Abstract:
A system and method of maintaining performance of an RF Distribution Network that combines signals (phases and amplitudes) passing through components that are subjected to parameters variations due to aging and temperature is disclosed; resultant distortion is reduced or eliminated via a real time calibration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 13/630,146 filed on Sep. 28, 2012, which in turn claims the benefit of U.S. Provisional Patent Application No. 61/652,743, filed on May 29, 2012, U.S. Provisional Patent Application No. 61/657,999, filed on Jun. 11, 2012 and U.S. Provisional Patent Application No. 61/665,592, filed on Jun. 28, 2012; this application is a continuation-in-part application of U.S. patent application Ser. No. 13/776,068 filed on Feb. 25, 2013, which is a continuation-in-part application of U.S. patent application Ser. No. 13/630,146 filed on Sep. 28, 2012, which in turn claims the benefit of U.S. Provisional Patent Application No. 61/652,743, filed on May 29, 2012, U.S. Provisional Patent Application No. 61/657,999, filed on Jun. 11, 2012 and U.S. Provisional Patent Application No. 61/665,592, filed on Jun. 28, 2012; U.S. patent application Ser. No. 13/776,068 further claims benefit from U.S. Provisional Patent Application No. 61/658,015 filed on Jun. 11, 2012, U.S. Provisional Patent Application No. 61/658,009 filed on Jun. 11, 2012, U.S. Provisional Patent Application No. 61/665,600 filed on Jun. 28, 2012 and U.S. Provisional Patent Application No. 61/671,417 filed on Jul. 13, 2012, all of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of radio frequency (RF) multiple-input-multiple-output (MIMO) systems and in particular to systems and methods for enhanced performance of RF MIMO systems using RF beamforming and/or digital signal processing. 
     BACKGROUND OF THE INVENTION 
     Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter. 
     The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity. 
     The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. 
     The term “beamformer” as used herein refers to RF circuitry that implements beamforming and usually includes a combiner and may further include switches, controllable phase shifters, and in some cases amplifiers and/or attenuators. 
     The term “Receiving Radio Distribution Network” or “Rx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above. 
     The term “hybrid MIMO RDN” as used herein is defined as a MIMO system that employs two or more antennas per channel (N is the number of channels and K is the total number of antennas and K&gt;N). This architecture employs a beamformer for each channel so that two or more antennas are combined for each radio circuit that is connected to each one of the channels. 
     In hybrid MIMO RDN receiving systems, when the phases of the received signals from each antenna are properly adjusted or tuned with respect to one another, the individual signals may be combined and result in an improved SNR for the receiving system. 
     SUMMARY 
       FIG. 2  shows an implementation whereby each antenna of an antenna array  201  as shown in  FIG. 1  is enhanced by adding a means to adjust the weight of its signal (amplitude and phase)  202  prior to applying it to a passive RF combiner  203 . In one embodiment the phase and amplitude of each antenna&#39;s signal is measured by the radio and baseband  204 , which performs a “channel estimation” function of the received signal. 
     The measurements allow the system to determine in advance the phase and optionally the amplitude of each signal prior to applying it to  203  in order to accomplish constructive signal combining To accomplish this effectively, it is important the system predicts the consequences of each setting. For this reason a calibration of phase and amplitude for all operational settings and for all operating conditions such as frequency and temperature is required. This invention describes the design and methods to provide a unit calibration to provide values to be used during operation to adjust the settings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and in order to show how it may be implemented, references are made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections. In the accompanying drawings: 
         FIG. 1  is a high level illustration of a simple MIMO receiving system with antenna array and RDN according to some embodiments of the invention; 
         FIG. 2  is an implementation for one embodiment of the system allowing setting of the weight of the signal from each antenna and channel estimation according to some embodiments of the invention; 
         FIG. 3  is a high level illustration of one embodiment allowing calibration of the gain from each antenna using a test generator and channel estimation. It also presents an alternative approach that replaces the test signal generator and channel estimator with a network analyzer according to some embodiments of the invention; 
         FIG. 4  is diagram of circuitry allowing calibration of the gain from each antenna separately according to some embodiments of the invention; 
         FIG. 5  is an implementation for one embodiment of the system allowing setting of the weight of the signal from each antenna and providing operational bypass of the RF combiner for channel estimation according to some embodiments of the invention; 
         FIG. 6  is a flowchart showing a method for calibration using the embodiment of  FIG. 2  according to some embodiments of the invention; 
         FIG. 7  is a flowchart showing a method for calibration using the embodiment of  FIG. 5  according to some embodiments of the invention; and 
         FIG. 8  is an implementation for one embodiment of the system allowing setting of the weight of the signal from each antenna and providing operational bypass of the RF combiner for channel estimation employing an RF signal combiner with a reconfigurable combining ratio according to some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With specific reference now to the drawings in detail, it is stressed that the particulars shown are for the purpose of example and solely for discussing the preferred embodiments of the present invention, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description taken with the drawings makes apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
     Before explaining the embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following descriptions or illustrated in the drawings. The invention is applicable to other embodiments and may be practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
       FIG. 1  is a high level block diagram illustrating a system according to embodiments of the present invention. A MIMO receiving system set in a hybrid MIMO RDN configuration is shown. In the hybrid MIMO RDN configuration, baseband module  110  receives N branches and is configured to operate, on the baseband level, in accordance with any known or legacy MIMO receiving scheme. The system includes a radio distribution network  130  (RDN) connected to baseband module  110  via radio circuits  20 - 1  to  20 -N. RDN  130  includes at least one beamformer such as  120 - 1  and  120 -N, being fed by two or more antennas such as  50 - 1 - 1  to  50 -N-K N , so that a total number of antennas in the system is greater than the number of the channels. Baseband module  110  further includes digital signal processing (DSP) modems  112  and an RF signal processing and control module  114  and a memory  116 , wherein control module  114  is configured to tune RDN  130  and also to calibrate the beamformers as will be explained in detail below. 
       FIG. 2  shows an implementation whereby each antenna of an antenna array  201  of the RDN beamformer system of  FIG. 1  (only one beamformer is shown in  FIG. 2 ) is enhanced by adding the ability to adjust the weight of its signal (amplitude and phase)  202  prior to applying it to a passive RF combiner  203 . In one embodiment the phase and amplitude of each antenna&#39;s signal is measured by the radio and baseband  204 , which performs a “channel estimation” function of the received signal. 
     The measurements allow the system to determine in advance the phase and optionally the amplitude of each signal prior to applying it to  203  in order to accomplish constructive signal combining To accomplish this effectively, it is important the system predicts the consequences of each setting. For this reason a calibration of phase and amplitude for all operational settings and for all operating conditions such as frequency and temperature is required. This invention describes the design and methods to provide a unit calibration to provide values to be used during operation to adjust the settings. 
     The system, as illustrated in  FIGS. 2 ,  5  and  8  that are explained in detail below, comprises a multiple-input-multiple-output (MIMO) receiving system comprising a MIMO baseband module having N branches (only one branch shown in  FIGS. 2 ,  5  and  8 ) or inputs associated with radio channels ( 20 - 1  to  20 -N as shown in  FIG. 1 ) and a radio distribution network (RDN) connected to the MIMO receiving system. 
     The RDN which includes a beamformer may be fed by two or more antennas, so that a total number of antennas in the system is K, wherein K is greater than N. 
     The beamformer may include at least a combiner configured to combine signals coming from the antennas into a combined signal. 
     The diagram of  FIG. 2  shows the elements of an RDN beamforming system. In operation the phases and (optionally) amplitudes of the signals from the antennas in array  201  are adjusted to ensure optimum alignment using the weighting adjustments provided by circuits  202 . Factors that affect the ability to achieve the alignments may include:
         Accuracy of the phase and amplitude adjustment settings of the weighting circuits  202     Variation of the phase and amplitude versus operational conditions of frequency and temperature   Variation of phase when adjusting amplitude and variation of signal amplitude when adjusting phase   Phase shift and loss differences through RF combiner  203  experienced by the antenna signals. This can be incidental due to combiner tolerances or difference in trace lengths or intentional by design (e.g., quadrature combiners introduce a 90 degree phase shift for some paths.)
 
Inspection of  FIG. 2  shows we can measure the end-to-end gain by applying a signal to each input separately (Input 1  through Input k ) and measure and record its change in phase and amplitude. The equipment to do this is shown in  FIG. 3 . The RF test signal from the Signal Generator  301  is routed through the Test RF Circuits  400 . As shown in the embodiment of  FIG. 4 , the signal can be applied individually to each input (Input 1  through Input k ). The flow chart of  FIG. 6  outlines the steps to employ the embodiment shown in  FIG. 3  using the internal channel estimation circuits to measure the voltage (amplitude and phase) for each signal path.
       

     Often, during operation, it is desirable to bypass the combiner when performing channel estimation of the signals received by each antenna. This is because the combiner  203  introduces a combining loss of 1/k to each input signal when the others are not present. For this reason the circuit of  FIG. 5  or similar have been proposed. Single pole switches  508  and multi-throw switch  509  are added to route each signal in order to bypass the combiner  503 . When the signals are bypassed (switch position B), the measured phase and amplitude by the channel estimation will be different than when the signals are routed through the combiner. For this reason the calibration tables generated must take these differences into account in order for “bypass” measurements to be applicable to setting the weights when operating in the “non-bypassed” configuration. 
     The flowchart of  FIG. 6  describes for one embodiment the steps to be followed to generate the calibration tables when using a test configuration as shown in  FIG. 3 . In this figure we see a baseline is established with the weight settings of  202  at maximum gain and the “zero” phase setting. Since the gain for each of the k antenna paths is measured under these conditions, a comparison of these measurements shows what offsets must be applied in order to align the signals to add constructively in the combiner  203 . Subsequent measurements determine the accuracy of the gain and amplitude weight settings and the interaction between the settings (i.e., the phase shift variation for different gain settings and the amplitude (gain) variations when adjusting the phase). 
     According to the flowchart of  FIG. 6 , the method may include operations such as configuring equipment as shown in  FIG. 3  and setting frequency and level of Test Generator (or alternatively the Network Analyzer as required and Test RF Circuits for Output to Input  610 ; setting the gain of the RDN to maximum and Phase to zero degrees  620 ; measuring and recording the voltage and phase at the Output port using the Internal Channel Estimator or alternatively the Network Analyzer  630 ; adjusting the gain of the RDN from maximum to minimum with Phase at zero degrees and measuring and recording for each gain setting (or a selected subset) the voltage and phase at the Output port as before  640 ; adjusting the gain of the RDN to maximum and for each possible phase setting measuring and recording the voltage and phase at the Output port as before  650 ; exercising all switches and measuring amplitude and phase of each antenna at each state  660 ; repeating for other measurement conditions of frequency and temperature  670 ; and loading calibration data in UE  680 . 
     The flowchart of  FIG. 7  describes the same factors for the circuit of  FIG. 5  that  FIG. 6  does for the circuit of  FIG. 2  with the addition of a measurement with the combiner  503  bypassed. This allows the system to perform channel estimation on received signals with the advantage of bypassing the combiner and to apply the proper settings operationally without the combiner bypassed. 
       FIG. 8  describes an embodiment of the invention for which the calibration procedure described in  FIG. 7  is also applicable. The structure of RDN  800  may include antennas  801  which are fed into variable gain amplifiers and phase shifters  802  and then to splitters  808  from which some of the signals are combined by combiner  803  to be later combined with other outputs of splitters  808  in a combiner  809 , exhibiting a total gain  807 , and into a channel estimator  804 . In operation the combiner coupling is adjusted to accommodate a different number of antennas to be combined to avoid combiner losses such as  805  and  806  as described in U.S. patent application Ser. No. 13/776,068. 
     In general each combiner configuration introduces a different coupling ratio and phase shift to its input signals. This means the calibration procedure described in  FIG. 7  must be repeated for each combiner configuration to be employed during unit operation. 
     According to some embodiments, the RDN circuits of  500  in  FIGS. 5 and 800  in  FIG. 8  may be contained within a module such as an integrated circuit (IC). In this case the calibration configuration as shown in  FIG. 3  and the processes outlined in  FIG. 6  and  FIG. 7  may be employed during module manufacture. 
     In one embodiment the calibration values may be stored within module in nonvolatile memory  116  to be retrieved when the RDN module is installed in user equipment (UE). A UE may be referred herein in a non-limiting manner as any device used directly by an end-user to communicate. A UE can be a hand-held telephone, a laptop computer equipped with a mobile broadband adapter, or any other suitable device. It is obvious the size of memory needed will depend on the RDN configuration and precision required. 
     In one embodiment the RDN of  FIG. 5  will provide five phase shifters each with four phase shifts of ninety degree increments. It would be possible to measure the phase shift provided by each phase setting with a resolution of thirty degrees. In this case the calibration would be required to measure twenty values of four bits each plus one word for the combiner bypass routing for each phase shifter and twice that total for two RDN&#39;s supporting MRC or 2×2 MIMO. In general the size of the memory required will depend on the module configuration and precision required as well as sufficient memory to store coefficients as necessary to characterize the module performance over operational conditions such as frequency and temperature. 
     In another embodiment calibration is performed in real time over the air during normal system operation using channel estimation. The need for such a procedure stems out of manufacturing limitations as well as of human operational mode:
         (a) RF circuitry calibration procedures of many Wireless devices are based on plugging probes between the radios ports and the antennas, in many cases leaving the antennas themselves un-calibrated; that lack of full calibration is sometimes due to the cost of radiated test.   (b) Human handling of a wireless device is often involving touching the area where an antenna is located, inadvertently modifying both amplitude and phase. Proximity sensor or other sensing means like VSWR monitoring may be used in order to trigger real time field calibration procedure.
 
Real time calibration can offer remedy in both of the above cases.
       

     During normal operation the transmitter (e.g. base station) transmits reference signals that allow the channel to be estimated. The estimated channel is the overall channel from the transmit antenna to the beamformer output. During calibration only one of the beamformer antennas is activated. Therefore, the overall channel comprises an external channel from the transmit antenna to the single active beamformer antenna, and an internal channel which is the signal path from the active beamformer antenna to the beamformer output. The internal signal path varies depending on the selected weights (amplitude and phase) and configuration (bypass or non-bypass). 
     Let R i , i=1 . . . k, be the number of internal paths to be calibrated from beamformer input i to its output, and p(i,j), i=1 . . . k, j=1 . . . R i , be the internal paths from input i to the output. Internal path p(i,j) can be expressed as A INT(i,j) e jφ     INT(i,j)   , wherein A INT(i,j)  and φ INT(i,j)  are respectively amplitude and phase of internal path p(i,j). Similarly, the external channel from the transmit antenna to beamformer input i is expressed as A EXT(i) e jφ     EXT(i)   . Therefore, the overall channel comprising the external channel and an internal path p(i,j) is given by A EXT(i) e jφ     EXT(i)   A INT(i,j) e jφ     INT(i,j)   . 
     A channel ratio Ratio (i, j1, j 2) between two channels, both including the same external channel from the transmit antenna to same input i, but having distinct internal paths p(i, j1) and p(i, j2), j1=1 . . . R i , j2=1 . . . R i , j1≠j2, is defined as: 
     
       
         
           
             
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     According to some embodiments, the calibration procedure may include the following stages:
         1. Define the calibration time slots. During the calibration time slots only a single beamformer antenna is activated and the system performs momentarily calibration function in addition to regular receiver functions. Since Ratio(i, j1, j2) is a ratio between two different channels, two separate channel estimations are required for estimating Ratio(i, j1, j2); therefore, in order to minimize variations in the external channel the time slots may be grouped in pairs for back to back channel estimation and more accurate estimation of Ratio(i, j1, j2).   2. Set i=1.   3. Select a path p(i, j1) from beamformer input i to its output as anchor.   4. Using channel estimation compute Ratio(i, j1, j2) for fixed j1 (selected in step 3 as anchor) and j2=1 . . . R i , j2≠j1, and store the computed values in the calibration table.   5. Repeat steps 3 and 4 for i=2 . . . k.   6. Repeat steps 2 to 5 periodically and average the measurements.       

     If an indication of channel mobility is available, it may be optionally used to add weights in the average: low mobility measurements may receive higher weights than high mobility measurements. If an indication of SINR is available, it may be optionally used to add weights in the average: high SINR measurements may receive higher weights than low SINR measurements. The temperature and frequency may be added to the calibration table, in which case the measurements may be averaged separately for different temperatures and frequencies. In some embodiments, the calibration table may be stored in non-volatile memory. 
     In some embodiments the real time calibration is performed only when slow fading is identified by a channel mobility detection procedure. 
     During regular RDN operation, the calibration table is used to correct channel estimation values before they are used for RDN weight settings. 
     In the above description, an embodiment is an example or implementation of the invention. 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. 
     Embodiments of the invention may include features from different embodiments disclosed above, and embodiments may incorporate elements from other embodiments disclosed above. The disclosure of elements of the invention in the context of a specific embodiment is not to be taken as limiting their used in the specific embodiment alone. 
     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. 
     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. 
     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. 
     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.