Abstract:
A system and method for compensating phase differences between multiple local oscillators and group delay differences between multiple transceivers. The system may include; an antenna array; a plurality of transceivers connected to said antennas and operatively associated each with a local oscillator (LO), wherein at least some of the transceivers do not share a common LO, and wherein at least some of the LOs are using a common reference oscillator; a common digital beamformer circuit connected to the transceivers; a baseband processor configured to operate the system at a specified communication scheme; and a calibration circuit and software modules configured to eliminate or reduce mismatches and phase deviations between the different transceivers, wherein the calibration circuit and software modules are incorporated in system such that the elimination or reduction of mismatches and phase deviations is non-interrupting with a continuous operation of the system at the specified communication scheme.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit from U.S. provisional patent application Ser. No. 61/898,802 filed Nov. 1, 2013, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to wireless communication, and more particularly to local oscillators in multiple channel transceivers. 
     BACKGROUND OF THE INVENTION 
     Implementations of a multiple-antenna beamformer may require the use of multiple radio circuits that feed these antennas. Some implementations are based on an array of discrete mixers controlled by a common LO (Local Oscillator) where the number of radio circuits is unlimited, but cost, power consumption, real estate and reliability are not optimal. Some other implementations offer an integrated RFIC (Radio-Frequency-Integrated-Circuit) controlled by a common integrated LO, yet, the number of radios is typically to few limited combinations e.g. 2 or 4 or 8 integrated radio circuits in a commercially available RFICs. Hence when a larger number of radio circuits is required, staking an array of RFICs together may be a preferable approach, provided the LOs in each of the RFICs can be locked to each other via external circuitry and procedures. 
     Such an array of RFICs may maintain frequency lock when all LOs are using a common crystal reference, yet in some cases phase lock mechanism must be implemented each time the radios are retuned, as well on a periodic basis that will counter the effect of phase drift. 
       FIG. 1  depicts a beamforming TD-LTE base station according to the prior art which includes multiple radio circuits  120 - 1 ,  120 - 2 , up to  120 -M, that feed or provide input to an antenna array of M elements  110 - 1 ,  110 - 2 , up to  110 -M, where the multiple radio circuits and antennas are used for beamforming  FIG. 1  further illustrates an example where the aforementioned beamforming capability may be used for the implementation of Multi-User MIMO, where up to N data streams are simultaneously served (e.g. in data communication with) by N baseband entities  140 - 1 ,  140 - 2 , up to  140 -N, where N&lt;M.  FIG. 1  further depicts an example implementation using a common local oscillator  150  configured to feed or provide input to the multiple radio circuits, so that digital beamformer  130  weights applied to baseband signals, are routed through the multiple radio frequency (RF) chains with a wells controlled resultant phases. 
       FIG. 2  depicts another possible block diagram similar to the one in  FIG. 1 , also in accordance with the prior art, where the radio circuits used are integrated Radio-Frequency-Integrated-Circuits (RFICs)  220 - 1 ,  220 - 2 , up to  220 -M, and where the LO are implemented inside and fed by a common reference clock  250 ; however, such a solution guarantees frequency lock but does not provide phase lock across the array, which makes it unfit for beamforming. The problem stems from the fact that while LOs are frequency locked, their relative phases may drift. Therefore, for beam forming purposes, the architecture shown in  FIGS. 1 and 2  will not work properly. 
     SUMMARY OF THE INVENTION 
     In accordance with embodiments of the present invention, a system for compensating phase differences between multiple local oscillators is provided. The system may include for example: an antenna array; a plurality of transceivers connected to the antennas and operatively associated each with a local oscillator (LO), wherein at least some of the transceivers do not share a common LO (e.g. connected to the same LO), and wherein at least some of the LOs are using a common reference oscillator; a common digital beamformer circuit connected to the transceivers; a baseband processor configured to operate the system at a specified communication scheme; and a calibration circuit and software modules executed by the baseband processor and configured to eliminate or reduce mismatches and phase deviations between the different transceivers, wherein the calibration circuit and software modules are incorporated in system such that the elimination or reduction of mismatches and phase deviations is non-interrupting with a continuous operation of the system at the specified communication scheme. In some embodiments, reduction of mismatches, phase deviations, etc., may include elimination of these phenomena. 
     Embodiments of the present invention includes a phase calibration apparatus and procedure that guarantees phase lock across an array of multiple separated radios, and a method of performing such calibration in multiple radios systems that operate in Time-Domain-Duplex (TDD). 
     Embodiments of the present invention include a digital beamforming array, serving a TDD air-protocol Base Station or User Equipment, where an antenna array of N antennas is fed by an array of N radio circuits, which in turn are digitized and fed into a common digital beamformer entity. The radio circuits may include an array of integrated RFICs or modules where their LOs are hooked into a common crystal reference. The phase of each integrated radio which may be different, is calibrated on a system level via a disclosed phase correction auxiliary circuitry and calibration procedure. 
     Embodiments of a phase correction system and method may calibrate the array of down-converting radio circuits&#39; signals so that phase variations created at the receive circuitries are known before received digital beamforming is performed; it may also calibrate the array of up-converting radio circuits so that phase variation created in the transmit circuitries are known before transmitted digital beamforming is performed. 
     Acquiring knowledge of the receiving radio circuits&#39; phase variations may be done by injection of a common known pilot signal via an auxiliary up-converter to each of the radios inputs, where the pilot is originated at the common digital beamforming entity, and measuring the output of each radio circuits&#39; receivers by the common digital beamforming entity, which then compares the input and output phases, to derive receive radio circuitries phase variation knowledge. 
     Acquiring knowledge of the transmission radio circuits&#39; phase variations may be done by feeding each of the transmission radio circuitries&#39; input with a common known pilot signal originated at the common digital beamforming entity, and injecting the outputs of the transmission radio circuitries into input an auxiliary down-converter circuitry that is digitized and fed back into the common digital beamforming entity, which then compares the input and output phases, to derive transmit radio circuitries phase variation knowledge. 
     Embodiments of the present invention further provide a method for TDD systems, where the up-converters&#39; and down-converters&#39; phase variations calibration, is done in way that does not require extra spectrum or extra bandwidth, via using the time gap between TDD transmission and reception for the injection of the calibration signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be more fully understood by reference to the following detailed description when read with the accompanying drawings in which: 
         FIG. 1 : A TDD multi antenna base station using discrete radio implementation in accordance with the prior art; 
         FIG. 2 : A TDD base station with separate local oscillators using a common reference clock accordance with the prior art; 
         FIG. 3 : Injecting a common pilot into TDD base station&#39;s receivers array&#39;s inputs in accordance with embodiments of present invention; 
         FIG. 4 : An example of injecting a pilot into a TDD base station&#39;s receivers, via an auxiliary up converter circuitry in accordance with embodiments of present invention; 
         FIG. 5 : An example of feeding a pilot into TDD base station&#39;s Tx array and measuring the output signals in accordance with embodiments of present invention; 
         FIG. 6 : An example of a calibration signal&#39;s modulation in accordance with embodiments of present invention; 
         FIG. 7 : A TD-LTE frame structure accordance with embodiments of present invention; 
         FIG. 8 : An example of Rx Mode Phase Compensation Timing Example for TD-LTE in accordance with embodiments of present invention; 
         FIG. 9 : An example of Tx Mode Phase Compensation Timing for TD-LTE with M LO&#39;s in accordance with embodiments of present invention; and 
         FIG. 10 : An example of three-stage calibration for TD-LTE with M LO&#39;s and radios in accordance with embodiments of present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, various aspects of the present invention will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the present invention. 
     Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining,” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulates and/or transforms data represented as physical, such as electronic, quantities within the computing system&#39;s registers and/or memories into other data similarly represented as physical quantities within the computing system&#39;s memories, registers or other such information storage, transmission or display devices. 
       FIG. 3  depicts an upgraded block diagram, where the circuitry of  FIG. 2 , including all of its components, is augmented by an auxiliary Tx/Rx RFIC  320 -(M+1) which feeds or provides input to an RF switch  370 , and is further augmented by an array of RF switches  360 - 1 ,  360 - 2 , up to  360 -M. The auxiliary radio circuit may be used to inject a calibration pilot signal into the RFICs inputs, as well as to receive another calibration pilot signal from the RFICs outputs, so that phases variations created by each of the integrated LOs, together with other phase deflection contributions, are measured in both transmission and receptions paths. 
       FIG. 4  depicts an example of a TDD base station with two antennas  410 - 1 ,  410 - 2 . The corresponding two down-converter parts of the radios  430 - 1 ,  430 - 2  are connected to their respective antennas via switches  420 - 1 ,  420 - 2 , where the switches may also be implemented as couplers; an auxiliary up-converter  430 - 3  and its front end RF splitter  440  are connected to the switches or couplers; both the down-converters and the auxiliary up-converter are fed by a common reference clock  450 ; the down-converters feed or provide input to the common digital beamformer entity, which compares it with the pilot&#39;s phase via phase comparisons  460 - 1 ,  460 - 2 , calculates phase variations per subcarrier ( 460 - 5 ), and stores the results in a calibration lookup table  460 - 6 . 
       FIG. 5  depicts an example of a TDD base station with two antennas  510 - 1 ,  510 - 2 . The corresponding two up-converter parts of the radios  530 - 1 ,  530 - 2  are connected to their respective antennas via switches  520 - 1 ,  520 - 2 , where the switches may also be implemented as couplers. An auxiliary down-converter  530 - 3  and its front end RF switch  540  are connected to the switches or couplers. Both the up-converters and the auxiliary down-converter are fed by a common reference clock  550 ; note that the reference  550  is the same one depicted in  FIG. 4  as reference clock  450 ; the common digital beamforming entity injects a Tx calibration pilot  560 - 2  via a selector  560 - 1  into each of the up-converters part of the  530 - 1  and  530 - 2  radios in a sequential order, and each time the output of the up-converters is fed into the input of the auxiliary down-converter  530 - 3 , which feeds or provides input to the resultant digital signal into a phase comparator  560 - 5  that calculates the phase difference between each up-converter path and the pilot signal, per subcarrier ( 560 - 4 ), to be stored at the Tx calibration lookup table. Note:  FIG. 5  is applicable for an apparatus with up to 48 antennas, as explained with respect to  FIG. 9 . 
     According to some embodiments of the present invention, the calibration circuit and baseband processor executing software modules may include hardware and software, enabling the common digital beamformer to generate calibration pilot signals injected to the inputs of receiving parts of the transceivers, via a digital-to-analog converter and an additional RF up-converter, and further configured to determine phase and amplitude differences between the plurality of transceivers&#39; receivers, based on the digital output of the receivers across the bandwidth of the transceiver. The specified communication scheme may be Time-Domain-Duplex (TDD) exhibiting a time gap between transmit and receive 
     According to other embodiments of the present invention the calibration circuitry and software modules may be based on auxiliary hardware and auxiliary software, enabling the common digital beamformer entity to pick up its own downlink signal from each transmitting part of the transceivers, via a calibration RF down-converter and an analog-to-digital converter, and further configured to determine phase and amplitude differences between the plurality of transceivers&#39; transmitters, based on the digital output of the transmitters across the bandwidth of the transceiver, where the down-converter input may be sequentially switched between each of the transmitting part of the transceivers. In some embodiments, the calibration pilot may include a narrowband signal. 
     According to some embodiments of the present invention the baseband processor may be configured to avoid interruption of the operation of the system for calibration purposes, for example by using the transmit-receive time gap for switching the receivers array away from the antenna array, and connecting them to outputs of the calibration RF up-converter, and feeding the input of the up-converter with a calibration pilot signal generated by the common digital beamformer, for a partial part of the time gap, and further measuring the digitized output of the receivers array by the common digital beamformer. 
     According to some embodiments of the present invention the baseband processor may be further configured to avoid interruption of the operation of the system for calibration purposes by sequentially feeding the transmitting part of the transceivers via an RF splitter to the input of the calibration down-converter and measure the down-converter digitized output during the time gap between transmit and receive. 
     According to other embodiments of the present invention, the time gap between transmit and receive may be divided up into several fields, so that the first field is left unused for guard time purposes, then the next field may be used for receivers calibration, then the next one is used for calibration processing by the common digital beamformer, then the next one may be used to apply phase adjustment to RF or digital parts of the system, and then the last field is not used to allow for guard time before switching back to active mode is taking place. 
     According to other embodiments of the present invention, the down-link RF output of a given transceiver may be fed into the calibration RF down-converter for at least few μsecs before the transmit timeslot is ending, and after transmission may be turned off, the digital output of the down-converter may be captured and processed by the common digital beamformer, and phase adjustment may be subsequently applied. 
     According to other embodiments of the present invention, a selection of a specific time period for wideband calibration may be based on measurements of temperature fluctuation and current fluctuations at the power amplifiers array, and the setting of thresholds that will increase sampling rate per increased fluctuation magnitude. Alternatively, the selection of specific time period for narrow band calibration addressing the LO phase alignment, may be based on factory measurements (e.g. calibration process during production) that determine inter-transceiver phase uncertainty over time. 
       FIG. 6  describes several examples of pilot modulation: A) depicts a sinusoidal waveform, B) depicts a 64 QAM modulation. 
     Using a narrowband sine wave pilot tone may allow accurate phase measurement for a given subcarrier, and it may be sufficient to gain knowledge of the LO phase shift versus a reference. Phase comparison may be implemented via correlations or via FFT. 
     Using a broadband pilot like a 64 QAM modulation which occupies the entire bandwidth, provides also wideband calibration, addressing the non-flat transfer function of the various RF components. The correction value can be computed using the following calculation:
         Given an input signal S in , a measured output signal S out , and an unknown circuit transfer function T, then S out =T*S in .   For each subcarrier i by applying a fast Fourier transform (FFT) of above equation, it becomes s out (i)=t(i)*s in (i)   Phase of the transfer function {circumflex over (t)}t(i) for subcarrier i can be estimated by Zero Forcing, e.g., {circumflex over (t)}(i)=s out (i)/s in (i), or MMSE in frequency domain.       

       FIG. 7  depicts the structure of a TD-LTE air protocol frame. In one embodiment the frame can be for example a 5 ms or a 10 ms switch point periodicity, and in both there is a time gap labeled GP. Embodiments of the present invention disclose a system and a method to use the GP gap for phase calibration of both transmission and reception parts of the transceivers in a periodic regime. 
       FIG. 8  describes an example of using the TD-LTE time gap between transmissions of down and up links for calibration of the receiving parts of the radios array, illustrated for the case of a 10 ms switch point (as with other embodiments described herein, other specific parameters may be used): 
     The time gap is assumed to be 285 μsec. 
     Starting with a guard time e.g. 50 μsec. 
     Continue with a simultaneous measurement of all receiving parts of the radio array, e.g. over 100 μsec. 
     Process the measurements in the common digital beamforming entity e.g. over 30 μsec. 
     Apply the weights according to the beamforming calibration lookup table, for example, over 5 μsec. 
     Leave approximately 100 μsec for a guard time before switching takes place. 
       FIG. 9  describes an example of using the TD-LTE time gap between transmissions of down link and uplink for calibration of the transmitting parts of the radios array, illustrated for the case of a 10 ms switch point: 
     The transmission calibrations may alternate with the reception calibrations. 
     Measure via the auxiliary down-converter live transmission of data during subframe 0 which includes Sequential TX Measurement Using Normal DL Signal. 
     During the 1 ms time period of subframe 0, alternating through the M&lt;15 antennas so that each is allocated with at least one full symbol; Calculate Relative phase adjustments based on factory calibration table and then apply phase. It should be noted that in  FIG. 9  maximum of 10 antennas are assumed so each is allocated with 100 μsecs. In case the number of antennas is larger than 15, continue calibrating the next batch of antennas during the next gap. 
     As long as the number antennas M does not exceed 45, then 3 switching cycles cover the transmission circuitry calibration within 3×10 ms intervals while the receiving circuitry calibration may be done each interval. The 4th can be used for reception circuitry calibration, yielding a total of 40 ms which is under the 50 ms assumed max duration; 
     When M&gt;45, then the embodiment in the  FIG. 5  block diagram may require augmentation by an additional auxiliary down-converter, which allows calibrations of Tx RF circuitries in pair, and so on and so forth. 
       FIG. 10  Depicts three stages calibration for TDD multiple LOs and radios. 
     The calibration goals of the downlink circuitry may be to guarantee sufficient compliance of the RF system with the digital processing system, e.g. that signals received by multiple antennas and fed into the inputs of a multichannel RF system, will be transformed into digital signals without distortion of each signal and its inter-relations with other signals, or that such distortions will be made know to the digital system. 
     Similarly, the calibration goals of the uplink circuitry, may be to guarantee that digital signals fed into the inputs of multichannel RF systems, are transferred to the antennas inputs without distortion of each signal and its inter-relations with other signals. 
     In one embodiment, metrics for a sufficient calibration may be based on estimating the RF and digital systems combined capabilities to create a deep enough null, e.g. to guarantee a minimum null depth. 
     For example, in an 8 arm multichannel beamforming system, with calibration that eliminates amplitude variations, and provides phase uncertainty of 2 degree or less, a null depth can be calculated as for example 20*log 10(1/57.30)=−35 dB. 
     The factors that govern RF circuitry phase uncertainty are temperature drifts, power supply voltage fluctuations, and loading; such variations may be slow or fast, e.g. may require calibration frequency of once per second or 20 times per second. 
     The factors that govern LOs coherency across the various RF channels are phase noise and LO frequency re-tuning rate. The fastest change may occur in the latter case every frame, therefore calibration must take place at the switching gap between transmit and receive. 
     Referring to  FIG. 10 , operation  1001  outlines a first stage calibration performed after initial installation, and later on at periodical maintenance. This stage is based on over-the-air transmission of one of the base station&#39;s antennas towards the others, in a round robin sequence, measuring wideband antennas coupling and mismatch, Power Amplifiers wideband non-linearity, and RF circuitry misalignments. 
     Operation  1002  outlines a second operation calibration where the wideband calibration, for both the receive circuitry and transmitting circuitry, takes place, every T2 millisecond, e.g. 50-1,000 milliseconds, where the specific T2 value may be calculated based on continuous sensing of Power Amplifier&#39;s (PA&#39;s) temperature and nonlinearity versus temperature factory measurements of the PAs, and further, on continuous measurement for current fluctuation thru the PAs current fluctuation and nonlinearity versus current factory measurements of the PAs. 
     Operation  1003  outlines the third stage of calibration, for the receive circuitry only, where narrowband calibration takes place during every frame&#39;s gap T1 of for example 5 or 10 milliseconds. Operation  1004  describes the application of calibration data derived from above three stages to RX channel for calculating reciprocal TX channel. Operation  1005 , and operation  1006  describe the T1 and T2 counters. 
     Advantageously, embodiments of the present invention may be implemented as a part of a base station or a subscriber unit. In the above description, an embodiment is an example or implementation of the inventions. The various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
     Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. 
     Reference in the specification to “some embodiments”, “an embodiment”, “one embodiment” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions. It will further be recognized that the aspects of the invention described hereinabove may be combined or otherwise coexist in embodiments of the invention. 
     The principles and uses of the teachings of the present invention may be better understood with reference to the accompanying description, figures and examples. 
     It is to be understood that the details set forth herein do not construe a limitation to an application of the invention. 
     Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. 
     It is to be understood that the terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, or integers or groups thereof and that the terms are to be construed as specifying components, features, steps or integers. 
     If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. 
     Where applicable, although state diagrams, flow diagrams or both may be used to describe embodiments, the invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. The descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only. 
     Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. 
     The present invention may be implemented in the testing or practice with methods and materials equivalent or similar to those described herein. While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.