Patent Publication Number: US-2022239328-A1

Title: Lo phase correction for aas with multiple rfic

Description:
This application is a continuation of U.S. patent application Ser. No. 16/972,882, filed Dec. 7, 2020, which is a 35 U.S.C. § 371 national phase filing of International Application No. PCT/EP2018/064795, filed Jun. 5, 2018, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     Systems and methods are disclosed herein that relate to an Advanced Antenna System (AAS) for a radio node such as, e.g., a base station in a cellular communications system and, more particularly, relate to systems and methods for Local Oscillator (LO) phase correction for an AAS with multiple Radio Frequency Integrated Circuits (RFICs) that share a common external LO reference. 
     BACKGROUND 
     An Advanced Antenna System (AAS) is an antenna system utilized by radio nodes, such as a base station in a cellular communications network, to perform analog beamforming. An AAS has many antenna elements, where each antenna element is connected to radio front-end that includes a transmitter and a receiver along with a phase tuner and a gain stage to apply desired phase and gain adjustments for, e.g., analog beamforming. Within the AAS, multiple Radio Frequency Integrated Circuits (RFICs) may be used, each having its own Radio Frequency (RF) and, in some cases, Intermediate Frequency (IF) mixing stages. A common Local Oscillator (LO) source is used to provide a reference LO signal to the RFICs. For very high frequencies, each RFIC may include circuitry (e.g., frequency multiplier(s) and/or divider(s)) used to translate the reference LO signal to the desired RF and, in some cases, IF frequencies. Generating the desired IF/RF frequencies on the RFIC is beneficial, particularly for very high frequencies, in order to avoid long routing and corresponding losses. 
     As described herein, the inventors have found that phase misalignment between different RFICs in such an AAS may occur. For instance, for high frequency, it is beneficial to use a scheme where each RFIC has circuitry to generate the RF LO, and in some cases the IF LO, from a lower frequency reference LO source. In such a case, the RF LO can be obtained by a frequency multiplier which is fed by the reference LO source, while the IF LO can be obtained from a frequency divider (divides by powers of 2) that is fed by RF LO. The inventors have found that, for LO generation circuitry designs that include a frequency divider, the generated LO may have a phase sift (e.g., a phase shift of 180 degrees in the case of a divide by 2). This can therefore lead to a situation where different RFICs that share a common external LO reference may not be phase aligned. This phase misalignment can adversely affect the performance of the system. As such, there is a need for systems and methods for correcting phase misalignment between multiple RFICs in an AAS. 
     SUMMARY 
     Systems and methods are disclosed for correcting Local Oscillator (LO) phase misalignment between different Radio Frequency Integrated Circuits (RFIC) of an Advanced Antenna System (AAS). In some embodiments, a system comprises a radio system comprising two or more RFICs. Each RFIC comprises LO generation circuitry, processing circuitry, and a plurality of antenna elements. The LO generation circuitry comprises a frequency divider. Using the frequency divider, the LO generation circuitry is configured to generate a LO signal based on a reference LO signal from an external LO source. The processing circuitry is configured to upconvert signals to be transmitted by the plurality of antenna elements and/or downconvert signals received via the plurality of antenna elements based on the LO signal. The system further comprises a processing unit adapted to, for a first RFIC pair comprising two of the two or more RFICs, namely a first RFIC and a second RFIC, obtain a first near-field power measurement via a receive antenna element located either in the first RFIC or the second RFIC while: (a) a test signal is transmitted via a first transmit antenna element located in the first RFIC and a second transmit antenna element located in the second RFIC, and (b) the phase state of the second RFIC is a first LO phase state. The first transmit antenna element for the first RFIC pair is one of the plurality of antenna elements located in the first RFIC that is configured as a transmit antenna element. The second transmit antenna element for the first RFIC pair is one of the plurality of antenna elements located in the second RFIC that is configured as a transmit antenna element. The receive antenna element for the first RFIC pair is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a receive antenna element or (b) one of the plurality of antenna elements comprised in the second RFIC that is configured as a receive antenna element. The processing unit is further adapted to, for the first RFIC pair, obtain a second near-field power measurement via the receive antenna element while a test signal is transmitted via the first transmit antenna element and the second transmit antenna element and the phase state of the second RFIC is a second LO phase state. In some embodiments, the second LO phase state is a state in which the phase of the LO signal for the second RFIC is shifted by 180 degrees relative to the phase of the LO signal for the second RFIC when the phase state of the second RFIC is the first LO phase state. The processing unit is further adapted to, for the first RFIC pair, determine which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC, and set the phase state of the second RFIC to the determined LO phase state. In this manner, LO phase misalignment between the pair of RFICs can quickly be detected and corrected. 
     In some embodiments, the first transmit antenna element, the second transmit antenna element, and the receive antenna element are chosen such that the coupling between the first transmit antenna element and the receive antenna element is symmetrical to the coupling between the second transmit antenna element and the receive antenna element. In addition or alternatively, in some embodiments, the first transmit antenna element, the second transmit antenna element, and the receive antenna element are chosen such that the coupling between the first transmit antenna element and the receive antenna element is not orthogonal to the coupling between the second transmit antenna element and the receive antenna element. 
     In some embodiments, for each RFIC of the two or more RFICs, the plurality of antenna elements comprised in the RFIC are dual-polarized antenna elements, the first transmit antenna element and the second transmit antenna element are configured in one polarization, while the receive antenna element is configured in the opposite polarization. 
     In some embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state; and, if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state. The determined phase state for the second RFIC is the second LO phase state if the first near-field power measurement is greater than the second near-field power measurement and is the first LO phase state if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some other embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state; and if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state. The determined phase state for the second RFIC is the first LO phase state if the first near-field power measurement is greater than the second near-field power measurement and is the second LO phase state if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some embodiments, the processing unit is further adapted to, for a second RFIC pair comprising two of the two or more RFICs, namely the first RFIC and a third RFIC, obtain a first near-field power measurement for the second RFIC pair via a receive antenna element for the second RFIC pair that is located either in the first RFIC or the third RFIC while: (a) a test signal is transmitted via a first transmit antenna element for the second RFIC pair that is located in the first RFIC and a second transmit antenna element for the second RFIC pair that is located in the third RFIC and (b) the phase state of the third RFIC is a first LO phase state. The first transmit antenna element for the second RFIC pair is one of the plurality of antenna elements comprised in the first RFIC that is configured as a transmit antenna element. The second transmit antenna element for the second RFIC pair is one of the plurality of antenna elements comprised in the third RFIC that is configured as a transmit antenna element. The receive antenna element for the second RFIC pair is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a receive antenna element or (b) one of the plurality of antenna elements in the third RFIC that is configured as a receive antenna element. The processing unit is further adapted to, for the second RFIC pair, obtain a second near-field power measurement via the receive antenna element for the second RFIC pair while a test signal is transmitted via the first transmit antenna element for the second RFIC pair and the second transmit antenna element for the second RFIC pair and the phase state of the third RFIC is a second LO phase state, wherein the second LO phase state is a state in which the phase of the LO signal for the third RFIC is shifted by 180 degrees relative to the phase of the LO signal for the third RFIC when the phase state of the third RFIC is the first LO phase state. The processing unit is further adapted to, for the second RFIC pair, determine which of the first LO phase state and the second LO phase state for the third RFIC results in phase alignment between the LO signals for the first RFIC and the third RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the third RFIC, and set the phase state of the third RFIC to the determined LO phase state. 
     In some embodiments, a system comprises a radio system comprising two or more RFICs. Each RFIC comprises LO generation circuitry, processing circuitry, and a plurality of antenna elements. The LO generation circuitry comprises a frequency divider. Using the frequency divider, the LO generation circuitry is configured to generate a LO signal based on a reference LO signal from an external LO source. The processing circuitry is configured to upconvert signals to be transmitted by the plurality of antenna elements and/or downconvert signals received via the plurality of antenna elements based on the LO signal. The system further comprises a processing unit adapted to, for a fist RFIC pair comprising two of the two or more RFICs, namely a first RFIC and a second RFIC, obtain a first near-field power measurement via a first receive antenna element located in the first RFIC and a second receive antenna element located in the second RFIC while: (a) a test signal is transmitted via a transmit antenna element located either in the first RFIC or the second RFIC and (b) a phase state of the second RFIC is a first LO phase state. The first receive antenna element for the first RFIC pair is one of the plurality of antenna elements in the first RFIC that is configured as a receive antenna element. The second receive antenna element for the first RFIC pair is one of the plurality of antenna elements in the second RFIC that is configured as a receive antenna element. The transmit antenna element for the first RFIC pair is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a transmit antenna element or (b) one of the plurality of antenna elements in the second RFIC that is configured as a transmit antenna element. The processing unit is further adapted to, for the first RFIC pair, obtain a second near-field power measurement via the first receive antenna element and the second receive antenna element while a test signal is transmitted via the transmit antenna element and the phase state of the second RFIC is a second LO phase state. In some embodiments, the second LO phase state is a state in which the phase of the LO signal for the second RFIC is shifted by 180 degrees relative to the phase of the LO signal for the second RFIC when the phase state of the second RFIC is the first LO phase state. The processing unit is further adapted to, for the first RFIC pair, determine which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC, and set the phase state of the second RFIC to the determined LO phase state. 
     In some embodiments, the first receive antenna element, the second receive antenna element, and the transmit antenna element are chosen such that the coupling between the first receive antenna element and the transmit antenna element is symmetrical to the coupling between the second receive antenna element and the transmit antenna element. In addition or alternatively, in some embodiments, the first receive antenna element, the second receive antenna element, and the transmit antenna element are chosen such that the coupling between the first receive antenna element and the transmit antenna element is not orthogonal to the coupling between the second receive antenna element and the transmit antenna element. 
     In some embodiments, for each RFIC of the two or more RFICs, the plurality of antenna elements comprised in the RFIC are dual-polarized antenna elements, the first receive antenna element and the second receive antenna element are configured in one polarization, and the transmit antenna element is configured in the opposite polarization. 
     In some embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state; and if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state. The determined phase state for the second RFIC is the second LO phase state if the first near-field power measurement is greater than the second near-field power measurement and is the first LO phase state if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state; and, if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state. The determined phase state for the second RFIC is the first LO phase state if the first near-field power measurement is greater than the second near-field power measurement and is the second LO phase state if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some embodiments, the processing unit is further adapted to, for a second RFIC pair comprising two of the two or more RFICs, namely the first RFIC and a third RFIC, obtain a first near-field power measurement for the second RFIC pair via a first receive antenna element for the second RFIC pair that is located in the first RFIC and a second receive antenna element for the second RFIC pair that is located in the third RFIC while: (a) a test signal is transmitted via a transmit antenna element for the second RFIC pair that is located either in the first RFIC or the third RFIC and (b) a phase state of the third RFIC is a first LO phase state. The first receive antenna element for the second RFIC pair is one of the plurality of antenna elements comprised in the first RFIC that is configured as a receive antenna element. The second receive antenna element for the second RFIC pair is one of the plurality of antenna elements comprised in the third RFIC that is configured as a receive antenna element. The transmit antenna element for the second RFIC pair is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a transmit antenna element or (b) one of the plurality of antenna elements in the third RFIC that is configured as a transmit antenna element. The processing unit is further adapted to, for the second RFIC pair, obtain a second near-field power measurement via the first receive antenna element for the second RFIC pair and the second receive antenna element for the second RFIC pair while: (a) a test signal is transmitted via the transmit antenna element for the second RFIC pair and the phase state of the third RFIC is a second LO phase state, wherein the second LO phase state is a state in which the phase of the LO signal for the third RFIC is shifted by 180 degrees relative to the phase of the LO signal for the third RFIC when the phase state of the third RFIC is the first LO phase state. The processing unit is further adapted to, for the second RFIC pair, determine which of the first LO phase state and the second LO phase state for the third RFIC results in phase alignment between the LO signals for the first RFIC and the third RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the third RFIC, and set the phase state of the third RFIC to the determined LO phase state. 
     Embodiments of a method for self-testing a system to correct for LO phase misalignment between RFICs is also provided. In some embodiments, a method for self-testing of a system is provided, where the system comprises a radio system comprising two or more RFICs. Each RFIC comprises LO generation circuitry, processing circuitry, and a plurality of antenna elements. The LO generation circuitry comprises a frequency divider. Using the frequency divider, the LO generation circuitry is configured to generate a LO signal based on a reference LO signal from an external LO source. The processing circuitry is configured to upconvert signals to be transmitted by the plurality of antenna elements and/or downconvert signals received via the plurality of antenna elements based on the LO signal. The method comprises, for a first RFIC pair comprising two of the two or more RFICs, namely a first RFIC and a second RFIC, obtaining a first near-field power measurement via a receive antenna element located either in the first RFIC or the second RFIC while: (a) a test signal is transmitted via a first transmit antenna element located in the first RFIC and a second transmit antenna element located in the second RFIC and (b) a phase state of the second RFIC is a first LO phase state. The first transmit antenna element for the first RFIC pair is one of the plurality of antenna elements located in the first RFIC that is configured as a transmit antenna element. The second transmit antenna element for the first RFIC pair is one of the plurality of antenna elements located in the second RFIC that is configured as a transmit antenna element. The receive antenna element for the first RFIC pair is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a receive antenna element or (b) one of the plurality of antenna elements in the second RFIC that is configured as a receive antenna element. The method further comprises, for the first RFIC pair, obtaining a second near-field power measurement via the receive antenna element while a test signal is transmitted via the first transmit antenna element and the second transmit antenna element and the phase state of the second RFIC is a second LO phase state. In some embodiments, the second LO phase state is a state in which the phase of the LO signal for the second RFIC is shifted by 180 degrees relative to the phase of the LO signal for the second RFIC when the phase state of the second RFIC is the first LO phase state. The method further comprises, for the first RFIC pair, determining which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the RF LO signals for the first RFIC and the second RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the RF LO signals for the first RFIC and the second RFIC, and setting the phase state of the second RFIC to the determined LO phase state. 
     In some embodiments, the first transmit antenna element, the second transmit antenna element, and the receive antenna element are chosen such that the coupling between the first transmit antenna element and the receive antenna element is symmetrical to the coupling between the second transmit antenna element and the receive antenna element. 
     In some embodiments, for each RFIC of the two or more RFICs, the plurality of antenna elements comprised in the RFIC are dual-polarized antenna elements, the first and second transmit antenna elements are configured in a first polarization, and the receive antenna element is configured in a second polarization. In addition or alternatively, in some embodiments, the first transmit antenna element, the second transmit antenna element, and the receive antenna element are chosen such that the coupling between the first transmit antenna element and the receive antenna element is not orthogonal to the coupling between the second transmit antenna element and the receive antenna element. 
     In some embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state; and, if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state. Determining which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC comprises: determining that the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is greater than the second near-field power measurement, and determining that the first LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state; and, if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state. Determining which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC comprises: determining that the first LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is greater than the second near-field power measurement, and determining that the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some embodiments, the method further comprises, for a second RFIC pair comprising two of the two or more RFICs, namely the first RFIC and a third RFIC, obtaining a first near-field power measurement for the second RFIC pair via a receive antenna element for the second RFIC pair that is located either in the first RFIC or the third RFIC while: (a) a test signal is transmitted via a first transmit antenna element for the second RFIC pair that is located in the first RFIC and a second transmit antenna element for the second RFIC pair that is located in the third RFIC and (b) a phase state of the third RFIC is a first LO phase state. The first transmit antenna element for the second RFIC pair is one of the plurality of antenna elements in the first RFIC that is configured as a transmit antenna element. The second transmit antenna element for the second RFIC pair is one of the plurality of antenna elements in the third RFIC that is configured as a transmit antenna element. The receive antenna element for the second RFIC pair is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a receive antenna element or (b) one of the plurality of antenna elements in the third RFIC that is configured as a receive antenna element. The method further comprises, for the second RFIC pair, obtaining a second near-field power measurement via the receive antenna element for the second RFIC pair while a test signal is transmitted via the first transmit antenna element for the second RFIC pair and the second transmit antenna element for the second RFIC pair and the phase state of the third RFIC is a second LO phase state, wherein the second LO phase state is a state in which the phase of the LO signal for the third RFIC is shifted by 180 degrees relative to the phase of the LO signal for the third RFIC when the phase state of the third RFIC is the first LO phase state. The method further comprises, for the second RFIC pair, determining which of the first LO phase state and the second LO phase state for the third RFIC results in phase alignment between the LO signals for the first RFIC and the third RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the third RFIC, and setting the phase state of the third RFIC to the determined LO phase state. 
     In some embodiments, a method for self-testing of a system is provided, where the system comprises a radio system comprising two or more RFICs. Each RFIC comprises LO generation circuitry, processing circuitry, and a plurality of antenna elements. The LO generation circuitry comprises a frequency divider. Using the frequency divider, the LO generation circuitry is configured to generate a LO signal based on a reference LO signal from an external LO source. The processing circuitry is configured to upconvert signals to be transmitted by the plurality of antenna elements and/or downconvert signals received via the plurality of antenna elements based on the LO signal. The method comprises, for a first RFIC pair comprising two of the two or more RFICs, namely a first RFIC and a second RFIC, obtaining a first near-field power measurement via a first receive antenna element located in the first RFIC and a second receive antenna element located in the second RFIC while: (a) a test signal is transmitted via a transmit antenna element located either in the first RFIC or the second RFIC and (b) a phase state of the second RFIC is a first LO phase state. The first receive antenna element is one of the plurality of antenna elements in the first RFIC that is configured as a receive antenna element. The second receive antenna element is one of the plurality of antenna elements in the second RFIC that is configured as a receive antenna element. The transmit antenna element is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a transmit antenna element or (b) one of the plurality of antenna elements in the second RFIC that is configured as a transmit antenna element. The method further comprises, for the first RFIC pair, obtaining a second near-field power measurement via the first receive antenna element and the second receive antenna element while a test signal is transmitted via the transmit antenna element and the phase state of the second RFIC is a second LO phase state. In some embodiments, the second LO phase state is a state in which the phase of the LO signal for the second RFIC is shifted by 180 degrees relative to the phase of the LO signal for the second RFIC when the phase state of the second RFIC is the first LO phase state. The method further comprises, for the first RFIC pair, determining which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC, and setting the phase state of the second RFIC to the determined LO phase state. 
     In some embodiments, the first receive antenna element, the second receive antenna element, and the transmit antenna element are chosen such that the coupling between the first receive antenna element and the transmit antenna element is symmetrical to the coupling between the second receive antenna element and the transmit antenna element. In addition or alternatively, in some embodiments, the first receive antenna element, the second receive antenna element, and the transmit antenna element are chosen such that the coupling between the first receive antenna element and the transmit antenna element is not orthogonal to the coupling between the second receive antenna element and the transmit antenna element. 
     In some embodiments, for each RFIC of the two or more RFICs, the plurality of antenna elements comprised in the RFIC are dual-polarized antenna elements, the first receive element and the second receive antenna element are configured in one polarization, and the transmit antenna element is configured in the opposite second polarization. 
     In some embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state; and, if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state. Determining which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC comprises: determining that the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is greater than the second near-field power measurement, and determining that the first LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some embodiments, the predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the second RFIC is that: if the first near-field power measurement is greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are aligned when the phase state of the second RFIC is the first LO phase state and misaligned when the phase state of the second RFIC is the second LO phase state; and, if the first near-field power measurement is not greater than the second near-field power measurement, then the phases of the LO signals for the first RFIC and the second RFIC are misaligned when the phase state of the second RFIC is the first LO phase state and aligned when the phase state of the second RFIC is the second LO phase state. Determining which of the first LO phase state and the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC comprises: determining that the first LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is greater than the second near-field power measurement, and determining that the second LO phase state for the second RFIC results in phase alignment between the LO signals for the first RFIC and the second RFIC if the first near-field power measurement is not greater than the second near-field power measurement. 
     In some embodiments, the method further comprises for a second RFIC pair comprising two of the two or more RFICs, namely the first RFIC and a third RFIC, obtaining a first near-field power measurement for the second RFIC pair via a first receive antenna element for the second RFIC pair that is located in the first RFIC and a second receive antenna element for the second RFIC pair that is located in the third RFIC while: (a) a test signal is transmitted via a transmit antenna element for the second RFIC pair that is located either in the first RFIC or the third RFIC and (b) a phase state of the third RFIC is a first LO phase state. The first receive antenna element for the second RFIC pair is one of the plurality of antenna elements in the first RFIC that is configured as a receive antenna element. The second receive antenna element for the second RFIC pair is one of the plurality of antenna elements in the third RFIC that is configured as a receive antenna element. The transmit antenna element for the second RFIC pair is either: (a) one of the plurality of antenna elements in the first RFIC that is configured as a transmit antenna element or (b) one of the plurality of antenna elements comprised in the third RFIC that is configured as a transmit antenna element. The method further comprises, for the second RFIC pair, obtaining a second near-field power measurement via the first receive antenna element for the second RFIC pair and the second receive antenna element for the second RFIC pair while a test signal is transmitted via the transmit antenna element for the second RFIC pair and the phase state of the third RFIC is a second LO phase state, wherein the second LO phase state is a state in which the phase of the LO signal for the third RFIC is shifted by 180 degrees relative to the phase of the LO signal for the third RFIC when the phase state of the third RFIC is the first LO phase state. The method further comprises, for the second RFIC pair, determining which of the first LO phase state and the second LO phase state for the third RFIC results in phase alignment between the LO signals for the first RFIC and the third RFIC based on a predetermined relationship between near-field power measurements and phase alignment between the LO signals for the first RFIC and the third RFIC, and setting the phase state of the third RFIC to the determined LO phase state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
         FIG. 1  illustrates one example of a radio system including an Advanced Antenna System (AAS) including multiple Radio Frequency Integrated Circuits (RFICs) in accordance with some embodiments of the present disclosure; 
         FIG. 2  illustrates one example of dual-polarized antenna elements; 
         FIG. 3  illustrates one example of the Intermediate Frequency (IF) and Radio Frequency (RF) Local Oscillator (LO) generation circuitry included in an RFIC of an AAS in accordance with some embodiments of the present disclosure; 
         FIG. 4  illustrates one example of two possible LO signals that can occur due to a frequency divider; 
         FIG. 5  illustrates one example of a system that performs a characterization and self-test procedure to correct the phase error between different RFICs of the AAS of  FIG. 1 ; 
         FIG. 6  is a flow chart that illustrates the operation of the processing unit of  FIG. 5  to perform a characterization and self-test procedure in accordance with embodiments of the present disclosure; 
         FIG. 7  illustrates one example of how the antenna elements of adjacent RFICs are configured for the characterization and self-test procedure; 
         FIG. 8  is a flow chart that illustrates a characterization procedure for transmit LOs in accordance with some embodiments of the present disclosure; 
         FIG. 9  is a flow chart that illustrates a characterization procedure for receive LOs in accordance with some embodiments of the present disclosure; 
         FIG. 10  is a flow chart that illustrates a self-test procedure for transmit LOs in accordance with some embodiments of the present disclosure; 
         FIG. 11  is a flow chart that illustrates a self-test procedure for receive LOs in accordance with some embodiments of the present disclosure; 
         FIG. 12  is a flow chart that illustrates a self-test procedure in accordance with some other embodiments of the present disclosure; 
         FIG. 13  graphically illustrates how the characterization and self-test procedures can be performed across many RFICs using one of the RFICs as a reference in accordance with some embodiments of the present disclosure; 
         FIG. 14  illustrates one example of a cellular communications network according to some embodiments of the present disclosure; 
         FIG. 15  is a schematic block diagram of a radio access node according to some embodiments of the present disclosure; 
         FIG. 16  is a schematic block diagram that illustrates a virtualized embodiment of the radio access node of  FIG. 15  according to some embodiments of the present disclosure; and 
         FIG. 17  is a schematic block diagram of the radio access node of  FIG. 15  according to some other embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments set forth below represent information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure. 
     Systems and methods for correcting phase misalignment between multiple Radio Frequency Integrated Circuits (RFICs) in an Advanced Antenna System (AAS) are disclosed. In this regard,  FIG. 1  illustrates one example of a radio system, which in this disclosure is referred to as a Phased Antenna Array Module (PAAM)  100 , that includes an AAS  102  including multiple RFICs  104 - 1  through  104 - 4  in accordance with some embodiments of the present disclosure. Note that while there are four RFICs  104  in this example, the PAAM  100  may include any number of two or more RFICs  104 . In this example, each of the RFICs  104  includes four Antenna Elements (AEs), which are referenced as AE 1 -AE 4 . Note, however, that each RFIC  104  may include any number of two or more AEs. As another example, each RFIC  104  may alternatively include 16 AEs. 
     The RFICs  104 - 1  through  104 - 4  receive a reference Local Oscillator (LO) signal from an external reference LO source  106  through a network of splitters  108  through  112 . In this example, the reference LO source  106  is “external” in that it is external to the RFICs  104 - 1  through  104 - 4 . In this example, the reference LO source  106  is external to the PAAM  100 . Note that the connectors that connect the outputs of the splitters  110  and  112  to the RFICs  104 - 1  through  104 - 4  are matched length connectors (i.e., have the same length). In absence of length matching, calibration data can be used to synchronize the LOs of different RFICs  104 . The AAS  102 , and in particular the RFICs  104 - 1  through  104 - 4 , are connected to an interface  114  to a processing unit (e.g., processing unit  500  of  FIG. 5 ). As discussed below, the interface  114  provides an interface to a processing unit (e.g., processing unit  500  of  FIG. 5 ), where the processing unit, for example, provides baseband transmit signals to the PAAM  100  for transmission by the AAS  102  and receives baseband receive signals from the PAAM  100 . In addition, as discussed below, the processing unit interacts with the PAAM  100  to perform a self-test procedure. 
     The RFICs  104 - 1  through  104 - 4  include LO generation circuitry  116 - 1  through  116 - 4 , Intermediate Frequency (IF) processing circuitry  118 - 1  through  118 - 4 , Radio Frequency (RF) processing circuitry  120 - 1  through  120 - 4 , and multiple AEs (AE 1  through AE 4 ). In the example embodiments described herein, the AEs are arranged in a matrix, or grid. For each RFIC  104 , the LO generation circuitry  116  generates an IF LO signal and an RF LO signal based on the reference LO signal using, e.g., one or more frequency multipliers and/or one or more frequency dividers. The IF processing circuitry  118  uses the IF LO signal to upconvert baseband transmit signals received from the processing unit from baseband to IF and to downconvert IF receive signals received from the RF processing circuitry  120  from IF to baseband. The RF processing circuitry  120  uses the RF LO signal to upconvert IF transmit signals received from the IF processing circuitry  118  from IF to RF and to downconvert RF receive signals received from the AEs (AE 1  through AE 4 ) from RF to IF. Note that, in the example embodiments described herein, the LO generation circuitry  116  generates separate IF LO signals for transmit and receive and generates separate RF LO signals for transmit and receive. 
     In some embodiments, the AEs (AE 1  through AE 4 ) of the RFICs  104 - 1  through  104 - 4  are dual-polarized. As an example,  FIG. 2  illustrates one example of the AEs (AE 1  through AE 4 ) of an RFIC  104  where the AEs (AE 1  through AE 4 ) are dual-polarized. 
       FIG. 3  illustrates one example of the LO generation circuitry  116 . In this example, the LO generation circuitry  116  includes a frequency multiplier  300  that translates the reference LO signal from some reference LO frequency to a desired frequency for translating from the desired transmit/receive RF frequency and a desired IF frequency of the transmit/receive signal. The resulting LO signal is referred to herein as the RF LO signal. The LO generation circuitry  116  also includes a frequency divider  302  that translates the RF LO signal to a desired frequency for translating between the desired IF frequency of the transmit/receive signal and either baseband or some other IF frequency (denoted in  FIG. 3  as IF 2 ). 
       FIG. 4  illustrates the LO phase mismatch problem that can occur between the different RFICs  104 - 1  through  104 - 4  that results from the circuitry used to implement the frequency divider  302 . In this example, the reference LO (REF LO) is first multiplied by 4 to generate the RF LO (4×LO). The RF LO is then divided by 2 to generate the divided signal (i.e., 2×LO). As can be seen in  FIG. 4 , as a result of the circuitry used to implement the frequency divider  302 , the divided signal (2×LO signal) could have either a rising edge or a falling edge in reference to the RF LO (i.e., the 4×LO signal). 
     Thus, when each RFIC  104  has its own LO generation circuitry  116  including a frequency multiplier/divider, sometimes the generated LO signals between RFICs  104  are out of phase with each other. This phase-misalignment of the LO signals in different RFICs  104  occurs even though the actual reference LO signals used by the RFICs  104  are phase aligned. Thus, inside the RFICs  104 , the IF/RF LO signals can be randomly out of phase. 
     As described herein, the LO phase state of the LO generation circuitry  116  is controlled such that respective LO signals of different RFICs  104  are phase-aligned. In particular, in the example of  FIG. 4 , the divided LO signal output by the frequency divider  302  is provided to one input of a multiplexer, or switch,  304 . In addition, phase-shifting circuitry  306  applies a  180  degree phase shift to the divided LO signal. The phase-shifting circuitry  306  may be implemented to, e.g., phase invert the divided LO signal by flipping p and n in a differential buffer, the positive and negative parts of a differential pair. The resulting phase-shifted, divided LO signal is provided to another input of the multiplexer  304 . The multiplexer  304  outputs the divided LO signal as the IF LO signal when configured in a first LO phase state and outputs the phase-shifted, divided LO signal as the IF LO signal when configured in a second LO phase state. The LO phase state of the LO generation circuitry  116  is controlled via a LO phase state select input (e.g., digital input written to a register), which is provided by, e.g., the processing unit via the interface  14 . 
     Note that the example of the LO generation circuitry  116  illustrated in  FIG. 3  is only an example. In general, the LO generation circuitry  116  is any circuitry that generates, based on reference LO signal provided by an external LO source, a LO signal using a frequency divider. Further, while the two LO phase states are provided by the multiplexer  304  and the phase-shifting circuitry  306  at the output of the frequency divider  302  in the example of  FIG. 3 , the present disclosure is not limited thereto. Other types of configurations for providing the different LO phase states can be used, as will be appreciated by one of ordinary skill in the art upon reading this disclosure. 
     It should also be noted that while a 180 degree phase shift is described herein, other phase-shift amounts may be used. For example, a 180 degree phase shift is suitable when the frequency divider  302  is a divide by 2. However, if the frequency divider  302  divides by some other number (e.g., 4, 8, 16, etc.), other phase-shift amounts (e.g., 90 degree phase shift, 45 degree phase shift, etc.) can be used. 
       FIG. 5  illustrates a system including the PAAM  100  of  FIG. 1  and a processing unit  500  including a characterization and self-test subsystem  502  that operates to correct the LO misalignment between the different RFICs  104 - 1  through  104 - 4  of the AAS  102  of the PAAM  100  in accordance with embodiments of the present disclosure. The characterization and self-test subsystem  502  may be implemented in hardware or a combination of hardware and software (e.g., software being executed by one or more processor(s)). Note that, while illustrated separately, in some embodiments, the processing unit  500  and, in particular, the characterization and self-test subsystem  502 , can be implemented in the PAAM  100 . 
     The characterization and self-test subsystem  502  includes a controller  504  that operates to, e.g., control the activation and configuration of the antenna elements of the RFICs  104 - 1  through  104 - 4  during characterization and self-testing, control a phase state of each of the RFICs  104 - 1  through  104 - 4 , etc. The characterization and self-test subsystem  502  also includes a signal generator  506  that operates to, e.g., generate test signals and provide those test signals to the PAAM  100  for transmission during calibration and self-testing. The test signals may be, e.g., pseudo-random signals or single tone signals. The characterization and self-test subsystem  502  also includes a measurement processing function  508  that receives near-field measurements from the PAAM  100  during calibration and self-testing and processes those measurements as described herein. Note that, as used herein, a “near field” measurement is a measurement obtained by a receive antenna element(s) in an RFIC(s)  104  of the PAAM  100  during transmission of a test signal via a transmit antenna element(s) in an RFIC(s)  104  of the PAAM  100 . Conversely, a “far field” measurement is: (a) a measurement made at a remote receiver during transmission of a test signal by the PAAM  100 ; or (b) a measurement made via a receive antenna element(s) of an RFIC(s)  104  of the PAAM  100  during transmission of a test signal by a remote transmitter. 
       FIG. 6  illustrates the operation of the processing unit  500 , and in particular the characterization and self-test subsystem  502 , in accordance with some embodiments of the present disclosure. Here, a dashed box represents an optional step. As illustrated, the characterization and self-test subsystem  502  characterizes a relationship between near-field power measurements and LO phase alignment for multiple RFIC pairs (step  600 ). Each RFIC pair includes a reference RFIC. The reference RFIC is one of the RFICs  104 . The characterization and self-test subsystem  502  performs a self-test procedure to determine whether to invert the phase of the LO(s) on each RFIC  104  (other than the reference RFIC) (step  602 ). Note that while illustrated together, the characterization procedure of step  600  and the self-test procedure of step  602  do not need to be performed by the same processing unit  500  for the same PAAM  100 . For example, one processing unit  500  may be used to perform the characterization procedure for one PAAM  100 , and the results of the characterization may be used by another processing unit  500  for another PAAM  100 . In other words, characterization may, in some embodiments, be performed once for a particular PAAM design and the results of the characterization then used for self-testing of each PAAM  100  produced for that PAAM design. 
       FIG. 7  illustrates an example antenna element configuration utilized for characterization and self-testing in accordance with some embodiments of the present disclosure. As illustrated, when performing transmit characterization (e.g., in step  600  of  FIG. 6 ), AE 4  on RFIC  104 - 1  is activated and configured as a receive antenna element (e.g., in vertical polarization (V-polarization), AE 1  on RFIC  104 - 1  is activated and configured as a (reference) transmit antenna element (e.g., in horizontal polarization (H-polarization), and the AE 1 s of RFICs  104 - 2 ,  104 - 3 , and  104 - 4  are, for respective iterations of the procedure, activated and configured as transmit antenna elements (e.g., in H-polarization). Note that the selection of AE 4  on RFIC  104 - 1  as the receive AE is only an example. Any AE having symmetrical or non-orthogonal coupling with the selected transmit AEs on RFICs  104 - 1  through  104 - 4  can be used. 
     More specifically, when performing transmit characterization, the controller  504  activates AE 4  on RFIC  104 - 1  and configures it as a receive antenna element (e.g., in vertical polarization (V-polarization). The controller  504  also activates AE 1  on RFIC  104 - 1  and configures it as a (reference) transmit antenna element (e.g., in H-polarization). Then, multiple iterations of the characterization procedure are performed for each pair of RFICs  104 . These pairs of RFICs  104  are: (IC 1 ,IC 2 ), (IC 1 ,IC 3 ), and (IC 1 ,IC 4 ). Note that RFIC  104 - 1  is also referred to herein as IC 1 , RFIC  104 - 2  is also referred to herein as IC 2 , etc. 
     More specifically, for a first iteration of transmit characterization, the controller  504  activates AE 1  on RFIC  104 - 2  and configures it as a transmit antenna element (e.g., in H-polarization). The signal generator  506  generates a test signal and provides the test signal to the PAAM  100  for transmission via AE 1  on RFIC  104 - 1  and AE 1  on RFIC  104 - 2 . Due to coupling between the AEs, during transmission of the test signal, a coupled signal is received via AE 4  on RFIC  104 - 1 . The power of this received signal is measured (e.g., on the PAAM  100 ), and the measurement is provided to the measurement processing function  508  where the measurement is stored as a first near-field power measurement. During characterization, a corresponding far-field power measurement is also obtained and stored as a first far-field power measurement. 
     The controller  504  then switches the LO phase state of RFIC  104 - 2 . More specifically, when obtaining the first near-field and far-field power measurements, the LO phase state of the RFIC  104 - 2  is set to some initial LO phase state, which is referred to here as a first LO phase state. The controller  504  switches the LO phase state of the RFIC  104 - 2  to a second LO phase state in which the phase of the LO signal for the RFIC  104 - 2  is shifted by 180 degrees with respect to the phase of the LO signal for the RFIC  104 - 2  when the RFIC  104 - 2  is in the first LO phase state. The switching of the phase state of the RFIC  104 - 2  may be performed by providing an appropriate digital input (e.g., the LO phase state select signal in the example of  FIG. 3 ) to the LO generation circuitry  116 . The signal generator  506  then generates a test signal and provides the test signal to the PAAM  100  for transmission via AE 1  on RFIC  104 - 1  and AE 1  on RFIC  104 - 2  while the RFIC  104 - 2  is in the second LO phase state. Again, due to coupling between the AEs, during transmission of the test signal, a coupled signal is received via AE 4  on RFIC  104 - 1 . The power of this received signal is measured (e.g., on the PAAM  100 ), and the measurement is provided to the measurement processing function  508  where the measurement is stored as a second near-field power measurement. A corresponding far-field power measurement is also obtained and stored as a second far-field power measurement. This process is then repeated for each of the remaining RFIC pairs. 
     During characterization, it is known that the signals transmitted from the two transmit AEs will constructively combine when the LO phases on the two RFICs IC 1  and ICx (where x=2, 3, or 4) are aligned. As such, for each RFIC pair (IC 1 ,ICx), if the first far-field power measurement is greater than the second far-field power measurement, it can be determined that the LO phases of the two RFICs IC 1  and ICx are aligned when the RFIC ICx is in the first LO phase state. Otherwise, it can be determined that that the LO phases of the two RFICs IC 1  and ICx are aligned when the RFIC ICx is in the second LO phase state. For each RFIC pair (IC 1 ,ICx), using this information and the first and second near-field power measurements for the RFIC pair (IC 1 ,ICx), the measurement processing function  508  can determine a relationship between near-field power measurements for the two RFICs IC 1  and ICx and the LO phase alignment of the two RFICs IC 1  and ICx. This can be expressed by the following truth table: 
                                     P 1,x,far  &gt; P 1,inv(x),far     P 1,x,near  &gt; P 1,inv(x),near     B 1,x                    False   False   True       False   True   False       True   False   False       True   True   True                    
where P 1,x,far  is the first far-field power measurement for the RFIC pair (IC 1 ,ICx), P 1,inv(x),far  is the second far-field power measurement for the RFIC pair (IC 1 ,ICx), P 1,x,near  is the first near-field power measurement for the RFIC pair (IC 1 ,ICx), P 1,inv(x),near  is the second near-field power measurement for the RFIC pair (IC 1 ,ICx), and B 1,x  is a Boolean variable that represents the relationship between the far-field and near-field measurements for the RFIC pair (IC 1 ,ICx). As can be seen from the table, the Boolean variable B 1,x  is TRUE when the far-field and near-field measurements are positively correlated, i.e. (P 1,x,near &gt;P 1,inv(x),near) ==(P 1,x,far &gt;P 1,inv(x),far ); and B 1,x  is FALSE when the far-field and near-field measurements are negatively correlated, i.e. (P 1,x,near &gt;P 1,inv(x),near ) is not equal to T(P 1,x,far &gt;P 1,inv(x),far ). Note that the far-field measurements are directly related to the LO phase state of the RFIC ICx for which the LOs for the RFIC pair (IC 1 ,ICx) are aligned, i.e. if (P 1,x,far &gt;P 1,inv(x),far ) is TRUE it means that the LOs of the RFIC pair (IC 1 ,ICx) are aligned when the LO phase state of the RFIC ICx is the first LO phase state, otherwise the LOs of the RFIC pair (IC 1 ,ICx) are aligned when the LO phase state of the RFIC ICx is the second LO phase state. Therefore, for an RFIC pair (IC 1 ,ICx), the Boolean variable B 1,x  captures the relationship between the near-field measurements and the LO phase state of the RFIC ICx for which the LO phases for the RFIC pair (IC 1 ,ICx) are aligned (i.e., the LO signals of RFICs IC 1  and ICx are phase-aligned). Once determined, the Boolean values B 1,x  are stored and used for subsequent self-testing of the PAAM  100  and/or used for subsequent testing of other PAAMs  100  (by storing the Boolean values B 1,x  in the processing units  500  used for other PAAMs  100 ).
 
     A similar characterization procedure can be performed for receive operation. 
     As illustrated, when performing transmit self-testing (e.g., in step  602  of  FIG. 6 ), AE 4  on RFIC  104 - 1  is activated and configured as a receive antenna element (e.g., in V-polarization), AE 1  on RFIC  104 - 1  is activated and configured as a (reference) transmit antenna element (e.g., in horizontal polarization (H-polarization), and the AE 1 s of RFICs  104 - 2 ,  104 - 3 , and  104 - 4  are, for respective iterations of the procedure, activated and configured as transmit antenna elements (e.g., in H-polarization). 
     More specifically, when performing transmit self-testing, the controller  504  activates AE 4  on RFIC  104 - 1  and configures it as a receive antenna element (e.g., in V-polarization). The controller  504  also activates AE 1  on RFIC  104 - 1  and configures it as a (reference) transmit antenna element (e.g., in H-polarization). Then, multiple iterations of the self-testing procedure are performed for each pair of RFICs  104 . These pairs of RFICs  104  are: (IC 1 ,IC 2 ), (IC 1 ,IC 3 ), and (IC 1 ,IC 4 ). Note that RFIC  104 - 1  is also referred to herein as IC 1 , RFIC  104 - 2  is also referred to herein as IC 2 , etc. 
     More specifically, for a first iteration of transmit self-testing, the controller  504  activates AE 1  on RFIC  104 - 2  and configures it as a transmit antenna element (e.g., in H-polarization). The signal generator  506  generates a test signal and provides the test signal to the PAAM  100  for transmission via AE 1  on RFIC  104 - 1  and AE 1  on RFIC  104 - 2 . Due to coupling between the AEs, during transmission of the test signal, a coupled signal is received via AE 4  on RFIC  104 - 1 . The power of this received signal is measured (e.g., on the PAAM  100 ), and the measurement is provided to the measurement processing function  508  where the measurement is stored as a first near-field power measurement. 
     The controller  504  then switches a LO phase state of the RFIC  104 - 2 . More specifically, when obtaining the first near-field and far-field power measurements, the LO phase state of the RFIC  104 - 2  is set to some initial LO phase state, which is referred to here as a first LO phase state. The controller  504  switches the LO phase state of the RFIC  104 - 2  to a second LO phase state in which the phase of the LO signal for the RFIC  104 - 2  is shifted by  180  degrees with respect to the phase LO signal for the RFIC  104 - 2  when the RFIC  104 - 2  is in the first LO phase state. The signal generator  506  then generates a test signal and provides the test signal to the PAAM  100  for transmission via AE 1  on RFIC  104 - 1  and AE 1  on RFIC  104 - 2  while the RFIC  104 - 2  is in the second LO phase state. Again, due to coupling between the AEs, during transmission of the test signal, a coupled signal is received via AE 4  on RFIC  104 - 1 . The power of this received signal is measured (e.g., on the PAAM  100 ), and the measurement is provided to the measurement processing function  508  where the measurement is stored as a second near-field power measurement. The process is repeated for each of the other RFIC pairs. 
     During self-testing, for each RFIC pair (IC 1 ,ICx), the expression P 1,x,near &gt;P 1,inv(x),near  is compared against the Boolean variable B 1,x  representing the relationship between the near-field measurements and the LO phase state of the RFIC ICx for which the LO phases of the RFIC pair (IC 1 ,ICx) are aligned. If the comparison is TRUE (i.e., if (P 1,x,near &gt;P 1,inv(x),near )==B 1,x ), then the controller  504  determines that the LO phases of the RFIC pair (IC 1 ,ICx) are aligned when the LO phase state of the RFIC ICx is the first LO phase state. Otherwise, the controller  504  determines that the LO phases of the RFIC pair (IC 1 ,ICx) are aligned when the LO phase state of the RFIC ICx is the second LO phase state. The controller  504  then sets the LO phase state of RFIC ICx to the phase state that results in LO phase alignment. 
       FIGS. 8 and 9  are flow charts that illustrate the characterization procedure of step  600  of  FIG. 6  in more detail in accordance with some embodiments of the present disclosure.  FIG. 8  illustrates a characterization procedure for the transmit LOs of the RFICs  104 , and  FIG. 9  illustrates a characterization procedure for the receive LOs of the RFICs  104 . 
     As illustrated in  FIG. 8 , for transmit, the controller  504  of the characterization and self-testing subsystem  502  initializes an index x to a value of 1 (step  800 ). If the value of x is not greater than the number of RFIC pairs to be processed for the characterization procedure (step  802 , NO), the controller  504  increments x (step  803 ) and sets an initial phase state for RFIC ICx to the first LO phase state (step  804 ). The controller  504  activates the reference antenna element (e.g., AE 1 ) on the reference RFIC IC 1  and activates a test antenna element (e.g., AE 1 ) on RFIC ICx and configures these two antenna elements as transmit antenna elements, e.g., in the H-polarization (step  806 ). In addition, the controller  504  activates an antenna element either on RFIC IC 1  or ICx (e.g., AE 4  on RFIC IC 1 ) and configures that antenna element as a receive antenna element, e.g., in the V-polarization (step  808 ). 
     While a test signal is transmitted by the PAAM  100  using the two transmit antenna elements and the LO phase state of RFIC ICx is set to the first LO phase state, the characterization and self-test subsystem  502  obtains a first near-field power measurement (P 1,x,near ) via the receive antenna element and also obtains a corresponding first far-field power measurement (P 1,x,far ) (step  810 ). More specifically, the controller  504  causes the signal generator  506  to generate a test signal and provide the test signal to the PAAM  100  for transmission via the two transmit antenna elements. Due to coupling, a resulting signal is received at the receive antenna element. A power of this signal is measured in the PAAM  100  (e.g., in the respective RFIC) to thereby provide the first near-field power measurement (P 1,x,near ), which is sent to the processing unit  500  and stored. The first far-field power measurement (P 1,x,far ) is made by a remote receiver and returned to the processing unit  500  and stored. 
     The controller  504  switches the LO phase state of the RFIC ICx from the first LO phase state to the second LO phase state such that the phase of the LO signal for the RFIC ICx is shifted by 180 degrees (step  812 ). While a test signal is transmitted by the PAAM  100  using the two transmit antenna elements and the LO phase state of RFIC ICx is set to the second LO phase state, the characterization and self-test subsystem  502  obtains a second near-field power measurement (P 1,inv(x),near ) via the receive antenna element and also obtains corresponding second far-field power measurement (P 1,inv(x),far ) (step  814 ). More specifically, the controller  504  causes the signal generator  506  to generate a test signal and provide the test signal to the PAAM  100  for transmission via the two transmit antenna elements. Due to coupling, a resulting signal is received at the receive antenna element. A power of this signal is measured in the PAAM  100  (e.g., in the respective RFIC) to thereby provide the second near-field power measurement (P 1,inv(x),near ), which is sent to the processing unit  500  and stored. The second far-field power measurement (P 1,inv(x),far ) is made by a remote receiver and returned to the processing unit  500  and stored. 
     The controller  504  then determines a relationship (e.g., B 1,x ) between the power level of the near-field power measurements for the RFIC pair (IC 1 ,ICx) and the LO phase alignment for the RFIC pair (IC 1 ,ICx) based on the near-field and far-field measurements, as discussed above (step  816 ). This relationship is stored. The process returns to step  802  and is repeated until the last RFIC pair has been processed. 
     As illustrated in  FIG. 9 , for receive characterization, the controller  504  of the characterization and self-test subsystem  502  initializes an index x to a value of 1 (step  900 ). If the value of x is not greater than the number of RFIC pairs to be processed for the characterization procedure (step  902 , NO), the controller  504  increments x (step  903 ) and sets an initial phase state for RFIC ICx to the first LO phase state (step  904 ). The controller  504  activates the reference antenna element (e.g., AE 1 ) on the reference RFIC IC 1  and activates a test antenna element (e.g., AE 1 ) on RFIC ICx and configures these two antenna elements as receive antenna elements, e.g., in the V-polarization (step  906 ). In addition, the controller  504  activates an antenna element either on RFIC IC 1  or ICx (e.g., AE 4  on RFIC IC 1 ) and configures that antenna element as a transmit antenna element, e.g., in the H-polarization (step  908 ). 
     While a test signal is transmitted by the PAAM  100  using the transmit antenna element and the LO phase state of RFIC ICx is set to the first LO phase state, the characterization and self-test subsystem  502  obtains a first near-field power measurement (P 1,x,near ) via the receive antenna elements and also obtains a corresponding first far-field power measurement (P 1,x,far ) (step  910 ). Note that the corresponding first far-field power measurement (P 1,x,far ) is obtained based on a separate signal transmitted to the PAAM  100  from a far-field transmitter. More specifically, the controller  504  causes the signal generator  506  to generate a test signal and provide the test signal to the PAAM  100  for transmission via the transmit antenna element. Due to coupling, resulting signals are received at the two receive antenna elements. These two receive signals are combined and a power measurement of this combined signal is made in the PAAM  100  (e.g., in the respective RFIC) to thereby provide the first near-field power measurement (P 1,x,near ), which is sent to the processing unit  500  and stored. The first far-field power measurement (P 1,x,far ) ismade based on a signal transmitted by a far-field transmitter that is received via the two receive antenna elements. 
     The controller  504  switches the LO phase state of the RFIC ICx from the first LO phase state to the second LO phase state such that the phase of the LO signal for the RFIC ICx is shifted by 180 degrees (step  912 ). While a test signal is transmitted by the PAAM  100  using the transmit antenna element and the LO phase state of RFIC ICx is set to the second LO phase state, the characterization and self-test subsystem  502  obtains a second near-field power measurement (P 1,inv(x),near ) via the two receive antenna elements and also obtains a corresponding second far-field power measurement (P 1,inv(x),far ) (step  914 ). 
     The controller  504  then determines a relationship (e.g., B 1,x ) between the power level of the near-field power measurements for the RFIC pair (IC 1 ,ICx) and the LO phase alignment for the RFIC pair (IC 1 ,ICx) based on the near-field and far-field measurements, as discussed above (step  916 ). This relationship is stored. The process returns to step  902  and is repeated until the last RFIC pair has been processed. 
       FIGS. 10 and 11  are flow charts that illustrate the self-test procedure of step  602  of  FIG. 6  in more detail in accordance with some embodiments of the present disclosure.  FIG. 10  illustrates a self-test procedure for the transmit LOs of the RFICs  104 , and  FIG. 11  illustrates a self-test procedure for the receive LOs of the RFICs  104 . 
     As illustrated in  FIG. 10 , for transmit, the controller  504  of the characterization and self-test subsystem  502  initializes an index x to a value of 1 (step  1000 ). If the value of x is not greater than the number of RFIC pairs to be processed for the characterization procedure (step  1002 , NO), the controller  504  increments x (step  1003 ) and sets an initial phase state for RFIC ICx to the first LO phase state (step  1004 ). The controller  504  activates the reference antenna element (e.g., AE 1 ) on the reference RFIC IC 1  and activates a test antenna element (e.g., AE 1 ) on RFIC ICx and configures these two antenna elements as transmit antenna elements, e.g., in the H-polarization (step  1006 ). In addition, the controller  504  activates an antenna element either on RFIC IC 1  or ICx (e.g., AE 4  on RFIC IC 1 ) and configures that antenna element as a receive antenna element, e.g., in the V-polarization (step  1008 ). 
     While a test signal is transmitted by the PAAM  100  using the two transmit antenna elements and the LO phase state of RFIC ICx is set to the first LO phase state, the characterization and self-test subsystem  502  obtains a first near-field power measurement (P 1,x,near ) via the receive antenna element (step  1010 ). More specifically, the controller  504  causes the signal generator  506  to generate a test signal and provide the test signal to the PAAM  100  for transmission via the two transmit antenna elements. Due to coupling, a resulting signal is received at the receive antenna element. A power of this signal is measured in the PAAM  100  (e.g., in the respective RFIC) to thereby provide the first near-field power measurement (P 1,x,near ), which is sent to the processing unit  500  and stored. 
     The controller  504  switches the LO phase state of the RFIC ICx from the first LO phase state to the second LO phase state such that the phase of the LO signal for the RFIC ICx is shifted by 180 degrees (step  1012 ). While a test signal is transmitted by the PAAM  100  using the two transmit antenna elements and the LO phase state of RFIC ICx is set to the second LO phase state, the characterization and self-test subsystem  502  obtains a second near-field power measurement (P 1,inv(x),near ) via the receive antenna element (step  1014 ). More specifically, the controller  504  causes the signal generator  506  to generate a test signal and provide the test signal to the PAAM  100  for transmission via the two transmit antenna elements. Due to coupling, a resulting signal is received at the receive antenna element. A power of this signal is measured in the PAAM  100  (e.g., in the respective RFIC) to thereby provide the second near-field power measurement (P 1,inv(x),near ), which is sent to the processing unit  500  and stored. 
     The controller  504  then determines which of the two near-field power measurements (P 1,x,near  and P 1,inv(x),near ) result in LO phase alignment between the two RFICs IC 1  and ICx based on the known relationship (e.g., B 1,x ) between power level of the near-field power measurements for the RFIC pair (IC 1 ,ICx) and LO phase alignment for the RFIC pair (IC 1 ,ICx) (step  1016 ). For example, as discussed above, in some embodiments, the relationship is expressed as a Boolean value B 1,x,  and the first LO phase state is determined to be the LO phase state that provides LO phase alignment if the expression (P 1,x,near &gt;P 1,inv(x),near )==B 1,x  is TRUE. Otherwise, the second LO phase state is determined to be the LO phase state that provides LO phase alignment. The controller  504  then sets the LO phase state of RFIC ICx to the LO phase stated determined to be the LO phase state that provides LO phase alignment between RFIC IC 1  and RFIC ICx (step  1018 ). The process returns to step  1002  and is repeated until the last RFIC pair has been processed. 
     As illustrated in  FIG. 11 , for receive self-test, the controller  504  of the characterization and self-test subsystem  502  initializes an index x to a value of 1 (step  1100 ). If the value of x is not greater than the number of RFIC pairs to be processed for the characterization procedure (step  1102 , NO), the controller  504  increments x (step  1103 ) and sets an initial phase state for RFIC ICx to the first LO phase state (step  1104 ). The controller  504  activates the reference antenna element (e.g., AE 1 ) on the reference RFIC IC 1  and activates a test antenna element (e.g., AE 1 ) on RFIC ICx and configures these two antenna elements as receive antenna elements, e.g., in the V-polarization (step  1106 ). In addition, the controller  504  activates an antenna element either on RFIC IC 1  or ICx (e.g., AE 4  on RFIC IC 1 ) and configures that antenna element as a transmit antenna element, e.g., in the H-polarization (step  1108 ). 
     While a test signal is transmitted by the PAAM  100  using the transmit antenna element and the LO phase state of RFIC ICx is set to the first LO phase state, the characterization and self-test subsystem  502  obtains a first near-field power measurement (P 1,x,near ) via the receive antenna elements (step  1110 ). More specifically, the controller  504  causes the signal generator  506  to generate a test signal and provide the test signal to the PAAM  100  for transmission via the transmit antenna element. Due to coupling, resulting signals are received at the receive antenna elements. These signals are combined, and the power of this combined signal is measured in the PAAM  100  to thereby provide the first near-field power measurement (P 1,x,near ), which is sent to the processing unit  500  and stored. 
     The controller  504  switches the LO phase state of the RFIC ICx from the first LO phase state to the second LO phase state such that the phase of the LO signal for the RFIC ICx is shifted by 180 degrees (step  1112 ). While a test signal is transmitted by the PAAM  100  using the transmit antenna element and the LO phase state of RFIC ICx is set to the second LO phase state, the characterization and self-test subsystem  502  obtains a second near-field power measurement (P 1,inv(x),near ) via the receive antenna elements (step  1114 ). More specifically, the controller  504  causes the signal generator  506  to generate a test signal and provide the test signal to the PAAM  100  for transmission via the transmit antenna element. Due to coupling, resulting signals are received at the receive antenna elements. These signals are combined, and a power of the combined signal is measured in the PAAM  100  (e.g., in the respective RFIC) to thereby provide the second near-field power measurement (P 1,inv(x),near ), which is sent to the processing unit  500  and stored. 
     The controller  504  then determines which of the two near-field power measurements (P 1,x,near  and P 1,inv(x),near ) result in LO phase alignment between the two RFICs IC 1  and ICx based on the known relationship (e.g., B 1,x ) between power level of the near-field power measurements for the RFIC pair (IC 1 ,ICx) and LO phase alignment for the RFIC pair (IC 1 ,ICx) (step  1116 ). For example, as discussed above, in some embodiments, the relationship is expressed as a Boolean value B 1,x , and the first LO phase state is determined to be the LO phase state that provides LO phase alignment if the expression (P 1,x,near &gt;P 1,inv(x),near )==B 1,x  is TRUE. Otherwise, the second LO phase state is determined to be the LO phase state that provides LO phase alignment. The controller  504  then sets the LO phase state of RFIC ICx to the LO phase stated determined to be the LO phase state that provides LO phase alignment between RFIC IC 1  and RFIC ICx (step  1118 ). The process returns to step  1102  and is repeated until the last RFIC pair has been processed. 
     Note that the basis for the characterization and self-test procedures described above with respect to  FIGS. 7 through 11  is as follows. For characterization and self-testing in the transmit direction, the transfer function of a system with two transmit antenna elements (AE 1  located in RFIC IC 1 , and AE 2  located in RFIC IC 2 ) and a single receive antenna element (AE 4  located in RFIC IC 1 ) can be expressed as: 
     
       
         
           
             
               M 
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     where: 
     Tx AE1_IC1  is the transfer function of the transmit antenna element AE 1  located in RFIC IC 1  (see  FIG. 7 ), 
     Tx AE1_IC2  is the transfer function of the transmit antenna element AE 1  located in RFIC IC 2  (see  FIG. 7 ), 
     C AE1_IC1,AE4_IC1  is the coupling between the transmit antenna element AE 1  located in RFIC IC 1  and the receive antenna element AE 4  located in RFIC IC 1 , 
     C AE1_IC2,AE4_IC1  is the coupling between the transmit antenna element AE 1  located in RFIC IC 2  and the receive antenna element AE 4  located in RFIC IC 1 , 
     Rx AE4_IC1  is the transfer function of the receive antenna element AE 4  located in RFIC IC 1 , 
     Δφ 1,2  is the LO phase difference between RFIC IC 1  and RFIC IC 2   
     Provided that Δφ 1,2  is either 0 or π radians, and assuming that the AAS  102  has already been calibrated, i.e. Tx AE1_IC1 ≅Tx AE1_IC2 , the transfer function Mx A1_IC1,A1_IC2  can be expressed as: 
         Mx   A1_IC1,A1_IC2   =Tx   AE1_IC1 ( C   AE1_IC1,AE4_IC1   ±C   AE1,IC2,AE4_IC1 ) Rx   AE4_IC1    
     Likewise, for the other RFIC pairs (IC 1 ,IC 3 ) and (IC 1 ,IC 4 ), the received signal at the receive antenna element can be expressed as: 
         Mx   A1_IC1,A1_IC3   =Tx   AE1_IC1 ( C   AE1_IC1,AE4_IC1   ±C   AE1_IC3,AE4_IC1 ) Rx   AE4_IC1    
       and 
         Mx   A1_IC1,A1_IC4   =Tx   AE1_IC1 ( C   AE1_IC1,AE4_IC1   ±C   AE1_IC4,AE4_IC1 ) Rx   AE4_IC1    
     From the equations above, it can be seen that, if the transmit antenna elements and the receive antenna element are selected such that there is symmetrical coupling (i.e., such that C IC1 ≅C IC2 ≅C IC3 ≅C IC4 ), then the power of the received signal will be small when ICx is in one LO phase state and relatively large when ICx is in the other phase state. 
     From the equations above, it can be seen that, as long as the couplings between the transmit antenna elements and the receive antenna element, i.e. C AE1_IC1,AE4_IC1  and C AE1_IC2,AE4_ICx , are not orthogonal to each other, the measured power of the received signal, which is proportional to the squared absolute value of the transfer function Mx AE1_IC1,AE1_ICx , can have two distinct values based on whether the LO phase state in the RFIC ICx is the first LO phase state or it is the second LO phase state. In other words, one power value will correspond to one LO phase state of the RFIC ICx, and a different power value will correspond to the opposite LO phase state of the RFIC ICx. 
     For the specific case, when the couplings between the transmit antenna elements and the receive antenna element, i.e. C AE1_IC1,AE4_IC1  and C AE1_IC2,AE4_ICx , are “symmetrical,” i.e. C AE1_IC1,AE4_IC1 ≈C AE1_IC1,AE4_ICx   e     tkπ    k=0,1, the two possible power values will be further apart from each other, allowing for a larger margin and thus making the method less sensitive to, for example, noise or external interference. Therefore, this is the preferred scenario when choosing the transmit and receive antenna elements. 
     Using this notation,  FIG. 12  is a flow chart that illustrates the self-testing procedure of step  602  of  FIG. 6  in accordance with some other embodiments of the present disclosure. As illustrated, the controller  504  of the characterization and self-test subsystem  502  sets a direction to perform LO phase alignment (step  1200 ) and set the polarization (e.g., H-polarization) (step  1202 ). The controller  504  activates a reference measurement antenna element in a different polarization than that set in step  1202  (e.g., V-polarization) (step  1204 ). The controller  504  activates a defined element in the reference RFIC IC 1  (step  1206 ) and sets an RFIC index x=1 (step  1208 ). 
     If x&lt;max (where max is the maximum number of RFICs to be tested) (step  1210 , YES), the controller  504  increments x (step  1211 ), and activates a defined antenna element in ICx (step  1212 ). The characterization and self-test subsystem  502  then obtains a first near-field power measurement for the RFIC pair (IC 1 ,ICx) as described above (step  1214 ). The controller  504  toggles, or switches, the LO phase state of the RFIC ICx (step  1216 ) and obtains a second near-field power measurement for the RFIC pair (IC 1 ,ICx) as described above (step  1218 ). The first and second near-field power measurements are checked against target logic (e.g., B 1,x ) stored in memory (step  1220 ). In this case, if XOR((M WT &gt;M T ),M)=1 (step  1222 , YES), then the polarity (i.e., the LO phase state) of RFIC ICx is kept at the toggled, or switched, LO phase state (step  1224 ). Otherwise (step  1222 , NO), the polarity of RFIC ICx is set to the default state (i.e., the first LO phase state) (step  1226 ). RFIC ICx is then deactivated (step  1228 ), and the process returns to step  1210 . Steps  1211  through  1228  are repeated for all of the RFICs to be tested. Once the last RFIC has been tested (i.e., step  1210 , NO), the polarization is changed (step  1232 ) and the process returns (step  1234 , YES) to step  1202  and the process is repeated for the new polarization. Once the last polarization is processed (step  1234 , NO), the direction is changed (e.g., from transmit self-test to receive self-test) (step  1236 ) and, if both transmit and receive directions have not yet been processed (step  1238 , YES), the process returns to step  1200  and is repeated for the new direction. 
       FIG. 13  graphically illustrates one example of how the characterization and self-test procedures of  FIGS. 7 through 12  can be performed across many RFICs  104  using one of the RFICs (e.g.,  104 - 1 ) as a reference in accordance with some embodiments of the present disclosure. As illustrated, 
     a first characterization/self-test is performed for RFICs IC 2 , IC 7 , and IC 8  using RFIC IC 1  as a reference; 
     a second characterization/self-test is performed for RFICs IC 3  and IC 9  using RFIC IC 2  (or RFIC IC 8 ) as the reference; 
     a third characterization/self-test is performed for RFICsIC 4  and IC 10  using RFIC IC 3  (or RFIC IC 9 ) as the reference; 
     a fourth characterization/self-test is performed for RFICsIC 5  and IC 11  using RFIC IC 4  (or RFIC IC 10 ) as the reference; 
     a fifth characterization/self-test is performed for RFICsIC 6  and IC 12  using RFIC IC 5  (or RFIC IC 11 ) as the reference; 
     a sixth characterization/self-test is performed for RFICs IC 13  and IC 14  using RFIC IC 7  (or RFIC IC  8 ) as the reference; 
     a seventh characterization/self-test is performed for RFIC IC 15  using RFIC IC 8  (or RFIC IC 9  or RFIC IC 14 ) as the reference; 
     an eighth characterization/self-test is performed for RFIC IC 16  using RFIC IC 9  (or RFIC IC 10  or RFIC IC 15 ) as the reference; 
     a ninth characterization/self-test is performed for RFIC IC 17  using RFIC IC 10  (or RFIC IC 11  or RFIC IC 16 ) as the reference; and 
     a tenth characterization/self-test is performed for RFIC IC 18  using RFIC IC 11  (or RFIC IC 12  or RFIC IC 17 ) as the reference. 
     In this manner, RFIC IC 1  becomes the main reference RFIC such that the LO phases of all of the RFICs are, after self-testing, aligned with the LO phase of the RFIC IC 1 . Note that the example described above with respect to  FIG. 13  is only an example. Any suitable pair-wise self-testing of the RFICs may be used. 
     Systems and methods are disclosed herein that provide a method for detecting and correcting LO phase misalignment between the RFICs  104  in the AAS  102 . As described herein, a test signal is transmitted using different combinations of a limited number of transmit antenna elements and measured with different combinations of a limited number of receive antenna elements. Then, power of the resulting received signal is measured. This process may be repeated after inverting the LO phase state of one or more of the RFICs. Then, the measured power value(s) are compared with information that defines a known relationship between the measurement value(s) and the LO phase-alignment of the RFICs. From this comparison, the RFIC(s) for which the LO phase is misaligned can be determined, and the LO phases of those RFIC(s) can be corrected. This process can be performed with a small number of steps and measurements and, as such, is very fast. 
     While not being limited to or by any particular advantage, embodiments of the present disclosure provide a number of advantages. For example, embodiments disclosed herein enable self-testing and correction of LO phase misalignment between RFICs in the AAS. The embodiments disclosed herein can be used for either analog or digital beamforming. The embodiments disclosed herein require a small number of steps and measurements and, therefore, the correction can be made very quickly. Embodiments of the present disclosure enable detection and correction of LO phase misalignment between the RFICs of an AAS for both uplink and downlink when different frequency multipliers and/or dividers are used for uplink and downlink. 
       FIG. 14  illustrates one example of a cellular communications network  1400  according to some embodiments of the present disclosure. In the embodiments described herein, the cellular communications network  1400  is a Fifth Generation (5G) New Radio (NR) network. In this example, the cellular communications network  1400  includes base stations  1402 - 1  and  1402 - 2 , which in Long Term Evolution (LTE) are referred to as enhanced or evolved Node Bs (eNBs) and in 5G NR are referred to as NR base stations (gNBs), controlling corresponding macro cells  1404 - 1  and  1404 - 2 . The base stations  1402 - 1  and  1402 - 2  are generally referred to herein collectively as base stations  1402  and individually as base station  1402 . Likewise, the macro cells  1404 - 1  and  1404 - 2  are generally referred to herein collectively as macro cells  1404  and individually as macro cell  1404 . The cellular communications network  1400  may also include a number of low power nodes  1406 - 1  through  1406 - 4  controlling corresponding small cells  1408 - 1  through  1408 - 4 . The low power nodes  1406 - 1  through  1406 - 4  can be small base stations (such as pico or femto base stations) or Remote Radio Heads (RRHs), or the like. Notably, while not illustrated, one or more of the small cells  1408 - 1  through  1408 - 4  may alternatively be provided by the base stations  1402 . The low power nodes  1406 - 1  through  1406 - 4  are generally referred to herein collectively as low power nodes  1406  and individually as low power node  1406 . Likewise, the small cells  1408 - 1  through  1408 - 4  are generally referred to herein collectively as small cells  1408  and individually as small cell  1408 . The base stations  1402  (and optionally the low power nodes  1406 ) are connected to a core network  1410 . 
     The base stations  1402  and the low power nodes  1406  provide service to wireless devices  1412 - 1  through  1412 - 5  in the corresponding cells  1404  and  1408 . The wireless devices  1412 - 1  through  1412 - 5  are generally referred to herein collectively as wireless devices  1412  and individually as wireless device  1412 . The wireless devices  1412  are also sometimes referred to herein as User Equipment devices (UEs). 
       FIG. 15  is a schematic block diagram of a radio access node  1500  according to some embodiments of the present disclosure. The radio access node  1500  may be, for example, a base station  1402  or  1406 . As illustrated, the radio access node  1500  includes a control system  1502  that includes one or more processors  1504  (e.g., Central Processing Units (CPUs), Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), and/or the like), memory  1506 , and a network interface  1508 . The one or more processors  1504  are also referred to herein as processing circuitry. In addition, the radio access node  1500  includes one or more radio units  1510  that each includes one or more transmitters  1512  and one or more receivers  1514  coupled to one or more antennas  1516 . The radio units  1510  may be referred to or be part of radio interface circuitry. In some embodiments, the radio unit(s)  1510  is external to the control system  1502  and connected to the control system  1502  via, e.g., a wired connection (e.g., an optical cable). However, in some other embodiments, the radio unit(s)  1510  and potentially the antenna(s)  1516  are integrated together with the control system  1502 . The one or more processors  1504  operate to provide one or more functions of a radio access node  1500  as described herein. In some embodiments, the function(s) are implemented in software that is stored, e.g., in the memory  1506  and executed by the one or more processors  1504 . 
       FIG. 16  is a schematic block diagram that illustrates a virtualized embodiment of the radio access node  1500  according to some embodiments of the present disclosure. This discussion is equally applicable to other types of network nodes. Further, other types of network nodes may have similar virtualized architectures. 
     As used herein, a “virtualized” radio access node is an implementation of the radio access node  1500  in which at least a portion of the functionality of the radio access node  1500  is implemented as a virtual component(s) (e.g., via a virtual machine(s) executing on a physical processing node(s) in a network(s)). As illustrated, in this example, the radio access node  1500  includes the control system  1502  that includes the one or more processors  1504  (e.g., CPUs, ASICs, FPGAs, and/or the like), the memory  1506 , and the network interface  1508  and the one or more radio units  1510  that each includes the one or more transmitters  1512  and the one or more receivers  1514  coupled to the one or more antennas  1516 , as described above. The control system  1502  is connected to the radio unit(s)  1510  via, for example, an optical cable or the like. The control system  1502  is connected to one or more processing nodes  1600  coupled to or included as part of a network(s)  1602  via the network interface  1508 . Each processing node  1600  includes one or more processors  1604  (e.g., CPUs, ASICs, FPGAs, and/or the like), memory  1606 , and a network interface  1608 . 
     In this example, functions  1610  of the radio access node  1500  described herein are implemented at the one or more processing nodes  1600  or distributed across the control system  1502  and the one or more processing nodes  1600  in any desired manner. In some particular embodiments, some or all of the functions  1610  of the radio access node  1500  described herein are implemented as virtual components executed by one or more virtual machines implemented in a virtual environment(s) hosted by the processing node(s)  1600 . As will be appreciated by one of ordinary skill in the art, additional signaling or communication between the processing node(s)  1600  and the control system  1502  is used in order to carry out at least some of the desired functions  1610 . Notably, in some embodiments, the control system  1502  may not be included, in which case the radio unit(s)  1510  communicate directly with the processing node(s)  1600  via an appropriate network interface(s). 
     In some embodiments, a computer program including instructions which, when executed by at least one processor, causes the at least one processor to carry out the functionality of radio access node  1500  or a node (e.g., a processing node  1600 ) implementing one or more of the functions  1610  of the radio access node  1500  in a virtual environment according to any of the embodiments described herein is provided. In some embodiments, a carrier comprising the aforementioned computer program product is provided. The carrier is one of an electronic signal, an optical signal, a radio signal, or a computer readable storage medium (e.g., a non-transitory computer readable medium such as memory). 
       FIG. 17  is a schematic block diagram of the radio access node  1500  according to some other embodiments of the present disclosure. The radio access node  1500  includes one or more modules  1700 , each of which is implemented in software. The module(s)  1700  provide the functionality of the radio access node  1500  described herein. This discussion is equally applicable to the processing node  1600  of  FIG. 16  where the modules  1700  may be implemented at one of the processing nodes  1600  or distributed across multiple processing nodes  1600  and/or distributed across the processing node(s)  1600  and the control system  1502 . 
     Any appropriate steps, methods, features, functions, or benefits disclosed herein may be performed through one or more functional units or modules of one or more virtual apparatuses. Each virtual apparatus may comprise a number of these functional units. These functional units may be implemented via processing circuitry, which may include one or more microprocessor or microcontrollers, as well as other digital hardware, which may include Digital Signal Processors (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as Read Only Memory (ROM), Random Access Memory (RAM), cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory includes program instructions for executing one or more telecommunications and/or data communications protocols as well as instructions for carrying out one or more of the techniques described herein. In some implementations, the processing circuitry may be used to cause the respective functional unit to perform corresponding functions according one or more embodiments of the present disclosure. 
     While processes in the figures may show a particular order of operations performed by certain embodiments of the present disclosure, it should be understood that such order is exemplary (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.). 
     At least some of the following abbreviations may be used in this disclosure. If there is an inconsistency between abbreviations, preference should be given to how it is used above. If listed multiple times below, the first listing should be preferred over any subsequent listing(s).
         5G Fifth Generation   AAS Advanced Antenna System   AE Antenna Element   AF Application Function   AMF Access and Mobility Management Function   AN Access Network   ASIC Application Specific Integrated Circuit   AUSF Authentication Server Function   CPU Central Processing Unit   DN Data Network   DSP Digital Signal Processor   eNB Enhanced or Evolved Node B   FPGA Field Programmable Gate Array   GHz Gigahertz   gNB New Radio Base Station   IF Intermediate Frequency   IP Internet Protocol   LO Local Oscillator   LTE Long Term Evolution   NEF Network Exposure Function   NF Network Function   NR New Radio   NRF Network Repository Function   NSSF Network Slice Selection Function   PAAM Phased Antenna Array Module   PCF Policy Control Function   QoS Quality of Service   RAM Random Access Memory   RAN Radio Access Network   RF Radio Frequency   RFIC Radio Frequency Integrated Circuit   ROM Read Only Memory   RRH Remote Radio Head   RTT Round Trip Time   SMF Session Management Function   UDM Unified Data Management   UE User Equipment       

     Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein.