Abstract:
The disclosure relates to an active antenna array for a mobile communication system which comprises a plurality of receive paths, a sounding signal generator generating a sounding signal, and a coupler for coupling the sounding signal into at least one of a plurality of receive paths. A sounding signal extractor substantially removes the sounding signal from digitized ones of the receive signals to form a wanted signal. The disclosure also provides a method for the calibration of the receive path of the active antenna array.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 12/751,391 entitled “ACTIVE ANTENNA ARRAY AND METHOD FOR CALIBRATION OF RECEIVE PATHS IN SAID ARRAY”, filed Mar. 31, 2010 and U.S. patent application Ser. No. 12/751,342 entitled “ACTIVE ANTENNA ARRAY AND METHOD FOR CALIBRATION OF THE ACTIVE ANTENNA ARRAY”, filed Mar. 31, 2010. 
     The entire contents of the two concurrently filed applications are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The field of the invention relates to an active antenna array and a method for calibration of the active antenna array. 
     BACKGROUND OF THE INVENTION 
     The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to reduce the running costs of the base station. One option to do this is to implement a radio system as an antenna-embedded radio forming an active antenna array. Many of the components of the antenna-embedded radio may be implemented on one or more chips. 
     Multiple receive paths in the antenna-embedded radio need to be synchronised in phase, delay and amplitude of signals travelling on the receive paths. Known techniques to establish variations in the phase, delay and amplitude of signals involve the injection of a known signal, termed the sounding signal, into one or more of the receive paths and, based on the comparison of the sounding signal and the received signal, the phase, delay and amplitude variations for the signals in the receive paths can be estimated. This allows for calibration of the receive paths by generation of correction coefficients to be applied to receive signals received along the multiple receive paths. 
     The sounding signal can have either the same frequency in a carrier signal spectrum or be at a different frequency than the carrier signal spectrum. In the first case (frequency of the sounding signal is in the carrier signal spectrum) then it is necessary to correctly adjust the power of the sounding signal. If the power of the sounding signal is too high, then the quality of the carrier signal can be degraded. On the other hand, if the power of the sounding signal is too low, the quality of the measurements of the phase, delay and amplitude variations is too low. 
     If the sounding signal is positioned in a frequency spectrum different from the carrier signal spectrum, then the frequency and phase response of the analogue receive filters in the receive paths can be slightly different at the different frequencies. This implies that the measurement results for the phase, delay and amplitude of the signals measured at the frequency of the sounding signal may be slightly different than the measurement results for the phase, delay and amplitude of the signals measured at the frequency of the carrier signal. In addition, it is necessary to ensure that the frequency of the sounding signal is different than any of the frequencies of the other carrier signals which might be measured at the antenna embedded radio. There is also a risk that blockers in the antenna embedded radio may block certain frequency bands and thus affect the quality of the error measurement. Finally the sounding signal might be unintentionally transmitted from a receive antenna and then be detectable at a receive port of another (unconnected) receiver, which might violate regulations. 
     A further known solution is to use a wide-band spectrum, for example a spread spectrum sounding signal, which is close to or below the noise floor of the carrier signals. In order to avoid the blockers, an extremely long sounding signal spreading code is necessary, in order to have sufficient processing gain. 
     SUMMARY OF THE INVENTION 
     An active antenna array is taught that comprises a plurality of receive paths carrying receive signals and a sounding signal generator for generating a sounding signal for sounding at least a portion of at least one of the plurality of receive paths. A coupler is used for coupling the sounding signal into at least one of the plurality of receive paths and a sounding signal extractor is used for substantially removing the sounding signal from digitised ones of the receive signals, to form a wanted signal. This allows the analysis of the sounding signal to take place after passage through the receive paths, to calculate correction coefficients, without it affecting the receive signal quality of the wanted received signals. 
     In one aspect of the invention the active antenna array further comprises a power controller for controlling the level of power of the sounding signal. This allows control of the power to avoid the “swamping” of a carrier signal. 
     The disclosure also teaches a method for the calibration of a receive path of an active antenna array which comprises generating an initial sounding signal, coupling the initial sounding signal into at least one of a plurality of the receive paths to generate an adjusted sounding signal, and comparing the adjusted sounding signal with the initial sounding signal for generating calibration parameters. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  shows an example of an active antenna array using the system for the calibration of a single signal receive path. 
         FIG. 2  shows a view of part of the circuit of the example of  FIG. 1 . 
         FIG. 3  shows an overview of the method used for the calibration of the single receive path. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described on the basis of the drawings. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protective scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that a feature or features of one aspect or embodiment of the invention can be combined with a feature or features of a different aspect or aspects and/or embodiments of the invention. 
       FIG. 1  shows an example of an aspect of the invention, in this instance for the calibration of a single receive path  30 - 1  in an active antenna array  10  by the generation of correction coefficients. The active antenna array  10  has a plurality of antenna elements  20  (only one  20 - 1  of which is shown in  FIG. 1 ) which are connected to a plurality of transceivers  25 . In the aspect shown in  FIG. 1  only one of the transceivers  25  is shown and is labelled as  25 - 1 . It will be appreciated that the teachings of this disclosure are relevant for an active antenna array  10  with any number of transceivers  25 . Typically there will be eight or sixteen transceivers  25 . 
     The transceiver  25 - 1  has a receive path  30 - 1  and a transmission path  50 - 1 . Both the receive path  30 - 1  and the transmission path  50 - 1  are connected to the antenna element  20 - 1  through a duplex switch  40 - 1 . The function of the duplex switch  40 - 1  is to switch the antenna element  20 - 1  between transmit signals being transmitted on the transmission path  50 - 1  and signals being received from the antenna element  20 - 1  and passed to the receive path  30 - 1 . 
     The active antenna array  10  has a digital signal processor  100 . The digital signal processor  100  is used to produce the signals for transmission on the antenna elements  20  and to process the radio signals received from the antenna element  20 . A beamforming block  107  in the digital signal processor  100  will use the correction coefficients calculated as described later in this disclosure in order to account for phase, delay and amplitude variations on the receive signals received on the receive path  30 - 1 . This function has been described in co-pending applications of Ubidyne and will be not discussed here in detail. 
     The active antenna array  10  has further a control unit  105  whose function is to produce a sounding signal  110 . The sounding signal  110  may take different forms. For example the sounding signal may be a broad-based signal covering all frequency bands in the spectrum of interest of the carrier signal. The sounding signal may also be a signal which is inserted between the frequencies of interest in the spectrum of the carrier signal. The control unit  105  is connected to a power controller  130 . The power controller  130  is connected to an auxiliary transceiver  27 . The auxiliary transceiver  27  is, however, used for the transmission of the sounding signal  110  as will be explained below. The sounding signal  110  is received from the power controller  130  and is converted by a digital-to-analogue converter (DAC)  140  from the digital domain to an analogue signal and is passed along an auxiliary transmission path  145  to an output  146  and then to a multi-way switch  150 . It will be noted at this stage that the auxiliary transceiver  27  also includes a receive path, but this receive path is not used in this aspect of the invention. 
     The multi-way switch  150  accepts the sounding signal  110  as an input and switches the sounding signal  110  to one of the plurality of the transceivers  25 - 1 ,  25 - 2 , . . . ,  25 -N. In the aspect depicted in  FIG. 1  the sounding signal  110  is passed through a coupler  155  to the duplex switch  40 - 1  of the first one  25 - 1  of the transceivers  25 . 
     It will be noted that the multi-way switch  150  has a number of other outputs which are labelled in  FIG. 1  as being passed to other ones of the plurality of the transceivers  25 - 2 , . . . ,  25 -N. 
     In the first transceiver  25 - 1  the sounding signal  110  is passed to the receive path  30 - 1  and then to an analogue-to-digital convertor  160 - 1  to produce a sounding signal  110 ′ in the digital domain. The receive path  30 - 1  will also carry a receive signal  120 - 1  from the antenna element  20 - 1  which is also converted to a digital signal in the analogue-to-digital convertor  160 - 1 . The sounding signal  110 ′ and the receive signal  120 - 1  (now in the digital domain) is passed further to the digital signal processor  100  for processing and to a calibration processing unit  180  via a coupler  170 . The digital signal processor  100  can carry out beam forming operations on the receive signal  120 - 1  and can also apply the correction coefficients as calculated below. 
     The calibration processing unit  180  receives the sounding signal  110 ′ after the sounding signal  110  has passed through the transceiver  25 - 1 , the digital-to-analogue convertor  140  and the analogue-to-digital convertor  160 - 1  as well as the transmission path  145  in the auxiliary transceiver  27 . The calibration processing unit  180  is able to compare this sounding signal  110 ′ with the originally injected sounding signal  110  received prior to the conversion to an analogue signal by the digital-to-analogue convertor  140 . The calibration unit  180  is therefore able to calculate the correction calibration coefficients that need to be supplied to the digital signal processor  100  in order to correct the beam forming vectors due to the passage of the receive signal  120 - 1  through the receive paths  30 - 1 . 
     The calibration processing unit  180  is also able to substantially remove the sounding signal  110 ′ from the receive path  30 - 1 . This is done by subtracting a signal substantially similar to the sounding signal  110 ′ using a subtractor  175 . 
     The power controller  130  compares the power of the sounding signal  110 ′ after the sounding signal  110 ′ has it is passed through circuitry in the radio head  25 - 1  with the power of the receive signal  120 - 1  in order to ensure that the energy in the sounding signal  110 ′ is not so large as to interfere with the receive signal  120 - 1  in the receive path  30 - 1 . The power of the initially injected sounding signal  110  can be adjusted by the power controller  130  in order to ensure that this interference is limited. 
     It will be appreciated that the receive path  30 - 1  can take many different forms. For example there may be direct conversion of the analogue receive signal from the antenna element  20 - 1  into a digital signal for passage to the digital signal processor  100 . There may be alternatively a single-stage or a multi-stage down conversion of the analogue receive signal and/or there may be a delta-sigma based analogue-to-digital conversion of the analogue receive signal. It will be appreciated that the precise design of the receive path  30 - 1  has no bearing on the ideas disclosed in this disclosure. 
       FIG. 2  shows an aspect of the calibration processing unit  180  as might be used in the circuit of  FIG. 1 . It will be seen that the sounding signal  110  in  FIG. 2  is a broad spectrum sounding pilot signal (see insert A) which is transmitted from a sounding signal generation unit in the control unit  105 . The sounding signal  110  is passed to the digital-to-analogue convertor  140  (not shown on  FIG. 2 , but shown in  FIG. 1 ) and also to the calibration unit  180 . The sounding signal  110  is passed through a splitter  230  to a first gain/phase controller or vector modulator  240  and to a correlator and control system  250 . The output of the first gain/phase controller or vector modulator  240  is passed to the subtracter  175 . 
     The correlator and control system  250  also receives an input from the coupler  170 . In addition, a calibration coefficient processing unit  260  is present and passes correction coefficients to the beamforming block  107  where the correction coefficients may be used to correct the receive signals  25 - 1 , as discussed above. 
     Insert B shows the combination of the receive signals  25 - 1  and the sounding signal  110  which have been converted from the analogue domain to the digital domain in the analogue-to-digital convertor  160 . It will be seen in this insert B that the power of the sounding signal  110  is substantially lower than the power of the receive signals  120 - 1  received by the antenna element  20 - 1 . 
     An adaptive filter  270  and a second gain/phase controller or vector modulator  280  are shown in the receive path  30 - 1  between the analogue-to-digital convertor  160  and the subtracter  175  in this  FIG. 2 . The adaptive filters  270  and the second gain/phase controller or vector modulator  280  are optional elements and may not be present in all implementations. The second gain/phase controller or vector modulator  280  may be used in place of the first gain/phase controller or vector modulator  240  without loss of functionality (in which case, the first gain/phase controller or vector modulator  240  could be omitted and replaced with a straight-through connection). In this case, the second gain/phase controller or vector modulator  280  would receive its control signals from the correlator and control system  250  in the same manner as is shown for the first gain/phase controller or vector modulator  240 , in  FIG. 2 . 
     The receive path  25 - 1  includes the subtractor  175  and the coupler  170 . An input of the subtractor  175  is connected to the output of the calibration processing unit  180 , and more particularly to an output of the first gain/phase controller or vector modulator  240 . The subtractor  175  is used to subtract the sounding signal  110 ′ from the receive path  25 - 1  such that the sounding signal  110 ′ is substantially removed from the signal, as is shown in the insert C, to leave substantially the receive signal  120 - 1 . The coupler  170  passes part of the signal to the calibration processing unit  180 , and more particularly to the correlator and control system  250  which calculates the values that need to be supplied to the subtractor  175  in order to substantially subtract the sounding signal  110  from the signal. The correlator and control system receives the sounding signal form the splitter  230 . 
     The output of the coupler  170  is substantially the receive signal  120 - 1  shown in the insert C which is then passed to the beam forming block  107  or further processing. 
     The output of the first correlator and control system  250  is also passed to the calibration coefficient processing unit  260  which is able to calculate the correction coefficients to be used to take into account any delays, phase changes or amplitude variations of the receive signal  120 - 1  passing through the receive path  30 - 1 . 
       FIG. 3  shows a method used for the measurement and calculation of the correction values for the phase, delay and amplitude of the receive signals  120 - 1  received by the antenna element  20 - 1  and passed along the receive path  25 - 1 . 
     In a first step  300  the sounding signal  110  is generated in the control unit  105 . The power of the sounding signal  110  is adjusted in step  305  by the power controller  130  which, as explained above, compares the power of the sounding signal  110  with the receive signal  120 - 1  in the receive path  30 - 1 . In step  315  the sounding signal  110  is passed through the transceiver  27  to the switch  150 . The switch  150  switches the sounding signal  110  into one of the plurality of receive paths  30 . In the example shown in  FIG. 1  and described in connection with  FIG. 3  the sounding signal  110  is switched to a first one of the receive paths  30 - 1  and coupled with the receive signal  120 - 1  in the coupler  155  before the sounding signal  110 ′ (with the receive signal  120 - 1 ) is passed through the transceiver  25 - 1 . The sounding signal  110  and the receive signal  120 - 1  are converted from the analogue domain to the digital domain in the analogue-to-digital convertor  160 . 
     In step  325  the converted sounding signal  110 ′ is compared in the calibration processing unit  180  with the original sounding signal  110  and the correction coefficients are calculated which can be passed to the digital signal processor  100 , as described above. The sounding signal  110  is subtracted from the receive signal  120 - 1  in step  330  and the receive signal  120 - 1  is passed in step  335  to the digital signal processor  100 . 
     The correction coefficients are applied to the receive signal  120 - 1  in step  340 , as well as a beam forming or other coefficients, as required. 
     It will be noted that the calculation of the correction coefficients should be carried out in a carrier-based manner because there could be differences in the power of the receive signals  120  from two different ones of the carrier signals. Therefore the power controller  130  should measure the power of the required carrier signal, i.e. at the carrier signal frequency. It will, of course, be noted that should more than one carrier&#39;s receive signals  120  be received by the antenna element  20  it could be possible to include more than one power controller  130  in order to measure the power of the carrier signals of the different carriers at different frequencies. The inclusion of more than one power controller  130  enables the calculation of the correction coefficients to be carried out for more than one carrier signal at the same time. This minimises the impact of the time required for the calculation of the correction coefficients for the received carrier signals. 
     It will be appreciated that in the event that the power of the received carrier signals is significantly changed during the calculation of the correction coefficients then the measurement may be corrupted. It would be possible for a trigger to be placed within, for example the control unit  105 , that triggers the calculation procedure only when there is a low probability of a significant change in the power of the received carrier signal. 
     In further refinements of this disclosure it will be appreciated that the sounding signal, its timing and its power can be selected such that any distortions due to the sounding signal in the receive signal are minimised. For example, when calibrating GSM signals it would be possible to choose a certain time slot for the calculation procedure. Similarly for the calculation of correction coefficients for LTE receive signals a certain specified time and frequency slot should be used. A spreading code that is not in use and is not intended to be used could be used for the generation of the sounding signal and the calculation of correction coefficients for WCDMA signals. Similarly a certain time slot and spreading code could be used for the generation of the sounding signal and the calculation of correction coefficients for TD-SCDMA signals. Of course, the skilled person will understand that with other types of radio signals there are opportunities for selecting the correct timing and power of the sounding signal as well as its structure. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant arts that various changes in form and detail can be made therein without departing from the scope of the invention. In addition to using hardware (e.g., within or coupled to a central processing unit (“CPU”), micro processor, micro controller, digital signal processor, processor core, system on chip (“SOC”) or any other device), implementations may also be embodied in software (e.g. computer readable code, program code, and/or instructions disposed in any form, such as source, object or machine language) disposed for example in a computer useable (e.g. readable) medium configured to store the software. Such software can enable, for example, the function, fabrication, modelling, simulation, description and/or testing of the apparatus and methods describe herein. For example, this can be accomplished through the use of general program languages (e.g., C, C++), hardware description languages (HDL) including Verilog HDL, VHDL, and so on, or other available programs. Such software can be disposed in any known computer useable medium such as semiconductor, magnetic disc, or optical disc (e.g., CD-ROM, DVD-ROM, etc.). The software can also be disposed as a computer data signal embodied in a computer useable (e.g. readable) transmission medium (e.g., carrier wave or any other medium including digital, optical, analogue-based medium). Embodiments of the present invention may include methods of providing the apparatus described herein by providing software describing the apparatus and subsequently transmitting the software as a computer data signal over a communication network including the internet and intranets. 
     It is understood that the apparatus and method described herein may be included in a semiconductor intellectual property core, such as a micro processor core (e.g., embodied in HDL) and transformed to hardware in the production of integrated circuits. Additionally, the apparatus and methods described herein may be embodied as a combination of hardware and software. Thus, the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 
     Reference Numeral 
     
         
           10  Active antenna array 
           20  Antenna elements 
           25  Transceiver 
           30  Receive path 
           40  Duplex switch 
           50  Transmission path 
           100  Digital signal processor 
           105  Control unit 
           110  Sounding signal 
           120  Receive signal 
           130  Power controller 
           140  DAC 
           145  Transmission path 
           146  Output path 
           155  Coupler 
           160  ADC 
           170  Coupler 
           175  Subtractor 
           180  Calibration processing unit 
           230  Splitter 
           240  First gain/phase controller 
           250  Correlator and control system 
           260  A calibration coefficient unit 
           270  Adaptive filter 
           280  Second gain/phase controller