JP4159567B2 - ADSL modem and transmission echo distortion noise reduction method in ADSL modem - Google Patents

Description

  The present invention relates to an ADSL modem and a transmission echo distortion noise reduction method in an ADSL modem for reducing noise when performing communication using a telephone line, and in particular, transmission echo mixed in a received signal using an echo canceller. The present invention relates to an ADSL modem and a transmission echo distortion noise reduction method in an ADSL modem in which components are removed.

  The telephone is equipped with an echo canceller to eliminate echo and howling that occur when the sound of the communication partner output from the speaker is input from the microphone. Even on a telephone line, there is a possibility that noise called “echo” on the output side may be mixed into the input side. Therefore, an ADSL modem (modulation / demodulation device) using ADSL (Asymmetric Digital Subscriber Line) employs a technique called FDM (Frequency Division Multiplexing) to completely separate the frequency band used in the upstream and the frequency used in the downstream. Separated to prevent mixing of echoes.

  By the way, in response to a request for an ADSL acceleration service, there is an ADSL acceleration technique called a spectrum overlap transmission system that improves the speed of a downlink signal by using the frequency band used for the uplink signal also for the downlink signal. Has appeared. In this ADSL speed-up technique, it is important how echo cancellation is performed in the overlapping part of the frequency band. In the XOL system, which is one of the technologies called Anex C (Seax X) (C.x) among the ADSL acceleration technologies, the received signal is overlapped with the transmission signal band only in the FEXT (Far End CrossTalk) time domain. ing. Further, in the technology called the DMB (OLM) -OL system in which both the FEXT and NEXT (Near End CrossTalk) time zones of Annex Sea are overlapped, further speed improvement is achieved.

  Also, Annex A, which is an ADSL standard for North America, has developed a technique called Annex A.ex that causes received signals to overlap a transmission band. Further, a technique for switching between a transmission band and a reception band without overlapping and switching a high-pass filter in accordance with a use band of a received signal is disclosed as a first proposal (for example, see Patent Document 1).

  By the way, in ADSL, a metal cable is used as a communication line. When laying a metal cable, a branch point called a bridge tap is widely attached in order to make it easy to branch the cable and take out the circuit. Further, when connecting a metal cable, a connection method (hereinafter referred to as “hand twist”) in which only a hand is twisted without using solder may be used. At these connection locations, the resistance value changes discontinuously at the connection portions, thereby causing signal reflection.

  FIG. 13 shows an example of reflection that occurs in the path between the ADSL modem and the central office. A metal cable 151 is laid between an ADSL modem (CPE: Customer Premises Equipment) 101 and a CO (Central Office). It is assumed that the bridge tap 152 is arranged in the middle of the metal cable 151 as shown in this figure. In this case, the transmission signal 153 from the ADSL modem 101 toward the CO 103 is reflected by the open end of the bridge tap 152 due to the discontinuity of the resistance value. Such reflection has a negative effect on high-speed communication.

  FIG. 14 shows how transmission echo components increase due to reflection of transmission signals in the ADSL modem. Each vertical axis represents a signal level. FIG. 14A shows a case where no reflection occurs, and the transmission echo component 131 is slightly above the reception component 141. However, when the transmission signal 153 (FIG. 13) is reflected, the transmission echo component 131 increases as indicated by an arrow 155 in FIG. 14 (b).

  FIG. 15 shows the length, line loss, and frequency characteristic when there is one bridge tap. In this figure, the curve 161 represents the characteristic when the bridge tap 152 does not exist between the ADSL modem 101 and the CO 103 in FIG. Curve 162 has one 80m (meter) bridge tap 152, curve 163 has one 100m bridge tap 152, and curve 164 has one 230m bridge tap 152. The curve 165 represents the case where there is one bridge tap 152 of 400 m. It is assumed that the bridge tap 152 exists at the center position between the ADSL modem 101 and the CO 103, respectively.

  On the other hand, FIG. 16 compares the case of one bridge tap and the case of two bridge taps with the case of zero. In this figure, a curve 171 represents the characteristic when the bridge tap 152 does not exist between the ADSL modem 101 and the CO 103 in FIG. A curve 172 represents a case where one 80 m bridge tap 152 exists, and a curve 173 represents a case where two 80 m bridge taps 152 exist. Here, it is assumed that one bridge tap 152 exists in the center of the ADSL modem 101 and the CO 103, and two bridge taps 152 exist at equal intervals between the ADSL modem 101 and the CO 103. In FIG. 16, the vertical axis represents line loss and the horizontal axis represents frequency. This is the same as FIG. As can be seen from FIG. 16, the bridge tap 152 not only reflects the transmission echo but also generates a loss having frequency characteristics.

FIG. 17 shows the respective signal levels in the prior art by dividing the length of the metal cable between the ADSL modem and the CO into three stages. In these figures, the vertical axis represents the signal level. FIG. 11A shows a case where the length of the metal cable 151 between the ADSL modem 101 and the CO 103 shown in FIG. 13 is relatively short, and FIG. FIG. 5B shows a case where the metal cable 151 has an intermediate length. Assuming that the levels of the transmission echo components 131 are the same, the signal level of the reception component 141 decreases by a value d _{1 when} the signal level changes from the figure (a) to the figure (b), and when the level becomes the figure (c). The signal level is reduced by a value d ₂ larger than the value d ₁ as compared with FIG.

  FIG. 17 shows an example in which the frequency of the reception component 141 is fixed. As is well known, attenuation due to the line length has a frequency characteristic. The higher the frequency is, the greater the attenuation of the reception component 141 is.

FIG. 18 shows an example of the positional relationship of the bridge taps in the cable arranged between the ADSL modem and the CO. In this example, a first bridged tap 152 ₁ to the metal cable 151 disposed between the ADSL modem (CPE) 101 and CO103, ₂ is a second bridged tap 152 is disposed. The distance between the _first bridge tap 152 ₁ and the second bridge tap 152 ₂ is 500 m, and the distance between the second bridge tap 152 ₂ and the CO 103 is 800 m. Furthermore, the distance between the ADSL modem 101 of the first bridge tap 152 ₁ is short enough to be ignored, the distance between the ADSL modem 101 and CO103 is assumed to be 1300 m. In this example, ₁ and the first bridged tap 152 during a second bridged tap 152 ₂ uses metal cable 151 _first diameter 0.65 mm, this part is insulated with plastic. Further, a 0.4 mm metal cable 151 ₂ is used between the second bridge tap 152 ₂ and the CO 103, and this portion is insulated by paper (paper).

FIG. 19 shows frequency characteristics when the lengths of the first and second bridge taps are changed under the arrangement state shown in FIG. In this figure, a curve 175 represents a characteristic in a state where both the first and second bridge taps 152 ₁ and 152 ₂ do not exist. A curve 176 shows the frequency characteristics when the 4 m first and second bridge taps 152 ₁ and 152 ₂ are connected. Curve 177 shows when 80 m of first and second bridge taps 152 ₁ , 152 ₂ are connected, and curve 178 shows that 100 m of first and second bridge taps 152 ₁ , 152 ₂ are connected. The frequency characteristics are shown respectively. Except for the curve 175, it can be seen that large attenuation occurs at each specified frequency.

  As described above, the actual line in which the bridge tap 152 exists between the ADSL modem 101 and the CO 103 exhibits complicated frequency characteristics. The bridge tap 152 has been described as an example. Similarly, when the metal cable is connected by hand twisting, a complicated frequency characteristic is generated in the reception component, or the metal cable is connected by a gain control circuit (not shown) in the ADSL modem 101. The transmission signal sent to the signal is amplified by a transmission echo component mixed in the circuit portion on the receiving side through a hybrid circuit (not shown) that performs 2-wire to 4-wire conversion. As a result, the bridge tap 152 or twisting may cause a significant decrease in the communication rate despite the adoption of the ADSL speed-up technique called the FDM system to the spectrum overlap transmission system Annex ADX. The problem that occurred.

  FIG. 20 shows a comparison between the FDM system and the spectral overlap transmission system when signal reflection occurs. FIGS. 20A to 20C sequentially show the process of removing the transmission echo component from the reception component in the FDM ADSL modem. (D) to (f) correspond to (a) to (c), respectively, and the process of removing the transmission echo component from the received component by the spectral overlap transmission method (OL method) is sequentially performed. It represents.

20A and 20D, a solid line represents a transmission echo waveform 181 including a transmission echo component generated by inputting a part of a transmission signal transmitted by the ADSL modem 101 to a reception receiver (not shown) therein. ing. The transmission echo waveform 181 is obtained by superimposing a waveform portion 183 generated by reflection on the waveform 182 of the transmission echo component when there is no reflection. The transmission echo waveform 181 further includes a transmission echo component 181a ₁ having a high wave height that exists in the original transmission band, and several transmission echo distortion noise components 181b ₁ to 181b ₁ that are derived from the transmission band. and a 181d _1. Looking at the transmission echo component 181a ₁ , the wave height raised by the waveform portion 183 generated by reflection becomes ΔH. The receiving receiver in the ADSL modem 101 receives signals sent from the CO 103 shown in FIG. 13 through the metal cable 151 as received signals 191 ₁ and 192 ₁ . The FDM method of FIG. (A), low frequency valence of the received signal 191 ₁ does not overlap with the transmission echo component 181a. On the other hand, in the spectrum overlap transmission system of FIG. 6D, the low frequency side of the received signal 192 ₁ is not clear in the drawing, but overlaps with the transmission echo component 181a ₁ .

FIGS. 20B and 20E show signal waveforms after the transmission echo component is filtered by the high-pass filter in the ADSL modem 101 and the signal level is adjusted by the gain control circuit described above. is there. Among them, in the FDM system of FIG. 5B, the frequency component of the transmission band is cut by the high-pass filter, so that the transmission echo component 181a ₁ is removed as shown by the broken line. Therefore, transmission echo distortion noise components 181b _{2 to} 181d ₂ whose signal levels have been changed through the gain control circuit and the like exist together with the reception signal 191 ₂ . Further, in the spectrum overlap transmission method of FIG. 9E, the transmission band is used as a part of the reception band, and therefore the frequency component of the transmission band is not cut by the high-pass filter. Therefore, the transmission echo components 181a _{3 to} 181d ₃ after the signal level is adjusted by the gain control circuit are present together with the reception signal 192 ₂ .

FIGS. 20C and 20F show signal waveforms after the transmission echo component is processed by an echo canceller processing unit (not shown) in the ADSL modem 101. FIG. Among them, in FIG. 9C, the transmission echo distortion noise components 181b _{2 to} 181d ₂ remaining in FIG. 10B are removed by the echo canceller processing unit, and only the intended received signal 191 ₃ remains. Also in FIG. 8F, the transmission echo components 181a _{3 to} 181d ₃ remaining in FIG. 8E are removed by the echo canceller processing unit, and only the intended received signal 192 ₃ remains.

  In FIG. 20, when looking at components mixed as transmission echoes on the receiving side in the ADSL modem 101, the echo components generated by the reflection of the bridge tap 152 and the like are as shown in FIGS. In the spectrum overlap transmission method, the ratio is increased by passing through the filter processing by the high-pass filter and the echo canceller processing section as shown in FIG. I am letting. As a result, when the spectrum overlap transmission method is used, when the spectrum overlap transmission method is used, the increase in the transmission echo component caused by the reflection occurs when the spectrum overlap transmission method is used. As a result, the dynamic range is reduced.

  However, most of the actual lines using metal cables include elements that reflect transmission echoes such as a bridge tap 152 and a hand twist. In addition, it is a middle-distance user of a metal cable that occupies the maximum number among users that is easily affected by a decrease in transmission speed. As a result, there has been a problem that the communication rate is sometimes greatly reduced although the FDM method is changed to the spectrum overlap transmission method in order to improve the communication rate.

Therefore, changing the characteristics of the high-pass filter according to the distance has been proposed as a second proposal (see, for example, Patent Document 2). In the second proposal, the distance between the ADSL modem 101 and the CO 103 is determined based on the level of the received signal, and the constant of the high-pass filter is changed at a middle distance as a specific distance range. That is, the received signal attenuates as the length of the metal cable between the ADSL modem and the CO increases. Therefore, in the second proposal, when the level of the received signal becomes equal to or lower than a predetermined level, the pass band or cutoff frequency of the high-pass filter is changed by adjusting the resistance value of the variable resistance circuit. Specifically, the reception level is increased by boosting the reception signal within a predetermined range. In addition, the communication rate is improved for users of medium and long distances.
Japanese Patent Laying-Open No. 2004-015786 (paragraph 0038, FIG. 4) JP 2004-032727 A (paragraphs 0009 to 0011, FIGS. 3, 4, and 6)

  In the second proposal, the problem of a decrease in communication rate can be solved by increasing the reception level only when the line is an ideal line in which only line loss occurs. However, when noise exists in the reception band, this noise is also amplified together with the amplification of the reception level. As a result, in a communication environment in which transmission echo distortion noise caused by a bridge tap or a twist is a problem, the second proposal cannot solve the problem of a decrease in communication rate. In addition, noise caused by a bridge tap, hand twist, and the like and simple attenuation of the received signal are completely different in the cause of the decrease in the communication rate. For this reason, it is impossible to cope with the method of determining the length of the metal cable and changing the characteristics of the high-pass filter according to the distance.

  In the technique for improving the apparent SNR (signal-to-noise ratio) according to the second proposal, the noise component is increased. As a result, communication errors frequently occur for non-stationary noise accompanied by frequency fluctuations. As a result, there is a problem that a very unstable communication state appears.

  Accordingly, an object of the present invention is to reduce transmission echo distortion noise caused by line reflection, particularly in an ADSL modem and an ADSL modem, which can effectively reduce transmission echo distortion noise when the cable distance is other than a short distance. There is to get a way.

  In the present invention, (a) an echo canceller that removes a transmission echo component of a transmission signal mixed in a reception band assigned to a reception signal sent from a telephone line, and (b) a transmission signal using a spectrum overlap transmission method. A transmission band blocking high-pass filter that blocks the transmission band in the received signal transmitted overlapping the allocated transmission band, and (c) whether the received signal can secure a predetermined dynamic range. And (d) filter control means for applying the transmission band cutoff high-pass filter to the received signal when the dynamic range ensuring presence / absence determining means determines that the predetermined dynamic range cannot be secured. In the ADSL modem.

  That is, in the present invention, when the transmission echo component of the transmission signal mixed in the reception band is removed using the echo canceller, the dynamic range ensuring presence / absence determining unit overlaps with the transmission band under the condition that the predetermined dynamic range cannot be secured. Therefore, the transmission band in the received signal transmitted is blocked by a transmission band blocking high-pass filter. Thereby, for example, transmission echo distortion noise in the mid-high range can be reduced.

  In the present invention, (a) training execution means for executing training prior to communication by ADSL, and (b) reception received from a telephone line based on the result of training executed by the training execution means. (C) dynamic range ensuring presence / absence determining means for determining whether a signal can secure a predetermined dynamic range; and (c) when the dynamic range ensuring presence / absence determining means determines that the received signal cannot secure a predetermined dynamic range. A transmission band cutoff high-pass filter that cuts off this overlapped transmission band when the signal overlaps the transmission band of the transmission signal assigned to a lower frequency range; and (d) the transmission band cutoff high pass. Retrain the received signal that passed the filter And training execution means, (e) is provided with a communication rate setting means for setting a communication rate in the communication by the ADSL from the execution result of the retraining execution means to ADSL modem.

  That is, in the present invention, when the received signal sent from the telephone line cannot secure a predetermined dynamic range from the result of the training executed by the training execution means, the received signal is assigned to a lower frequency than this. In addition to functioning the transmission band blocking high-pass filter that blocks this overlapping transmission band when it overlaps the transmission band of the transmitted signal, the transmission echo distortion noise can be achieved by performing training again in this state. ADSL communication can be performed using an optimal communication rate in a state in which the amount of communication is reduced.

  In the present invention, (a) a training execution step for executing training prior to communication by ADSL, and (b) reception received from a telephone line based on the results of the training executed in this training execution step. A dynamic range ensuring presence / absence determining step for determining whether or not a signal can secure a predetermined dynamic range; and (c) when the received signal is determined to be unable to secure a predetermined dynamic range by the dynamic range ensuring / notifying step. When the signal overlaps the transmission band of the transmission signal assigned to the lower band, the transmission band blocking step for blocking this overlapping transmission band, and (d) processing in this transmission band blocking step Re-execute training on the received signal. And training execution step, (e) is provided with a communication rate setting step of setting a communication rate in the communication by the ADSL from the execution result by the retraining execution step to transmit echo distortion noise reduction method in the ADSL modem.

  That is, in the present invention, when the received signal sent from the telephone line cannot secure a predetermined dynamic range from the result of the training executed in the training execution step, the received signal is assigned to a lower frequency than this. In addition to functioning the transmission band blocking high-pass filter that blocks this overlapping transmission band when it overlaps the transmission band of the transmitted signal, the transmission echo distortion noise can be achieved by performing training again in this state. ADSL communication can be performed using an optimal communication rate in a state in which the amount of communication is reduced.

  As described above, according to the present invention, the transmission band in the received signal transmitted overlapping the transmission band is cut off by the transmission band blocking high-pass filter under a predetermined condition. For example, transmission echo distortion noise in the mid-high range can be reduced, the communication distance using ADSL can be extended, and communication quality can be improved.

  Hereinafter, the present invention will be described in detail with reference to examples.

  FIG. 1 shows an ADSL modem for ADSL and its peripheral configuration in an embodiment of the present invention. In this ADSL modem (CPE: Customer Premises Equipment) 201, a CO (Central Office) 203 and a metal cable 251 are connected via a distributor called a splitter 202 that divides a voice band and an ADSL band. . The ADSL modem 201 is connected to a transformer 211 connected to the splitter 202, a hybrid circuit (HYB) 212 that performs two-wire to four-wire conversion, and a host interface 210 that realizes an interface with a terminal (not shown). A digital signal processing unit 213 for processing digital equalization and inverse Fourier transform and Fourier transform of the digital signal is provided. A digital transmission signal 214 output from the digital signal processing unit 213 is converted into an analog signal by a digital-analog converter (DAC) 215, and a high-frequency component higher than a transmission band used by the transmission signal by a low-pass filter (LPF) 216. Is then amplified by the transmission driver 217 and input to the hybrid circuit 212. The hybrid circuit 212 outputs the transmission signal to the splitter 202 side via the transformer 211.

  On the other hand, the reception signal input to the ADSL modem 201 via the splitter 202 is sent from the hybrid circuit 212 to the reception receiver 218 and received. A signal 219 that has passed through the reception receiver 218 is branched and input to a first high-pass filter (HPF) 221 and a second high-pass filter (HPF) 222. The first high-pass filter 221 passes the first high-frequency band as it is, the second high-pass filter 222 passes the second high-frequency band through the switch 223, and the signal that has passed through each is input to the selector 224. To be processed.

  Here, the first high-pass filter 221 is a filter having a cut-off frequency of about 50 kHz at the maximum in order to reduce low-frequency transmission echo that reduces the amount of echo cancellation suppression. The second high-pass filter 222 is a transmission band when the ADSL modem 201 of this embodiment performs communication by the conventional FDM system without using the spectrum overlap transmission system (hereinafter abbreviated as EC system). It is a filter used for blocking. In the present embodiment, the second high-pass filter 222 is also used in the EC method under a predetermined condition. A switch 223 is used for this control. The switch 223 is turned on when the dynamic margin of the received signal calculated by the digital signal processing unit 213 during the EC system training cannot secure a predetermined dynamic range threshold. The predetermined dynamic range threshold is a threshold determined from various line model verifications affected by an increase in echo due to reflection. The switch 223 is controlled by the switch control signal 225 output from the digital signal processing unit 213, and the output side of the second high-pass filter 222 is turned on (conducted) thereby, so that the transmission band is cut off. Such an EC system of the present embodiment is referred to as a “new EC system” when distinguished from a conventional EC system that does not perform on / off control of the second high-pass filter 222.

  The selector 224 is a circuit element having the following functions. First, the signal sent from the first high-pass filter 221 is the G.P. During handshake procedure processing of 994.1 (G.hs), it is always sent to the gain control circuit 226. Next, when performing communication using the existing EC method, the signal transmitted from the first high-pass filter 221 is allowed to pass because the switch 223 is turned off, but from the second high-pass filter 222. Do not let incoming signals pass through. In the new EC method, when a predetermined dynamic range is not ensured, a signal transmitted from the second high-pass filter 222 is passed. Specifically, on / off control of the switch 223 may be performed in relation to the distance between the ADSL modem 201 and the CO 203. A gain control circuit 226 arranged at the subsequent stage of the selector 224 amplifies the received signal received, converts the received signal into a digital received signal 228 by an analog-digital converter (ADC) 227, and then inputs the digital received signal to the digital signal processor 213. It will be.

  In the ADSL modem 201, the transmission signal 214 and the reception signal 228 are input to the echo canceller processing unit 229, and the output signal 230 is input to the digital signal processing unit 213. Cancellation is to be made. This is to prevent echo and howling due to a part of the transmission signal input from the transmission driver 217 to the hybrid circuit 212 being input to the reception receiver 218 as the transmission echo component 231.

  By the way, the ADSL modem 201 for ADSL of this embodiment selects two types of communication methods for communication. The two types of communication systems employed in the present embodiment are a new EC system obtained by improving the EC system used conventionally and an FDM system employed conventionally. As described in the prior art, the EC method is an evolution of the FDM method. Therefore, in this embodiment, communication is performed by the FDM method only when the two types of EC methods are not possible.

  FIG. 2 shows a state of selection control of two types of communication methods performed in this embodiment. First, the ADSL modem 201 for ADSL shown in FIG. Communication with the CO 203 is performed according to a handshaking procedure of 994.1 (G.hs) (step S301), and it is determined whether or not communication using the EC method is possible (step S302). Specifically, it is determined whether or not communication using the EC method can be performed between the ADSL modem 201 and the CO 203 and both are within the distance range permitted by the EC method. The latter determination is made by the ADSL modem 201 detecting the level of the handshake signal sent from the CO 203. When it is determined that communication using the EC method is possible in this way (step S302: Y), first, the switch 223 is turned off by the control of the switch control signal 225, and the second high-pass filter 222 The output is turned off (step S303), and training of the ADSL line is performed in this state (step S304). If a predetermined dynamic range can be secured from this result (step S305: Y), the EC method is selected, and communication via the ADSL line is performed with the output of the second high-pass filter 222 remaining off. (Step S306). In conclusion, in this case, communication using the conventional EC method is performed.

  On the other hand, when a predetermined dynamic range cannot be ensured as a result of training in step S305 (N), the switch 223 is turned on by the switch control signal 225 to turn on the output of the second high-pass filter 222 ( Step S307). In this state, the ADSL line is retrained (step S308). In this retraining, a communication rate suitable for a new EC method in which the output of the second high-pass filter 222 is turned on is determined. Then, communication using the ADSL line in the new EC method in which the output of the second high-pass filter 222 is turned on at this communication rate is performed (step S309).

  On the other hand, if it is determined in step S302 that communication using the EC method is impossible (N), the switch 223 is turned on by control of the switch control signal 225 (step S310), and training is performed (step S311). This is because the received signal does not overlap with the band of the transmission signal in the FDM system, and the second high-pass filter 222 may be turned on when obtaining the received signal. When training is performed, a communication rate suitable for the FDM system is determined thereby, and communication is performed using the ADSL line of the FDM system (step S312).

  As apparent from the above control, in the ADSL modem 201 of this embodiment, when the EC method can be adopted, only the output side of the first high-pass filter 221 is turned on as in the case of the conventional EC method. Training is performed with the output side of the high-pass filter 222 turned off. As a result, when the dynamic margin of the received signal cannot secure a predetermined dynamic range threshold due to the influence of echo increase due to reflection or the like, the output side of the second high-pass filter 222 is turned on to cut off the transmission band. To do. As a result, it is possible to suppress the distortion that the transmission echo due to the influence of the bridge tap and the hand twist has on the reception band. In addition, by performing echo cancellation processing together with this, it is possible to prevent the communication rate from deteriorating when compared with the FDM system under a specific line environment, and secure communication with a higher communication rate than the FDM system. Yes.

  If the EC method cannot be adopted (step S302: N), communication is performed using the FDM method (step S312). In this case, both the output sides of the first and second high-pass filters 221 and 222 are turned on as shown in step S310. Thereby, the transmission echo is cut off, the dynamic range is secured, and the communication speed (communication rate) is determined by training (step S311).

FIG. 3 shows the state of signal processing in the circuit portion on the receiving side of the ADSL modem when communication is performed using the ADSL line by the FDM method in step S312. The upper half of the same figure (a)-(c) represents the signal waveform, and the horizontal axis represents the frequency. The frequency increases as it goes to the right in the figure. Transmitting the echo component 231a ₁ ~231d _1, this represents the waveform when present in solid lines and the removed state of the signal by a broken line. The waveform expressed by a one-dot chain line represents the receiving component 241 _{1-243 1.} The lower half of FIGS. 9A and 9B shows the level of the received signal received by the ADSL modem 201 and the level of the transmission echo, and the vertical axis shows those signal levels.

First, the upper half of FIG. 3A shows a signal 219 that appears on the output side of the reception receiver 218 in FIG. The transmission echo component 231 ₁ is a transmission echo signal component 231a ₁ having a high peak value shown in the lowest transmission band (left end side in the figure) in FIG. It is composed of several transmission echo distortion noise components 231b _{1 to} 231d ₁ (three are representatively shown in the figure) generated on the high frequency side. Here, the transmission echo signal component 231a ₁ having a high peak value is composed of a frequency lower than the cutoff frequency of the low-pass filter 216. Receiving component 241 ₁ FDM scheme has a frequency component higher than the cutoff frequency of the low pass filter 216. That is, the transmission echo signal component 231a ₁ and the reception component 241 ₁ having a high peak value are separated in frequency. However, due to distortion of the transmission echo signal component 231a ₁ , transmission echo distortion noise components 231b _{1 to} 231d ₁ are generated as distortion noise in a frequency band in which only the reception signal originally exists.

The lower half of FIG. 3 (a), the receiving component 241 ₁ and transmit echo component 231 ₁ of each signal level is shown. In this example, the signal level of the transmission echo component 231 ₁ is higher than the signal level of the reception component 241 ₁ . Thus, reading of the receiving component 241 ₁ is difficult.

On the other hand, the received signal 228 output from the analog-digital converter 227 in FIG. 1 is shown in the upper half of FIG. In the FDM system performed when the EC system is impossible in this embodiment, the switch 223 shown in FIG. 1 is turned on. Therefore, both the first high-pass filter 221 and the second high-pass filter 222 function as filters. As a result, the transmission echo signal component 231a ₁ having a high peak value is removed. As a result, the signal level of the reception component 242 ₁ is higher than that of the reception component 241 _{1 in} FIG. 9A due to signal amplification by the gain control 226, and the same applies to the transmission echo distortion noise components 231b _{2 to} 231d _2. but the signal level of the transmission echo component 231 ₂ as a whole is reduced. As a result, if the dynamic range 245 ₁ as the level difference between the reception component 242 ₁ and the transmission echo component 231 ₂ is large to some extent, the received signal can be obtained with satisfactory quality.

Finally, the output signal 230 after processing of the echo canceller processing unit 229 is shown in the upper half of FIG. The echo canceller processing unit 229 removes transmission echo distortion noise components 231b _{2 to} 231d ₂ (see FIG. 5A) having a low peak value. Thus, the receiving component 243 ₁ can ensure good communication rate quality by training.

  Next, a case where the conventional EC method can be adopted will be described. In the EC system, the reception band can partially overlap the transmission band in order to improve the communication rate of the reception signal. In order to receive such overlapping received signals, it is preferable to secure the dynamic margin by turning off the switch 223 and shutting off the output side of the second high-pass filter 222.

FIG. 4 shows an ADSL line in which the output of the second high-pass filter remains off as a result of using the EC method and securing a predetermined dynamic range in step S305 as shown in step S306 of FIG. 2 illustrates a state of signal processing in a circuit portion on the receiving side of the ADSL modem when performing communication according to the above. 4A to 4C, as in FIGS. 3A to 3C, the horizontal axis represents the frequency in the upper half of these drawings. The frequency increases as it goes to the right in the figure. The transmission echo components 231a _{2 to} 231d ₂ are represented by solid lines when they are present, and by broken lines when signals are removed. In addition, the lower half of FIGS. 9A to 9C shows the level of the received signal received by the ADSL modem 201 and the level of the transmission echo, and the vertical axis shows the signal level.

First, the upper half of FIG. 4A shows a signal 219 that appears on the output side of the receiver 218 in FIG. The transmission echo component 231 ₃ is a transmission echo signal component 231a ₃ having a high peak value shown on the lowest frequency side (left side in the figure) in FIG. Are composed of several transmission echo distortion noise components 231b _{3 to} 231d ₃ ( _three are representatively shown in the figure). Here, the transmission echo signal component 231 a ₃ having a high peak value is composed of a frequency lower than the cutoff frequency of the low-pass filter 216. Due to distortion of the transmission echo signal component 231a ₃ , transmission echo distortion noise components 231b _{3 to} 231d ₃ are generated as distortion noise in a frequency band in which only the reception signal originally exists.

In the lower half of FIG. 4A, signal levels of the reception component 241 ₂ and the transmission echo component 231 ₃ are shown. In this example, the signal level of the transmission echo component 231 ₃ is higher than the signal level of the reception component 241 ₂ . Thus, reading of the receiving component 241 ₂ becomes difficult.

On the other hand, the received signal 228 output from the analog-digital converter 227 in FIG. 1 is shown in the upper half of FIG. In this example, the EC method is employed, and the switch 223 shown in FIG. 1 is off. Therefore, only the first high-pass filter 221 functions as a filter. At this time, the transmission echo components 231a _{4 to} 231d ₄ are not removed. In this figure, these signal levels are increased as compared with the figure (a) as a result of signal amplification by the gain control 226. As a result of this, towards the receiving component 242 ₂ transmission than echo components 231 ₄ remains in state signal level high, is difficult state read of the receive component 242 ₂ is not improved.

FIG. 4C shows the output signal 230 after processing by the echo canceller processing unit 229. The echo canceller processing unit 229 removes the transmission echo signal component 231a ₄ and the transmission echo distortion noise components 231b _{4 to} 231d ₄ shown in FIG. As a result, as shown in the lower part of the FIG. 4 (c), the lower than the signal level received component 243 _{and second} signal levels of the transmission echo component 231 ₅ remaining at this point for transmission echo. Therefore, if the dynamic range 245 ₂ is larger than a predetermined size, the received signal can be obtained with satisfactory quality.

As described in step S305 in FIG. 2, the dynamic range 245 ₂ may not be able to secure a predetermined value. This is the case where transmission echoes such as the bridge tap 152 and hand twist described above with reference to FIG. 13 are reflected. When EC scheme that can be employed, if such were not able to secure a sufficient dynamic range 245 ₂ turns on the switch 223 by the switch control signal 225, the second high-pass filter 222 It will shift to a new EC system with the output turned on. Then, the dynamic range 245 ₂ shown in FIG. 4C is secured to a predetermined value or more based on the result of training shown in step S311. In this embodiment, whether the output of the second high-pass filter 222 is turned on or off is controlled according to the length of the metal cable between the ADSL modem 201 and the CO 203.

  FIG. 5 shows the length of the metal cable between the ADSL modem and the CO in three stages, and shows the three communication systems that can be adopted in this embodiment and the characteristics of the received signal finally obtained for each. is there. In this figure, the vertical row (a) indicates the case where the length of the metal cable between the ADSL modem 201 and the CO 203 shown in FIG. 1 is relatively short, and the row (b) in FIG. It shows the case where the cable has an intermediate length. The column in FIG. 3C shows a case where the metal cable is relatively long. In FIG. 5, the row (α) in the horizontal direction indicates the FDM method in step S312 in FIG. 2, and the column in FIG. 5 (β) indicates the output of the second high-pass filter 222 shown in step S306. 2 shows a conventional EC system with the side turned off. FIG. 5 (γ) shows the new EC method shown in step S309. However, in the figure (γ), the output side of the second high-pass filter 222 is turned on only in the figure corresponding to the figure (b). In the same figure (γ), the output side of the second high-pass filter 222 is off in the remaining lengths of the metal cables shown in FIGS. In these diagrams showing the signal characteristics in each case in the matrix form, the broken line shows the waveform of the transmission echo component 231 (see FIGS. 3 and 4) from which the dashed line is removed, and the alternate long and short dash line shows the reception component 243 ( FIG. 3 and FIG. 4).

  First, in the FDM system represented by the row (α) in the figure, the transmission echo signal component 231a and the reception component having a high peak value are shown in each of the columns (a) to (c) in FIG. 243 is divided in frequency with a predetermined frequency as a boundary. Further, in this FDM system, the attenuation of the high frequency component of the signal transmitted through the metal cable becomes remarkable. Therefore, the reception component 243 has a wider frequency range for reception as the length of the metal cable is shorter, and the communication rate can be kept high. It is like that.

  Next, looking at the conventional EC method in the row of FIG. (Β), the attenuation of the reception component 243 has the same tendency as the FDM method in the row of FIG. However, as compared with the row in FIG. 9A, the low frequency side (left side in these figures) of the reception component 243 overlaps the transmission echo signal component 231a having a high peak value existing in the transmission band. As a result, the communication rate of the reception component 243 is faster than that of the FDM system in the row of FIG.

  In the new EC system in the row (γ) in the figure, the same tendency as in the row (β) in the figure (β) is applied to the attenuation of the reception component 243 and the frequency overlap with the low-frequency side transmission signal 231a existing in the transmission band. It is in. However, in the case of the row (γ) in the figure, the filter function of the second high-pass filter 222 is used together with the first high-pass filter 221 when the length of the metal cable in the figure (b) is intermediate. ing. For this reason, there is an advantage that the dynamic range of the received signal is increased when the metal cable of FIG. 5B has an intermediate length compared to FIG.

  From FIG. 5, the following can be understood. The influence of transmission echo reflection due to the bridge tap 152 and hand twist shown in FIG. 13 on the deterioration of the communication rate is generally large in the column of FIG. 13B, and FIG. When the length of the metal cable between the ADSL modem 201 and the CO 203 is short or long as in the column c), the influence is small. Further, when considering the speed (downlink speed) of the received signal of the ADSL modem 201, when the length of the metal cable is relatively short (a), the conventional and new EC systems have almost the same downlink speed. This is superior to the downlink speed of the FDM system. When the metal cables in the row in FIG. 5B have an intermediate length, the new EC method is the fastest in terms of downstream speed, followed by the FDM method. In this regard, the downlink speed of the conventional EC method is the slowest. Finally, when the metal cables in the row (c) are relatively long, the conventional and new EC schemes have almost the same downlink speed, which is superior to the FDM downlink speed.

  In addition, the overlap transmission method in which a part of the reception band overlaps the transmission band is a conventional method that always overlaps, and a new EC method that overlaps only when the metal cable has an intermediate length If the superiority and inferiority of the received signal of the FDM system that does not always overlap and the ADSL modem 201 are compared, the length of each metal cable is as follows. First, when the metal cable of FIG. 5A is short, the present embodiment and the conventional overlap method are superior to the FDM method at the reception speed. When the metal cable shown in FIG. 5B has an intermediate length, the method of the present embodiment that uses the filter function of the second high-pass filter 222 is superior to the FDM method, and this FDM method is a conventional overlap. It will be superior to the transmission method. When the metal cable shown in FIG. 5C is long, the reception speed of the system of this embodiment and the conventional overlap system is superior to the FDM system.

  6 to 8 show the frequency band of the received signal when the length of the metal cable between the ADSL modem and the CO is in each case. In these figures, the vertical axis represents the signal level, and the horizontal axis represents the change in frequency in the reception band. The frequency is higher on the right side of the horizontal axis.

  Among these, FIG. 6 shows the frequency band of the received signal when the metal cable between the ADSL modem 201 and the CO 203 is relatively short. In the figure, a section 261 is a band (OL band) where the transmission echo component 231a and the received signal 264 are not shown in this figure, and the section 262 has a higher frequency than this and only the received signal 264 can exist. It is a non-overlap band (non-OL band). As shown in this figure, when the distance between the ADSL modem 201 and the CO 203 is short, the attenuation is small and the level of the reception signal 264 is large, which is sufficient over almost the entire frequency range with respect to the transmission echo distortion noise 265 and the background noise 266. A dynamic range 267 can be ensured. Therefore, the reception signal 264 is not easily affected by the transmission echo distortion noise 265. In such a line situation, it is possible to obtain a preferable communication result when the output side of the second high-pass filter 222 is turned off even in the new EC system as in the conventional EC system.

  On the other hand, FIG. 7 shows a case where the metal cable has an intermediate length. Compared with FIG. 6, the transmission echo distortion noise 265 and the background noise 266 are not changed, but the reception signal 271 is attenuated by the length of the metal cable. Therefore, the transmission echo distortion noise 265 becomes dominant in securing the dynamic range 272 in the middle / high range with respect to the reception signal 271. “Dominant” means that the transmission echo distortion noise 265 has a greater influence on the dynamic range 272 than the background noise 266 in the region where the reception signal 271 exists.

  By the way, in ADSL, a multicarrier (carrier wave) having an interval of 4.3125 kHz is used. These carriers are subjected to QAM (phase amplitude) modulation so that information of 15 bits or more is placed on one carrier. This is called bit loading. However, if the noise is large, if many bits are modulated, errors will increase and communication will become unstable. Therefore, the SNR (signal-to-noise ratio) is determined in advance by the system, and bit loading is reduced for a noisy carrier wave. On the other hand, bit loading is increased for a carrier with less noise.

  In FIG. 7, since the transmission echo distortion noise 265 is dominant in securing the dynamic range in the mid-high range, the bit loading in the mid-high range is reduced so that errors do not increase, thereby degrading the communication rate. As a result, in the worst case, the communication rate is worse than that of the FDM system. Therefore, in this embodiment, such a line status is determined based on the level of the dynamic range of each received carrier calculated during the training performed in step S304 in FIG. If the predetermined dynamic range threshold value cannot be secured in step S305, the process proceeds to step S307 to turn on the output of the second high-pass filter 222. As a result, the received signal is cut off from the overlapping transmission band. However, the transmission echo distortion noise 265 is reduced, and bit loading to the mid-high range becomes possible. Therefore, if this bit loading is superior to the overlapping bit loading in the transmission band, a higher communication rate can be obtained by turning on the output of the second high-pass filter 222. Moreover, in this case, since echo cancellation processing is performed, bit loading can be performed to the limit of the transmission band, which contributes to the realization of a high communication rate.

  As shown in FIG. 8, when the metal cable is relatively long, the transmission echo distortion noise 265 and the background noise 266 are not different from those in FIGS. 6 and 7, but the reception signal 275 is greatly attenuated in the high band. Bit loading cannot be performed in the band affected by the transmission echo distortion noise 265 in the received signal 275 due to the error rate. Therefore, a higher communication rate can be obtained when bit loading is performed in the section 261 where the overlap is performed in order to effectively use the bandwidth. Therefore, it is possible to obtain a preferable communication result when the output side of the second high-pass filter 222 is turned off even in the new EC system as in the conventional EC system.

  From the above results, in this embodiment, as described with reference to FIG. 5 (γ), when the metal cable between the ADSL modem 201 and the CO 203 has an intermediate length, the output side of the second high-pass filter 222 Is turned on, and when the metal cable has a length other than this, it is turned off.

  Next, the on / off control of the second high-pass filter 222, which is a feature of the present invention, will be described more specifically. In ADSL, as described above, communication is performed by modulating a carrier wave at intervals of 4.3125 kHz. The reception band needs to be 8 to 16 times the transmission band because the data capacity on the downstream side needs to be larger than that on the upstream side. During training, the dynamic range of each subcarrier is calculated, and the number of modulation bits that can be stably communicated is determined. Therefore, a dynamic range threshold value for determining whether the second high-pass filter 222 is on or off may be determined for each subcarrier.

  However, if the threshold value of the dynamic range for determining whether the second high-pass filter 222 is turned on or off is determined for each subcarrier in this way, there is a problem that data processing becomes heavy. Therefore, in the case where such a problem occurs, in this embodiment, data processing is made light by using a predetermined dynamic range for several subcarriers.

FIG. 9 is a diagram for explaining a case where the function of the second high-pass filter is controlled on / off using a dynamic range of five points when the reception band is eight times the transmission band. is there. In this figure, the horizontal axis represents the frequency, and in the figure of the reception signal band 281, the left end portion is the transmission signal band 282. The reception signal band 281 is assigned a frequency eight times that of the transmission signal band 282. First to fifth points P _{1 to} P ₅ are set from the frequency close to the transmission signal band 282 toward the high band side in the reception signal band 281 excluding the transmission signal band 282. The transmission echo distortion is distributed in a higher harmonic band than the transmission band. Therefore, the first to fifth points P _{1 to} P ₅ are appropriately changed according to the transmission band.

In the present embodiment, the on / off state of the second high-pass filter 222 is determined by using the dynamic ranges of the _first to fifth points P _{1 to} P ₅ . For this purpose, a dynamic range threshold value for determining on / off of the second high-pass filter 222 is set for each of the _first to fifth points P _{1 to} P ₅ individually.

These first to fifth dynamic range threshold points P ₁ to P for ₅ is set by a variety of verification at the laboratory level. In this case, the frequency characteristics, the communication rate, the bit loading situation, and the bridge tap in the pseudo line based on the arrangement of the bridge tap between the ADSL modem 201 and the CO 203 shown in FIG. Compare and verify when it exists and when it does not exist. In this way, not only can the dynamic range threshold values for the _first to fifth points P _{1 to} P _{5 be} set in advance in the pseudo line, but the field test can be performed on the real line. Is fed back, and the dynamic range threshold value can be determined with a more accurate determination criterion. The dynamic range threshold value for the _first to fifth points P _{1 to} P ₅ determined in this way is a dynamic range affected by reflection of transmission echo for each subcarrier or for each specific frequency. The threshold model is stored in a memory (not shown) in the digital signal processing unit 213 shown in FIG. It may be stored in another memory in the ADSL modem 201.

By the way, the waveform shown by the broken line in FIG. 9 is the transmission echo component 231a existing in the transmission signal band 282, and the transmission echo distortion noise component at the _first to fifth points P _{1 to} P ₅ higher than this. 231b to 231f exist. The reception signals 243a to 243c show reception examples in different frequency bands. Among these, in the case of the received signal 243a whose reception band is widened to the highest frequency side, it can be determined that a sufficient dynamic range can be secured by using the _first to fifth points P _{1 to} P _5. it can. Therefore, the second high-pass filter 222 is turned off for a received signal having a signal waveform such as the received signal 243a.

If the received signal 243b may be a dynamic range is determined to be low in the second and third band of points P _2, P ₃ of the. Therefore, for the received signal having a signal waveform such as the received signal 243b, the second high-pass filter 222 is turned on (step S307 in FIG. 2), retraining is performed (step S308), and optimum bit loading is performed. The bitmap to be performed is configured.

In the case of the received signal 243c, the optimum bit map is obtained by overlapping the received signal 243c with the transmission signal band 282 rather than turning on the second high-pass filter 222 in relation to the _first point P1. Should be configured. Therefore, in this case, the second high-pass filter 222 remains off (step S306).

  Next, the effect of reducing transmission echo distortion noise according to the present embodiment will be described with a specific example.

FIG. 10 shows a specific example of the positional relationship of the bridge taps in the cable arranged between the ADSL modem and the CO. In this example, a first bridge tap 252 ₁ and a second bridge tap 252 ₂ are arranged on a 2500 m metal cable arranged between the ADSL modem 201 and the CO 203. It is assumed that the distance between the _first bridge tap 252 ₁ and the second bridge tap 252 ₂ is 500 m, and the distance between the second bridge tap 252 ₂ and CO 203 is 2000 m. Further, it is assumed that the distance between the ADSL modem (CPE) 201 and the first bridge tap 252 ₁ is negligibly short. In this example, a metal cable 251 ₁ having a diameter of 0.65 mm is used between the first bridge tap 252 ₁ and the second bridge tap 252 ₂ , and this portion is insulated with plastic. Also, a 0.4 mm metal cable 251 ₂ is used between the second bridge tap 252 ₂ and the CO 203, and this portion is insulated by paper (paper). Further, both the first bridge tap 252 ₁ and the second bridge tap 252 ₂ are assumed to be 100 m.

  FIG. 11 shows experimental data in which the effect of the present invention is quantitatively expressed as a line graph on the pseudo line shown in FIG. In these figures, “bin #” on the horizontal axis represents a carrier number. For example, “bin # 32” represents a carrier wave having a frequency of 138 kHz obtained by multiplying 4.3125 kHz by 32. The vertical axis represents the number of bits.

  FIG. 7A shows a comparison between the spectrum overlap transmission method and the FDM method when the output side of the second high-pass filter 222 is not turned on, and FIG. This is a comparison between the spectrum overlap transmission system of the present invention and the FDM system when the output side is turned on. In both figures, a broken line 291 indicates the FDM system. Moreover, the case where the broken line 292 of the same figure (a) communicates by the EC method which has existed conventionally is shown, and the case where the broken line 293 of the same figure (b) communicates by the new EC method is shown. Yes.

  In the conventional EC system of FIG. 6A, the communication rate is lower than 5152 kbps (kilobits / second) and 5440 kbps of the FDM system. The reason why the bit loading in the mid-high range in the spectrum overlap transmission system is lower than the bit loading in the mid-high range in the FDM system is because the transmission echo distortion noise reduces the dynamic margin. The calculation of the communication rate is the sum of the number of bits of information transmitted per unit time by each carrier wave. For example, for the carrier wave of “bin # 32”, 11-bit information is modulated as shown in FIG. Each carrier wave sends out information at a modulation speed of 4000 times per second. Therefore, for the carrier wave of “bin # 32”, 48.4 kbps is obtained by multiplying the modulation rate by the amount of information. By summing up the optical lenses obtained for each carrier wave, a communication rate of 5152 kbps of the conventional EC method is obtained. The communication rate of 5440 kbps in the FDM system is calculated in the same way.

  On the other hand, the new EC method shown in FIG. 5B is equivalent to the FDM method in terms of bit loading in the middle and high range. However, since the echo cancellation process is operating, even if the output side of the second high-pass filter 222 that cuts off the transmission band is turned on, bit loading is performed to the limit of the transmission band. As a result, a communication rate of 6112 kbps is calculated by the above-described calculation method. As a result, it can be seen that the new EC scheme achieves the best communication rate.

  <Modification of the invention>

  FIG. 12 shows a transmission / reception bitmap in the modification of the present invention. In this modification, the present invention is applied to the XOL system. Here, the XOL system is an Annex C DBM (Dual BitMap) system that uses one of the ADSL standards for Japan. In this XOL system, the concept of time division is adopted for overlap, and this is overlapped with the band of the transmission signal 402 only in the FEXT (Far End CrossTalk) time region of the received signal 401 as shown in FIG. I am letting. As shown in FIG. 5B, the reception signal 403 is not overlapped with the band of the transmission signal 402 in the NEXT (Near End CrossTalk) time domain.

  FIGS. 7C to 7F show a conventional XOL system and a XOL system to which the present invention is applied when transmission echo is reflected by a bridge tap or a hand twist. As shown in these drawings, in addition to the transmission echo component 411a in the transmission band due to reflection of the transmission echo, several transmission echo distortion noise components 411b to 411d that are derived in the higher frequency region than the transmission band are derived. Has occurred. These transmission echo component 411a and transmission echo distortion noise components 411b to 411d have their peak values increased from the original waveform by the amount of transmission echo reflection. In these figures, the waveform is indicated by a broken line with a broken line because it shows both the original transmission echo distortion noise component and the transmission echo distortion noise component raised by reflection.

In both the conventional XOL system shown in FIGS. 7C and 7D and the XOL system of the present invention shown in FIGS. 6E and 6F, echo cancellation processing is performed in both the FEXT time domain and the NEXT time domain, The transmission echo component 411a and the transmission echo distortion noise components 411b to 411d are removed. Here, the communication rate in the FEXT time region in which the received signals 401 ₁ and 401 ₂ are overlapped with the transmission bands shown in (c) and (e) of FIG. Both of these ensure the communication rate with the overlap of the transmission band.

On the other hand, in the NEXT time zone, the XOL system of the present invention shown in FIG. 5F turns on the output side of the second high-pass filter 222 as compared to the conventional XOL system shown in FIG. As a result of this, it is possible to increase the bit loading of the NEXT time domain as shown from the received signal 403 ₂ of the same as compared to the received signal 403 ₁ of the (d) of FIG Figure (f). Therefore, a dynamic range can be ensured even when transmission echoes are reflected by a bridge tap or hand twist.

  In the embodiment described above, the switch 223 is arranged on the output side in order to turn on / off the function of the second high-pass filter 222, but this is not necessary, and a similar switch can be provided on the input side. It is. Further, whether the function of the second high-pass filter 222 is on or off is not limited to the embodiment. Next, an example in which this determination is simplified will be described.

  In ADSL, G. During the hs handshake procedure, the distance from the CO 203 shown in FIG. 1 is detected based on the attenuation level of the tone signal used for the handshake. Then, using the result of this distance determination, whether the reception method is a quad spectrum method, an annex eye (G.992.1 Annex I) method or an annex sea (G.992 Annex C) method, Alternatively, it is determined whether the EC method is possible. By adopting the detection result as a condition for turning on / off the function of the second high-pass filter 222, the processing can be simplified. That is, the point of the dynamic range used for the determination of turning on / off the function of the second high-pass filter 222 is one point on the low frequency side where the distortion of the transmission echo is determined to be the largest, and the dynamic range is predetermined. If it is lower than the threshold and within a certain distance range, the function may be turned on. The specific distance range here is preferably determined by using various verification results in the same manner as described in the determination of the dynamic range. Thereby, although the accuracy of discrimination is inferior to that in the embodiment, not only can the processing be simplified, but also good reception characteristics can be maintained.

  Further, in the embodiment, the on / off control of the function of the second high-pass filter is performed using the dynamic range of five points in FIG. 9, but the point dividing point and the number of divisions may be changed as appropriate. Good. The band of the transmission signal varies depending on the communication mode, such as (a) 25 kHz to 138 kHz, (b) 25 kHz to 276 kHz, and (c) 25 kHz to 552 kHz. When the transmission band changes in this way, the transmission echo distortion distribution also changes accordingly. Therefore, by optimizing the division points and the number of divisions in accordance with this, it is possible to reduce distortion noise that is more suitable for the transmission distortion noise distribution.

1 is a block diagram showing an ADSL modem for ADSL and its peripheral configuration in an embodiment of the present invention. FIG. It is a flowchart showing the mode of selection control of two types of communication systems performed in a present Example. It is explanatory drawing showing the mode of the signal processing in the circuit part of the receiving side of an ADSL modem in the case of performing communication by an ADSL line by FDM system. It is explanatory drawing showing the mode of the signal processing in the circuit part of the receiving side of an ADSL modem in the case of performing communication by an ADSL line using the conventional EC system. It is explanatory drawing showing the characteristic of the received signal finally obtained about each of three communication systems and dividing each of the length of a metal cable into three steps in a present Example. It is the characteristic view which showed the frequency band of the received signal when the length of the metal cable between an ADSL modem and CO is comparatively short in a present Example. It is the characteristic view which showed the frequency band of the received signal in case a length of the metal cable between an ADSL modem and CO is intermediate in the present embodiment. It is the characteristic view which showed the frequency band of the received signal when the length of the metal cable between ADSL modem and CO is comparatively long in a present Example. It is explanatory drawing in the case of performing on / off control of the function of a 2nd high pass filter using the dynamic range of 5 points in a present Example. It is explanatory drawing showing the positional relationship of the bridge tap in the cable arrange | positioned between ADSL modem and CO in a present Example. FIG. 11 is a characteristic diagram as experimental data that quantitatively represents the effect of the present invention in the pseudo line shown in FIG. 10. It is explanatory drawing which showed the transmission / reception bit map in the modification of this invention. It is explanatory drawing which showed the reflection which generate | occur | produces in the path | route between an ADSL modem and a telephone office. It is explanatory drawing showing a mode that the transmission echo component by the reflection of a transmission signal increases in the conventional ADSL modem. It is a characteristic diagram showing the length, line loss, and frequency characteristic in the case of one bridge tap. It is the characteristic view which compared the case with one bridge tap and the case with two with the case of zero. It is explanatory drawing which divided each length of the metal cable between an ADSL modem and CO into three steps, and showed each conventional signal level. It is explanatory drawing showing an example of the positional relationship of the bridge tap in the cable arrange | positioned between an ADSL modem and CO. FIG. 19 is a characteristic diagram showing frequency characteristics when the lengths of the first and second bridge taps are changed under the arrangement state shown in FIG. 18. It is a wave form diagram which contrasted the FDM system and the conventional spectrum overlap transmission system.

Explanation of symbols

201 ADSL modem 203 CO (Central Office)
212 Hybrid Circuit 213 Digital Signal Processing Unit 217 Transmission Driver 218 Reception Receiver 221 First High Pass Filter 222 Second High Pass Filter (Transmission Band Cutoff High Pass Filter)
223 Switch 224 Selector 225 Switch control signal 226 Gain control circuit 229 Echo canceller processing unit 231 Transmission echo component 241 to 243 Reception component 251 Metal cable

Claims (8)

An echo canceller that removes the transmission echo component of the transmission signal mixed in the reception band assigned to the reception signal sent from the telephone line;
A transmission band blocking high-pass filter that blocks the transmission band in the received signal transmitted in an overlapped manner with the transmission band assigned to the transmission signal in a spectrum overlap transmission system;
Dynamic range securing presence / absence judging means for judging whether the received signal can secure a predetermined dynamic range,
An ADSL modem comprising: filter control means for applying the transmission band cutoff high-pass filter to the received signal when the dynamic range ensuring presence / absence determining means determines that the predetermined dynamic range cannot be secured. 2. The ADSL modem according to claim 1, further comprising bit loading means for bit-loading the received signal to the boundary of the transmission band when the transmission band blocking high-pass filter is applied to the received signal . 2. The ADSL modem according to claim 1, wherein the dynamic range ensuring presence / absence determining means determines using a dynamic range threshold value of each carrier calculated during training . 2. The ADSL modem according to claim 1, wherein the dynamic range ensuring presence / absence determining means determines using a dynamic range threshold value of one or more subcarriers determined in advance . 5. The ADSL modem according to claim 3, further comprising a dynamic range threshold memory for storing the dynamic range threshold in advance . Training execution means for executing training prior to communication by ADSL;
Dynamic range securing presence / absence determining means for determining whether a received signal sent from a telephone line can secure a predetermined dynamic range based on a result of training executed by the training executing means,
When it is determined by the dynamic range ensuring presence / absence determining means that the received signal cannot secure a predetermined dynamic range, when the received signal overlaps the transmission band of the transmission signal allocated to a lower frequency range A transmission band blocking high-pass filter that blocks the overlapped transmission band of
Retraining execution means for executing training again on the received signal that has passed through the transmission band cutoff high-pass filter;
Communication rate setting means for setting a communication rate in communication by the ADSL from the execution result of the retraining execution means
ADSL modem, characterized in that it comprises a door. A training execution step for executing training prior to communication by ADSL;
Based on the result of training executed in this training execution step, a dynamic range ensuring presence / absence determining step for determining whether a received signal sent from the telephone line can secure a predetermined dynamic range;
When it is determined in the dynamic range ensuring presence / absence determining step that the received signal cannot secure a predetermined dynamic range, the received signal overlaps the transmission band of the transmission signal assigned to a lower frequency than this A transmission band blocking step of blocking the overlapping transmission band of
A retraining execution step of executing training again on the received signal processed in the transmission band cut-off step;
Communication rate setting step for setting a communication rate in communication by the ADSL from the execution result of the retraining execution step
A transmission echo distortion noise reduction method in an ADSL modem. 8. The transmission echo distortion noise reduction method in an ADSL modem according to claim 7, further comprising an echo canceller processing step for removing a transmission echo component and a transmission echo distortion noise component from the received signal.

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