Pulse or digital communications – Receivers
Reexamination Certificate
2003-01-10
2004-03-30
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
C375S326000, C375S329000
Reexamination Certificate
active
06714601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to 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 distortions of an output signal of the modulator from an ideal one are determined and possibly corrected during a normal operation of the modulator. The method is also suited for use in a quadrature demodulator mixer, for measuring and correcting its imperfections. The invention also relates to a digital quadrature modulator and a quadrature demodulator, the imperfections of which are determined and possibly corrected with a method of the invention.
2. Background
In quadrature modulators appear a lot of imperfections, the impact of which is difficult to remove during the normal operation of the modulator. The removing of the imperfections is problematic because the characteristics of the modulator change due to carrier wave frequency, temperature, ageing, etc., and in practice such changes cannot be compensated for without regular maintenance of the modulator being required, in addition to its original tuning.
The imperfections include the following:
A. Offset voltages appearing in the I and/or Q channels in the quadrature modulator, said voltages causing carrier wave leak into the output signal.
B. Differences in amplification and thereby differences in signal levels appear in the I and Q channels, causing distortion in a constellation generated on an I/Q plane by amplitude values of the output signal, said distortion disturbing reception.
C. Deviation from a desired 90° phase shift appears between the I and Q channels, causing crosstalk in reception between the I and Q channels and thereby disturbing reception.
D. The output power of the modulator (or the entire transmitter) can vary.
The problems are known from before and for instance the following solutions have been proposed to them:
U.S. Pat. No. 5,012,208 provides a solution to the carrier wave leak (problem A) of the quadrature modulator by a correlation, performed by analog means, of the output signal amplitude variations with signals of the I and Q channels. The apparatus in question does not, however, offer any help for solving the other problems (B, C and D).
U.S. Pat. No. 5,442,655 describes a quadrature demodulator, in which the offset voltage in the I and Q channels is measured and corrected using a two-phase procedure. In the first phase the average voltages of the I and Q channels are measured, without the time of measurement being linked to a symbol clock. The average values are then subtracted from real values and the roughly corrected values are fed into a fine correction phase. In the fine correction phase the output signals (an I/Q voltage pair) of the rough correction phase are divided into phase angle sectors on the basis of the ratio between the I and Q voltages. An average distance of signal points in a sector in relation to an axis (I or Q) is then measured from an opposite axis (Q or I) and an offset voltage is calculated as a difference of the distances in the opposite sectors. The offset voltages are then subtracted from the I and Q voltages. The publication thus describes a solution applied in a quadrature demodulator to problem A.
EP patent application 608577 A1 presents a solution, similar to the one in U.S. Pat. No. 5,012,208, to all the above mentioned problems (A, B, C, D), only its function is based on the idea that the normal operation of the modulator is interrupted for the duration of the tuning and a plurality of known test signals are run through the modulator. The apparatus in question is thus unable to correct imperfections emerging during continuous operation of the apparatus and it also requires a separate tuning to be performed in connection with the manufacturing or initialisation of the modulator.
EP patent application 0503588 A2 also describes a solution to all the above mentioned problems (A, B, C, D), only there the measurement of the modulator errors is based on two alternative implementations: (1) using specific, known test signals interrupting the normal operation, or (2) causing a slight periodic interference, separately in each adjusting parameter of the modulator, during the normal operation and examining the impact of each adjusting parameter interference on the output signal. The above solutions require either the normal operation to be interrupted or interference to be added to a normal signal, and they lead to a fairly complex implementation.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method with which in a digital quadrature modulator utilizing the method all the above mentioned problems A, B, C and D can be solved in a simple manner which can be easily automated and made digital. This is achieved with a method of the invention 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.
The method advantageously also comprises a step wherein 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.
The invention also relates to method for measuring imperfections in the operation of a digital quadrature demodulator in which an output signal of a quadrature mixer of the demodulator comprises quadrature-phase I and Q channels, whereby from the output signal of the demodulator quadrature mixer are determined, during a normal operation of the demodulator, 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. The method comprises the 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, at a rate based on a symbol clock of the demodulator, numerous momentary samples;
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 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.
The method advantageously also comprises a step wherein 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.
The invention is based on the monitoring of the output amplitude of the modulator/demodulator and, above all, on the observation that all the above mentioned imperfections (A, B, C and D) can be measured, using simple inference rules, on an I/Q plane (from what is known as a constellation figure) from a large quantity of momentary samples taken from a modulated signal at a rate based on a symbol clock.
As a solution according to the invention to problem A, 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 in relation to a respective axis.
As a solution to problem B, the differences in the signal levels of the I and Q channels are determined 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 incr
Bocure Tesfaldet
Nokia Telecommunications Oy
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