing circuitry  26  is configured to linearly transform the received downlink transmission, i.e., the received downlink signal in question, by applying a transpose of the precoder matrix to the received downlink transmission. Note, however, that in at least some embodiments, the wireless device  12  does not necessarily apply uplink beamforming when transmitting reference signals. In cases where the received downlink transmission corresponds to an uplink reference signal that was transmitted by the wireless device  12  without the use of beamforming, the wireless device  12  skips the compensation processing—i.e., skips the linear transformation of the received downlink transmission—and applies its demodulation processing to the received downlink transmission. 
         [0029]    In at least one such embodiment, the processing circuitry  26  is configured to decide to use uplink beamforming for transmitting the reference signal, in dependence on at least one of: a location of the wireless device  12  with respect to the network node  10 , channel conditions observed at the wireless device  12  with respect to the network node  10 , configuration information stored at the wireless device  12 , and control signaling received from the wireless communication network. In one example of location dependency, the wireless device  12  uses uplink beamforming for its reference signal transmissions when operating in cell edge areas. In one example of channel conditions dependency, the wireless device  12  uses uplink beamforming for its reference signal transmissions when downlink signals from the network node  10  are received below a threshold signal strength or quality. In one example of network signaling dependency, the network node  10  sends an Information Element, IE, flag, or other indicator, implicit or explicit, that enables or disables reference-signal beamforming at the wireless device  12 . 
         [0030]    Similarly, in some embodiments, the processing circuitry  26  in one or more embodiments is configured to select the precoder matrix to use for uplink beamforming of the reference signal according to control signaling received from the wireless communication network. Additionally, or alternatively, the wireless device  12  selects the precoder based on channel estimate information determined by the wireless device  12  with respect to the network node  10 . In at least one such embodiment, the wireless device  12  autonomously selects the precoder and uses it, unless or until the network signals an overriding precoder selection. 
         [0031]    In at least some embodiments, the processing circuitry  26  is configured to maintain linking information in the wireless device  12  that logically links the received downlink transmission to the reference signal and indicates whether or not uplink beamforming was used for transmission of the reference signal. In such embodiments, the processing circuitry  26  is further configured to account for the effective channel by applying the linear transform to the received downlink transmission in dependence on determining that the linking information indicates that uplink beamforming was used for transmission of the reference signal. In an example case, a predefined relationship exists between uplink reference signal transmissions by the wireless device  12  and corresponding downlink transmissions by the network node  10  towards the wireless device  12 . For example, a scheduled downlink transmission corresponds to a particular uplink reference signal transmission if it occurs a defined number of transmission time intervals—e.g., a defined number of radio signal subframes—after transmission of the uplink reference signal. 
         [0032]    For any given downlink transmission received at the wireless device  12 , the processing circuitry  26  in one or more embodiments is configured to determine whether the given downlink transmission corresponds to a prior transmission of a beamformed reference signal by the wireless device  12 . If so, the processing circuitry  26  obtains a corresponding compensated received downlink transmission from the given received downlink transmission for demodulation, according to the precoder matrix used for the beamformed uplink reference signal. 
         [0033]    In another embodiment, a non-transitory computer-readable medium stores a computer program. The program comprises program instructions for execution by a processor of a wireless device  12 , e.g., the processing circuitry  26 . The wireless device  12  receives a downlink transmission that is beamformed by a network node  10  in a network according to a channel estimate determined from a reference signal transmitted by the wireless device  12 . Correspondingly, the program instructions configure the wireless device  12  to obtain a compensated received transmission by linearly transforming the received downlink transmission according to a precoder matrix used by the wireless device  12  for beamforming the reference signal. Demodulation processing performed by the wireless device  12  in such embodiments may depend on further computer program instructions that configure the wireless device  12  to demodulate the compensated received transmission, for recovering information conveyed in the downlink transmission. 
         [0034]    Whether or not it is implemented according to the example circuitry of  FIG. 1 , a wireless device  12  as contemplated herein is configured for operation in a wireless communication network and is adapted to transmit a reference signal to the wireless communication network using uplink beamforming. Further, the wireless device  12  is adapted to receive a downlink transmission from the wireless communication network that was beamformed in dependence on an effective channel arising from the use of uplink beamforming on the reference signal. Still further, the wireless device  12  is adapted to account for the effective channel by applying a linear transform to the received downlink transmission, to thereby obtain a compensated received transmission. Here, the linear transform is based on a precoder matrix used for the uplink beamforming and the compensated received transmission may be used, e.g., for demodulation and information recovery at the wireless device  12 . 
         [0035]      FIG. 2  illustrates a method  200  performed by a wireless device  12 , such as the wireless device  12  illustrated in  FIG. 1 . The method  200  includes transmitting (Block  202 ) a reference signal to a wireless communication network using uplink beamforming, receiving (Block  204 ) a downlink transmission from network that was beamformed in dependence on an effective channel arising from the use of uplink beamforming on the reference signal, and accounting (Block  206 ) for the effective channel by applying a linear transform to the received downlink transmission to obtain a compensated received transmission, wherein the linear transform is based on a precoder matrix used by the wireless device  12  for the uplink beamforming. In the same or another embodiment, the method  200  includes demodulating the compensated received transmission, to recover the information conveyed in the downlink transmission. 
         [0036]      FIG. 3  illustrates another method  300 , which may be regarded as one example of a more detailed implementation of the method  200 . Here, the wireless device  12  receives (Block  302 ) the downlink transmission on two or more of the receive antennas  14  of the wireless device  12 —denoted here as “antennas y”. Further, the reference signal transmitted on the uplink by the wireless device  12  is a Sounding Reference Signal or SRS, and the wireless device  12  device receives a downlink transmission from the network node  10  that is channel-adapted by the network node  10 , in dependence on the SRS transmission by the wireless device  12 . 
         [0037]    The method  300  includes determining (Block  304 ) whether the SRS was precoded—i.e., whether the wireless device  12  transmitted the SRS with or without uplink beamforming. The determination involves, for example, the wireless device  12  maintaining linking information that logically links the SRS to the received downlink transmission—e.g., based on knowing that the downlink transmission was received at a defined number of subframes after transmission of the SRS. 
         [0038]    If the SRS was precoded (YES from Block  304 ), processing continues with the wireless device  12  setting a linear transform matrix equal to the precoder—an antenna-weighting matrix—used for precoding the SRS (Block  306 ). From there, processing continues with compensating the received downlink transmission with the transpose of the precoder matrix. Such processing is seen in Block  308  as, 
         [0000]    
       
      
       z=A 
       T 
       y,  
      
     
         [0000]    where y is a matrix of received signal samples received on two or more of the antennas  14 , A is the precoder matrix of antenna weights used for beamforming the SRS from two or more of the antennas  14 , A T  is the transpose the precoder matrix, and z is the resulting matrix of compensated signal samples. Processing further continues with demodulating (Block  310 ) the compensated received transmission z. 
         [0039]    If the SRS was not precoded (NO from Block  304 ), processing continues with the wireless device  12  setting the linear transform matrix A equal to the identity matrix I (Block  312 ), and then applying A T  to the received signal y (Block  308 ). Application of the identity matrix transpose I T  does not change the received signal y, and is equivalent to skipping compensation and using the downlink transmission y directly in the demodulation processing seen in Block  312 . One advantage of using the identity matrix in cases where the SRS was not precoded is simplification of the program code used to carry out the overall processing—i.e., the wireless device  12  always applies A T  to the received signal y, and simply decides whether A T  is the identity matrix or the transpose of an actual precoder matrix in dependence on whether or not the SRS was precoded. Equivalently, in one or more embodiments, the wireless device  12  advances on along the “NO” processing path from Block  304  directly to demodulation of the received downlink transmission y, without calculating A T  as I T  and without forming z as I T y. 
         [0040]    Such an approach is encompassed in the method  400  illustrated in  FIG. 4 . The method  400  can also be understood as a more detailed example of the method  200 . The method  400  includes receiving a downlink transmission (Block  402 ) that is beamformed by a network node  10  according to a channel estimate determined from an uplink reference signal previously transmitted by the wireless device  12 . At Block  404 , the wireless device  12  determines whether the uplink reference signal in question was beamformed. If so, the wireless device  12  obtains a compensated downlink transmission (Block  406 ) and demodulates the compensated downlink transmission (Block  408 ) to obtain the information conveyed in the downlink transmission. If not, the wireless device  12  demodulates the received downlink transmission (Block  410 ), i.e., it skips the compensation processing. 
         [0041]      FIG. 5  illustrates an example of the wireless communication network referenced above, identified in the diagram as a wireless communication network  70 . As a non-limiting example, the network  70  comprises a cellular communication network based on the LTE specifications promulgated by the 3GPP. 
         [0042]    The network  70  includes a Radio Access Network, RAN,  72  and a Core Network, CN,  74 . The RAN  72  includes one or more radio base stations  76 , e.g.,  76 - 1  and  76 - 2 , with each radio base station  76  providing service in one or more cells  78 . Here, the radio base station  76 - 1  provides cellular communication services in a cell  78 - 1  and the radio base station  76 - 2  provides cellular communication services in an adjacent cell  78 - 2 . One or more wireless devices  12  operate within the wireless communication network  70 , e.g.,  12 - 1 ,  12 - 2 , etc. The CN  74  includes a number of nodes supporting the communication services, including a Mobility Management Entity, MME,  80 , a Serving Gateway, SGW,  82 , and one or more other nodes  84 . The CN  74  may, of course, include any number of nodes not illustrated or discussed here, and it will be appreciated that the CN  74  is not germane to the focus of this disclosure. 
         [0043]    One or more of the radio base stations  76  is configured to operate as the aforementioned network node  10 . A given one of the wireless devices  12  receives a downlink transmission from a radio base station  76  is beamformed in dependence on an effective channel arising from the use of uplink beamforming on a reference signal transmitted by the wireless device  12 . Further, the wireless device  12  accounts for the effective channel by applying a linear transform to the received downlink transmission to obtain a compensated received transmission. The linear transform is based on a precoder matrix used for the uplink beamforming. 
         [0044]    In at least one example of such operation, the wireless device  12  beamforms the uplink reference signal using a precoder that maximizes the average path gain to a serving base station  76 . For example, the wireless device  12  receives Cell-specific Reference Signals, CRS, on the downlink from the base station  76  and uses the CRS to estimate the downlink channel between it and the base station  76 . In turn, the wireless device  12  selects a particular precoder—e.g., from a defined codebook, or based on calculations—based on the channel estimate. For base stations having potentially many antenna elements, the wireless device  12  may be configured to beamform towards the “average” channel, or the wireless device  12  may be configured to try multiple precoders and receive feedback from the base station  76  as to which one it should use. For example, the base station  76  determines which precoder yields the highest signal strength or best Signal-to-Interference-plus-Noise-Ratio, SINR. 
         [0045]    The base station  76 , operating as an example network node  10 , also may control whether or not the wireless device  12  uses uplink beamforming for one or more reference signal transmissions. For example, an example network node  10  monitors channel conditions, e.g., based on tracking received signal strength and/or quality, with respect to a particular wireless device  12  and indicates to the wireless device  12  as to whether or not it should beamform its uplink SRS transmissions. Not beamforming has the advantage of saving processing power at the wireless device  12 , hence, beamforming may be activated only in relatively poor conditions or when the wireless device  12  is known to be at the cell edge. 
         [0046]    To better appreciate an example implementation of the network node  10  in such embodiments, refer back to  FIG. 1 . There, the network node  10  includes one or more communication interfaces  50  that include one or more radiofrequency receivers  52  and transmitters  54  and associated protocol processing circuitry that are adapted to support the uplink and downlink air interfaces implemented within the wireless communication network  70  in which the network node  10  operates. Processing circuitry  56  is operatively associated with the communication interface(s)  50  and comprises fixed circuitry, programmed circuitry, or a combination of fixed and programmed circuitry. 
         [0047]    In an example embodiment, the processing circuitry  56  is at least partly implemented using programmed circuitry and comprises, for example, one or more processor circuits, such as one or more microprocessors, Digital Signal Processors or DSPs, Application Specific Integrated Circuits or ASICs, Field Programmable Gate Arrays or FPGAs, or other digital processing circuitry. Correspondingly, the processing circuitry  56  includes or is associated with one or more types of computer-readable media—“STORAGE  58 ” in the figure—such as one or more types of memory circuits such as FLASH, EEPROM, SRAM, DRAM, etc. Additionally, or alternatively, the storage  58  comprises hard disk storage, Solid State Disk, SSD, storage, etc. 
         [0048]    In general, the storage  58  provides both working memory and longer-term storage. In at least one embodiment, the storage  58  provides non-transitory storage for a computer program  60  and one or more items of configuration data  62 . Here, non-transitory does not necessarily mean permanent or unchanging storage but does mean storage of at least some persistence—i.e., holding information for subsequent retrieval. The computer program  60 , which may comprise a number of related or supporting programs, comprises program instructions that, when executed by the processing circuitry  56 , configure the network node  10  to operate according to the examples described herein. For example, the network node  10  is configured to selectively control whether individual wireless devices  12  or groups of wireless devices  12  use uplink beamforming for SRS transmissions. 
         [0049]    Consider the case where there are M antennas  16  available at a network node  10  for making downlink transmissions towards a wireless device  12 . Representing the number of antennas  14  available at the wireless device  12  for receiving these downlink transmissions by n, the downlink channel between the network node  10  and the wireless device  12  can be represented as a M×n complex matrix H, where each matrix element H i,j  denotes the downlink channel gain between the i-th transmit antenna  14  and the j-th receive antenna  16 . 
         [0050]    Assuming reciprocity between the uplink and downlink channels, the uplink channel G equals H T . Pilot symbols, s device  transmitted by a wireless device  12  on the uplink as reference signals are then received at a network node  10  as 
         [0000]        y   node   =H   T   s   device   +e   node , 
         [0000]    where y node  denotes the received signal vector at the network node  10 , s device  denotes the vector of pilot symbols transmitted across the involved antennas  16  at the wireless device  12 , and e node  denotes additive noise at the network node  10 . If the uplink reference signal s device  is beamformed by the wireless device  12  using a precoder W device  of antenna weights, then 
         [0000]        y   node   =H   T   W   device   s   device   +e   node . 
         [0051]    The network node  10  estimates the downlink channel H based on y node , and uses that estimate to compute or select a downlink precoder W node  that comprises the antenna weights used for beamforming a downlink transmission s node  from the network node  10  to the wireless device  12 . Here, the downlink data transmission s node  comprises, e.g., a scheduled data transmission being adapted based on the channel estimated derived from y node . The beamformed signal is denoted as x node  and is received at the wireless device  12  as 
         [0000]        y   device   ={tilde over (H)}x   node   +e   device . 
         [0000]    Here, {tilde over (H)} denotes the “effective” downlink channel arising from the network node  10  basing its estimation of the downlink channel on receiving a beamformed reference signal from the wireless device  12 . In other words, {tilde over (H)}≡W device   T H. 
         [0052]    Advantageously, the wireless device  12  backs out or otherwise compensates for the influence of W device   T  on the received signal samples comprising y device  by applying the linear transformation taught herein. Consequently, the teachings herein provide for enhanced CSI acquisition at the network node  10 , i.e., via beamforming of the uplink reference signal, and compensation of a corresponding downlink transmission transmitted over the effective downlink channel arising as a consequence of the uplink beamforming. The wireless device  12  may store or otherwise remember the precoder that it used for uplink beamforming a particular reference signal transmission, and may link that precoder to correspondingly receive downlink transmission, such that that precoder is used to linearly transform the received downlink transmission to remove the influence of the effective channel. 
         [0053]    While such processing may be particularly advantageous in FD-MIMO systems, it should be appreciated that the teachings herein are applicable in any system that uses uplink reference signal transmissions for downlink channel estimation and corresponding adaptation of downlink data transmissions. Further, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is/are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.