Patent Publication Number: US-2023155652-A1

Title: Systems and methods for beamforming of broadcast and synchronization channels

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application Ser. No. 63/279,520, filed on Nov. 15, 2021, and titled “SYSTEMS AND METHODS FOR BEAMFORMING OF BROADCAST AND SYNCHRONIZATION CHANNELS,” the contents of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     A centralized or cloud radio access network (C-RAN) is one way to implement base station functionality. Typically, for each cell (that is, for each physical cell identifier (PCI)) implemented by a C-RAN, one or more baseband unit (BBU) entities (also referred to here simply as “BBUs”) interacts with multiple remote units (also referred to here as “RUs,” “radio units,” “radio points,” or “RPs”) implement a base station entity in order to provide wireless service to various items of user equipment (UEs). The BBU entities may comprise a single entity (sometimes referred to as a “baseband controller” or simply a “baseband band unit” or “BBU”) that performs Layer-3, Layer-2, and some Layer-1 processing for the cell. The BBU entities may also comprises multiple entities, for example, one or more central unit (CU) entities that implement Layer 3 and non-time critical Layer 2 functions for the associated base station and one or more distribution units (DU) that implement the time critical Layer 2 functions and at least some of the Layer 1 (also referred to as the Physical Layer) functions for the associated base station. Each CU can be further partitioned into one or more user-plane and control-plane entities that handle the user-plane and control-plane processing of the CU, respectively. Each such user-plane CU entity is also referred to as a “CU-UP,” and each such control-plane CU entity is also referred to as a “CU-CP.” In this example, each RU is configured to implement the radio frequency (RF) interface and the physical layer functions for the associated base station that are not implemented in the DU. The multiple remote units are typically located remotely from each other (that is, the multiple remote units are not co-located), and the BBU entities are communicatively coupled to the remote units over a fronthaul network. The remote units may also be collocated (for example, in instances where each remote unit processes different carriers or time slices). 
     SUMMARY 
     In one example, a system includes at least one baseband unit (BBU), one or more radio units communicatively coupled to the at least one BBU, and a plurality of antennas communicatively coupled to the one or more radio units. Each respective radio unit of the one or more radio units is communicatively coupled to a respective subset of the plurality of antennas. The at least one BBU, the one or more radio units, and the plurality of antennas are configured to implement a base station for wirelessly communicating with user equipment. The base station is configured to map a synchronization signal to the plurality of antennas during a sequence of time periods. The base station is further configured to transmit the synchronization signal via each respective subset of the plurality of antennas with a first antenna beam pattern during a first time period of the sequence of time periods. The base station is further configured to transmit the synchronization signal via each respective subset of the plurality of antennas with a second antenna beam pattern via the plurality of antennas during a second time period of the sequence of time periods. The first antenna beam pattern is different than the second antenna beam pattern. 
     In another example, a method includes mapping a synchronization signal to a plurality of antennas during a sequence of time periods. The plurality of antennas is communicatively coupled to one or more radio units. The one or more radio units are communicatively coupled to at least one baseband unit. Each respective radio unit of the one or more radio units is communicatively coupled to a respective subset of the plurality of antennas. The at least one baseband unit, the one or more radio units, and the plurality of antennas are configured to implement a base station for wirelessly communicating with user equipment. The method further includes transmitting the synchronization signal via the plurality of antennas with a first antenna beam pattern during a first time period of the sequence of time periods. The method further includes transmitting the synchronization signal with a second antenna beam pattern via the plurality of antennas during a second time period of the sequence of time periods. The first antenna beam pattern is different than the second antenna beam pattern. 
    
    
     
       DRAWINGS 
       Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG.  1    is a block diagram illustrating an example radio access network; 
         FIG.  2    is a block diagram illustrating an example radio access network; 
         FIGS.  3 A- 3 B  are diagrams of example antenna beam patterns for transmission of synchronization signals; 
         FIG.  4    is a diagram of example antenna beam patterns for transmission of synchronization signals; 
         FIG.  5    is a diagram of example antenna beam patterns for transmission of synchronization signals; and 
         FIG.  6    is a flow diagram of an example method for beamforming synchronization signals. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be used and that logical, mechanical, and electrical changes may be made. The following detailed description is, therefore, not to be taken in a limiting sense. 
     In fifth generation (5G) New Radio (NR) systems, synchronization and master cell information is broadcast periodically (for example, using the Synchronization Signal Physical Broadcast Channel (SS-PBCH) Block or the Synchronization Signal Block (SSB)) by the base station (for example, gNodeB (gNB)) to enable the UEs to acquire the cell. The UEs are configured to receive, demodulate, and decode these synchronization signals transmitted from the base station (for example, on the Physical Broadcast Channel (PBCH), Primary Synchronization Signal (PSS), and Secondary Synchronization Signal (SSS)). 
     The synchronization information is typically mapped by the base station to a single logical transmission port that may be beamformed in a specific direction using N physical antennas mapped to that logical port. For millimeter wave (MM wave) transmissions in 5G NR systems, multiple versions of the SSB are used and each respective SSB is precoded as a beam in a different direction. The UEs typically receive one or two of these beams and respond at a timing that that corresponds to the beam that is favorable to the particular UE. 
     The 3GPP specification TS 38.211 for 5G NR indicates that a single logical transmission port (logical entity) should be used to transmit the SSB, and there is a specific precoding (or no precoding) that occurs (or mapping that occurs) between the logical antenna port and the physical antenna ports. Typically for cells and macro deployments, the logical antenna port is mapped to a single physical antenna port. Since every logical antenna port has limited power, the SSB block will be transmitted with less total power if multiple antennas are used. For example, if four physical antennas are coupled to the logical antenna port, then each physical antenna will transmit the signals with one-quarter of the total power. The reach of this SSB block is smaller due to the lower overall power to the UE compared to other channels that use all ports. This limits the overall size of the cell, which is undesirable. 
     To cover a larger cell area, the gNB can repeat a SSB several times in each period with consecutive time slots and each SSB is beamformed in a different direction. However, this approach incurs significant overhead since the additional SSB resources cannot be used for payload data. This overhead is typically deemed too wasteful in small cells where a small number of transmit antennas (for example, four or less antennas), and the overhead is particularly problematic in the lower frequency range (for example, FR1) due to the wider beams being used. 
     While the problems described above involves the SSB in 5G NR systems, a similar problem exists in LTE with the synchronization sequences. Therefore, although the following embodiments are primarily described as being implemented for use to provide 5G NR service, it is to be understood the techniques described here can be used with other wireless interfaces (for example, fourth generation (4G) Long-Term Evolution (LTE) service) and references to “gNB” can be replaced with the more general term “base station” or “base station entity” and/or a term particular to the alternative wireless interfaces (for example, “enhanced NodeB” or “eNB”). Furthermore, it is also to be understood that 5G NR embodiments can be used in both standalone and non-standalone modes (or other modes developed in the future) and the following description is not intended to be limited to any particular mode. Also, unless explicitly indicated to the contrary, references to “layers” or a “layer” (for example, Layer 1, Layer 2, Layer 3, the Physical Layer, the MAC Layer, etc.) set forth herein refer to layers of the wireless interface (for example, 5G NR or 4G LTE) used for wireless communication between a base station and user equipment). 
       FIG.  1    is a block diagram illustrating an example base station  100  in which the techniques for beamforming the synchronization signals described herein can be implemented. In the particular example shown in  FIG.  1   , the base station  100  includes one or more baseband unit (BBU) entities  102  communicatively coupled to a RU  106  via a fronthaul network  104 . The base station  100  provides wireless service to various items of user equipment (UEs)  108  in a cell  110 . Each BBU entity  102  can also be referred to simply as a “BBU.” 
     In the example shown in  FIG.  1   , the one or more BBU entities  102  comprise one or more central units (CUs)  103  and one or more distributed units (DUs)  105 . Each CU  103  implements Layer 3 and non-time critical Layer 2 functions for the associated base station  100 . Each DU  105  is configured to implement the time critical Layer 2 functions and at least some of the Layer 1 (also referred to as the Physical Layer) functions for the associated base station  100 . Each CU  103  can be further partitioned into one or more control-plane and user-plane entities  107 ,  109  that handle the control-plane and user-plane processing of the CU  103 , respectively. Each such control-plane CU entity  107  is also referred to as a “CU-CP”  107 , and each such user-plane CU entity  109  is also referred to as a “CU-UP”  109 . 
     The RU  106  is configured to implement the control-plane and user-plane Layer-1 functions not implemented by the DU  105  as well as the radio frequency (RF) functions. The RU  106  is typically located remotely from the one or more BBU entities  102 . In the example shown in  FIG.  1   , the RU  106  is implemented as a physical network function (PNF) and is deployed in or near a physical location where radio coverage is to be provided in the cell  110 . In the example shown in  FIG.  1   , the RU  106  is communicatively coupled to the DU  105  using a fronthaul network  104 . In some examples, the fronthaul network  104  is a switched Ethernet fronthaul network (for example, a switched Ethernet network that supports the Internet Protocol (IP)). 
     The RU  106  includes or is coupled to a set of antennas  112  via which downlink RF signals are radiated to UEs  108  and via which uplink RF signals transmitted by UEs  108  are received. In some examples, the set of antennas  112  includes two or four antennas. However, it should be understood that the set of antennas  112  can include two or more antennas  112 . In one configuration (used, for example, in indoor deployments), the RU  106  is co-located with its respective set of antennas  112  and is remotely located from the one or more BBU entities  102  serving it. In another configuration (used, for example, in outdoor deployments), the antennas  112  for the RU  106  are deployed in a sectorized configuration (for example, mounted at the top of a tower or mast). In such a sectorized configuration, the RU  106  need not be co-located with the respective sets of antennas  112  and, for example, can be located at the base of the tower or mast structure, for example, and, possibly, co-located with its serving one or more BBU entities  102 . 
     In the example shown in  FIG.  1   , at least one of the BBU entities  102  is configured to map synchronization signals to the antennas  112  coupled to the RU  106  and transmit the synchronization signals via the antennas  112  coupled to the RU  106  during particular time periods. The time periods where the transmission of synchronization signals occurs are non-consecutive time slots that repeat with some periodicity. In some examples, the time periods repeat every 10 milliseconds, 20 milliseconds, or a different amount of time depending on the system. In some examples, the time periods correspond to SSB transmission opportunities utilized by the base station  100 . 
     The base station  100  is configured to transmit the same synchronization signal (for example, the same SSB) via the antennas  112  using different antenna beam patterns in different time periods. In some examples, the respective antenna beam patterns are produced by applying respective phase differences to the synchronization signal for the antennas  112 . In some examples, the respective phase differences correspond to respective precoding vectors. In some such examples, the respective phase differences correspond to respective existing precoding vectors in a codebook from a 3GPP specification for 4G LTE or 5G NR (for example, for spatial multiplexing MIMO use cases). These respective existing precoding vectors are utilized by the base station  100  for other types of transmission, so the respective existing precoding vectors are already in the system for the base station  100 . In some examples, the respective phase differences are applied to the antennas using digitally controlled analog phase shifters. 
     The particular antenna beam patterns to be used by the base station  100  during operation are selected prior to commissioning the base station  100 . In some examples, the antenna beam patterns are selected based on the number of antennas  112  coupled to the RU  106 , the assumed geometry of the antennas  112 , and a simulated coverage in the cell  110  that can be achieved with the different antenna beam patterns using the antennas  112 . In some examples, the selected antenna beam patterns are orthogonal to each other. In general, the selected antenna beam patterns should include different points around the base station  100  in the cell  110  where nulls and enhancements occur to provide better coverage in the cell  110  compared to using a single antenna beam pattern. 
     Two or more distinct antenna beam patterns are used by the base station  100  for transmission of the synchronization signal via the antennas  112 . For 5G NR, the UEs  108  are instructed to attempt to read the information over four consecutive SSB opportunities, so using between two and four distinct antenna beam patterns would yield significant benefits compared to using a single antenna beam pattern. If the distinct antenna beam patterns are selected appropriately, using three antenna beam patterns will provide better performance compared to using two antenna beam patterns and using four antenna beam patterns will provide better performance for 5G NR implementations compared to using two or three antenna beam patterns. Example antenna beam patterns are discussed in more detail below with respect to  FIGS.  3 A- 5   . 
     During operation of the base station  100 , the base station  100  is configured to sequentially switch between the predetermined antenna beam patterns for transmission of the synchronization signal during the different time periods. The sequence of antenna beam patterns is static and repeats after a particular number of time periods that depends on the number of antenna beam patterns used. For example, if the base station  100  implements four distinct antenna beam patterns for transmission of the synchronization signal, then the base station  100  will transmit the synchronization signal with a different antenna beam pattern every time period and the sequence will repeat after four time periods. If the time periods occur every 20 milliseconds in such an example, then the sequence will repeat every 80 milliseconds. 
     The base station  100  is also configured to receive an uplink transmission from a UE  108  in response to the synchronization signal. For example, the base station  100  is configured to receive a Random Access Channel (RACH) response during a RACH opportunity that follows the transmission of the synchronization signal. In some examples, the base station  100  is configured to determine which particular antenna beam pattern was used where the best reception of the synchronization signal by a UE  108  and to transmit a subsequent response message (for example, the Physical Downlink Shared Channel (PDSCH) response) to that UE  108  based on that determination. In some such examples, the base station  100  is configured to determine that the UE  108  had the best reception of the synchronization signal with the particular antenna beam pattern used for the most recent transmission of the synchronization signal preceding the response from the UE  108 . In such examples, the base station  100  is configured to transmit a response message to the UE  108  using that particular antenna beam pattern. By using this technique, the base station  100  can increase the signal quality of the response message received at the UE  108 . 
     While the example shown in  FIG.  1    shows a single CU-CP  107 , a single CU-UP  109 , a single DU  105 , and a single RU  106  for the base station  100 , it should be understood that this is an example and other numbers of BBU entities, components of the BBU entities, and/or RUs can also be used. 
       FIG.  2    is a block diagram illustrating an example base station  200  in which the techniques for beamforming the synchronization signals described herein can be implemented. In the particular example shown in  FIG.  2   , the base station  200  includes one or more baseband unit (BBU) entities  202  communicatively coupled to multiple radio units (RUs)  206  via a fronthaul network  204 . The base station  200  provides wireless service to various items of user equipment (UEs)  208  in a cell  210 . Each BBU entity  202  can also be referred to simply as a “BBU.” 
     In the example shown in  FIG.  2   , the one or more BBU entities  202  comprise one or more central units (CUs)  203  and one or more distributed units (DUs)  205 . Each CU  203  implements Layer 3 and non-time critical Layer 2 functions for the associated base station  100 . Each DU  205  is configured to implement the time critical Layer 2 functions and at least some of the Layer 1 (also referred to as the Physical Layer) functions for the associated base station  200 . Each CU  203  can be further partitioned into one or more control-plane and user-plane entities  207 ,  209  that handle the control-plane and user-plane processing of the CU  203 , respectively. Each such control-plane CU entity  207  is also referred to as a “CU-CP”  207 , and each such user-plane CU entity  209  is also referred to as a “CU-UP”  209 . 
     The RUs  206  are configured to implement the control-plane and user-plane Layer-1 functions not implemented by the DU  205  as well as the radio frequency (RF) functions. Each RU  206  is typically located remotely from the one or more BBU entities and located remotely from other RUs  206 . In the example shown in  FIG.  2   , each RU  206  is implemented as a physical network function (PNF) and is deployed in or near a physical location where radio coverage is to be provided in the cell  210 . In the example shown in  FIG.  2   , the RUs  206  are communicatively coupled to the DU  205  using a fronthaul network  204 . In some examples, the fronthaul network  204  is a switched Ethernet fronthaul network (for example, a switched Ethernet network that supports the Internet Protocol (IP)). 
     Each of the RUs  206  includes or is coupled to a respective set of antennas  212  via which downlink RF signals are radiated to UEs  208  and via which uplink RF signals transmitted by UEs  208  are received. In some examples, each set of antennas  212  includes two or four antennas. However, it should be understood that each set of antennas  212  can include two or more antennas  212 . In one configuration (used, for example, in indoor deployments), each RU  206  is co-located with its respective set of antennas  212  and is remotely located from the one or more BBU entities  202  serving it. In another configuration (used, for example, in outdoor deployments), the sets of antennas  212  for the RUs  206  are deployed in a sectorized configuration (for example, mounted at the top of a tower or mast). In such a sectorized configuration, the RUs  206  need not be co-located with the respective sets of antennas  212  and, for example, can be located at the base of the tower or mast structure, for example, and, possibly, co-located with the serving one or more BBU entities  202 . Other configurations can be used. 
     In the example shown in  FIG.  2   , at least one of the BBU entities  202  is configured to map synchronization signals to the antennas  212  coupled to the RUs  206  and transmit the synchronization signals via each respective set of antennas  212  coupled to the RUs  206  during particular time periods. The time periods where the transmission of synchronization signals occurs are non-consecutive time slots that repeat with some periodicity. In some examples, the time periods repeat every 10 milliseconds, 20 milliseconds, or a different amount of time depending on the system. In some examples, the time periods correspond to SSB transmission opportunities by the base station  200 . 
     The base station  200  is configured to transmit the same synchronization signal (for example, the same SSB) via each respective set of antennas  212  using different antenna beam patterns in different time periods. In some examples, the different antenna beam patterns are produced by applying phase differences to the synchronization signals for the antennas  212 . In some examples, the respective phase differences correspond to respective precoding vectors. In some such examples, the respective phase differences correspond to respective existing precoding vectors in a codebook from a 3GPP specification for 4G LTE or 5G NR (for example, for spatial multiplexing MIMO use cases). These respective existing precoding vectors are utilized by the base station  200  for other types of transmission, so the respective existing precoding vectors are already in the system for the base station  200 . In some examples, the respective phase differences are applied to the antennas using digitally controlled analog phase shifters. 
     The particular antenna beam patterns to be used by the base station  200  during operation are selected prior to commissioning the base station  200 . In some examples, the antenna beam patterns are selected based on the number of antennas is each respective set of antennas  112  coupled to the RUs  206 , the assumed geometry of each respective set of antennas  212 , and a simulated coverage in the cell  210  that can be achieved with the different antenna beam patterns using the respective sets of antennas  212 . In some examples, the selected antenna beam patterns are orthogonal to each other. In general, the selected antenna beam patterns should include different points around the base station  200  in the cell  210  where nulls and enhancements occur to provide better coverage in the cell  210  compared to using a single antenna beam pattern. 
     Two or more distinct antenna beam patterns are used by the base station  200  for transmission of the synchronization signal via the respective sets of antennas  212 . For 5G NR, the UEs  208  are instructed to attempt to read the information over four consecutive SSB opportunities, so using between two and four distinct antenna beam patterns would yield significant benefits compared to using a single antenna beam pattern. If the distinct antenna beam patterns are selected appropriately, using three antenna beam patterns will provide better performance compared to using two antenna beam patterns and using four antenna beam patterns will provide better performance for 5G NR implementations compared to using two or three antenna beam patterns. Example antenna beam patterns are discussed in more detail below with respect to  FIGS.  3 A- 5   . 
     During operation of the base station  200 , the base station  200  is configured to sequentially switch between the predetermined antenna beam patterns for transmission of the synchronization signal during the different time periods. The sequence of antenna beam patterns is static and repeats after a particular number of time periods that depends on the number of antenna beam patterns used. For example, if the base station  200  implements four distinct antenna beam patterns for transmission of the synchronization signal, then the base station  200  will transmit the synchronization signal with a different antenna beam pattern via each respective set of antennas  212  every time period and the sequence will repeat after four time periods. If the time periods occur every 20 milliseconds in such an example, then the sequence will repeat every 80 milliseconds. 
     The base station  200  is configured to receive an uplink transmission from a UE  208  in response to the synchronization signal. For example, the base station  200  is configured to receive a Random Access Channel (RACH) response during a RACH opportunity that follows the best reception of the synchronization signal. In some examples, the base station  200  is configured to determine which particular antenna beam pattern was used where the best reception of the synchronization signal by a UE  208  and to transmit a subsequent response message (for example, the Physical Downlink Shared Channel (PDSCH) response) to the UE  208  based on that determination. In some such examples, the base station  200  is configured to determine that the UE  208  had the best reception of the synchronization signal with the particular antenna beam pattern used for the most recent transmission of the synchronization signal preceding the response from the UE  208 . In such examples, the base station  200  is configured to transmit a response message to the UE  208  using that particular antenna beam pattern. By using this technique, the base station  200  can increase the signal quality of the response message received at the UE  208 . 
     The base stations  100 ,  200  that include the components shown in  FIGS.  1 - 2    can be implemented using a scalable cloud environment in which resources used to instantiate each type of entity can be scaled horizontally (that is, by increasing or decreasing the number of physical computers or other physical devices) and vertically (that is, by increasing or decreasing the “power” (for example, by increasing the amount of processing and/or memory resources) of a given physical computer or other physical device). The scalable cloud environment can be implemented in various ways. For example, the scalable cloud environment can be implemented using hardware virtualization, operating system virtualization, and application virtualization (also referred to as containerization) as well as various combinations of two or more of the preceding. The scalable cloud environment can be implemented in other ways. For example, the scalable cloud environment is implemented in a distributed manner. That is, the scalable cloud environment is implemented as a distributed scalable cloud environment comprising at least one central cloud, at least one edge cloud, and at least one radio cloud. 
     In some examples, one or more components of the one or more BBU entities  102 ,  202  (for example, the CU  103 ,  203 , CU-CP  107 ,  207 , CU-UP  109 ,  209 , and/or DU  105 ,  205 ) are implemented as a software virtualized entities that are executed in a scalable cloud environment on a cloud worker node under the control of the cloud native software executing on that cloud worker node. In some such examples, the DU  105 ,  205  is communicatively coupled to at least one CU-CP  107 ,  207  and at least one CU-UP  109 ,  209 , which can also be implemented as software virtualized entities. In some other examples, one or more components of the one or more BBU entities  102 ,  202  (for example, the CU-CP  107 ,  207 , CU-UP  109 ,  209 , and/or DU  105 ,  205 ) are implemented as a single virtualized entity executing on a single cloud worker node. In some examples, the at least one CU-CP  107 ,  207  and the at least one CU-UP  109 ,  209  can each be implemented as a single virtualized entity executing on the same cloud worker node or as a single virtualized entity executing on a different cloud worker node. However, it is to be understood that different configurations and examples can be implemented in other ways. For example, the CU  103 ,  203  can be implemented using multiple CU-UP VNFs and using multiple virtualized entities executing on one or more cloud worker nodes. Moreover, it is to be understood that the CU  103 ,  203  and DU  105 ,  205  can be implemented in the same cloud (for example, together in a radio cloud or in an edge cloud). In some examples, the DU  105 ,  205  is configured to be coupled to the CU-CP  107 ,  207  and CU-UP  109 ,  209  over a midhaul network  111 ,  211  (for example, a network that supports the Internet Protocol (IP)). Other configurations and examples can be implemented in other ways. 
       FIGS.  3 A- 3 B  illustrate two simulated antenna beam patterns  300 ,  310  for an RU  106 ,  206  with two antennas  112 ,  212 . In particular,  FIG.  3 A  shows an antenna beam pattern  300  with two antennas  112 ,  212  transmitting with no phase difference, and  FIG.  3 B  shows an antenna beam pattern  310  with two antennas  112 ,  212  transmitting with  7 C phase difference. In examples where the two antenna beam patterns  300 ,  310  shown in  FIGS.  3 A- 3 B  are used, the base station  100 ,  200  is configured to switch between the antenna beam pattern  300  shown in  FIG.  3 A  and the antenna beam pattern  310  shown in  FIG.  3 B . In some such examples, the sequence repeats after two time periods. In other examples, the base station  100 ,  200  is configured to transmit the synchronization signal with the antenna beam pattern  300  shown in  FIG.  3 A  for multiple time periods (for example, two time periods) before switching to and transmitting the synchronization signal with the antenna beam pattern  310  shown in  FIG.  3 B  for multiple time periods (for example, two time periods). In other examples, the base station  100 ,  200  is configured to shift the antenna beam patterns  300 ,  310  (for example, by 45 degrees) for transmission over two time periods and then shift the antenna beam patterns  300 ,  310  back to the original directions such that the antenna beam patterns  300 ,  310  repeat every four time periods. The examples where the antenna beam patterns are shifted (for example, by 45 degrees) would likely increase the probability that UEs will be close to the peak power. 
       FIG.  4    illustrates four simulated antenna beam patterns  400 ,  410 ,  420 ,  430  for an RU  106 ,  206  with two antennas  112 ,  212 . In particular,  FIG.  4    shows an antenna beam pattern  400  with two antennas  112 ,  212  transmitting with no phase difference, an antenna beam pattern  410  with two antennas  112 ,  212  transmitting with π phase difference, an antenna beam pattern  420  with two antennas  112 ,  212  transmitting with π/2 phase difference, an antenna beam pattern  430  with two antennas  112 ,  212  transmitting with −π/2 phase difference. In examples where the four antenna beam patterns  400 ,  410 ,  420 ,  430  shown in  FIG.  4    are used, the base station  100 ,  200  is configured to switch between the antenna beam patterns  400 ,  410 ,  420 ,  430  in a sequential manner over four time periods. It should be understood that the switching sequence used by the base station  100 ,  200  could switch between the antenna beam patterns  400 ,  410 ,  420 ,  430  in any order as long as each antenna beam pattern is selected over four time periods and the sequence repeats. In some examples, the base station  100 ,  200  is configured to shift the antenna beam patterns  400 ,  410 ,  420 ,  430  (for example, by 45 degrees) for transmission over four time periods and then shift the antenna beam patterns  400 ,  410 ,  420 ,  430  back to the original directions such that the antenna beam patterns  400 ,  410 ,  420 ,  430  repeat in a particular direction every eight time periods. The examples where the antenna beam patterns are shifted (for example, by 45 degrees) would likely increase the probability that UEs will be close to the peak power. 
       FIG.  5    illustrates four simulated antenna beam patterns  500 ,  510 ,  520 ,  530  for an RU  106 ,  206  with four antennas  112 ,  212 . In particular,  FIG.  5    shows an antenna beam pattern  500  with four antennas  112 ,  212  transmitting with no phase difference between adjacent antennas, an antenna beam pattern  510  with four antennas  112 ,  212  transmitting with  7 C phase difference between adjacent antennas, an antenna beam pattern  520  with four antennas  112 ,  212  transmitting with π/2 phase difference between adjacent antennas, an antenna beam pattern  530  with four antennas  112 ,  212  transmitting with −π/2 phase difference between adjacent antennas. In examples where the four antenna beam patterns  500 ,  510 ,  520 ,  530  shown in  FIG.  5    are used, the base station  100 ,  200  is configured to switch between the antenna beam patterns  500 ,  510 ,  520 ,  530  in a sequential manner over four time periods. It should be understood that the switching sequence used by the base station  100 ,  200  could switch between the antenna beam patterns  500 ,  510 ,  520 ,  530  in any order as long as each antenna beam pattern is selected over four time periods and the sequence repeats. In some examples, the base station  100 ,  200  is configured to shift the antenna beam patterns  500 ,  510 ,  520 ,  530  (for example, by 45 degrees) for transmission over four time periods and then shift the antenna beam patterns  500 ,  510 ,  520 ,  530  back to the original directions such that the antenna beam patterns  500 ,  510 ,  520 ,  530  repeat in a particular direction every eight time periods. The examples where the antenna beam patterns are shifted (for example, by 45 degrees) would likely increase the probability that UEs will be close to the peak power. 
       FIG.  6    is a flow diagram of an example method  600  for beamforming synchronization signals. The common features discussed above with respect to the base stations in  FIGS.  1 - 5    can include similar characteristics to those discussed with respect to method  600  and vice versa. In some examples, the method  600  is performed by a base station (for example, base station  100 ,  200 ). 
     The method  600  begins with mapping a synchronization signal to a plurality of antennas (block  602 ). In some examples, mapping the synchronization signal to a plurality of ports includes mapping the synchronization signal from a single logical port (for example, a single logical antenna port) to physical antenna ports for each of the plurality of antennas. 
     The method  600  includes transmitting the synchronization signal with a first antenna beam pattern (block  604 ). In some examples, the first antenna beam pattern is produced by applying a first phase difference to the synchronization signal for each antenna of the plurality of antennas. The transmission with the first antenna beam pattern occurs during the first time period of a sequence of transmission time periods. In some examples where multiple RUs are included in the base station (for example, as shown in  FIG.  2   ), each respective subset of antennas associated with a respective RU transmits the synchronization signal with the first antenna beam pattern during the first time period. 
     The method  600  further includes transmitting the synchronization signal with a second antenna beam pattern (block  606 ). The second antenna beam pattern is different than the first antenna beam pattern. In some examples, the second antenna beam pattern is produced by applying a second phase difference to the synchronization signal for each antenna of the plurality of antennas, and the second phase difference is different than the first phase difference. The transmission with the second antenna beam pattern occurs during a second time period of the sequence of transmission time periods. In some examples where multiple RUs are included in the base station (for example, as shown in  FIG.  2   ), each respective subset of antennas associated with a respective RU transmits the synchronization signal with the second antenna beam pattern during the second time period. 
     When only two antenna beam patterns are used, the method  600  either proceeds back to block  604  and repeats blocks  604 - 606  for subsequent time periods or proceeds to block  612  (discussed below) to shift the first and second antenna beam patterns prior to repeating blocks  604 - 606 . 
     The method  600  optionally includes transmitting the synchronization signal with a third antenna beam pattern (block  608 ). The third antenna beam pattern is different than the first antenna beam pattern and the second antenna beam pattern. In some examples, the third antenna beam pattern is produced by applying a third phase difference to the synchronization signal for each antenna of the plurality of antennas, and the third phase difference is different than the first phase difference and the second phase difference. The transmission with the third antenna beam pattern occurs during a third time period of the sequence of transmission time periods. In some examples where multiple RUs are included in the base station (for example, as shown in  FIG.  2   ), each respective subset of antennas associated with a respective RU transmits the synchronization signal with the third antenna beam pattern during the third time period. 
     When only three antenna beam patterns are used, the method  600  either proceeds back to block  604  and repeats blocks  604 - 608  for subsequent time periods or proceeds to block  612  (discussed below) to shift the first, second, and third antenna beam patterns prior to repeating blocks  604 - 608 . 
     The method  600  optionally includes transmitting the synchronization signal with a fourth antenna beam pattern (block  610 ). The fourth antenna beam pattern is different than the first antenna beam pattern, the second antenna beam pattern, and the third antenna beam pattern. In some examples, the fourth antenna beam pattern is produced by applying a fourth phase difference to the synchronization signal for each antenna of the plurality of antennas, and the third phase difference is different than the first phase difference, the second phase difference, and the third phase difference. The transmission with the fourth antenna beam pattern occurs during a fourth time period of the sequence of transmission time periods. In some examples where multiple RUs are included in the base station (for example, as shown in  FIG.  2   ), each respective subset of antennas associated with a respective RU transmits the synchronization signal with the fourth antenna beam pattern during the fourth time period. 
     Following block  610 , the method  600  either proceeds back to block  604  and repeats blocks  604 - 610  for subsequent time periods or proceeds to block  612  (discussed below) to shift the first, second, third, and fourth antenna beam patterns prior to repeating blocks  604 - 610 . 
     As discussed above, the method  600  optionally includes shifting the antenna beam patterns prior to repeating the sequence (block  612 ). In some examples, each antenna pattern in the sequence is shifted by the same amount for the subsequent time period where that antenna beam pattern is used for transmission. For example, each antenna beam pattern in the sequence is shifted by 45 degrees prior to the subsequent time period where that antenna beam pattern is used for transmission. 
     Following block  612 , the method  600  proceeds back to block  604  and repeats the blocks corresponding to the number of antenna beam patterns used for transmission. 
     Other examples are implemented in other ways. 
     The example techniques described herein reduce the bandwidth used for transmitting synchronization signals to UEs compared to previous techniques that transmit multiple SSBs in consecutive time slots. The example techniques described herein also reduce null spots for synchronization signals in the cell and increase the size of the cell compared to previous techniques that did not use multiple antenna beam patterns for synchronization signals. Overall, the bandwidth used for synchronization signals and signal strength of the synchronization signals at the UEs is improved using the example techniques described herein. 
     The methods and techniques described here may be implemented in digital electronic circuitry, or with a programmable processor (for example, a special-purpose processor or a general-purpose processor such as a computer) firmware, software, digitally controlled analog phase shifters, or in combinations of them. Apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. A process embodying these techniques may be performed by a programmable processor executing a program of instructions to perform desired functions by operating on input data and generating appropriate output. The techniques may advantageously be implemented in one or more programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Generally, a processor will receive instructions and data from a read-only memory and/or a random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and DVD disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs). 
     EXAMPLE EMBODIMENTS 
     Example 1 includes a system, comprising: at least one baseband unit; one or more radio units communicatively coupled to the at least one baseband unit; and a plurality of antennas communicatively coupled to the one or more radio units, wherein each respective radio unit of the one or more radio units is communicatively coupled to a respective subset of the plurality of antennas; wherein the at least one baseband unit, the one or more radio units, and the plurality of antennas are configured to implement a base station for wirelessly communicating with user equipment; map a synchronization signal to the plurality of antennas during a sequence of time periods; transmit the synchronization signal via each respective subset of the plurality of antennas with a first antenna beam pattern during a first time period of the sequence of time periods; and transmit the synchronization signal via each respective subset of the plurality of antennas with a second antenna beam pattern during a second time period of the sequence of time periods, wherein the first antenna beam pattern is different than the second antenna beam pattern. 
     Example 2 includes the system of Example 1, wherein the base station is configured to: produce the first antenna beam pattern by applying first phase differences to the synchronization signal for each respective subset of the plurality of antennas; and produce the second antenna beam pattern by applying second phase differences to the synchronization signal for each respective subset of the plurality of antennas, wherein the first phase differences are different than the second phase differences. 
     Example 3 includes the system of Example 2, wherein the first phase differences for each respective subset of the plurality of antennas correspond to a first precoding vector, wherein the second phase differences for each respective subset of the plurality of antennas correspond to a second precoding vector. 
     Example 4 includes the system of Example 3, wherein the first precoding vector and the second precoding vector each correspond to a respective precoding vector in a 3GPP specification for 4G LTE or 5G NR. 
     Example 5 includes the system of any of Examples 1-4, wherein the base station is configured to: transmit the synchronization signal via each respective subset of the plurality of antennas with a third antenna beam pattern during a third time period of the sequence of time periods, wherein the third antenna beam pattern is different than the first antenna beam pattern and the second antenna beam pattern; and transmit the synchronization signal via each respective subset of the plurality of antennas with a fourth antenna beam pattern during a fourth time period of the sequence of time periods, wherein the fourth antenna beam pattern is different than the first antenna beam pattern, the second antenna beam pattern, and the third antenna beam pattern. 
     Example 6 includes the system of Example 5, wherein the third antenna beam pattern corresponds to a shifted version of the first antenna beam pattern, wherein the fourth antenna beam pattern corresponds to a shifted version of the second antenna beam pattern. 
     Example 7 includes the system of any of Examples 1-6, wherein the first antenna beam pattern and the second antenna beam pattern are static during operation of the system. 
     Example 8 includes the system of any of Examples 1-7, wherein the base station is configured to: receive an uplink transmission from a first user equipment in response to the synchronization signal; determine whether the first user equipment received the synchronization signal via the first antenna beam pattern or the second antenna beam pattern; and transmit a response message to the first user equipment based on the determination of whether the first user equipment received the synchronization signal via the first antenna beam pattern or the second antenna beam pattern. 
     Example 9 includes the system of any of Examples 1-8, wherein the synchronization signal includes a 5G NR Synchronization Signal Block (SSB). 
     Example 10 includes the system of any of Examples 1-9, wherein the synchronization signal includes a 4G LTE synchronization signal. 
     Example 11 includes a method, comprising: mapping a synchronization signal to a plurality of antennas during a sequence of time periods, wherein the plurality of antennas is communicatively coupled to one or more radio units, wherein the one or more radio units are communicatively coupled to at least one baseband unit, wherein each respective radio unit of the one or more radio units is communicatively coupled to a respective subset of the plurality of antennas, wherein the at least one baseband unit, the one or more radio units, and the plurality of antennas are configured to implement a base station for wirelessly communicating with user equipment; transmitting the synchronization signal via the plurality of antennas with a first antenna beam pattern during a first time period of the sequence of time periods; and transmitting the synchronization signal with a second antenna beam pattern via the plurality of antennas during a second time period of the sequence of time periods, wherein the first antenna beam pattern is different than the second antenna beam pattern. 
     Example 12 includes the method of Example 11, further comprising: producing the first antenna beam pattern by applying first phase differences to the synchronization signal for each respective subset of the plurality of antennas; and producing the second antenna beam pattern by applying second phase differences to the synchronization signal for each respective subset of the plurality of antennas, wherein the first phase differences are different than the second phase differences. 
     Example 13 includes the method of Example 12, wherein the first phase differences for each respective subset of the plurality of antennas correspond to a first precoding vector, wherein the second phase differences for each respective subset of the plurality of antennas correspond to a second precoding vector. 
     Example 14 includes the method of Example 13, wherein the first precoding vector and the second precoding vector each correspond to a respective precoding vector in a 3GPP specification for 4G LTE or 5G NR. 
     Example 15 includes the method of any of Examples 11-14, further comprising: transmitting the synchronization signal via each respective subset of the plurality of antennas with a third antenna beam pattern during a third time period of the sequence of time periods, wherein the third antenna beam pattern is different than the first antenna beam pattern and the second antenna beam pattern; and transmitting the synchronization signal via each respective subset of the plurality of antennas with a fourth antenna beam pattern during a fourth time period of the sequence of time periods, wherein the fourth antenna beam pattern is different than the first antenna beam pattern, the second antenna beam pattern, and the third antenna beam pattern. 
     Example 16 includes the method of Example 15, wherein the third antenna beam pattern corresponds to a shifted version of the first antenna beam pattern, wherein the fourth antenna beam pattern corresponds to a shifted version of the second antenna beam pattern. 
     Example 17 includes the method of any of Examples 11-16, wherein the first antenna beam pattern and the second antenna beam pattern are static during operation. 
     Example 18 includes the method of any of Examples 11-17, further comprising: receiving an uplink transmission from a first user equipment in response to the synchronization signal; determining whether the first user equipment received the synchronization signal via the first antenna beam pattern or the second antenna beam pattern; and transmitting a response message to the first user equipment based on the determination of whether the first user equipment received the synchronization signal via the first antenna beam pattern or the second antenna beam pattern. 
     Example 19 includes the method of any of Examples 11-18, wherein the synchronization signal includes a 5G NR Synchronization Signal Block (SSB). 
     Example 20 includes the method of any of Examples 11-19, wherein the synchronization signal includes a 4G LTE synchronization signal. 
     A number of embodiments of the invention defined by the following claims have been described. Nevertheless, it will be understood that various modifications to the described embodiments may be made without departing from the spirit and scope of the claimed invention. Accordingly, other embodiments are within the scope of the following claims.