Abstract:
A method and system of merging signals feeding the antenna elements by using a combiner network and to create an antenna reference point representing the signal representation in the air. This merged signal will be easily accessed for measurement purposes. An additional signal combiner connected to the output terminals of the distributed power amplifier modules of the active antenna creates a measured virtual antenna reference point which may be utilized in testing and typing active antennas having distributed electronics for a certification according to standards for GSM, PDC and DCS. The corresponding method for measuring active receive antennas is also disclosed. Also discloses a measurement system for application of the new method of obtaining a virtual transmit or receive antenna.

Description:
TECHNICAL FIELD 
     The present invention relates to a method and a measurement system for creating a virtual antenna reference point to be used in testing of active antennas containing distributed amplifier modules. 
     BACKGROUND 
     Today radio performance typing of the present generation of base stations according to standard requirements (for instance GSM, PDC and DCS) is performed by measurement in an antenna reference point. This point is defined by the current standard and is constituted by the antenna connector terminal and should regarding transmission be located after active electronics, but regarding reception be located before the electronics. 
     When utilizing active antennas having distributed amplifiers this approach leads to problems. Each transmitter and receiver module, respectively, is connected to one or more antenna elements and the merging of the signals takes place in the air. For this type of antennas a new type of antenna reference point must be defined. 
     One method of measurement could be to individually measure each radiating element interface and state a type acceptance for each unit as such. This would lead to a very large number of measurements and produce a result which would not be representative for the signal environment resulting of the merging in the air. The position in space and relative amplitudes of spuriouses and main signal will not be measured, however this is entirely of vital importance for the resulting signal representation. 
     Another method is to measure radiation pattern by means of an antenna test range. This method is very difficult as it is necessary to maintain a safe control of undesired disturbance sources. That is practically impossible as certain measurements are performed at signal levels of −110 dBm and less. If the measurements are performed in an unshielded room or outdoors it will be disturbed by present active mobile telephones and when performing transmit tests, telephone systems operating in the neighborhood may be disturbed by such a test. 
     There will be found a number of documents relating to calibration of phased array antennas, for instance U.S. Pat. No. 5,063,529 disclosing utilization of automated processing techniques to compute calibration coefficients based on a generalized model of the array. However the documents relating to such calibrations are not dealing with systems having active antennas with distributed amplifiers. 
     Therefore there is a large need for a new method for performing a reliable test method for an active antenna system containing distributed amplifiers for instance for use in base stations for radio telephone systems like GSM, PDC or DCS. 
     SUMMARY OF THE INVENTION 
     A method according to the present invention proving to be practically usable is a merging of signals feeding the antenna elements in transmit by means of a combiner network or feeding distributed low noise amplifiers by means of a combiner network substituting the antenna elements, to thereby create an antenna reference point. The result of this signal merging will be easily accessed for measurement purposes. 
     Consequently, the present invention discloses a method and a measurement system creating a virtual antenna reference point which for testing and typing active antennas having distributed electronics for a certification according to standards for GSM, DCS and PDC. The present method is set forth in the independent claim  1  and a measurement system for applying the method is set forth in the independent claims  11  and  12 , while different embodiments of the method according to the present invention are set forth in the dependent claims  2 - 10 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, features and advantages of the present invention as mentioned above will become apparent from a detailed description of the invention to be contemplated with reference to the accompanying drawings wherein like reference numerals are used throughout to designate like parts. In the drawings: 
     FIG. 1 illustrates an example of an arrangement according to the present invention for creating a virtual antenna reference point for an active transmit antenna; 
     FIG. 2 illustrates an arrangement according to the present invention presenting a measurement configuration having an over-dimensioned matrix for an active antenna for transmit; 
     FIG. 3 illustrates an arrangement according to the present invention presenting a measurement configuration having a distribution network for equal phase; 
     FIG. 4 illustrates an arrangement according to the present invention presenting a measurement configuration having a distribution network for one phase gradient; 
     FIG. 5 illustrates an example of an active transmit antenna having a non-constant amplitude distribution; 
     FIG. 6 illustrates an example of a measurement configuration with a distribution network of an active transmit antenna according to FIG. 5 for one phase gradient; 
     FIG. 7 illustrates an example of an arrangement according to the present invention for creating a virtual antenna reference point for an active receive antenna; and 
     FIG. 8 illustrates another example of an arrangement according to the present invention for creating a virtual antenna reference point for an active receive antenna corresponding to the arrangement in FIG. 6 shown for a transmit antenna. 
    
    
     DETAILED DESCRIPTION 
     The present invention demonstrates a new strategy for primarily testing transmitting performance of an active antenna system utilizing distributed power amplifiers. According to the method the output characteristics of the system are measured virtually by introducing an additional signal combiner network forming a spatial transform of a signal fed to the initial antenna system power distribution circuitry. FIG. 1 illustrates an example of a measurement arrangement for virtually measuring output characteristics of an active antenna transmit system having such a distributing circuitry  10  feeding distributed Power Amplifier Modules  15 . The distributing circuitry  10  is provided with a test signal at input terminal S 0 . According to the invention an additional combiner network  20  having a number of input and output terminals is introduced. Each one of the outputs S 1 , S 2  . . . S n  of the distributed power amplifier modules  15  is feeding an input terminal IN 1 , IN 2  . . . IN n  of the additional combiner network  20  instead of a normally connected radiation element or group of radiation elements. In a first basic embodiment the additional combiner network  20  constitutes for instance a Butler matrix having a number of input ports IN 1 , IN 2  . . . IN n , and output ports OUT 1 , OUT 2  . . . OUT n  corresponding to the number of distributed power amplifier modules  15  of the active distributed antenna system to be measured. 
     When feeding a test signal to the input terminal S 0  of the distributing circuitry  10  of the active antenna transmit system, amplitude and phase values can be measured at the output ports OUT 1 , OUT 2  . . . OUT n  of the additional combiner network  20 , while the ordinary radiating elements or groups of radiating elements are disconnected from the output terminals of the power amplifier modules  15 . The additional signal combiner network  20  performs a spatial transform, preferably a Fourier transform, of the signal applied at S 0 . It should be noted that each output terminal OUT 1 , OUT 2  . . . OUT n  corresponds to a direction in space. These directions are in a wellknown way defined by the realization of the combiner network  20  and by the internal distances of the radiating elements. The achieved measurements of amplitude and phase values then are, by means of a standard calculation method, used to obtain a calculated measure representing the performance of the active transmit antenna system. To establish a desired description of signal and signal characteristics of the corresponding antenna system, the measured amplitude and phase values additionally are combined in this calculation with the theoretically estimated or measured radiator system normally connected to the ports S 1 , S 2  . . . S n  of the distributed power amplifier module system. This will efficiently create a virtual antenna reference point which can be used for typing of active antenna transmit systems in quality certifications according to standards like GSM, DCS and PDC. 
     In the embodiment of FIG. 1 an active transmit antenna system having five power amplifier modules will be measured. The measurement is performed according to the state of the art using commercially available equipment to measure phase and amplitude at the output terminals of the additional combiner network  20 , for instance a Butler matrix. 
     In testing active transmit antennas having a number n of distributed power amplifiers modules PAM corresponding to radiators or groups of radiators, the same additional combiner network, e.g., Butler matrix, may be used if it is dimensioned for a maximum port number of n. FIG. 2 illustrates a measurement arrangement using such an over-dimensioned combiner for measurements of an active transmit antenna with only three power amplifier modules. Here outputs S 2 , S 3  and S 4  from corresponding distributed power amplifier modules  15  are feeding inputs IN 2 , IN 3  and IN 4  of the additional combiner network  20  having n input terminals and n output terminals. Unused input terminals and output terminals are terminated according to the state of the art when operating with an over-dimensioned matrix. 
     If only a signal level in a straight forward direction from the active antenna is desired the additional combiner network may be reduced to a simpler network of equal phase with n input terminals and one output terminal. This is illustrated in another embodiment of the present invention as demonstrated by FIG.  3 . 
     FIG. 4 demonstrates still another embodiment of the present invention for measurement of the transmit signal level in an arbitrary direction from the active antenna. The additional combiner network is reduced to an additional measurement combiner network  21  having a phase gradient. By changing the phase gradient by means of phase shifters, delay lines or the like, the signal strength will be obtained in another chosen direction of radiation. Equal to FIG. 3 the combiner network  21  of FIG. 4 has n input terminals and one output terminal. 
     Antenna systems having passive distribution networks between the radiation elements and the power amplifier modules PAM may also be tested by introducing power dividers and/or phase shifting members to imitate the antenna function. This method may be combined with the cases illustrated in FIGS. 2-4. An active antenna for transmission with a non-constant amplitude distribution is illustrated in FIG. 5 and a corresponding arrangement for measurement according to the present invention using an additional measurement combiner network  21  having a phase gradient. It should be noted that the input terminals of the additional combiner network are connected to the points where the individual radiators  31  are connected to the passive combiners  30 . Thus, in the example there will be needed a number of input terminals of the additional combiner network  23  corresponding to the number of radiators  31  as is demonstrated in FIG.  6 . This measurement arrangement will produce a measured signal for one phase gradient and one signal direction at a time. Thus, an additional combiner according to the invention having a configuration adopted for the particular active transmit antenna array will in a simple way be able to produce measures in production and testing of such active transmit antenna arrays. 
     FIG. 7 demonstrates a corresponding measurement according to the present invention for an active receive antenna setup utilizing the additional combiner network for inserting a test signal R 0  into the entire array of distributed low noise amplifiers, which via a receiver combiner combines all the amplified antenna signals into an output terminal for a receiver. In FIG. 7 terminals corresponding to inputs IN 1 , IN 2  . . . IN n  of the additional combiner network according to FIG. 1 are connected to a corresponding contact of one radiating element. The contact is connected directly or via a passive combiner to a low noise amplifier, LNA  17 , of the active receive antenna. The radiator configuration in a receive mode may for instance be corresponding to the transmit configuration of FIG.  5 . Exchanging then only the PAM:s to LNA:s and changing the direction of arrow T 0  in the opposite direction we obtain a configuration of FIG. 7 with the arrow R x . Then a suitable terminal, previously used as output of the additional combiner network  23  when examining transmit characteristics, is connected to the test signal generator supplying a test signal R 0  for the measurement of the performance of the active receive antenna. 
     In other words the method according to the present invention for measuring characteristics of an active transmit antenna described above may also in a corresponding way be utilized to measure and determine the receive characteristics of the system with the help of a virtual receive antenna reference point. 
     According to the present invention a measurement of an active receive antenna corresponding to the active transmit antenna demonstrated in FIG. 5 is illustrated in FIG. 8. A test signal R 0  is applied to the additional combiner network  23 , the ports of which will then correspond to the radiators A 1 -A 9  of FIG. 5 but in this case connected to low noise amplifiers  17  instead of power amplifier modules  15  as in the transmit case. Like in FIG.  6  radiating elements A 1  and A 2 , A 3  and A 4 , A 6  and A 7 , A 8  and A 9 , respectively, are combined by means of passive combiners  30 . This gives in this case five receive signals R 1 -R 5  applied to the low noise amplifiers  17  the outputs of which then being combined by means of a further receive combiner  13 . The output obtained at the output terminal R x  will reproduce the response of the active receive antenna. Thus the utilization of the method using the additional combiner network will produce a simple and accurate way of establishing measurements according to standards for active receive antennas. A procedure according to the state of the art for a simultaneous measurement of the entire active receive antenna system would be time consuming and costly and require specific space for setting up an environment for the antenna measurement test range permitting test signals down to the order of at least −110 dBm. 
     Thus, an additional combiner according to the invention having a configuration adopted for the particular active transmit or receive antenna array will simply be able, in production and testing, to produce characteristics of such an active distributed antenna system according to a standard procedure. 
     A measurement system for applying the present method will simply be built up. Such a measurement system will comprise an additional combiner network according to the alternative embodiments of the method disclosed. The additional combiner has a number of input and output terminals and terminating means to terminate unused input and output terminals. Additionally the measurement system incorporates a generator means (not shown) generating a first test signal S 0  to be applied to a primary test input terminal at a distributing circuitry feeding distributed power amplifiers of an active antenna system for measurement of transmit characteristics of the active transmit antenna, or generating a second test signal R 0  to be applied to a secondary test signal input terminal of the additional combiner network for measurement of active receive antenna characteristics in an active antenna system. A person skilled in the art will realize that as a generator means may, for instance, be used the actual transmit driver intended for the active antenna system if suitably adapted for the measurement to be performed. Finally the measurement system contains a metering system (not shown) for evaluating amplitude or phase values at a measurement output terminal of said additional combiner network for obtaining a measured virtual active transmit antenna reference point or for evaluating amplitude values at a measurement output terminal of a distributed receiving amplifier combining circuitry for obtaining a measured virtual active receive antenna reference point. A person skilled in the art will realize that as the metering system a standard vector voltmeter may be utilized or even the actual receiver system for the active antenna could be utilized as a measurement tool if properly adapted. The measurement system disclosed will form an excellent tool in production and in testing for certifications according to standards like GSM, DCS.and PDC in mobile telephone service systems. 
     A measurement system according to the present method may utilize different types of additional combiner network, for instance a network providing an equal phase combination, a network providing a controllable phase gradient or a Butler matrix. 
     It will be appreciated by those of ordinary skill in the art that the present invention can be embodied in other specific forms, e.g. radiating elements or groups of elements placed on nonplanar surfaces, without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.