JP2007243662A - Frequency variable filter circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a frequency variable filter circuit capable of suppressing an influence of an adjacent jamming electric wave by a simplified method compared with a conventional technology. <P>SOLUTION: There is provided a frequency variable filter circuit including a frequency variable filter for band-pass filtering a high frequency communication signal from a plurality of sub carrier signals digital-modulated with a predetermined pass band and inputted. A control means in a filter control circuit 18 includes a filter control signal generation table for storing frequencies in a pass band of the frequency variable filter set for each primary modulation set information common to each subcarrier signal and for each set frequency information. Referring to a filter control signal generation table memory 19 on the basis of the set frequency information and the primary modulation set information common to each subcarrier signal, the frequencies in pass band of the frequency variable filter is controlled to be set so as to suppress an influence of an adjacent electric jamming wave. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a frequency variable filter circuit provided to reduce the influence of adjacent interference waves in a communication modem apparatus such as a cable modem apparatus for the CATV (Cable Television) Internet.

  The conventional communication modem apparatus attenuates the frequency band of other signals by passing only the frequency band used for the communication signal so as not to disturb other signals and from other signals. A band pass filter is provided.

  For example, Patent Document 1 discloses a tunable filter according to the prior art. The tunable filter can change the resonance frequency and the frequency bandwidth by using a varactor diode whose capacitance is variable by voltage control. Therefore, the tunable filter described above switches the frequency band of adjacent channels by switching the pass frequency band of the filter in terrestrial broadcasting, satellite broadcasting, cable television broadcasting, etc. including digital multi-channel broadcasting using a broadband transmission medium. The signal of a desired channel can be extracted without including it.

  Specifically, when multiple communication channels are used, or when used in combination with other signals such as sharing with a broadcast wave on a coaxial cable, if the frequency band used is close, It is disturbed or interferes with other signals. At this time, the tunable filter described above changes the setting of the desired wave to the communication channel one higher when the disturbing wave is lower in frequency than the desired wave, while the desired wave is changed when the disturbing wave is higher in frequency than the desired wave. By changing the setting to the next lower communication channel, the influence of interference can be suppressed.

JP 2002-9573 A.

  In the above-described tunable filter according to the prior art, when the frequency band to be used is close to the frequency band of another signal and the frequency band to be used is changed to a different communication channel, addition of a different clock or modulation signal There was a problem that modification of the control algorithm was necessary.

  An object of the present invention is to provide a variable frequency filter circuit that solves the above-described problems and can reduce the influence of adjacent interfering waves by a simpler method compared to the prior art.

A frequency variable filter circuit according to the present invention is a frequency variable filter circuit including a frequency variable filter that performs band-pass filtering of a high-frequency communication signal including a plurality of subcarrier signals that are each digitally modulated and input in a predetermined pass band. ,
Control means for controlling to set a frequency of band pass of the frequency variable filter based on setting frequency information of a modem that demodulates the high-frequency communication signal is provided.

  In the frequency variable filter circuit, the control means stores a first passband frequency of the frequency variable filter to be set for each primary modulation setting information common to the set frequency information and each subcarrier signal. A band pass of the frequency tunable filter so as to reduce adjacent interfering waves with reference to the first table based on the set frequency information and primary modulation setting information common to each subcarrier signal. It is characterized by controlling so that the frequency of this may be set.

  In the frequency variable filter circuit, the control means includes a second table for storing the frequency of the pass band of the frequency variable filter to be set for each set frequency information, and based on the set frequency information. Referring to the second table, control is performed so as to set the bandpass frequency of the frequency variable filter so as to reduce adjacent interference waves.

  Furthermore, in the frequency variable filter circuit, the control means measures line quality for each subcarrier signal of the input high-frequency communication signal, and based on the set frequency information and the measured line quality, Control is performed so as to set the bandpass frequency of the frequency variable filter so as to reduce the interference wave.

  Here, the line quality is a bit error rate (BER).

  Still further, in the frequency variable filter circuit. Based on the set frequency information and individual primary modulation setting information for each subcarrier signal of the input high-frequency communication signal, the control means performs bandpass of the frequency variable filter so as to reduce adjacent interference waves. It is characterized by controlling so that the frequency of this may be set.

  Therefore, according to the frequency variable filter circuit of the present invention, the frequency provided with the frequency variable filter that performs band-pass filtering of a high-frequency communication signal composed of a plurality of subcarrier signals that are each digitally modulated and input in a predetermined pass band. The variable filter circuit includes control means for controlling to set a band pass frequency of the frequency variable filter based on setting frequency information of a modem that demodulates the high frequency communication signal. Therefore, by changing the frequency band of the signal extracted by the frequency variable filter according to the frequency band usage situation of the desired wave and the jamming wave, it is affected by the neighboring jamming wave or the neighboring jamming wave. Therefore, a part of the frequency band that could not be used until now can be used effectively, and more communication channels can be used with a simple configuration.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In addition, in each following embodiment, the same code | symbol is attached | subjected about the same component.

First embodiment.
FIG. 1 is a block diagram showing the configuration of the cable modem device according to the first embodiment of the present invention. The cable modem device 1 according to the first embodiment receives a high-frequency communication signal from the head end of a cable television company via a coaxial cable 20 of a CATV network, demodulates and converts it into a baseband signal, This is a communication terminal device for extracting predetermined user data from the inside and outputting it via a LAN cable 21 to a personal computer provided in the user's home.

FIG. 2 is a diagram showing a relative gain with respect to frequency, showing an example of a band pass characteristic of the frequency variable filter 12 of FIG. In FIG. 2, for example, when the desired wave signal band 30 is from 875 MHz to 925 MHz, the interference wave signal band 31 is set from 975 MHz to 1025 MHz. In the present embodiment, the interference wave signal in which the influence of interference or interference is generated by the frequency variable filter 12 of FIG. 1 capable of controlling the center frequency and the frequency bandwidth for band-pass filtering based on an external filter control signal. The subcarrier signal on the band 31 side is attenuated. Here, for the subcarrier signal to be attenuated,
(A) A modem signal (for example, an OFDM (Orthogonal Frequency Division Multiplexing) signal including a plurality of subcarrier signals) set by the modem 13a in the RF processing circuit 13, and the coaxial cable 20 is connected as is well known. Which subcarrier signal is used according to the channel quality (frequency characteristics and bit error rate (BER)) of the high-frequency communication signal (training signal) transmitted through the network, and which modulation method is used for each subcarrier signal Use center frequency and frequency bandwidth of the desired wave, and
(B) Center frequency and frequency bandwidth of the interference wave to be attenuated (based on the center frequency of the interference wave that is preset according to the use center frequency and frequency bandwidth of the desired wave and detected by the RF processing circuit 13, for example. Controller 17).
(C) Depending on the common primary modulation setting information of each subcarrier signal of the modem 13a (for example, 128QAM, 64QAM, 32QAM, 16QAM, 8PSK, 4PSK, 2PSK, etc. when the OFDM signal is used). For example, as shown in Table 1 below, a center frequency and a frequency bandwidth for band pass filtering to be set in the frequency variable filter 12 are determined in advance, and a filter control signal generation table including these data is filtered. The filter control signal generation table memory 19 connected to the circuit 18 is stored in advance.

  Here, the use center frequency and frequency bandwidth of the desired wave and the center frequency and frequency bandwidth of the interference wave are referred to as “frequency setting information”. A method for generating the filter control signal generation table will be described later in detail. Note that the frequency bandwidth of the desired wave and the frequency bandwidth of the interference wave are set in advance, and need not be stored in the filter control signal generation table.

  In FIG. 1, a high-frequency communication signal from the CATV network is input to the frequency variable filter 12 via the coaxial cable 20. Based on the filter control signal input from the filter control circuit 18, the frequency variable filter 12 band-filters the high-frequency communication signal input from the coaxial cable 20 for the frequency band corresponding to the filter control signal, Output to the RF processing circuit 13. Next, the RF processing circuit 13 includes a modem that modulates and demodulates the high-frequency communication signal. Based on the control signal input from the controller 17, the low-noise amplification process and the frequency are performed on the high-frequency communication signal input from the frequency variable filter 12. After performing the conversion process and the demodulation process in the digital demodulation method corresponding to the primary modulation setting information of the modem 13 a determined based on the training signal and output to the controller 17, the data is output to the BB processing circuit 14. Further, the BB processing circuit 14 converts the high-frequency communication signal input from the RF processing circuit 13 into a baseband signal based on the control signal input from the controller 17, and then outputs it to the MAC processing circuit 15. Further, the MAC processing circuit 15 analyzes the header portion of the baseband signal input from the BB processing circuit 14 based on the control signal input from the controller 17, extracts user data in the baseband signal, and extracts the ether interface 16. Output to.

  Further, the Ethernet interface 16 converts user data input from the MAC processing circuit 15 into a LAN signal based on a control signal input from the controller 17, and then a personal computer or the like via a LAN cable 21 such as an Ethernet cable. (Not shown). Here, the user data is, for example, data described in HTML, e-mail data, or the like. As described above, the controller 17 controls the operations of the RF processing circuit 13, the BB processing circuit 14, the MAC processing circuit 15, and the Ethernet interface 16.

  In addition, a filter control circuit 18 is connected to the controller 17. Based on the frequency setting information (only the center frequency of the desired wave and the center frequency of the disturbing wave) and the common primary modulation setting information input from the controller 17, the filter control circuit 18 stores in the filter control signal generation table memory 19. The filter control signal corresponding to the frequency setting information and the common primary modulation setting information is searched for, and the filter control signal obtained by searching is output to the frequency variable filter 12. In response to this, the frequency variable filter 12 band-pass-filters the input high-frequency communication signal for the frequency band corresponding to the filter control signal, and then outputs it to the RF processing circuit 13.

Next, the filter control signal generation table stored in the filter control signal generation table memory 19 will be described below. In the first embodiment, the modulation scheme to be used is, for example, a multi-carrier modulation scheme by OFDM, and the cable modem device 1 checks the transmission path state of each frequency band at startup and sets primary modulation common to all subcarriers. . Under a thermal noise environment, as is apparent from the carrier-to-noise power ratio (C / N) characteristics with respect to a known bit error rate (BER) whose characteristics vary depending on the modulation method, the bit error rate (BER) = 10 ⁻⁴ . In order to satisfy the channel quality with BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), and 128 QAM (Quadrature Amplitude Modulation), the carrier-to-noise power ratio (C / N) is 8.5 dB and 11.5 dB, respectively. , 27 dB is required. Here, the bit error rate (BER) = 10 ⁻⁴ is a value that can be regarded as almost error-free when a Reed-Solomon code is added.

For example, when the threshold for determining whether or not each subcarrier signal is used is a bit error rate (BER) = 10 ⁻⁴ and communication is performed with 128QAM in the primary modulation, the frequency-variable filter 12 is used to perform carrier-to-noise. By reducing the power ratio (C / N) to 27 dB or less, the subcarrier signal becomes unused, and the usable frequency bandwidth can be limited. When the received signal level is 70 dBμV and the noise level is 20 dBμV, in order to satisfy the carrier-to-noise power ratio (C / N) of 27 dB or less, the relative gain of the subcarrier signal to be unused by the frequency variable filter 12 is set. It may be attenuated by 23 dB or more. In addition, when a filter is added to both the transmitter side and the receiver side of the modem 13a, the subcarrier signal becomes unused by adding a filter that attenuates the relative gain by 11.5 dB or more. The noise level depends on the characteristics of the cable modem device 1. Therefore, the filter control signal generation table memory 19 stores a filter control signal generation table in which the frequency variable filter 12 satisfies the above characteristics.

  Usually, out-of-band radiation exists in a high-frequency communication signal, and in multi-carrier modulation schemes such as OFDM signals, particularly out-of-band radiation exists near both ends of the occupied frequency bandwidth of the carrier wave. Generally, the out-of-band radiation is very large in the range of 10 to 15% of the occupied frequency bandwidth. Therefore, in the channels having the occupied bandwidths adjacent to each other, the adjacent subcarrier signal is greatly affected by out-of-band radiation.

  FIG. 3 is a diagram showing a relative gain with respect to frequency, showing an example of the out-of-band radiation component 35 of the subcarrier signal input to the frequency variable filter 12 of FIG. In FIG. 3, when the signal occupying frequency band 33 of the signal transmission channel is 875 MHz to 925 MHz and the signal occupying frequency band 34 of the adjacent channel is 925 MHz to 975 MHz, the sub-band from the frequency 915 MHz to the frequency 925 MHz in the signal of the center frequency 900 MHz. The carrier is likely to be influenced by the signal band out-of-band radiation component 35 having the center frequency of 950 MHz. The adjacent channel is a high-frequency communication signal used in another network. Further, in order to attenuate the relative gain of 11.5 dB or more from the slope characteristic of the frequency variable filter 12, a frequency of 10 MHz is required. The filter control signal generation table based on the above conditions is Table 1 described above.

  As apparent from Table 1, for example, when the center frequency of the desired wave is 900 MHz, the frequency bandwidth is 50 MHz, the center frequency of the disturbing wave is 950 MHz, the frequency bandwidth is 50 MHz, and the common primary modulation is 128 QAM, the filter control signal The center frequency is 890 MHz, and the frequency bandwidth is 30 MHz. Since the slope characteristic of the frequency variable filter 12 depends on the bandpass filter to be mounted, it is desirable to optimize the parameters of the filter control signal generation table for each communication terminal device.

  As described above, according to the first embodiment, the filter control circuit 18 uses the filter control signal generation table of the filter control signal generation table memory 19 based on the frequency setting information and the common primary modulation setting information. A filter control signal is generated and output to the frequency variable filter 12, and in response to this, the frequency variable filter 12 band-pass-filters and outputs the frequency component of the high-frequency communication signal corresponding to the filter control signal. Therefore, it is possible to reduce the possibility that the desired wave will be affected by the adjacent interference wave or affect the adjacent interference wave, thereby effectively utilizing a part of the frequency band that could not be used so far. Therefore, the present invention has a specific effect that more communication channels can be used with a simple configuration as compared with the prior art.

In the first embodiment described above, the threshold for determining whether or not each subcarrier signal is used is set to bit error rate (BER) = 10 ⁻⁴ , but the present invention is not limited to this, and the BER is set to 10 ^It may be larger than ^-4 or smaller.

  In the first embodiment described above, the modulation schemes used for the primary modulation are PSK and QAM. However, the present invention is not limited to this, and examples include ASK (Amplitude Shift Keying) and FSK (Frequency Shift Keying). Other modulation schemes may be used.

Second embodiment.
FIG. 4 is a block diagram showing a configuration of a cable modem device according to the second embodiment of the present invention. Compared with the cable modem device 1 according to the first embodiment of FIG. 1, the cable modem device 1A according to the second embodiment has a filter control circuit 18 and a filter control signal generation table 19 as shown in FIG. Instead, a filter control circuit 18A for calculating a filter control signal based on the frequency setting information and the bit error rate (BER) of each subcarrier signal is provided. Hereinafter, differences from the first embodiment will be described in detail.

  In FIG. 4, the modem 13a in the RF processing circuit 13 measures the bit error rate (BER) for each subcarrier signal using the training signal of the high-frequency communication signal, and the measured value together with the frequency setting information is sent to the controller 17. Output to. The controller 17 outputs the measured bit error rate (BER) and frequency setting information for each subcarrier signal to the filter control circuit 18A. In response to this, the filter control circuit 18A calculates the filter control signal based on the input frequency setting information and the bit error rate (BER) of each subcarrier signal, as described in detail later. The filter control signal is output to the frequency variable filter 12. In response to this, the frequency variable filter 12 transmits a high-frequency communication signal input from the coaxial cable 20 based on the filter control signal input from the filter control circuit 18 in a frequency band corresponding to the filter control signal. After passing and filtering, the signal is output to the RF processing circuit 13.

Next, a calculation method of the filter control signal in the filter control circuit 18A will be described below. In the second embodiment, the modulation scheme to be used is, for example, multi-carrier modulation of an OFDM signal, and the cable modem device 1A checks the transmission path state of each frequency band at startup and sets the primary modulation common to all subcarriers. To do. Under a thermal noise environment, bit error rate (BER) = 10 ⁻⁴ is a value that can be regarded as almost error-free when a Reed-Solomon code is added. Therefore, if the threshold for determining whether or not each subcarrier signal is used is bit error rate (BER) = 10 ⁻⁴ , the filter control circuit 18A has a bit error rate (BER) of each subcarrier signal of 10 ^−4. If it is larger than 10 ⁻⁴ , it is set to a subcarrier signal that attenuates the subcarrier signal.

Next, the attenuation amount of the subcarrier signal to be attenuated is calculated. The center frequency of the desired wave and the frequency bandwidth of the desired wave are Fca and Bwa, respectively, the number of all subcarrier signals, the number of subcarrier signals with a bit error rate (BER) of 10 ⁻⁴ or less at a frequency higher than the subcarrier signals to be attenuated. When the number of subcarrier signals having a bit error rate (BER) of 10 ⁻⁴ or less at a frequency lower than the subcarrier signal to be attenuated is Nt ₁ , Nu ₁ , Nd ₁ , the upper limit frequency of the bandpass filter of the frequency variable filter 12 Fhigh and the lower limit frequency Flow of the bandpass filter are respectively expressed by the following equations.

  Accordingly, the center frequency Fc and the frequency bandwidth Bw of the filter control signal are respectively expressed by the following equations.

  As described above, according to the second embodiment, the filter control circuit 18A generates a filter control signal on the basis of the frequency setting information and the bit error rate (BER) of each subcarrier signal to generate the frequency variable filter 12. Output to. Therefore, it is possible to reduce the possibility that the desired wave is affected by the adjacent jamming wave or affects the neighboring jamming wave, so that a part of the frequency band that could not be used until now can be reduced. It can be used effectively, and has a specific effect that more communication channels can be used with a simple configuration as compared with the prior art.

In the above second embodiment, the threshold for determining whether or not each subcarrier signal is used is set to bit error rate (BER) = 10 ⁻⁴ , but the present invention is not limited to this, and the bit error rate is not limited to this. (BER) may be larger or smaller than 10 ⁻⁴ .

  In the second embodiment described above, the modulation scheme used for the primary modulation is PSK and QAM. However, the present invention is not limited to this. For example, ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), etc. Other modulation schemes may be used.

Third embodiment.
FIG. 5 is a block diagram showing a configuration of a cable modem device according to the third embodiment of the present invention. Compared with the cable modem device 1 according to the first embodiment of FIG. 1, the cable modem device 1B according to the third embodiment has a filter control circuit 18 and a filter control signal generation table 19 as shown in FIG. Instead of the filter control signal generation table memory 19B for storing only 8PSK, QPSK and BPSK data in Table 1, and the filter control circuit 18B to which the filter control signal generation table memory 19B is connected. It is a feature. Hereinafter, differences from the first embodiment will be described in detail.

In FIG. 5, the controller 17 changes the modulation method from 8PSK to QPSK when the bit error rate (BER) becomes, for example, 10 ⁻⁴ or more based on the bit error rate (BER) of the training signal measured by the modem 13a. Further, for example, when it becomes 10 ⁻³ or more, it is adaptively controlled to change from QPSK to BPSK, and the frequency setting information is output to the filter control circuit 18B. In response to this, the filter control circuit 18B refers to the filter control signal generation table in the filter control signal generation table memory 19B based on the input frequency setting information, and thereby performs filter control corresponding to the frequency setting information. The signal is searched, and the filter control signal obtained by the search is output to the frequency variable filter 12. In response to this, the frequency variable filter 12 band-pass-filters the input high-frequency communication signal for the frequency band corresponding to the filter control signal, and then outputs it to the RF processing circuit 13.

Next, processing of the controller 17 and the filter control circuit 18B will be described below. The modulation method used is, for example, multi-carrier modulation of an OFDM signal, and each subcarrier signal has a modulation method of primary modulation according to the channel quality (specifically, bit error rate (BER)) of the transmission path condition. Adaptive control is performed to change from 8PSK to QPSK and from QPSK to BPSK. The subcarrier signal to be attenuated is preset according to the frequency to be used. In order to satisfy the bit error rate (BER) = 10 ⁻⁴ with BPSK under a thermal noise environment, a carrier-to-noise power ratio (C / N) = 8.5 dB is required. The bit error rate (BER) = 10 ⁻⁴ is a value that can be regarded as almost error-free when a Reed-Solomon code is added.

In this embodiment, when the threshold value for switching the setting of the modulation method of the primary modulation is bit error rate (BER) = 10 ⁻⁴ , and when the bit error rate (BER) ≧ 10 ⁻⁴ , 8PSK to QPSK , QPSK is changed so that the number of modulation levels of BPSK and primary modulation is reduced. When the carrier-to-noise power ratio (C / N) is the lowest, the modulation method of the primary modulation is BPSK. Therefore, the frequency variable filter 12 has the carrier-to-noise power ratio (C / N) = BPSK threshold = By reducing the signal level to 8.5 dB or less, the subcarrier signal becomes unused, and the use frequency bandwidth can be limited.

  For example, when the received signal level is 70 dBuV and the noise level is 20 dBuV, in order to satisfy the carrier-to-noise power ratio (C / N) = 8.5 dB or less, the signal level of the subcarrier to be unused by the frequency variable filter 12 May be reduced by 41.5 dB or more. In addition, when a filter is added to both the transmitter side and the receiver side of the modem 13a, the subcarrier signal becomes unused by adding a filter that attenuates 20.75 dB or more. Therefore, the filter control signal generation table 19B is generated so that the frequency variable filter 12 satisfies the above characteristics. By referring to the generated filter control signal generation table 19B, outputting the filter control signal to the frequency variable filter 12, and filtering the high-frequency communication signal through the band, greatly reducing the influence of interference and interference. Can do.

  As described above, according to the third embodiment, the filter control circuit 18B generates a filter control signal based on the frequency setting information using the filter control signal generation table of the filter control signal generation table memory 19B. The frequency variable filter 12 is controlled. Therefore, it is possible to reduce the possibility that the desired wave is affected by the adjacent jamming wave or affects the neighboring jamming wave, so that a part of the frequency band that could not be used until now can be reduced. It can be used effectively, and has a specific effect that more communication channels can be used with a simple configuration as compared with the prior art.

In the third embodiment described above, the threshold for determining the change of the modulation scheme of each subcarrier signal is set to bit error rate (BER) = 10 ⁻⁴ . However, the present invention is not limited to this, and bit error rate is not limited to this. The rate (BER) may be larger or smaller than 10 ⁻⁴ .

  In the third embodiment described above, the modulation scheme used for the primary modulation is PSK and QAM. However, the present invention is not limited to this. For example, ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), etc. Other modulation schemes may be used.

Fourth embodiment.
FIG. 6 is a block diagram showing a configuration of a cable modem device according to the fourth embodiment of the present invention. Compared with the cable modem device 1A according to the second embodiment in FIG. 4, the cable modem device 1C according to the fourth embodiment has frequency setting information instead of the filter control circuit 18A as shown in FIG. And a filter control circuit 18C for calculating a filter control signal based on primary modulation setting information (indicating a modulation method) for each subcarrier signal. Hereinafter, differences from the second embodiment will be described in detail.

  In FIG. 6, the controller 17 measures the bit error rate (BER) for each subcarrier signal using the training signal of the high-frequency communication signal, and the modem 13 a in the RF processing circuit 13. The primary modulation setting information for each carrier signal is output to the controller 17. The controller 17 outputs the primary modulation setting information and frequency setting information for each measured subcarrier signal to the filter control circuit 18C. In response to this, the filter control circuit 18C calculates the filter control signal based on the input frequency setting information and the primary modulation setting information of each subcarrier signal, as will be described in detail later, and calculates the calculated filter. The control signal is output to the frequency variable filter 12. In response to this, the frequency variable filter 12 transmits a high-frequency communication signal input from the coaxial cable 20 based on the filter control signal input from the filter control circuit 18 in a frequency band corresponding to the filter control signal. After passing and filtering, the signal is output to the RF processing circuit 13.

  Next, a method for calculating a filter control signal in the filter control circuit 18C will be described below. In the fourth embodiment, the modulation scheme to be used is, for example, multi-carrier modulation of an OFDM signal, and the cable modem device 1C checks the transmission path state of each frequency band at the time of activation and modulates the primary modulation of each subcarrier signal. Set.

FIG. 7 is a diagram showing a relative gain with respect to frequency, showing an example of the band pass characteristic 32C of the frequency variable filter 12 of FIG. When the modulation multi-level number of the primary modulation of the subcarrier signal is 1/2 ² times or less than the modulation multi-level number of the primary modulation of the other subcarrier signal, it is due to the influence of interference of other signals instead of white noise. In many cases, primary modulation changes due to C / N degradation. Thus, primary modulation of the modulation multi-level number is set to the sub-carrier signal to attenuate the maximum 1/2 ² times the sub-carrier signals than the. In FIG. 7, when the modulation multi-level number of the primary modulation is 128 at maximum, the band-pass characteristic 32C capable of attenuating five subcarrier signals having the modulation multi-level number of primary modulation of 32 or less is variable in frequency by the filter control signal. Set to filter 12. That is, four subcarrier signals whose primary modulation method is 32QAM and one subcarrier signal whose primary modulation method is 16QAM are subcarrier signals to be attenuated.

Next, the attenuation amount of the subcarrier signal to be attenuated is calculated. The center frequency of the desired wave and the frequency bandwidth of the desired wave are Fca and Bwa, respectively, and the total number of subcarrier signals and the number of modulation multilevels of the primary modulation at a frequency higher than the subcarrier signal to be attenuated are ½ ^2. times following sub-carrier signal number, the modulation level of the primary modulation at a frequency lower than the sub-carrier signal for attenuating the largest ones than 1/2 ² times the subcarrier signal number respectively Nt _2, Nu _2, Nd ₂ Then, the upper limit frequency Fhigh of the frequency variable filter 12 and the lower limit frequency Flow of the frequency variable filter 12 are respectively expressed by the following equations.

  Therefore, the center frequency Fc and the frequency bandwidth Bw of the filter control signal are expressed by Expression (3) and Expression (4), respectively.

  As described above, according to the fourth embodiment, the filter control circuit 18C generates a filter control signal based on the frequency setting information and the primary modulation setting information of each subcarrier signal, and the frequency variable filter 12 Control filter characteristics. Therefore, it is possible to reduce the possibility that the desired wave is affected by the adjacent jamming wave or affects the neighboring jamming wave, so that a part of the frequency band that could not be used until now can be reduced. It can be used effectively, and has a specific effect that more communication channels can be used with a simple configuration as compared with the prior art.

  In the fourth embodiment described above, the modulation scheme used for the primary modulation is PSK and QAM. However, the present invention is not limited to this, and other modulation schemes such as ASK (Amplitude Shift Keying) and FSK (Frequency Shift) are used. Keying) may be used.

In the fourth embodiment described above, although primary modulation of the modulation level was set to the sub-carrier signal to attenuate the maximum 1/2 ² times the sub-carrier signals than the present invention will now not limited to, may be greater than 1/2 ^2-fold, it may be reduced.

  The frequency variable filter circuit according to the present invention has an effect of reducing the influence of interference and enabling effective use of a plurality of channels in communication using, for example, multicarrier modulation of an OFDM signal, and is added to a communication terminal device Useful as a filter.

It is a block diagram which shows the structure of the cable modem apparatus which concerns on the 1st Embodiment of this invention. It is a figure which shows an example of the band pass characteristic of the frequency variable filter 12 of FIG. 1, and shows the relative gain with respect to a frequency. It is a figure which shows a relative gain with respect to a frequency which shows an example of the out-of-band radiation component of the subcarrier signal input into the frequency variable filter 12 of FIG. It is a block diagram which shows the structure of the cable modem apparatus which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the cable modem apparatus which concerns on the 3rd Embodiment of this invention. It is a block diagram which shows the structure of the cable modem apparatus which concerns on the 4th Embodiment of this invention. It is a figure which shows an example of the band pass characteristic of the frequency variable filter of FIG. 6, and shows the relative gain with respect to a frequency.

Explanation of symbols

1, 1A, 1B, 1C ... cable modem device,
12 ... Frequency variable filter,
13 ... RF processing circuit,
13a ... modem,
14 ... BB processing circuit,
15 ... MAC processing circuit,
16 ... Ether interface,
17 ... Controller,
18, 18A, 18B, 18C ... filter control circuit,
19, 19B ... Filter control signal generation table memory,
20 ... Coaxial cable,
21 ... LAN cable,
30 ... Desired signal band,
31 ... Interference signal band,
32, 32C: Band pass characteristics of the variable frequency filter,
33: Signal occupying frequency band of the signal transmission channel,
34 ... Signal occupying frequency band of adjacent channel,
35: Out-of-band radiation component.

Claims (6)

In a frequency variable filter circuit including a frequency variable filter that performs band-pass filtering of a high-frequency communication signal composed of a plurality of subcarrier signals that are each digitally modulated and input in a predetermined pass band,
A frequency variable filter circuit comprising control means for controlling to set a frequency of band pass of the frequency variable filter based on setting frequency information of a modem that demodulates the high frequency communication signal.   The control means includes a first table for storing a frequency of a pass band of the frequency variable filter to be set for each set of primary modulation setting information common to the set frequency information and each subcarrier signal, and the set frequency Based on the information and primary modulation setting information common to each subcarrier signal, the band passing frequency of the frequency variable filter is set so as to reduce adjacent interference waves with reference to the first table. 2. The frequency variable filter circuit according to claim 1, wherein the frequency variable filter circuit is controlled.   The control means includes a second table that stores the frequency of the pass band of the frequency variable filter to be set for each set frequency information, and refers to the second table based on the set frequency information. 2. The frequency variable filter circuit according to claim 1, wherein control is performed so as to set a band-pass frequency of the frequency variable filter so as to reduce adjacent interference waves.   The control means measures line quality for each subcarrier signal of the input high-frequency communication signal, and reduces the adjacent interference wave based on the set frequency information and the measured line quality. 2. The frequency variable filter circuit according to claim 1, wherein control is performed so as to set a band pass frequency of the variable filter.   5. The frequency variable filter circuit according to claim 4, wherein the channel quality is a bit error rate (BER).   Based on the set frequency information and individual primary modulation setting information for each subcarrier signal of the input high-frequency communication signal, the control means performs bandpass of the frequency variable filter so as to reduce adjacent interference waves. 2. The frequency variable filter circuit according to claim 1, wherein the frequency variable filter circuit is controlled so as to set a frequency.

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