JP7102072B2 - Adaptive equalizer and carrier regenerative circuit - Google Patents

Description

The present invention relates to an adaptive equalizer used for reproducing a carrier wave / received wave in digital wireless transmission, and a carrier wave regenerative circuit provided with the adaptive equalizer.

In recent years, wireless traffic has been increasing more and more, and from the viewpoint of improving the efficiency of frequency utilization, there is an increasing demand for high-speed transmission by the high multi-value QAM (Quadrature Amplitude Modulation) method in digital wireless transmission. .. In this high multi-level QAM method, the demodulation performance may deteriorate due to the phase noise (phase error) of the carrier wave generated in the transmitting device or the receiving device. Therefore, there is known a carrier regenerative circuit that improves demodulation performance (bit error rate) based on the degree of influence of phase noise and thermal noise (see, for example, Patent Document 1).

In this carrier wave reproduction circuit, the loop filter control unit controls the bandwidth of the loop filter based on the phase error detected by the phase error detector and the amplitude error detected by the amplitude error detector, thereby causing phase noise and heat. The bandwidth is set appropriately according to the noise to improve the demodulation performance.

Japanese Unexamined Patent Publication No. 2011-101177

By the way, when only thermal noise is present, a small circular signal distribution region centered on the ideal point R exists as shown in FIG. 7A when the C / N (carrier-to-noise ratio) is high. When the C / N is low, as shown in FIG. 7B, there is a large circular signal distribution region centered on the ideal point R. Further, when thermal noise and phase noise are present, as shown in FIG. 8, there is a circular thermal noise distribution region centered on the ideal point R, and a circle centered on the origin C on the complex plane. There is a phase noise distribution region that is arcuate and passes through the ideal point R.

Further, in the high multi-value modulation, the phase error detection range of the carrier wave / carrier reproduction is remarkably narrowed. That is, in the case of a low multi-value, the phase error detection range is wide because the distance between adjacent ideal points is large, but in the case of a high multi-value, the phase error detection range is small because the distance between adjacent ideal points is small. Becomes narrower. Since the phase error detection range is remarkably narrowed, phase jitter (phase fluctuation) as shown in FIG. 9 increases in a phase noise environment, which may lead to out-of-synchronization of carrier wave reproduction.

That is, based on the ideal point R closest to the output signal S of the adaptive equalizer, for example, the signal point S (FIG. 9) is located in a region far from the origin C on the complex plane shown in FIG. 9 and greatly affected by the phase rotation. The tap coefficient is updated with reference to the erroneous ideal point R for the black circle surrounded by the middle square broken line, the output signal), and an event occurs in which the tap coefficient converges to an unintended signal point arrangement. On the other hand, in the carrier wave reproduction circuit described in Patent Document 1, the specifications of the carrier wave reproduction loop at the time of high multi-value are switched depending on whether the reduction of thermal noise is prioritized or the reduction of phase noise is prioritized. Unstable operation due to a decrease in the phase error detection range due to the increase in value is not considered.

Therefore, an object of the present invention is to provide an adaptive equalizer and a carrier regenerative circuit provided with the adaptive equalizer, which can realize stable and high demodulation performance even at high multi-values.

In order to solve the above problems, the invention according to claim 1 includes an adaptive equalization unit that compensates for the frequency characteristics of the input signal, an ideal point arranged on a complex plane, and an output signal of the adaptive equalization unit. When the phase noise of the output signal of the adaptive equalization unit is larger than a predetermined state, the tap update unit includes a tap update unit that updates the tap coefficient with respect to the adaptive equalization unit by an algorithm based on the error of the above. , The output signal of the adaptive equalization unit is rotated around the origin on the complex plane, the ideal point closest to the output signal of the adaptive equalization unit is selected, and the selected ideal point and the adaptation are selected. The adaptive equalizer is characterized in that the tap coefficient is updated based on an error with the output signal of the equalization unit.

The invention according to claim 2 compensates for the frequency characteristics of a first phase rotator that rotates the phase of the input signal and a phase rotation signal that is an input signal whose phase is rotated by the first phase rotator. The adaptive equalizer according to claim 1, a phase error detector that detects a phase error included in the phase rotation signal compensated by the adaptive equalizer, and a phase rotation control signal based on the phase error. A rotation signal generation unit to be generated and a second phase rotator that rotates the phase of the input signal based on the phase rotation control signal are provided, and the first phase rotator is used as the phase rotation control signal. The carrier reproduction circuit is characterized in that the phase of the input signal is rotated based on the above.

According to the invention of claim 1, when the phase noise of the output signal of the adaptive equalization unit is larger than a predetermined state, the output signal of the adaptive equalization unit is rotated around the origin on the complex plane. Select the ideal point closest to the output signal of the adaptive equalization unit. In other words, when the phase noise (phase jitter) is large, the ideal point that is closest to the output signal of the adaptive equalization unit is not simply selected, but the corresponding ideal point is selected by rotating the output signal, so the correct ideal point is selected. Can be selected. Then, by updating the tap coefficient based on the error between the correct ideal point and the output signal of the adaptive equalizer unit, it is possible to obtain a stable output from the adaptive equalizer. As a result, it is possible to realize highly accurate and stable demodulation performance and carrier wave reproduction even at high multi-values.

According to the invention of claim 2, the phase rotation control signal is generated and the phase of the input signal is rotated based on the phase error of the phase rotation signal whose frequency characteristic is compensated by the adaptive equalizer, so that the phase is faded. Even if there is waveform distortion due to the above, it is possible to estimate the phase noise of the carrier wave with high accuracy and realize high demodulation performance and carrier wave reproduction. Moreover, as described above, since a stable adaptive equalizer output can be obtained, it is possible to realize more accurate and stable demodulation performance and carrier wave reproduction. Further, since the frequency characteristic of the phase rotation signal is compensated by the adaptive equalizer, the influence of thermal noise can be reduced.

It is a schematic block diagram which shows the carrier wave regenerative circuit which concerns on embodiment of this invention. It is a schematic block diagram which shows the microwave radio system which includes the carrier wave regenerative circuit of FIG. It is a schematic block diagram which shows the adaptation equalizer of the carrier wave regenerative circuit of FIG. It is a figure for demonstrating the method of estimating the magnitude of phase noise by the C / N estimation part of the adaptive equalizer of FIG. It is a figure for demonstrating the method of selecting an ideal point by the rotating part of the adaptive equalizer of FIG. It is a figure which shows the ideal point selected by the rotating part of the adaptive equalizer of FIG. 3 in the region where the influence of a phase rotation is large. It is a figure which shows the signal distribution area (a) when C / N is high, and the signal distribution area (b) when C / N is low, when only thermal noise is present. It is a figure which shows the thermal noise distribution region and the phase noise distribution region in the case where thermal noise and phase noise are present. It is a figure which shows the case where an erroneous ideal point is used as a reference with respect to a signal point in a region where the influence of a phase rotation is large.

Hereinafter, the present invention will be described based on the illustrated embodiment.

1 to 6 show an embodiment of the present invention, FIG. 3 is a schematic block diagram showing an adaptive equalizer 3 according to this embodiment, and FIG. 1 is a schematic block diagram showing the adaptive equalizer 3. It is a schematic block diagram which shows the carrier wave regenerative circuit 1. The carrier wave regeneration circuit 1 is a circuit that reproduces a carrier wave in digital wireless transmission, and is provided in the receiving device 102 of the microwave wireless system shown in FIG. Here, the microwave wireless system will be briefly described. The transmitted signal mapped and modulated in the transmitting device 101 is analog-converted, multiplied by the carrier wave W1, and transmitted from the antenna. Then, when it is received by the antenna of the receiving device 102 via the multipath fading environment, it is multiplied by the carrier wave W2, frequency-converted, digitally converted by the ADC, demodulated by the carrier wave reproduction circuit 1, and demapped. Is.

The carrier wave reproduction circuit 1 mainly includes a first phase rotator 2, an adaptive equalizer 3, a phase error detector 4, an LPF 5, an NCO (rotation signal generator) 6, and a second phase rotator. 7 and an equalizer 8 are provided.

The first phase rotator 2 is a rotator / multiplier that rotates the phase of the input signal, and rotates the phase of the input signal based on the phase rotation control signal of NCO6 described later. Specifically, for each of the I-channel baseband signal and the Q-channel baseband signal converted into digital signals, phase rotation is performed based on the sine wave and cosine wave of the phase rotation control signal of NCO6. Is.

The adaptive equalizer 3 compensates for the frequency characteristics of the phase rotation signal, which is an input signal whose phase has been rotated by the first phase rotation unit 2, that is, equalizes to eliminate waveform distortion and data error of the phase rotation signal. It is a vessel. Here, the adaptive equalizer 3 is composed of a determination feedback equalizer (DFE: Decision Feedback Equalizer) and a linear equalizer, and the tap coefficient is updated as described later.

The phase error detector 4 is a detector that detects the phase error included in the phase rotation signal compensated by the adaptive equalizer 3. A specific detection method is a well-known technique. For example, a signal point corresponding to an output signal is selected from the signal point arrangement of the modulation method used between the transmission / reception devices 101 and 102, and the coordinates of the selected signal point are selected. And the coordinates of the input signal point are compared to calculate the phase error value.

The LPF 5 is a filter that removes high-frequency components of the phase error detected by the phase error detector 4 according to a predetermined bandwidth, and is composed of a low-pass filter (Low Pass Filter).

The NCO 6 is a generator that generates a phase rotation control signal based on the phase error from which the high frequency component is removed by the LPF 5, and is composed of an NCO (Numerically Controlled Oscillator). Specifically, the phase rotation by the first phase rotator 2 is controlled by generating an antiphase sine wave and a cosine wave based on the phase error from the LPF 5 and outputting them to the first phase rotator 2. To do. Further, the generated phase rotation control signal is output to the second phase rotation device 7.

The second phase rotator 7 is a rotator / multiplier that rotates the phase of the input signal, and rotates the phase of the input signal based on the phase rotation control signal from the NCO 6 to compensate for the frequency characteristics. Output to vessel 8. That is, the phase of the input signal is rotated based on the sine wave and the cosine wave from the NCO 6 based on the phase error detected by the frequency characteristic compensation (waveform distortion and the like are eliminated) by the adaptive equalizer 3. In this way, the adaptive equalizer 3 is mounted in the carrier wave reproduction loop (first phase rotor 2, phase error detector 4, LPF5 and NCO6 loop), thereby compensating for the frequency characteristics. The phase noise of the input signal is canceled based on the phase error value estimated later.

Next, a method of updating the tap coefficient in the adaptive equalizer 3 will be described. As shown in FIG. 3, the adaptive equalizer 3 includes an adaptive equalizer unit (equalizer body, filter) 31 that compensates for the frequency characteristics of the input signal, and a tap update that updates the tap coefficient for the adaptive equalizer unit 31. A unit 32 is provided.

The tap update unit 32 is an algorithm (determination-oriented algorithm) based on the error between the ideal point (reference signal) R arranged on the complex plane and the output signal S of the adaptive equalization unit 31, and is a tap for the adaptive equalization unit 31. It is an update unit that updates the coefficient (updates the previous tap coefficient). That is, the tap coefficient is calculated and updated so that the error power between the output signal S and the ideal point R is minimized by using a judgment-oriented algorithm based on the least mean squares error (MMSE: Minimum Mean Square Error). As a determination-oriented algorithm, an LMS (Least Mean Square) algorithm or an RLS (Recursive First Square) algorithm is adopted.

At this time, in normal times, the ideal point R closest to the output signal S of the adaptive equalization unit 31 is simply selected from the ideal points R arranged in a grid pattern at predetermined equal intervals, and this ideal point R is used. The tap coefficient is updated based on the error with the output signal S. On the other hand, when the phase noise of the output signal S of the adaptive equalization unit 31 is larger than the predetermined state, the output signal S of the adaptive equalization unit 31 is rotated around the origin C on the complex plane to rotate the adaptive equalization unit 31. The ideal point R closest to the output signal S of 31 is selected, and the tap coefficient is updated based on the error between the selected ideal point R and the output signal S of the adaptive equalization unit 31.

Specifically, first, it is determined whether or not the phase noise of the output signal S of the adaptive equalization unit 31 is larger than the predetermined state. In this embodiment, the phase noise is larger than the predetermined state when the phase noise level is larger than the thermal noise level, and it is determined by the C / N estimated by the C / N estimation unit 321. That is, as shown in FIG. 4, the influence of phase rotation (rotational movement amount) due to phase noise is small around the origin C on the complex plane (inside the region C1), and the output signal S in this region C1 is used. Estimate the C / N. Then, when the C / N is equal to or more than a predetermined value (when the thermal noise is small), it is determined that the phase noise is larger than the predetermined state, and when the C / N is smaller than the predetermined value (when the thermal noise is large). Determines that the phase noise is not greater than the predetermined state.

Here, the predetermined value is set to a value at which the phase noise level can be determined to be larger than the thermal noise level by C / N, and the predetermined value is set so that the level difference becomes 1.5 times, 2 times, or the like. You may. Then, the determination result determined in this way, that is, whether or not the phase noise of the output signal S is larger than the predetermined state is input to the changeover switch 323.

Next, in the rotating unit 322, the output signal S of the adaptive equalization unit 31 is rotated around the origin C on the complex plane, and the ideal point R closest to the output signal S of the adaptive equalization unit 31 is selected. .. That is, as shown in FIG. 5, the output signal S is rotated clockwise and counterclockwise around the origin C on the complex plane, and the ideal point R where the output signal S comes closest to is set at the smallest rotation angle. Select. For example, in the case of the first output signal S1 shown in FIG. 5, the first ideal point R1 that comes closest to the smallest rotation angle when rotated clockwise is selected.

As described above, in this embodiment, the output signal S is rotated by the rotating unit 322 to select the ideal point R closest to the output signal S, but the following may be used. That is, the rotating unit 322 calculates the rotation direction and the rotation angle required for each output signal S to come closest to the ideal point R, and the determination unit 324, which will be described later, rotates the output signal S by this rotation angle. The ideal point may be selected.

On the other hand, the changeover switch 323 inputs the output signal S and the ideal point R of the adaptive equalization unit 31 to the determination unit 324 as they are, or inputs the ideal point R and the output signal S selected by the rotation unit 322 to the determination unit 324. Can be switched freely. That is, based on the determination result by the C / N estimation unit 321, the output signal S and the ideal point R are directly input to the determination unit 324 in normal times, and when the phase noise of the output signal S is larger than the predetermined state, The ideal point R and the output signal S selected by the rotating unit 322 are input to the determination unit 324.

Next, the determination unit 324 updates the tap coefficient based on the tap update error between the input output signal S and the ideal point R. As a result, in normal times, that is, when the phase noise of the output signal S is small, it is based on the error between the ideal point R closest to the output signal S among the ideal points R arranged in a grid pattern and the output signal S. And update the tap coefficient. On the other hand, when the phase noise of the output signal S is large, the tap coefficient is updated based on the error between the ideal point R selected by rotating the output signal S by the rotating unit 322 and the output signal S. Then, the tap coefficient updated in this way is input to the adaptive equalization unit 31.

As described above, according to the adaptive equalizer 3, when the phase noise of the output signal S of the adaptive equalizer 31 is larger than the predetermined state, the adaptive equalizer 31 is centered on the origin C on the complex plane. The output signal S of the above is rotated to select the ideal point R closest to the output signal S of the adaptive equalization unit 31. That is, when the phase noise (phase jitter) is large, the ideal point R closest to the output signal S of the adaptive equalization unit 31 is not simply selected, but the corresponding ideal point R is selected by rotating the output signal S. Therefore, it is possible to select the correct ideal point R. For example, as shown in FIG. 6, the correct ideal point R can be selected even in a region far from the origin C on the complex plane and greatly affected by the phase rotation, unlike the case of FIG. Then, by updating the tap coefficient based on the tap update error between the correct ideal point R and the output signal S of the adaptive equalizer unit 31, it is possible to obtain a stable output from the adaptive equalizer 3. .. As a result, it is possible to realize highly accurate and stable demodulation performance and carrier wave reproduction even at high multi-values.

Further, according to the carrier wave reproduction circuit 1, a phase rotation control signal is generated based on the phase error of the phase rotation signal whose frequency characteristics are compensated (the waveform distortion and the like are eliminated) by the adaptive equalizer 3, and the input signal is used. Since the phase is rotated, even if there is waveform distortion due to fading, it is possible to estimate the phase noise of the carrier wave with high accuracy (detected by the phase error detector 4) and realize high demodulation performance and carrier wave reproduction. .. Moreover, as described above, since a stable adaptive equalizer output can be obtained, it is possible to realize more accurate and stable demodulation performance and carrier wave reproduction. Further, since the frequency characteristic of the phase rotation signal is compensated by the adaptive equalizer 3, the influence of thermal noise can be reduced.

Although the embodiment of the present invention has been described above, the specific configuration is not limited to the above-described embodiment, and even if there is a design change or the like within a range that does not deviate from the gist of the present invention, this embodiment is described. Included in the invention. For example, in the above embodiment, the output signal S is always rotated in advance by the rotating unit 322 to select the ideal point R, but the C / N estimation unit 321 determines that the phase noise is larger than the predetermined state. Only in this case, the ideal point R may be selected by the rotating unit 322 and input to the determination unit 324.

1 Carrier regenerative circuit 2 First phase rotator 3 Adaptation equalizer 31 Adaptation equalization unit 32 Tap update unit 321 C / N estimation unit 322 Rotation unit 323 Changeover switch 324 Judgment unit 4 Phase error detector 5 LPF
6 NCO (Rotation signal generator)
7 Second phase rotator 8 Equalizer S Output signal (signal point)
R ideal point

Claims (2)

An adaptive equalization unit that compensates for the frequency characteristics of the input signal,
An algorithm based on the error between the ideal point arranged on the complex plane and the output signal of the adaptive equalization unit, the tap update unit that updates the tap coefficient for the adaptive equalization unit, and the tap update unit.
When the phase noise of the output signal of the adaptive equalization unit is larger than a predetermined state, the tap update unit rotates the output signal of the adaptive equalization unit around the origin on the complex plane. , The ideal point closest to the output signal of the adaptive equalization unit is selected, and the tap coefficient is updated based on the error between the selected ideal point and the output signal of the adaptive equalization unit.
An adaptive equalizer characterized by that. A first phase rotator that rotates the phase of the input signal,
The adaptive equalizer according to claim 1, which compensates for the frequency characteristics of the phase rotation signal, which is an input signal whose phase is rotated by the first phase rotation device.
A phase error detector that detects the phase error included in the phase rotation signal compensated by the adaptive equalizer, and a phase error detector.
A rotation signal generator that generates a phase rotation control signal based on the phase error,
A second phase rotator that rotates the phase of the input signal based on the phase rotation control signal, and
The first phase rotator rotates the phase of the input signal based on the phase rotation control signal.
A carrier wave regenerative circuit characterized by that.

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