Apparatus and method for compensating for limiter induced non-linear distortion in a wireless data communication system

• US 5,533,048 A
• Filed: 02/17/1994
• Issued: 07/02/1996
• Est. Priority Date: 07/28/1993
• Status: Expired due to Term

• Current Assignee: Alpha Industries Incorporated (Skyworks Solutions Incorporated)
• Original Assignee: Celeritas Technologies, Ltd.
• Inventors: Dolan, Michael G.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Bocure, Tesfaldet
• Application Number:
  US08/198,085
• Time in Patent Office: 866 Days
• Field of Search: 375/229, 375/231, 375/261, 375/222, 375/223, 375/358, 375/224, 375/225, 375/296, 375/346, 375/377, 375/285, 375/345, 375/371, 375/295, 375/298, 375/316, 375/340, 375/226, 379/97, 379/93, 379/98
• US Class Current: 375/222
• CPC Class Codes: H04B 1/38 Transceivers, i.e. devices in which transmitter and receiver form a structural unit and in which at least one part is used for functions of transmitting and receiving
• CPC Class Codes: H04B 1/62 for providing a predistortion of the signal in the transmitter and corresponding correction in the receiver, e.g. for improving the signal/noise ratio
• CPC Class Codes: H04L 1/0002 by adapting the transmission rate
• CPC Class Codes: H04L 1/0026 Transmission of channel quality indication
• CPC Class Codes: H04L 27/368 adaptive predistortion
• CPC Class Codes: H04L 27/38 Demodulator circuits; Receiver circuits

Abstract

A system and method for reducing amplitude distortion in data signals transmitted via cellular analog communication systems are embodied in several forms. In one embodiment, pre-emphasis and de-emphasis functions of the cellular system are offset by complementary de-emphasis and post-emphasis circuits to preclude clipping of high frequency components of the data signal by the limiter. Alternatively, the limiter circuit is selectively bypassed when a data signal is transmitted. Alternatively, a pilot tone is combined with the incoming data signal. The amplitude of the pilot tone is constant and always sets the gain of the limiter, thereby keeping the gain of the limiter circuit constant. Alternatively, a pair of signals at different frequencies are transmitted on a telephone line, and the relative amplitudes of the return echoes are measured to determine the frequency roll-off of the line and adjust the spectral shaping accordingly. Alternatively, a pair of modems vary their respective spectral characteristics until the eye quality values of the two modems are satisfactory. Alternatively, a receiving modem performs an auto-equalization routine which varies to ignore outer points on a signal constellation that are likely to have a reduced amplitude because of limiting. Alternatively, the threshold amplitudes of an auto-equalizer are varied in accordance with phase changes that correspond to new data points by varying the threshold or by varying the amplitude of the data compared to the threshold.

Claims

1. A method of compensating for varying spectral characteristics of data signals received by a receiving modem from a transmitting modem via an analog communications link wherein said data signals comprise amplitude and phase modulation of at least one carrier signal, said amplitude and phase modulation defining a signal constellation at said receiving modem wherein predetermined points on said signal constellation represent combinations of binary data, said signal constellation including points having varying phases and amplitudes that differ from phases and amplitudes of said predetermined points in the presence of non-ideal spectral characteristics of said communication link between said transmitting modem and said receiving modem, said non-ideal spectral characteristics including non-linear amplitude limiting of portions of said data signals, said method comprising the steps of:

generating at said receiving modem said data signal constellation to provide a first plurality of data points having respective amplitudes and phases;

detecting at said receiving modem a second plurality of points included in said first plurality of points having amplitudes that differ sufficiently from said predetermined points to indicate that amplitudes of data signals that generated said second plurality of points have been nonlinearly limited;

calculating in said receiving modem spectral shaping coefficients for use by said receiving modem using amplitude measurements from a third plurality of points that are included in said first plurality of points but not included in said detected second plurality of points; and

modifying in said receiving modem spectral shaping characteristics of said receiving modem using said calculated spectral shaping coefficients so that said third plurality of points have phases and amplitudes that more closely correspond to phases and amplitudes of said predetermined points.

2. The method as defined in claim 1, wherein said first points are detected sequentially in time, and wherein said step of calculating spectral shaping characteristics further includes the step of multiplying said amplitude of one of said second plurality of points by a factor that varies in accordance with a phase change between said one of said second plurality of points and an immediately preceding one of said first plurality of points to generate a modified amplitude, said modified amplitude being used to calculate said spectral shaping characteristics.

3. The method as defined in claim 1, wherein said first points are detected sequentially in time, and wherein said step of calculating spectral shaping characteristics further includes the step of multiplying said amplitude of one of said second plurality of points by a factor that varies in accordance with a phase change between said one of said second plurality of points and an immediately preceding one of said first plurality of points to generate a modified amplitude and in accordance with said amplitude of said one of said second plurality of points, said modified amplitude being used to calculate said spectral shaping characteristics.

4. A method of establishing amplitude thresholds in a data signal constellation of a receiving modem that receives data signals from a transmitting modem via an analog communications link, wherein said data signals comprise amplitude and phase modulation of at least one carrier signal, said amplitude and phase modulation defining said data signal constellation at said receiving modem wherein predetermined points on said data signal constellation represent combinations of binary data, said receiving modem establishing a plurality of amplitude thresholds on said data signal constellation between points having like phases and different amplitudes wherein a point having an amplitude less than one of said thresholds is recognized by said receiving modem as a first data combination and wherein a point having an amplitude greater than said one of said thresholds is recognized as a second data combination, said plurality of amplitude thresholds being substantially midway between amplitudes of adjacent points at like phases, said data signal constellation including points having varying phases and amplitudes that differ from phases and amplitudes of said predetermined points in the presence of non-ideal spectral characteristics of said communication link, said non-ideal spectral characteristics including non-linear amplitude limiting of portions of said data signals, said method comprising the steps of:

measuring in said receiving modem amplitudes and phase changes for known data combinations having predictable predetermined amplitudes and phase changes;

based upon said measured phase changes, setting in said receiving modem a range of amplitudes for each predictable amplitude for each phase change, each range having an upper limit and a lower limit;

for each measured phase change, establishing in said receiving modem a respective new threshold value for said each predictable predetermined amplitude so that said new threshold value is below the lower limit of said range of amplitudes for said predictable predetermined amplitude and above the upper limit of said range of amplitudes for an adjacent point at a lower predictable predetermined amplitude; and

using said new threshold values in said receiving modem when receiving unknown data from said transmitting modem to determine which of said predetermined points is represented by said unknown data.

5. A method of discriminating data points in a signal constellation, said signal constellation having ideal inner and outer points at various phases and amplitudes, said constellation having a configuration depending upon the data exchange rate between a transmitting modem and a receiving modem, said transmitting modem and said receiving modem communicating over a communication link having non-ideal characteristics resulting in amplitude deviations of said data points from said ideal inner and outer points, said method comprising the steps of:

receiving data signals having phase and amplitude characteristics transmitted from said transmitting modem;

defining at said receiving modem a signal constellation configuration having inner and outer ideal points corresponding to lesser and greater amplitudes, respectively, at various phase angles;

assigning at said receiving modem data point positions in said signal constellation to said data signals based upon said phase and amplitude characteristics;

measuring at said receiving modem said phase and amplitude characteristics of said received data signals to generate measured inner and outer points;

calculating at said receiving modem said amplitude deviations of said measured inner and outer points from said ideal inner and outer points;

dynamically adjusting at said receiving modem an amplitude threshold value between said measured inner and outer points based upon the measured amplitude deviations of said measured inner and outer points from said ideal inner and outer points to generate adjusted thresholds; and

assigning at said receiving modem data points of incoming data to ideal constellation points based upon said dynamically adjusted thresholds.

6. The method as defined in claim 5, wherein each of said dynamically adjusted thresholds is selected so that a ratio of an amplitude difference between a respective ideal inner point and said threshold to an amplitude difference between said threshold and a respective ideal outer point is substantially equal to a ratio between a standard deviation of said measured inner data points from said ideal inner point to a standard deviation of said measured outer data points from said ideal outer point.

7. The method as defined in claim 5, further comprising the step of receiving at said receiving modem known data from said transmitting modem prior to said measuring step so that said ideal inner points and said ideal outer points are predictable.

8. A method of compensating for varying spectral characteristics of data signals received by a receiving modem from a transmitting modem via an analog communications link wherein said data signals comprise amplitude and phase modulation of at least one carrier signal, said amplitude and phase modulation defining a signal constellation at said receiving modem wherein predetermined points on said signal constellation represent combinations of binary data, said signal constellation comprising outer points and inner points, said outer points having greater amplitudes than said inner points, said signal constellation including points having varying phases and amplitudes that differ from phases and amplitudes of said predetermined points in the presence of non-ideal spectral characteristics of said communication link between said transmitting modem and said receiving modem, said non-ideal spectral characteristics including non-linear amplitude limiting of portions of said data signals, said method comprising the steps of:

generating said data signal constellation in said receiving modem to provide a first plurality of data points having respective amplitudes and phases, said first plurality of points comprising said inner and outer points;

identifying in said receiving modem a second plurality of said first plurality of points, said second plurality of points comprising selected ones of said outer points resulting from phase changes greater than a selected phase change, said second plurality of points having amplitudes likely to have been limited;

identifying in said receiving modem a third plurality of points, said third plurality of points comprising points included in said first plurality of points but not included in said second plurality of points; and

performing in said receiving modem auto-equalization on said data signals, said auto-equalization using coefficients predicated upon error signals primarily derived from said third plurality of points.