Patent Publication Number: US-2022217028-A1

Title: Cellular radio employing multiple power amplifiers for legacy communications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Patent Application No. 63/199,520, filed Jan. 5, 2021 and titled “CELLULAR RADIO EMPLOYING MULTIPLE POWER AMPLIFIERS FOR LEGACY COMMUNICATIONS,” 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 (FR1) or in the range of about 24.25 GHz to 52.6 GHz for 5G communications using Frequency Range 2 (FR2). 
     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 antenna, a second antenna, and a front-end system including a first transmit path operable to transmit a first radio frequency signal of a first communication standard on the first antenna in a first mode, and a second transmit path operable to transmit a second radio frequency signal of the first communication standard on the second antenna in the first mode. The first transmit path and the second transmit path further are operable to synchronously transmit a third radio frequency signal of a second communication standard on the first antenna and the second antenna in a second mode. 
     In some embodiments, the second communication standard is second generation (2G). According to a number of embodiments, the first communication standard is third generation (3G). In accordance with several embodiments, the first communication standard is fourth generation (4G). According to various embodiments, the first communication standard is fifth generation (5G). In accordance with a number of embodiments, the front-end system does not include any explicit 2G power amplifier module. 
     In various embodiments, in the second mode a first radio wave transmitted from the first antenna and a second radio wave transmitted from the second antenna combine with constructive interference in a far field. 
     In several embodiments, the first radio frequency signal is a multiple input multiple output (MIMO) signal. 
     In some embodiments, the first radio frequency signal is an EN-DC signal. 
     In various embodiments, the first radio frequency signal is an uplink carrier aggregation signal. 
     In a number of embodiments, the first transmit path includes a first power amplifier and the second power amplifier includes a second power amplifier. 
     In several embodiments, the mobile phone further includes a transceiver configured to generate the first radio frequency signal, the second radio frequency signal, and the third radio frequency signal. 
     In various embodiments, the first transmit path is further operable to transmit a fourth radio frequency signal of the first communication standard on the first antenna in a third mode, and the second transmit path is further operable to transmit a fifth radio frequency signal of a third communication standard on the second antenna in the third mode. According to a number of embodiments, the first communication standard is 4G, the second communication standard is 2G, and the third communication standard is 5G. In accordance with several embodiments, the first communication standard is 5G, the second communication standard is 2G, and the third communication standard is 4G. 
     In another aspect, the present disclosure relates to a method of legacy communications in a mobile phone. The method includes transmitting a first radio frequency signal of a first communication standard on a first antenna using a first transmit path in a first mode, transmitting a second radio frequency signal of the first communication standard on a second antenna using a second transmit path in the first mode, and synchronously transmitting a third radio frequency signal of a second communication standard on the first antenna and the second antenna using the first transmit path and the second transmit path in a second mode. 
     In various embodiments, the second communication standard is second generation (2G). According to a number of embodiments, the first communication standard is third generation (3G). In accordance with several embodiments, the first communication standard is fourth generation (4G). According to some embodiments, the first communication standard is fifth generation (5G). 
     In several embodiments, in the second mode a first radio wave transmitted from the first antenna and a second radio wave transmitted from the second antenna combine with constructive interference in a far field. 
     In a number of embodiments, the first radio frequency signal is a multiple input multiple output (MIMO) signal. 
     In some embodiments, the first radio frequency signal is an EN-DC signal. 
     In various embodiments, the first radio frequency signal is an uplink carrier aggregation signal. 
     In several embodiments, the first transmit path includes a first power amplifier and the second power amplifier includes a second power amplifier. 
     In a number of embodiments, the method further includes generating the first radio frequency signal, the second radio frequency signal, and the third radio frequency signal using a transceiver. 
     In some embodiments, the method further includes transmitting a fourth radio frequency signal of the first communication standard on the first antenna using the first transmit path in a third mode, and transmitting a fifth radio frequency signal of a third communication standard on the second antenna using the second transmit path in the third mode. According to various embodiments, the first communication standard is 4G, the second communication standard is 2G, and the third communication standard is 5G. In accordance with a number of embodiments, the first communication standard is 5G, the second communication standard is 2G, and the third communication standard is 4G. 
     In another aspect, a front-end system is provided. The front-end system includes a first antenna port, a second antenna port, a first transmit path operable to provide a first radio frequency signal of a first communication standard to the first antenna port in a first mode, and a second transmit path operable to provide a second radio frequency signal of the first communication standard to the second antenna port in the first mode. The first transmit path and the second transmit path further operable to synchronously provide a third radio frequency signal of a second communication standard to the first antenna and the second antenna in a second mode. 
     In some embodiments, the second communication standard is second generation (2G). According to a number of embodiments, the first communication standard is third generation (3G). In accordance with several embodiments, the first communication standard is fourth generation (4G). According to various embodiments, the first communication standard is fifth generation (5G). In accordance with several embodiments, the front-end system does not include any explicit 2G power amplifier module. 
     In various embodiments, the first radio frequency signal is a multiple input multiple output (MIMO) signal. 
     In several embodiments, the first radio frequency signal is an EN-DC signal. 
     In some embodiments, the first radio frequency signal is an uplink carrier aggregation signal. 
     In a number of embodiments, the first transmit path includes a first power amplifier and the second power amplifier includes a second power amplifier. 
     In various embodiments, the first transmit path is further operable to provide a fourth radio frequency signal of the first communication standard to the first antenna port in a third mode, and the second transmit path is further operable to provide a fifth radio frequency signal of a third communication standard to the second antenna port in the third mode. According to a number of embodiments, the first communication standard is 4G, the second communication standard is 2G, and the third communication standard is 5G. 
     In several embodiments, the first communication standard is 5G, the second communication standard is 2G, and the third communication standard is 4G. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of one example of a communication network. 
         FIG. 2A  is a schematic diagram of one example of a communication link using carrier aggregation. 
         FIG. 2B  illustrates various examples of uplink carrier aggregation for the communication link of  FIG. 2A . 
         FIG. 2C  illustrates various examples of downlink carrier aggregation for the communication link of  FIG. 2A . 
         FIG. 3A  is a schematic diagram of one example of a downlink channel using multi-input and multi-output (MIMO) communications. 
         FIG. 3B  is schematic diagram of one example of an uplink channel using MIMO communications. 
         FIG. 3C  is schematic diagram of another example of an uplink channel using MIMO communications. 
         FIG. 4  is a schematic diagram of an example dual connectivity network topology. 
         FIG. 5  is a schematic diagram of one embodiment of a mobile device. 
         FIG. 6  is a schematic diagram of another embodiment of a front-end system. 
         FIG. 7  is a schematic diagram of another embodiment of a mobile device. 
         FIG. 8  is a schematic diagram of a power amplifier system according to one embodiment. 
         FIG. 9  is a schematic diagram of one example of a power amplifier powered by a power amplifier supply voltage. 
     
    
    
     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 introduced Phase 2 of 5G technology in Release 16. 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 (for instance, as between mobile device  2   g  and mobile device  2   f ). 
     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 (FR1), Frequency Range 2 (FR2), 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. 
     In certain implementations, the communication network  10  supports supplementary uplink (SUL) and/or supplementary downlink (SDL). For example, when channel conditions are good, the communication network  10  can direct a particular UE to transmit using an original uplink frequency, while when channel condition is poor (for instance, below a certain criteria) the communication network  10  can direct the UE to transmit using a supplementary uplink frequency that is lower than the original uplink frequency. Since cell coverage increases with lower frequency, communication range and/or signal-to-noise ratio (SNR) can be increased using SUL. Likewise, SDL can be used to transmit using an original downlink frequency when channel conditions are good, and to transmit using a supplementary downlink frequency when channel conditions are poor. 
       FIG. 2A  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. 2A , 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. 2A  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. 2A , 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. 2B  illustrates various examples of uplink carrier aggregation for the communication link of  FIG. 2A .  FIG. 2B  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. 2B  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. 2B , 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. 2C  illustrates various examples of downlink carrier aggregation for the communication link of  FIG. 2A . 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. 2C  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. 2A-2C , 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 secondary 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. 3A  is a schematic diagram of one example of a downlink channel using multi-input and multi-output (MIMO) communications.  FIG. 3B  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. 3A , 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. 3A  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. 3B , 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. 3B  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. 3C  is schematic diagram of another example of an uplink channel using MIMO communications. In the example shown in  FIG. 3C , 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. 3C  illustrates an example in which multiple base stations cooperate to facilitate MIMO communications. 
     With the introduction of the 5G NR air interface standards, 3GPP has allowed for the simultaneous operation of 5G and 4G standards in order to facilitate the transition. This mode can be referred to as Non-Stand-Alone (NSA) operation or E-UTRAN New Radio-Dual Connectivity (EN-DC) and involves both 4G and 5G carriers being simultaneously transmitted from a user equipment (UE). 
     In certain EN-DC applications, dual connectivity NSA involves overlaying 5G systems onto an existing 4G core network. For dual connectivity in such applications, the control and synchronization between the base station and the UE can be performed by the 4G network while the 5G network is a complementary radio access network tethered to the 4G anchor. The 4G anchor can connect to the existing 4G network with the overlay of 5G data/control. 
       FIG. 4  is a schematic diagram of an example dual connectivity network topology. This architecture can leverage LTE legacy coverage to ensure continuity of service delivery and the progressive rollout of 5G cells. A UE  13  can simultaneously transmit dual uplink LTE and NR carrier. The UE  13  can transmit an uplink LTE carrier Tx 1  to the eNB  11  while transmitting an uplink NR carrier Tx 2  to the gNB  12  to implement dual connectivity. Any suitable combination of uplink carriers Tx 1 , Tx 2  and/or downlink carriers Rx 1 , Rx 2  can be concurrently transmitted via wireless links in the example network topology of  FIG. 1 . The eNB  11  can provide a connection with a core network, such as an Evolved Packet Core (EPC)  14 . The gNB  12  can communicate with the core network via the eNB  11 . Control plane data can be wireless communicated between the UE  13  and eNB  11 . The eNB  11  can also communicate control plane data with the gNB  12 . Control plane data can propagate along the paths of the dashed lines in  FIG. 4 . The solid lines in  FIG. 4  are for data plane paths. 
     In the example dual connectivity topology of  FIG. 4 , any suitable combinations of standardized bands and radio access technologies (e.g., FDD, TDD, SUL, SDL) can be wirelessly transmitted and received. This can present technical challenges related to having multiple separate radios and bands functioning in the UE  13 . With a TDD LTE anchor point, network operation may be synchronous, in which case the operating modes can be constrained to Tx 1 /Tx 2  and Rx 1 /Rx 2 , or asynchronous which can involve Tx 1 /Tx 2 , Tx 1 /Rx 2 , Rx 1 /Tx 2 , Rx 1 /Rx 2 . When the LTE anchor is a frequency division duplex (FDD) carrier, the TDD/FDD inter-band operation can involve simultaneous Tx 1 /Rx 1 /Tx 2  and Tx 1 /Rx 1 /Rx 2 . 
     As discussed above, EN-DC can involve both 4G and 5G carriers being simultaneously transmitted from a UE. Transmitting both 4G and 5G carriers in a UE, such as a phone, typically involves two power amplifiers (PAs) being active at the same time. Traditionally, having two power amplifiers active simultaneously would involve the placement of one or more additional power amplifiers specifically suited for EN-DC operation. Additional board space and expense is incurred when designing to support such EN-DC/NSA operation. 
     Cellular Radios Employing Multiple Power Amplifiers for Legacy Communications 
     A radio frequency (RF) communication device can include multiple antennas for supporting wireless communications. Additionally, the RF communication device can include a radio frequency front-end (RFFE) system for processing signals received from and transmitted by the antennas. The RFFE system can provide a number of functions, including, but not limited to, signal filtering, signal partitioning and combining, controlling component connectivity to the antennas, and/or signal amplification. 
     RFFE systems can be used to handle RF signals of a wide variety of types, including, but not limited to, wireless local area network (WLAN) signals, Bluetooth signals, and/or cellular signals. RFFE systems are also referred to herein as front-end systems. 
     RFFE systems can be used to process signals of a wide range of frequencies. For example, certain RFFE systems can operate using one or more low bands (for example, RF signal bands having a frequency content of 1 GHz or less, also referred to herein as LB), one or more mid bands (for example, RF signal bands having a frequency content between 1 GHz and 2.3 GHz, also referred to herein as MB), one or more high bands (for example, RF signal bands having a frequency content between 2.3 GHz and 3 GHz, also referred to herein as HB), and one or more ultrahigh bands (for example, RF signal bands having a frequency content between 3 GHz and 7.125 GHz, also referred to herein as UHB). In certain implementations, modules operate over mid band and high band frequencies (MHB). 
     RFFE systems can be used in a wide variety of RF communication devices, including, but not limited to, smartphones, base stations, laptops, handsets, wearable electronics, and/or tablets. 
     An RFFE system can be implemented to support a variety of features that enhance bandwidth and/or other performance characteristics of the RF communication device in which the RFFE system is incorporated. 
     For example, to support wider bandwidth, an increasing number of uplink carrier aggregation scenarios have been developed to support wider bandwidth. Additionally, the bandwidths for uplink and downlink cannot be arbitrarily sent since there is a minimum uplink bandwidth for maintaining a reliable link supported by the transport layer&#39;s ACK/NACK traffic. Thus, in 4G/5G, wideband uplink carrier aggregation should be supported to achieve higher bandwidth for downlink carrier aggregation. 
     Thus, an RFFE system can be implemented to support both uplink and downlink 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, for instance, up to five carriers. 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. 
     Transition from 4G to 5G is through non-standalone (NSA) operation, rather than directly to full standalone (SA) operation. Current networks operate in 4G and 5G concurrently by communicating with an eNodeB and a gNodeB simultaneously in an EN-DC mode of operation. Thus, 4G and 5G transmitters operate concurrently is such a phone. 
     To provide such feature support, an RFFE system can be implemented to support EN-DC. 
     Support for EN-DC can cover a wide range of frequency bands, including using a 4G band in the LB, MHB, HB, or UHB frequency ranges in combination with a 5G band in the LB, MHB, HB, or UHB frequency ranges. Thus, various combinations of EN-DC including, but not limited to, LB-LB EN-DC, MHB-MHB EN-DC, LB-MHB EN-DC, LB-UHB EN-DC, MHB-UHB EN-DC, and UHB-UHB EN-DC, are possible. 
     Moreover, in certain dual uplink transmission scenarios, it can be desirable to provide flexibility between swapping which antenna transmits a first RF transmit signal (for instance, one of a 4G signal or a 5G signal) on a first side of a phone board assembly and which antenna transmits a second RF transmit signal (for instance, the other of the 4G signal or the 5G signal) on a side of the phone board assembly. To provide such flexibility, an RFFE system can support a transmit swap function to selectively switch which antenna a particular RF transmit signal is transmitted from. 
     Another technique for increasing uplink capacity is uplink multiple-input multiple-output (MIMO) communications, in which multiple (for instance, two) power amplifiers transmit two different signals simultaneously on the same frequency using different antennas. 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. MIMO order refers to a number of separate data streams sent or received. 
     Certain legacy communication standards operate using very high transmit powers compared to modern communication standards. For example, GSM/E-GPRS/2G radio access technology (RAT) uses extremely high output antenna powers of GMSK modulation from the UE (for example, 33 dBm for LB and 30 dBm for MB) relative to the average power of 3G, 4G, or 5G technologies. For example certain 3G LB/MB transmissions use 24 dBm, while certain 4G LB/MB transmissions use 23 dBm. 
     Although such legacy communications operate using high transmit powers, certain UE are specified to support such communications for purposes of backwards compatibility with 2G network infrastructure. 
     In comparison to 2G GMSK modulations which are constant envelope, 3G, 4G, and 5G transmissions have variable signal envelope with relatively large peak-to-average ratios (PAR). The varying signal envelope results in stringent requirements for linearity and emissions from power amplifiers. As a result, 3G, 4G, and 5G power amplifiers have insufficient power capability to be re-used for 2G transmissions. Moreover, if the 3G/4G linear power amplifier were to be re-tuned to deliver the higher powers of the 2G RAT, the efficiency of the FDD 3G/4G bands would be compromised and the resulting performance unacceptable. 
     According, conventional UE supporting 2G transmissions includes an explicit 2G power amplifier module (PAM) in the RFFE. The inclusion of the 2G PMA results in an extra cost and area penalty. 
     Cellular radios employing multiple power amplifiers for legacy communications are disclosed herein. In certain embodiments, a mobile phone includes a first antenna, a second antenna, and an RFFE including a first transmit path operable to transmit a first RF signal on the first antenna using a first communication standard in a first mode, and a second transmit path operable to transmit a second RF signal on the second antenna using the first communication standard in the first mode. Furthermore, in a second mode the RFFE is implemented to reuse the first transmit path and the second transmit path to synchronously transmit a third RF signal on the first antenna and the second antenna using a second communication standard. 
     Thus, the first transmit path and the second transmit path can be used for a variety of signal communications using the first communication standard (for instance, 3G, 4G or 5G) in the first mode, while being reused to synchronously transmit using the second communication standard (for instance, 2G) in the second mode. For example, the same 2G signal can be synchronously sent through two separate power amplifiers onto two separate LB antennas to increase the total radiated power from the mobile phone by the 3 dB needed to reach the 2G power levels. 
     Accordingly, in the far field, the total uplink transmit 2G power would be seen to be +33 dBm (minus path loss that all radiated channels suffer). 
     By implementing the mobile phone in this manner, the 2G PAM can be entirely eliminated. Thus, benefits in terms of cost and area reduction can be achieved. 
     The first transmit path and the second transmit path can correspond to a wide variety of communication paths. For example, multiple communications paths can be included in a mobile phone to support a wide variety of features, such as LB-LB EN-DC, uplink carrier aggregation scenarios, MIMO features, and/or to leverage the supply domain connectivity of combining specific LB paths with other MB and/or UHB paths in EN-DC or uplink carrier aggregation. 
     The mobile phone can be further implemented to support additional modes. 
     For example, with respect to inter-RAT transmissions, the mobile phone can be further implemented to support 2G transmission (at backed off power) on the first antenna and 3G, 4G, or 5G transmission on the second antenna. Additionally or alternatively, the mobile phone can be further implemented to support any combination of 3G, 4G, or 5G on the first antenna and a different RAT (3G, 4G, or 5G) on the second antenna. Such combinations can include, but are not limited to, 4G (E-UTRA) plus 5G (NR) and EN-DC use cases. 
     Furthermore, with respect to intra-RAT transmissions, the mobile phone can support 2×2 UL-MIMO (different data layers on the same uplink channel) for 3G, 4G, and/or 5G with layer one on the first antenna and layer two on the second antenna. Additionally or alternatively, the mobile phone can be implemented to support dual transmit coherent transmission where same data and signal is transmitted on both the first antenna and the second antenna with phase adjusted for maximum coherence and uplink diversity gain. Such dual transmit coherent transmission includes 2G delivered in uplink with 30 dBm burst power from each of the first antenna and the second antenna so the sum from the entire UE is 33 dBm back to the base station. 
     The aforementioned transmit modes can support a wide variety of bands including, but not limited to, LB MB, MHB, and/or HB. 
       FIG. 5  is a schematic diagram of one embodiment of a mobile device  220 . The mobile device  220  includes a baseband processor  201 , a transceiver  202 , a front-end system  203 , and antennas  204 . 
     As shown in  FIG. 5 , the front-end system  203  includes a first transmit path  211  and a second transmit path  212 . Additionally, the antennas  204  include a first antenna  213  and a second antenna  214 . 
     In the embodiment of  FIG. 5 , the front-end system  203  is implemented such that the first transmit path  211  transmits a first RF signal on the first antenna  213  using a first communication standard in a first mode, and such that the second transmit path  212  transmits a second RF signal on the second antenna  214  using the first communication standard in the first mode. Furthermore, in a second mode the front-end system  203  is implemented to reuse the first transmit path  211  and the second transmit path  212  to synchronously transmit a third RF signal on the first antenna  213  and the second antenna  214  using a second communication standard. 
     Thus, the first transmit path  211  and the second transmit path  212  can be used for a variety of signal communications using the first communication standard (for instance, 3G, 4G or 5G) in the first mode, while being reused to synchronously transmit using the second communication standard (for instance, 2G) in the second mode. 
     For example, in the second mode the same 2G signal can be synchronously through two separate transmit paths (and thus separate power amplifiers) onto two separate LB antennas to increase the total radiated power from the mobile phone  220  by the 3 dB needed to reach the 2G power levels. 
     Accordingly, in the far field, the total uplink transmit 2G power would be seen to be +33 dBm (minus path loss that all radiated channels suffer). 
     By implementing the mobile phone  220  in this manner, the 2G PAM can be entirely eliminated. Thus, benefits in terms of cost and area reduction can be achieved. 
     The first transmit path  211  and the second transmit path  212  can correspond to a wide variety of communication paths. For example, multiple communications paths can be included in the mobile phone  220  to support a wide variety of features, such as LB-LB EN-DC, uplink carrier aggregation scenarios, MIMO features, and/or to leverage the supply domain connectivity of combining specific LB paths with other MB and/or UHB paths in EN-DC or uplink carrier aggregation. 
     The mobile phone  220  can be further implemented to support additional modes. 
     For example, with respect to inter-RAT transmissions, the mobile phone  220  can be further implemented to support 2G transmission (at backed off power) on the first antenna  213  and 3G, 4G, or 5G transmission on the second antenna  214 . Additionally or alternatively, the mobile phone  220  can be further implemented to support any combination of 3G, 4G, or 5G on the first antenna  213  and a different RAT (3G, 4G, or 5G) on the second antenna  214 . Such combinations can include, but are not limited to, 4G (E-UTRA) plus 5G (NR) and EN-DC use cases. 
     Furthermore, with respect to intra-RAT transmissions, the mobile phone  220  can support 2×2 UL-MIMO (different data layers on the same uplink channel) for 3G, 4G, and/or 5G with layer one on the first antenna  213  and layer two on the second antenna  214 . Additionally or alternatively, the mobile phone  220  can be implemented to support dual transmit coherent transmission where same data and signal is transmitted on both the first antenna  213  and the second antenna  214  with phase adjusted for maximum coherence and uplink diversity gain. Such dual transmit coherent transmission includes 2G delivered in uplink with 30 dBm burst power from each of the first antenna  213  and the second antenna  214  so the sum from the entire mobile phone  220  is 33 dBm back to the base station. 
     The aforementioned transmit modes can support a wide variety of bands including, but not limited to, LB MB, MHB, and/or HB. 
       FIG. 6  is a schematic diagram of another embodiment of a front-end system  260 . The front-end system  260  includes a first LB module  231   a , a second LB module  231   b , a first diplexer  232   a , and a second diplexer  232   b.    
     As shown in  FIG. 6 , the first LB module  231   a  receives a first LB transmit signal LB_TX 1  from a transceiver, and provides a first LB receive signal LB_RX 1  to the transceiver. The first LB module  231   a  is coupled to a first antenna terminal ANT 1  by way of the first diplexer  232   a . The first LB module  231   a  includes a power amplifier  241   a , a low noise amplifier  242   a , a transmit switch  243   a , a transmit filter  244   a , a duplexer  245   a , a receive switch  246   a , a first receive filter  247   a , a second receive filter  248   a , and an antenna switch  249   a.    
     With continuing reference to  FIG. 6 , the second LB module  231   b  receives a second LB transmit signal LB_TX 2  from the transceiver, and provides a second LB receive signal LB_RX 2  to the transceiver. The second LB module  231   b  is coupled to a second antenna terminal ANT 2  by way of the second diplexer  232   b . The second LB module  231   b  includes a power amplifier  241   b , a low noise amplifier  242   b , a transmit switch  243   b , a transmit filter  244   b , a duplexer  245   b , a receive switch  246   b , a first receive filter  247   b , a second receive filter  248   b , and an antenna switch  249   b.    
     When the front-end system  260  operates in a first mode, the first LB transmit signal LB_TX 1  and the second LB transmit signal LB_TX 2  can correspond to two different transmit signals associated with a first communication standard, such as 3G, 4G, or 5G. For example, the front-end system  260  can be used for uplink carrier aggregation, EN-DC, or MIMO communications in the first mode. 
     With continuing reference to  FIG. 6 , the front-end system  260  can simultaneously transmit the same 2G transmit signal in a second mode. In particular, the 2G transmit signal can be provided to both the first LB module  231   a  and the second LB module  231   b  such that the 2G transmit signal is transmitted on two antennas. The two separate LB antennas to increase the total radiated power from the mobile phone  220  by the 3 dB needed to reach the 2G power levels. 
     Accordingly, in the far field, the total uplink transmit 2G power would be seen to be about +33 dBm (minus path loss that all radiated channels suffer). 
     By implementing the mobile phone  220  in this manner, the 2G PAM can be entirely eliminated. Thus, benefits in terms of cost and area reduction can be achieved. 
     Although an example using LB is depicted, the teachings herein are applicable to other frequency ranges. 
       FIG. 7  is a schematic diagram of another 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 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. 7  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 in 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. 7 , 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). The power management system  805  can include PMUs implemented in accordance with the teachings herein. Thus, the power management system  805  can be implemented in accordance with any of the embodiments herein, and serves as a power management sub-system for UE. 
     As shown in  FIG. 7 , 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. 8  is a schematic diagram of a power amplifier system  860  according to one embodiment. The illustrated power amplifier system  860  includes a baseband processor  841 , a transmitter/observation receiver  842 , a power amplifier (PA)  843 , a directional coupler  844 , front-end circuitry  845 , an antenna  846 , a PA bias control circuit  847 , and a PA supply control circuit  848 . The illustrated transmitter/observation receiver  842  includes an I/Q modulator  857 , a mixer  858 , and an analog-to-digital converter (ADC)  859 . In certain implementations, the transmitter/observation receiver  842  is incorporated into a transceiver. 
     The baseband processor  841  can be used to generate an in-phase (I) signal and a quadrature-phase (Q) signal, which can be used to represent a sinusoidal wave or signal of a desired amplitude, frequency, and phase. For example, the I signal can be used to represent an in-phase component of the sinusoidal wave and the Q signal can be used to represent a quadrature-phase component of the sinusoidal wave, which can be an equivalent representation of the sinusoidal wave. In certain implementations, the I and Q signals can be provided to the I/Q modulator  857  in a digital format. The baseband processor  841  can be any suitable processor configured to process a baseband signal. For instance, the baseband processor  841  can include a digital signal processor, a microprocessor, a programmable core, or any combination thereof. Moreover, in some implementations, two or more baseband processors  841  can be included in the power amplifier system  860 . 
     The I/Q modulator  857  can be configured to receive the I and Q signals from the baseband processor  841  and to process the I and Q signals to generate an RF signal. For example, the I/Q modulator  857  can include digital-to-analog converters (DACs) configured to convert the I and Q signals into an analog format, mixers for upconverting the I and Q signals to RF, and a signal combiner for combining the upconverted I and Q signals into an RF signal suitable for amplification by the power amplifier  843 . In certain implementations, the I/Q modulator  857  can include one or more filters configured to filter frequency content of signals processed therein. 
     The power amplifier  843  can receive the RF signal from the I/Q modulator  857 , and when enabled can provide an amplified RF signal to the antenna  846  via the front-end circuitry  845 . 
     The front-end circuitry  845  can be implemented in a wide variety of ways. In one example, the front-end circuitry  845  includes one or more switches, filters, duplexers, multiplexers, and/or other components. In another example, the front-end circuitry  845  is omitted in favor of the power amplifier  843  providing the amplified RF signal directly to the antenna  846 . 
     The directional coupler  844  senses an output signal of the power amplifier  823 . Additionally, the sensed output signal from the directional coupler  844  is provided to the mixer  858 , which multiplies the sensed output signal by a reference signal of a controlled frequency. The mixer  858  operates to generate a downshifted signal by downshifting the sensed output signal&#39;s frequency content. The downshifted signal can be provided to the ADC  859 , which can convert the downshifted signal to a digital format suitable for processing by the baseband processor  841 . Including a feedback path from the output of the power amplifier  843  to the baseband processor  841  can provide a number of advantages. For example, implementing the baseband processor  841  in this manner can aid in providing power control, compensating for transmitter impairments, and/or in performing digital pre-distortion (DPD). Although one example of a sensing path for a power amplifier is shown, other implementations are possible. 
     The PA supply control circuit  848  receives a power control signal from the baseband processor  841 , and controls supply voltages of the power amplifier  843 . In the illustrated configuration, the PA supply control circuit  848  generates a first supply voltage V CC1  for powering an input stage of the power amplifier  843  and a second supply voltage V CC2  for powering an output stage of the power amplifier  843 . The PA supply control circuit  848  can control the voltage level of the first supply voltage V CC1  and/or the second supply voltage V CC2  to enhance the power amplifier system&#39;s PAE. 
     The PA supply control circuit  848  can employ various power management techniques to change the voltage level of one or more of the supply voltages over time to improve the power amplifier&#39;s power added efficiency (PAE), thereby reducing power dissipation. 
     One technique for improving efficiency of a power amplifier is average power tracking (APT), in which a DC-to-DC converter is used to generate a supply voltage for a power amplifier based on the power amplifier&#39;s average output power. Another technique for improving efficiency of a power amplifier is envelope tracking (ET), in which a supply voltage of the power amplifier is controlled in relation to the envelope of the RF signal. Thus, when a voltage level of the envelope of the RF signal increases the voltage level of the power amplifier&#39;s supply voltage can be increased. Likewise, when the voltage level of the envelope of the RF signal decreases the voltage level of the power amplifier&#39;s supply voltage can be decreased to reduce power consumption. 
     In certain configurations, the PA supply control circuit  848  is a multi-mode supply control circuit that can operate in multiple supply control modes including an APT mode and an ET mode. For example, the power control signal from the baseband processor  841  can instruct the PA supply control circuit  848  to operate in a particular supply control mode. 
     As shown in  FIG. 8 , the PA bias control circuit  847  receives a bias control signal from the baseband processor  841 , and generates bias control signals for the power amplifier  843 . In the illustrated configuration, the bias control circuit  847  generates bias control signals for both an input stage of the power amplifier  843  and an output stage of the power amplifier  843 . However, other implementations are possible. 
       FIG. 9  is a schematic diagram of one example of a power amplifier  1132  powered by a power amplifier supply voltage V CC_PA . As shown in  FIG. 9 , an inductor  1127  is used to provide the power amplifier supply voltage V CC_PA  from a PMU to the power amplifier  1132 , which is terminated using an output impedance matching circuit  1131 . 
     The illustrated power amplifier  1132  includes a bipolar transistor  1129  having an emitter, a base, and a collector. As shown in  FIG. 9 , the emitter of the bipolar transistor  1129  is electrically connected to a power low supply voltage V 1 , which can be, for example, a ground supply. Additionally, an RF signal (RF IN ) is provided to the base of the bipolar transistor  1129 , and the bipolar transistor  1129  amplifies the RF signal to generate an amplified RF signal at the collector. The bipolar transistor  1129  can be any suitable device. In one implementation, the bipolar transistor  1129  is a heterojunction bipolar transistor (HBT). 
     The output impedance matching circuit  1131  serves to terminate the output of the power amplifier  1132 , which can aid in increasing power transfer and/or reducing reflections of the amplified RF signal generated by the power amplifier  1132 . In certain implementations, the output impedance matching circuit  1131  further operates to provide harmonic termination and/or to control a load line impedance of the power amplifier  1132 . 
     The inductor  1127  can be included to provide the power amplifier  1132  with the power amplifier supply voltage V CC_PA  while choking or blocking high frequency RF signal components. The inductor  1127  can include a first end electrically connected to the envelope tracker  1102 , and a second end electrically connected to the collector of the bipolar transistor  1129 . In certain implementations, the inductor  1127  operates in combination with the impedance matching circuit  1131  to provide output matching. 
     Although  FIG. 9  illustrates one implementation of the power amplifier  1132 , skilled artisans will appreciate that the teachings described herein can be applied to a variety of power amplifier structures, such as multi-stage power amplifiers and power amplifiers employing other transistor structures. For example, in some implementations the bipolar transistor  1129  can be omitted in favor of employing a field-effect transistor (FET), such as a silicon FET, a gallium arsenide (GaAs) high electron mobility transistor (HEMT), or a laterally diffused metal oxide semiconductor (LDMOS) transistor. Additionally, the power amplifier  1132  can be adapted to include additional circuitry, such as biasing circuitry. 
     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 any other systems or apparatus that have needs for UHB architectures. 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.