Patent Publication Number: US-2023163918-A1

Title: Sounding reference signal switching

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/646,115, filed Dec. 27, 2021, titled “SOUNDING REFERENCE SIGNAL SWITCHING,” which is a continuation of U.S. patent application Ser. No. 16/828,135, filed Mar. 24, 2020, titled “SOUNDING REFERENCE SIGNAL SWITCHING,” which claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Patent Application No. 62/826,750, filed Mar. 29, 2019 and titled “SOUNDING REFERENCE SIGNAL SWITCHING,” which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the invention relate to electronic systems, and in particular, to radio frequency electronics. 
     Description of Related Technology 
     Radio frequency (RF) communication systems can be used for transmitting and/or receiving signals of a wide range of frequencies. For example, an RF communication system can be used to wirelessly communicate RF signals in a frequency range of about 30 kHz to 300 GHz, such as in the range of about 410 MHz to about 7.125 GHz for fifth generation (5G) communications using Frequency Range  1  (FR 1 ). 
     Examples of RF communication systems include, but are not limited to, mobile phones, tablets, base stations, network access points, customer-premises equipment (CPE), laptops, and wearable electronics. 
     SUMMARY 
     In certain embodiments, the present disclosure relates to a mobile device. The mobile device includes a first transmit chain including a first power amplifier, a second transmit chain including a second power amplifier, and a baseband system configured to generate a first sequence of symbols for transmission by way of the first transmit chain and a second sequence of symbols for transmission by way of the second transmit chain. The baseband system is further configured to stagger the first sequence of symbols and the second sequence of symbols, and to include one or more sounding reference signal symbols in the second sequence of symbols. 
     In various embodiments, the baseband system is further configured to stager symbol transmissions of the first sequence of symbols relative to the second sequence of symbols with substantially no delay between symbol transmissions. 
     In several embodiments, the first transmit chain is a first uplink multi-input multiple-output chain, and the second transmit chain is a second uplink multiple-input multiple-output chain. 
     In some embodiments, the baseband system is further configured to provide the second sequence of symbols for transmission when an uplink multiple-input multiple-output mode is disabled. 
     In a number of embodiments, the mobile device further includes a first antenna and a second antenna, and the baseband system is further configured to transmit at least a portion of the first sequence of symbols using the first antenna and to transmit at least a portion of the second sequence of symbols using the second antenna. 
     In several embodiments, the baseband system is further configured to include one or more sounding reference signal symbols in the first sequence of symbols. 
     In some embodiments, the baseband system is further configured to include no sounding reference signal symbols in the first sequence of symbols. 
     In various embodiments, the baseband system is further configured to include at least one Physical Uplink Shared Channel (PUSCH) symbol in the first sequence of symbols. 
     In a number of embodiments, the baseband system is further configured to include at least one Physical Uplink Control Channel (PUCCH) symbol in the first sequence of symbols. 
     In several embodiments, the baseband system is further configured to include no blank symbols in either of the first sequence of symbols or the second sequence of symbols. 
     In some embodiments, the baseband system is further configured to encode the first sequence of symbols and the second sequence of symbols in accordance with a Fifth Generation (5G) communication standard. 
     In several embodiments, the baseband system is further configured to alternate symbol transmissions of the first sequence of symbols and the second sequence of symbols. 
     In various embodiments, the mobile device further includes a first antenna and two or more antennas in addition to the first antenna, the baseband system further configured to transmit the first sequence of symbols using the first antenna and to transmit the second sequence of symbols using the two or more antennas. 
     In several embodiments, the mobile device further includes a first group of two or more antennas and a second group of two or more antennas, the baseband system further configured to transmit the first sequence of symbols using the first group and to transmit the second sequence of symbols using the second group. 
     In some embodiments, the baseband system is further configured to receive a capability inquiry from a base station, and to transmit capability information to the base station in response to the capability inquiry. According to a number of embodiments, the baseband system is further configured to receive transmit configuration information from the base station, and to configure the first transmit chain and the second transmit chain to operate with a switching time indicated by the transmit configuration information. In accordance with various embodiments, the transmit configuration information indicates transmissions duplexed using time-division duplexing. According to several embodiments, the transmit configuration information indicates transmissions duplexed using frequency-division duplexing. In accordance with a number of embodiments, the transmit configuration information indicates a switching time of 0 microsecond. According to various embodiments, the capability information indicates switching capability for each of a plurality of subcarrier spacings. In accordance with several embodiments, the capability information indicates switching capability for each of a plurality of frequency bands. According to a number of embodiments, the capability information indicates whether or not the user equipment complies with a switching time threshold. 
     In certain embodiments, the present disclosure relates to a method of sounding reference signal switching in a mobile device. The method includes transmitting a first sequence of symbols using a first transmit path through a first power amplifier, and transmitting a second sequence of symbols using a second transmit path through a second power amplifier, including staggering symbol transmissions of the second sequence of symbols with respect to the first sequence of symbols, and transmitting one or more sounding reference signal symbols in the second sequence of symbols. 
     In some embodiments, the method further includes staggering symbol transmissions of the second sequence of symbols with respect to the first sequence of symbols with substantially no delay between symbol transmissions. 
     In several embodiments, the first transmit path is a first uplink multi-input multiple-output path, and the second transmit path is a second uplink multiple-input multiple-output path. 
     In various embodiments, the method further includes transmitting the second sequence of symbols when an uplink multiple-input multiple-output mode is disabled. 
     In a number of embodiments, the method further includes transmitting at least a first portion of the first sequence of the symbols on a first antenna, and transmitting at least a first portion of the second sequence of symbols on a second antenna. 
     In some embodiments, the method further includes transmitting one or more sounding reference signal symbols in the first sequence of symbols. 
     In several embodiments, the method further includes transmitting no sounding reference signal symbols in the first sequence of symbols. 
     In a number of embodiments, the method further includes transmitting at least one Physical Uplink Shared Channel (PUSCH) symbol in the first sequence of symbols. 
     In various embodiments, the method further includes transmitting at least one Physical Uplink Control Channel (PUCCH) symbol in the first sequence of symbols. 
     In some embodiments, the method further includes transmitting no blank symbols in either of the first sequence of symbols or the second sequence of symbols. 
     In several embodiments, the method further includes transmitting the first sequence of symbols and the second sequence of symbols over a Fifth Generation (5G) network. 
     In a number of embodiments, the method further includes alternating symbol transmissions of the first sequence of symbols and the second sequence of symbols. 
     In several embodiments, the method further includes transmitting the first sequence of symbols using a first antenna, and transmitting the second sequence of symbols using two or more antennas each different from the first antenna. 
     In some embodiments, the method further includes transmitting the first sequence of symbols using a first group of two or more antennas, and transmitting the second sequence of symbols using a second group of two or more antennas, each antenna of the second group different from each antenna in the first group. 
     In various embodiments, the method further includes receiving a capability inquiry from a base station, and transmitting capability information to the base station in response to the capability inquiry. According to some embodiments, the method further includes receiving transmit configuration information from the base station, and configuring a front end system of the mobile device to operate with a switching time indicated by the transmit configuration information, the front end system including the first transmit path and the second transmit path. In accordance with a number of embodiments, the transmit configuration information indicates transmissions duplexed using time-division duplexing. According to several embodiments, the transmit configuration information indicates transmissions duplexed using frequency-division duplexing. In accordance with some embodiments, the transmit configuration information indicates a switching time of 0 microsecond. According to a number of embodiments, the capability information indicates switching capability for each of a plurality of subcarrier spacings. In accordance with several embodiments, the capability information indicates switching capability for each of a plurality of frequency bands. According to some embodiments, the capability information indicates whether or not the user equipment complies with a switching time threshold. 
     In certain embodiments, the present disclosure relates to a front end system. The front end system includes a plurality of terminals including a first transmit terminal, a second transmit terminal, a first antenna terminal, and a second antenna terminal. The front end system further includes a first power amplifier configured to amplify a first radio frequency transmit signal received from the first transmit terminal and to provide a first amplified radio frequency transmit signal to the first antenna terminal, the first radio frequency transmit signal carrying a first sequence of symbols. The front end system further includes a second power amplifier configured to amplify a second radio frequency transmit signal received from the second transmit terminal and to provide a second amplified radio frequency transmit signal to the second antenna terminal, the second radio frequency signal carrying a second sequence of symbols that is staggered with respect to the first sequence of symbols and that includes one or more sounding reference signal symbols. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of one example of a communication network. 
         FIG.  2 A  is a schematic diagram of one example of a communication link using carrier aggregation. 
         FIG.  2 B  illustrates various examples of uplink carrier aggregation for the communication link of  FIG.  2 A . 
         FIG.  2 C  illustrates various examples of downlink carrier aggregation for the communication link of  FIG.  2 A . 
         FIG.  3 A  is a schematic diagram of one example of a downlink channel using multi-input and multi-output (MIMO) communications. 
         FIG.  3 B  is schematic diagram of one example of an uplink channel using MIMO communications. 
         FIG.  3 C  is schematic diagram of another example of an uplink channel using MIMO communications. 
         FIG.  4    is a schematic diagram illustrating two examples of multiple access schemes for a communication network. 
         FIG.  5 A  is a schematic diagram of one example of a communication system that operates with beamforming. 
         FIG.  5 B  is a schematic diagram of one example of beamforming to provide a transmit beam. 
         FIG.  5 C  is a schematic diagram of one example of beamforming to provide a receive beam. 
         FIG.  6 A  is a diagram depicting two examples of symbol blanking for time slots including sounding reference signal (SRS) symbols. 
         FIG.  6 B  is a table depicting one example of symbol duration versus subcarrier spacing (SCS). 
         FIG.  6 C  is a table depicting one example of various communication parameters versus SCS. 
         FIG.  6 D  is a diagram of one example of ON to ON timing for SRS. 
         FIG.  7 A  is a schematic diagram of one example of a communication system operating with SRS for one transmit four receive (1T4R). 
         FIG.  7 B  is one example of a timing diagram for the communication system of  FIG.  7 A . 
         FIG.  8 A  is a schematic diagram of one example of a communication system operating with SRS for two transmit four receive (2T4R). 
         FIG.  8 B  is one example of a timing diagram for the communication system of  FIG.  8 A . 
         FIG.  9 A  is a schematic diagram of one embodiment of a communication system operating with SRS for 2T4R. 
         FIG.  9 B  is one example of a timing diagram for the communication system of  FIG.  9 A . 
         FIG.  10 A  is a diagram of one example of an impact of transients on an uplink physical layer. 
         FIG.  10 B  is a diagram of another example of an impact of transients on an uplink physical layer. 
         FIG.  10 C  is a table of one example of an impact of transients on an uplink physical layer. 
         FIG.  11 A  is a schematic diagram of another embodiment of a communication system operating with SRS for 2T4R. 
         FIG.  11 B  is one example of a timing diagram for the communication system of  FIG.  11 A . 
         FIG.  12 A  is a schematic diagram of another embodiment of a communication system operating with SRS for 2T4R. 
         FIG.  12 B  is one example of a timing diagram for the communication system of  FIG.  12 A . 
         FIG.  13    is a graph of one example of uplink demodulation performance for different transient switching times. 
         FIG.  14    is a schematic diagram of one embodiment of a mobile device. 
         FIG.  15    is a schematic diagram of one embodiment of an RF communication system. 
         FIG.  16    is a schematic diagram of another embodiment of an RF communication system. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The following detailed description of certain embodiments presents various descriptions of specific embodiments. However, the innovations described herein can be embodied in a multitude of different ways, for example, as defined and covered by the claims. In this description, reference is made to the drawings where like reference numerals can indicate identical or functionally similar elements. It will be understood that elements illustrated in the figures are not necessarily drawn to scale. Moreover, it will be understood that certain embodiments can include more elements than illustrated in a drawing and/or a subset of the elements illustrated in a drawing. Further, some embodiments can incorporate any suitable combination of features from two or more drawings. 
     The International Telecommunication Union (ITU) is a specialized agency of the United Nations (UN) responsible for global issues concerning information and communication technologies, including the shared global use of radio spectrum. 
     The 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunications standard bodies across the world, such as the Association of Radio Industries and Businesses (ARIB), the Telecommunications Technology Committee (TTC), the China Communications Standards Association (CCSA), the Alliance for Telecommunications Industry Solutions (ATIS), the Telecommunications Technology Association (TTA), the European Telecommunications Standards Institute (ETSI), and the Telecommunications Standards Development Society, India (TSDSI). 
     Working within the scope of the ITU, 3GPP develops and maintains technical specifications for a variety of mobile communication technologies, including, for example, second generation (2G) technology (for instance, Global System for Mobile Communications (GSM) and Enhanced Data Rates for GSM Evolution (EDGE)), third generation (3G) technology (for instance, Universal Mobile Telecommunications System (UMTS) and High Speed Packet Access (HSPA)), and fourth generation (4G) technology (for instance, Long Term Evolution (LTE) and LTE-Advanced). 
     The technical specifications controlled by 3GPP can be expanded and revised by specification releases, which can span multiple years and specify a breadth of new features and evolutions. 
     In one example, 3GPP introduced carrier aggregation (CA) for LTE in Release 10. Although initially introduced with two downlink carriers, 3GPP expanded carrier aggregation in Release 14 to include up to five downlink carriers and up to three uplink carriers. Other examples of new features and evolutions provided by 3GPP releases include, but are not limited to, License Assisted Access (LAA), enhanced LAA (eLAA), Narrowband Internet of things (NB-IOT), Vehicle-to-Everything (V2X), and High Power User Equipment (HPUE). 
     3GPP introduced Phase 1 of fifth generation (5G) technology in Release 15, and plans to introduce Phase 2 of 5G technology in Release 16 (targeted for 2020). Subsequent 3GPP releases will further evolve and expand 5G technology. 5G technology is also referred to herein as 5G New Radio (NR). 
     5G NR supports or plans to support a variety of features, such as communications over millimeter wave spectrum, beamforming capability, high spectral efficiency waveforms, low latency communications, multiple radio numerology, and/or non-orthogonal multiple access (NOMA). Although such RF functionalities offer flexibility to networks and enhance user data rates, supporting such features can pose a number of technical challenges. 
     The teachings herein are applicable to a wide variety of communication systems, including, but not limited to, communication systems using advanced cellular technologies, such as LTE-Advanced, LTE-Advanced Pro, and/or 5G NR. 
       FIG.  1    is a schematic diagram of one example of a communication network  10 . The communication network  10  includes a macro cell base station  1 , a small cell base station  3 , and various examples of user equipment (UE), including a first mobile device  2   a , a wireless-connected car  2   b , a laptop  2   c , a stationary wireless device  2   d , a wireless-connected train  2   e , a second mobile device  2   f , and a third mobile device  2   g.    
     Although specific examples of base stations and user equipment are illustrated in  FIG.  1   , a communication network can include base stations and user equipment of a wide variety of types and/or numbers. 
     For instance, in the example shown, the communication network  10  includes the macro cell base station  1  and the small cell base station  3 . The small cell base station  3  can operate with relatively lower power, shorter range, and/or with fewer concurrent users relative to the macro cell base station  1 . The small cell base station  3  can also be referred to as a femtocell, a picocell, or a microcell. Although the communication network  10  is illustrated as including two base stations, the communication network  10  can be implemented to include more or fewer base stations and/or base stations of other types. 
     Although various examples of user equipment are shown, the teachings herein are applicable to a wide variety of user equipment, including, but not limited to, mobile phones, tablets, laptops, IoT devices, wearable electronics, customer premises equipment (CPE), wireless-connected vehicles, wireless relays, and/or a wide variety of other communication devices. Furthermore, user equipment includes not only currently available communication devices that operate in a cellular network, but also subsequently developed communication devices that will be readily implementable with the inventive systems, processes, methods, and devices as described and claimed herein. 
     The illustrated communication network  10  of  FIG.  1    supports communications using a variety of cellular technologies, including, for example, 4G LTE and 5G NR. In certain implementations, the communication network  10  is further adapted to provide a wireless local area network (WLAN), such as WiFi. Although various examples of communication technologies have been provided, the communication network  10  can be adapted to support a wide variety of communication technologies. 
     Various communication links of the communication network  10  have been depicted in  FIG.  1   . The communication links can be duplexed in a wide variety of ways, including, for example, using frequency-division duplexing (FDD) and/or time-division duplexing (TDD). FDD is a type of radio frequency communications that uses different frequencies for transmitting and receiving signals. FDD can provide a number of advantages, such as high data rates and low latency. In contrast, TDD is a type of radio frequency communications that uses about the same frequency for transmitting and receiving signals, and in which transmit and receive communications are switched in time. TDD can provide a number of advantages, such as efficient use of spectrum and variable allocation of throughput between transmit and receive directions. 
     In certain implementations, user equipment can communicate with a base station using one or more of 4G LTE, 5G NR, and WiFi technologies. In certain implementations, enhanced license assisted access (eLAA) is used to aggregate one or more licensed frequency carriers (for instance, licensed 4G LTE and/or 5G NR frequencies), with one or more unlicensed carriers (for instance, unlicensed WiFi frequencies). 
     As shown in  FIG.  1   , the communication links include not only communication links between UE and base stations, but also UE to UE communications and base station to base station communications. For example, the communication network  10  can be implemented to support self-fronthaul and/or self-backhaul. 
     The communication links can operate over a wide variety of frequencies. In certain implementations, communications are supported using 5G NR technology over one or more frequency bands that are less than 6 Gigahertz (GHz) and/or over one or more frequency bands that are greater than 6 GHz. For example, the communication links can serve Frequency Range  1  (FR 1 ), Frequency Range  2  (FR 2 ), or a combination thereof. In one embodiment, one or more of the mobile devices support a HPUE power class specification. 
     In certain implementations, a base station and/or user equipment communicates using beamforming. For example, beamforming can be used to focus signal strength to overcome path losses, such as high loss associated with communicating over high signal frequencies. In certain embodiments, user equipment, such as one or more mobile phones, communicate using beamforming on millimeter wave frequency bands in the range of 30 GHz to 300 GHz and/or upper centimeter wave frequencies in the range of 6 GHz to 30 GHz, or more particularly, 24 GHz to 30 GHz. 
     Different users of the communication network  10  can share available network resources, such as available frequency spectrum, in a wide variety of ways. 
     In one example, frequency division multiple access (FDMA) is used to divide a frequency band into multiple frequency carriers. Additionally, one or more carriers are allocated to a particular user. Examples of FDMA include, but are not limited to, single carrier FDMA (SC-FDMA) and orthogonal FDMA (OFDMA). OFDMA is a multicarrier technology that subdivides the available bandwidth into multiple mutually orthogonal narrowband subcarriers, which can be separately assigned to different users. 
     Other examples of shared access include, but are not limited to, time division multiple access (TDMA) in which a user is allocated particular time slots for using a frequency resource, code division multiple access (CDMA) in which a frequency resource is shared amongst different users by assigning each user a unique code, space-divisional multiple access (SDMA) in which beamforming is used to provide shared access by spatial division, and non-orthogonal multiple access (NOMA) in which the power domain is used for multiple access. For example, NOMA can be used to serve multiple users at the same frequency, time, and/or code, but with different power levels. 
     Enhanced mobile broadband (eMBB) refers to technology for growing system capacity of LTE networks. For example, eMBB can refer to communications with a peak data rate of at least 10 Gbps and a minimum of 100 Mbps for each user. Ultra-reliable low latency communications (uRLLC) refers to technology for communication with very low latency, for instance, less than 2 milliseconds. uRLLC can be used for mission-critical communications such as for autonomous driving and/or remote surgery applications. Massive machine-type communications (mMTC) refers to low cost and low data rate communications associated with wireless connections to everyday objects, such as those associated with Internet of Things (IoT) applications. 
     The communication network  10  of  FIG.  1    can be used to support a wide variety of advanced communication features, including, but not limited to, eMBB, uRLLC, and/or mMTC. 
       FIG.  2 A  is a schematic diagram of one example of a communication link using carrier aggregation. Carrier aggregation can be used to widen bandwidth of the communication link by supporting communications over multiple frequency carriers, thereby increasing user data rates and enhancing network capacity by utilizing fragmented spectrum allocations. 
     In the illustrated example, the communication link is provided between a base station  21  and a mobile device  22 . As shown in  FIG.  2 A , the communications link includes a downlink channel used for RF communications from the base station  21  to the mobile device  22 , and an uplink channel used for RF communications from the mobile device  22  to the base station  21 . 
     Although  FIG.  2 A  illustrates carrier aggregation in the context of FDD communications, carrier aggregation can also be used for TDD communications. 
     In certain implementations, a communication link can provide asymmetrical data rates for a downlink channel and an uplink channel. For example, a communication link can be used to support a relatively high downlink data rate to enable high speed streaming of multimedia content to a mobile device, while providing a relatively slower data rate for uploading data from the mobile device to the cloud. 
     In the illustrated example, the base station  21  and the mobile device  22  communicate via carrier aggregation, which can be used to selectively increase bandwidth of the communication link. Carrier aggregation includes contiguous aggregation, in which contiguous carriers within the same operating frequency band are aggregated. Carrier aggregation can also be non-contiguous, and can include carriers separated in frequency within a common band or in different bands. 
     In the example shown in  FIG.  2 A , the uplink channel includes three aggregated component carriers f UL1 , f UL2 , and f UL3 . Additionally, the downlink channel includes five aggregated component carriers f DL1 , f DL2 , f DL3 , f DL4 , and f DL5 . Although one example of component carrier aggregation is shown, more or fewer carriers can be aggregated for uplink and/or downlink. Moreover, a number of aggregated carriers can be varied over time to achieve desired uplink and downlink data rates. 
     For example, a number of aggregated carriers for uplink and/or downlink communications with respect to a particular mobile device can change over time. For example, the number of aggregated carriers can change as the device moves through the communication network and/or as network usage changes over time. 
       FIG.  2 B  illustrates various examples of uplink carrier aggregation for the communication link of  FIG.  2 A .  FIG.  2 B  includes a first carrier aggregation scenario  31 , a second carrier aggregation scenario  32 , and a third carrier aggregation scenario  33 , which schematically depict three types of carrier aggregation. 
     The carrier aggregation scenarios  31 - 33  illustrate different spectrum allocations for a first component carrier f UL1 , a second component carrier f UL2 , and a third component carrier f UL3 . Although  FIG.  2 B  is illustrated in the context of aggregating three component carriers, carrier aggregation can be used to aggregate more or fewer carriers. Moreover, although illustrated in the context of uplink, the aggregation scenarios are also applicable to downlink. 
     The first carrier aggregation scenario  31  illustrates intra-band contiguous carrier aggregation, in which component carriers that are adjacent in frequency and in a common frequency band are aggregated. For example, the first carrier aggregation scenario  31  depicts aggregation of component carriers f UL1 , f UL2 , and f UL3  that are contiguous and located within a first frequency band BAND 1 . 
     With continuing reference to  FIG.  2 B , the second carrier aggregation scenario  32  illustrates intra-band non-continuous carrier aggregation, in which two or more components carriers that are non-adjacent in frequency and within a common frequency band are aggregated. For example, the second carrier aggregation scenario  32  depicts aggregation of component carriers f UL1 , f UL2 , and f UL3  that are non-contiguous, but located within a first frequency band BAND 1 . 
     The third carrier aggregation scenario  33  illustrates inter-band non-contiguous carrier aggregation, in which component carriers that are non-adjacent in frequency and in multiple frequency bands are aggregated. For example, the third carrier aggregation scenario  33  depicts aggregation of component carriers f UL1  and f UL2  of a first frequency band BAND 1  with component carrier f UL3  of a second frequency band BAND 2 . 
       FIG.  2 C  illustrates various examples of downlink carrier aggregation for the communication link of  FIG.  2 A . The examples depict various carrier aggregation scenarios  34 - 38  for different spectrum allocations of a first component carrier f DL1 , a second component carrier f DL2 , a third component carrier f DL3 , a fourth component carrier f DL4 , and a fifth component carrier f DL5 . Although  FIG.  2 C  is illustrated in the context of aggregating five component carriers, carrier aggregation can be used to aggregate more or fewer carriers. Moreover, although illustrated in the context of downlink, the aggregation scenarios are also applicable to uplink. 
     The first carrier aggregation scenario  34  depicts aggregation of component carriers that are contiguous and located within the same frequency band. Additionally, the second carrier aggregation scenario  35  and the third carrier aggregation scenario  36  illustrates two examples of aggregation that are non-contiguous, but located within the same frequency band. Furthermore, the fourth carrier aggregation scenario  37  and the fifth carrier aggregation scenario  38  illustrates two examples of aggregation in which component carriers that are non-adjacent in frequency and in multiple frequency bands are aggregated. As a number of aggregated component carriers increases, a complexity of possible carrier aggregation scenarios also increases. 
     With reference to  FIGS.  2 A- 2 C , the individual component carriers used in carrier aggregation can be of a variety of frequencies, including, for example, frequency carriers in the same band or in multiple bands. Additionally, carrier aggregation is applicable to implementations in which the individual component carriers are of about the same bandwidth as well as to implementations in which the individual component carriers have different bandwidths. 
     Certain communication networks allocate a particular user device with a primary component carrier (PCC) or anchor carrier for uplink and a PCC for downlink. Additionally, when the mobile device communicates using a single frequency carrier for uplink or downlink, the user device communicates using the PCC. To enhance bandwidth for uplink communications, the uplink PCC can be aggregated with one or more uplink secondary component carriers (SCCs). Additionally, to enhance bandwidth for downlink communications, the downlink PCC can be aggregated with one or more downlink SCCs. 
     In certain implementations, a communication network provides a network cell for each component carrier. Additionally, a primary cell can operate using a PCC, while a secondary cell can operate using a SCC. The primary and second cells may have different coverage areas, for instance, due to differences in frequencies of carriers and/or network environment. 
     License assisted access (LAA) refers to downlink carrier aggregation in which a licensed frequency carrier associated with a mobile operator is aggregated with a frequency carrier in unlicensed spectrum, such as WiFi. LAA employs a downlink PCC in the licensed spectrum that carries control and signaling information associated with the communication link, while unlicensed spectrum is aggregated for wider downlink bandwidth when available. LAA can operate with dynamic adjustment of secondary carriers to avoid WiFi users and/or to coexist with WiFi users. Enhanced license assisted access (eLAA) refers to an evolution of LAA that aggregates licensed and unlicensed spectrum for both downlink and uplink. 
       FIG.  3 A  is a schematic diagram of one example of a downlink channel using multi-input and multi-output (MIMO) communications.  FIG.  3 B  is schematic diagram of one example of an uplink channel using MIMO communications. 
     MIMO communications use multiple antennas for simultaneously communicating multiple data streams over common frequency spectrum. In certain implementations, the data streams operate with different reference signals to enhance data reception at the receiver. MIMO communications benefit from higher SNR, improved coding, and/or reduced signal interference due to spatial multiplexing differences of the radio environment. 
     MIMO order refers to a number of separate data streams sent or received. For instance, MIMO order for downlink communications can be described by a number of transmit antennas of a base station and a number of receive antennas for UE, such as a mobile device. For example, two-by-two (2×2) DL MIMO refers to MIMO downlink communications using two base station antennas and two UE antennas. Additionally, four-by-four (4×4) DL MIMO refers to MIMO downlink communications using four base station antennas and four UE antennas. 
     In the example shown in  FIG.  3 A , downlink MIMO communications are provided by transmitting using M antennas  43   a ,  43   b ,  43   c , . . .  43   m  of the base station  41  and receiving using N antennas  44   a ,  44   b ,  44   c , . . .  44   n  of the mobile device  42 . Accordingly,  FIG.  3 A  illustrates an example of m×n DL MIMO. 
     Likewise, MIMO order for uplink communications can be described by a number of transmit antennas of UE, such as a mobile device, and a number of receive antennas of a base station. For example, 2×2 UL MIMO refers to MIMO uplink communications using two UE antennas and two base station antennas. Additionally, 4×4 UL MIMO refers to MIMO uplink communications using four UE antennas and four base station antennas. 
     In the example shown in  FIG.  3 B , uplink MIMO communications are provided by transmitting using N antennas  44   a ,  44   b ,  44   c , . . .  44   n  of the mobile device  42  and receiving using M antennas  43   a ,  43   b ,  43   c , . . .  43   m  of the base station  41 . Accordingly,  FIG.  3 B  illustrates an example of n×m UL MIMO. 
     By increasing the level or order of MIMO, bandwidth of an uplink channel and/or a downlink channel can be increased. 
     MIMO communications are applicable to communication links of a variety of types, such as FDD communication links and TDD communication links. 
       FIG.  3 C  is schematic diagram of another example of an uplink channel using MIMO communications. In the example shown in  FIG.  3 C , uplink MIMO communications are provided by transmitting using N antennas  44   a ,  44   b ,  44   c , . . .  44   n  of the mobile device  42 . Additional a first portion of the uplink transmissions are received using M antennas  43   a   1 ,  43   b   1 ,  43   c   1 , . . .  43   m   1  of a first base station  41   a , while a second portion of the uplink transmissions are received using M antennas  43   a   2 ,  43   b   2 ,  43   c   2 , . . .  43   m   2  of a second base station  41   b . Additionally, the first base station  41   a  and the second base station  41   b  communication with one another over wired, optical, and/or wireless links. 
     The MIMO scenario of  FIG.  3 C  illustrates an example in which multiple base stations cooperate to facilitate MIMO communications. 
       FIG.  4    is a schematic diagram illustrating two examples of multiple access schemes for a communication network. Examples of frequency versus voltage versus time for OFDMA and SC-FDMA are depicted in  FIG.  4   . 
     The examples are shown for an illustrated transmit sequence of different QPSK modulating data symbols, in this embodiment. As shown in  FIG.  4   , SC-FDMA includes data symbols occupying greater bandwidth (N*B KHz, where N=4 in this example) relative to OFDMA data symbols (B KHz). However, the SC-FDMA data symbols occupy the greater bandwidth for a fraction of time (1/N) relative to that of the OFDMA data symbols.  FIG.  4    has also been annotated to show times of transmitting a cyclic prefix (CP). 
       FIG.  5 A  is a schematic diagram of one example of a communication system  110  that operates with beamforming. The communication system  110  includes a transceiver  105 , signal conditioning circuits  104   a   1 ,  104   a   2  . . .  104   a n,  104   b   1 ,  104   b   2  . . .  104 bn,  104   m   1 ,  104   m   2  . . .  104 mn, and an antenna array  102  that includes antenna elements  103   a   1 ,  103   a   2  . . .  103 an,  103   b   1 ,  103   b   2  . . .  103 bn,  103   m   1 ,  103   m   2  . . .  103 mn. 
     Communications systems that communicate using millimeter wave carriers (for instance, 30 GHz to 300 GHz), centimeter wave carriers (for instance, 3 GHz to 30 GHz), and/or other frequency carriers can employ an antenna array to provide beam formation and directivity for transmission and/or reception of signals. 
     For example, in the illustrated embodiment, the communication system  110  includes an array  102  of m×n antenna elements, which are each controlled by a separate signal conditioning circuit, in this embodiment. As indicated by the ellipses, the communication system  110  can be implemented with any suitable number of antenna elements and signal conditioning circuits. 
     With respect to signal transmission, the signal conditioning circuits can provide transmit signals to the antenna array  102  such that signals radiated from the antenna elements combine using constructive and destructive interference to generate an aggregate transmit signal exhibiting beam-like qualities with more signal strength propagating in a given direction away from the antenna array  102 . 
     In the context of signal reception, the signal conditioning circuits process the received signals (for instance, by separately controlling received signal phases) such that more signal energy is received when the signal is arriving at the antenna array  102  from a particular direction. Accordingly, the communication system  110  also provides directivity for reception of signals. 
     The relative concentration of signal energy into a transmit beam or a receive beam can be enhanced by increasing the size of the array. For example, with more signal energy focused into a transmit beam, the signal is able to propagate for a longer range while providing sufficient signal level for RF communications. For instance, a signal with a large proportion of signal energy focused into the transmit beam can exhibit high effective isotropic radiated power (EIRP). 
     In the illustrated embodiment, the transceiver  105  provides transmit signals to the signal conditioning circuits and processes signals received from the signal conditioning circuits. As shown in  FIG.  5 A , the transceiver  105  generates control signals for the signal conditioning circuits. The control signals can be used for a variety of functions, such as controlling the gain and phase of transmitted and/or received signals to control beamforming. 
       FIG.  5 B  is a schematic diagram of one example of beamforming to provide a transmit beam.  FIG.  5 B  illustrates a portion of a communication system including a first signal conditioning circuit  114   a , a second signal conditioning circuit  114   b , a first antenna element  113   a , and a second antenna element  113   b.    
     Although illustrated as included two antenna elements and two signal conditioning circuits, a communication system can include additional antenna elements and/or signal conditioning circuits. For example,  FIG.  5 B  illustrates one embodiment of a portion of the communication system  110  of  FIG.  5 A . 
     The first signal conditioning circuit  114   a  includes a first phase shifter  130   a , a first power amplifier  131   a , a first low noise amplifier (LNA)  132   a , and switches for controlling selection of the power amplifier  131   a  or LNA  132   a . Additionally, the second signal conditioning circuit  114   b  includes a second phase shifter  130   b , a second power amplifier  131   b , a second LNA  132   b , and switches for controlling selection of the power amplifier  131   b  or LNA  132   b.    
     Although one embodiment of signal conditioning circuits is shown, other implementations of signal conditioning circuits are possible. For instance, in one example, a signal conditioning circuit includes one or more band filters, duplexers, and/or other components. 
     In the illustrated embodiment, the first antenna element  113   a  and the second antenna element  113   b  are separated by a distance d. Additionally,  FIG.  5 B  has been annotated with an angle θ, which in this example has a value of about 90° when the transmit beam direction is substantially perpendicular to a plane of the antenna array and a value of about 0° when the transmit beam direction is substantially parallel to the plane of the antenna array. 
     By controlling the relative phase of the transmit signals provided to the antenna elements  113   a ,  113   b , a desired transmit beam angle θ can be achieved. For example, when the first phase shifter  130   a  has a reference value of 0°, the second phase shifter  130   b  can be controlled to provide a phase shift of about −2πf(d/v)cosθ radians, where f is the fundamental frequency of the transmit signal, d is the distance between the antenna elements, v is the velocity of the radiated wave, and π is the mathematic constant pi. 
     In certain implementations, the distance d is implemented to be about ½λ, where k is the wavelength of the fundamental component of the transmit signal. In such implementations, the second phase shifter  130   b  can be controlled to provide a phase shift of about −πcosθ radians to achieve a transmit beam angle θ. 
     Accordingly, the relative phase of the phase shifters  130   a ,  130   b  can be controlled to provide transmit beamforming. In certain implementations, a baseband processor and/or a transceiver (for example, the transceiver  105  of  FIG.  5 A ) controls phase values of one or more phase shifters and gain values of one or more controllable amplifiers to control beamforming. 
       FIG.  5 C  is a schematic diagram of one example of beamforming to provide a receive beam.  FIG.  5 C  is similar to  FIG.  5 B , except that  FIG.  5 C  illustrates beamforming in the context of a receive beam rather than a transmit beam. 
     As shown in  FIG.  5 C , a relative phase difference between the first phase shifter  130   a  and the second phase shifter  130   b  can be selected to about equal to −2πf(d/v)cosθ radians to achieve a desired receive beam angle θ. In implementations in which the distance d corresponds to about ½λ, the phase difference can be selected to about equal to −πcosθ radians to achieve a receive beam angle θ. 
     Although various equations for phase values to provide beamforming have been provided, other phase selection values are possible, such as phase values selected based on implementation of an antenna array, implementation of signal conditioning circuits, and/or a radio environment. 
     Sounding Reference Signal Switching 
     In cellular networks, such as 5G networks, sounding reference signal (SRS) features can be enabled to determine channel qualities of a communication link between UE (for example, a wireless device such as a mobile phone) and a base station. SRS symbols are transmitted on uplink and processed by the network to estimate the quality of the wireless channel at different frequencies. For instance, the SRS symbols transmitted by the UE can be used by the base station to estimate the quality of the uplink channel for large bandwidths outside the assigned frequency span to the UE. 
     Although SRS provides a number of benefits, SRS also places a burden on data transport capacity. 
     For example, for 3GPP 5G Release 15, ON to ON timing for consecutive SRS symbols is 15 microseconds (μs) for Frequency Range  1  (FR 1 ). For a subcarrier spacing (SCS) of 15 kilohertz (kHz), the cyclic prefix (CP) and 10 μs of the preceding data symbol is consumed. At 30 kHz and 60 kHz SCS 15 μs the ON to ON timing constraint corresponds to about half a symbol and a full symbol, respectively. Thus, a full symbol can be lost or blanked when 30 kHz or 60 kHz SCS is enabled. 
     Apparatus and methods for SRS switching are provided. In certain embodiments, transmit path resources of UE are used to reduce or eliminate the impairment of SRS upon transport capacity. Furthermore, the transmit path resources can be used for other purposes, and thus SRS switching time can be reduced by re-using transmit path resources that may be included for other purposes. The teachings herein can be used to achieve SRS switching of 0 μs, thereby eliminating the impact of switching timing constraints for SRS symbols on transport capacity. 
     In certain implementations, the UE includes a first transmit path associated with a first power amplifier, and a second transmit path associated with a second power amplifier. Additionally, when the second transmit path is not in use for other purposes, symbol transmissions are staggered using the first transmit path and the second transmit path, with at least the second transmit path used for transmitting SRS symbols. Thus, a power amplifier associated with an antenna not in operation for data transport can be used for SRS signaling. Implementing SRS in this manner can provide a number of advantages, including, but not limited to, 0 μs SRS switching. 
     In certain implementations, the first transmit path and the second transmit path correspond to transmit paths used for transmitting MIMO signals. For example, in the context of a UE capable of UL MIMO and not in MIMO mode, the first power amplifier (PA 1 ) is used for data transport activities while the second power amplifier (PA 2 ) is engaged for SRS. 
     Thus, a UE capable of UL MIMO and not in MIMO mode alternates transmit path resources to provide SRS. By using the other power amplifier, SRS can be achieved without overhead on data transport. 
     Such low overhead provides a number of advantages. For example, 0 μs SRS switching can be realized to achieve lower latency and enhanced performance relative to an implementation in which time is set aside to permit SRS on a particular antenna by shortening or blanking a symbol. 
       FIG.  6 A  is a diagram depicting two examples of symbol blanking for time slots including SRS symbols. The depicted transmit sequences show the sequence of transmitted symbols, starting on the left and ending on the right. 
     Certain cellular networks are implemented with an uplink physical layer that includes multiple physical channels. In one example, a cellular network includes a Physical Uplink Shared Channel (PUSCH) and a Physical Uplink Control Channel (PUCCH). Additionally, the PUSCH is used for transmitting user traffic data, while PUCCH carriers Uplink Control Information (UCI) indicating channel quality and other parameters. 
     The left-hand side of  FIG.  6 A  depicts an example of a first time slot in which a transmit sequence includes three initial PUSCH/PUCCH symbols transmitted on a first antenna, followed by a first SRS symbol on the first antenna, followed by a blank symbol (GAP), and followed by a second SRS symbol on a second antenna. The right-hand side of  FIG.  6 A  depicts an example of a second time slot in which two PUSCH/PUCCH symbols, a first blank symbol, a first SRS symbol, a second blank symbol, and a second SRS symbol are transmitted using various antennas as indicated. 
     Table 1 below shows one example of SCS and symbol blanking versus numerology. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 numerology 
                 SCS [kHz] 
                 Y [symbol] 
               
               
                   
               
             
            
               
                 0 
                 15 
                 1 
               
               
                 1 
                 30 
                 1 
               
               
                 2 
                 60 
                 1 
               
               
                 3 
                 120  
                 2 
               
               
                   
               
            
           
         
       
     
     In this example, one symbol blanking is permitted for SCS of 30 kHz and SCS of 60 kHz. Additionally, two symbol blanking is permitted for SCS of 120 kHz. 
       FIG.  6 B  is a table depicting one example of symbol duration versus SCS. The table depicts symbol duration for half of a time slot. 
     As shown in  FIG.  6 B , 15 kHz SCS operates with a first OFDM symbol that is 16TS (0.521 μs for SCS of 15 kHz) longer than each of the other symbols in the time slot. The table includes information for SCS of 15 kHz, 30 kHz, and 60 kHz. As shown in the table, symbol duration scales linearly with SCS. 
       FIG.  6 C  is a table depicting one example of various communication parameters versus SCS. 
     In the example shown in  FIG.  6 C , CP scales linearly with SCS. 
       FIG.  6 D  is a diagram of one example of ON to ON timing for SRS. 
     As shown in  FIG.  6 D , SRS symbol used during switching (port ‘y’) is truncated by 5 μs+5 μs=10 μs. The first 5 μs CP duration at SCS of 15 kHz, while the second 5 μs arises from symbol impairment. Additionally, SRS symbols on prior and post switching transients suffer from 10 μs+5 μs=15 μs penalty. 
     ON to ON timing for consecutive SRS symbols is 15 μs for FR 1  in Release 15 of 5G. For an SCS of 15 kHz, the CP is consumed and 10 μs of the preceding data symbol is consumed. At 30 kHz and 60 kHz SCS 15 μs of ON to ON timing corresponds to about half a symbol and a full symbol, respectively. Thus, a full symbol can be lost or blanked when 30 kHz or 60 kHz SCS is enabled. 
     Moreover, in Release 15, symbol blanking is the default assumption for all UE types. Thus, uRLLC performance is degraded when scheduler applies SRS default symbol blanking to all UE types. 
     In certain implementations, the UE provides binary reporting of SRS switching latency. In one example, the binary reporting includes four states: 0 μs/less than 3 μs/less than 5 μs/less than 15 μs. In certain implementations, binary reporting is provided per frequency band. 
       FIG.  7 A  is a schematic diagram of one example of a communication system  510  operating with SRS for one transmit four receive (1T4R).  FIG.  7 B  is one example of a timing diagram for the communication system  510  of  FIG.  7 A . 
     With reference to  FIGS.  7 A and  7 B , the communication system  510  includes a power amplifier  501  that is connected to a main antenna  505 , a diversity antenna  506 , a first MIMO antenna  507 , and a second MIMO antenna  508  by a multi-throw switch  504 . 
     When sounding all four antennas  503 - 506  at 15 kHz SCS, 4 symbols are used with whole CP and 10 μs of the preceding symbol affected. For 30 kHz and 60 kHz SCS, 7 symbols are used, 3 of which are blanks. 
       FIG.  8 A  is a schematic diagram of one example of a communication system  520  operating with SRS for two transmit four receive (2T4R).  FIG.  8 B  is one example of a timing diagram for the communication system  520  of  FIG.  8 A . 
     With reference to  FIGS.  8 A and  8 B , the communication system  520  includes a first power amplifier  511  that is connected to a main antenna  515  and a first MIMO antenna  517  by a first multi-throw switch  513 . Additionally, the communication system  520  further includes a second power amplifier  512  that is connected to a diversity antenna  516  and a second MIMO antenna  518  by a second multi-throw switch  514 . 
     When sounding all four antennas  515 - 518  at 15 kHz SCS, 2 symbols are used with whole CP and 10 μs of preceding symbol affected. For 30 kHz and 60 kHz SCS, 4 symbols are used, 2 of which are blanks. 
       FIG.  9 A  is a schematic diagram of one embodiment of a communication system  530  operating with SRS for 2T4R.  FIG.  9 B  is one example of a timing diagram for the communication system  530  of  FIG.  9 A . 
     With reference to  FIGS.  9 A and  9 B , the communication system  530  includes a first power amplifier  521  that is connected to a main antenna  525  and a first MIMO antenna  527  by a first multi-throw switch  523 . Additionally, the communication system  530  further includes a second power amplifier  522  that is connected to a diversity antenna  526  and a second MIMO antenna  528  by a second multi-throw switch  524 . 
     When sounding all four antennas  525 - 528  at 15 kHz SCS, 4 symbols are used with SRS switching of about 0 μs. For 30 kHz and 60 kHz SCS, 4 symbols are used, with no blanks and SRS switching of about 0 μs. Moreover, the ON/OFF switching is performed with less than 10 μs when uplink MIMO is supported. 
       FIG.  10 A  is a diagram of one example of an impact of transients on an uplink physical layer. In the example of  FIG.  10 A , when transitioning from a PUSCH/PUCCH symbol to an SRS symbol and then back to a PUSCH/PUCCH symbol, no antenna switching occurs (antenna ‘x’ used for each transmission). 
       FIG.  10 B  is a diagram of another example of an impact of transients on an uplink physical layer. In the example of  FIG.  10 B , when transitioning from a PUSCH/PUCCH symbol to an SRS symbol and then back to a PUSCH/PUCCH symbol, antenna switching occurs (from antenna ‘x’ to antenna ‘y’ and then back to antenna ‘x’). 
       FIG.  10 C  is a table of one example of an impact of transients on an uplink physical layer. The table depicts examples of impact of transient times on PUSCH/PUCCH symbol duration for SCS scenarios for both FR 1  and FR 2 . 
       FIG.  11 A  is a schematic diagram of another embodiment of a communication system  540  operating with SRS for 2T4R.  FIG.  11 B  is one example of a timing diagram for the communication system  540  of  FIG.  11 A . 
     With reference to  FIGS.  11 A and  11 B , the communication system  540  includes a first power amplifier  531  that is connected to a main antenna  535  and a first MIMO antenna  537  by a first multi-throw switch  533 . Additionally, the communication system  540  further includes a second power amplifier  532  that is connected to the main antenna  535 , a diversity antenna  536 , the first MIMO antenna  537 , and a second MIMO antenna  538  by a second multi-throw switch  532 . 
     In comparison to the second multi-throw switch  524  of the communication system  530  of  FIG.  9 A , the second multi-throw switch  534  of the communication system  540  of  FIG.  11 A  further includes two additional throws. By including the additional throws, operability for 0 μs PUSCH/PUCCH is provided, even when the switches have a 15 μs switching time. 
     The timing diagram of  FIG.  11 B  depicts SRS transient for an UL MIMO capable UE not in MIMO transmission mode. 
       FIG.  12 A  is a schematic diagram of another embodiment of a communication system  550  operating with SRS for 2T4R.  FIG.  12 B  is one example of a timing diagram for the communication system  550  of  FIG.  12 A . 
     With reference to  FIGS.  12 A and  12 B , the communication system  550  includes a first power amplifier  541  that is connected to a main antenna  545  and a first MIMO antenna  547  by a first multi-throw switch  543 . Additionally, the communication system  550  further includes a second power amplifier  542  that is connected to a diversity antenna  546 , the first MIMO antenna  547 , and a second MIMO antenna  548  by a second multi-throw switch  544 . 
     The timing diagram of  FIG.  12 B  depicts SRS transient for an UL MIMO capable UE not in MIMO transmission mode. 
       FIG.  13    is a graph of one example of uplink demodulation performance for different transient switching times. The graph depicts an impact of power masks of various widths on uplink demodulation for an example using 64 quadrature amplitude modulation (64QAM). The plot for the case of no mask corresponds to a 0 μs transient time. 
       FIG.  14    is a schematic diagram of one embodiment of a mobile device  800 . The mobile device  800  includes a baseband system  801 , a transceiver  802 , a front end system  803 , antennas  804 , a power management system  805 , a memory  806 , a user interface  807 , and a battery  808 . The mobile device  800  can be implemented in accordance with any of the embodiments herein. 
     The mobile device  800  can be used communicate using a wide variety of communications technologies, including, but not limited to, 2G, 3G, 4G (including LTE, LTE-Advanced, and LTE-Advanced Pro), 5G NR, WLAN (for instance, WiFi), WPAN (for instance, Bluetooth and ZigBee), WMAN (for instance, WiMax), and/or GPS technologies. 
     The transceiver  802  generates RF signals for transmission and processes incoming RF signals received from the antennas  804 . It will be understood that various functionalities associated with the transmission and receiving of RF signals can be achieved by one or more components that are collectively represented in  FIG.  14    as the transceiver  802 . In one example, separate components (for instance, separate circuits or dies) can be provided for handling certain types of RF signals. 
     The front end system  803  aids is conditioning signals transmitted to and/or received from the antennas  804 . In the illustrated embodiment, the front end system  803  includes antenna tuning circuitry  810 , power amplifiers (PAs)  811 , low noise amplifiers (LNAs)  812 , filters  813 , switches  814 , and signal splitting/combining circuitry  815 . However, other implementations are possible. 
     For example, the front end system  803  can provide a number of functionalities, including, but not limited to, amplifying signals for transmission, amplifying received signals, filtering signals, switching between different bands, switching between different power modes, switching between transmission and receiving modes, duplexing of signals, multiplexing of signals (for instance, diplexing or triplexing), or some combination thereof. 
     In certain implementations, the mobile device  800  supports carrier aggregation, thereby providing flexibility to increase peak data rates. Carrier aggregation can be used for both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), and may be used to aggregate a plurality of carriers or channels. Carrier aggregation includes contiguous aggregation, in which contiguous carriers within the same operating frequency band are aggregated. Carrier aggregation can also be non-contiguous, and can include carriers separated in frequency within a common band or in different bands. 
     The antennas  804  can include antennas used for a wide variety of types of communications. For example, the antennas  804  can include antennas for transmitting and/or receiving signals associated with a wide variety of frequencies and communications standards. 
     In certain implementations, the antennas  804  support MIMO communications and/or switched diversity communications. For example, MIMO communications use multiple antennas for communicating multiple data streams over a single radio frequency channel. MIMO communications benefit from higher signal to noise ratio, improved coding, and/or reduced signal interference due to spatial multiplexing differences of the radio environment. Switched diversity refers to communications in which a particular antenna is selected for operation at a particular time. For example, a switch can be used to select a particular antenna from a group of antennas based on a variety of factors, such as an observed bit error rate and/or a signal strength indicator. 
     The mobile device  800  can operate with beamforming in certain implementations. For example, the front end system  803  can include amplifiers having controllable gain and phase shifters having controllable phase to provide beam formation and directivity for transmission and/or reception of signals using the antennas  804 . For example, in the context of signal transmission, the amplitude and phases of the transmit signals provided to the antennas  804  are controlled such that radiated signals from the antennas  804  combine using constructive and destructive interference to generate an aggregate transmit signal exhibiting beam-like qualities with more signal strength propagating in a given direction. In the context of signal reception, the amplitude and phases are controlled such that more signal energy is received when the signal is arriving to the antennas  804  from a particular direction. In certain implementations, the antennas  804  include one or more arrays of antenna elements to enhance beamforming. 
     The baseband system  801  is coupled to the user interface  807  to facilitate processing of various user input and output (I/O), such as voice and data. The baseband system  801  provides the transceiver  802  with digital representations of transmit signals, which the transceiver  802  processes to generate RF signals for transmission. The baseband system  801  also processes digital representations of received signals provided by the transceiver  802 . As shown in  FIG.  14   , the baseband system  801  is coupled to the memory  806  of facilitate operation of the mobile device  800 . 
     The memory  806  can be used for a wide variety of purposes, such as storing data and/or instructions to facilitate the operation of the mobile device  800  and/or to provide storage of user information. 
     The power management system  805  provides a number of power management functions of the mobile device  800 . In certain implementations, the power management system  805  includes a PA supply control circuit that controls the supply voltages of the power amplifiers  811 . For example, the power management system  805  can be configured to change the supply voltage(s) provided to one or more of the power amplifiers  811  to improve efficiency, such as power added efficiency (PAE). 
     As shown in  FIG.  14   , the power management system  805  receives a battery voltage from the battery  808 . The battery  808  can be any suitable battery for use in the mobile device  800 , including, for example, a lithium-ion battery. 
       FIG.  15    is a schematic diagram of one embodiment of an RF communication system  910 . The RF communication system  910  includes a baseband system  900 , a first transmit chain  901 , a second transmit chain  902 , switches  903 , and antennas  904   a ,  904   b , . . .  904   n . The RF communication system  910  represents a wireless device of a cellular network, such as a mobile phone. The RF communication system  910  can be implemented in accordance with any of the embodiments herein. 
     As shown in  FIG.  15   , the baseband system  900  generates a first transmit signal and a second transmit signal, which in certain implementations are represented each using a pair of in-phase (I) and quadrature-phase (Q) signals. 
     With continuing reference to  FIG.  15   , the first transmit chain  901  includes a first power amplifier  905 , and the second transmit chain  902  includes a second power amplifier  906 . The first power amplifier  905  is used to amplify a first RF transmit signal carrying a first sequence of symbols (SEQ 1 ). Additionally, the second power amplifier  906  is used to amplify a second RF transmit signal carrying a second sequence of symbols (SEQ 2 ). 
     As shown in  FIG.  15   , the switches  903  are used to selectively connect the first power amplifier  905  and the second power amplifier  906  to desired antenna(s) chosen from the antennas  904   a ,  904   b , . . .  904   n . Although the RF communication system  910  is depicted as included three antennas, more or fewer antennas can be included as indicated by the ellipses. 
     The baseband system  900  controls generation of the first RF transmit signal and the second RF transmit signal such that the first sequence of symbols and the second sequence of symbols are staggered with one or more sounding reference signal symbols in the second sequence of symbols in accordance with the teachings herein. 
     As shown in  FIG.  15   , the baseband system  900  is further configured to receive a base station capability inquiry from a base station, and to control transmission of capability information to the base station in response to the base station capability inquiry. In certain implementations, the baseband system  900  can further receive transmit configuration information from the base station in response to sending the compatibility information. The baseband system  900  can configure first transmit chain  901 , second transmit chain  902 , and/or the switches  903  based on the inquiry and/or transmit configuration information. 
       FIG.  16    is a schematic diagram of another embodiment of an RF communication system  1000 . The RF communication system  1000  includes a baseband system  940 , a transceiver  950 , a front end system  970 , and antennas  981   a ,  981   b , . . .  981   n . The RF communication system  1000  represents a wireless device of a cellular network, such as a mobile phone. The RF communication system  1000  can be implemented in accordance with any of the embodiments herein. 
     As shown in  FIG.  16   , the baseband system  940  generates a first pair of in-phase (I) and quadrature-phase (Q) signals representing a first transmit signal. Additionally, the baseband system  940  processes a first pair of I and Q signals representing a first receive signal. Furthermore, the baseband system  940  generates a second pair of I and Q signals representing a second transmit signal. Additionally, the baseband system  940  processes a second pair of I and Q signals representing a second receive signal. 
     With continuing reference to  FIG.  16    the transceiver  950  modulates the first pair of I and Q signals representing the first transmit signal to generate a first RF transmit signal provided to the front end system  970  at a first transmit terminal  991 . The first RF transmit signal carries a first sequence of symbols (SEQ 1 ). Additionally, the transceiver  950  demodulates a first RF receive signal from a first receive terminal  993  of the front end system  970  to generate the first pair of I and Q signals representing the first receive signal. Furthermore, the transceiver  950  modulates the second pair of I and Q signals representing the second transmit signal to generate a second RF transmit signal provided to the front end system  970  at a second transmit terminal  992 . The second RF transmit signal carriers a second sequence of symbols (SEQ 2 ). Additionally, the transceiver  970  demodulates a second RF receive signal from a second receive terminal  994  of the front end system  970  to generate the second pair of I and Q signals representing the second receive signal. 
     As shown in  FIG.  16   , the front end system  970  includes a first power amplifier  953 , a second power amplifier  954 , a first transmit/receive switch  955 , a second transmit/receive switch  956 , a first band filter  957 , a second band filter  958 , an antenna switch  959 , a first low noise amplifier  961 , and a second low noise amplifier  962 . 
     Although one embodiment of a front end system  970  is shown, other implementations of front end systems are possible. For example, a wide range of components and circuitry can be present between an output of a power amplifier and an antenna. Examples of such components and circuitry include, but are not limited to, switches, matching networks, harmonic termination circuits, filters, resonators, duplexers, detectors, directional couplers, bias circuitry, and/or frequency multiplexers (for instance, diplexers, triplexers, etc.). Furthermore, multiple instantiations of one or more components or circuits can be included. Moreover, a wide range of components and circuitry can be present between the transceiver and an input to a power amplifier. 
     As shown in  FIG.  16   , the antenna switch  959  is used to selectively connect the first power amplifier  953  and the second power amplifier  954  to desired antenna(s) chosen from the antennas  981   a ,  981   b , . . .  981   n . The front end system  970  is coupled to the antennas  981   a ,  981   b , . . .  981   n  at antenna terminals  995   a ,  995   b , . . .  995   n , respectively. Although the RF communication system  1000  is depicted as included three antennas, more or fewer antennas can be included as indicated by the ellipses. 
     In the illustrated embodiment, the RF communication system  1000  includes a first transmit path through the first power amplifier  953  and a second transmit path through the second power amplifier  954 . The first transmit path is for the first RF transmit signal carrying the first sequence of symbols (SEQ 1 ) and the second transmit path is for the second RF transmit signal carrying the second sequence of symbols (SEQ 2 ). 
     The baseband system  940  controls generation of the first RF transmit signal and the second RF transmit signal such that the first sequence of symbols and the second sequence of symbols are staggered with one or more sounding reference signal symbols in the second sequence of symbols. 
     As shown in  FIG.  16   , the baseband system  940  is further configured to receive a base station capability inquiry from a base station, and to control transmission of capability information to the base station in response to the base station capability inquiry. In certain implementations, the baseband system  940  can further receive transmit configuration information from the base station. The baseband system  940  can configure the transceiver  950  and/or the front end system  970  based on the inquiry and/or transmit configuration information. 
     Applications 
     Some of the embodiments described above have provided examples in connection with mobile devices. However, the principles and advantages of the embodiments can be used for a wide range of RF communication systems. Examples of such RF communication systems include, but are not limited to, mobile phones, tablets, base stations, network access points, customer-premises equipment (CPE), laptops, and wearable electronics. 
     Conclusion 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” The word “coupled”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Likewise, the word “connected”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     Moreover, conditional language used herein, such as, among others, “may,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     The above detailed description of embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative embodiments may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times. 
     The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments. 
     While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.