Abstract:
The disclosure relates to an active antenna array for a mobile communication system which comprises a plurality of receive paths, a control unit for generating a sounding signal, and a coupler for coupling the sounding signal into at least one of a plurality of receive paths. A switch is used to switch the output of one the plurality of receive paths between one of a receiver and a calibration unit. The disclosure also provides a method for the calibration of the active antenna array which comprises generating an initial sounding signal, coupling the initial sounding signal into at least one of a plurality of receive paths to generate an adjusted sounding signal, comparing the adjusted sounding signal with an initial sounding signal and then generating calibration parameters.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. Patent Application No. 12/751,368 entitled: “ACTIVE ANTENNA ARRAY AND METHOD FOR CALIBRATION OF THE ACTIVE ANTENNA ARRAY”, filed Mar. 31, 2010 and U.S. Patent Application No. 12/751,391 entitled “ACTIVE ANTENNA ARRAY AND METHOD FOR CALIBRATION OF RECEIVE PATH IN SAID ARRAY”, filed Mar. 31, 2010. 
     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 beat a different frequency than the carrier signal spectrum. In the first case (frequency of the sounding signal is in the carrier signal spectrum) than it is necessary to correctly adjust power of the sounding signal. If the power of the sounding signal is too high, than 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 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 
     The active antenna array of this disclosure comprises a plurality of receive paths, a control unit for generating a sounding signal, and a coupler for coupling the sounding signal into at least one of the plurality of receive paths. At least one switch is located in one of the plurality of receive paths for switching the one of the plurality of receive paths between one of a receiver and a calibration unit. This switch allows the sounding signal to be passed to each one of the receive paths to enable the receive paths to be separately calibrated. 
     In one aspect of the disclosure, the active antenna array comprises a power meter for monitoring the average power of receive signals on at least one of the plurality of receive paths. This allows the power of the sounding signal to be kept at a level which does not interfere with the receive signals. The active antenna array may also include a power control for generating a power offset signal and adding the power offset signal to the sounding signal. 
     The disclosure also teaches a method for calibration 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 receive paths to generate an adjusted sounding signal and comparing the adjusted sounding signal with the initial sounding signal, thus generating correlation coefficients. The correlation coefficients can be applied to the receive signals in a digital signal processor to correct of variations in phase, amplitude and delay along the various receive paths. 
     The method may also comprise measuring power of receive signals over at least one of the plurality of receive paths and adding an offset power signal to the initial sounding signal. 
     The comparing of the adjusted sounding signal with the initial sounding signal comprises storing of the initial values of the initial sounding signals and the storing of the adjusted values of the adjusted sounding signals and comparing the initial values with the adjusted values. 
    
    
     
       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 an overview of the method used for the calibration of the single receive path. 
         FIG. 3  shows another aspect of the active antenna array. 
     
    
    
     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 features of one aspect or embodiment of the invention can be combined with a feature 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 of which  20 - 1  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  through a switch  40 - 1 . The function of the switch  40 - 1  is to switch the antenna element  20  between transmit signals being transmitted on the transmission path  50 - 1  and receive signals being received from the antenna element  20  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 transmit signals for transmission on the antenna elements  20  and to process the receive signals received from the antenna element  20 . A beamforming block  107  in the digital signal processor  100  will use 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 control unit  105  is connected to a first FIFO memory  120  and to a power controller  130 . The power controller  130  is connected to an auxiliary transceiver  27 . The sounding signal  110  is received from the power controller  130  and is converted by a digital-analogue-controller (DAC)  140  to an analogue signal and is passed along an auxiliary transmission path  145  to an output  146  and than 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 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 filter  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 the Figure 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 than to an analogue-digital-convertor  160 - 1 . The sounding signal  110  (now in digital form) is passed further to the digital signal processor  100  for processing or to a second FIFO memory  180 . A power meter  170  measures the power on the receive path  30 - 1  in the digital domain and passes the result of the power measurement to the control unit  105 . The switch  190  is controlled by a signal from the control unit  105 . 
     Both the first FIFO memory  120  and the second FIFO memory  180  are connected to the control unit  105  processor  100  and the results can be compared with each other, as will be discussed below, in order to calibrate correction values for the signals received along the receive path  25 - 1 . The first FIFO memory  120  and the second FIFO memory  180  together with the control unit  105  collectively form a calibration unit. 
       FIG. 2  shows a method which is used for the measurement and thus calculation of the compensation values for the phase, delay and amplitude of the signals received along the receive path  25 - 1 . In a first step  200  the control unit  105  receives a trigger signal to indicate that a measurement needs to be started. 
     In step  205  the control unit  105  reads the power Prx of the receive signals on the receive path  30 - 1  by means of the power meter  170 . The control unit  105  uses this power measurement Prx to configure the power control  130  to send the sounding signal  110  with a power of Prx plus an offset power Pd. The offset power Pd is an offset amount which is used to optimise the power of the sounding signal  110  for the active antenna array  10  being used. It will be noted at this time that only the power control  130  has been configured. No sounding signal  110  is yet sent. 
     In step  215  a gate signal is sent from the control unit  105  which activates the calculation procedure. The power control  130  sends the sounding signal  110  with the specified power Prx+Pd through the auxiliary transceiver  27  and the multi-way switch  150  to the required one of the transceivers  25  which is to be calibrated. It was noted above that the aspect shown in  FIG. 1  is of the transceiver  25 - 1 . It will be further noted that the multi-way switch  150  can switch the sounding signal  110  to any one of the other transceivers  25 - 2 , . . . ,  25 -B and will generally do this in a round-robin-manner so that in the course of time all of the transceivers  25 - 1 ,  25 - 2 , . . . ,  25 -N will be calibrated using the teachings of this disclosure. 
     The switch  190  is open and thus the receive signals on the receive paths  30  are not passed through to the digital signal processor  100  but instead the values are collected by the second FIFO memory  180 . The reason for the open switch  190  is to ensure that no distortions of the receive signals are passed through to the digital signal processor  100  during collections in the second FIFO memory  180 . The first FIFO memory  120  will have obtained the values of the sounding signal  110  before the sounding signal  110  was passed through the auxiliary transceiver  27 . 
     In step  220  the gate signal is deactivated and the switch  190  is closed to allow the receive signals to pass normally to the digital signal processor  100 . The values in the first FIFO memory  120  and the second FIFO memory  180  are read out and compared with each other in step  230  in order to calculate the changes in the phase, delay and amplitude of the sounding signal passing through the receive path  30 - 1  of the transceiver  25 . This corresponds to variations in the phase, delay and amplitude of the receive signals which pass along the receive path  30 - 1 . This allows the correction coefficients to be calculated which can be used to adjust the values of the phase, frequency and amplitude of the receive signals of the carrier signals received from the antenna element  20 . 
       FIG. 3  shows a further aspect of the invention in which the generation of the sounding signal  110  by the control unit  105  is replaced by the extraction of part of the receive signals received on the receive part in order to generate the sounding signal. This is done by passing the stored values in the second FIFO  150  through a second switch  195  to the auxiliary transceiver  27  as the sounding signal  110 . The stored values from the second FIFO  150  are also passed to the first FIFO  110  so that the sounding signal  110  passed to the auxiliary transceiver can be compared with the sounding signal received after passage through the receive path  30 - 1 . 
     This aspect of the invention reduces the hardware required since there is no need to have a separate circuit to generate a separate sounding signal. Furthermore there is no need to adjust the power of the sounding signal  110  as the strength of the sounding signal  110  generated from the values in the second FIFO  150  are approximately the same as those of the receive signal. 
     The control unit  105  is used to activate the calibration procedure. It does this by closing the second switch  195  so that values from the second FIFO  150  are passed to the auxiliary transceiver  27  and opening the first switch  190  so that none of the sounding signal  110  is passed through to the digital signal processor  100 . The receive signal is captured in the second FIFO  150  and, after a short delay, passed through the transmission path of the auxiliary transceiver  27  to the multi-way switch  150 . The values received in the second FIFO  150  are compared to the transmitted values stored in the first FIFO  110  to calculate the correction coefficients. 
     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 from two different ones of the carrier signals. Therefore the power meter  170  should be measuring 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 be received by the antenna element  20  it could be possible to include more than one power meter  170  in order to measure the power of the carrier signals of the different carriers at different frequencies. The inclusion of more than one power meter  170  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 and also the impact of the calibration of the receive 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.