WO2003017494A1

WO2003017494A1 - Method and device for frequency synchronisation

Info

Publication number: WO2003017494A1
Application number: PCT/DE2002/002885
Authority: WO
Inventors: Christian Steinbauer
Original assignee: Siemens Aktiengesellschaft
Priority date: 2001-08-10
Filing date: 2002-08-06
Publication date: 2003-02-27
Also published as: DE10139467A1

Abstract

The invention relates to a method and device for frequency synchronisation of a mobile station with a base station. The frequency synchronisation is also possible with a grossly-detuned oscillator crystal by selection of an appropriate seek range.

Description

description

Method and device for frequency synchronization

The present invention relates to a method and a device for frequency synchronization of a mobile station with a base station, in particular under extreme temperature conditions.

To carry out a transmission in the field of mobile communication, the mobile station must be synchronized with the base station.

In the GSM system, the base station sends signals on the frequency carrier of the Broadcast Control Channel (BCCH) that allow a mobile station to transmit or

Adapt the reception frequency to a reference frequency specified by the base station. For this purpose, the base station sends a so-called frequency correction burst (FB) in the GSM system, by means of which the mobile station can determine the frequency offset with respect to the base station.

This frequency correction burst consists of a predefined bit sequence from which the GMSK modulator (Gaussian Minimum Shift Keying) used in the GSM system generates a pure sinusoidal signal. This then corresponds to an unmodulated carrier (frequency channel) shifted with respect to the reference frequency. In the GSM system, the shift is 67.7kHz.

In the mobile station, once the sine signal has been measured, the digital signal processor can then calculate back to the reference frequency.

This reference frequency is then produced by means of a frequency synthesis circuit.

Ultimately, the output frequency f_out of the frequency synthesis circuit shown in FIG. 1 should match the reference frequency specified by the base station. Of course, the facts presented here are not limited to the GSM system; the term "mobile station" also includes a terminal or, in general, any mobile unit in a communication network. The term “base station” is generally understood to mean a central facility in a communication network.

The simplified concept of a frequency synthesis circuit can now be seen in FIG. The heart of the frequency synthesis circuit is a PLL (Phase Locked Loop). The PLL has a frequency / phase comparator PD, a filter LF, a high-frequency oscillator HFO and a divider D. The externally fed oscillator frequency of an oscillator crystal VCXO is compared with the frequency in the PLL loop by means of the phase / frequency comparator PD. The filter LF, the so-called loop filter, serves to filter out undesired high-frequency components. The high frequency oscillator HFO is used to synthesize the high frequency. The high frequency is divided down by means of the divider D in order to approximate it to the external, that is to say produced outside of the PLL, oscillator frequency of the VCXO, so that they can be compared with one another by means of the phase / frequency comparator. In most cases, the oscillator frequency is also divided down using a further divider D2 in order to be able to use technologically simple, energy-saving and inexpensive components in the PLL due to the lower input frequency.

The frequency f_out on the high-frequency side is specified as a reference frequency by the base station. f_out is produced on the basis of the input frequency f_in, which is given by the oscillator frequency. f_in, the oscillator frequency, can be varied by changing the voltage on the oscillator VCXO.

The oscillator crystal is already calibrated at room temperature during manufacture. For this, the voltage is measured, the is necessary to achieve a certain oscillator frequency. This happens at only a few calibration points in order to keep the costs as low as possible. For the sake of simplicity, this procedure is explained when using three calibration points:

The calibration values for frequency f_min at the minimum tuning voltage, center frequency f_c at medium tuning voltage and frequency f_max at maximum absorption voltage are determined. Only the value f_c is used for the frequency synchronization; the other calibration values for the oscillator frequencies f_min and f_max are used to approximate the frequency behavior of the oscillator in a limited voltage range.

For frequency synchronization, the reference frequency is now generated on the basis of the center frequency f_c of the oscillator, for example 13 MHz, or by means of the oscillator frequency divided by the divider D2.

The disadvantage here is that the oscillator frequency depends not only on the voltage applied, but also on other factors, in particular the temperature. The oscillator crystal is therefore adjusted in relation to the calibration created during production if there is a different temperature than that which was assumed during the calibration.

A different tuning voltage is therefore required at the oscillator crystal to generate f_c than that which was determined during the calibration. To do this, the tuning voltage is now varied by the voltage value associated with f_c.

Since a changed temperature is usually the most prominent reason for detuning the oscillator crystal, the temperature in the mobile station is measured, for example, using a thermistor and using a corresponding appropriate temperature compensation algorithm compensates for the temperature-dependent error of the oscillator crystal VCXO.

The following occurs with frequency synchronization: First the temperature is measured, whereby the voltage correction value is determined. This voltage correction value is given to the tuning voltage for the middle frequency f.

Disadvantageously, this method is complex on the one hand, and on the other hand the temperature compensation itself or precisely does not provide the desired result, in particular at the limits of the temperature range of approximately 0 to 55 ° C. which is considered normal. The main reasons for this are:

A) The approximation of the course of the oscillator frequency over the temperature with a polynomial, as is suggested from theoretical considerations, shows considerable deviations at the temperature limits. B) The tuning steepness of the oscillator crystal in

Hertz / volt, which is used in the temperature compensation algorithm to compensate for frequency deviations, changes over temperature, which means that a curve in which the oscillator frequency is plotted against the tuning voltage has a different course for different temperatures having. To take this into account, these curves would have to be stored for different temperatures in order to determine the tuning slope as a function of the tuning voltage, oscillator frequency and temperature.

C) The calibration values obtained in production, i.e. the frequency f_min at the minimum tuning voltage, the center frequency f_c at the medium tuning voltage and the frequency f_max at the maximum tuning voltage, are not obtained exactly at 25 ° C, as is the case for the exact one Error correction using a polynomial would be necessary. D) The hardware of the oscillator, such as the oscillator crystal itself, capacitance diodes, transistor and so on, are subject to aging, in the course of which the properties of the oscillator change. E) The digital-to-analog converter of the baseband chip set, on which the frequency synthesis circuit shown in FIG. 1 is located, is subject to strong scatter, that is to say the average tuning voltage of the oscillator crystal can vary from chip set to chip set and thus shift the center frequency of the oscillator crystal.

F) The measurement of the temperature in the mobile station by means of a thermistor is not arbitrarily precise, since here too the temperature behavior is approximated using a polynomial, which of course is flawed at the temperature limits. In particular, the incorrect measurements here affect the temperature drift and the inaccuracy of the analog-digital converter, by means of which the thermistor voltage is digitized. This error is fully included in the temperature compensation calculation algorithm, i.e. the oscillator crystal is incorrectly compensated.

With regard to the points mentioned above, it must also be noted that in order to keep the error contributions small, very high quality components are used.

In summary, the main disadvantages of frequency synchronization are that temperature compensation must take place and the algorithm for temperature compensation is complex; furthermore, that this temperature compensation algorithm delivers unsatisfactory results, especially in the case of deviations from the room temperature; and that the components used must be of high quality and therefore also have a high price. It is therefore an object of the present invention to simplify the frequency synchronization of a mobile station with a base station and to enable it in particular even in the case of non-standard temperature conditions.

This task is solved by the independent claims. Further developments result from the dependent claims.

According to the invention, a movable unit, for example a mobile station, is synchronized with a central unit, for example a base station, with respect to the frequency in which the central unit emits signals which indicate the reference frequency which it predefines. In the movable unit, these signals are processed to set the reference frequency and the reference frequency is searched for by segmenting all or part of the available frequency range and searching for the reference frequency in at least one frequency range segment.

This also has the following advantage: If additional frequency ranges, which are based, for example, on further calibration points, are opened up as a new segment range, frequency synchronization is also possible with a strongly disjointed oscillator crystal. This detuning can now be based on the temperature behavior of the oscillator crystal, as well as on a low quality of the oscillator crystal. As a result, frequency synchronization is made possible in particular at temperatures deviating from normal, as is the use of inferior oscillator crystals which are less reproducibly detunable, but of course also cheaper.

A further development provides that the search for the reference frequency is ended when the reference frequency has been found by the mobile station. This avoids unnecessary bills. In another development, the mobile station contains an oscillator device with an oscillator, by means of which an oscillator frequency is provided. The frequency range in which the reference frequency is searched is changed by changing the oscillator frequency.

For example, a change between the individual segments of the entire frequency range is realized by applying a different tuning voltage to the oscillator crystal. The term "oscillator" also includes the oscillator crystal itself, as well as capacitance diodes, transistors and the like.

Another development provides for the frequency range in which the reference frequency is searched to be selected or also changed as a function of external parameters. The search for the desired reference frequency can thus be shortened if, for example, there is a relationship between external parameters and the frequency behavior of the oscillator.

A corresponding device has an oscillator circuit, by means of which an externally predetermined reference frequency is synthesized. This oscillator circuit has an oscillator which provides an oscillator frequency, on the basis of which the reference frequency is searched for. The oscillator frequency is changed by means of a processor unit in the device if the search for the reference frequency with the current oscillator frequency has been unsuccessful. A new search for the reference frequency is then carried out based on the changed oscillator frequency.

Because of the cheapness of the components used and the low computational complexity, this device is particularly suitable for installation in a mobile radio device. Further details, features and advantages of the invention result from the following description of preferred embodiments with reference to the drawing. Show it:

Figure 1 is a schematic representation of a frequency synthesis circuit with its essential components, in particular the oscillator crystal VCXO

Figure 2 shows the ranges in which the oscillator frequency is varied in order to search for the reference frequency.

FIGS. 2a and 2b show the windows W1 and W1, W2 and W3, within which the tuning voltage U, which is applied to the oscillator VCXO, is varied in order to achieve the desired oscillator frequency.

The tuning voltage U is plotted on the lower axis in FIG. 2a. If the value of the tuning voltage is U (f_c), then according to the calibration, the oscillator crystal has the center frequency f_c. As already explained at the beginning, the calibration usually takes place at three frequency values, to which a voltage is then assigned. The two other calibration points are here designated U (f_max), which characterizes the maximum tuning voltage, and U (f_min), which characterizes the minimum tuning voltage. The oscillator crystal can be tuned to a maximum within this range, which means that there is a calibration in this voltage range.

On the axis above is the deviation from the center frequency, as is achieved with the voltage calibration value U (f_c) and the temperature compensation.

If both calibration and temperature compensation are correct, the deviation relates to that of the oscillator crystal assigned oscillator frequency of, for example, 13 MHz. The deviation from this should ideally be zero Hz, the absolute frequency as reference point should be 13 MHz.

On the basis of 13 MHz, after the transformation into the high frequency range by the PLL, the reference frequency transmitted by the base station is also obtained.

Due to the reasons given at the beginning under points A) to F), the theoretically defined voltage value U (f_c), which should actually cause a frequency of exactly 13 MHz, can cause a different frequency than the intended 13 MHz for the oscillator. In this case, frequency synchronization with the base station would only work under certain conditions. These are explained below:

Depending on the amount of the resulting frequency difference between the mobile station and the base station, the frequency synchronization behavior of the mobile station changes accordingly. With increasing frequency difference, the signal energy received in the digital signal processor for evaluation decreases in a non-linear manner over the frequency shift due to the power density distribution of the GMSK modulated received signal used for example in GSM, so that the synchronization behavior gradually deteriorates until finally no synchronization at all from a certain frequency shift is more possible. This is the case when the synchronization signal of the base station can no longer be received or can be meaningfully evaluated. The frequency of the reference oscillator is then outside the capture range of the automatic frequency correction control, ie the range within which frequency shifts can be compensated. The capture range and thus the maximum permissible frequency offset of the oscillator is determined by the quality of the detector used. algorithm as well as the signal conditions at the receiving location

In summary, two cases can be distinguished:

a) If the value of the applied tuning voltage U (f_c) causes a frequency difference between the mobile station and the received base station that is smaller than the capture range of the control loop, the frequency synchronization can nevertheless take place. The control loop of the automatic frequency correction (AFC) will change the value of the tuning voltage U (f_c) step by step according to a certain algorithm until the frequency error is less than a certain threshold value, which depends on the mobile radio system used.

b) If the value of the oscillator frequency and the associated tuning voltage during the attempt to synchronize lie outside the capture window of the control loop, synchronization with the base station is no longer possible under the present system conditions. A log in let alone a data transfer can then not take place.

An exemplary embodiment of the method according to the invention is shown in FIG. 2b. In this case, the tuning voltage is not only varied in a window by the tuning voltage U (f_c) for the center frequency, but further windows are used on the basis of the other values of the tuning voltage, which were set during the calibration. In the exemplary embodiment shown here, the window W1 is defined analogously to the case shown in FIG. 2a. In addition, two further windows W2 and W3 were defined on the basis of the minimum tuning voltage and the maximum tuning voltage. Due to the overlap of two adjacent windows it is guaranteed that the desired oscillator frequency is found in any case.

Let the tuning voltage, which leads to a value of the oscillator frequency of 13 MHz for the oscillator crystal VCXO, be marked as point OS. In this case, the tuning voltage determined by means of the calibration and temperature compensation achieves a value shifted from the target oscillator frequency of 13 MHz. In particular, this value has such a shift with respect to the target oscillator frequency that it no longer lies within the first window W1, the capture range, and consequently the base station can no longer be determined by the search algorithm previously used.

Finally, it should be shown how significant the frequency deviations in the high-frequency range are when the oscillator frequency is detuned in the ppm range. For example, the point OS from FIG. 2b lies at a deviation of 20 ppm.

It is assumed that the frequency in the GSM 1800 system is transformed to 1880 MHz: the divider D2 has a division ratio of 65, that is to say the input frequency f_in is 200 kHz. The division ratio of D is 1880MHz / 200kHz. With a deviation of 20ppm, there is a deviation of (20ppm * 13MHz) / 65 * (1880MHz / 0.2MHz) = 37.6kHz in the high-frequency range, which corresponds to about a fifth of the bandwidth of a GSM channel.

The range from -Δ_ max to + Δ_ max, which covers the entirety of the windows, should therefore advantageously be selected such that a substantial range of the bandwidth of the channel is covered. Corresponding tuning voltage values are selected to achieve this. In addition to the exemplary embodiments described, there are of course a large number of further embodiment variants within the scope of the invention, which are not described further here. However, they can easily be put into practice by a specialist on the basis of the exemplary embodiments.

Claims

claims

1. Method for frequency synchronization of a mobile station with a base station, in which the base station sends out signals which describe a reference frequency,

the mobile station processes these signals to set the reference frequency,

- First, the reference frequency is searched in a first frequency range using a search algorithm,

- and the reference frequency is searched for in at least one further frequency range using a search algorithm if the reference frequency was not found in the first frequency range.

2. The method of claim 1, wherein the search for the reference frequency is terminated when the reference frequency is found.

3. The method according to any one of claims 1 or 2,

- in which the mobile station has an oscillator device with an oscillator which provides an oscillator frequency, - in which the frequency range in which the reference frequency is sought is changed by changing the oscillator frequency.

4. The method according to any one of the preceding claims, in which the frequency range in which the reference frequency is sought is selected as a function of external parameters.

5. Device with an oscillator circuit for providing a reference frequency, the value of which is predefined externally, the oscillator circuit comprising an oscillator which has a Oscillator frequency, on the basis of which the reference frequency is sought, with a processor unit which is designed such that the oscillator frequency is changed if the search for the reference frequency with the unchanged oscillator frequency is unsuccessful and a new search is carried out on the basis of the changed oscillator frequency.

6. Mobile radio device with a device according to claim 5.