JP2007274454A - Distortion compensation coefficient training signal generator - Google Patents

Abstract

In a radio transmitter that performs digital distortion compensation in a baseband unit, a training signal generation device is provided to prevent a change in amplification characteristics of a power amplifier and perform distortion compensation appropriately.
In a training signal generator for a power amplifier that performs digital distortion compensation in a baseband part, by providing means for outputting a training signal composed of a basic waveform signal part and a no-signal part, The effective value is reduced, and the change in the amount of heat generated by the power amplifier after the input to the power amplifier is switched to the training signal is suppressed.
[Selection] Figure 1

Description

  The present invention relates to distortion compensation control in a power amplifier of a radio transmitter.

  In the field of wireless communication technology, in recent years, in response to demands such as an increase in data transmission speed and an increase in subscriber capacity, data multilevel technology, spread spectrum systems such as CDMA, multi-band such as OFDM, etc. Carrier transmission systems and the like have been developed, and the use of modulation systems combining these has increased. These modulation signals are characterized in that the dynamic range of the modulation signal is large and the peak factor represented by the ratio between the peak value and the average value of the power is large as compared with the conventional analog modulation system.

  In such an amplifier that amplifies the modulation signal, linearity for amplifying a wideband / high dynamic range signal with low distortion and high power efficiency for reducing power consumption of the apparatus are required. However, in a general amplifier, improvement in linearity and improvement in efficiency are in a contradictory relationship, and a distortion compensation technique has been developed as a means for simultaneously realizing the two.

  Distortion compensation is intended to improve distortion characteristics by applying a reverse characteristic of the distortion characteristics of a power amplifier to a modulated signal as an input signal for a power amplifier having high efficiency but non-linear characteristics. Distortion compensation using digital technology mainly includes a polynomial system that generates a reverse characteristic of a power amplifier using a polynomial and an LUT system that generates an inverse characteristic using a look-up table (LUT). The polynomial method has advantages such as the high speed of calculating the reverse characteristic of the power amplifier and the small amount of memory used for the calculation, and the LUT method can calculate the reverse characteristic for any power amplifier. There are advantages. High-efficiency power amplifiers are highly nonlinear, and power amplifier characteristics (AM-AM / AM-PM characteristics, etc.) often have very steep fluctuation characteristics. Digital distortion compensation is often used.

  The LUT method is a method in which a distortion compensation coefficient with respect to an input signal amplitude is sequentially calculated, and the inverse characteristic is calculated by repeatedly storing and updating in the LUT. Although it is possible to use an actual input signal for updating the LUT, in recent modulation systems having a large peak factor, the frequency of appearance of signals close to the maximum amplitude value is low, and the frequency of updating the LUT must be low. There was no problem.

  In order to solve the above problems, the conventional distortion compensation method updates the LUT using a training signal instead of the modulation signal. For example, Patent Document 1 corresponds to this.

  This will be described with reference to FIG. A training signal generator 115 used for the equalizer 122 and change-over switches 116, 118, and 120 are provided, and all these switches are connected to the A side during training. As a result, the training signal output from the training signal generation unit 115 is sent to the modulation unit 117, subjected to predetermined modulation, and then output from the power amplifier 119. The training signal is input to the demodulator 121 via the change-over switch 120, and at this time, the waveform distortion is received by the band limiting filter of the demodulator 121. The equalizer 122 uses the training signal of the training signal generation unit 115, performs a training calculation process with the timing strobe signal, derives an optimum parameter, and further stores it in the parameter storage unit 123. After the training is completed, all the change-over switches 116, 118, 120 are switched to the B side, and the apparatus can be used as a normal radio. In this way, the distortion characteristic is compensated using the training signal.

In order to grasp the reverse characteristic of the power amplifier, a signal having the same number of steps as the resolution of the LUT and a maximum amplitude of which is the maximum input limit of the power amplifier is used as the training signal. In addition, since the linearity of the input / output signal can be verified relatively easily, a triangular wave or a sawtooth wave is often used for the training signal. In addition, in order to accurately calculate the inverse characteristic, a predetermined signal waveform included in the training signal is repeatedly input to the power amplifier. FIG. 5 shows an example of a general training signal. FIG. 6 shows an example in which a triangular wave approximation waveform is configured by steps corresponding to the case where the LUT resolution is 128 points (7 bits). By using the training signal having such a configuration, it is possible to grasp the amplitude characteristic corresponding to each step from the minimum value to the maximum value of the input signal at the same frequency. Although the example has been described based on a triangular wave, similar training can be performed using a sawtooth wave having similar steps.
JP-A-7-183833

  However, when the input to the modulation unit is switched from the baseband signal to the training signal, the amplification characteristic of the power amplifier changes, and there is a problem that information for appropriately performing distortion compensation of the baseband signal cannot be acquired. It was.

  The above problem will be further described. In the case of a general training signal, in addition to the difference in signal waveform from the baseband signal, there is also a difference in the effective value of the signal. For example, when the training signal is a triangular wave, the peak factor, which is the ratio of the peak of the signal to the effective value, is 4.8 dB, and the peak factor of the baseband signal is about 8 dB for CDMA and about 11 dB for OFDM, depending on the detailed method. If the peak value is the same, there is a difference of 2 to 4 times in the effective value.

  When signals having different effective values are modulated by the modulation unit and then input to the power amplifier, the amount of heat generated by the power amplifier also changes, and the gain of the power amplifier also changes. Even if the reverse characteristic of the power amplifier is grasped from the training signal, when the input is subsequently switched to the baseband signal, the gain of the power amplifier also changes due to the change in the effective value of the signal, and the amplitude characteristic of the power amplifier also changes again. Therefore, even if the inverse characteristic grasped by using the training signal is held in the LUT and distortion compensation is performed thereby, sufficient distortion compensation cannot be performed due to the characteristic change of the power amplifier.

  As a specific example, a change in amplitude characteristics of a power amplifier when a triangular wave is used as a training signal is shown. FIG. 7 shows the power amplifier input signal, and FIG. 8 shows the power amplifier output signal. The horizontal axis is time, and the vertical axis is amplitude. In FIG. 7, a training signal is input in a region where the amplitude is 1.0 and constant, and a baseband signal is input in a region thereafter. As shown in FIG. 8, the gain of the power amplifier tends to decrease in the input region of the training signal, and after the input signal is switched to the baseband signal, the gain of the power amplifier gradually increases. .

  Due to the nature of the training signal, which grasps the characteristics of the power amplifier, its maximum amplitude is generally set to be the maximum input limit of the power amplifier, but the maximum amplitude of the baseband signal prevents clipping of the amplified waveform. From the standpoint of stable operation of the amplifier and the like, it is set lower than the maximum input of the power amplifier, and the difference between the effective values of both signals may be even greater.

  The present invention provides a training signal generation method capable of accurately calculating a distortion compensation coefficient and obtaining excellent linearity in a transmitter that performs distortion compensation using an LUT, and training capable of generating the training signal. An object of the present invention is to provide a signal generation device and a transmitter in which the device is mounted.

  The present invention is a training signal generator for a radio transmitter that performs digital distortion compensation in a baseband unit, and an output training signal is composed of a basic waveform signal unit and a no-signal unit. Take the configuration.

  Also, the training signal generator of the present invention employs a configuration characterized by having a basic waveform signal generator and a basic waveform signal delay processor.

  Further, the training signal generator of the present invention employs a configuration characterized by having means for determining the configuration of the basic waveform signal portion and the no-signal portion so that the effective amplitude value of the training signal becomes a desired value.

  In addition, the training signal generator of the present invention employs a configuration having a baseband signal detection unit and means for setting a desired amplitude effective value to an amplitude effective value of the detected baseband signal.

Further, the present invention includes the training signal generator, and further,
A switching unit for switching between a training signal and a baseband signal;
A multiplier that multiplies the baseband signal and the distortion compensation coefficient from the LUT;
A modulator for converting to a radio frequency band;
A power amplifier having nonlinear characteristics;
A demodulator for demodulating the output from the power amplifier;
An adder that compares the signal generator and demodulator output and calculates the error amount;
An address generator for generating an address for the amplitude value output from the signal generator;
A LUT for storing a distortion compensation coefficient for an address value based on the error amount output by the adder
And a wireless transmitter configured to perform digital distortion compensation in the baseband unit.

  The training signal generation method of the present invention is a training signal generation method for a radio transmitter that performs digital distortion compensation in a baseband unit, and an output training signal is configured by a basic waveform signal unit and a no-signal unit. A configuration characterized by having steps is adopted.

  In the training signal generation method of the present invention, the step of configuring the training signal by the basic waveform signal part and the no-signal part includes a basic waveform signal generation step and a basic waveform signal delay processing step. Take the configuration.

  The training signal generation method of the present invention employs a configuration characterized in that it has a step of setting the effective amplitude value of the training signal to a desired effective amplitude value.

  The training signal generation method of the present invention further includes a step of detecting an effective amplitude value of the baseband signal and a step of setting the delay processing amount of the basic waveform signal as the effective amplitude value of the baseband signal in the training signal generating method. The structure characterized by having is taken.

  According to the training signal generating apparatus or the training signal generating method of the present invention, the heat generation amount of the power amplifier generated after the input to the power amplifier is switched to the training signal by reducing the effective value of the training signal used for the LUT training. Can be suppressed.

  As a result, it is possible to suppress a change in the output characteristics of the power amplifier that occurs when the input to the power amplifier is further switched from the training signal to the input signal, and distortion compensation using the distortion compensation coefficient calculated based on the training signal can be suppressed. The effect can be enhanced.

  Further, according to the training signal generating apparatus or the training signal generating method of the present invention, the heat generation and output characteristics of the amplifier when the input signal is switched to the training signal by further setting the effective value of the training signal to a desired value. The change range of can be controlled.

  As a result, it is possible to further enhance the effect of distortion compensation using the distortion compensation coefficient calculated based on the training signal. In particular, when the effective value of the input signal can be theoretically grasped, the effect of distortion compensation can be further enhanced by setting the effective value of the training signal to the theoretical value.

  In addition, according to the training signal generation apparatus or the training signal generation method of the present invention, it further includes a signal detection unit, and by making the effective amplitude value of the training signal the detected effective amplitude value, the input signal is converted into the training signal. It is possible to eliminate the change in the amount of heat generated by the amplifier when switching.

  Thereby, the output characteristic of the amplifier when switched to the training signal can be matched with the input signal, and the effect of distortion compensation by the distortion compensation coefficient can be further enhanced.

Example 1
The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a transmitter including a first embodiment of the apparatus of the present invention.

  The training signal generation device 101 according to the embodiment of the present invention generates a training signal including a basic waveform signal part such as a triangular wave and a non-signal part. By setting the effective value of the training signal to a level equivalent to that of the baseband signal, a change in the amount of heat generated by the power amplifier 107 when the input signal is switched is suppressed. In the training signal generator 101, the basic waveform signal generator 112 generates a basic waveform signal such as a triangular wave, the delay generator 113 delays the basic waveform signal, and adds a no-signal part to the training signal.

  The generated training signal is input to the distortion compensation system 104 via the changeover switch 103. The input signal is multiplied by the distortion compensation coefficient output from the LUT unit 111 storing the distortion compensation coefficient by the multiplication unit 105, and a distortion compensation signal is generated and input to the power amplifier 107. Part of the output of the power amplifier 107 is input to the demodulator 108 and decoded. The decoded signal is converted into an inverted signal of a substantially input signal of the distortion compensation system 104, and is input to the adder 109 together with the training signal, and an error is output. On the other hand, the training signal is input to the address generation unit 110, generates an address value corresponding to the input amplitude value, and gives an address for the distortion compensation coefficient corresponding to the output error. This operation is repeated while the training signal is input, and the update of the LUT unit is sequentially repeated. This operation is a general adaptive operation, and can be performed using a technique such as LMS or RLS.

  After completion of the training signal, the original baseband signal 102 is input to the distortion compensation system 104, the distortion compensation coefficient stored in the LUT unit 111 is read based on the address value generated by the address generation unit 110, and the compensation coefficient is set. The baseband signal 102 is multiplied to generate a distortion compensation signal. As a result, an output with improved linearity can be obtained from the power amplifier 107. At this time, the LUT may be updated as in the case of the training signal.

  Next, the setting of the effective value in the present invention will be described. The peak factor of the baseband signal is P, the peak factor of the basic waveform signal is Ps, and the ratio between the maximum amplitude value of the basic waveform signal and the maximum amplitude value of the baseband signal is r. Assuming that the ratio of the basic signal waveform in the training signal on the time axis is X, by setting X = r ^ 2 * 10 ^ ((Ps−P) / 10), the same effective value as the baseband signal is obtained. A training signal can be obtained.

  The above will be described with a specific example. The peak factor of the triangular wave is 10 * log10 (3) = 4.8 dB. It is assumed that the peak factor of the baseband signal targeted for distortion compensation is 9 dB. Assuming that the peak values of the respective signals are the same, in order to realize the same amplitude effective value as that of the baseband signal by the training signal having a triangular wave as the basic waveform, the ratio of the basic waveform signal on the time axis is expressed as X = 10 ^ ((4.8-9) / 10) = 0.38, and the remaining 0.62 portion may be a no-signal portion. FIG. 2 shows an example of the configured training signal. The training signal having such a configuration makes it possible to operate the power amplifier without causing a gain change due to switching between the training signal and the baseband signal.

  In the above embodiment, the training signal generating apparatus 101 is configured by the basic waveform signal generating unit 112 and the delay unit 113 including the digital delay circuit, and the no-signal unit is added to the training signal by delay processing. Instead of 113, a means for ON-OFF control of the basic waveform signal may be provided. The training signal generator 101 may control the timing by a clock counter and generate a training signal composed of a basic waveform signal part and a no-signal part.

(Example 2)
FIG. 3 is a block diagram showing a second embodiment of the training signal generator 101 of the present invention. The second embodiment is different from the training signal generator of the first embodiment in that the waveform of the training signal can be changed based on the switching information of the baseband signal.

  In general, in a mobile communication base station, the number of carriers that are simultaneously transmitted in the form of single carrier, two carriers, four carriers, and the like varies depending on subscriber capacity and the like. Along with this, the degree of signal multiplexing varies, and the peak value or effective value of the baseband signal also changes. For this reason, the composition ratio between the basic waveform generated in the basic waveform signal generation unit 112 and the no-signal unit added by the delay generation unit 113 is adjusted based on the carrier information 114.

  The carrier information 114 may be a theoretical peak factor and maximum amplitude value of the corresponding baseband signal, or may be an actual measurement value of the effective amplitude value of the baseband signal. Assuming that the effective amplitude value of the baseband signal is Sb and the effective amplitude value of the basic waveform signal is S, the proportion of the basic signal waveform in the training signal on the time axis can be set to X = Sb / S.

  Although an example in the case of dealing with a change in the number of carriers in the baseband signal is shown here, the same correspondence can be made, for example, when the modulation method is switched between CDMA and OFDM. With such a configuration, it is possible to perform training corresponding to baseband signals having various signal characteristics.

  The present invention can be applied to a wireless transmitter that performs distortion compensation for improving nonlinearity of a power amplifier using a training signal. Specifically, the present invention can be applied to mobile communication base stations, terrestrial digital transmitters, wireless LAN base stations, and the like.

1 is a block diagram of a wireless transmitter including a training signal generator according to a first embodiment. The figure which shows an example of the training signal in Example 1. Block diagram of training signal generator of embodiment 2 Block diagram of a wireless transmitter that performs distortion correction using a conventional training signal Diagram showing an example of a conventional training signal The figure which shows an example of the training signal comprised based on the resolution of LUT The figure which shows an example of the change of the input signal of a power amplifier The figure which shows an example of the change of the amplification characteristic of a power amplifier

Explanation of symbols

101 Training signal generator
102 Baseband signal 103, 116, 118, 120 Changeover switch 104 Distortion compensation system 105 Multiplication unit 106, 117 Modulation unit
107, 119 Power amplifier 108, 121 Demodulation unit 109 Addition unit 110 Address generation unit 111 LUT unit 112 Basic waveform signal generation unit 113 Delay generation unit 114 Carrier information 115 Training data generation unit 122 Equalizer 123 Parameter storage unit 124 Baseband signal Processing part

Claims (9)

  A training signal generator for a radio transmitter that performs digital distortion compensation in a baseband unit, wherein an output training signal is composed of a basic waveform signal unit and a no-signal unit. apparatus.   The training signal generation apparatus according to claim 1, wherein the training signal generation apparatus includes a basic waveform signal generation unit and a delay processing unit for the basic waveform signal.   3. The training signal generation according to claim 1, further comprising means for determining a configuration of the basic waveform signal portion and the no-signal portion so that an effective amplitude value of the training signal becomes a desired value. apparatus.   4. The training signal generating apparatus according to claim 3, further comprising a baseband signal detecting unit, and means for setting the desired amplitude effective value to an amplitude effective value of the detected baseband signal. A training signal generator according to claim 4, further comprising:
A switching unit for switching between a training signal and a baseband signal;
A multiplier that multiplies the baseband signal and the distortion compensation coefficient from the LUT;
A modulator for converting to a radio frequency band;
A power amplifier having nonlinear characteristics;
A demodulator for demodulating the output from the power amplifier;
An adder that compares the signal generator and demodulator output and calculates the error amount;
An address generator for generating an address for the amplitude value output from the signal generator;
A LUT for storing a distortion compensation coefficient for an address value based on the error amount output by the adder
A wireless transmitter that performs digital distortion compensation in a baseband unit.   A training signal generation method for a radio transmitter that performs digital distortion compensation in a baseband unit, comprising a step of configuring an output training signal by a basic waveform signal unit and a no-signal unit.   7. The training signal generation according to claim 6, wherein the step of configuring the training signal by a basic waveform signal part and a no-signal part includes a basic waveform signal generation step and a delay processing step of the basic waveform signal. Method.   The training signal generation method according to claim 6, further comprising a step of setting an effective amplitude value of the training signal to a desired effective amplitude value. 9. The training signal generation method according to claim 8, further comprising: a step of detecting an effective amplitude value of the baseband signal; and a step of setting a delay processing amount of the basic waveform signal as an effective amplitude value of the baseband signal.

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