Abstract:
Variations in received signal amplitude caused by filter characteristics are reduced by a C/I measuring apparatus for a signal having a fluctuating carrier-frequency, comprising a filter for filtering the signal and means for splitting the signal into an RSS signal representing the signal strength and a BB signal representing the momentary carrier frequency deviation, comprising means for measuring the RSS signal unaffected by the frequency dependent attenuation in the IF filter. A method of measuring the C/I of a received signal is also disclosed. The method comprises the steps of filtering the signal using an IF filter to obtain one signal channel, feeding the signal to the FM detector and the RSSI unit, and measuring the signal strength of the received signal at a selected, substantially constant, carrier frequency.

Description:
TECHNICAL FIELD 
     The present invention relates to radio communication systems and in particular to the detection of disturbances in such systems in which Carrier over Interference (C/I) ratio is used to measure disturbances. 
     DESCRIPTION OF RELATED ART 
     In, for example, most Frequency Division Multiple Access (FDMA) systems the received signal on each channel is filtered out using an Intermediate Frequency (IF) filter. Variations in the amplitude of the received signal are then used to detect possible disturbance. Ideally, the IF filter should let the entire signal through in the appropriate frequency range of the filter and filter out all signals having frequencies outside of this range. With an ideal IF filter, therefore, an undisturbed signal would have a uniform amplitude for all frequencies within the frequency range of the channel. Since the IF filters used are not ideal, the signal is attenuated to a varying degree, depending on the frequency, so that the signal strength varies. Since the frequency of the signal is never entirely stable, the amplitude of the signal varies. The frequency of the signal varies, for example, with the amplitude of the modulated signal. 
     Such fluctuations in the amplitude of the signal are often mistaken for disturbances from another radio transmitter transmitting on the same frequency. The variations in frequency differ between different types of systems. 
     The same type of problems may occur in any type of radio communications network, where other type of transmitters than base stations may be used for communication with the mobile terminals. 
     Various measures can be taken to compensate for disturbances. Often a mobile telephone is ordered to change its frequency when a disturbance is detected. If there is another base station in the vicinity, the mobile telephone may be ordered to connect to this other base station instead of the one to which it is connected. The mobile telephone may also be ordered to increase its output power, to increase the C/I ratio. In extreme cases a radio transmitter may be ordered to disconnect from the base station. 
     These measures may be effective if taken when there is really a disturbance. If a disturbance is erroneously detected, they are, of course, useless. In prior art mobile telephone systems a mobile telephone may be ordered to change channels again and again, because a disturbance is erroneously detected. 
     Attempts to solve this problem include building better IF-filters. Building ideal filters is expensive, or even impossible. 
     It has also been attempted to compensate in the received signal for the imperfection of the filter. This requires very high processor capacity since each individual filter has different characteristics. Delay and beat must be taken into account, which is complicated. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and an apparatus for the reliable measurements of the disturbances to a received signal. 
     This object is achieved according to the invention by a C/I measuring apparatus for a signal having a fluctuating carrier frequency, comprising a filter for filtering the signal and means for splitting the signal into one part (RSS) representing the signal strength and one baseband part (BB) representing the momentary carrier frequency deviation, comprising means for measuring the RSS signal unaffected by the frequency dependent attenuation in the IF filter. 
     The object is also achieved according to the invention by a method of measuring the C/I of a received signal comprising the following steps: 
     Filtering the signal is filtered using an IF filter to obtain one signal channel. feeding the signal to the FM detector and the RSSI unit, 
     measuring the signal strength of the received signal at a selected, substantially constant, carrier frequency. 
     According to a preferred embodiment the C/I measuring apparatus comprises means for determining at least one point in time when the BB signal is substantially equal to a constant carrier frequency deviation. 
     By measuring the received signal at points in time where the filter attenuation is substantially equal, the measurements become more reliable without the need for compensation. 
     In a first embodiment the C/I measuring apparatus comprises a level detecting unit for monitoring the BB signal and a sampling unit, the level detecting unit controlling the sampling unit in such a way that when the amplitude of the BB signal is equal to the level specified in the level detecting unit, the sampling unit samples the RSS signal. 
     In the first embodiment of the invention the method comprises the step of determining at least one point in time at which the BB signal is substantially equal to a constant carrier frequency deviation and measuring the RSS signal at this point in time. 
     This first embodiment can be implemented by software change only, to a prior art receiver unit. 
     In a second embodiment, the C/I measuring apparatus comprises means for determining at least one point in time at which the BB signal is substantially equal to the level specified in the level detecting unit and measuring the RSS signal at this point in time. 
     In the second embodiment the method comprises the steps of 
     dividing the BB into measurement sequences; 
     taking at least one sample in each measurement sequence the BB signal; 
     selecting for each measurement sequence at least one sample nearest to the zero level; and 
     sampling the RSS signal at the point or points in time corresponding to the at least one selected sample of the BB signal. 
     This is a hardware-based embodiment, which requires less processing power than the software-based embodiment, since the sampling is performed by hardware units. 
     In the second embodiment the C/I measuring device may also comprise means for 
     dividing the BB signal into measurement sequences; 
     taking at least two samples in each measurement sequence of the BB signal; and 
     interpolating between at least two samples in each measurement sequence to obtain at least one point in which the carrier frequency deviation is constant and hence the IF filter attenuation is constant, of the BB signal; 
     sampling the RSS signal at the point or points in time corresponding to the at least one calculation point of the BB signal. 
     This enables an more reliable approximation of the value of the signal at the exactly right point in time. 
     Preferably, the carrier frequency is selected so that the attenuation of the filter is substantially stable for frequencies near this carrier frequency. 
     This reduces the effects of small fluctuations of the frequency. 
     The selected carrier frequency may be substantially the centre frequency of the filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in more detail in the following, by way of preferred embodiments, and with particular reference to the drawings, in which: 
     FIG. 1 shows a prior art receiving system for radio waves, for example in a mobile telephone; 
     FIG. 2 shows a typical IF filter attenuation curve 
     FIG. 3 shows the FM signal, the disturbed RSS signal, and the RSS signal without any disturbance; 
     FIG. 4 shows the principle of measuring the signal amplitude according to a first preferred embodiment of the invention; 
     FIG. 5 shows a first, software based embodiment of the apparatus according to the invention; 
     FIG. 6 shows a second, hardware based embodiment of the apparatus according to the invention; 
     FIG. 7 is a flow chart of the actions taken to carry out the method according to a general embodiment of the invention; 
     FIG. 8 is a flow chart of the actions taken to carry out the method according to the first preferred embodiment of the invention; 
     FIG. 9 is a flow chart of the actions taken to carry out the method according to a further development of the first preferred embodiment of the invention; 
     FIG. 10 is a flow chart of the actions taken to carry out the method according to the second preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1 shows a prior art radio receiving device which may be used in a first embodiment of the invention. As common in the art, an antenna  1  receives the incoming radio signal, which is amplified in a low-noise amplifying unit  2  and then filtered in a filter unit  3  to produce a signal comprising the signals received on all channels. The received signal is then mixed in a mixing unit  4  with a reference signal generated by a local oscillator  5 . The output from the mixing unit  4  is an intermediate signal with a frequency equal to the difference in frequency between the incoming signal from the antenna  1  and the reference signal. If, for example, the incoming signal has the frequency 900 MHz, and the reference signal has the frequency 830 MHz, the intermediate signal has a frequency of 70 MHz. 
     The intermediate signal is then filtered, in an Intermediate Frequency (IF) filter  7 , to obtain each channel separately. 
     In the following description, only one channel will be discussed, for clarity. The output signal from the IF filter  7  is fed to an FM detector  9  and to a Received Signal Strength Indicator (RSSI) circuit  11 . The output signal from the FM detector  9  is a baseband (BB) signal, and the output signal from the RSSI circuit  11  is a Received Signal Strength (RSS) signal. The BB signal and the RSS signal then pass through A/D conversion units  13  and  15 , respectively, to a processing unit  17 . The BB signal and the RSS signal will be described in more detail in connection with FIG.  3 . The processing unit receives the BB signal and the RSS signal and calculates C/I based on these signals. 
     FIG. 2 shows three different IF filter attenuation curves. The dashed line shows the characteristics of an ideal filter, which substantially cuts off all frequencies outside of the passband completely and lets through all frequencies within the passband with no attenuation. The solid line indicates the characteristics of a realistic filter, which lets through the frequencies within the passband with a varying attenuation which is lower near the centre frequency of the passband and increases towards the edges of the passband. The attenuation increases outside the passband, but frequencies outside the passband are not entirely cut off. The dotted line indicates the characteristics of another realistic filter, for which the filter curve is not centred, that is, the attenuation is lowest around a frequency different from the centre frequency. 
     As shown by the two filter attenuation curves representing realistic curves, the properties of such a filter are individual, and it is difficult to predict, or compensate for, the unideal properties of the filter. Because of the variations in attenuation, the signal strength of the received signal varies with the varying frequency of the signal. The frequency varies more or less depending on the type of system, but the principle problem is the same. 
     When determining the C/I ratio, the signal strength is sampled continuously. This variation in the detected signal strength resulting from the imperfection of the filter is often erroneously interpreted as a result of interference with another radio signal. 
     The solution according to the invention is to base the calculations only on the signal strength when the received signal has one particular frequency. This frequency could in theory be any frequency within the passband. However, it is most feasible to select a frequency near the centre of the passband where the attenuation is relatively stable for small variations in frequency. In this way, the precision of the measurements becomes less critical. 
     FIG. 3 shows the demodulated FM signal, the RSS signal with and without the variations caused by the filter attenuation. 
     The dashed line shows the baseband signal, as a function of time. As can be seen, the baseband signal varies with time, at a relatively high frequency, but the mean value is substantially constant. 
     The solid line shows the signal strength of the carrier, that is, the RSS signal with the variations caused by the frequency dependent attenuation in the IF filter super-posed on it as a function of time. 
     The dash-dotted line shows RSS signal as it would be without frequency dependent variations, as a function of time. This is the result that would ideally be achieved according to the invention. The fluctuations in signal strength seen in the solid curve are often interpreted as the result of a disturbing radio transmitter. 
     FIG. 4 shows a part of FIG. 3 magnified for clarity. 
     FIG. 4 illustrates how the undisturbed RSS signal shown by the dash-dotted line in FIG. 3 can be obtained according to the invention. The dashed line illustrates a sequence of the demodulated FM signal shown in FIG.  3 . The solid line illustrates the corresponding sequence of the RSS signal with the fluctuations caused by the filter attenuation. 
     According to a first embodiment, the FM sequence shown in FIG. 4 has been sampled eighteen times at constant time intervals. Of course, an arbitrary number of samples may be taken, and the time between the samples may vary, if desired. The sampling points are indicated in the FIG. 4 by small x&#39;es and numbered p 1 , p 2 , . . . , p 18 . 
     The sampling points are used to determine when the RSS signal should be sampled. The samples of the RSS signal are then used to construct the RSS signal without any disturbances from the filter, that is, the RSS signal that would have been received if the filter had been ideal. 
     The simplest solution for determining when the RSS signal should be sampled is to select the sampling point nearest to the mean baseband signal and to sample the RSS signal at the same point in time when this sampling point was taken. In the example shown in FIG. 4, the first sampling point p 1  is identified as the one nearest to the mean signal. At the same point in time when this sampling point p 1  was taken, the RSS signal is therefore sampled. This sample will not be taken at exactly the right time, but the approximation may be sufficiently good, depending on the filter quality and the frequency variations. 
     A more sophisticated solution is to interpolate between two or more sampling points to a higher accuracy in the calculation of the sampling reference point, which is usually zero. The RSS signal is then also interpolated to achieve the most accurate signal strength value corresponding to a fixed frequency deviation. The possibility of selecting more than one sampling point in the RSS signal is of course advantageous, in addition to the advantage of a more exact determination of the sampling points in time. The interpolation can be carried out according to any suitable interpolation algorithm known in the art. 
     These two solutions can both be implemented with only software changes. Functions may be implemented in the processor  17  shown in FIG. 1 to perform the sampling of the two signals and the interpolation if desired. 
     A preferred embodiment for carrying out the embodiments described above, with software changes only, is shown in FIG.  5 . 
     As in FIG. 1, an antenna  101  receives the incoming radio signal, which is amplified in a low-noise amplifying unit  102  and then filtered in a filter unit  103  to produce a signal comprising the signals received on all channels. The received signal is then mixed in a first mixing unit  104  with a reference signal generated by a first local oscillator  105 . The output from the mixing unit  104  is an intermediate signal with a frequency equal to the difference in frequency between the incoming signal from the antenna  101  and the reference signal. 
     The intermediate signal is then filtered, in an Intermediate Frequency (IF) filter  106 , to obtain each channel separately. In this embodiment, the IF filter  106  comprises several elements. The first element is a first IF filter in itself comprising a first part IF filter  107 , an amplifier  109  and a second part IF filter  111 . Letting the filter consist of two filters and amplifying the signal between them improves the filter characteristics. The output from the first part filter is amplified in an amplifying unit  109 , before it is filtered again in a second IF filter  111 . The output from the second IF filter  111  is mixed in a second mixing unit  113  with a reference signal generated by a second local oscillator  115 , to produce a desired frequency which in this embodiment is 450 kHz. If the resulting frequency from the first mixing unit  103  is 70 MHz, as described in connection with FIG. 1, then the frequency of the reference signal generated by the second local oscillator  115  should be 70 MHz-450 kHz. 
     This implementation of the IF filter is of course only an example. Any implementation of the IF filter can be used with the apparatuses according to the invention. 
     In the following description, only one channel will be discussed, for clarity. The output signal from the IF filter unit  106  is fed to an FM detector  119  and to a Received Signal Strength Indicator (RSSI) circuit  121 . The output signal from the FM detector  129  is a baseband (BB) signal, and the output signal from the RSSI circuit  121  is a Received Signal Strength (RSS) signal. The BB signal and the RSS signal then pass through A/D conversion units  123  and  125 , respectively, to a processing unit  127 . The BB signal and the RSS signal are similar to the ones described in more detail in connection with FIG.  3 . The processing unit  127  receives the BB signal and the RSS signal and calculates C/I based on these signals. 
     According to a second embodiment, the points in time at which to sample the RSS signal can be determined more exactly. This embodiment, however, requires additional hardware. By continuously monitoring the FM signal, the points in time at which this signal is zero may be determined exactly. At the same points in time the RSS signal is then sampled. 
     FIG. 6 shows an apparatus for determining the points in time when the RSS signal should be sampled, according to this second embodiment. As in FIG. 1, an antenna  201  receives the incoming radio signal, which is amplified in a low-noise amplifying unit  202  and then filtered in a filter unit  203  to produce a signal comprising the signals received on all channels. The signal is then mixed in a mixing unit  204  with a reference signal generated by a local oscillator  205 . This creates an intermediate signal with a frequency equal to the difference in frequency between the incoming signal from the antenna  201  and the reference signal. If, for example, the incoming signal has the frequency 900 MHz, and the reference signal has the frequency 830 MHz, the intermediate signal has a frequency of 70 MHz. 
     The intermediate signal is then filtered, in an IF filter  207  to obtain each channel separately. 
     In the following discussion only one channel will be considered, for clarity. The signal is fed to an FM detector  209  and to an RSSI circuit  211 . The output signal from the FM detector is a baseband signal indicating the frequency deviation of the carrier, and the output from the RSSI circuit is an RSS signal. These signals were described in more detail in connection with FIG.  3 . As in FIG. 1 the signals pass through A/D converters  213  and  215  respectively to a processing unit  217 . 
     The baseband signal, which is the output signal from the FM detector  209 , representing the frequency deviation of the carrier, which is used to determine the attenuation in the IF filter, is monitored by a level detector  219 . Each time the amplitude of the signal passes a specified level, the level detector  219  triggers a sampling unit  221  which samples the RSS signal in the other branch at exactly the time when the baseband signal passes the level. From the samples of the RSS signal, the RSS signal can be generated as it would look if the IF filter had been almost ideal. This RSS signal can then be used to determine if any real disturbances have occurred. The specified level may, for example, be selected as the zero level. 
     The baseband signal is continuously monitored by the level detecting unit  219 . When the amplitude of the baseband signal is the same as the level specified in the level detector, it triggers a sampling unit  221  to sample the RSS signal. The sample is passed through an A/D converter  223  to the processing unit  217  in which the samples are used to regenerate the RSS signal. 
     FIG. 7 is a flow chart showing the general principle of the method to regenerate the RSS signal without the influence of the attenuation ripple in the IF filter. For simplification, it is assumed that the level to be detected by the level detector  9 ,  119 ,  209  has been set to zero. The procedure starts between the mixing unit  3 ,  103 ,  203  and the IF filter  7 ,  106 ,  207 , in FIG. 1,  5 , and  6 , respectively. Therefore, the first two steps S 61  and S 62  are carried out in ways known in the art. 
     Step S 61 : The signal is filtered using an IF filter to obtain one signal channel. 
     Step S 62 : The signal is transmitted to the FM detector, and the RSSI circuit. The BB and RSS signals are obtained. 
     Step S 63 : At least one point in time in which the BB signal is 0, or substantially 0, is determined. 
     Step S 64 : The signal strength of the RSS signal is measured at the point or points in time found in step S 63 . 
     Step S 65 : Should the procedure be repeated for other parts of the signal? If yes, go to step S 63 , if no go to step S 66 . 
     Step S 66 : Use the sampling points obtained in step S 64  to regenerate the RSS signal. 
     FIG. 8 shows the method according to the first embodiment of the invention. This flow chart starts after step S 62  in FIG. 7, and as before, it is assumed that the level has been set to zero. 
     Step S 71 : Divide the BB signal into sequences. 
     Step S 72 : Sample a sequence of the BB signal n times. 
     Step S 73 : Find the sample having a value nearest zero of the samples taken in step S 72 . 
     Step S 74 : Find the value of the RSS signal at the same point in time as the sample of the BB signal found in step S 73 . 
     Step S 75 : Should the procedure be repeated for other parts of the signal? If yes, go to step S 72 , if no go to step S 76 . 
     Step S 76 : Use the sampling points obtained in step S 74  to regenerate the RSS signal. 
     FIG. 9 shows the method according to a development of the first embodiment of the invention. This flow chart starts at the same point as FIG.  8 . 
     Step S 81 : Divide the BB signal into sequences. 
     Step S 82 : Sample a sequence of the BB signal n times. 
     Step S 83 : Interpolate between at least two of the samples obtained in step S 82  to find at least one point in time at which the FM signal is zero. 
     Step S 84 : Find the value of the RSS signal at the point or points in time found in step S 83 . 
     Step S 85 : Should the procedure be repeated for other parts of the signal? If yes, go to step S 82 , if no go to step S 86 . 
     Step S 86 : Use the sampling points obtained in step S 84  to regenerate the RSS signal. 
     FIG. 10 is a flow chart illustrating the steps of regenerating the RSS signal according to the second embodiment of the invention. 
     Step S 91 : Measure the BB signal strength continually. 
     Step S 92 : Is the signal strength zero? If yes, go to step S 93 ; if no, go to step S 91 . 
     Step S 93 : Measure the signal strength of the RSS signal at exactly the point in time at which the BB signal is zero, that is, when the level detector is triggered. 
     Step S 94 : When the measurements are finished, use the values obtained in step S 93  to regenerate the RSS signal. 
     For FIGS. 7-10 above, to keep the discussion simple, it has been assumed that the zero level has been selected as the level to be detected. As stated above, of course, any level could be selected. 
     The method may be adapted to a system using antenna diversity. Because of the phase relationship between the carrier and the interfering signal at the antenna, the two signals received are measured individually. The two calculated C/I ratio values are added at a later stage by a diversity processing algorithm. 
     According to a preferred embodiment, a Maximum Ratio Combining (MRC) algorithm is used, taking the average for 1000 samples of RSSI_A and RSSI_B, respectively. Such algorithms, and other diversity processing algorithms are well known in the art.