The invention relates to a base station for a radio system, the base station comprising, for super- vising the quality of a radio link between the base station and mobile stations, a generator means for generating a supervising signal having a desired baseband frequency; measuring means for monitoring the level of the baseband supervising signal; a transmitting means for frequency-modulating a trans¬ mitting-frequency carrier by said baseband super¬ vising signal and transmitting it to the mobile radio station; a receiving means for detecting the super¬ vising signal transmitted back from the mobile radio station and modulated to a receiving frequency; and signal processing means for determining the quality of the used radio link.

In the NMT (Nordic Mobile Telephone) mobile telephone system, the quality of the radio link be- tween a base station and a mobile telephone is super¬ vised by special supervising signals. The base station generates a supervising signal e.g. by low- pass-filtering from a clock signal having a desired supervising signal frequency. The NMT system utilizes four supervising signal frequencies with spacings of 30 Hz. The base station transmits a supervising signal to the mobile telephone, which returns the supervising signal immediately to the base station, and the base station determines the quality of the used radio link, especially the signal-to-noise ratio, by means of the received supervising signal.

If the level of the baseband supervising signal generated at the base station changes for some reason before transmission, this change in level appears as deviation variations in the radio-frequency signal

transmitted from the base station to the mobile tele¬ phone and frequency-modulated by the supervising signal and also affects the measuring accuracy of the S/N ratio of the radio link. In order to ensure that the measured S/N ratio describes the quality of the radio link sufficiently accurately, the NMT specifi¬ cations define that the deviation caused by the supervising signal must not deviate from a set value by more than 10%. Today the level of the supervising signal is monitored by a measuring circuit comprising a rectifier and comparators, for which appropriate threshold voltages corresponding to the lowest and the highest allowable level of the supervising signal are set in connection with calibration by means of adjustable resistors. Such a measuring circuit is, however, slow and difficult to calibrate, as it re¬ quires at least two adjusters and four adjusting pro¬ cedures.

The object of the present invention is a base station enabling a more accurate, more reliable, and more easily calibrateable control of the supervising signal level.

In the first embodiment of the invention, this is achieved by a base station which is characterized in that the measuring means comprise an A/D converter for deriving digital sample values from the baseband supervising signal, and a control means which controls, by means of at least one reference value stored in a memory in connection with the calibra- tion, that the difference between said sample values and a digital sample value corresponding to a super¬ vising signal level causing a desired deviation in the output signal of the transmitting means is no more than a predetermined difference value. In the preferred embodiment of the invention, a

microprocessor used as the control means calculates alarm limit values from a reference value stored in the memory in the calibration of the supervising signal, the reference value being a digital sample value provided by the A/D converter from a supervising signal level causing a desired deviation. During the calibration of the level of the supervising signal, the microprocessor is merely indicated when the measured deviation caused by the supervising signal is such as desired, whereby the microprocessor stores the sample value obtained by the A/D converter in its memory as the above- mentioned reference value. The invention thus requires no manual adjustments. In addition, such a digital comparison is inherently more accurate, stable in time and more insensitive to temperature changes.

In another embodiment of the invention, the microprocessor compares digital samples with refer- ence values stored in the memory in the calibration of the supervising signal, the reference values being digital sample values obtained by the A/D converter from the supervising signal levels causing the smallest and the largest allowable deviation. In the calibration of the supervising signal, it is necessary in this version to separately determine the supervising signal levels corresponding to the smallest and the largest allowable deviation, and the microprocessor then stores the sample values cor- responding to these signal levels in its memory as alarm limit values. This embodiment requires more calibration steps than the preceding one; it, how¬ ever, avoids difficult manual adjustments.

In a further embodiment of the invention, the measuring device measuring deviation indicates the

control means directly when the deviation is such as desired. In still another embodiment the control means controls a supervising signal generator on the basis of the deviation information it receives from the measuring device in such a way that the desired deviation is achieved. Thus the whole calibration takes place automatically and can be initiated by remote control from outside the base station, if desired. The invention will now be described in greater detail by means of illustrating embodiments with reference to the attached drawing, in which

Figure 1 shows a block diagram of a base station section required for processing supervising signals; and

Figure 2 shows a measuring circuitry according to the invention for monitoring the level of the supervising signal and required calibration means.

The NMT mobile telephone system (Nordic Mobile System) is a so-called cellular telephone system in which the geographical area covered by the system is divided into smaller adjoining geographical areas or cells, each containing one ore more fixed base stations in communication with mobile telephones within the cell. The structure of the NMT system will not be described in more detail in this connection but the following publications are referred to in this respect:

[1] NMT Doc.4.1981 Technical specification for the base station equipment

[2] NMT Doc. 900-4 Technical specification for the base station equipment

[3] Addendum to NMT Doc. 900-4 Revised 1988-04- 01. [4] NMT Doc. 900-1.

The above specifications are also referred to regarding the signalling between the base station and the mobile telephone and especially regarding the supervising signal. Referring to Figure 1, the generation of the supervising signal at the base station will be dis¬ cussed first. The base station comprises a source generating a clock signal, such as a crystal oscil¬ lator 16 which applies a clock signal f _c to an adjustable divider circuit 20. The divider circuit 20 divides the clock signal f _c by a divisor term determined by a frequency setting signal FSET to produce a main clock signal CLK. The main clock signal CLK is divided by a fixed divider circuit 2 which produces a supervising-signal-frequency square wave signal CK and one or more filtering clock signals mCK having preferably a frequency which is the multiple of the frequency of the signal CK. The square wave signal CK is lowpass-filtered by a low- pass-filtering block 1 containing a Switched capac¬ itor lowpass filter and a level adjustment. The SC filter is controlled by the clock signal mCK, whereby its cut-off frequency is adapted automatically to the frequency of the square wave signals. The output signal of the filtering block 1 is a sinusoidal supervising signal CSr _p , which during normal operation is applied through a switching unit 14 to a con¬ tinuous lowpass filter 12, which filters the clock signal mCK out of the supervising signal. The output signal of the filter 12 is applied to a transmitting unit 15 at the base station for transmission through a transmitting antenna 18 to a mobile radio station.

The supervising signal CS _T is also applied to a level indicator 13 which monitors the level of the supervising signal, thus ensuring that it is in a

window defined between predetermined limit values. If the level of the supervising signal falls outside this window, the indicator 13 generates an alarm signal X3. The supervising signal returned from the mobile station and received at the base station by a receiv¬ ing antenna 19 and a receiver section 17 is applied through filters 11 and 3 and the switching unit 14 to a signal processing circuitry comprising a mixer 4; filters 5, 6 and 10; signal processing blocks 8 and 9; and a level indicator 7, which signal processing circuitry indicates the frequency of the supervising signal and measures the S/N ratio.

The structure and operation of the equipment described above are described more closely in Finnish Patent Application 900620.

The present invention relates mainly to the realization of the level supervising circuitry 13 and the calibration of the level of the supervising signal. Figure 2 shows a block diagram illustrating the principal features of the base station equipment required for the purpose. A generator block 21 illustrates generally the generation of the super¬ vising signal and may contain e.g. the components 1, 2, 16 and 20 disclosed in connection with Figure 1. The output of the generator block 21 is connected to a modulating input in a frequency modulator 27, and the transmitting-frequency frequency-modulated output signal of the modulator 27 is applied through a high- frequency amplifier 28 to a transmitting antenna 31. The output of the generator block 21 is further con¬ nected through a rectifier 22 to an analog input in an analog-to-digital converter 23. The operation of the A/D converter 23 is controlled by a micro- processor 25 which reads at regular intervals a

digital sample value derived by the A/D converter 23 from the voltage level of the supervising signal and compares it with a reference value stored in an electronic memory 24, the reference value being a digital sample value derived by a D/A converter from the voltage level of a supervising signal causing a desired deviation in the output signal of the modu¬ lator. The microprocessor 25 generates an alarm signal X3 if the sample value derived from the super- vising signal during the operation of the quality control of the radio link deviates from said refer¬ ence values more than by a predetermined difference value. This difference value is preferably ±10% of the reference value. The microprocessor 25 calculates automatically the alarm limits from the stored refer¬ ence value. The alarm limits may be calculated each time the comparison is made; alternatively, they can be stored in the memory 24 after the first calcula¬ tion, whereby the sample values can subsequently be compared directly with these stored alarm limits.

To calibrate the voltage level of an outgoing supervising signal, a modulation analyzer, such as Hewlett-Packard HP 8903, is connected to the output of the modulator 27 or the high-frequency amplifier 28 for measuring a frequency deviation caused by the voltage level of the modulating baseband supervising signal in the outgoing radio-frequency signal. In the simplest calibration system, the voltage level of the supervising signal generated by the generator 21 is adjusted manually until the modulation analyzer 29 indicates that the deviation caused by the super¬ vising signal is such as desired. Thereafter the performer of the calibration applies a control signal from an operating device 30, such as a service monitor, to the microprocessor to indicate that the

calibration of the deviation and the voltage level of the supervising signal is completed. On receiving the control signal, the microprocessor 25 reads from the A/D converter 23 a digital sample value corresponding to the present voltage level of the supervising signal and stores this sample value in the memory 24 for use as the above-mentioned reference value.

In the figure, the operating device 30 is con¬ nected to the microprocessor 25 through another microprocessor 26 which controls the operation of all speech and control channel units of the base station in a centralized manner. Each channel unit comprises its own supervising signal generation, signal pro¬ cessing and level supervising circuitry. In the other embodiment of the invention, the modulation analyzer 29 may be connected through the microprocessor 26 to the microprocessor 25, as is shown by a broken line 32. The analyzer 29 may thus directly indicate the microprocessor of the comple- tion of the calibration of the deviation and the supervising signal level in a desired manner, and thus initiate the storage procedure described above. The microprocessor 25 may further control the gener¬ ator 21 automatically in response to the deviation information it receives from the analyzer 29 so as to cause it to change the voltage level of the super¬ vising signal in such a way that the desired devia¬ tion is achieved. It is thereby possible to carry out the calibration by remote control from outside the base station.

Alternatively, the microprocessor 25 may monitor the level of the supervising signal by comparing the digital samples it reads from the A/D converter 23 directly with alarm limit values stored in the memory 24 and being digital sample values

derived from the supervising signal levels causing the smallest and the largest allowable deviation. The calibration can again be carried out by any one of the above-described calibration systems, except that the supervising signal level now first has to be adjusted to the values causing the smallest and the largest allowable deviation before the actual calibration to the nominal value to derive digital sample values from these voltage levels and to store them in the memory 24 as the above-mentioned alarm limits. This alternative solution thus requires more adjustments than the above-described preferred embodiment.

The rectifier 22 is not necessary in the measuring circuit but it can be omitted if a suf¬ ficiently rapid A/D converter is used.