BACKGROUND

[0001 ] Base stations that implement multi-user multiple-input-multiple-output (MU-MIMO) can concurrently transmit downlink signals to multiple user equipment using spatial diversity, e.g., beamforming. For example, the base station can implement multiple physical antennas that are configured to transmit independent data streams to the user equipment. The base station can transmit signals to multiple user equipment at different locations using multiple spatial channels (e.g., beams) corresponding to the directions from the base station to the user equipment. In the case of 2x2 MU- MIMO, the base station implements two physical antennas and the user equipment implement two physical antennas. Two antenna ports corresponding to the base station's two physical antennas are allocated to each of the user equipment for transmission of downlink signals. If the base station is concurrently transmitting to two user equipment, antenna ports 8 and 9 are allocated to the first user equipment and antenna ports 10 and 1 1 are allocated to the second user equipment. If the base station is concurrently transmitting to more than two user equipment, some of the antenna ports are shared between more than one user equipment. For example, if the base station is concurrently transmitting to three user equipment, the first and third user equipment can share antenna ports 8 and 9.

SUMMARY OF EMBODIMENTS [0002] The following presents a simplified summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

[0003] In some embodiments, a method is provided that includes mapping a first antenna port to a first physical antenna and a second antenna port to a second physical antenna for downlink transmission to a first user equipment, wherein the first and second physical antennas are orthogonally polarized. The method also includes mapping the first antenna port to the second physical antenna and the second antenna port to the first physical antenna for downlink transmission to a second user equipment. The method further includes concurrently transmitting reference signals to the first and second user equipment in a first resource element allocated to the first antenna port and a second resource element allocated to the second antenna port.

[0004] In some embodiments, an apparatus is provided that includes a transceiver that provides a first antenna port and a second antenna port for downlink transmission using a first physical antenna and a second physical antenna. The first and second physical antennas are orthogonally polarized. The apparatus also includes a processor configured to map the first antenna port to the first physical antenna and the second antenna port to the second physical antenna for downlink transmission to a first user equipment. The processor is also configured to map the first antenna port to the second physical antenna and the second antenna port to the first physical antenna for downlink transmission to a second user equipment. The transceiver is configured to concurrently transmit reference signals to the first and second user equipment in a first resource element allocated to the first antenna port and a second resource element allocated to the second antenna port.

[0005] In some embodiments, a method is provided that includes assigning antenna ports to physical antennas for downlink transmission to first user equipment. The physical antennas are orthogonally polarized. The method also includes remapping the antenna ports to the physical antennas for downlink transmission to second user equipment. The remapping changes polarizations of the physical antennas mapped to the antenna ports. The method further includes concurrently transmitting reference signals to the first and second user equipment in resource elements allocated to the antenna ports.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items. [0007] FIG. 1 is a block diagram of a wireless communication system according to some embodiments.

[0008] FIG. 2 is of a physical resource block according to some embodiments.

[0009] FIG. 3 illustrates mappings of antenna ports to physical antennas that can be assigned to user equipment that share the antenna ports according to some embodiments. [0010] FIG. 4 illustrates signals and interference produced at user equipment due to concurrent transmission of reference signals in resource elements associated with corresponding antenna ports according to some embodiments.

[001 1 ] FIG. 5 is a flow diagram of a method for assigning shared antenna ports to user equipment for downlink transmission according to some embodiments. [0012] FIG. 6 is a diagram of a non-transitory computer readable storage medium that is used to store software for configuring a processor to perform aspects of the techniques described herein according to some embodiments. DETAILED DESCRIPTION

[0013] Base stations transmit reference signals associated with antenna ports to allow user equipment to estimate channels used to receive downlink signals transmitted by the base stations. The reference signals are sequences that are allocated to the user equipment and transmitted using predetermined resources of the air interface. For example, the physical resource blocks used for downlink transmission are defined by a frequency range and a time interval that is referred to as a subframe. The subframes include two slots and each slot includes seven symbols. The frequency range is subdivided into twelve subcarriers and each symbol on each subcarrier is referred to as a resource element. The resource elements for all of the subcarriers for the third symbol in each physical resource block are reserved to transmit demodulation reference signals (DMRS) to the user equipment, which use the DMRS signals for channel estimation. Each antenna port transmits a different DMRS so that the user equipment can independently estimate the channels to the physical antennas. The antenna ports are allocated a predetermined set of symbols for transmitting the DMRS. For example, antenna port 8 transmits a DMRS on subcarriers 1 , 5, and 9 of symbol 2 in the physical resource blocks, antenna port 9 transmits a DMRS on subcarriers 2, 6, and 10 of symbol 2, antenna port 10 transmits a DMRS on subcarriers 3, 7, and 1 1 of symbol 2, and antenna port 1 1 transmits a DMRS on subcarriers 4, 8, and 12 of symbol 2.

[0014] The DMRS signals for different user equipment can be concurrently transmitted in the same resource elements if the base station is concurrently transmitting downlink signals to more than two user equipment. For example, if the base station is concurrently transmitting to three user equipment, the first and third user equipment can be allocated the same antenna ports (e.g., antenna ports 8 and 9). The base station can therefore concurrently transmit the different DMRS to the first and third user equipment in the same resource elements of the physical resource block. Spatial diversity (and in some cases polarization diversity) between the beams used to transmit downlink signals to the first and third user equipment reduces interference between the DMRS signals transmitted to the user equipment. The spatial channels are typically assumed to be perfectly orthogonal to each other so that signals transmitted on one spatial channel do not interfere with signals transmitted on another spatial channel. However, spatial channels frequently interfere with each other (at least to a small degree) in real-world deployments. Even small amounts of interference between the DMRS signals transmitted in shared antenna ports can degrade the quality of the channel estimation performed by the user equipment, which in turn can increase the likelihood of decoding errors in subsequently received data.

[0015] The most common MU MIMO antenna configuration is two physical antennas transmitting signals with polarizations that are orthogonal to each other, i.e., the two physical antennas are cross polarized. For example, a first physical antenna can transmit with +45° polarization and a second physical antenna can transmit with -45° polarization. A receiver that implements cross polarized antennas can effectively discriminate between signals received from the first and second physical antennas. The enhanced discrimination between cross polarized signals can be leveraged to reduce the interference between reference signals transmitted by a base station to first and second user equipment that share antenna ports by assigning the first and second user equipment different mappings of the antenna ports to physical antennas implemented by the base station. For example, the base station can allocate first and second antenna ports to the first user equipment and the first mapping can map the first and second antenna ports to first and second physical antennas, respectively. The base station also allocates the first and second ports to the second user equipment, but the second mapping maps the first antenna port to the second physical antenna and the second antenna port to the first physical antenna. Consequently, DMRS signals transmitted to the first and second user equipment via the shared first port in a first resource element are transmitted by the first and second physical antennas, respectively, and the DMRS signals transmitted to the first and second user equipment via the shared second port in a second resource element are transmitted by the second and first physical antennas, respectively. The DMRS signals transmitted in the same resource element are therefore transmitted by physical antennas that have different polarizations. Using cross polarized antennas provides additional cross polarization discrimination that significantly reduces interference between the downlink signals transmitted via the same antenna port and resource element. For example, the cross polarization of the first and second physical antennas can provide an additional 5-20 dB discrimination at the user equipment between DMRS signals transmitted by the first and second physical antennas in the same resource element.

[0016] FIG. 1 is a block diagram of a wireless communication system 100 according to some embodiments. The wireless communication system 100 includes a base station 105 that uses a cross polarized antenna system 110 to transmit and receive signals over an air interface. The cross polarized antenna system 1 10 includes at least two physical antennas having orthogonal polarizations. For example, the cross polarized antenna system 1 10 can have a first physical antenna having a polarization direction of +45° and a second physical antenna having a polarization direction of -45°. However, other orthogonal polarizations can be used to implement the cross polarized antenna system 1 10. The wireless communication system 100 implements MU MIMO to concurrently communicate with multiple user equipment 115, 120 using different spatial channels or beams 125, 130. Some embodiments of the wireless communication system 100 operate according to the standards established by the Verizon 5G Technical Forum (5G-TF), e.g., as set forth in Technical Specification V5G.201 V1.0, which is incorporated herein by reference in its entirety.

[0017] The base station 105 includes a transceiver 131 that is connected to the cross polarized antenna system 1 10 for transmitting and receiving signals. The transceiver 131 can be implemented as a single integrated circuit (e.g., using a single application-specific integrated circuit, ASIC, or field programmable gate array, FPGA) or as a system-on-a-chip (SOC) that includes different modules for implementing the functionality of the transceiver 131 . The base station 105 also includes a processor 132 that is coupled to a memory 133. The processor 132 can be used to execute instructions stored in the memory 133 and to store information in the memory 133 such as the results of the executed instructions. For example, the memory 133 includes a random access memory (RAM) 134 and a read-only memory (ROM) 135, which can store a copy of instructions from a program code 136 that is to be executed by the processor 132.

[0018] In some embodiments, the base station 105 and the cross polarized antenna system 1 10 are implemented in different locations, in which case the cross polarized antenna system 1 10 can be referred to as a remote radio head (RRH) in a distributed base station architecture. In that case, the cross polarized antenna system 1 10 is coupled to the base station 105 by an optical interface implemented using optical fibers. The interface carries optical signals such as Control, Management, Sync, and IQ signals. Some embodiments of the RRH implement multiple wireless standards or technologies. The RRH contains radiofrequency (RF) circuitry including filters plus analog-to-digital or digital-to-analog converters and up/down converters and antenna(s). For example, the RRH can implement Digital Up Conversion(DUC), Digital Down Conversion(DDC), Crest Factor

Reduction(CFR), Digital Pre-Distortion(DPD), as well as other operation and management functions. A distance between the base station 105 and the RRH can be on the order of ten kilometers so that the RRH can be used to extend the coverage of the base station 105 to remote rural areas. Some embodiments of the RRH are implemented using FPGAs.

[0019] The user equipment 115, 120 include transceivers 141 , 142 connected to cross polarized antenna systems 145, 146 for transmitting and receiving signals. For example, the cross polarized antenna systems 145, 146 can each implement a first physical antenna having a polarization direction of +45° and a second physical antenna having a polarization direction of -45°. The transceivers 141 , 142 can be implemented as a single integrated circuit (e.g., using a single application-specific integrated circuit, ASIC, or field programmable gate array, FPGA) or as a system-on-a-chip (SOC) that includes different modules for implementing the functionality of the transceivers 141 , 142. The user equipment 115, 120 also include processors 151 , 152 that are coupled to corresponding memories 161 , 162. The processors 151 , 152 can be used to execute instructions stored in the memories 161 , 162, respectively, and to store information in the memories 161 , 162, respectively, such as the results of the executed instructions. For example, the memory 161 includes a random access memory (RAM) 162 and a read-only memory (ROM) 163, which can store a copy of instructions from a program code 164 that is to be executed by the processor 151 . The memory 162 can also include a similar memory structure, which is not shown in FIG. 1 in the interest of clarity. [0020] The transceiver 131 supports a set of logical antenna ports that are allocated to the user equipment 115, 120, e.g., for downlink transmission over the air interface. The logical antenna ports are associated with physical antennas in the cross polarized antenna system 110 so that signals transmitted using the different logical antenna ports are radiated by different physical antennas (or different combinations of the physical antennas). The processor 132 in the base station 105 is configured to generate reference signals associated with the antenna ports to allow the user equipment 115, 120 to estimate channels used to receive downlink signals transmitted by the base station 105. Some embodiments of the processor 132 generate demodulation reference signals (DMRS) for transmission to the user equipment 115, 120. For example, the DMRS can be generated according to the standards defined by the Verizon 5G-TF in Technical Specification V5G.21 1 V1 .7, which is incorporated herein by reference in its entirety.

[0021 ] The antenna ports are allocated predetermined resources of the air interface for transmission of reference signals such as the DMRS. In some embodiments, the antenna ports are allocated predetermined subsets of the reference elements in a physical resource block for transmission of the reference signals. However, there are a limited number of antenna ports and corresponding reference elements available to transmit reference signals. If the number of antenna ports needed to provide mutually exclusive subsets of antenna ports to all of the user equipment being served by the base station 105 is larger than the number of antenna ports supported by the transceiver 131 , more than one user equipment can be assigned to share the same subset of antenna ports. For example, the user equipment 1 15 and the user equipment 120 can both be assigned to the same pair of antenna ports. The reference signals transmitted to the user equipment 1 15 and the user equipment 120 using the shared antenna ports (and associated resource elements) can interfere with each other, which may reduce the likelihood that the user equipment 115, 120 are able to successfully decode the reference signals.

[0022] In order to reduce interference between reference signals transmitted to the user equipment 1 15, 120 using the same antenna ports and associated resource elements, antenna ports are assigned to physical antennas in the cross polarized antenna system 1 10 for downlink transmission to the user equipment 1 15 and the antenna ports are remapped to the physical antennas for downlink transmission to the user equipment 120 so that the polarizations of the physical antennas that are used to transmit reference signals to the user equipment 1 15, 120 in the same resource element are transmitted using physical antennas having different (orthogonal) polarizations. The reference signals can then be transmitted concurrently to the user equipment 1 15, 120. Since the user equipment 115, 120 also implement cross polarized antenna systems 145, 146, the user equipment 1 15, 120 can use cross polarization discrimination to reduce interference from the reference signals that are intended for the other user equipment. In some embodiments, the base station 105 selects the user equipment 120 to share the antenna ports that are allocated to the user equipment 115 on the basis of a degree of spatial separation between the user equipment 1 15, 120. For example, the base station 105 can select the user equipment 115, 120 to share a subset of antenna ports if an angle between directions of the beams 125, 130 exceeds a threshold angle.

[0023] FIG. 2 is of a physical resource block 200 according to some embodiments. The vertical direction indicates frequency and the horizontal direction indicates time. The physical resource block 200 is subdivided into resource elements 205 (only one indicated by a reference numeral in the interest of clarity) that span a predetermined time interval (conventionally referred to as a symbol) on a predetermined frequency (conventionally referred to as a subcarrier). In the illustrated embodiment, the physical resource block 200 represents a subframe that includes fourteen symbols (0-13) and twelve subcarriers (1 -12). For example, if the physical resource block 200 is defined according to 5G- TF TS V5G.201 V1 .0, a single component carrier bandwidth of 100MHz is supported and the physical resource block 200 spans twelve subcarriers with a subcarrier bandwidth of 75kHz over a duration of 0.1 ms. A radio frame consists of 50 subframes and has a length of 10 ms. Each subframe has a length of 0.2ms and link direction (downlink or uplink) for data transmission can be dynamically switched on a subframe basis. [0024] The resource elements in the symbol 2 are reserved for transmission of reference signals such as DMRS signals. Subsets of the resource elements in symbol 2 are associated with or assigned to different antenna ports. For example, a first antenna port indicated by the crosshatched box 210 is associated with the resource elements at subcarriers 1 , 5, and 9, a second antenna port indicated by the crosshatched box 215 is associated with the resource elements at subcarriers 2, 6, and 10, a third antenna port indicated by the crosshatched box 220 is associated with the resource elements at subcarriers 3, 7, and 1 1 , and a fourth antenna port indicated by the crosshatched box 225 is associated with the resource elements at subcarriers 4, 8, and 12. However, different resource elements in the physical resource block 200 can be reserved for transmission of reference signals in some embodiments. [0025] FIG. 3 illustrates mappings 300, 305 of antenna ports 310, 315 to physical antennas 320, 325 that can be assigned to user equipment that share the antenna ports 310, 315 according to some embodiments. The physical antennas 320, 325 are orthogonally polarized and are used to implement some embodiments of the cross polarized antenna system 110 shown in FIG. 1 . The first mapping 300 maps the antenna port 310 to the physical antenna 320 (as indicated by the arrow 330) and maps the antenna port 315 to the physical antenna 325 (as indicated by the arrow 335). The second mapping 305 maps the antenna port 310 to the physical antenna 325 (as indicated by the arrow 340) and maps the antenna port 315 to the physical antenna 320 (as indicated by the arrow 345). As discussed herein, assigning the different mappings 300, 305 to different user equipment reduces interference between reference signals that are concurrently transmitted to the user equipment in the resource elements associated with the antenna ports 310, 315 due to the improved cross polarization interference discrimination at the user equipment.

[0026] FIG. 4 illustrates signals and interference produced at user equipment 400, 405 due to concurrent transmission of reference signals in resource elements 410, 41 1 associated with corresponding antenna ports according to some embodiments. [0027] In the resource element 410, a first antenna port is mapped to a first physical antenna 415 for downlink transmission to the user equipment 400 and the first antenna port is mapped to a second physical antenna 420 for downlink transmission to the user equipment 405. Thus, reference signals transmitted by the physical antenna 415 in the resource element 410 are received as signals 430 by the user equipment 400 and as interference 435 by the user equipment 405. Similarly, reference signals transmitted by the physical antenna 420 in the resource element 410 are received as signals 440 by the user equipment 405 and as interference 445 by the user equipment 400. Since the signal 430 and the interference 445 are transmitted with orthogonal polarizations by the physical antennas 415, 420, respectively, the user equipment 400 is able to use cross polarization discrimination to reduce the impact of the interference 445 on the received signal 430. Similarly, the interference 435 and the signal 440 are transmitted with orthogonal polarizations by the physical antennas 415, 420, respectively, which enables the user equipment 405 to use cross polarization discrimination to reduce the impact of the interference 435 on the received signal 440.

[0028] In the resource element 41 1 , a second antenna port is mapped to the second physical antenna 420 for downlink transmission to the user equipment 400 and the second antenna port is mapped to the first physical antenna 415 for downlink transmission to the user equipment 405. Thus, reference signals transmitted by the physical antenna 415 in the resource element 41 1 are received as interference 450 by the user equipment 400 and as signals 455 by the user equipment 405. Similarly, reference signals transmitted by the physical antenna 420 in the resource element 41 1 are received as interference 460 by the user equipment 405 and as signals 465 by the user equipment 400. Since the interference 450 and the signal 465 are transmitted with orthogonal polarizations by the physical antennas 415, 420, respectively, the user equipment 400 is able to use cross polarization discrimination to reduce the impact of the interference 450 on the received signal 465. Similarly, the signal 455 and the interference 460 are transmitted with orthogonal polarizations by the physical antennas 415, 420, respectively, which enables the user equipment 405 to use cross polarization discrimination to reduce the impact of the interference 460 on the received signal 455.

[0029] FIG. 5 is a flow diagram of a method 500 for assigning shared antenna ports to user equipment for downlink transmission according to some embodiments. The method 500 is implemented in some embodiments of the base station 105 that implements the cross polarized antenna system 1 10 shown in FIG. 1 .

[0030] The method 500 starts at block 505. At this point, a base station has one or more user equipment that are to be assigned to antenna ports for downlink transmission. Other user equipment may or may not already be assigned to antenna ports for downlink transmission.

[0031 ] At decision block 510, the base station determines whether there are unassigned antenna ports available to be assigned to the user equipment. If so, the method 500 flows to block 515. If the base station determines that the antenna ports have all been assigned to other user equipment for downlink transmission, the method 500 flows to block 520. [0032] At block 515, the base station assigns a subset of antenna ports to the user equipment for downlink transmission. The subset of antenna ports is not shared with any other user equipment and so the subset of antenna ports uses an initial mapping of the antenna ports to physical antennas used by the base station for downlink transmission. For example, the subset of antenna ports can include antenna ports that are unassigned and can therefore be assigned to physical antennas according to a default mapping. [0033] At block 520, the base station determines that all of the antenna ports are currently assigned to other user equipment and so the base station chooses a subset of the antenna ports that are to be shared by the user equipment and one of the other user equipment. The base station assigns the shared subset of the antenna ports to the user equipment. The base station also reverses the mapping of the antenna ports (or remaps the antenna ports) to the physical antennas so that the antenna ports in the shared subset are mapped to physical antennas having different polarizations than the polarizations of the physical antennas that are associated with the corresponding antenna ports for transmission to the other user equipment.

[0034] At decision block 525, the base station determines whether there are additional user equipment to be assigned to antenna ports. If so, the method 500 flows to decision block 510. If not, the method 500 flows to block 530 and the method 500 ends.

[0035] In some embodiments, certain aspects of the techniques described above are implemented by one or more processors of a processing system executing software. The software comprises one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer readable storage medium. The software can include the instructions and certain data that, when executed by the one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer readable storage medium can include, for example, a magnetic or optical disk storage device, solid state storage devices such as Flash memory, a cache, random access memory (RAM) or other non-volatile memory device or devices, and the like. The executable instructions stored on the non-transitory computer readable storage medium may be in source code, assembly language code, object code, or other instruction format that is interpreted or otherwise executable by one or more processors.

[0036] FIG. 6 is a diagram of a non-transitory computer readable storage medium 600 that is used to store software for configuring a processor to perform aspects of the techniques described herein according to some embodiments. The non-transitory computer readable storage medium 600 may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and/or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu- Ray disc), magnetic media (e.g., floppy disc , magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. In the illustrated embodiment, the non-transitory computer readable storage medium 600 is a CD or DVD. The computer readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)). [0037] Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure. [0038] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.