JP2005079935A - Adaptive predistortion type distortion compensation power amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an adaptive predistortion type distortion compensation power amplifier in which convergence time at cold start is shortened by reducing memory capacity and power efficiency is maximized within a range where an adjacent channel leak power ratio satisfies a legal value. <P>SOLUTION: Expansion into complex power series is used to model an input-output characteristic of a power amplifier and an input-output characteristic of a predistortion amplifier for performing predistortion processing, and a fundamental wave coefficient of the predistortion amplifier, that is, the backoff of an input signal of the predistortion amplifier is controlled so as to be maximized within the range where an adjacent channel leak power ratio of the power amplifier does not exceed the legal value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a distortion compensation power amplifier used in a linear transmitter or the like of a radio communication apparatus, and more particularly to an adaptive predistortion distortion compensation power amplifier.

With the rapid spread of digital radio such as mobile radio, there is an increasing demand for higher power and higher efficiency of the power amplifier of the transmitter, and development is actively conducted in both devices and circuits. In digital radios, the linearity of the input / output amplitude characteristics of the power amplifier is strictly required in order to prevent adjacent channel interference due to higher-order distortion caused by the nonlinear input / output amplitude characteristics of the power amplifier.
However, since it is difficult to increase power and increase efficiency while maintaining good linearity, it is important to apply a nonlinear compensation technique using a distortion compensator. Methods for compensating the nonlinear characteristics of the power amplifier are roughly classified into a feed forward method, a feedback method, and a predistortion method.

The feed-forward method is a method that takes out a part of the output of the power amplifier, subtracts a separately generated undistorted signal component to create a distortion component, and subtracts this from the output of the power amplifier to compensate for the distortion. The method is a method of compensating for distortion by applying negative feedback to the power amplifier. The predistortion method is a method in which a nonlinear characteristic opposite to the distortion characteristic generated in the power amplifier is given to the input signal in advance and then input to the power amplifier, and the signal processing is performed before the power amplifier. This is also called an adaptive (or adaptive) predistortion scheme.
In particular, in recent years, adaptive predistortion type distortion compensation technology has attracted attention as a technology for simultaneously realizing high power efficiency and high linearity in a power amplifier.

FIG. 1 is a diagram showing a configuration of a general adaptive predistortion type distortion compensation power amplifier, and the principle of the adaptive predistortion type distortion compensation power amplifier will be described based on this figure.
A baseband signal that is a digital signal is input to the distortion compensation circuit 1, and a signal that has been subjected to distortion compensation by a distortion compensation control signal from the distortion compensation control unit 2 is converted into an analog signal by a D / A converter. The analog modulation signal is mixed with a local oscillation signal from a local oscillator by a frequency mixer, frequency-converted to an RF band, and then input to a power amplifier 3 (target amplifier).
The output signal (RF signal) from the power amplifier 3 is frequency-converted into a baseband signal, converted into a digital signal by an A / D converter, and fed back to the distortion compensation controller 2, and the signal, the input baseband signal, Thus, the distortion compensation coefficient in the distortion compensation circuit 1 is determined. By repeating this, the output distortion from the power amplifier 3 is controlled to be as small as possible. Note that the distortion compensation circuit 1 that performs distortion compensation, the distortion compensation control unit 2, and the like may be configured by a DSP (Digital Signal Processor) suitable for high-speed processing.
In the configuration as described above, a table lookup method is used in which the distortion compensation coefficient is determined by referring to the lookup table in the distortion compensation control unit of the conventional adaptive predistortion type distortion compensation power amplifier.
JP2002-290161 Kosaka, Doan, Sakaguchi, Takada, Araki, “Implementation of Look-Up-Table Digital Predistortion Linearizer”, Shingaku Zendai, March 2003 AAMSaleh, J. Salz, “Adaptive linearization of power amplification in digital radio systems,” Bell systemtechnical journal, vol. 62, No. 4, pp. 1019-1033, Apr. 1983.

However, in the adaptive predistortion type distortion compensating power amplifier using the table look-up method, the circuit scale of the memory for the look-up table becomes large, and the circuit cannot be reduced in size. There is also a problem that the convergence characteristic of the distortion compensation algorithm at the cold start is poor (it takes time to converge).
The present invention has been made in view of the problems of the prior art, and the object of the present invention is to introduce an algorithm based on complex power series expansion for distortion compensation and to optimize backoff, To provide an adaptive predistortion type distortion compensation power amplifier that reduces the amount of memory, shortens the convergence time at cold start, and maximizes the power efficiency within a range where the adjacent channel leakage power ratio satisfies the legal value. is there.

  The present invention relates to an adaptive predistortion type distortion compensation power amplifier, and the object of the present invention is to generate a distortion compensation coefficient from an input signal and a feedback signal obtained by feeding back a part of an output signal that has passed through the power amplifier. In an adaptive predistortion type distortion compensating power amplifier that removes nonlinear distortion of the power amplifier by performing predistortion processing on an input signal, input / output characteristics of the power amplifier and predistortion processing that performs the predistortion processing are performed. The input / output characteristics of the distortion amplifier are modeled using complex power series expansion, and the fundamental wave coefficient of the predistortion amplifier, that is, the back-off of the input signal of the predistortion amplifier is the adjacent channel leakage power of the power amplifier. The largest in the range where the ratio does not exceed the legal value It is achieved by the adaptive predistortion type distortion compensation power amplifier, characterized in that it is controlled as determined as.

The above-described object of the present invention, the adjacent channel leakage power ratio of the power amplifier ACPR, the legal value of the adjacent channel leakage power ratio when the ACPRr, back-off formula B ₁ number 1 of the predistortion amplifier By controlling to determine sequentially, it is achieved effectively.

Since the adaptive predistortion type distortion compensating power amplifier according to the present invention employs an algorithm based on complex power series expansion, it is about 1/1300 of the memory amount (about 130 kbit) required in the conventional table look-up method. (About 100 bits) can be reduced, and the circuit can be downsized.
Further, the convergence time of the distortion compensation operation in the conventional table lookup method can be shortened to about 1/1600.
Furthermore, by setting the back-off that has been set by experience and KAN so far by the control method based on the adjacent channel leakage power ratio, the adjacent channel leakage power ratio can be set to the legal value without relying on experience and KAN. Since the back-off can be set so as to maximize the transmission power within the range to be satisfied, there is an advantage that the power efficiency can be maximized.

同様に、歪補償回路１の入力xと出力yとの関係も複素べき級数を用いて（２）式のように表される。

ここで、式（２）を式（１）へ代入すると、適応プリディストーション型歪補償電力増幅器の入出力関係は、式（３）のように表される。

ここで、システム全体の係数Ckは、Ak及びBkを用いて（４）式のように表される。

前記歪補償回路１は、（４）式におけるC₃, C₅…がゼロになるように、後述のB₃, B₅…の値を計算することにより歪補償を行っている。 An embodiment of an adaptive predistortion type distortion compensating power amplifier according to the present invention will be described in detail with reference to the drawings.
FIG. 2 is a diagram modeling a distortion compensation algorithm based on the complex power series expansion adopted in the adaptive predistortion type distortion compensating power amplifier according to the present invention.
In FIG. 2, a baseband input signal x is input to the distortion compensation circuit 1 and given a distortion having a characteristic opposite to that of the nonlinear distortion of the power amplifier (also referred to as a target amplifier) 3, and then output to y, The signal y is input to the power amplifier 3 and cancels out with the nonlinear distortion characteristic of the power amplifier 3 to become a distortion compensated output z.
The relationship between the output z of the power amplifier 3 and the output of the distortion compensation circuit 1 (= input of the power amplifier 3) y is expressed as in equation (1) using a complex power series.

Similarly, the relationship between the input x and the output y of the distortion compensation circuit 1 is also expressed by equation (2) using a complex power series.

Here, when Expression (2) is substituted into Expression (1), the input / output relationship of the adaptive predistortion type distortion compensating power amplifier is expressed as Expression (3).

Here, the coefficient Ck of the entire system is expressed as in equation (4) using Ak and Bk.

The distortion compensation circuit 1 performs distortion compensation by calculating values of B ₃ , B ₅ ... Described later so that C ₃ , C ₅ .

FIG. 3 is a block diagram showing a configuration example of an adaptive predistortion type distortion compensating power amplifier according to the present invention. In FIG. 3, reference numeral 1 corresponds to the distortion compensating circuit in FIG. 1, and shows a back-off amplifier 11 and a predistortion. And an amplifier 12.
Further, 2 corresponds to the distortion compensation control unit in FIG. 1, and a K-th order complex power series coefficient learning unit 21 for calculating the coefficient Ak in the equation (1), Ak and backoff B1 (setting method will be described later) The K-order inverse function calculation unit 22 that calculates distortion compensation coefficients B3, B5,... From the above, the evaluation function from the adjacent channel leakage power ratio (ACPR) obtained from the output z of the power amplifier 3 and the legal value (ACPRr) of ACPR And an evaluation function calculation unit 23 for calculating (CF) and a back-off control unit 24 for sequentially determining the back-off B1 according to a predetermined formula so that the CF approaches zero.

次に、前記Ｋ次逆関数計算部２２におけるBk（k=3,5,…）は、（６）式に基づいて計算される。なお、（６）式は、（４）式においてC₃, C₅…をゼロとすることにより求められる。

次に、前記Ｋ次逆関数計算部２２で求められたBk（k=3,5,…）とB1とから、歪補償回路１において、（２）式に基づく歪補償が行われ、出力yが前記電力増幅器３に入力される。 The calculation of Ak in the K-th order complex power series coefficient learning unit 21 is obtained by solving simultaneous equations expressed by the following equation (5).

Next, Bk (k = 3, 5,...) In the Kth-order inverse function calculation unit 22 is calculated based on the equation (6). Note that (6) is obtained by a C _3, C ₅ ... zero in equation (4).

Next, distortion compensation based on the equation (2) is performed in the distortion compensation circuit 1 from Bk (k = 3, 5,...) And B1 obtained by the Kth-order inverse function calculation unit 22, and the output y Is input to the power amplifier 3.

この評価関数ＣＦは、ＡＣＰＲの測定値が法定値（ＡＣＰＲｒ）に近づくほどゼロに近づく。つまり、ＣＦをゼロに近づけるようにすれば隣接チャネル漏洩電力比が法定値を満たす範囲内で、電力効率を最大にすることが可能となる。
バックオフB1^dBと評価関数ＣＦとの関係は、下の（８）式で規定され、初めに適当な初期値B1^dB(1)を与え、次に、ＣＦがゼロに近づくようにB1^dB(2)を決め、これを繰り返して、バックオフB1^dBを逐次的に決定する。（８）式においてＮは繰り返し回数である。なお、上記（６）式におけるB1と（８）式のB1^dBとの関係は、以下の通りである。

なお、前記歪補償制御部２の各コンポーネントはハードウェア又はソフトウェアのいずれで構成してもよいことは言うまでもない。 On the other hand, in the evaluation function calculation unit 23, the evaluation function CF is calculated from the ACPR calculated from the output z of the power amplifier 3 and the ACPRr which is a legal value of the ACPR based on the equation (7).

The evaluation function CF approaches zero as the measured value of ACPR approaches the legal value (ACPRr). That is, if the CF is made close to zero, the power efficiency can be maximized within the range where the adjacent channel leakage power ratio satisfies the legal value.
The relationship between the back-off B1 ^dB and the evaluation function CF is defined by the following equation (8). First, an appropriate initial value B1 ^dB (1) is given, and then B1 ^dB ( 2) and repeat this to determine the backoff B1 ^dB sequentially. In the formula (8), N is the number of repetitions. The relationship between B1 in the above equation (6) and B1 ^{dB in the} equation (8) is as follows.

Needless to say, each component of the distortion compensation controller 2 may be configured by either hardware or software.

FIG. 4 is a diagram showing the convergence characteristics of the adjacent channel leakage power ratio (ACPR). As can be seen from this figure, in this embodiment, when distortion compensation up to the fifth order is performed (when K = 5). ) Regardless of the initial value of the backoff, it converges about 5 times (about 5 frames). That is, both the case where the initial value of the back-off is −12 dB ((a) in FIG. 4) and the case where the initial value of the back-off is 0 dB ((b) in FIG. 4) Has converged to a legal value of −45 dB ((c) in FIG. 4). This indicates that the convergence time is extremely short.

5A to 5C are diagrams showing output power spectra (computer simulation) when a W-CDMA signal is input to the adaptive predistortion type distortion compensating power amplifier according to the present invention. 5A shows the output characteristics of the power amplifier 3 alone, FIG. 5B shows the output characteristics of the predistortion amplifier 12, and FIG. 5C shows the output characteristics of the entire system. However, ACPRr = −45 dBc, and the backoff uses a convergence value of about −6 dB.
According to these figures, in the adaptive predistortion type distortion compensating power amplifier according to the present invention, the output power is maximized within a range in which the adjacent channel leakage power ratio (ACPR) keeps the legal value while reducing unnecessary out-of-band radiation. It can be seen that the power efficiency is maximized.

It is a figure showing the structure of a general adaptive predistortion type distortion compensation power amplifier. It is a figure for demonstrating the relationship of the input / output to a distortion compensation circuit and a power amplifier. It is a block diagram which shows the structural example of the adaptive predistortion type distortion compensation power amplifier which concerns on this invention. It is a figure which shows the convergence characteristic of adjacent channel leakage power ratio (ACPR). It is a figure which shows the output power spectrum of the signal of W-CDMA (power amplifier). It is a figure which shows the output power spectrum of the signal of W-CDMA (predistortion amplifier). It is a figure which shows the output power spectrum of the signal of W-CDMA (the whole system).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Distortion compensation circuit 2 Distortion compensation control part 3 Power amplifier 11 Back-off amplifier 12 Predistortion amplifier 21 Kth order complex power series coefficient learning part 22 Kth order inverse function calculation part 23 Evaluation function calculation part 24 Backoff control part

Claims (2)

Generates distortion compensation coefficients from the input signal and the feedback signal obtained by feeding back a part of the output signal that passed through the power amplifier, and removes non-linear distortion of the power amplifier by performing predistortion processing on the input signal In the adaptive predistortion type distortion compensating power amplifier,
The input / output characteristics of the power amplifier and the input / output characteristics of the predistortion amplifier that performs the predistortion processing are modeled using a complex power series expansion,
Control is performed so that the fundamental wave coefficient of the predistortion amplifier, that is, the back-off of the input signal of the predistortion amplifier, is determined to be the largest within a range where the adjacent channel leakage power ratio of the power amplifier does not exceed the legal value. An adaptive predistortion type distortion compensating power amplifier. When the adjacent channel leakage power ratio of the power amplifier is ACPR and the legal value of the adjacent channel leakage power ratio is ACPRr, control is performed so that the back-off B _{1 of the} predistortion amplifier is sequentially determined by the equation (1). The adaptive predistortion type distortion compensating power amplifier according to claim 1, wherein

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