JP2007158931A - Transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To properly perform correction even when there is a nonlinear point in a transmitter which corrects a DC offset generated in a signal to be transmitted. <P>SOLUTION: An arithmetic means 14, 17 computes a correction value of a DC offset for a signal and a detection means detects a quantity reflected with the corrected DC offset. A control means iteratively defines a signal point with a minimum quantity to be detected when a value corresponding to each of signal points as a correction value as a next reference signal point among a plurality of signal points disposed at intervals of a step size to a reference signal point and the signal point and changes the step size into a smaller value in accordance with reduction of a quantity to be detected, thereby setting a value with a small quantity to be detected as a correction value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a transmitter that corrects a DC offset component generated in a signal to be transmitted, and particularly corrects the DC offset component appropriately even when there is a discontinuous point in a D / A converter. Regarding the transmitter.

For example, in a base station apparatus or mobile station apparatus of a mobile communication system, a signal is wirelessly transmitted by a wireless transmitter, or a signal is wirelessly received by a wireless receiver.
As an example, the wireless transmitter has a configuration similar to that described in Japanese Patent Application No. 2005-198591 which is not publicly known. Here, for convenience of explanation, reference is made to FIG. 1 for explaining an embodiment of the present invention to be described later, and the same reference numerals as those shown in FIG. 1 are used for explanation, but there is no intention to limit the present invention.

In a wireless transmitter, when a digitally processed signal is converted into an analog signal through D / A (Digital to Analog) converters 2 and 3, a DC component (DC offset in D / A conversion) is generated. When the analog signal is quadrature-modulated through the analog quadrature modulator (AQM) 4, a reference frequency leak (carrier leak) occurs. In this example, these two are collectively referred to as a DC offset.
The controller 7 can remove the DC offset component by changing the DC offset correction value in the carrier leak correction circuit 1.

FIGS. 5A to 5D show an example of a carrier leak cancellation algorithm performed by the control unit 7 during low-level transmission. The signal is composed of an I-phase (in-phase) component and a Q-phase (orthogonal) component.
In this example, a certain point on the IQ plane and a range of points that are ± 1 in the I-axis direction and ± 1 in the Q-axis direction are considered, and these five points are used as DC offset correction values in order. Set in the leak correction circuit 1, find a point where the power value of the feedback signal (remaining DC offset power value) is the minimum among these five points, and then consider the same five points centered on that point. The same operation is repeated, and a DC offset correction value that minimizes the remaining DC offset power value is searched for. Here, the width of ± 1 described above is called a step size.

  Specifically, as shown in FIG. 5 (a), the DC remaining when each point is set in the carrier leak correction circuit 1 for each of the five points where the distance between the points is 1 on the IQ plane. The power value of the offset is measured, and the point where the power value is minimum is set as the next center point. Next, as shown in FIG. 5 (b), the point where the same minimum power is obtained for the same five points with the next center point as a reference is searched, and as shown in FIG. 5 (c), Let that point be the next center point. By repeating the same search, as shown in FIG. 5D, it is possible to search for a point (correct answer value) at which the remaining DC offset power value is the smallest.

However, the wireless transmitter has a problem in that there is a situation in which the DC offset component cannot be appropriately removed even when an algorithm as shown in FIGS. 5A to 5D is used.
FIG. 6 shows an example of a frequency spectrum when the I-phase DC offset correction value is a correct value but the Q-phase DC offset correction value cannot escape from the nonlinear point. In the figure, the horizontal axis represents frequency and the vertical axis represents level (power). As shown in the figure, a large amount of carrier leakage due to the remaining DC offset remains.

In such a problematic situation, a phenomenon occurs in which the DC offset correction value of at least one of the I-phase and the Q-phase stagnates at or near 0 instead of the correct value. This occurs because, for example, the DC offset correction value that minimizes the power value of the remaining DC offset is erroneously detected due to the non-linear characteristics of the D / A converters 2 and 3.
The non-linear characteristic of the D / A converter is because, for example, the weight of each bit is not exactly a power of two, and the one that occurs near the zero output at the bipolar output is the most serious.
FIG. 7 shows an example of the correction value versus carrier leak characteristic. The horizontal axis of the figure shows the Q-phase DC offset correction value, and the vertical axis shows the measured value (measured power value) of the leaked carrier power value.

As the measured power value, the power value of the signal obtained by feeding back the output signal from the quadrature modulator 4 via the amplifier 5 and the A / D (Analog to Digital) converter 6, that is, the transmission power value itself is used. A feedback signal, for example, a digital quadrature demodulation, the average of the sum of squares of the I-phase and Q-phase (Ii 2 + ^{Qi 2)} becomes the measured power values, if either sufficiently small input to the carrier leak compensation circuit 1 is no, the measurement The power value sufficiently reflects the power value of carrier leak.

The smaller the carrier leak, the better. In the example of FIG. 7, the correct value of the Q-phase DC offset correction value is in the left direction in the figure, but with the algorithm described above, it is possible to escape from the minimum point (nonlinear point) at the correction value = 0. Therefore, the correct value cannot be reached, and the DC offset component cannot be appropriately corrected.
The present invention has been made to solve such a conventional problem, and provides a transmitter capable of appropriately correcting a DC offset component even when there is a nonlinear point, for example. With the goal.

In order to achieve the above object, the transmitter according to the present invention corrects a DC offset generated in a signal to be transmitted with the following configuration.
That is, the calculation means calculates a DC offset correction value for the signal. The detecting means detects an amount (carrier leak level) reflecting the DC offset in the signal after the correction value is calculated by the calculating means and after the DC offset is generated. The control unit calculates a value corresponding to each signal point by the calculation unit among a plurality of signal points including a reference signal point and a signal point arranged at a step size interval with respect to the signal point. When the correction value of the DC offset is used, the signal point that minimizes the amount detected by the detection means is repeatedly set as the next reference signal point, and in this case, the detection means A DC offset correction in which the step size is changed to a small value in response to a decrease in the detected amount, and a value with a small amount detected by the detection unit is calculated by the calculation unit. Set as a value.

  Therefore, the signal point that corrects the DC offset best when it is used as a DC offset correction value among a plurality of signal points consisting of a reference signal point and surrounding signal points set by the step size is the following. When the DC offset correction value is searched by repeatedly performing the above-described reference, and the calculated DC offset correction value is set and calculated for the signal to be transmitted, the remaining amount of the DC offset after correction is calculated. For example, even when there is a non-linear point, the DC offset component is set appropriately by switching to a smaller step size when the remaining amount of the DC offset after correction becomes smaller using a larger step size. Can be corrected.

Here, the DC offset to be corrected may be generated by various processing units, for example, can be generated by a D / A converter or a quadrature modulator. .
Further, after the calculation of the correction value of the DC offset and after the occurrence of the DC offset, it is possible to grasp the extent (remaining amount) by which the generated DC offset is reduced by detecting the carrier leak level. .
The connection order (processing order) between the processing unit that calculates the correction value of the DC offset and the processing unit that generates the DC offset may be arbitrary, and the generated DC offset is corrected overall. It will be corrected by the value.
Further, as the amount (evaluation value) reflecting the DC offset, it is desirable to use the power or amplitude level of the leaked carrier. However, if the amount changes monotonously with the remaining DC offset, | I The average value of | + | Q | or the average value (DC component) of each phase can be used as long as I / Q can be correctly separated.

In addition, as an initial value of a reference signal point, for example, a signal point as an initial value is set in advance, or a signal point searched as good in the past is used as an initial value. Can be used.
Various signal points may be used as the signal points arranged at the step size intervals with respect to the reference signal point. For example, one of the upper, lower, left, and right of the reference signal point is used. In the above, signal points separated by a step size can be used.
In addition, various numbers may be used as the number of the plurality of signal points that are a range for searching for signal points.
Further, an arbitrary number of times may be used as the number of times the signal point search is repeatedly performed.

Further, as a signal point to be finally searched (DC offset correction value), for example, a signal point at which the level of carrier leak after correction, that is, the residual DC offset is minimized is preferably used. A signal point at which the level of carrier leak after correction is reduced to a practically effective level may be used. As an example, a signal point at which the level of carrier leak after correction is equal to or less than a predetermined threshold value (or less). Can be used.
In addition, the search for signal points (DC offset correction values) may be performed using, for example, a signal that is actually transmitted to a communication partner, or specified in advance for performing the search. The signal is actually transmitted to the communication partner after the signal point (DC offset correction value) is finally set after the search is performed using the received signal as the signal to be transmitted. Also good.

In addition, various times may be used as the number of times the step size value is switched to a small value when the amount that reflects the corrected DC offset becomes small.
As an example, the number of times is set to one time, and a large step size is initially used, and the step size is switched to a small value when the corrected carrier leak level is equal to or less than a predetermined threshold value (or less). Such an embodiment can be used.
Further, the number of times is set to two times or more, and the step size is switched to a value corresponding to each of the two or more threshold values when the corrected carrier leak level is equal to or less than (or less than) the respective threshold value. Embodiments can be used.
As another example, the number of times is not particularly limited, and a value obtained by multiplying the DC offset level by a proportionality constant (for example, converting it to an integer by rounding up) for each correction may be used as the step size.

The transmitter according to the present invention has the following configuration as one configuration example.
That is, the amount reflecting the DC offset is larger than the magnitude of the signal to be transmitted. That is, the signal to be transmitted is small. Here, as the magnitude of the signal to be transmitted, the same unit as the amount reflecting the DC offset is used. For example, units such as power and absolute amplitude can be used.
The signal to be transmitted consists of an I-phase component and a Q-phase component. The signal point is a signal point on the IQ plane. The DC offset correction value includes an I-phase component and a Q-phase component. The computing means adds (or subtracts) the I-phase component of the DC offset correction value to the I-phase component of the signal, and the DC offset correction value to the Q-phase component of the signal. Means for adding (or subtracting) the Q-phase components.
The transmitter includes a D / A converter that performs D / A conversion on the signal for which the DC offset correction value is calculated by the calculator, and a quadrature modulation of the signal that is D / A converted by the D / A converter. Quadrature modulation means.
The detection means detects an amount reflecting a DC offset for a signal subsequent to the quadrature modulation means.

  Therefore, when the signal to be transmitted is a digital signal composed of an I-phase component and a Q-phase component, it is possible to satisfactorily correct the DC offset generated by the D / A conversion unit and the quadrature modulation unit.

  As described above, according to the transmitter of the present invention, when correcting the DC offset generated in the signal to be transmitted, the function of calculating the DC offset correction value for the signal, and the correction value It has a function of detecting the amount that reflects the DC offset in the signal after the calculation and after the DC offset is generated, and is arranged at a step size interval with respect to the reference signal point and the signal point. Among the plurality of signal points, the signal point at which the detected amount is minimum when the value corresponding to each signal point is used as the correction value of the DC offset to be calculated is the following reference In this case, the step size is changed to a smaller value when the detected amount becomes smaller. As a result, the value that decreases the detected amount is set as the correction value of the DC offset to be calculated. For example, even when there is a nonlinear point, The DC offset component can be corrected appropriately.

Embodiments according to the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration example of a wireless transmitter according to an embodiment of the present invention.
The wireless transmitter of this example includes a carrier leak correction circuit 1, an I-phase D / A converter 2, a Q-phase D / A converter 3, an analog quadrature modulator (AQM) 4, and a power amplifier ( PA 5), an A / D converter 6, and a control unit 7.
The carrier leak correction circuit 1 includes a multiplier 11, an adder 12, a multiplier 13, an adder 14, a multiplier 15, a multiplier 16, and an adder 17.

An example of the operation performed in the wireless transmitter of this example is shown.
The I-phase component (in-phase component) and Q-phase component (orthogonal component) of the digital signal to be transmitted are input to the carrier leak correction circuit 1.
The multiplier 11 multiplies tan θ as a correction value (orthogonality correction value) for correcting the orthogonality (orthogonality of IQ) with respect to the input Q-phase component, and outputs the multiplication result to the adder 12. To do.
The adder 12 adds the input I-phase component and the multiplication result input from the multiplier 11 to correct the orthogonality (IQ orthogonality), and outputs the addition result to the multiplier 13. .
The multiplier 13 multiplies the addition result input from the adder 12 by a correction value (I-phase gain correction value) for correcting the I-phase gain (IQ gain ratio), and adds the multiplication result. Output to the device 14.
The adder 14 adds the multiplication result input from the multiplier 13 and a correction value (I-phase DC offset correction value) for correcting the I-phase DC offset, and sends the addition result to the D / A converter 2. Output.

The multiplier 15 multiplies the input Q-phase component by (1 / cos θ) as a correction value (orthogonality correction value) for correcting the orthogonality (orthogonality of IQ), and multiplies the multiplication result. To the device 16.
The multiplier 16 multiplies the multiplication result input from the multiplier 15 by a correction value (Q-phase gain correction value) for correcting the Q-phase gain (IQ gain ratio), and adds the multiplication result. To the device 17.
The adder 17 adds the multiplication result input from the multiplier 16 and a correction value (Q-phase DC offset correction value) for correcting the Q-phase DC offset, and sends the addition result to the D / A converter 3. Output.

The D / A converter 2 converts the corrected I-phase component signal input from the adder 14 from a digital signal to an analog signal and outputs the converted signal to the quadrature modulator 4.
The D / A converter 3 converts the corrected Q-phase component signal input from the adder 17 from a digital signal to an analog signal and outputs the converted signal to the quadrature modulator 4.
The quadrature modulator 4 performs quadrature modulation on the I-phase component input from the D / A converter 2 and the Q-phase component input from the D / A converter 3, and outputs a signal of the quadrature modulation result to the amplifier 5.

The amplifier 5 amplifies the signal input from the quadrature modulator 4 and outputs the amplified signal. This output signal is transmitted by radio from an antenna (not shown), for example.
A part of the signal output from the amplifier 5 is acquired by a coupler (coupler) or the like for feedback and input to the A / D converter 6.
The A / D converter 6 converts the input feedback signal from an analog signal to a digital signal. The input to the A / D converter 6 is appropriately converted to IF, but the center frequency is set to be different from that of the IF output by the D / A converters 2 and 3. As a result, it is possible to distinguish which DC offset has occurred.

The control unit 7 detects the power value of the digital feedback signal acquired by the A / D converter 6 as a quantity reflecting the DC offset remaining after correction.
In this example, the control unit changes (updates) the I-phase DC offset correction value and the Q-phase DC offset correction value in the carrier leak correction circuit 1 so as to reduce the amount detected in this way. ) Then, an I-phase DC offset correction value and a Q-phase DC offset correction value that minimize the power value are set.

Here, in this example, a DC component (DC offset in D / A conversion) generated when a digitally processed signal is converted into an analog signal through the D / A converters 2 and 3, and the analog signal is The offset (shift) of the direct current component due to the leak of the reference frequency (carrier leak) that occurs when the quadrature modulation is performed through the analog quadrature modulator (AQM) 4 is generically called a DC offset.
In the wireless transmitter of this example, since there is a main feature point in the configuration related to the setting of the DC offset correction value of the I phase and the Q phase, the setting of the orthogonality correction value and the gain correction value of the I phase or the Q phase is set. Although detailed explanation is omitted, various modes may be used.

Next, an algorithm for removing the DC offset component by correction in the wireless transmitter of this example will be described.
In this example, when changing the I-phase DC offset correction value and the Q-phase DC offset correction value, the step size that is the width of the change is not fixed to ± (plus or minus) 1, but is obtained from the feedback signal. The step size is changed according to the remaining DC offset.

FIG. 2 shows an example of the relationship between the step size and whether or not the nonlinear point can be removed. The horizontal axis in the figure shows the Q-phase DC offset correction value, and the vertical axis shows the measured power value of the remaining carrier leak.
As shown in FIG. 2, if the step size is set to a small value such as ± 1, the correction value is not changed beyond the non-linear point in the middle of the search. It is not possible to detect an incorrect correction value. On the other hand, if the step size is set to a large value such as ± 5, the correction value may be changed beyond the non-linear point in the middle of the search. It becomes possible to detect the correction value. The step size of 1 here corresponds to the minimum resolution (the weight of the least significant bit) of the D / A converters 2 and 3, and is the minimum unit of the amount that can be handled. Therefore, 5 is 5 times the minimum unit.

  In addition, for example, when the correction value approaches the correct value after exiting the nonlinear point using a large step size, if the step size remains large, the fluctuation range is large, so that it is detected even near the correct value. The power value of the DC offset will fluctuate greatly. In view of this, the present example focuses on the fact that the power of the remaining DC offset component decreases as the correction value is closer to the correct value, and the power of the remaining DC offset component increases as the correction value is farther from the correct value. Then, the step size is changed according to the detected power value of the DC offset.

FIGS. 3A to 3D show an example of an algorithm for carrier leak cancellation at the time of low level transmission used in the wireless transmitter of this example.
In this example, a certain point on the IQ plane and a range of points that are ± z in the I-axis direction and ± z in the Q-axis direction are considered, and these five points are used as DC offset correction values in order. Set in the leak correction circuit 1, find a point where the power value of the feedback signal (remaining DC offset power value) is the minimum among these five points, and then consider the same five points centered on that point. The same operation is repeated to search for a DC offset correction value that minimizes the remaining DC offset power value. At this time, the step size z is changed from a large value to a small value as the power value (remaining carrier leak power value) of the feedback signal changes from a large value to a small value.

Specifically, as shown in FIG. 3A, at the initial stage or at a certain point in time, the center point set on the IQ plane and the distance between the points are X (= step size z). For each of the four points in total, the DC offset power value remaining when each point is set in the carrier leak correction circuit 1 is measured, and the point at which the power value is minimum is defined as the next central point. To do. Here, X is a value greater than 1 (X> 1). The initial center point is set in advance, for example, and the center point at a point other than the initial point is a point determined by the previous search.
Further, these five points are assumed to have the coordinates of the center point on the IQ plane (x0, y0), point (x0, y0), point (x0-X, y0), point (x0 + X, y0), A point (x0, y−X0) and a point (x0, y0 + X) are obtained.
When the point (x, y) on the IQ plane is set in the carrier leak correction circuit 1, the value x is set as the I-phase DC offset correction value and the value y is set as the Q-phase DC offset correction value. Set.

Next, it is determined whether or not the power value (minimum power value) at the next center point detected in FIG. 3A is smaller than a preset threshold value, for example. In this example, it is assumed that the minimum power value is greater than or equal to the threshold value, and therefore, X described above is used as the step size z.
As shown in FIG. 3 (b), the step size X (= z) is used to search for the point where the minimum power is obtained in the same manner as described above with respect to the same five points with respect to the next center point. The next center point.

Next, it is determined whether or not the power value (minimum power value) at the next center point detected in FIG. 3B is smaller than a preset threshold value, for example. In this example, it is assumed that the minimum power value is smaller than the threshold value. For this reason, the step size z is set to be smaller than X and 1 is used as the step size z.
As shown in FIG. 3C, using the step size 1 (= z), the same five points as described above (however, the step sizes are different) with the next center point as a reference are the same as above. The point with the minimum power is searched for as the next center point.

By repeatedly performing a search similar to the above, the detected minimum power value gradually decreases and converges to the minimum value.
In this way, as shown in FIG. 3D, it is possible to search for a point where the power value of the remaining DC offset is the smallest (correct value point). By setting such correct value point values as the I-phase DC offset correction value and the Q-phase DC offset correction value of the carrier leak correction circuit 1, the DC offset component can be accurately removed.

FIG. 4 shows an example of the operation result of the carrier leak cancellation algorithm at the time of low-level transmission performed by the wireless transmitter of this example. In the figure, the horizontal axis indicates the frequency, and the vertical axis indicates the level of carrier leak due to the remaining DC offset.
It can be seen that the carrier leak is suppressed to almost the same level as the noise floor.

  As described above, in the wireless transmitter of this example, the correction value (I-phase DC offset) for correcting the DC offset generated in the signal to be transmitted due to the nonlinear characteristics of the D / A converters 2 and 3 and the like. By devising a configuration for setting the correction value and the Q-phase DC offset correction value), the influence of the nonlinear characteristics of the D / A converters 2 and 3 is avoided, and the DC offset component is accurately corrected and removed. Communication quality can be improved.

  In particular, the carrier leak, which is a tone signal, can be detected with a large amplitude stably even with the same power as the normal modulation signal (transmission signal). Therefore, the DC offset is corrected so as to minimize the transmission power itself including the carrier leak. In this example of setting a value, an algorithm that normally converges even at low-level transmission can be realized.

  In the wireless transmitter of this example, the calculation means for calculating the correction value of the DC offset is configured by the functions of the adders 14 and 17 of the carrier leak correction circuit 1, and the control unit 7 controls the A / D converter 6. The detecting means is configured by the function of detecting the amount that reflects the corrected DC offset via the control unit 7, and the control unit 7 searches for a good correction value (signal point) of the DC offset based on the detected amount. A control unit is configured by the function to be set, a D / A conversion unit is configured by the function of the D / A converters 2 and 3, and an orthogonal modulation unit is configured by the function of the orthogonal modulation unit 4.

It is a figure which shows the structural example of the radio | wireless transmitter provided with the carrier leak correction circuit which concerns on one Example of this invention. It is a figure which shows an example of the relationship between step size and non-linear point omission possibility. (A)-(d) is a figure for demonstrating an example of the algorithm of the carrier leak cancellation at the time of the low level transmission which concerns on one Example of this invention. It is a figure which shows an example of the operation result of the algorithm of the carrier leak cancellation at the time of the low level transmission which concerns on one Example of this invention. It is a figure for demonstrating an example of the algorithm of the carrier leak cancellation at the time of low level transmission. It is a figure which shows an example of a frequency spectrum when a correction value cannot escape from a nonlinear point. It is a figure which shows an example of a correction value versus carrier leak characteristic.

Explanation of symbols

  1 ·· Carrier leak correction circuit 2 · 3 ·· D / A converter 4 ·· Quadrature modulator 5 ·· Amplifier 6 ·· A / D converter 7 ·· Control unit 11, 13, 15 16 .... multiplier, 12, 14, 17 ... adder,

Claims (2)

In a transmitter for correcting a DC offset generated in a signal to be transmitted,
Computing means for computing a correction value of a DC offset for the signal;
Detecting means for detecting an amount reflecting the DC offset in a signal after the correction value is calculated by the calculating means and after the DC offset is generated;
Among a plurality of signal points including a reference signal point and signal points arranged at step size intervals with respect to the signal point, a value corresponding to each signal point is calculated by the calculation means. When used as a correction value, the signal point that minimizes the amount detected by the detection means is repeatedly set as the next reference signal point, and in this case, the amount detected by the detection means The step size is changed to a smaller value in response to a decrease in the value, and a value with a smaller amount detected by the detection means is set as a DC offset correction value calculated by the calculation means. Control means;
A transmitter characterized by comprising: The transmitter of claim 1, wherein
The amount reflecting the DC offset is larger than the magnitude of the signal to be transmitted,
The signal to be transmitted consists of an I-phase component and a Q-phase component,
The signal point is a signal point on the IQ plane,
The correction value of the DC offset is composed of an I-phase component and a Q-phase component,
The computing means adds or subtracts the I-phase component of the DC offset correction value with respect to the I-phase component of the signal, and the Q-phase of the DC offset correction value with respect to the Q-phase component of the signal. Means for adding or subtracting components;
The transmitter includes a D / A conversion unit that D / A converts a signal in which the correction value of the DC offset is calculated by the calculation unit;
Orthogonal modulation means for orthogonally modulating the signal D / A converted by the D / A conversion means,
The detection means detects an amount reflecting a DC offset for a signal subsequent to the quadrature modulation means.
A transmitter characterized by that.

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