Method and device for training an adaptive equalizer by means of an unknown data signal in a quadrature amplitude modulation transmission system

• US 4,227,152 A
• Filed: 05/23/1979
• Issued: 10/07/1980
• Est. Priority Date: 06/13/1978
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Godard, Dominique N., Thirion, Philippe E.
• Primary Examiner(s): Pitts, Harold I.
• Application Number:
  US06/041,585
• Time in Patent Office: 503 Days
• Field of Search: 325/323, 325/324, 325/42, 333/18, 178/67
• US Class Current: 375/231
• CPC Class Codes: H04L 25/0305 using blind adaptation
• CPC Class Codes: H04L 27/38 Demodulator circuits; Receiver circuits

Abstract

A method and a device for training the equalizer of a receiver used in a QAM transmission system, by means of an unknown data signal, thereby enabling in particular said equalizer to be trained in the cases described above. In accordance with the method of the present invention, in a data receiver wherein, under normal operating conditions, the data is detected with respect to a first constellation, the equalizer is trained by detecting the data with respect to a second constellation which comprises fewer points than said first constellation and by adjusting the coefficients of the equalizer by means of an error signal produced as a result of the detection operation performed with respect to the second constellation.

In accordance with a first embodiment of the present invention, the second constellation is comprised of those points of the first constellation that are farthest from its center.

In accordance with a second embodiment of the invention, the points of the second constellation are obtained as follows: the first constellation is divided into L sets of points S^l, l=1, 2, . . . , L and a point b^l of the second constellation is associated with each set S^l in accordance with the relation ##EQU1## where the a^k '"'"'s are those points of the first constellation that belong to the set S^l.

Claims

1. In a data receiver for a data transmission system using a QAM technique, of the type wherein the received signal is, in particular, equalized by an adaptive equalizer and then fed as an input signal to a data detection device which supplies the detected data after comparing said input signal with a first constellation, a method of training said equalizer, characterized in that it includes the steps of:

a. comparing said input signal with a second constellation comprising fewer points than said first constellation;

b. selecting that point of said second constellation which is nearest the point representative of said input signal;

c. determining an error signal which is a function of the distance between the point representative of said input signal and the point of said second constellation selected during step b.; and

d. deriving from said error signal control signals serving to adjust the coefficients of the equalizer.

2. Method according to claim 1, characterized in that said second constellation is comprised of those points of said first constellation which are farthest from its center.

3. Method according to claim 2, characterized in that the coefficients of the equalizer are not adjusted where the point representative of said input signal is located within an area that includes all of those points of said first constellation which do not belong to said second constellation.

4. Method according to claim 1, characterized in that said second constellation is derived from said first constellation:

by dividing said first constellation into L sets of point S^l, l=1, . . . , L, and

by associating with each set S^l one point b^l of said second constellation defined as ##EQU13## where points a^k are those points of said first constellation which belong to set S^l.

5. In a data receiver for a data transmission system using a QAM technique, of the type that includes an adaptive equalizer and a data detection device which receives as input signal the received signal after the latter has been, in particular, equalized, and provides the detected data after comparing said input signal with a first constellation, a device for training the equalizer, characterized in that it includes:

an auxiliary data detection device wherein said input signal is compared with a second constellation comprising fewer points than said first constellation and which selects that point of said second constellation which is nearest the point representative of said input signal and generates an error signal that is dependent upon the distance between said point representative of said input signal and the selected point of said second constellation, and

means for deriving from said error signal control signals for adjusting the coefficients of the equalizer.

7. Device according to claim 5 or 6, characterized in that said second constellation is comprised of those points of said first constellation which are farthest from its center.

8. Device according to claim 7, characterized in that the coefficients of the equalizer are not adjusted where the point representative of said input signal is located within an area which includes all of those points of said first constellation which do not belong to said second constellation.

9. Device according to claim 5 or 6, characterized in that said second constellation is derived from said first constellation:

by dividing said first constellation into L sets of points S^l, l=1, . . . , L, and

by associating with each set S^l one point b^l of said second constellation defined as ##EQU14## where points a^k are those points of said first constellation which belong to set S^l.

6. In a data receiver for a data transmission system using a QAM technique, of the type that includes an adaptive equalizer, means for subtracting an estimated phase error from the phase of the equalized signal and for supplying a signal called input signal, a data detection device which receives said input signal as input and supplies the detected data after comparing said input signal with a first constellation, and means for supplying said estimated phase error, a device for training the equalizer, characterized in that it includes:

an auxiliary data detection device wherein said input signal is compared with a second constellation comprising fewer points than said first constellation, and which selects that point of said second constellation which is nearest the point representative of said input signal and generates an error signal that is dependent upon the distance between the point representative of said input signal and the selected point of said second constellation,

means for adding said estimated phase error to the phase of said error signal, and

means for deriving from the signal supplied by the latter means control signals for adjusting the coefficients of the equalizer.

10. Device according to claim 6, characterized in that said input signal is simultaneously applied to said data detection device and to said auxiliary data detection device so that said data detection device will control said means for supplying said estimated phase error while said auxiliary data detection device will control the adjustment of the coefficient of the equalizer.