Method in digital quadrature modulator and demodulator, and digital quadrature modulator and demodulator

• US 20030118121A1
• Filed: 01/10/2003
• Published: 06/26/2003
• Est. Priority Date: 11/29/1996
• Status: Active Grant

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Telecommunications Oy (Nokia Corporation)
• Inventors: Makinen, Jarmo
• Granted Patent: US 6,714,601 B2
• Time in Patent Office: Days
• US Class Current: 375/261
• CPC Class Codes: H04L 2027/0016 Stabilisation of local oscillators
• CPC Class Codes: H04L 27/364 Arrangements for overcoming imperfections in the modulator, e.g. quadrature error or unbalanced I and Q levels
• CPC Class Codes: H04L 27/3863 Compensation for quadrature error in the received signal

Abstract

The invention relates to a method for measuring imperfections in a digital quadrature modulator and demodulator operation, and to a digital quadrature modulator and quadratue demodulator. The modulator of the invention comprises means (11) for modulating quadrature-phase I and Q input signals to generate an output signal comprising quadrature-phase I and Q components; means (12) for taking momentary amplitude samples from the output signal of the modulator (11) at a rate based on a symbol clock (SCLK) of the modulator, means (13) for classifying a direction angle of the symbol to be modulated at a particular time into a particular direction angle sector, means (13) for linking the amplitude samples of the output signal to the direction angle sector corresponding to the symbol to be transmitted at a particular time; and means (13) for comparing the amplitudes of the samples belonging to each direction angle sector with other direction angle sectors or with an ideal value for determining the imperfections of the output signal of the modulator.

Claims

1. A method for measuring imperfections in a digital quadrature modulator operation in which an input signal of the modulator comprises quadrature-phase I and Q channels, whereby during a normal operation of the modulator a local oscillator carrier wave leak, an imbalance in I and Q channel amplitudes, a quadrature error between the I and Q channels and an amplitude error are determined from an output signal of the modulator, characterized in that the method comprises the steps in which

from an amplitude of the output signal are taken, at a rate based on a symbol clock of the modulator, numerous momentary samples;

a direction angle of a transmission signal corresponding to the samples is divided, on the basis of data bits to be transmitted or modulator input signals, into different direction angle sectors; and

from amplitude sample deviations between the different direction angle sectors or from a nominal value are calculated the magnitudes of the distortions in the modulator operation.

2. A method according to claim 1, characterized in that the measurement results of the imperfections in the modulator operation are used for adjusting the operation of the modulator by generating feedback loops, which correct all said imperfections simultaneously in an iterative manner.

5. A method according to any one of claims 1 to 4, characterized in that DC offsets of the I and Q channels are determined from a difference in the amplitudes of the samples inside two opposite direction angle sectors around a respective axis.

6. A method according to any one of claims 1 to 4, characterized in that the differences in the signal levels of the I and Q channels are determined on the basis on the basis of how much the sum of the amplitudes of the samples of one channel inside two opposite direction angle sectors around a respective axis increases or decreases in relation to the other channel.

7. A method according to any one of claims 1 to 4, characterized in that a phase error in a phase difference of 90 degrees between the carrier waves of the I and Q channels is determined on the basis of how much the sum of the amplitudes of the samples of one channel inside direction angle sector pairs of opposite directions around one of two axes which are in an angle of 45 degrees to the I and Q axes increases or decreases in relation to the other axis.

8. A method according to any one of claims 1 to 4, characterized in that a change in the output power of the modulator/demodulator is defined based on the amplitude of the samples.

9. A method according to any one of claims 1 to 8, characterized in that the sample group is reduced in such a way that only samples relating to symbols that are known to have the same nominal amplitude are accepted for calculation.

3. A method for measuring imperfections in the operation of a digital quadrature demodulator when an output signal of a quadrature mixer of the demodulator comprises quadrature-phase I and Q channels, whereby during a normal operation of the demodulator from the output signal of the quadrature mixer of the demodulator is determined offset voltages, an imbalance in the amplitudes of the I and Q channels, a quadrature error between the I and Q channels and a total amplitude error, characterized in that the method comprises steps in which

from the amplitude of the output signal of the demodulator quadrature mixer, i.e. from the amplitude of a vector formed by the I and Q channels, are taken numerous momentary samples at a rate based on a symbol clock of the demodulator;

a direction angle of the demodulator output signal vector corresponding to the samples is divided, on the basis of the voltages of the I and Q channels, into different direction angle sectors; and

from the amplitude sample deviations between the different direction angle sectors or from a nominal value are calculated the magnitudes of the distortions in the demodulator operation.

4. A method according to claim 3, characterized in that the measurement results of the imperfections in the demodulator operation are used for adjusting the operation of the demodulator by generating feedback loops, which correct all said imperfections simultaneously in an iterative manner.

10. A digital quadrature modulator comprising means (11) for modulating quadrature-phase I and Q input signals to generate an output signal comprising quadrature-phase I and Q components, characterized in that the modulator further comprises

means (12) for taking momentary amplitude samples from the output signal of the modulator (11), at a rate based on a symbol clock (SCLK) of the modulator;

means (13) for classifying a symbol to be modulated at a particular time into a particular direction angle sector;

means (13) for linking the amplitude samples of the output signal to the direction angle sector corresponding to the symbol to be transmitted at a particular time; and

means (13) for comparing the amplitudes of the samples belonging to a particular direction angle sector with other direction angle sectors or with an ideal value, for determining from the output signal of the modulator a local oscillator carrier wave leak, an amplitude imbalance between I and Q channels, a quadrature error between the I and Q channels and an amplitude error.

11. A modulator according to claim 10, characterized in that it further comprises means (14) for correcting the adjusting parameters of the modulator in response to the predetermined distortions by generating feedback loops, which correct all said imperfections simultaneously in an iterative manner.

14. A modulator or demodulator according to any one of claims 10 to 13, characterized in that it further comprises means (13 or 23) for selecting the amplitude samples that have the same-nominal amplitude.

12. A digital quadrature demodulator comprising a quadrature mixer (21) for generating analog quadrature-phase I and Q output signals from a quadrature modulated input signal, characterized in that the demodulator comprises

means (22) for taking momentary samples from the I and Q output signals of the quadrature mixer (21) at a rate based on a symbol clock (MCLK) of the demodulator;

means (22) for calculating total amplitude samples of the output signal from I and Q samples;

means (22) for classifying a direction angle of the symbol received at a particular time into a particular direction angle sector;

means (23) for linking the amplitude samples of the output signal of the quadrature mixer (21) to a particular direction angle sector; and

means (23) for comparing the amplitudes of the samples belonging to a particular direction angle sector with an ideal value or with other sectors, for determining from the output signal of the quadrature mixer offset voltages, an amplitude imbalance between I and Q channels, a quadrature error between the I and Q channels and a total amplitude error.

13. A demodulator according to claim 12, characterized in that it further comprises means (24) for correcting the adjusting parameters of the demodulator in response to the predetermined distortions by generating feedback loops, which correct all said imperfections simultaneously in an iterative manner.