Patent Publication Number: US-2019173535-A1

Title: System and Method for Multi-User Multiple Input Multiple Output Communications

Description:
This application is a continuation of U.S. patent application Ser. No. 14/724,639, filed on May 28, 2015 and entitled “System and Method for Multi-User Multiple Input Multiple Output Communications,” which claims priority to U.S. Provisional Application No. 62/082,647, filed on Nov. 21, 2014, entitled “MU-MIMO Split-Beam Antenna System and Method,” both of which applications are hereby incorporated herein by reference herein as if reproduced in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to digital communications, and more particularly to a system and method for multi-user multiple input multiple output (MU-MIMO) communications. 
     BACKGROUND 
     In modern communications systems, such as The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) compliant communications systems, MU-MIMO with multiple antennas is a key component in improving overall communications system throughput. However, the performance improvement for communications systems with small numbers of antennas, such as 4 or 8 antennas, per evolved NodeB (eNB) may be disappointing in light of the increased complexity and cost of the communications system. This has limited the popularity of such communications systems for deployment. 
     SUMMARY OF THE DISCLOSURE 
     Example embodiments of the present disclosure provide a system and method for multi-user multiple input multiple output (MU-MIMO) communications. 
     In accordance with an example embodiment of the present disclosure, a method for operating a communications controller in a wireless communications system is provided. The method includes scheduling a pair of user equipments (UE) located in different ones of a plurality of split beams using an appropriate code pair that produces the the plurality of split beams for multi-user multiple-input multiple output (MU-MIMO) mode transmission, and transmitting data packets to the pair of UEs in accordance with the appropriate code pair. 
     In accordance with another example embodiment of the present disclosure, a method for configuring a 3-sector wireless communications system by a designing device is provided. The method includes generating a plurality of split beams covering a first sector of the 3-sector communications system, mapping between the plurality of split beams and baseband antenna ports to equalize reference signal coverage between the baseband antenna ports, and prompting use of the plurality of split beams and the baseband antenna port mapping. 
     In accordance with another example embodiment of the present disclosure, a communications controller in a wireless communications system is provided. The communications controller includes a processor and a transmitter chain, and a computer readable storage medium storing programming for execution by the processor. The programming including instructions to schedule a pair of user equipments (UE) located in different ones of a plurality of split beams using an appropriate code pair that produces the plurality of split beams for multi-user multiple-input multiple output (MU-MIMO) mode transmission, and transmit data packets to the pair of UEs in accordance with the appropriate code pair. The communications controller includes a plurality of antennas coupled to the processor. The plurality of antennas transmits the plurality of split beams. 
     Practice of the foregoing example embodiments provide a split beam design along with novel mappings between baseband ports to antenna ports that improve MU-MIMO performance in communications systems with small number of antennas. 
     Moreover the example embodiments provide novel configurations for implementing power sharing between split beams while simplifying hardware design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  illustrates an example communications system according to example embodiments described herein; 
         FIG. 2  illustrates an example four transmitter (4T) 3GPP LTE system employing a two-column±45° cross-polarized antenna; 
         FIG. 3  illustrates an example sectorization change for an eNB site according to example embodiments described herein; 
         FIG. 4 a    illustrates an example antenna beam width configuration for an eNB site with different sectorization arrangements according to example embodiments described herein; 
         FIG. 4 b    illustrates example coverage areas of a 3 sector communications system with a split beam antenna according to example embodiments described herein; 
         FIG. 5  illustrates plot of antenna gain as a function of angle for example antenna beams according to example embodiments described herein; 
         FIG. 6  illustrates a polar plot of an example split beam antenna pattern according to example embodiments described herein; 
         FIG. 7 a    illustrates a graph of example beam patterns of reference signals according to example embodiments described herein; 
         FIG. 7 b    illustrates a graph of example beam patterns of UE data using a precoding codebook according to example embodiments described herein; 
         FIG. 7 c    illustrates a graph of example beam patterns using precoding codebooks according to example embodiments described herein; 
         FIG. 8  illustrates an example coverage area map for a 3 sector communications system with different down-tilt angles according to example embodiments described herein; 
         FIG. 9  illustrates a flow diagram of example operations occurring in the configuration of a communications system using split beam antennas according to example embodiments described herein; 
         FIG. 10  illustrates a flow diagram of example operations occurring in a communications controller of a communications system using split beams communicating with UEs according to example embodiments described herein; 
         FIG. 11 a    illustrates an example 4T 3GPP LTE system with a two-column cross-polarized antenna according to example embodiments described herein; 
         FIG. 11 b    illustrates a circuit diagram of an example 90-degree hybrid coupler according to example embodiments described herein; 
         FIG. 11 c    illustrates a first example 4T 3GPP LTE system with 90-degree hybrid couplers according to example embodiments described herein; 
         FIG. 11 d    illustrates a second example 4T 3GPP LTE system with 90-degree hybrid couplers according to example embodiments described herein; 
         FIG. 11 e    illustrates a third example 4T 3GPP LTE system with 90-degree hybrid couplers according to example embodiments described herein; 
         FIG. 12  illustrates a diagram of an example extension of a 4T 3GPP LTE system to include sharing between two different polarizations according to example embodiments described herein; 
         FIG. 13  illustrates a diagram of a modification of a 4T 3GPP LTE system for compatibility with LTE codebook according to example embodiments described herein; 
         FIG. 14  illustrates an example 4T 3GPP LTE system for use with a 3GPP LTE standard 4T codebook according to example embodiments described herein; 
         FIG. 15  illustrates a block diagram of an embodiment processing system  1500  for performing methods described herein; 
         FIG. 16  illustrates a block diagram of a transceiver  1600  adapted to transmit and receive signaling over a telecommunications network according to example embodiments described herein; 
         FIG. 17  illustrates a block diagram of an embodiment method for operating an evolved NodeB (eNB) in a wireless communication systems; and 
         FIG. 18  illustrates a block diagram of another embodiment method for operating an eNB in a wireless communications system. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The operating of the current example embodiments and the structure thereof are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific structures of the disclosure and ways to operate the disclosure, and do not limit the scope of the disclosure. 
     One embodiment of the disclosure relates to MU-MIMO communications. For example, a communications controller schedules a pair of user equipments (UE) located in different ones of a plurality of split beams using an appropriate code pair that produces the the plurality of split beams for multi-user multiple-input multiple output (MU-MIMO) mode transmission, and transmits data packets to the pair of UEs in accordance with the appropriate code pair. 
     The present disclosure will be described with respect to example embodiments in a specific context, namely communications systems that use split beams to implement MU-MIMO with small number of antennas. The disclosure may be applied to standards compliant communications systems, such as those that are compliant with Third Generation Partnership Project (3GPP), IEEE 802.11, and the like, technical standards, and non-standards compliant communications systems, that use split beams to implement MU-MIMO with small number of antennas. 
       FIG. 1  illustrates an example communications system  100 . Communications system  100  includes an evolved NodeB (eNB)  105  serving a plurality of user equipments (UEs)  110 ,  112 ,  114 ,  116 . In a first operating mode, transmissions for UEs as well as transmissions by UEs pass through the eNB. The eNB allocates communications resources for the transmissions to or from the UEs. eNBs may also be commonly referred to as base stations, Node-Bs, transmission points, remote radio heads, or access points, and the like, while UEs may also be commonly referred to as mobiles, mobile stations, terminals, subscribers, users, wireless devices, and the like. Communications resources may be time resources, frequency resources, code resources, time-frequency resources, and the like. Communications system  100  can also include communication between UEs, such as UE  114  and UE  120 . As an illustrative example, UE  114  and UE  120  are engaged in device to device communication and/or discovery, and UE  114  can relay messages between UE  120  and eNB  105 . 
     Communications system  100  may include a designing device  130 . Designing device  130  may be configured to design split beam antennas for communications system  100 . Designing device  130  may also map split beams to baseband antenna ports to ensure coverage of reference signals. Designing device  130  may prompt the use of the split beam antennas, e.g., save the configuration of the split beam antennas to memory for subsequent use, provide the configuration of the split beam antennas to eNBs, and the like. Designing device  130  may be a stand-alone device as shown in  FIG. 1 . Alternatively, designing device  130  may be co-located with a network entity, such as an eNB. 
     While it is understood that communications systems may employ multiple eNBs capable of communicating with a number of UEs, only one eNB, and a number of UEs are illustrated for simplicity. 
     Generally, in MU-MIMO, an eNB (or multiple eNBs) simultaneously transmits to multiple independent UEs in order to achieve multi-user gain and increased overall communications system performance. In order to implement MU-MIMO, an eNB is required to have a plurality of transmit antennas. However, complexity and cost increase dramatically as the number of antennas increase. Hence, there is a tendency for communications system providers to limit the number of antennas per eNB. 
       FIG. 2  illustrates an example four transmitter (4T) 3GPP LTE system  200  employing a two-column±45° cross-polarized antenna. System  200  includes transmitters, such as transmitter  210 , which are configured for filtering, equalizing, and the like, of signals being transmitted and/or received. System  200  also includes power amplifiers, such as power amplifier  215 , duplexers, such as duplexer  220 , and antennas, such as antenna  225 . Each antenna usually has a beam width of 65°. The antenna spacing is typically between ½ and 1.5 times wavelength (λ). The downlink throughput gain of such a system over a traditional two transmitter (2T) system employing a single column antenna is typically about 20-30%. However, applying MU-MIMO techniques to a 4T LTE system may not significantly improve performance. Additionally, there is a significant cost increase over a traditional 2T LTE system. Therefore, 4T LTE systems are not very attractive to operators. 
       FIG. 3  illustrates an example sectorization change for an eNB site  300 . Another way to increase system capacity is to increase sectorization. A first eNB site  305  has a typical three-sector sectorization with three 120-degree sectors. It may be straightforward to double the number of sectors to produce a second eNB site  310  that has six-sector sectorization with six 60-degree sectors. As an illustrative example, this approach has been employed in many code division multiple access (CDMA) communications systems and has yielded good results. If each sector retrains a 2T configuration, the total number of transmitters and receivers (TRX) is 12 (6 sectors*2 TRX per sector), which is the same as a three-sector site in a 4T configuration. 
       FIG. 4 a    illustrates an example antenna beam width configuration for an eNB site  400  with different sectorization arrangements. In a sector  405  of an eNB site with 120-degree sectors, an example antenna beam  410  may have a 65-degree antenna beam width antenna beam, while in a sector  415  of an eNB site with 60-degree sectors, example antenna beams  420  and  425  may each be 35-degree antenna beam width antenna beams. The doubling of the antenna beams effectively double the number of available communications resources. UEs that are under the coverage areas of the two beams (in the overlap area  430 ) may be scheduled to use the same frequency resource simultaneously. 
     When the two sectors operate completely independently in terms of their scheduling, however, they may interfere with each other due to the overlap in coverage. Therefore, the overall capacity gain is less than 100%, with normal simulation results for capacity gain being in the range of 60 to 80 percent for LTE communications systems, depending on the antenna beams being used and the angle spread in the channel. Furthermore, the use of eNB sites with six-sector sectorization requires network re-planning and re-optimization, which may increase deployment costs significantly. Additionally, the peak and cell-edge throughput is not improved dramatically. 
     According to an example embodiment, multi-column antennas are coupled to a radio frequency (RF) network that produces split beams in the horizontal dimension, and optionally vertical dimension, and which are then mapped to baseband ports. 
     According to an example embodiment, one or more of the following elements are included:
         a split beam antenna with patterns that are similar to that of a six-sector antenna is used in a three-sector 4T or 8T communications system supporting MU-MIMO;   a mapping between the antenna beams and baseband antenna ports so that the coverage of reference signals is proper for standard MIMO processing is used;   different down tilt angles for beams with different polarizations are used;   a RF network is used after amplification to enable sharing of power amplifier (PA) resources among the beams; and   a MU-MIMO pairing algorithm that allows for the pairing of UEs that can be simultaneously served with minimal mutual interference is used.       

       FIG. 4 b    illustrates example coverage areas  450  of a 3 sector communications system with a split beam antenna. Coverage areas  450  show that a 3 sector communications system utilizing a split beam antenna is able to achieve a coverage pattern similar to that of a 6 sector communications system. A first coverage area  455  displays the coverage pattern for a 3 sector communications system with a single tilt angle split beam antenna and a second coverage area  460  displays the coverage pattern for a 3 sector communications system with a split beam antenna with a low tilt angle and a high tilt angle. When combined with different polarizations, the use of the split beam antenna with two tilt angles may enable more vertical dimension functionality and gain. 
       FIG. 5  illustrates plot  500  of antenna gain as a function of angle for example antenna beams. Plot  500  illustrates antenna gain as a function of angle for two example antenna beams (beam [1, j]  505  and beam [1, −j]  510 ) of a conventional horizontal 4T communications system supporting MU-MIMO that employs 2 column cross-polarized antennas spaced ½ wavelength apart as illustrated in  FIG. 2 . Each antenna has beams with a horizontal beam width of 65 degrees. Beams  505  and  510  correspond to 2 of the possible beams generated using a 3GPP LTE Release-8 4T codebook. Beam  505  may be generated by code [1, j], while beam  510  may be generated by code [1, −j]. Therefore, the beams may be referred by their code, such as beam  505  may be referred to as beam [1, j] and beam  510  [1, −j] or simply by their codes, such as code [1, j] for beam  505  and code [1, −j] for beam  510 . The imaginary number, j (square root (−1)) can sometimes be denoted i, either notation can be used without confusion. 
     MU-MIMO gain may depend on being able to schedule two (or more) UEs simultaneously on the same resource block(s) (RB) with low mutual interference. As an illustrative example, a first UE scheduled using beam  505  (corresponding to code [1,j]) can be paired with a second UE scheduled using beam  510  (corresponding to code [1,−j]). However, the mutual interference is low only if the two UEs are located close to each other&#39;s null, which is limited to a narrow range around +30 degrees and −30 degrees. Due to availability of only discretized feedback from UEs, the eNB generally knows nothing about their positions relative to each other&#39;s null. Furthermore, due to angle spread in the propagation environment, the nulls disappear, and the mutual interference is even worse. Generally, there is not a single pair of beams that would work well, i.e., produce low mutual interference, therefore, resulting in poor MU-MIMO performance. 
     According to an example embodiment, the use of a split beam antenna produces antenna beams that have low mutual interference under realistic cellular propagation environment and discretized UE feedback conditions. The use of such beams to schedule multiple UEs simultaneously may significantly increase overall communications system throughput. 
       FIG. 6  illustrates a polar plot  600  of an example split beam antenna pattern. According to an example embodiment, a split beam antenna includes two separate antennas, each antenna with 35-degree beamwidth. Alternatively, a split beam antenna includes a single multi-column antenna (e.g., 3 or 4 columns) where the two 35-degree beams are generated through an RF feeding network or digital beam forming techniques. Polar plot  600  shows a split beam antenna pattern with two semi-overlapping traces  605  and  610 , resulting in an overall split beam antenna pattern  615 , which is a sum of two semi-overlapping traces  605  and  610 . 
     As discussed previously, the coverage of reference signals may need to be adjusted in order to ensure proper standard MIMO processing. Reference signals may be used for synchronization, timing advance, and the like, therefore, coverage of the reference signals is important for operation. 
     According to an example embodiment, reference signal coverage is equalized between different baseband antenna ports by employing a mapping between baseband antenna ports and the antenna beams as presented herein. In a cross-polarized antenna case, the 4 narrow antenna beams with 2 cross polarizations may be denoted as: A/, B/, A\, and B \, where “I” represents the +45 degree polarization and “\” represents the −45 degree polarization. An example mapping is as follows: 
     Port 0: A/−j*B/; 
     Port 1: B/−j*A/; 
     Port 2: A\−j*B\; and 
     Port 3: B\−j*A\. 
     The resulting beams for all 4 baseband antenna ports have identical beam pattern magnitudes and hence, the same coverage in practice. 
     According to an example embodiment, communications resource re-use is increased to improve communications system capacity. Both MU-MIMO and sectorization use and/or communications system resource re-use can be used to increase overall performance. However, MU-MIMO may compare unfavorably to sectorization in terms of an equivalent number of transmitters. 
     As an illustrative example, changing from a 3 sector system to a 6 sector system may yield about a 60-80 percent capacity gain. However, operators of communications systems may be reluctant to change to the 6 sector system due to having to redo network planning and optimization. But, traditional 4T 3GPP LTE systems, such as the one illustrated in  FIG. 2 , operating with MU-MIMO appears to have produced far less gain but also uses the same number of transmitters (4) as a 6 sector system. 
       FIG. 7 a    illustrates a graph  700  of example beam patterns of reference signals. Graph  700  highlights beam A  705 , beam B  710 , and A+jB and A−jB (which is simply B−jA multiplied by −j) (both are superimposed on each other as trace  715 ).  FIG. 7 b    illustrates a graph  750  of example beam patterns of UE data using a precoding codebook. Graph  750  displays beam patterns with consideration being given to 1 polarization. From graph  750 , it may be evident that only one pair of codes or beams that produce very low mutual interference (even under large angle spread), namely [1,1]  755  and [1,−1]  760 . It is noted that the mapping used in graph  750  is port 0=A\+j*B\, port 1=A\−j*B\, port 2=A/+j*B/, and port 3=A/−j*B/ and that only 1 polarization is shown. 
       FIG. 7 c    illustrates a graph  775  of example beam patterns using precoding codebooks. The scheduling and pairing of UEs may be critical to achieving good MU-MIMO performance and achieving high capacity. According to an example embodiment, the improved performance arises from several different aspects. A first aspect is that two UEs from two halves of the sector can always be scheduled on codes [1,1] and [1,−1] simultaneously. Their mutual interference may not be worse than in a 6 sector communications system case. The UE feedback using the codebook may clearly identify the UE location in the azimuth angle, enabling the pairing of the UEs. When ignoring certain 3GPP LTE technical standards release 8 (R8) limitations on MU-MIMO scheduling, this may result in at least the same level of performance as the 6 sector communications system case. 
     A second aspect is that the UEs feedback code [1,j] may be located at the bore side of the antenna and would be edge users with relatively poor performance in the overlap region in the 6 sector communications system case. According to an example embodiment, the performance of these UEs is significantly improved by scheduling them in a single user (SU) mode. This may be similar to the 6 sector communications system solution with joint scheduling between two adjacent sectors, which should yield better performance than a 6 sector communications system with independent scheduling. 
     A third aspect is that for future releases of the 3GPP LTE technical standards which may include flexible MU-MIMO pairing and scheduling, the performance can be further enhanced by pairing UEs with different power levels depending on their azimuth angles. 
     Therefore, a properly implemented MU-MIMO solution with an antenna remote unit (ARU) type of antenna can generally achieve performance that is similar to or better than a 6 sector communications system solution. 
     According to an example embodiment, multiple polarizations are combined with different down-tilt angles to obtain additional vertical dimension functionality and gain.  FIG. 8  illustrates an example coverage area map  800  for a 3 sector communications system with different down-tilt angles. As shown in  FIG. 8 , the use of a split beam antenna and the combination of two different polarizations and two different down-tilt angles results in two different high tilt beams and two different low tilt beams per sector. Different UEs can be served in each of the four beams. It is noted that more than two horizontal beams and two tilt angles may be used to further increase gain. 
     As an illustrative example, 4 narrow beams are represented as follows:
         A\H—Left beam with −45 polarization and small down-tilt angle;   B\L—Right beam with −45 polarization and large down-tilt angle;   A/L—Left beam with +45 polarization and large down-tilt angle; and   B/H—Right beam with +45 polarization and small down-tilt angle.       

     Multiple mappings between the baseband antenna ports and the antenna beams are possible. As an illustrative example, one mapping is as follows:
         Port 0: A/L−j*A\H−j*B/H−B\L;   Port 1: −j*A/L−A\H+B/H−j*B\L;   Port 2: −A/L−j*A\H−j*B/H+B\L; and   Port 3: −j*A/L+A\H−B/H−j*B\L.       

     In general, the coverage area of the reference signals of all of the baseband antenna ports is substantially the same. Due to the additional isolation introduced by the different down-tilt angles, more simultaneous UEs (e.g., 4 UEs per sector as shown in coverage area map  800 ) may be scheduled with low mutual interference, further enhancing communications system throughput and capacity. 
       FIG. 9  illustrates a flow diagram of example operations  900  occurring in the configuration of a communications system using split beam antennas. Operations  900  may be indicative of operations occurring in a device, such as a designing device, an eNB, or another network entity, involved in the configuration of a communications system using split beam antennas. 
     Operations  900  may begin with the device generating a pair of split beams that cover each sector of a multi-sector communications system (block  905 ). As an example, the split beams may have a beam pattern as shown by beam  605  and  610  of  FIG. 6 . A single pair of split beams may be used for each sector of the multi-sector communications system. Each split beam of the pair of split beams may be referred to by its corresponding code, such as a code from a 3GPP LTE codebook. The device may map the pair of split beams to antenna baseband ports (block  910 ). The device may use any of the mappings discussed herein to map the pair of split beams to antenna baseband ports. The device may generate widebeams covering each sector of the multi-sector communications system (block  915 ). The widebeams may simply be combinations of the pair of split beams. The widebeams may be referred to by their corresponding codes, such as codes from a 3GPP LTE codebook. The device may prompt the use of the pair of split beams, port mappings, and widebeams (block  920 ). The device may store information regarding the pair of split beams, port mappings, and widebeams to a memory where they can be subsequently provided to eNBs. The device may alternatively forward the information regarding the pair of split beams, port mappings, and widebeams to the eNBs. 
       FIG. 10  illustrates a flow diagram of example operations  1000  occurring in a communications controller of a communications system using split beams communicating with UEs. Operations  1000  may be indicative of operations occurring in a communications controller of a communications system using split beams communicating with UEs. 
     Operations  1000  may begin with the communications controller initializing (block  1005 ). Initializing may include retrieving information regarding the pair of split beams, port mappings, and widebeams. The communications controller may schedule a pair of UEs to a pair of split beams (block  1010 ). The pair of split beams may have associated codes that are used to generate the pair. Scheduling the pair of UEs may include determining a need for the UEs in the pair of UEs to communicate with the communications controller (in other words, there is data to be transmitted to the UEs or there is data at the UEs to be transmitted to the communications controller). Scheduling the pair of UEs may also include selecting the pair of UEs from a plurality of UEs that have a need to communicate. The selection of the pair of UEs may be in accordance with selection criteria, including but not limited to, amount of data, UE service history, UE priority, Quality of Service (QoS) requirements of the UEs, communications system load, communications system traffic, UE position and/or location, and the like. Scheduling the pair of UEs may also include allocating one or more communications system resources to the UEs and informing the UEs about the allocated communications system resources. In general, the communications system may need to provide the UEs with sufficient information to enable the UEs to communicate. The information may include the allocated communications system resources, and other information, including a modulation and coding scheme (MCS) level, rank information (for MIMO operation), precoder information, and so on. 
     The communications controller may schedule a UE to a widebeam (block  1015 ). The widebeam may have have a code used to generate the widebeam. In situations where it may be more effective to schedule a single UE rather than a pair of UEs, such as when the single UE is at the edge of the coverage area, only a single UE meets the selection criteria, and the like, the communications controller schedules the single UE to a widebeam. The widebeam may correspond to a pair of split beams that cover the sector. The scheduling of the UE may be similar to the scheduling of the pair of UEs. The communications controller may communicate with the scheduled UEs (block  1020 ). 
       FIG. 11 a    illustrates an example 4T 3GPP LTE system  1100  with a two-column cross-polarized antenna. 4T 3GPP LTE system  1100  may be modified to implement the port mappings described previously when the use of 90-degree hybrid couplers.  FIG. 11 b    illustrates a circuit diagram of an example 90-degree hybrid coupler  1120 . 90-degree hybrid coupler  1120  includes two input ports and two output ports. If the inputs are labeled X and Y, then the two output ports are X−j*Y and Y−j*X. 
       FIG. 11 c    illustrates a first example 4T 3GPP LTE system  1140  with 90-degree hybrid couplers. 4T 3GPP LTE system  1140  includes a pair of 90-degree hybrid couplers  1145  coupled in between PAs  1147  and duplexers  1149 . The configuration of 4T 3GPP LTE system  1140  may be referred to as an “after power amplification” configuration.  FIG. 11 d    illustrates a second example 4T 3GPP LTE system  1160  with 90-degree hybrid couplers. 4T 3GPP LTE system  1160  includes a pair of 90-degree hybrid couplers  1165  coupled in between transmitters  1167  and PAs/duplexers  1169 . The configuration of 4T 3GPP LTE system  1160  may be referred to as an “in small signal radio frequency (RF) domain” configuration.  FIG. 11 e    illustrates a third example 4T 3GPP LTE system  1180  with 90-degree hybrid couplers. 4T 3GPP LTE system  1180  includes a pair of 90-degree hybrid couplers  1185  before transmitters/PAs/duplexers  1187 . The configuration of 4T 3GPP LTE system  1180  may be referred to as an “in digital intermediate frequency (IF) domain” configuration. It is noted that the after power amplification configuration (4T 3GPP LTE system  1140 ) has an inherent benefit of PA sharing between the split beams, which may allow for significantly improved coverage over a conventional 6 sector communications system configuration, since in the 6 sector communications system configuration, a UE can receive signals from at most 2 PAs (e.g., the two PAs driving the A/ and A\ antennas). However, in 4T 3GPP LTE system  1140 , all four PAs can deliver the signals to a single UE through the A/ and A\ antennas. 
       FIG. 12  illustrates a diagram  1200  of an example extension of a 4T 3GPP LTE system to include sharing between two different polarizations. A 4T 3GPP LTE system  1205  may be extended to include sharing between two different polarizations (as shown in 4T 3GPP LTE system  1210 ) by adding a second pair of 90-degree hybrids  1215  coupled between an existing pair of 90-degree hybrids  1217  and duplexers  1219 . Although shown in  FIG. 12  as being positioned immediately after existing pair of 90-degree hybrids  1217 , second pair of 90-degree hybrids  1215  may be coupled between existing pair of 90-degree hybrids  1217  and PAs  1221 . 4T 3GPP LTE system  1210  may realize the mapping as follows:
         Port 0: A/−j*A\−j*B/−B\;   Port 1: −j*A/−A\+B/−j*B\;   Port 2: −A/−j*A\−j*B/+B\; and   Port 3: −j*A/+A\−B/−j*B\.       

       FIG. 13  illustrates a diagram  1300  of a modification of a 4T 3GPP LTE system for compatibility with LTE codebook. Diagram  1300  displays a 4T 3GPP LTE system  1305  that supports sharing between two different polarizations. In order to generate the four basic beams (i.e., A/, A\, B/, and B\) as data beams for individual UEs for the purpose of optimal MU-MIMO operation, the encoding may need slight modifications. 4T 3GPP LTE system  1310  includes additional phase shifts (as implemented by phase shifters  1315  and  1317 ) that may be necessary in the baseband. Actual modification to 4T 3GPP system  1305  is not necessary. 4T 3GPP LTE system  1310  may realize the mapping as follows:
         Port 0: A/−j*A\−j*B/−B\;   Port 1: A/+j*A\−j*B/+B\;   Port 2: −A/−j*A\−j*B/+B\; and   Port 3: −A/+j*A\−j*B/−B\.       

       FIG. 14  illustrates an example 4T 3GPP LTE system  1400  for use with a 3GPP LTE standard 4T codebook. 4T 3GPP LTE system  1400  is similar to 4T 3GPP LTE system  1310  of  FIG. 13  with the exception of phase shifters  1405  and  1407  of 4T 3GPP LTE system  1400  being coupled to ports 2 and 3 instead of phase shifters  1315  and  1317  being coupled to ports 1 and 3 of 4T 3GPP LTE system  1310 . A table below shows the mapping of baseband ports to achieve the 4 basic beams. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 Baseband Port 
                 A\H 
                 A/L 
                 B\H 
                 B/L 
               
               
                   
               
             
            
               
                 0 
                 1 
                 −j 
                 −j 
                 −1 
               
               
                 1 
                 −j 
                 1 
                 −1 
                 −j 
               
               
                 2 
                 1 
                 −j 
                  j 
                  1 
               
               
                 3 
                 −j 
                 1 
                  1 
                  j 
               
               
                   
               
            
           
         
       
     
       FIG. 15  illustrates a block diagram of an embodiment processing system  1500  for performing methods described herein, which may be installed in a host device. As shown, the processing system  1500  includes a processor  1504 , a memory  1506 , and interfaces  1510 - 1514 , which may (or may not) be arranged as shown in  FIG. 15 . The processor  1504  may be any component or collection of components adapted to perform computations and/or other processing related tasks, and the memory  1506  may be any component or collection of components adapted to store programming and/or instructions for execution by the processor  1504 . In an embodiment, the memory  1506  includes a non-transitory computer readable medium. The interfaces  1510 ,  1512 ,  1514  may be any component or collection of components that allow the processing system  1500  to communicate with other devices/components and/or a user. For example, one or more of the interfaces  1510 ,  1512 ,  1514  may be adapted to communicate data, control, or management messages from the processor  1504  to applications installed on the host device and/or a remote device. As another example, one or more of the interfaces  1510 ,  1512 ,  1514  may be adapted to allow a user or user device (e.g., personal computer (PC), etc.) to interact/communicate with the processing system  1500 . The processing system  1500  may include additional components not depicted in  FIG. 15 , such as long term storage (e.g., non-volatile memory, etc.). 
     In some embodiments, the processing system  1500  is included in a network device that is accessing, or part otherwise of, a telecommunications network. In one example, the processing system  1500  is in a network-side device in a wireless or wireline telecommunications network, such as a base station, a relay station, a scheduler, a controller, a gateway, a router, an applications server, or any other device in the telecommunications network. In other embodiments, the processing system  1500  is in a user-side device accessing a wireless or wireline telecommunications network, such as a mobile station, a user equipment (UE), a personal computer (PC), a tablet, a wearable communications device (e.g., a smartwatch, etc.), or any other device adapted to access a telecommunications network. 
     In some embodiments, one or more of the interfaces  1510 ,  1512 ,  1514  connects the processing system  1500  to a transceiver adapted to transmit and receive signaling over the telecommunications network.  FIG. 16  illustrates a block diagram of a transceiver  1600  adapted to transmit and receive signaling over a telecommunications network. The transceiver  1600  may be installed in a host device. As shown, the transceiver  1600  comprises a network-side interface  1602 , a coupler  1604 , a transmitter  1606 , a receiver  1608 , a signal processor  1610 , and a device-side interface  1612 . The network-side interface  1602  may include any component or collection of components adapted to transmit or receive signaling over a wireless or wireline telecommunications network. The coupler  1604  may include any component or collection of components adapted to facilitate bi-directional communication over the network-side interface  1602 . The transmitter  1606  may include any component or collection of components (e.g., up-converter, power amplifier, etc.) adapted to convert a baseband signal into a modulated carrier signal suitable for transmission over the network-side interface  1602 . The receiver  1608  may include any component or collection of components (e.g., down-converter, low noise amplifier, etc.) adapted to convert a carrier signal received over the network-side interface  1602  into a baseband signal. The signal processor  1610  may include any component or collection of components adapted to convert a baseband signal into a data signal suitable for communication over the device-side interface(s)  1612 , or vice-versa. The device-side interface(s)  1612  may include any component or collection of components adapted to communicate data-signals between the signal processor  1610  and components within the host device (e.g., the processing system  1500 , local area network (LAN) ports, etc.). 
     The transceiver  1600  may transmit and receive signaling over any type of communications medium. In some embodiments, the transceiver  1600  transmits and receives signaling over a wireless medium. For example, the transceiver  1600  may be a wireless transceiver adapted to communicate in accordance with a wireless telecommunications protocol, such as a cellular protocol (e.g., long-term evolution (LTE), etc.), a wireless local area network (WLAN) protocol (e.g., Wi-Fi, etc.), or any other type of wireless protocol (e.g., Bluetooth, near field communication (NFC), etc.). In such embodiments, the network-side interface  1602  comprises one or more antenna/radiating elements. For example, the network-side interface  1602  may include a single antenna, multiple separate antennas, or a multi-antenna array configured for multi-layer communication, e.g., single input multiple output (SIMO), multiple input single output (MISO), multiple input multiple output (MIMO), etc. In other embodiments, the transceiver  1600  transmits and receives signaling over a wireline medium, e.g., twisted-pair cable, coaxial cable, optical fiber, etc. Specific processing systems and/or transceivers may utilize all of the components shown, or only a subset of the components, and levels of integration may vary from device to device. 
       FIG. 17  is a flowchart of an embodiment method  1700  for operating an eNB. At step  1710 , the eNB concurrently transmit a first reference signal and a second reference signal in the same horizontal beam direction but at different antenna down-tilt angles. The first reference signal has a different polarization than the second reference signal. At step  1720 , the eNB receives feedback associating the first reference signal with a first UE and the second reference signal with a second UE. At step  1730 , the eNB schedules the first UE and the second UE to a code pair for a multi-user multiple-input multiple output (MU-MIMO) mode transmission in accordance with the feedback. At step  1740 , the eNB transmits data packets to the first UE and the second UE in accordance with the code pair for MU-MIMO mode transmission. 
       FIG. 18  is a flowchart of another embodiment method  1800  for operating an eNB. At step  1810 , the eNB schedules a pair of UEs using a code pair that produces a plurality of split beams for multi-user multiple-input multiple output (MU-MIMO) mode transmission. At step  1820 , the eNB transmits data packets on a first beam to a first UE and on a second beam to a second UE in accordance with the code pair. The first UE and the second UE belong to the pair of UEs, and the first beam and the second beam belong to the code pair. At step  1830 , the eNB retrieves information about the plurality of split beams and the mapping between the plurality of split beams and a plurality of baseband antenna ports. At step  1840 , the eNB applies a different down-tilt angle to each of the two polarizations for each one of the plurality of split beams. At step  1850 , the eNB schedules a second pair of UEs using the code pair. 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.