DUPLEX FREQUENCY DIVISION MULTIPLEX MODEM SYSTEM WITH ECHO CANCELLATION

Abstract: A two-wire full duplex frequency division multiplex (FDM) modem system includes two modem units (MODEM) connected through a two-wire full duplex transmission line system (EX OFFICE). The modem system includes an echo cancelling unit (12) for cancelling a transmission signal taking into consideration the line distortion characteristics. Each modem unit includes a modulator (10) for modulating transmission data (DATA T ), a hybrid (5) for feeding the modulated transmission signal and a reception signal to and from the line, an adder (7) for adding the signal from the hybrid and a cancelling signal (SECHO) and a demodulator (11) for demodulating the reception signal. The echo cancelling unit generates the echo cancelling signal for cancelling a transmission echo signal included in the signal from the hybrid, in response to the modulated signal from the modulating unit and the signal from the adding unit. The adding unit forms the difference between the echo cancellation signal and the signal from the hybrid. The system may also include a band pass filter (14) for receiving the signal from the adder unit and for passing a signal within the transmission frequency band to the echo cancellation signal generation unit as a signal for compensating the line characteristics.

Description: This invention relates to a data communication system and, more particularly, to a two-wire full duplex frequency division multiplex (FDM) modulator and demodulator (modem) system having echo cancellation means, particularly for use in a relatively low-speed data transmission system. For two-wire full duplex modem systems using telephone lines and having a relatively low data transmission rate, such as that disclosed in EP-A-091014, the CCITT recommends an FDM modem system according to, for example, the standard V.22 bis: 2400 bits per second duplex modem standardised for use in general switched telephone networks and leased circuits, and an echo cancellation modem system according to, for example, the standard V.26 ter. In the FDM modem system, a frequency band of 0.3 to 3.4kHz on the telephone line is divided into two. One part of the divided frequency band is used as a transmission frequency band, and the other part of the divided frequency band is used as a reception frequency band. Accordingly, the FDM modem system has the merit of simple and accurate extraction of a received signal at the receiving modem. On the other hand, since precise frequency discrimination is needed, complex and costly filters must be provided in the modem. The echo cancellation can be achieved by a digital signal processing technique, and the costly filters therefore become unnecessary. Nevertheless, difficult digital processing is required and a high precision extraction of the received signal cannot be realistically achieved. Both systems described above suffer from antinomic problems. In addition, public telephone transmission networks connect to a variety of modems. In order to facilitate the connection between and through a variety of modems, the establishment of standard hardware for a modem system is strongly required. It is known from EP-A-091014 to use an FDM system with partially overlapping frequency bands, whereby an echo canceller is used to cancel the unwanted signals in the overlapping portion. An object of the present invention is to provide an improved relatively low transmission speed two-wire full duplex transmission system. According to the present invention, there is provided a frequency division multiplex modem system comprising a first modem (MODEM) for modulating a transmission signal having a frequency in a first frequency band and demodulating a reception signal having a second frequency in a second frequency band different from the first frequency band and closely adjacent the first frequency band; means (EX OFFICE) including a two-wire duplex line structure for transmitting the transmission signal and the reception signal therethrough; and a second modem (MODEM) for demodulating the said transmission signal in the first frequency band transmitted by the first modem, and modulating a transmission signal of the second frequency to form the said reception signal; each modem comprising means for modulating transmission data (DATA T ); signal feeding means coupled to the modulating means and the transmission line means, for feeding the modulated transmission signal and the reception signal to and from the transmission line means; means for demodulating the reception signal; and echo cancelling means coupled to the modulating means for generating an echo cancelling signal for cancelling a transmission echo signal included in the signal from the signal feeding means, the echo cancelling signal being generated from the signal modulated at the modulating means and being subtracted from the signal from the signal feeding means: characterised in that the first and second frequency bands do not overlap, in that each modem includes adding means connected to the signal feeding means, for adding the signal from the signal feeding means and the echo cancelling signal; in that each modem includes a digital-to-analog converter coupled between the modulating means and the signal feeding means for converting the modulated signal having a digital form to an analog signal, an analog-to-digital converter coupled between the adder and the demodulating means and the echo cancellation signal generating means, for converting the analog signal from the adder means to a digital signal, and a digital-to-analog converter coupled to the output of the echo cancelling means to convert the echo cancelling signal (SECHO) into an analog signal, whereby the modulating means, the demodulating means and the echo cancelling means perform digital signal processing, and the signal feeding means and the adder means perform analog signal processing so that the echo cancellation is carried out in the analog regime; and in that each modem includes band-pass filter means to receive the signal from the adder means, and to pass a signal having only the transmitted frequency band to the echo cancellation signal generation means as a adaptation signal for adapting for line characteristics. Each modem may include a digital-to-analog converter, coupled between the modulating means and the synthesizing means, for converting the modulated signal from a digital signal into an analog signal, and an analog-to-digital converter, coupled to the adder, the demodulating means and the echo cancellation signal generating means, for converting the analog signal from the adder unit into a digital signal, so that the modulating means and the demodulating means perform digital signal processing, and the synthesizing means and the adder means perform analog signal processing. The echo cancellation signal generating means may include a transversal-type filter having a plurality of series-connected delaying portions, a first of which receives the modulated transmission signal from the modulating means, a plurality of coefficient multipliers each connected to an output of a corresponding delay portion to receive the signal from the adding means, and an adder to receive the coefficient multiplied signals from the coefficient multipliers. The demodulating means may include a demodulating portion, a roll-off filter and an automatic equalizer having a transversal-type filter. The frequency division multiplex modem system may further include means coupled to the echo cancellation signal generating unit and the adder means, for receiving the signal from the adder means, passing a signal having the transmission frequency band, and supplying same to the echo cancellation signal generation unit as a signal for compensating the line characteristics. The modem unit may also include a second digital-to-analog converter, operatively connected between the echo cancellation signal generating means and the adder unit, converting the echo cancellation signal of a digital form into an analog signal, so that the echo cancellation signal generating means also performs digital signal processing. The band pass filter means may include a first multiplier multiplying the signal from the analog-to-digital converter and a carrier having a centre frequency of the transmission frequency band, a transversal-type filter receiving the multiplied signal and a second multiplier multiplying the signal from the transversal-type filter and the carrier. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a block schematic diagram of a two-wire full duplex modem system to which an FDM modem of the present invention can be applied; Fig. 2 is a graph illustrating an example of a frequency band used by an FDM modem of the present invention; Fig. 3 is a block schematic diagram of a prior FDM modem; Fig. 4 is a block schematic diagram of an FDM modem illustrating the concept of the present invention; Fig. 5 is a block schematic diagram of an echo canceller forming part of the modem of Fig. 4; Figs. 6a and 6b are graphs illustrating a transmission signal waveform in the FDM modem of Fig. 4 and an echo cancelling signal waveform generated in the echo canceller of Fig. 5; Fig. 7 is a block schematic diagram of the preferred embodiment of an FDM modem according to the present invention; and Fig. 8 is a block schematic diagram of a band pass filter shown in Fig. 7. Referring to Fig. 1, a two-wire full duplex modem system includes a modem, an exchange office (EX OFFICE), another modem, and public telephone communication lines between the modem and the exchange office and between the exchange office and the other modem. The reference H denotes a hybrid circuit, and each line consists of two wires. The system of Fig. 1 is thus a so-called two-wire and four-wire hybrid communication system. The FDM modem system of Fig. 1 uses a frequency band between 0.3kHz and 3.4kHz, as shown in Fig. 2. The frequency band is divided into two frequency bands: a frequency band having a centre frequency f₁ in a transmission channel of the modem, and conversely, a reception channel of another modem, and another frequency band having a centre frequency f₂ using a reception channel for the modem, and conversely, a transmission channel for another modem. Before describing preferred embodiments of the present invention, a description will be given of the prior art and the differences between the prior art and the present invention distinguished. Fig. 3 is a circuit diagram of an FDM modem of the prior art such as may be installed at each end of the system in Fig. 1. The FDM modem includes a processor 1 having a modulator 10 and a demodulator 11, a digital-to-analog (D/A) converter 21, a transmission filter 3, a hybrid circuit 5, a reception filter 4 and an analog-to-digital (A/D) converter 22. Preferably, a low pass filter is provided between the transmission filter 3 and the hybrid circuit 5, and another low pass filter is provided between the hybrid circuit 5 and the reception filter 4. The modulator 10 modulates transmission data DATA T in digital form with a first carrier CR1 having a frequency of, for example, f₁ shown in Fig. 2. The modulated digital transmission data is converted to analog data at the D/A converter 21. The analog transmission data is frequency-limited between 0.3 and 1.85 kHz as shown in Fig. 2 at the transmission filter 3. The frequency-limited analog transmission data SD T is transmitted to another FDM modem through the hybrid circuit 5. Conversely, a reception signal SD R on the transmission line from another FDM modem and having a center frequency of f₂ is received at the hybrid circuit 5 and frequency-limited between 1.85 and 3.4 kHz at the reception filter 4. The frequency-limited reception signal is converted to an analog signal at the A/D converter 22. The converted reception signal is demodulated with another carrier having a frequency of f₂ to obtain digital reception data DATA R . Since the frequency pass band is narrow, as shown in Fig. 2, and the frequency characteristic has steep slopes, the transmission filter 3 and the reception filter 4 must have high frequency discrimination characteristics. In addition, both filters must be low loss type. Accordingly, both filters must be mechanical filters, better-worse filters having a large number of steps, etc. As a result, the FDM modem suffers from a disadvantage of high cost. Preferred embodiments of the present invention will be described. Figure 4 is a circuit diagram of an FDM modem illustrating the basis of the concept of the present invention. In Fig. 4, the FDM modem includes an echo canceller 12 in a processor 1a, a D/A converter 23, and a signal synthesizer (or adder) 7, in addition to the FDM modem of Fig. 3. Low pass line filters 24 to 26 are optionally provided to limit the signals therethrough in a frequency band of the telephone line: 0.3 kHz to 3.4 kHz. These filters can be provided in the prior FDM modem of Fig. 3. Note that the transmission filter 3 and the reception filter 4 in the FDM modem of Fig. 3 are omitted. The demodulator ll includes a demodulating portion lla, a roll-off filter llb and an automatic equalizer llc. The demodulating portion lla demodulates a received signal from the A/D converter 22 with the carrier CR2 having a frequency f₂. The roll-off filter llb cuts a high frequency component of the demodulated signal and makes a wave form shape. The automatic equalizer llc includes a transversal type equalizer and rejects the interference between codes to obtain reception data DATA R . Figure 5 is a circuit diagram of the echo canceller l2. The echo canceller l2 includes n steps of taps l2l₁ to l2l n , n steps of coefficient multiplier l22₁ to l22 n , and an adder l23. The echo canceller l2 receives a modulated transmission signal from the modulator l0. Each tap delays the modulated transmission signal. Each coefficient multiplier includes a multiplier Ml, an adder Al, a tap T, and a multiplier M2. The echo canceller l2 also receives a D/A converted reception signal from the A/D converter 22. The multiplier Ml multiplies the delayed signal of the tap l2l₁ and the reception signal from the A/D converter 22. The tap T delays one previous data with a delay Dl. The adder Al adds the multiplied signal from the multiplier Ml and the delayed signal from the tap T. The multiplier M2 multiplies the signal delayed at the tap l2l₁ and the signal added at the adder Al as a tap coefficient. The signal multiplied at the multiplier M2 is supplied to the adder l23. The adder l23 receives n output signals from the coefficient multipliers l22₁ to l22 n , adds the same and outputs an echo cancellation signal SECHO. As can be easily understood from the above description, the echo canceller l2 has a transversal type variable filter the same as that of the automatic equalizer llc. The echo canceller l0 receives the transmission signal, compensates the same by the transmission line delay characteristics including the delay characteristics of the transmission lines, another FDM modem oppositely provided, and the D/A converters 2l, 23, the A/D converter 22, the filters 24 to 26, the hybrid circuit 5 and the adder 7 in the FDM modem in question, on the basis of the transmission signal from the A/D converter 22, and outputs the echo cancellation signal SECHO with respect to the transmission signal. Referring back to Fig. 4, upon receipt of the transmission data DATA T having a digital form, the modulator l0 modulates the same with the carrier CRl having the frequency f₁. The modulated signal as shown in Fig. 6a is supplied to the D/A converter 2l and the echo canceller l2. The D/A converter 2l converts the modulated transmission signal to an analog signal, and the analog signal is passed through the low pass filter 24 to impose a frequency limit of between 0.3 and 3.4 kHz. The frequency limited analog transmission signal is supplied to the hybrid circuit 5 and joined with the reception signal having the frequency f₂ from another FDM modem, and transmitted to another FDM modem through the telephone line. The signals of the transmission signal having the frequency f₁ and the reception signal having the frequency f₂ combined at the hybrid circuit 5 are supplied to the adder 7. Simultaneously, the echo canceller l2 generates the echo cancellation signal SECHO depending on the transmission signal from the modulator l0. The echo cancellation signal SECHO is converted to an analog signal at the D/A converter 23. The D/A converted signal is supplied to an inverted terminal of the adder 7 through the low pass filter 25. The adder 7 subtracts the echo cancellation signal from the combined signals from the hybrid circuit 5. As a result, the adder 7 outputs the reception signal of frequency f₂ and a noise signal due to the distortion of the line characteristics and having approximately a frequency f₁. The signals from the adder 7 are frequency-limited between 0.3 and 3.4 kHz at the low pass filter 26. The frequency-limited signals are converted into digital signals at the D/A converter 22. The noise signal due to the line delay characteristics and having approximately a frequency f₁ is supplied to the echo canceller l2. The echo canceller l2 compensates the transmission signal from the modulator l0 on the basis of the noise signal and outputs the compensated transmission signal as shown in Fig. 6b, thus completely cancelling the transmission signal from the hybrid circuit 5. In Fig. 6b, curve el represents a first echo cancellation signal for cancelling the transmission signal immediately output from the hybrid circuit 5. Curve e2 represents a second cancellation signal for cancelling the transmission signal returned from the communication lines and another FDM modem, and thus distorted due to the line characteristics. The reception signal from the A/D converter 22 is demodulated by the carrier of frequency f₂ at the demodulating portion lla, filtered at the roll-off filter llb, and automatic-equalized at the automatic equalizer llc. As a result, the reception signal DATA R can be obtained. In a pure echo cancellation MODEM system, the same frequency is used for a transmission signal and a reception signal, and accordingly, echo cancellation is difficult and a complex echo cancellation process is needed. Conversely, the FDM system uses different frequency bands for the transmission signal and the reception signal. In addition, in a system having a relatively low transmission rate equal to 2400 bits per second or lower, an eye pattern is open. Accordingly, even if the S/N ratio is bad, it is enough to obtain the echo cancellation for locally strong echoes. In other words, high order paths in the transversal filter of the echo canceller 12 can be omitted. Furthermore, the roll-off filter 11b per se has a cancellation function. As a result, the echo canceller 12 can carry out the cancellation of a noise signal of approximately 20 ms to 30 ms. For example, the echo canceller 12 can provide 20 to 30 of the taps 121₁ to 121₂₀ or 121₃₀ and 20 to 30 of the coefficient multipliers 122₁ to 122₂₀ or 122₃₀ , when each tap has 1 ms delay characteristics. The echo canceller 12 naturally performs an automatic phase adjustment. The A/D converter 22 can be provided between the hybrid circuit 5 and the adder 7 instead of between the low pass filter 26 and the modulator 11. In this case, however, a high precision and large bit A/D converter must be installed. In the FDM MODEM system using the FDM modem set forth above, the modulator 10 carries out an amplitude phase modulation for 2400 bits per second, of 16 multilevels: four by four according to V.22 bis, or four multilevels according to V.26 ter. In the above description, the FDM modem at the one end using a center frequency of f₁ for the data transmission and a center frequency of f₂ is described. Another FDM modem at another end, using a center frequency of f₂ for the data transmission and a center frequency of f₁ , naturally carries out the echo cancellation for the frequency of f₂. Figure 7 shows the preferred embodiment of an FDM modem according to the present invention In Figure 7 the echo cancellation is available for the random pattern transmission data. In other words, the echo cancellation is unavailable for the regular pattern transmission data, for example, 75 bits per second modem, because the number of taps in the echo canceller 12 must be fixed. In order to apply the FDM modem set forth above, another FDM modem as shown in Fig. 7 including a band pass filter 14 is provided in addition to the FDM modem shown in Fig. 4. The band pass filter 14 may be a finite impulse response (FIR) type filter or an infinite impulse response (IIR) type filter. Figure 8 is a circuit diagram of the FIR type filter. The FIR type band pass filter includes a multiplexer 141, a transversal type filter 142, a multiplexer 147 and a low pass filter 148. The transversal type filter 142 includes a plurality of taps 143₁ to 143 k each of which may have 1 ms delay characteristics, a plurality of multipliers 144₁ to 144 k each of which multiplies a delayed signal from each tap and a coefficient c₁ to c k each of which is constant. A carrier having the frequency f₁ is supplied to the multipliers 141 and 147. The signal from the D/A converter 22 is multiplied by the carrier at the multiplier 141, reducing a low frequency signal. The frequency-reduced signal is supplied to the transversal type filter 142 and filtered thereat. The filtered signal is multiplied by the carrier at the multiplier 147, increasing a high frequency signal and thus restoring the frequency to that from the A/D converter 22. The signal from the multiplier 147 is preferably passed through the low pass filter 148 and supplied to the echo canceller 12. The band pass filter 14 extracts the signal having the frequency band of the transmission signal from the signal of the A/D converter 22, and provides only the noise signal due to the line delay characteristics. The echo canceller l2 uses the noise signal for generating the echo cancellation signal SECHO. The band pass filter l4 can be provided in not only the FDM modem for treating the regular pattern transmission data but also the FDM modem for treating the random pattern transmission data. The echo canceller l2 includes the transversal type filter as shown in Fig. 5. The band pass filter l4 also includes the transversal type filter as shown in Fig. 8. These transversal type filters are similar to the filter in the automatic equalizer llc. Accordingly, the echo canceller l2 and the band pass filter l4 are easily realized by applying the technology for making the automatic equalizer llc. This facilitates the production of the FDM modem. The FDM modems set forth above are easily applicable to any type of two-wire duplex MODEM systems having a relatively low transmission speed, by changing the parameters of the echo canceller and the band pass filter. This contributes to the establishment of a standard FDM modem. The above processor, which may include the modulator l0, the demodulator ll, the echo canceller l2 and the band pass filter l4, can be realized by a digital processing processor large scaled integrated circuit (LSI) of one chip, such as Fujitsu MB-8764.

Claims: A frequency division multiplex modem system comprising a first modem (MODEM) for modulating a transmission signal (f₁) having a frequency in a first frequency band and demodulating a reception signal (f₂) having a second frequency in a second frequency band different from the first frequency band and closely adjacent the first frequency band; means (EX OFFICE) including a two-wire duplex line structure for transmitting the transmission signal and the reception signal therethrough; and a second modem (MODEM) for demodulating the said transmission signal (f₁) in the first frequency band transmitted by the first modem, and modulating a transmission signal of the second frequency to form the said reception signal (f₂); each modem comprising means (10) for modulating transmission data (DATA T ); signal feeding means (5) coupled to the modulating means and the transmission line means, for feeding the modulated transmission signal and the reception signal to and from the transmission line means; means (11) for demodulating the reception signal; and echo cancelling means (12) coupled to the modulating means for generating an echo cancelling signal for cancelling a transmission echo signal included in the signal from the signal feeding means, the echo cancelling signal being generated from the signal modulated at the modulating means and being subtracted from the signal from the signal feeding means: characterised in that the first and second frequency bands do not overlap, in that each modem includes adding means (7) connected to the signal feeding means, for adding the signal from the signal feeding means and the echo cancelling signal; in that each modem includes a digital-to-analog converter (21) coupled between the modulating means (10) and the signal feeding means (5) for converting the modulated signal having a digital form to an analog signal, an analog-to-digital converter (22) coupled between the adder (7) and the demodulating means (11) and the echo cancellation signal generating means (12), for converting the analog signal from the adder means to a digital signal, and a digital-to-analog converter (23) coupled to the output of the echo cancelling means (12) to convert the echo cancelling signal (SECHO) into an analog signal, whereby the modulating means, the demodulating means and the echo cancelling means perform digital signal processing, and the signal feeding means and the adder means perform analog signal processing so that the echo cancellation is carried out in the analog regime; and in that each modem includes band-pass filter means (14) to receive the signal from the adder means (7), and to pass a signal having only the transmitted frequency band to the echo cancellation signal generation means (12) as a adaptation signal for adapting for line characteristics. A system according to claim 1, characterised in that the echo cancellation signal generating means (12) includes a transversal type filter having a plurality of series-connected delaying portions (121₁-121 n ), a first of which receives the modulated transmission signal from the modulating means (10), a plurality of coefficient multipliers (122₁-122 n ), each connected to an output of a corresponding delay portion to receive the signal from the adding means (7), and an adder (123) to receive the coefficient multiplied signals from the coefficient multipliers. A system according to claim 1 or 2, characterised in that the demodulating means (11) includes a demodulating portion (11a), a roll-off filter (11b) and an automatic equalizer (11c) having a transversal type filter. A system according to any preceding claim, characterised in that each modem includes a first digital-to-analog converter (21) coupled between the modulating means (10) and the signal feeding means (5) for converting the modulated signal having a digital form to an analog signal, a second digital-to-analog converter (23) coupled between the echo cancellation signal generating means (12) and the adder means (7) for converting the echo cancellation signal having a digital form to an analog signal, and an analog-to-digital converter (22) coupled between the adder and the demodulating means and the echo cancellation signal generating means (12) for converting the analog signal from the adder means to a digital signal, whereby the modulating means, the echo cancellation signal generating means and the demodulating means perform digital signal processing, and the signal feeding means and the adder means perform analog signal processing. A system according to claim 4, characterised in that the echo cancellation signal generating means (12) includes a transversal type filter having a plurality of series-connected delaying portions (121₁-121 n ), a first of which receives the modulated transmission signal from the modulating means (10), a plurality of coefficient multipliers (122₁-122 n ) each connected to an output of a corresponding delay portion to receive the signal from the adding means, and an adder (123) to receive the coefficient multiplied signals from the coefficient multipliers; and the band pass filter means (14) includes a first multiplier (141) for multiplying the signal from the analog-to-digital converter and a carrier having a centre frequency of the transmission frequency band, a transversal type filter (142) to receive the multiplied signal and a second multiplier (147) for multiplying the signal from the transversal type filter and the carrier.

Inventors: KAKU, TAKASHI
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: H04B3
IPC codes: H04L5, H04B3, H04B1