IES83526Y1 - A method for preparing pre-coder filter coefficients and pre-equaliser filter coefficients for PCM modem communications - Google Patents

Abstract

ABSTRACT A method and apparatus for preparing pre-coder filter coefficients and pre-equaliser filter coefficients for use by a client modem (2) in a client/server PCM modem communication in a client to server direction. The method comprises transmitting a first training signal of predetermined values by the client modem (2) through the channel (5) to the server modem (3), and preparing a model of the channel (5) in the server modem (3) in a feed-forward filter (6) which is representative of distortion in the channel (5) in the client to server direction. A second training signal generated by the server modem (3) is passed through a shaping filter (18), and in turn through the channel model feed-forward filter (6), and the output signal from the channel model filter (6) is fed through an equaliser (16) and passed to an adder (40) where the output of the shaping filter (18) is subtracted from the signal from the equaliser (16). The output from the adder (40) is fed back to the equaliser (16) where the coefficients of feed-forward amplifiers and feedback amplifiers of the equaliser (16) are determined for providing the pre-coder and pre equaliser filter coefficients respectively for correcting for distortion in the channel (5).

Description

A method for preparing pre—coder filter coefficients and pre~equa|iser filter coefficients for PCM modem communications The present invention relates to a method for preparing pre—coder filter coefficients and pre-equaliser filter coefficients for use in PCM modem communications, and in particular, for use in a client modem in client/server PCM modem communications in a client to server direction.

PCM upstream modulation used by V.92 modems for upstream transmission between a client and a server, in other words, upstream communications in a client to server direction requires that pre—coder and pre-equaliser filter coefficients be prepared by the server modem. The prepared pre—coder and pre-equaliser filter coefficients are transmitted by the server modem to the client modern. The respective coefficients are used by the client modem in the pre—coder and pre- equaliser filters of the client modem in order to pre—compensate for the effect of channel distortions on transmissions from the client modem to the server modem. In this way the sewer modem receives a clean undistorted signal from the client modem. This form of modulation used in PCM upstream by V.92 modems is different to modulation used by other communications systems. In most communications systems, channel distortions are compensated for by the receiving modem, rather than in the case of PCM upstream modulation used by V.92 modems by the transmitting modem. However, in PCM upstream modulation used by V.92 modems, since it is the server modem which prepares the pre—coder and pre- equaliser filter coefficients, the training signal is transmitted by the client modem to the sewer modem, which then prepares the respective coefficients from the training signal for subsequent transmission to the client modem. in prior art arrangements the pre-coder filter used by the client modem is a feedback type filter, while the pre—equaliser filter used by the client modem is a feed—forvvard type filter. The client modem minimises the power at the output of the pre-coder feedback filter by choosing the point from an allowed selection of constellation points, which generates the smallest output power at the output of that filter.

Typically, the server modem uses a decision feedback equaliser in the preparation of the pre-coder and pre—equaliser filter coefficients. The decision feedback equaliser is trained by the training signal from the client modem in order to prepare the pre— coder and pre—equaliser filter coefficients, which are then subsequently transmitted to the client modem. The decision feedback equaliser is trained using an adaptive least mean square algorithm in order to minimise the errors from the decision feedback equaliser in the server modem. The errors are calculated by comparing the output of the feedback equaliser in the receiver of the server modem while receiving the training signal from the client modem with a known reference or with decisions made based on the known constellation. This method while it allows for the establishment of communications between client and server modems, it results in poor signal quality at the receiving modem, and connection speeds are relatively slow. A typical telecommunications line contains strong attenuation (zeros) around DC and 4kHz frequencies. The V.92 upstream signal, unlike most other types of modem signals, contains energy around DC and at 4kHz frequencies, and therefore, an ideal equaliser would have to amplify these frequencies relatively strongly, up to, for example, 40dBs to 50dBs in order to compensate for the channels attenuation.

Typical least mean squares algorithms are unsuitable since by amplifying the DC and 4kHz frequency components noise at these frequencies is correspondingly amplified. Accordingly, the maximum obtainable amplification using known methods is typically limited to 20dBs. Since the client modem pre—coder filter minimises its output transmit power, the spectral characteristic of which is flat, unpre—coded signals have a —20dBs attenuation around DC frequencies. Since a channel could have attenuation of —50dBs around the DC frequencies, a significant amount of information at these lower frequencies is lost. This results in an increase in noise being observed by the server modem. Accordingly, in known methods, in order to avoid this problem, it is necessary to minimise the information being sent around these highly attenuated frequencies, which is undesirable.

Accordingly, there is a need for a method for preparing pre—coder filter coefficients and pre—equaliser filter coefficients for PCM upstream modulation for use by V.92 modems for use in PCM upstream communications in the client to server direction.

The present invention is directed towards providing such a method, and the invention is also directed towards apparatus for carrying out the method.

According to the invention there is provided a method for preparing pre—coder filter coefficients and pre—equaliser filter coefficients for use by a client modem in a client/server PCM modem communication in a client to server direction, the method comprising the steps of preparing a model of the channel in the server modem representative of distortions in the channel in the client to server direction, the channel model being prepared by reading a first training signal of predetermined values transmitted by the client modem through the channel to the server modem, and preparing the channel model to correct for distortions in the received first training signal so that the channel model is representative of the channel distortions, passing a second training signal generated in the server modem through the channel model, and using the output signal from the channel model in response to the second training signal for preparing the pre—coder filter coefficients and the pre- equaliser filter coefficients in the server modem.

In one embodiment of the invention the channel model is prepared in a filter, and preferably, in a feed-forward filter, which advantageously, is a feed—forward filter having up to one hundred and twenty-eight coefficients. Preferably, the method includes the step of adapting the feed—forward filter in which the channel model is prepared by using a least mean squares algorithm. A feed—forward filter is sufficient for preparing the channel model, since the channel, in general, only contains attenuation which can be modelled effectively by a feed—fonivard filter, typically, an all zero filter.

Preferably, a separate simple background equaliser is provided in the server modem for decoding training signals received from the client modem, the value of which are not predetermined, and which are received after the first training signals.

In one embodiment of the invention a shaping filter is provided in the sewer modern, and the second training signal is passed through the shaping filter prior to being passed through the channel model filter. Preferably, the shaping filter is adapted to amplify the frequencies which are expected to be attenuated most in the Channel, and preferably, the shaping filter is adapted to amplify frequencies around the DC and the 4kHz level relative to its mid—band gain. Preferably, the shaping filter has an amplification in the range of 15dBs to 20dBs around DC and 4kHz frequencies.

In one embodiment of the invention the second training signal comprises a normal pseudo-random white training sequence.

In another embodiment of the invention the equaliser in the server modem which receives the second training signal for training thereof is a feed-forvvard and a feedback coefficient filter, and preferably, is a feed-fon/vard and a feedback filter having one hundred and twenty»eight coefficients.

In another embodiment of the invention the method is used for preparing pre—coder and pre-equaliser filter coefficients for use in communications between V.92 modems, and in particular, for PCM upstream modulation used by V.92 modems for the upstream transmission in the client to server direction.

The invention also provides apparatus for preparing pre~coder filter coefficients and pre-equaliser filter coefficients for use by a client modem in a client server PCM modem communication in the client to server direction, the apparatus comprising a means in the server for preparing a channel model representative of distortions in the channel in the client to server direction, the means for preparing the channel model being responsive to a first training signal received from the client modem, a means in the server modem for generating a second training signal, and an equaliser for receiving the second training signal through the channel model for preparing the pre—coder and pre—equaliser filter coefficients in response to the second training signal received from the channel model.

In one embodiment of the invention the means for preparing the channel model comprises a filter, and preferably, a feed-fonivard filter, which ideally, comprises up to one hundred and twenty-eight coefficients. in another embodiment of the invention a simple background equaliser is provided in the server modem for decoding training signals received from the client modem, the value of which are not predetermined, and which are received after the first training signals.

Preferably, the equaliser in the server modem which receives the second training signal for training thereof is a feed—forward and a feedback coefficient filter, and preferably, is a feed-fon/vard and a feedback filter having one hundred and twenty- eight coefficients.

The invention will be more clearly understood from the following description of an embodiment thereof, which is given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a block representation of a circuit for preparing pre-coder and pre- equaliser filter coefficients in a server modem for use by a client modem, Fig. 2 is a circuit diagram of a portion of the circuit of Fig. 1, Fig. 3 is a circuit diagram of another portion of the circuit of Fig. 1, Fig. 4 is a circuit diagram of a further portion of the circuit of Fig. 1, and Fig. 5 is a block representation of the circuit of Fig. 1 in use.

Referring to the drawings, there is illustrated apparatus according to the invention which is provided by a circuit, indicated generally by the reference numeral 1, for preparing pre-coder and pre-equaliser filter coefficients for use in PCM upstream modulation for V.92 modems for the upstream transmission in the client to server direction from a client modem 2 to a server modem 3. In the PCM upstream modulation used for V.92 modems for upstream transmission from the client modem 2 to the server modem 3, the pre-coder and pre—equaliser filter coefficients must be determined in the server modem 3 based on a first training signal transmitted by the client modem 2 to the server modem 3. The circuit 1 for preparing the pre-coder and pre—equaliser coefficients is located in the server modem 3. The circuit 1 comprises a means for modelling a channel 5 over which communication between the client modem 2 and the server modem 3 takes place in the client to server direction, which is representative of distortions in the channel 5 which distort signals transmitted by the client modem 2 to the server modem 3. In this embodiment of the invention the channel modelling means comprises a feed-forward filter 6 which for convenience is hereinafter referred to as a channel model filter 6. The channel model filter 6 is a one hundred and twenty-eight coefficient feed—forward filter, and a first switch 7 switches the channel model filter 6 to the channel 5 for receiving a first training signal from the client modem 2 of predetermined values, which are known to the server modem. A second switch 8 switches the output of the channel model filter 6 to a microprocessor 9 in the server modem 3 for comparing the output of the channel model filter 6 with the input signal from the channel 5 for determining the one hundred and twenty-eight coefficients for the channel model filter 6, in order that the channel model filter 6 is a model of the channel and is representative of the errors in the channel.

Referring now to Fig. 2, a circuit diagram of the channel model filter 6 is illustrated.

The channel model filter 6 comprises a plurality of amplifiers 10, in this embodiment of the invention one hundred and twenty-eight amplifiers 10 for which the coefficients are determined by the microprocessor 9 in response to the first training signal. The training signal is fed from the channel 5 to the amplifiers 10 through corresponding time delay elements 11. The output of the amplifiers 10 are summed in adders 12 and the summed signal is outputted from the channel filter 6 on an output 13. The microprocessor 9 determines the one hundred and twenty-eight coefficients for the channel model filter 6 so that the signals on the output 12 contains the corrected values corresponding to the original values of the first training signal which is transmitted from the client modem 2. Thus, when the channel model filter 6 has been trained, the channel model filter 6 is a model of the channel 5 and includes the distortions of the channel 5. The construction and training of such a feed-forvvard filter will be well known to those skilled in the art.

A simple background equaliser 14 is provided in the server modem 3 for decoding training signals received from the client modem, the value of which are not predetermined, and which are received after the first training signals.

The server modem 3 comprises a means for generating a second training signal 15, namely, a signal generator 15 which is used in conjunction with the channel model filter 6 and an equaliser 16 for determining the pre—coder and pre-equaliser filter coefficients. in this embodiment of the invention the second training signal is a white training signal and comprises a normal pseudo random white training sequence.

The equaliser 16 is a one hundred and twenty—eight coefficient feed-forward and feedback filter which is illustrated in Fig. 3 and will be described below, and the pre- coder and pre—equa|iser coefficients are determined in the equaliser 16. A shaping filter 18, which is illustrated in Fig. 4, which is a two coefficient feed-forward and feedback filter is provided for receiving the second training signal from the signal generator 15 for feeding to the channel model filter 6. The shaping filter 18 amplifies the second training signal at the frequencies which are expected to be attenuated by the channel model 6, namely, frequencies around the DC and 4kHz frequencies at an amplification of approximately 15dBs to 20dBs relative to its mid—band gain.

During training of the equaliser 16 to prepare the pre—coder and pre—equaliser coefficients, the first and second switches 7 and 8 are switched for switching the second training signal from the shaping filter 18 to the channel model filter 6 and then to the equaliser 16.

Referring now to Fig. 3, the equaliser 16 comprises one hundred and twenty—eight feed-forward amplifiers 21, and one hundred and twenty-eight feedback amplifiers 22. The input signal to the equaliser 16 from the channel model filter 6 is fed to the feed-forward amplifiers 21 through time delay elements 23. The outputs of the feed- fon/vard and feedback amplifiers 21 and 22 are summed in adders 24 and outputted on an output 25. The signals on the output 25 of the equaliser 16 are fed back to the feedback amplifiers 22 through time delay elements 26, which correspond to the time delay elements 23. The construction and training of such equalisers will be well known to those skilled in the art.

Referring now to Fig. 4, the shaping filter 18 comprises two feed-forward amplifiers 31, and two feedback amplifiers 32. The second training signals to the shaping filter 18 from the signal generator 15 are fed to the feed-forward amplifiers 31 through time delay elements 33. The output of the feed-forward and feedback amplifiers 31 and 32 are summed in adders 34 and outputted on an output 35. The signals from the output 35 of the pre-equaliser 18 are fed back to the feedback amplifiers 32 through time delay elements 36, which correspond to the time delay elements 33.

The construction and operation of such shaping filters will be well known to those skilled in the art.

In use, the first training signals of predetermined value are transmitted by the client modem 2 through the channel 5 to the server modem 3, followed by training signals, the value of which are not predetermined. Using the first training signals of predetermined value and the training signals not of predetermined value, the server modem prepares the channel model in the channel model filter 6. On completion of the channel model, the second training signal is generated in the signal generator , and is fed from the signal generator 15 to the shaping filter 18 where the DC and 4kHz frequencies are amplified at approximately 15dBs to 20dBs amplification. The amplified signal is fed from the shaping filter 18 to the channel model filter 6 where it is subject to similar distortions as though it had been fed through the channel 5. The distorted signal is fed from the output 13 of the channel model filter 6 to the equaliser 16. The second training signal 15 which has passed through the shaping filter 18 is summed in an adder 40 with the output of the equaliser 16 and is fed back to the equaliser 16 where the one hundred and twenty—eight coefficients for the feed- forward amplifiers 21 and the feedback amplifiers 22 are determined. The coefficients determined for the feed—forward and feedback amplifiers 21 and 22 are the pre-coder and pre—equa|iser filter coefficients, respectively, for correcting for distortion in the channel 5. The server modem 3 then transmits the determined pre- coder and pre—equaIiser filter coefficients to the client modem 2 which are used by the client modem 2 in a pre-coder filter and pre-equaliser filter (neither of which are shown) of the client modem 2 for compensating for distortion in the channel 5 for transmissions being transmitted on the channel 5 by the client modem 2 to the server modem 3.

The advantages of the invention are many. By virtue of the fact that the equaliser 16 of the server modem 3 is trained using the output signal from the channel model filter 6, rather than a training signal passed through the channel itself, the training signal used to train the equaliser is not subjected to channel noise which would otherwise disrupt training, since the channel model is substantially free of noise. Additionally, since the channel model is prepared, any training signal can be used in the training of the equaliser, and in particular, the provision of the channel model permits a training signal to be generated by the server modem itself. Thus, training of the equaliser 16 is entirely independent of the actual channel itself, and permits any training signal to be used which can be generated in the server modem 3. Thus, a significantly more suitable training signal can be used than that which is provided by a conventional training signal outputted by a client modem. Additionally, since the training of the equaliser can be carried out entirely in the server modem independently of the client modem and the channel, there is no limitation on the training time, and furthermore, more energy is available near channel null points.

Additionally, the method according to the invention allows the pre-coder and pre- equaliser filter coefficients to be prepared which are suitable for providing the appropriate amplification over the entire bandwidth.

While the channel model filter 6 and the equaliser 16 have been described as being one hundred and twenty-eight coefficient devices, they may be any desired number of coefficient devices, although one hundred and twenty-eight coefficients is the optimum. Additionally, while the shaping filter has been described as being a two coefficient device, the shaping filter may comprise any desired number of n 5030291 coefficients, however, in general, a relatively low number of coefficients is sufficient.

The invention is not limited to the embodiment hereinbefore described, which may be varied in construction and detail.

Claims (5)

Claims

1. A method for preparing pre—coder filter coefficients and pre-equaliser filter coefficients for use by a client modem in a client/server PCM modem communication in a client to server direction, the method comprising the steps of preparing a model of the channel in the server modem representative of distortions in the channel in the client to server direction, the channel model being prepared by reading a first training signal of predetermined values transmitted by the client modem through the channel to the server modem, and preparing the channel model to model the distortions in the received first training signal so that the channel model is representative of the channel distortions, passing a second training signal generated in the server modem through the channel model, and using the output signal from the channel model in response to the second training signal for preparing the pre—coder filter coefficients and the pre-equaliser: filter coefficients in the server modem. ,9

2. A method as claimed in Claim 1 in which the channel model is prepared in a feed-fon/vard filter.

3. A method for preparing pre—coder filter coefficients and pre-equaliser filter coefficients for use by a client modem in a client/server PCM modem communication in a client to server direction, the method being substantially as described herein with reference to and as illustrated in the accompanying drawings.

4. Apparatus for preparing pre—coder filter coefficients and pre-equaliser filter coefficients for use by a client modem in a client/server PCM modem communication 15 in the client to server direction, the apparatus comprising a means in the server for preparing a channel model representative of distortions in the channel in the client to server direction, the means for preparing the channel model being responsive to a first training signal received from the client modem, a means in the server modem for generating a second training signal, and an equaliser for receiving the second training signal through the channel model for preparing the pre-coder and pre- equaliser filter coefficients in response to the second training signal received from the channel model.

5. Apparatus for preparing pre-coder filter coefficients and pre-equaliser filter coefficients for use by a client modem in a client/server PCM modem communication in the client to server direction, the apparatus being substantially as described herein with reference to and as illustrated in the accompanying drawings. F.F. GORMAN & CO.