METHOD AND ARRANGEMENT FOR FORMING CALIBRATION SIGNAL

Abstract: The present invention relates to a method and an arrangement for forming a calibration signal, in which arrangement a CDMA signal is used. The arrangement comprises means (200) for placing the calibration signal in the frequency band of the CDMA signal and amplification means (300) for amplifying the CDMA signal, in which CDMA signal information is sent. The arrangement comprises means (100) for attenuating the frequency band of the CDMA signal at least at one point and means (200) for forming and placing the calibration signal in the attenuated point of the frequency band.

Description: METHOD AND ARRANGEMENT FOR FORMING CALIBRATION SIGNAL FIELD OF THE INVENTION The present invention relates to a method for forming a calibration signal, which method uses a CDMA signal, in the frequency band of which the calibration signal is placed, in which method the CDMA signal is amplified, by means of which CDMA signal information is sent. The present invention further relates to an arrangement for forming a calibration signal, in which arrangement a CDMA signal is used, which arrangement comprises means for placing the calibration signal in the frequency band of the CDMA signal and amplification means for amplifying the CDMA signal, in which CDMA signal information is sent. BACKGROUND OF THE INVENTION It is previously known to use calibration signals for adjusting amplifiers or attenuation in various apparatuses and systems, for example. The purpose of adjustment is generally to maintain the signal obtained from the amplifier at a predetermined reference value, for example. In cellular radio systems employing a spread spectrum CDMA system (Carrier Division Multiple Access) calibration signals are used for adjusting CDMA linear amplifiers, for example. Linear amplifiers cause less distortion to a signal in comparison with amplifiers generally in use. The forming of intermodulation distortion is in linear amplifiers smaller in comparison with intermodulation distortion formed in ordinary amplifiers. In linear amplifiers the forming of intermodulation distortion is decreased by specific linearization methods. Known linearization methods of intermodulation distortion are feedforward, predistortion and Cartesian feedback, for example. Different linearization methods are based on signal processing. Various amplitude and phase changes are nevertheless produced in signal processing. Said amplitude and phase changes are detrimental to the correct operation of the system. It is thus known to use different methods with which the harmful effects of amplitude and phase changes on the operation of the system can be eliminated. The amplitude and phase changes produced in linearization of intermodulation distortion are eliminated by using the calibration signal formed in the system. Elimination takes place in such a manner that the formed calibration signal is arranged to pass through the system. After having passed through the system, the calibration signal includes information about the amplitude and phase changes that have occurred in the system. After this the amplitude and phase changes included in the calibration signal are detected and they are further utilized when adjusting the system. Calibration signals are narrowband and continuous signals, for example. Calibration signals can also be signals realized by the spread spectrum technique, for instance. If a narrowband signal is used as a calibration signal, the frequency of the calibration signal is set as such that a signal is not sent simultaneously on the frequency used by the calibration signal. Therefore it is necessary to set the calibration signal outside the frequency band of the signal sent by the system. It is also possible that the level of the calibration signal is also much higher than the level of the transmitting signal. In the above-mentioned cases it is necessary for the system to build a specific arrangement with which the calibration signal is eliminated and removed from the output of the system. The arrangement used for removing the calibration signal from the system will, however, become complicated. If the calibration signal is implemented with the spread spectrum technique, the calibration signal behaves as noise. In this case, it is not necessary to remove calibration signals. The forming and detection of a calibration signal implemented by the spread spectrum technique is considerably more complicated and slower than the detection of a narrowband and continuous calibration signal. It is also necessary for the calibration signal implemented by the spread spectrum technique to be as far as possible from the actual signal when filtering, in which case calibration accuracy will deteriorate. CHARACTERISTICS OF THE INVENTION The object of the present invention is to provide a method for realizing a calibration signal in such a manner that the forming and detection of a calibration signal becomes easier and the formed calibration signal does not need to be removed from the output of the system. This will be attained with the method as disclosed in the preamble which is characterized in that the frequency band of the CDMA signal is attenuated at least at one point and the calibration signal is placed in the attenuated point of the frequency band. The arrangement of the invention is characterized in that the arrangement comprises means for attenuating the frequency band of the CDMA signal at least at one point and means for forming and placing the calibration signal in the attenuated point of the frequency band. Considerable advantages are attained with the solution of the invention. In the method of the solution a digitally-implemented narrowband and continuous calibration signal and a CDMA signal are formed, the frequency band of the CDMA signal being attenuated at least at one point. The formed calibration signal is placed in the attenuated point of the frequency band, whereby it does not disturb the CDMA signal. The calibration signal is thus placed inside the frequency band of the CDMA signal. The formed calibration signal need not be removed from the CDMA signal as it does not cause disturbances to the system. As the calibration signal need not be removed, the system need not build a complicated arrangement for removing the calibration signal. In case the calibration signal is to be removed, it is easy to make the removal in the solution of the invention by a digital filter because the signal is narrowband. The formed calibration signal can be used in various linerization methods, such as predistortion. It is also possible in the method to form several calibration signals which are placed in different points of the frequency band of the CDMA signal for improving the adjustment of the system. The preferred embodiments of the method of the invention also appear from the appended dependent claims, and the preferred embodiments of the arrangement of the invention appear from the appended dependent claims. BRIEF DESCRIPTION OF THE FIGURES In the following, the preferred embodiment of the invention will be explained in more detail by means of the appended drawings, wherein Figure 1 illustrates a prior art arrangement for forming a narrowband calibration signal, Figure 2 illustrates an arrangement of the invention for forming a narrowband calibration signal, and Figure 3 illustrates a signal spectrum after filtering the arrangement of the invention, to which signal a narrowband calibration signal is added. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 illustrates a prior art arrangement for forming a narrowband calibration signal. A CDMA signal is carried to the arrangement, and the arrangement comprises means 11 for forming a narrowband calibration signal, amplification means 12 for amplifying the signal, detection means 13 and means 14. The calibration signal formed by the means 11 is combined to the CDMA signal and the combined signal is conveyed to the amplification means 12. Changes, such as amplitude and phase changes are produced to the signal formed by the CDMA signal and the calibration signal in the amplification means 12. The calibration signal having passed through the amplification means 12 is taken to the detection means 13 after amplification, after which the calibration signal is removed in the means 14. The removal of the calibration signal carried out by the means 14 is in practice done for example by filtering the calibration signal. The removal of the calibration signal can also be done by summing a signal with the opposite phase of the calibration signal, that is, a signal in the phase transfer of 180 degrees. The detection means 13 are arranged to detect in the amplification means 12 the amplitude and phase changes formed in the calibration signal. The amplitude and phase changes detected in the detection means 13 are utilized when adjusting the system, in practice, the means 14 are complicated and difficult to be built. Figure 2 illustrates the arrangement of the invention for forming a calibration signal and for placing a calibration signal in a signal formed with the CDMA method, by means of which signal information is transmitted. The arrangement shown in Figure 2 comprises filtering means 100, means 200, amplification means 300 and detection means 400. A broadband signal formed by the CDMA method and carried to the filtering means 100 is conveyed to the arrangement. The filtering means 100 are connected to the means 200 and the amplification means 300. The amplification means 300 are connected to the detection means 400. The filtering means 100 filter the broadband CDMA signal received at the filtering means 100. The filtering means 100 carry out digital filtering for the incoming signal. The filtering means 100 are in practice implemented by a notch filter, for example. The means 200 form a calibration signal and place the calibration signal formed by them at a predetermined point in the CDMA signal filtered by the filtering means 100. The level of the calibration signal can be as high as or lower than the level of the signal transmitted from the arrangement. It is especially significant that the calibration signal and the broadband signal have a sufficiently great level difference when being examined on the same frequency. The processing and further processing of the calibration signal formed by the means 200 is simple as the calibration signal is continuous and narrowband. If the calibration signal is to be removed from the broadband output signal, the output signal is mixed to the baseband and the calibration signal is separated by a digital filter. For example, a band-pass filter may in practice be a digital filter. The calibration signal does not, however, cause any problems even if it were not removed from the output signal as the level of the calibration signal is as high as or lower than the level of the signal to be transmitted. The used frequency of the calibration signal can be selected freely, but so that the frequency is inside the frequency band of the CDMA signal. The frequency selection of the calibration signal can be adaptive, for example, in which case in the arrangement only the frequency of the calibration signal and the coefficients of the digital filter are changed. The means 200 can form preferably several calibration signals on a different frequency in addition to one calibration signal. By the use of several calibration signals, a frequency response is obtained in the system which is used to control a frequency-selective control element or a recursive filter, for example. The filtering means 100 filter the CDMA signal received at the filtering means 100. In the filtering, the CDMA signal is attenuated on a very narrow band inside the frequency band of the CDMA signal. The width of the attenuated band is very small in comparison to the band of the signal that is not attenuated. The means 200 are arranged to place the calibration signal formed by them in the narrow band attenuated by the filtering means 100. The filtering means 100 attenuate the frequency band of the CDMA signal at various points, in which case the means 200 place the CDMA signal at a point of the attenuated narrow band in the calibration signal. The means 200 place the calibration signal formed by them preferably at the point of the narrow band attenuated by the filtering means 100 where the attenuation is at its greatest. The calibration signals placed in the attenuated bands of the CDMA signal operate on different frequencies. The calibration signal collects information about amplitude and phase changes of the signal in the arrangement when the calibration signal travels through the arrangement. The information collected by the calibration signal are used to help to adjust the amplification means 300. The amplitude and phase changes in the calibration signal are detected in the detection means 400. After this, the amplification means 300 are adjusted by the manner determined by the changes detected by the detection means 400. Figure 3 illustrates the spectrum of a CDMA signal transmitted from the filtering means 100. It can be seen from the spectrum that the filtering means 100 have attenuated the CDMA signal only on a narrow frequency band. In the solution shown in the figure the CDMA signal is attenuated at one point only. To the position of the attenuated spectrum, the means 200 add a narrowband and continuous calibration signal formed by them which signal is used to help to adjust the amplification means 300. It can also be seen in the figure that the band attenuated by the filtering means 100 is very narrow. The filtering means 100 carry out digital filtering on the baseband of digital filtering, and in practice, the filtering means 100 are implemented by a notch filter. In the spectrum of the figure, the level of the calibration signal formed by the means 200 is lower in comparison with the level of the CDMA signal on the same frequency. As in the solution of the invention the level of the calibration signal is selected to be suitable with respect to the level of the CDMA signal, the formed calibration signal need not be removed or eliminated in any way. Furthermore, the calibration signal can be placed in any position inside the CDMA frequency band. Although the invention has been explained above with reference to the example of the accompanying drawings, it is evident that the invention is not restricted thereto, but it can be varied in several ways within the inventive idea disclosed in the appended claims.

Claims: CLAIMS 1. A method for forming a calibration signal, which method uses a CDMA signal, in the frequency band of which the calibration signal is placed, in which method the CDMA signal is amplified, by means of which CDMA signal information is sent, characterized in that the frequency band of the CDMA signal is attenuated at least at one point and the calibration signal is placed in the attenuated point of the frequency band. 2. A method according to claim 1, characterized in that the width of the attenuated frequency band is small with respect to the width of the unattenuated frequency band. 3. A method according to claim 1, characterized in that the calibration signal is placed at the point of the frequency band where the attenuation is essentially at its greatest. 4. A method according to claim 1, characterized in that the calibration signal collects information about amplitude and phase changes. 5. A method according to claim 1, characterized in that in the method several calibration signals are formed which are placed at different points of the frequency band to improve the adjustment of the system. 6. A method according to claim 1, characterized in that the calibration signal formed in the method is continuous and narrowband. 7. An arrangement for forming a calibration signal, in which arrangement a CDMA signal is used, which arrangement comprises means (200) for placing the calibration signal in the frequency band of the CDMA signal and amplification means (300) for amplifying the CDMA signal, in which CDMA signal information is sent, characterized in that the arrangement comprises means (100) for attenuating the frequency band of the CDMA signal at least at one point and means (200) for forming and placing the calibration signal in the attenuated point of the frequency band. 8. An arrangement according to claim 7, characterized in that the means (100) attenuate the frequency band on a very narrow width with respect to the unattenuated part of the frequency band. 9. An arrangement according to claim 7, characterized in that the means (200) place the calibration signal at the point of the frequency band where the attenuation is essentially at its greatest. 10. An arrangement according to claim 7, characterized in that the calibration signals formed by the means (200) collect information about amplitude and phase changes in the amplification means (300). 11. An arrangement according to claim 7, characterized in that the means (200) form several calibration signals which are placed at different points of the frequency band to improve adjustment. 12. An arrangement according to claim 7, characterized in that the calibration signal formed by the means (200) is continuous and narrowband. 13. An arrangement according to claim 7, characterized in that the amplification means (300) are in practice implemented by a linear amplifier.

Inventors: PEKKARINEN JARI, PEKKARINEN, JARI
Applicants: NOKIA TELECOMMUNICATIONS OY, PEKKARINEN JARI, PEKKARINEN, JARI
Application year: 1997
Main classification: H04J13
IPC codes: H04B1, H04B7, H04B17, H04J13