MXPA97003890A - Digital modulated frequency receiver condesmodulator and method for demodulating a signal ra - Google Patents

Abstract

The present invention relates to a digital FM receiver having a demodulator that receives a radio signal and generates from there a phase sample whose behavior determines a demodulator output, an apparatus for removing a low frequency interference signal from the sample of phase, characterized in that it comprises: means for feeding to receive the phase sample, first means for converting the phase sample into a frequency sample, high-pass filter means, for withdrawing from the frequency sample, energy associated with frequencies below a predetermined frequency, the high-pass filter means provide a filtered frequency sample at a filter output, and second means for converting the filtered frequency sample into a filtered phase sample.

Description

OE MODULATED DIGITAL FREQUENCY RECEIVER WITH DEMODULATOR AND METHOD FOR DEMODULATING A RADIO SIGNAL The present invention relates to techniques for removing low frequency interference of a received angle modulated signal, which transports digital information and more particularly techniques for removing this interference in samples. of phase of the signal modulated in received angle. Techniques for communicating digital information when using it to modulate a carrier signal are well known. A modulator-demodulator (modem) is a well-known device designed for this purpose. In response to the increased demand for mobile capacity, modern have been designed with an interphase to communicate information about a wireless medium such as cell phone technology. In this aspect it is known to use Manchester coding (also known as split-phase coding) of binary data in the data transmission. An example of one of these systems that uses this technique is the advanced mobile phone service system (AMPS • Advanced Mobile Phone Service) in North America. Manchester coding first applies as it represents each bit of information as a codeword of two bits or symbols: a bit "1" is represented by the symbol "10a and a bit" 0 * is represented by the symbol "OÍ". The digital information REF: 24701 encoded is then printed on the carrier radio frequency by frequency modulation (FM). A number of techniques for demodulating a Manchester encoded digital FM signal is known. A preferred technique is described in the U.S. patent application. No. 08 / 053,860 by Paul W. Dent, entitled "Demodulator For Manchester-Coded FM Signáis" (Demodulator For Manchester-Coded FM Signals), filed on April 29, 1993, which is hereby expressly incorporated by reference. One of the characteristics of this preferred technique is the fact that instead of aplying the received radio signal to a frequency discriminator as in the prior art, the demodulation on the contrary is based on the behavior of the complex vector value or phase of the received signal. The polarities of the information bits can be determined by measuring the phase excursions in the halves of the Manchester symbols. a phase reference is established from a plurality of candidate phase references as a basis for comparison of the average symbol phase. The phase can be measured at the start points and end points of the symbols and averaged, or measured a plurality of times during each symbol period, to generate a reference phase. Important for the proper operation of a broadband data modem employing the preferred demodulation technique is the ability to receive a signal that is substantially free of interference. Unfortunately, when the signal is received from a wireless medium, interference from a variety of sources may arise. In particular, it has been found that a particular type of cell phone system base station currently in use introduces a large, slowly variable residual FM component that can cause serious problems for a broadband data modem. This residual FM component has been found to be sinusoidal with a frequency of 36 Hz and a deviation of. maximum frequency of approximately 1 KHz, although these parameters may vary with time. When the received Manchester encoded signal is demodulated, this residual FK deterioration causes a frequency (and therefore phase) error that varies more rapidly and in a large proportion than can be handled by the phase-averaging circuit of the demodulator (which includes a frequency error estimator). As a result, approximately 50 percent of the white-and-burst messages may be lost, which of course is unacceptable. C-HP-a-Olo OF THE INVENTION It is therefore an object of the present invention to provide a technique for removing low frequency interference in a digital FM receiver, which employs a demodulation technique based on the behavior of the value of complex vector or phase of the received signal.

A further object of the present invention is to further provide a technique for detecting the presence of low frequency interference in a received digital FM signal. According to one aspect of the present invention, the above and other objects are achieved by an apparatus in a digital FM receiver having a demodulator that receives a radio signal and generates thereon a phase sample whose behavior determines a output of the demodulator, the apparatus is for removing a low frequency interference signal from the phase sample. The apparatus converts the phase sample into a frequency sample that is then filtered from a high pass to produce a filtered frequency sample. The filtered frequency sample is then converted to a filtered phase sample, which may additionally be demodulated according to known techniques. Converting the phase sample into a frequency sample can be done through a different circuit of the first order. The conversion of the filtered frequency sample into the filtered phase sample can be done by an integrator. Because the demodulator can produce degraded performance when the apparatus of the invention is applied to a radio signal that does not have low frequency interference, in another aspect of the invention, the demodulator further provides a circuit for detecting the presence of the low frequency interference signal in the radio signal, and alternately choosing in response to it, either the original or filtered phase sample that is supplied to additional demodulation circuits. According to one modality, this detection is performed by low pass filtering of the frequency sample and measuring the energy in the band where low frequency deterioration would appear, if present. This measured energy value is then compared with a threshold value. If the measured energy exceeds the threshold value, then the low frequency interference signal is present. Alternatively, this detection can be performed by including, within the demodulator, a memory for storing a table of base station system identification values (IDs) of those known base stations for generating the low frequency interference. The signal deterioration detection is then performed by comparing a received system ID with those contained in the table. A correspondence indicates the presence of low frequency interference. In another alternate embodiment, the apparatus for removing a low frequency interference signal from the base sample is provided in a demodulator to produce a corrected demodulated signal. The demodulator is further provided with means for demodulating the radio signal without benefit of the apparatus of the invention, thereby producing an uncorrected demodulated signal. Each of the corrected and uncorrected demodulated signals is verified for error content (for example by performing a cyclic redundancy check in each) and a demodulator output is chosen from the unmodified and corrected demodulated signals, based on to their respective error contents. ftRKVK fíBfirWIPCIOH? * QS DRAWINGS The objectives and advantages of the invention will be understood upon reading the following detailed description in conjunction with the drawings, wherein: FIGURES l (a) and l (b) show illustrative embodiments of a low interference separator. frequency according to one aspect of the invention; FIGURE 2 is a block diagram of an alternate preferred embodiment of an inventive low frequency interference separator, in accordance with the present invention; FIGURE 3 is a graph of test results for a demodulator using the preferred embodiment of the inventive low frequency interference separator, which is operated in plane fading with a vehicle speed of 100 kra / hr; FIGURE 4 is a block diagram of a demodulator using the inventive low frequency interference separator and further including circuits that are capable of detecting the presence or absence of deterioration, according to another aspect of the invention; FIGURE 5 is a block diagram of one embodiment of the deterioration detector according to the invention; FIGURE 6 is a block diagram of an alternative embodiment of the deterioration detector according to the present invention; FIGURE 7 is a block diagram of a demodulator incorporating another method for reducing performance degradation associated with high pass filtering of a non-deteriorated received signal, according to another aspect of the present invention. ngwPTtvrmí IMWIT.T-.I_- The various features of the invention will now be described with respect to the figures, in which like parts are identified with the same reference characters. A first embodiment of the inventive low frequency interference suppressor 100 will now be described with respect to the block diagram illustrated in FIGURE 1 (a). The low frequency interference suppressor 100 has a power to receive phase samples 101 that are preferably produced by a demodulator for Manchester encoded FM signals (not shown) according to the techniques described in the US patent application. Serial No. 08 / 053,860, which has been incorporated herein by reference. It is considered that the phase samples 101 are deteriorated by the presence of the undesired low frequency interference described above. The phase samples 101 are converted into frequency samples 105 by a first-order difference circuit 103, which determines the difference between successive samples. That is, each of the frequency samples 105 represents change of frequency per sample. The frequency samples 105 are reduced module 2w, and then supplied to a low pass filter 107 which filters a velocity of 80 Km / second in order to allow oversampling of Manchester encoded data of 10 Kbits / second by a factor of eight. The low pass filter 107 is preferably a second order Butterworth filter mapped to the digital domain by a bilinear transformation. The corner frequency of this filter whose purpose is to pass only the deterioration, is 150 Hz. The output of the low-pass filter 107 is supplied to an integrator 113 which generates modulo 2 values of the estimated phase deterioration 115. Integration is performed at the speed of 80 KHz, allowing the estimated phase deterioration 115 to be at the correct sample rate for subtraction of time-aligned copies of the original phase samples 101 in subtractor 117. (Time alignment is necessary to compensate for delays associated with generating the estimated phase deterioration). To accomplish this, the output of the integrator 113 is supplied to a subtractor feed of the subtractor 117. The other (positive) feed of the subtractor 117 receives the output of a delay line 153, which aligns the original phase samples 101 in time, with the estimated phase impairments 115. The output of subtractor 117, after reducing this module 27, is the corrected phase signal 119 which is supplied back to the demodulator for Manchester encoded FM signals, to complete the demodulation process. The circuit illustrated in FIGURE 1 (a) is useful for explaining the operation theory of this aspect of the invention, but it is not preferred due to the higher processing requirements imposed by the need to perform a low pass filtering operation. second order at a sampling rate of 80 KHz. A less intense circuit in processing operating in accordance with the theory described above with respect to FIGURE 1 (a), is illustrated in FIGURE 1 (b). Here, the low frequency interference suppressor 150 includes many of the same components described above with respect to FIGURE 1 (a). One difference, however, is that the low pass filter 155 (equivalent to the low pass filter 107 illustrated in FIGURE 1 (a)) operates on frequency samples that are supplied at a rate of 16 KHz instead of 80 KHz. In practice, these samples can be easily available. However, for testing purposes of this assembly, these samples can be easily generated by supplying frequency samples of 80 KHz, 105, from the first order difference circuit 103 to a finite impulse response low pass filter, smoothing (FIR) 151, which will allow a reduction of sample rate by the decimation filter 109. For this purpose, the FIR low-pass filter 151 eß preferably a 7-phase linear filter with a cutoff frequency of 3- dB of 6 KHz. The low pass filter 155 then operates on the 16 KHz output samples of the decimation filter 109. The low pass filter 155 has the same frequency response as the low pass filter 107 described above with respect to FIGURE 1 (FIG. a), but has to be scaled in the bilinear transformation for the sampling rate below 16 KHz. The 16 KHz output of the low pass filter to 155 is then supplied to a sample and hold circuit 111 that repeats each of the 16 KHz samples five times, to generate an 80 KHz supply of the filtered samples that is supplied to the integrator 113. As described above with respect to FIGURE 1 (a), the integrator 113 generates modulo values 2w of the estimated phase deterioration 115 that is subtracted from the time aligned copies of the original phase samples 101. The scheme above described for removing low frequency interference in a digital FM receiver has several disadvantages. First, it is sensitive to the delay of the low pass filter 155. For example, in a test of this type of system, it was necessary to adjust the delay line 153 to retard the original phase signals 101 by 122 samples, in order to achieve alignment of time with the estimated phase deterioration 115. Also, since the low-pass filter 155 was of the Infinite Impulse Response (IIR) variety, it was impossible to provide a linear phase response (group delay). constant). Therefore, the interference passed through this filter undergoes a phase change and can not generally cancel that in the original phase waveform, even if the two signals are aligned in time. An alternate mode of an inventive low frequency interference suppressor 200 will now be described with respect to FIGURE 2. This alternate mode is preferred because it avoids the problems of time delay and distortion of the interference phase when converting the samples from phase 101 to frequency samples 105, directly by removing the unwanted residual FM component, and then converting the corrected frequency samples into corrected phase samples, which are then demodulated according to the preferred technique. In this way, as with the previously described embodiments, the low frequency interference suppressor 200 has a power to receive phase samples 101 that are preferably produced by a demodulator for Manchester encoded FM signals (not shown), in accordance with the techniques described in the US patent application Serial No. 08 / 053,860, which has been incorporated herein by reference. Again it is considered that the phase samples 101 are deteriorated by the presence of the undesired low frequency interference described above. The phase samples 101 are converted into frequency samples 105 by a first-order interference circuit 103, which determines the difference between successive samples. The frequency samples 105 ß reduce module 2 *, and then supply in a high-pass filter 201 that filters a velocity of 80 Km / second. In order to reduce processing requirements, the high-pass filter 201 preferably has a first-order Butterworth filter in the high-pass configuration. Of course, a second or third order filter can also be used instead, but this will impose higher processing requirements on the low frequency interference suppressor 200. The Butterworth filter is preferred because it exhibits a flat response in the passband. However, this type of filter is not a requirement; any type of high pass filter can be used instead. It has been empirically determined that a corner frequency (ie 3-dB) of about 750 Hz works best for a first order filter in this application. A higher corner frequency damages the broadband data too much, and a lower corner frequency inappropriately attenuates deterioration. Laa mueßtraß frequency filtered 203 then supplied to an integrator 113 whose output after reduction module 2r, is the sequence of corrected phase samples 205. Despite the simplicity of the preferred embodiment illustrated in FIGURE 2, this assembly operates completely at the speed of 80 KHz sampled mueßtraß laß of FAAE entry 101, and therefore imposes more demands on the processing hardware that is used to implement various components loe. The performance of each of the modalities described above is estimated by simulation. First, without applying any correction, the performance of a demodulator for FM signals Manchester encoding is simulated for a received signal without deterioration in a static gaussian channel. These numbers baseline showed that, in the case of no impairment, demodulation of data may be performed more or less perfectly beginning at E / No below 10 dB, where Ec is defined as (energy señalpr B_ < Uß) / (bit rate), and N "is defined as (interference energy) / (interference bandwidth). When the deterioration interference is added to the received signal, however, each message of a hundred frames is lost for each value of E_ / N0 up to and including 31 dB. Use of an early version (absent the delay line 153) or suppressor low frequency interference 100 (shown in Figure l (a)), was able to reduce the rate of lost considerably at high values of messages E "/ N0, but still produces a 91 percent loss of the frames at 10 dB in a static Gaussian channel. Some of this performance it seems attributable to a misalignment between the phase samples 101 and the deterioration of estimated phase 115, which is presented to substrator 117. For this reason, the delay line 153 is introduced in an attempt to align in time the samples e original phase 101 with the estimated phase deterioration of 115. This greatly helped the performance, giving a lost message rate of 22 percent to 10 dB Eb / N ,. The modality of FIGURE 1 (b) is also simulated to determine how the performance of a demodulator would affect the Manchester encoding FM signals.

After optimizing the amount of delay produced by delay line 153, it was found that the lost message rate over 10 dB E "/ N0, in a static Gaussian channel could be returned to zero, but at 10 dB, the system suffered a frame rate lost of 99 percent. A similar static Gaussian simulation was performed for the preferred mode of the low frequency interference suppressor 200 illustrated in FIGURE 2. It was found that this configuration reduced the lost message rate to 10 dB Eb / N "up to 8 percent. The test results for this flat fading mode with a vehicle speed of 100 km / hr are summarized in the graphs illustrated in FIGURE 3. It can be seen that although the preferred technique is capable of restoring performance to an acceptable level of lost messages when the received signal is deteriorated by interference, a price is paid for the high-pass filtering of a portion of the broadband data when no deterioration is present. That is, when there is no deterioration present, some deterioration in performance occurs with respect to the system without any correction. According to another aspect of the invention, the disadvantage that is introduced by the high-pass filtering of a received non-deteriorated signal is greatly reduced. Now with reference to FIGURE 4, this is achieved by deploying the low frequency interference suppressor 200 in a system which is also capable of detecting the presence or absence of deterioration. When the deterioration detector 401 determines that the received signal has been deteriorated by the introduction of interference, its output (present deterioration 405) controls the multiplexer (MUX) 403 to choose the output of the low frequency interference suppressor 200. The output of the MUX 403 is then supplied to additional demodulation circuits (not shown) that operate in accordance with known techniques. If deterioration is not detected however, then the deterioration detector 401 causes the MUX 403 to choose the original lattice of the 101 façes, thus avoiding the degraded performance that would otherwise be introduced by the use of the low frequency interference suppressor. 200. An embodiment of the deterioration detector 401 will now be described with respect to FIGURE 5. In this embodiment, the deterioration detector 401 receives as a feed the frequency samples 105, which are produced by the first-order difference circuit. (see FIGURE 2). A low pass filter 501 filters the frequency samples 105 to allow only those frequency components that pass, which are in the band where the low frequency deterioration will appear. The filtered frequency switches 503 are then supplied to an energy measuring circuit 505 which measures the intensity of these signals, preferably by determining the sum of square samples. This energy measurement 507 is supplied to a power supply of a comparator 509, such that it can be compared to a threshold value 511. When the energy measurement 507 exceeds the threshold value 511, the comparator generates the present deterioration signal 405 that is It supplies the MUX 403 as illustrated in FIGURE 4. To determine an appropriate threshold value, the energy in the deterioration band can be measured with and without the present deterioration for a large number of channel conditions. Then, histograms of energy distributions can be drawn for the two cases. A threshold value that differentiates between the two cases can then be identified and selected. An alternate mode of the deterioration detector 401 will now be described with respect to FIGURE 6. In this mode, the deterioration detector 401", receives as feed the system identification (ID) 601 of the base station (not shown), with which the receiving equipment communicates This information is typically provided by a base station to the mobile station, which caters in such a way that the mobile station will know in which cell it is, a processor 603 compares the system ID 601 with a table of IDS 607 that are stored in a memory 605, which is preferably a read-only memory (ROM) The ID table 607 is generated by collecting the system IDs of all the cells ß that are known to contain the particular type of equipment of base station that generates unwanted low-frequency interference.

If the processor 603 determines that the system ID 601 corresponds to any of the ß IDs stored in table 607, then it estimates the present deterioration signal 405. It is noted that, while this solution is good in theory, it may not always produce results reliable because cell phone system operators occasionally move the base station equipment from one cell site to another. As a result, table 607 may not always represent the most up-to-date allocation of base station physical equipment. However, in a geographic region where the type of base station equipment used in a given cell is stable, this modality offers a practical solution. Another method for reducing performance degradation associated with high pass filter of a received signal not deteriorated, is illustrated in FIGURE 7. Here, phase samples 101 are produced as described above. These are supplied to each of two parallel processing paths. In a trajectory, laß mueßtraß of faße 101 ße directly supply a first demodulator circuit 701-1. In another path, the faecal displays 101 are supplied to the low frequency interference suppressor 200, which output is supplied to a second demodulator circuit 701-2 which is functionally identical to the first demodulator circuit 701-1. The outputs of each of the first and second demodulator circuits 701-1, 701-2, are then supplied to respective two cyclic redundancy check circuits (CRC) 703-1, 703-2, as well as to respective first and second circuits MUX 705 feeds. The outputs of the two CRC circuits 703-1, 703-2, are supplied to a processor 709 that determines the identity of the path that produces the most error-free result. A select signal 707 is generated by the processor 709 which causes the MUX 705 to pass the most error free signal as the demodulated output 709 from the demodulator 700. While this latter approach is not the most economical or efficient of the solutions described above However, it is viable. The invention has been described with reference to a particular embodiment. However, it will be readily apparent to those skilled in the art that it is possible to incorporate the invention into specific forms different from those of the preferred embodiment described above. This can be done without departing from the spirit of the invention. The preferred modality is simply illustrative and should not be considered restrictive in any way. The scope of the invention is given by the appended claims, rather than by the foregoing description, and all variations and equivalents that fall within the range of the claim are intended to be encompassed therein.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (16)

1. glVINDICATIONS 1. In a digital FM receiver that has a demodulator that receives a radio signal and generates from there a phase sample whose behavior determines a demodulator output, an apparatus for removing a low frequency interference signal from the phase sample , characterized in that it comprises: feeding means for receiving the phase sample; first means for converting the phase sample into a frequency sample; high-pass filter means, for withdrawing from the frequency sample, energy associated with frequencies below a predetermined frequency, the high-pass filter means provide a filtered frequency sample at a filter output; and second means for converting the filtered frequency sample into a filtered phase sample. The apparatus according to claim 1, characterized in that the first means comprise a first order difference circuit. The apparatus according to claim 1, characterized in that the second means comprise an integrator. 4. In a digital FM receiver, a demodulator apparatus characterized in that it comprises: power means for receiving a radio signal; means for converting the radio signal into a phase sample; means for removing a low frequency interference signal from the phase sample, comprising: first means for converting the phase sample to a frequency sample; high-pass filter means for removing from the frequency sample, energy associated with frequencies below a predetermined frequency, the high-pass filter means supplies a filtered frequency sample at a filter output; and second means for converting the filtered frequency sample into a filtered phase sample; detection means for the presence of a low frequency interference signal in the radio signal; selection means, operatively coupled with the detection means, for alternately selecting as output from the selection means, the phase sample when the low frequency interference signal is not detected in the radio signal, or the sample of filtered phase, when the low frequency interference signal is detected in the radio signal; and demodulation means for generating a demodulated signal based on the output of the selection means. The demodulator apparatus according to claim 4, characterized in that the detection means comprise: feeding means for receiving the frequency sample; low-pass filter means for removing, from the frequency sample, energy associated with frequencies on a second predetermined frequency, the low-pass filter means supplies a second filtered frequency sample to a low-pass filter output; means for measuring an energy value of the second filtered frequency sample; and means for comparing the energy value of the second filtered frequency sample with a predetermined energy value and generating an indication of the presence of the low frequency interference signal in the radio signal, when the energy value of the second Filtered frequency screen exceeds the predetermined energy value. The demodulator apparatus according to claim 4, characterized in that the detection means comprise: means of feeding to receive a system identification value associated with a base station transmitting the radio signal; table medium for storing one or more predetermined system identification values; means to determine whether the received system identification value corresponds to any one or more priority system identification values that are stored in the middle of the table; and if a correspondence is found, then indicate the presence of the low frequency interference signal on the radio signal. 7. In a digital FM receiver, a demodulator apparatus characterized in that it comprises: power means for receiving a radio signal; means for converting the radio signal into an original façster wall; means for removing a low frequency interference signal from the original phase sample, comprising: first means for converting the original phase sample into a frequency sample; high-pass filter means for removing from the frequency sample, energy associated with frequencies below a predetermined frequency, the high-pass filter means supplies a filtered frequency sample to a filter output; and second means for converting the filtered frequency gate into a filtered phase sample; first demodulation means for generating a corrected demodulated signal from the filtered faeus gate; second demodulation means for generating a unmodified dsmodulated signal from the original phase gate; first means for error detection, for detecting a first quantity of errors contained in the corrected modulated signal; second means for error detection, for detecting a second quantity of errors contained in the uncorrected demodulated signal; selection means, operatively coupled to the first and second demodulation means and to the first and second error detection means, to alternatively choose as an output of the selection means, either the corrected demodulated signal or the uncorrected demodulated signal, based on the comparison of the first quantity of errors with the second quantity of errors. 8. The demodulator apparatus according to claim 7, characterized in that the selection means comprise: means for alternately choosing the corrected demodulated signal when the first number of errors is less than the second amount of errors, or the demodulated signal not corrected when The first number of errors is not less than the second number of errors. 9. In a digital FM receiver having a demodulator that receives a radio signal and hence generates a phase sample whose behavior determines a demodulator output, a method for removing a low frequency interference signal from the sample of phase; characterized in that it comprises the steps of: receiving the fae sample; converting the façe mueÚtra into a frequency unit; Remove from the frequency gate, energy associated with frequencies below a predetermined frequency, to produce a filter of filtered frequency and convert the filtered frequency sample into a filtered faeged bed * 10. The method according to the claim 9, characterized in that the step of converting the phase sample into a frequency sample, comprises generating a difference between the faeze sample and a previous phase stage. 11. The method according to claim 9, characterized in that the step of converting the filtered frequency sample into a filtered phase sample comprises the step of integrating the filtered frequency sample. 12. In a digital FM receiver, a method for demodulating a radio signal, characterized in that it comprises the steps of: receiving the radio signal; converting the radio signal into a phase sample; performing a method for removing a low frequency interference signal from the phase sample, the method comprising the steps of: converting the phase sample into a frequency sample; withdrawing from the frequency sample energy associated with frequencies below a predetermined frequency to produce a filtered frequency sample; and converting the filtered frequency sample into a filtered faeze sample; detecting the presence of the low frequency interference signal in the radio signal; alternatively choose to use the phase sample as a phase-selective gate, when the low-frequency interference signal does not detect in the radio signal or the filtered faeze sample when the low-frequency interference signal is detected in the signal of radio; and generating a demodulated signal based on the selected phase sample. 13. The method according to claim 12, characterized in that the step of detecting the presence of the low frequency interference signal in the radio signal comprises the steps of: withdrawing from the frequency sample, energy associated with frequency over a second frequency predetermined to produce a second filtered frequency sample; measuring an energy value of the second filtered frequency sample; and comparing the energy value of the second filtered frequency sample with predetermined energy value and generating an indication of the presence of the low frequency interference signal in the radio signal, when the energy value of the second frequency sample filtered exceeds the predetermined energy value. The method according to claim 12, characterized in that the step of detecting the presence of the low frequency interference signal in the radio signal comprises the steps of: receiving a system identification value associated with a base station that transmits the radio signal; determine whether the received system identification value corresponds to at least one predetermined system identification value that is stored in a table and if a correspondence is found, then indicate the presence of the low frequency interference signal in the radio signal . 15. In a digital FM receiver, a method for demodulating a radio signal characterized in that it comprises the steps of: receiving the radio signal; converting the radio signal into an original phase sample; performing a method for removing a low frequency interference signal from the original phase sample, the method comprising the steps of: converting the original phase sample to a frequency carrier; removing from the frequency plate, energy associated with frequencies below a predetermined frequency to produce a filtered frequency motor; and converting the filtered frequency unit into a filtered faeuse unit; generating a corrected demodulated signal from the filtered phase sample; generate a demodulated uncorrected signal of the original phase card; detecting a first quantity of errors contained in the corrected demodulated signal; detect a second amount of errors contained in the demodulated uncorrected signal; alternatively choose either the corrected demodulated signal or the demodulated signal not corrected based on a comparison of the first number of errors with the second amount of erroreß. The method according to claim 15, characterized in that the step of choosing alternatively comprises the step of alternatively choosing the corrected demodulated signal when the first erroreß ßß amount is smaller than the second amount of errors or the uncorrected signal demodulated when the first amount of errors is not less than the second amount of errors. TO? HTW ^ TWMCTQH In a digital FM receiver that has a demodulator that receives a radio signal and generates from there a phase sample whose behavior determines a demodulator output, an apparatus for removing a low frequency interference signal from the The phase sample converts the phase sample into a frequency sample that is then filtered from a high pass to produce a filtered frequency filter. The frequency filtered filter is then converted to a filtered phase sample which can also be demodulated according to known techniques. The conversion of the phase sample into a frequency sample can be done by a first-order difference circuit. The conversion of the filtered frequency sample into the filtered phase sample can be done by an integrator. Because the demodulator can produce degraded performance when the apparatus of the invention is applied to a radio signal that does not have low frequency interference, in another aspect of the invention, the demodulator is further provided with a circuit for detecting the presence of the low frequency interference signal in the radio signal and alternatively select in response thereto, either the original or filtered phase sample that is supplied to additional demodulation circuits. Various methods for detecting the presence of the low frequency interference signal are described.