The present application is directed to systems and methods for calibrating one or more antennas of a massive multiple-input multiple-output (MIMO) radio unit. More particularly, the present application is directed to systems and methods of reducing noise when calibrating the one or more antennas of a massive MIMO radio unit.
Massive MIMO technology is recognized as a solution for the limited capacity of radio networks. However, transceiver lines of massive MIMO units may need to be calibrated frequently, as the transmitter and receiver lines may need to be synchronized for effective beamforming.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to limit the scope of the claimed subject matter. The foregoing needs are met, to a great extent, by the exemplary embodiments of the present application described in more detail below.
According to one or more exemplary embodiments of the application, the antennas of a radio unit (e.g., a MIMO radio unit) may be calibrated for beamforming. To calibrate the antennas, the antennas may transmit calibration signals. A combiner of the radio unit may rotate the calibration signals to phases. The phases may be phases that may cause non-coherent combining of noise of the calibration signals in an instance in which the calibration signals are combined. The combiner may combine the calibration signals. Based on the combined calibration signals, the antennas may be calibrated.
According to an example embodiment of the application, a method may comprise rotating a plurality of calibration signals received from a plurality of antennas of a radio unit to obtain different phases, associated with the plurality of calibration signals, configured to cause non-coherent combining of noise of the plurality of calibration signals. The method may comprise combining the plurality calibration signals comprising the non-coherent combined noise. The method may comprise calibrating at least one antenna of the plurality of antennas based on the combined plurality of calibration signals.
According to an example embodiment of the application, a radio unit may comprise a plurality of antennas and at least one combiner. The combiner may be configured to receive a plurality of calibration signals transmitted from the plurality of antennas. The combiner may be configured to rotate the plurality of calibration signals to obtain different phases, associated with the plurality of calibration signals, to cause non-coherent combining of noise of the plurality of calibration signals. The combiner may be configured to combine the plurality of calibration signals comprising the non-coherent combined noise. The radio unit may calibrate at least one antenna of the plurality of antennas based on the combined plurality of calibration signals.
According to an example embodiment of the application, a combiner may be configured to rotate a plurality of calibration signals received from a plurality of antennas of a radio unit to obtain different phases, associated with the plurality of calibration signals, configured to cause non-coherent combining of noise of the plurality of calibration signals. The combiner may be configured to combine the plurality calibration signals comprising the non-coherent combined noise. The combiner may calibrate at least one antenna of the plurality of antennas based on the combined plurality of calibration signals.
There has thus been outlined, rather broadly, certain embodiments in order that the detailed description thereof may be better understood, and in order that the present contribution to the art may be better appreciated.
In order to facilitate a more robust understanding of the application, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed to limit the application and are intended only to be illustrative.
A detailed description of the illustrative embodiment is discussed in reference to various figures, embodiments, and aspects herein. Although this description provides detailed examples of possible implementations, it should be understood that the details are intended to be examples and thus do not limit the scope of the application.
Reference in this specification to “one embodiment,” “an embodiment,” “one or more embodiments,” “exemplary embodiments,” “an aspect” or the like may mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Moreover, the term “embodiment” in various places in the specification is not necessarily referring to the same embodiment. That is, various features are described which may be exhibited by some embodiments and not by the other embodiments.
Generally, the present application describes exemplary embodiments having improved methods of calibrating massive MIMO radio unit antenna(s). Existing calibration methods may suffer from reduced accuracy as a result of the coherent combining of noise from calibration signals. The described methods of the exemplary embodiments address this shortcoming by rotating calibration signals to phases such that noise combines non-coherently, allowing for more accurate calibration of one or more antennas (e.g., MIMO antennas).
According to the present application, it is understood that any or all of the systems, methods and processes described herein may be embodied in the form of computer executable instructions, e.g., program code, stored on a computer-readable storage medium which instructions, when executed by a machine, such as a computer, server, transit device or the like, perform and/or implement the systems, methods and processes described herein. Specifically, any of the steps, operations or functions described herein may be implemented in the form of such computer executable instructions. Computer readable storage media may include volatile and nonvolatile, removable, and non-removable media implemented in any method or technology for storage of information, but such computer readable storage media does not include signals. Computer readable storage media may include, but are not limited to, random access memory (RAM), read only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technology, compact disc read only memory (CD ROM), digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other physical medium which may be used to store the desired information and which may be accessed by a computer.
MIMO technology is designed to multiply a radio link's capacity through the use of multiple transmitting and receiving antennas. Using the multiple antennas and multi-path propagation, more than one data signal may be sent and/or received simultaneously over the same radio channel. MIMO may be used in Institute of Electrical and Electronics Engineers (IEEE) 802.11ac (W-Fi), IEEE 802.11n (W-Fi), Worldwide Interoperability for Microwave Access (WiMAX), High Speed Packet Access Plus, Third Generation (HSPA+) (3G), Fourth generation (4G) Long Term Evolution (4G LTE), and Fifth generation (5G) Long Term Evolution (5G LTE) and other communication technologies. MIMO may also be used for power line communication for 3 wire deployments, as part of HomePlug AV2 specification and International Telecommunication Union (ITU) G.hn standard.
Massive MIMO is an extension of the MIMO technique, with better spectrum efficiency and increased throughput by combining receiver and transmitter antennas. Massive MIMO antennas are capable of augmenting system capacity, improving throughput, enhancing spectral efficiency, increasing resistance, and reducing fading. Massive MIMO technology is used for both mobile devices and base stations.
Beamforming is a signal processing technique for directional signal transmission and/or reception.
For example,
As another example, radio unit 103 is shown having 4 antennas that are broadcasting with a 0.5 wavelength separation and with 90 degrees phase shift per antenna. Radio unit 103 has constructive interference (e.g., lobes) at −30 degrees, 30 degrees, and 50 degrees from the 0 degree azimuth. Radio unit 103 has destructive interference in other directions, such as along the 0 degree azimuth, 30 degrees, −90 degrees, and 90 degrees from the 0 degree azimuth.
Signals from different transmitters may be amplified by different weights. For example, a transmitter pointing in the intended direction may be amplified by a higher weight than another transmitter of a radio unit pointing in another direction. When receiving signals, information from different antennas may be combined in a way that the expected pattern of radiation is preferentially observed.
Massive MIMO radio unit transceiver lines may need to be calibrated frequently as the transmitter and/or receiver lines may need to be synchronized (e.g., having same phase and magnitude) for effective beamforming. Otherwise, there may be an error in the direction of the transmission. Synchronization also may improve beamforming gain, prevent sidelobes, and/or improve beamforming performance. Calibrating may involve the antennas transmitting calibration signals, receiving and/or combining the calibration signals in a particular direction, observing the strength of the combined calibration signal, and adjusting the phase and/or magnitude on which the antennas are transmitting to direct the signals in a desired direction and may minimize side lobes. A side lobe may be an unintended transmission caused by at least a portion of a transmitted signal not going toward the intended direction. Existing approaches of calibration that utilize combiners may have the effect of combining noise from calibration signals coherently. The coherent combining of the noise, in these existing approaches, may increase the noise floor, which may reduce accuracy of the calibration as it may lower the signal to noise ratio, making it difficult to determine the phase and magnitude of the combined calibration signal.
The exemplary embodiments of the present application enables calibration of antennas of massive MIMO radio units with accuracy higher than traditional/existing methods. The accuracy may be achieved by reducing noise of the calibration signals by using transmission feedback signals (e.g., calibration signals) with different phases so that noise may be combined non-coherently. Non-coherent combining of noise may be combining of noise from different sources or non-correlated sources.
The combined signal may be transmitted to a circulator 204, which may redirect the Tx1 and Rx1 signals to their corresponding branches (e.g., Tx1 to PA 202 and Rx1 to LNA 210 and PA 202) and control the direction of the flow of the signal. The circulator 204 may send the combined signal to a duplexer/filter 205. The duplexer/filter 205 may enable antenna sharing between the transmit and/or receive paths of the radio unit, such as by isolating the receive path from interference caused by the transmit signal path and suppressing out-of-band signals on both paths.
The duplexer/filter 205 may send the combined signal to a DPD coupler 206. The DPD coupler 206 may combine the combined signal with signals from other antenna(s) of the radio unit. The DPD coupler 206 may send the combined signal to a calibration transmitter (TxCal) 207. Although
The TDD TRx line 200 may include the receiver Rx1. The receiver Rx1 may be part of an antenna(s). The receiver Rx1 and the transmitter Tx1 may be components of the same antenna(s). The receiver Rx1 may be a dedicated receiver.
The circulator 204 may send the combined signal to a single pole double throw (SPDT) switch 208 of the receiver Rx1. The circulator 204 may receive signals from antenna and direct towards LNA 210 and/or Rx1. The circulator 204 may send the combined signal to the SPDT switch 208 and the duplexer/filter 205 simultaneously.
The SPDT switch 208 may be connected to a resistor and a ground point 209. The SPDT switch 208 may send the combined signal to a low noise amplifier (LNA) 210. The LNA 210 may amplify the combined signal. The LNA 210 may send the amplified combined signal to a duplexer/filter 211. The duplexer/filter 211 may send the signal to an analog-to-digital (ADC) convertor, such as for digitization and/or to a Baseband processing unit.
The calibration coupler 306a may send the combined signal to a transmitter calibration combiner 313. The transmitter calibration combiner 313 may also receive a signal from another transceiver TRx2. TRx2 may be of a different antenna than TRx1 and/or antenna 312a. Tx2 may include a calibration coupler r 306b that receives a signal from an antenna 312b. The signal from the antenna 312b may be combined with the signal from Tx2 at a calibration coupler 306b. The combined signal may be sent from the calibration coupler 306b to the transmitter calibration combiner 313.
The transmitter calibration combiner 313 may collect and combine the signal from TRx1, the signal from TRx2, and/or the signal from one or more other transceivers TRxn, such as of a radio unit. The noise from the different transceivers TRx1, TRx2, and/or TRxn may combine coherently when combined by the transmitter calibration combiner 313. The noise may combine coherently because phase noise of a local oscillator (LO) may come/originate from a same source. When coherently combined, the noise may be enhanced by a factor of 20 log 10(Ntrx), where Ntrx is the number of transceivers and/or antennas of a radio unit.
The transmitter calibration combiner 313 may send the combined signal to an ADC unit 314. The ADC unit 314 may make a digitized version of the combined signal. The ADC unit 314 may send an indication of the received signal to another device or component of the radio unit (e.g., radio unit 1110 of
Because of the enhanced noise, the device or component may not be able to calibrate the radio unit very accurately. For example, the calibration may be limited to accuracy of a few degrees for phase accuracy.
There are two main approaches to antenna calibration according to the exemplary embodiments.
Even though some of the transmission lines Tx1, Tx2, TxN may not be calibrated or transmitting on the calibration resource blocks 414, they may create noise 416 in the calibration resource blocks. The noise 416 may be the result of LO noise or other leaked noise. The transmissions may be combined by a calibration combiner 406 (e.g., DPD coupler 206 in
When calibrating using partial bandwidth, it may not be possible to turn off power amplifiers (PA)s. When calibrating using partial bandwidth, the power amplifiers (PA)s on the antennae that may not be calibrated may be turned off to avoid LO leakage. Yet, noise may leak from PA's that are being calibrated simultaneously and which may be unable to be turned off.
The signals may be combined by a calibration combiner 506 (e.g., DPD coupler 206 in
As an illustrative example, resource blocks 1-10 may be assigned for calibration of transmission antenna 1 (TxAnt1) of a radio unit. All other antennas of the radio unit may be using resource blocks 11-100 to transmit normal traffic (e.g., data signals). As such, in this example only TxAnt1 may be transmitting on resource blocks 1-10 (e.g., transmitting a calibration training sequence), while the other antennas may be silent resource blocks 1-10. Although the other antennas may be logically silent, these antennas may physically inject noise in the calibration resource blocks 1-10.
The noise may be represented by an error vector magnitude (EVM). The EVM may represent an in-band noise floor. The EVM may be set to 2.5% (e.g., −32 dBc, in which dBc denotes decibels relative to a carrier). The radio unit may have 64 antennas. The noise floor created by a calibration combiner may be added to the calibration training sequence. The EVM noise may mostly be from phase noise (generated by LO's), clipping noise (generated by a BB unit), and/or coherent combination of LO phase noise/clipping noise. Therefore, using the noise enhancement formula of 20 log 10(Ntrx), the noise may be anywhere in the range of 10 log 10(64)=18 dBc to 20 log 10(64)=36 dBc. This level of noise may result in degrading or flooding of the training sequence (e.g., the calibration training sequence). With non-coherent combining or independent clipping noise, the noise floor may be raised from −32 dBc to −14 dBc, derived from −32 dBc −18 dBc (where 18 dBc is a positive value). If the EVM is set to −42 dBc, coherent combining may raise the floor as high as −42 dBc+36 dBc=−6 dBc (EVM+high end of range). A strong training sequence may be needed to lower the noise level. A strong training sequence may be one that has the ability to archive processing gain in a BB unit. Another problem associated with traditional calibration techniques is that data from other data resource blocks of all antenna may be combined via a combiner to process the data by one reference receiver and such that the data has a similar reference calibration path. The combined data may consume the dynamic range of an observation receiver (e.g., ORx in
The phases of the signals may be determined by calculating phases that when summed may yield a noise level of 0. The phase of each input port/output port pair (e.g., transfer function) of the radio unit may be described as 6i, where i is a number from 1 to N, where N is the number antenna/transceivers. For example, as shown in
Alternatively, the first stage combiner 716 may split the input port/output port pairs of the radio unit into signals having differences of 0 degrees, 90 degrees, 180 degrees, or 270 degrees. The second stage combiners 717 may split the signals from the first stage combiner 716 into 1:N/4 signals. The phases of the transfer functions may be jumbled or shifted by a constant value. The phases of the transfer functions may be jumbled or shifted by a spread phase unit (e.g., spread phase unit 716 in
A similar method may be performed for the ports of a first stage combiner, or M ports. The first stage combiner 716 may split each transfer function into signals having phase differences of 0,
The second stage combiners 717 may split the signals from the first stage combiner 716 into N/M signals. M (the number of ports) may be set equal to N (the number of antennas/transceivers). The second stage combiner 717 may not be used.
At step 1002, a plurality of antennas may transmit calibration signals. The calibration signals may be transmitted using all bandwidth available to the radio unit (e.g., antenna calibration using full bandwidth). The calibration signals may be transmitted using a portion of bandwidth available to the radio unit (e.g., antenna calibration using partial bandwidth). Other portions of the bandwidth may be used for normal traffic (e.g., data traffic).
At step 1004, a plurality of phases may be determined. The plurality of phases may be phases associated with the calibration signals. The plurality of phases may be determined by a combiner of the radio unit. The combiner may be DPD coupler 206, calibration coupler 306a, calibration coupler 306b, first stage combiners 816 or any other suitable combiner described herein. The plurality of phases may be determined by a processor (e.g., controller 1120 of
At step 1006, the plurality of calibration signals may be rotated to the determined phases. The signals may be rotated by the combiner (e.g., of the radio unit). Rotating the signals may include causing the signals to propagate at different angles. The angles may differ by 360/m, where m is the number of calibration signals and/or the number of antennas being calibrated. The angles may differ by 90 degrees.
Rotating the calibration signals may comprise splitting each calibration signal into two or more signals having different phases. Rotating the signals may include splitting the split calibration signals again (e.g., into two or more other signals). Splitting the calibration signals again may allow for a faster calibration process.
At step 1008, the rotated calibration signals may be combined. The signals may be combined by the combiner. The combiner may be DPD coupler 206, calibration coupler 306a, calibration coupler 306b, first stage couplers 816. The signals may be combined by a secondary combiner of the radio unit. The secondary combiner may be second stage combiner 817. When the signals are combined, the noise may combine non-coherently. The noise that may be combined non-coherently may be cancelled by the radio unit. Summation of coherent noise may cause subtraction of the noise when it is combined. For example, noise may have a phase of 0 and a magnitude of 1 when entering a 1:2 spreading phase combiner. The combiner may change/rotate the phase of one transmission to −180, which may convert the magnitude to −1. When noise from various transmission is combined, the magnitudes of 1 and −1 may be added, equaling 0 and cancelling out the noise.
Optionally, at step 1010, the antennas may be calibrated. The antennas may be calibrated based on the combined signals. Due to the noise cancellation, the calibration may be more accurate than with traditional/existing methods/techniques.
The transmitter 1116 may be configured to transmit signals (e.g., signals weighted from application of beamforming weights) to corresponding ones of the plurality of antennas 1112(1)-1112(M) for transmission. The transmitter 1116 may include individual transmitter circuits that provide signals to corresponding ones of a plurality of antennas 1112(1)-1112(M) for transmission.
The controller 1120 may be configured to provide data to the modem 1118 to be transmitted. The controller 1120 may process data recovered by the modem 1118 from received signals. The controller 1120 may perform other transmit and/or receive control functionality. In some exemplary embodiments, there may be analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) in the various signal paths to convert between analog and digital signals.
The memory 1122 may store data used for the techniques described herein. The memory 1122 may be separate or part of the controller 1120. In addition, instructions for noise enhancement solution logic module 1123 (also referred to herein noise enhancement solution logic 1123 or process logic 1123) may be stored in the memory 1122 for execution by the controller 1120. The controller 1120 may supply the phases, described herein, to the modem 1118 and the modem 1118 may rotate the calibration signals, described herein, before they are sent to the transmitter 1116 for transmission by corresponding ones of the plurality of antennas 1112(1)-1112(M).
The memory 1122 may be a memory device that may include read only memory (ROM), random access memory (RAM), magnetic disk storage media devices, optical storage media devices, flash memory devices, electrical, optical, or other physical/tangible (e.g., non-transitory) memory storage devices. The controller 1120 may be, for example, a microprocessor or microcontroller that executes instructions for the process noise enhancement solution logic 1123 stored in memory 1122. Thus, in general, the memory 1122 may include one or more computer readable storage media (e.g., a memory device) encoded with software including computer executable instructions and when the software is executed (by the controller 1120) it is operable to perform the operations described herein in connection with process logic 1123.
The functions of the controller 1120 may be implemented by logic 1123 encoded in one or more tangible media (e.g., embedded logic such as an application specific integrated circuit, digital signal processor instructions, software that is executed by a processor, etc.), wherein the memory 1122 may store data used for the computations described herein (and/or to store software or processor instructions that are executed to carry out the computations described herein). Thus, the process logic 1123 may be implemented with fixed logic or programmable logic (e.g., software/computer instructions executed by a processor) and the controller 1120 may be a programmable processor, programmable digital logic (e.g., field programmable gate array) or an application specific integrated circuit (ASIC) that includes fixed digital logic, or a combination thereof. Some or all of the controller functions described herein, such as those in connection with the process logic 1123, may be implemented in the modem 1118.
While the systems and methods have been described in terms of what are presently considered to be specific aspects, the application need not be limited to the disclosed aspects. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. The present disclosure includes any and all aspects of the following claims.