Patent Publication Number: US-7911376-B2

Title: Systems and methods for antenna array calibration

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to antenna arrays, and more particularly to performing antenna array calibration for a range of potential operating conditions. 
     BACKGROUND 
     A phased antenna array is a group of antennas in which the relative phases of the respective radio frequency (RF) signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. One prominent use of such antenna arrays is to enable beamforming and beamsteering. Beamforming is the coherent summing of directionality such that signals are additive rather than random. When transmitting, a beamformer controls the phase and relative gain of the RF signal at each antenna element, based upon various algorithms, in order to create a coherent pattern in the wavefront. Beamsteering, on the other hand, refers to the concept of changing the direction of the main lobe of a radiation pattern by switching antenna elements or by changing the relative phases of the RF signals driving the elements. 
     With multiple antenna elements and related circuitry, calibration is an issue since it is difficult to achieve the same output power across each antenna in the array for a given gain setting. If the phase of the RF signal provided to the antenna elements is not properly calibrated to account for such phase offsets, the directional or omnidirectional beam patterns emanating from the antenna may be distorted and/or misdirected. 
     Antenna arrays can be implemented using individual antenna elements on an integrated circuit (IC), on a printed wiring board (PWB), or as separate components. In the case of an IC antenna array with IC-based amplifiers and related circuitry, matching can be approached by very precise attention to design and fabrication. While this may produce the desired results, it significantly increases manufacturing costs. In the case of a PWB implementation, controlled impedance and equal trace lengths can contribute to making each circuit perform identically. However, this can be time consuming, costly to implement and difficult to simulate. Calibration is further complicated when performed for antenna arrays which are expected to be placed in service under varying operating conditions. 
     As such, what is needed is a system and method for calibrating antenna arrays in order to overcome one or more of the aforementioned drawbacks. 
     BRIEF SUMMARY OF THE INVENTION 
     Disclosed and claimed herein are systems and methods for providing antenna array calibration. In one embodiment, a method providing a plurality of test signals from an antenna array to a receiving sensor while varying ambient operating conditions over a predetermined range of ambient operating conditions, where the antenna array includes a plurality of antenna elements. The method also includes measuring signal properties of the plurality of test signals and measuring a plurality of ambient operating conditions under which the plurality of test signals are provided. In addition, the method includes determining signal offsets for each of the plurality of antenna elements corresponding to each of the measured plurality of ambient operating conditions, and then storing calibration information in a memory of the antenna array corresponding to said determined signal offsets. 
     Other aspects, features, and techniques of the invention will be apparent to one skilled in the relevant art in view of the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
         FIG. 1A  depicts one embodiment of a calibration system configured in accordance with the principles of the invention; 
         FIG. 1B  depicts a block diagram of the antenna array of  FIG. 1A , configured in accordance with one embodiment of the invention; 
         FIG. 2  is one embodiment of a process for implementing an antenna array calibration process in accordance with the principles of the invention; and 
         FIG. 3  is one embodiment of a process for how the calibration information of  FIG. 2  may be utilized. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Disclosure Overview 
     One aspect of the present disclosure relates to providing a calibration process for antenna arrays such that the array will operate more efficiently in that transmission range will tend to increase for a given input power, and power consumption decrease for a fixed range application. In certain embodiments, one or more test signals may be provided from the antenna array to be calibrated to a receiving sensor while varying ambient operating conditions over some predetermined range of ambient operating conditions. The signal properties of these test signals may be measured by the receiving sensor or associated spectrum analyzer, and the ambient operating conditions under which the test signals are provided may be similarly measured. The RF signal properties to be measured may include measurement of the wavefront pattern, range and signal directionality produced by the antenna array. 
     Thereafter, signal offsets for each of the antenna array&#39;s elements may be determined as a function of the measured ambient operating condition. These signal offsets may be based on whether the actual produced signal wavefront is within a predetermined tolerance of a desired wavefront. In certain embodiments, this determination may comprise comparing the wavefront pattern, range and/or signal directionality of the received RF test signals to corresponding expected values. 
     Calibration information corresponding to these signal offsets may then be stored in a memory of the antenna array for use during operation of the antenna array. This calibration information may be in the form of a lookup table or a curve-fitting equation (or even a combination thereof). 
     As used herein, the terms “a” or “an” shall mean one or more than one. The term “plurality” shall mean two or more than two. The term “another” is defined as a second or more. The terms “including” and/or “having” are open ended (e.g., comprising). The term “or” as used herein is to be interpreted as inclusive or meaning any one or any combination. Therefore, “A, B or C” means “any of the following: A; B; C; A and B; A and C; B and C; A, B and C”. An exception to this definition will occur only when a combination of elements, functions, steps or acts are in some way inherently mutually exclusive. 
     Reference throughout this document to “one embodiment”, “certain embodiments”, “an embodiment” or similar term means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of such phrases or in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner on one or more embodiments without limitation. 
     In accordance with the practices of persons skilled in the art of computer programming, the invention is described below with reference to operations that are performed by a computer system or a like electronic system. Such operations are sometimes referred to as being computer-executed. It will be appreciated that operations that are symbolically represented include the manipulation by a processor, such as a central processing unit, of electrical signals representing data bits and the maintenance of data bits at memory locations, such as in system memory, as well as other processing of signals. The memory locations where data bits are maintained are physical locations that have particular electrical, magnetic, optical, or organic properties corresponding to the data bits. 
     When implemented in software, the elements of the invention are essentially the code segments to perform the necessary tasks. The code segments can be stored in or on a “computer storage medium,” which may include any medium that can store or transfer information. Examples of the computer storage medium include an electronic circuit, a semiconductor memory device, a ROM, a flash memory or other non-volatile memory, a floppy diskette, a CD-ROM, an optical disk, a hard disk, a fiber optic medium, a radio frequency (RF) link, etc. 
     Exemplary Embodiments 
     With reference to  FIG. 1A , depicted is a calibration system  100  for performing the calibration operation according to one embodiment. As shown, an antenna array  110  is comprised of multiple antenna elements (ANT 1 -ANT n ), each having its own antenna and related RF signal-transmission circuitry (not shown), as is generally known in the art. In the context of a calibration operation, the antenna array  110  may be configured to transmit one or more test signals to a receiver sensor  120 , which itself may be comprised of an antenna and related RF signal-receiving circuitry (not shown), as is generally known in the art. In certain embodiments, the antenna array  110  may be placed some minimum distance  130  away from the receiver sensor  120  such that the center array element is essentially normal to the receiving plane  140  of the receiver sensor  120 , and further that the antenna angles for the outlying antenna elements (i.e., θ 1  and θ n ) are sufficiently close to 90 degrees from the receiving plane to be considered normal (e.g., ±2 degrees). 
     By way of providing a non-limiting example, in the case of a millimeter wave IC antenna, the individual antenna elements may be approximately 1 or 2 mm apart in a grid of, for example, 52 antennas spanning an area of about 23 mm×23 mm. In that case, the minimum distance  130  may be about 1 meter, and still maintain the center array element in an essentially normal orientation to the receiving plane  140 . Additionally, and as will be described in more detail below, the receiver sensor  120  may be used as a calibration sensor for each of the individual antenna elements ANT 1 -ANT n  as a first order approximation. 
     Each of the antenna elements (ANT 1 -ANT n ) may be configured with its own RF signal processing control circuitry for providing appropriately-phased signals to each of the respective antenna elements (ANT 1 -ANT n ) to form a desired directional or omnidirectional beam pattern. 
     The receiver sensor  120  may be implemented as a single receiver, as shown in  FIG. 1 , or alternatively may itself be comprised of an array of receiving sensors. The receiver sensor  120  may be operated as part of, or in connection with test equipment configured to carry out the calibration procedure described herein. Such test equipment may include, for example, the receiver sensor  120  and/or other signal receiving circuitry, one or more ambient condition sensors (not shown), a spectrum or signal analyzer, etc. 
     Additionally, the receiver sensor  120  may be configured to rotate at a fixed distance (yaw) so as to perform the calibration operation across an arc of the x-y plane. Similarly, tilting the sensor  120  at a fixed distance (pitch) may be done to perform the calibration across an arc along the z-axis. As will be described in more detail below, since the test signals provided by the antenna elements (ANT 1 -ANT n ) to the receiver sensor  120  have known characteristics, any deviations or distortions therefrom that are detected by the receiver sensor  120  can, in turn, be used to determine offset values (e.g., phase and/or gain) on a per-element basis. 
     With reference now to  FIG. 1B , depicted is a more detailed diagram of an exemplary embodiment of the antenna array  110  of  FIG. 1A . In particular, antenna array  110  includes a plurality of individual antenna elements (ANT 1 -ANT n ) and related RF signal-transmission circuitry, the details of which are generally known in the art. The antenna array  110  may be implemented as an IC, PWB or in separate components. 
     The antenna array  110  further includes control logic  150  for controlling/adjusting the RF signal characteristics (e.g., phase, gain, etc.) of each individual antenna elements (ANT 1 -ANT n ). In certain embodiments, the control logic  150  may be configured to retrieve and execute instructions stored in the memory  160  for operating the antenna elements (ANT 1 -ANT n ), in accordance with various beamforming and beamsteering algorithms to direct each of the antenna elements (ANT 1 -ANT n ) to produce a desired wavefront pattern. Additionally, and as will be described in more detail below with reference to  FIG. 2 , the control logic  150  may be configured to adjust the RF signal characteristics (e.g., phase offsets, power gain, etc.) based on pre-stored offset values (e.g., determined during a calibration operation) that are specific to each of the individual antenna elements, as well as to the then-current ambient conditions. It should further be appreciated that the control logic  150  may be comprised of any number and type of processors, including but not limited to integrated circuit microprocessor(s), microcontroller(s), digital signal processor(s), etc. Similarly, memory  160  may include any combination of different memory storage devices, such as hard drives, random access memory (RAM), read only memory (ROM), FLASH memory, or any other type of volatile and/or nonvolatile memory. 
     Antenna array  110  may be optionally coupled to one or more ambient condition sensors  170 , as depicted in  FIG. 1B , for providing ambient condition information (e.g., temperature, humidity, etc.) to control logic  150 . As will be described in more detail below with reference to  FIG. 3 , the ambient condition sensors  170  may be used to adjust for the actual ambient operating conditions, which can materially affect RF signal characteristics. 
     Referring now to  FIG. 2 , depicted is one embodiment of a process for implementing an antenna array calibration scheme in accordance with the principles of the invention. In one embodiment, process  200  may be implemented as a factory-level calibration process, and may be performed by a calibration system (e.g., system  100 ) to calibrate an antenna array (e.g., antenna array  110 ). Alternatively, process  200  may be implemented at any point in time in which calibration may be desirable and/or beneficial. In certain embodiments, the calibration process  200  may be used to cause each antenna array to perform very similarly (e.g., within the manufacturer&#39;s tolerances) in a cost effective manner. Additionally, the calibration process of the current disclosure will tend to reduce design time, and hence reduce time to market. A properly aligned array will operate more efficiently in that transmission range will tend to increase for a given input power, and power consumption decrease for a fixed range application. 
     Process  200  begins at block  210  where the starting ambient operating conditions for the system under test (e.g., calibration system  100 ) may be set. In certain embodiments, this may comprise setting one or both of the temperature and humidity under which the system will operate. While this initial ambient operating condition may correspond to the average or expected operating condition for the antenna array under test, it may alternatively be associated with either end of a predetermined operating range (e.g., coldest expected operating temperature, highest expected operating temperature, lowest expected humidity, highest expected humidity, etc.). 
     Once the starting ambient condition is set, process  200  may continue to block  220  where each of the antenna elements (e.g., ANT 1 -ANT n ) provides one or more test signals with known characteristics to a signal sensor (e.g., receiver sensor  120 ). It should further be appreciated that the test signals to be provided at block  220  may be provided over a range of sensor positions and orientations. For example, during transmission of the test signals, the receiving sensor may be rotated along the x-y plane (yaw) and/or tilted along the z-axis (pitch). 
     Once received by the receiving sensor, these test signals may then be measured at block  230 , which may include any known equipment capable of receiving a test signal and measuring the RF signal properties and characteristics (e.g., spectrum analyzer, network analyzer, etc.). The RF signal properties to be measured may include measurement of the wavefront pattern, range and signal directionality produced by the antenna array. 
     Additionally, the then-current ambient conditions (e.g., temperature, humidity, etc.) may be measured as well (block  240 ). In the initial iteration of process  200 , the values measured at block  240  should be consistent with the operating conditions set above at block  210 . In certain embodiments, the ambient conditions may be measured by one or more sensors (e.g., ambient condition sensor(s)  170 ) coupled to the antenna array. Alternatively, the ambient condition sensor may be associated with the receiving sensor, or located in the general proximity to the overall calibration system. While ambient condition measurement operation of block  240  is shown as being performed after the signal measurement operation of block  230 , it should be appreciated that these order of these operations may be reversed or even performed simultaneously. 
     Process  200  may then continue to block  250  where a determination may be made as to whether the actual produced signal wavefront is within a predetermined tolerance of a desired wavefront, where the predetermined tolerance may be set by the manufacturer. In certain embodiments, this determination may comprise comparing the wavefront pattern, range and/or signal directionality of the received RF test signals to corresponding expected values. 
     If it is determined at block  250  that the test signal properties are not within tolerance (e.g., desired signal wavefront achieved), then process  200  may continue to block  260  where one or both of the individual antenna elements&#39; phases and gains may be adjusted by a predetermined increment (e.g., ±1 degree, ±1 dB, etc.). Process  200  may then return to block  220  where a new set of test signals may be provided by the antenna array using the newly-incremented signal offsets. The operations described above with reference to blocks  220 - 250  are then repeated. 
     If, on the other hand, it is determined at block  250  that the test signal properties are within tolerance (e.g., desired signal wavefront achieved), then process  200  may continue to block  270  where the current ambient conditions measured at block  240  and the current signal offsets (as previously adjusted at block  260 ) for each of the antenna elements that make up the array under test may be recorded for later use. 
     Thereafter, a determination may be made at block  280  as to whether phase and/or gain offsets have been recorded for the antenna array over an entire predetermined ambient operating condition range. If not, process  200  may continue to block  290  where the current ambient operating conditions may be incremented by a predetermined incremental value (e.g., ±predetermined number of degrees, ±predetermined percentage of humidity, etc.). Thereafter, process  200  may return to block  220  where a new set of test signals may be provided by the antenna array for subsequent measurement and analysis, as described above with reference to blocks  220 - 250 , in an iterative fashion. 
     If on the other hand, offset values have been collected for the antenna array over the entire predetermined ambient operating condition range, then process  200  may continue to block  295  where a memory of the antenna array (e.g., memory  160 ) may be programmed with element-specific calibration information for the antenna array under test. This calibration information may be in the form of a lookup table or a curve-fitting equation (or even a combination thereof). In the case of lookup table, the ambient operating condition values (from block  240 ) and corresponding signal offsets (from block  260 ) recorded at block  270  for each of the antenna elements may be stored/tabulated in a lookup table. This lookup table may then be accessed, during normal operation to optimize the array&#39;s performance, as will be described in more detail below with reference to  FIG. 3 . While it should be appreciated that such a lookup table may take many different forms, in one embodiment it may include a tabulation of signal offset values (e.g., phase and/or gain) for each of the individual antenna elements as a function of ambient conditions. 
     In the case of a curve-fitting equation, the ambient operating condition values and corresponding signal offsets from block  270  may be used to generate a curve-fitting equation using known regression analysis, interpolation and/or extrapolation techniques. Once an applicable equation is known, it may be stored in the antenna array&#39;s memory for use during normal operation, as will be described below with reference to  FIG. 3 . The choice as to whether a lookup table or curve-fitting equation is used may be based on the device&#39;s available memory and processing power. That is, while the lookup table may require more memory, it will require a relatively low amount of processing power. Conversely, use of a curve-fitting equation may require less memory, but more processing power. 
     In this above-described manner, the calibration process  200  of  FIG. 2  provides a cost efficient approach to ensuring that a manufacturer&#39;s antenna arrays perform very similarly (e.g., within the manufacturer&#39;s tolerances) while in use. Moreover, a properly-aligned array will operate more efficiently in that transmission range will tend to increase for a given input power, and power consumption decrease for a fixed range application. 
     Referring now to  FIG. 3 , depicted is a process for transmitting RF signals from an antenna array (e.g., antenna array  110 ) that has been calibrated in accordance with the principles of the invention (e.g., according to process  200 ). Process  300  may be implemented in normal operation in an uncontrolled environment, such as would be the case at a user location. Process  300  may performed once, such as upon initialization of the antenna array, or on a continuous or periodic basis during operation of the antenna array. 
     Process  300  begins at block  310  where the ambient conditions (e.g., temperature, humidity, etc.) may be measured using one or more sensors (e.g., ambient condition sensor(s)  170 ) coupled to the antenna array. Using these ambient condition values at block  320 , calibration information may be retrieved from a memory (e.g., memory  160 ) of the antenna array corresponding to the individual antenna elements (e.g., ANT 1 -ANT n ). As previously described, this calibration information may be in the form of tabulated values in a lookup table, or alternatively may be comprised of a curve-fitting equation. In either case, the array&#39;s processing logic (e.g., control logic  150 ) may be configured to perform the necessary lookup operation (when using a lookup table) or the mathematical computation (when using an equation) to determine the applicable signal (e.g., phase and/or gain) offsets for each of the individual array elements (block  330 ). 
     Thereafter, the array&#39;s processing logic may further be configured to apply the corresponding signal offsets to the signal processing logic of each of the array&#39;s elements in order to produce an optimal and coherent array wavefront (block  340 ). Since the signal offsets that are utilized are both element-specific as well as ambient condition specific, manufacturing imperfections and operating condition variations may be corrected for in a cost-effective manner. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art. Trademarks and copyrights referred to herein are the property of their respective owners.