JP2000022659A - Ofdm modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an orthogonal frequency division multiplex OFDM system modulator where nonlinear distortion is excellently eliminated from a transmission signal by compensating distortion corresponding to a temporal change in a nonlinear characteristic of a high output amplifier. SOLUTION: The OFDM modulator 100 is provided with an orthogonal demodulation means 40 that demodulates a transmission signal passing through a high output amplifier 62 with a nonlinear characteristic. A demodulation signal outputted from the orthogonal demodulation means 40 and an output of a pre-distortion circuit 21 that provides distortion in advance to the transmission signal are given to a modeling means 50. The modeling means 50 models the nonlinear characteristic of the high output amplifier 62 based on the received signal and reflects the result of modeling on the pre-distortion circuit 21. That is, the nonlinear characteristic changing every moment is dynamically obtained to excellently eliminate the nonlinear distortion from the transmission signal and pre-distortion in following to it is conducted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an orthogonal frequency division multiplexing quadrature modulator, and more particularly to an OFDM modulator having a function of effectively compensating for nonlinear distortion of a signal generated with high-power signal amplification.

[0002]

2. Description of the Related Art Orthogonal frequency division multiplexing (orthogonal frequency division multiplexing) has been conventionally used as a frequency multiplexing modulation method which is resistant to the interference of multipath interference and has high frequency use efficiency.
xing; hereinafter, referred to as OFDM).

[0003] An OFDM modulator encodes a digital data stream to be transmitted into multi-level digital data, modulates it with a plurality of carriers having mutually orthogonal frequencies, and frequency-multiplexes a large number of modulated signals.

[0004] The multiplexed modulated signal produced is amplified by a high-power amplifier and then sent out as a transmission signal to a transmission line.

However, since a high-power amplifier usually has nonlinear characteristics, a signal passing through the high-power amplifier is
Subject to non-linear distortion due to non-linear characteristics. in this case,
Not only is the transmission signal of the channel distorted, but also so-called adjacent channel interference in which unnecessary frequency components occur in adjacent channels. For this reason, a phenomenon that the communication of the adjacent channel is interrupted also occurs.

In particular, multicarrier transmission is more susceptible to non-linear distortion than a single carrier system because a plurality of subcarriers are commonly amplified in a transmitter.

As one of the techniques for solving this problem, there is, for example, Giard's patent (see US Pat. No. 4,462,001). In Gird's patent, a pre-distortion is performed in the quadrature modulator to pre-distort the transmission signal so that the distortion is eliminated when the signal passes through the nonlinear part. More specifically, Girdard's patent covers high-power amplifiers for non-linear distortion compensation, and eliminates non-linear distortion based on an envelope model or non-linear characteristic published by a high-power amplifier manufacturer. Performs pre-distortion.

By the way, the nonlinear characteristics of a high-output amplifier are as follows.
It has the property of changing over time instead of being constant. However, the Giard patent does not mention any measures against the aging of the nonlinear characteristic. Therefore, the Gird patent has a problem that nonlinear distortion of a transmission signal cannot be compensated for a long time.

In order to cope with the change with time of the characteristics of the non-linear section such as a high-output amplifier, the technique of Won et al. Has been proposed. (See, for example, Won et al. “An A
daptive Data Predistorter for Compensation of Nonl
inear Distortion in OFDM Systems, ”IEEE Trans. Co
mmun .. vol. 45, pp. 1167-1171, Oct. 1997) Fig. 1
9 and 20 are block diagrams for explaining a predistortion method proposed by Won in an OFDM system.

FIG. 19 is a schematic block diagram showing a configuration of a general OFDM system. Referring to FIG. 19, after a digital data sequence inputted is encoded by QAM encoder 800, a conversion circuit (hereinafter referred to as an S / P conversion circuit) 81 for converting serial data into a parallel data sequence.
At 0, it is converted into N parallel data strings equal to the number of carriers.

After these data strings are modulated by N carriers 820 having mutually orthogonal frequencies,
The data is serialized in a conversion circuit (hereinafter referred to as a P / S converter) 830 for converting a parallel data string into serial data, and
It becomes an FDM signal. The OFDM signal is supplied to the D / A conversion circuit 8
40, and are sent to a high output amplifier 860 via a quadrature modulation circuit 850. The high-power amplifier 860 having nonlinear characteristics
The transmission signal is amplified to a high output and transmitted to the transmission line 870.

Quadrature demodulation circuit 880 to QAM decoding circuit 9
Reference numeral 30 extracts a digital data string by performing demodulation which is the reverse operation of the above-described OFDM modulation.

FIG. 20 is a schematic block diagram of a system that applies predistortion to the OFDM modulator of FIG.

Referring to FIG. 20, P / S conversion circuit 830
After being pre-distorted by the address generation circuit 960 and the RAM table 970, the OFDM signal serialized by the above is sent to the D / A conversion circuit 840. That is, the digital signal xn to be transmitted is converted into a signal yn which has been previously distorted so that the transmission signal transmitted through the high power amplifier 860 and transmitted to the transmission line 870 does not have nonlinear distortion.

Further, a high-power amplifier 86 as a non-linear section
The signal that has passed 0 is returned to the original digital signal by the quadrature demodulation circuit 880 and the A / D conversion circuit 890. The addition circuit 950 obtains an error between this signal and the signal xn to be transmitted.

The technique of Won et al. Updates the data in the RAM table 970 by reflecting the error signal, thereby reducing nonlinear distortion even when the nonlinear characteristic of the high-power amplifier circuit 860 changes over time. It is intended to compensate.

[0017]

However, when pre-distortion is performed by the Won technique as described above, a difference based on data after passing through the non-linear section is subtracted from a signal to be transmitted. Therefore, the nonlinear distortion cannot be completely compensated. This is because, when the difference component passes through the non-linear section, nonlinear distortion is generated with respect to the difference, so that it is not possible to compensate for the distortion generated in the difference.

Since there is a time delay between the demodulated transmission signal compared with the addition signal 950 and the signal to be transmitted, the nonlinear distortion is completely compensated also from this point. It is difficult.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to compensate for nonlinear characteristics of a high-output amplifier in response to aging and to reduce nonlinear distortion from a transmission signal. OFDM that can be removed well
It is to provide a modulator.

[0020]

An OFDM according to claim 1 is provided.
An IDFT means for receiving the digital data sequence to be transmitted, converting the digital data sequence into multivalued digital data which is a complex digital signal comprising I-axis and Q-axis data, and further performing an inverse discrete Fourier transform to output a baseband signal; ,
Predistortion means for receiving a baseband signal and outputting a predistortion signal obtained by predistorting the baseband signal in order to compensate for non-linear distortion generated by the non-linear section, and quadrature modulation processing on the output of the predistortion means To generate an OFDM signal and send the OFDM signal to the non-linear section, and an orthogonal modulating section that receives the OFDM signal passed through the non-linear section and performs an orthogonal demodulation process which is a process opposite to the orthogonal modulation process. A demodulating unit; and a modeling unit that receives an output of the predistortion unit and a signal restored by the quadrature demodulating unit, obtains a nonlinear characteristic of the nonlinear unit, and outputs the nonlinear characteristic to the predistortion unit.

The OFDM modulator according to a second aspect of the present invention is the OFDM modulator according to the first aspect, further comprising delay means for receiving a predistortion signal, delaying the predistortion signal, and sending the predistortion signal to the modeling means.

The OFDM modulator according to a third aspect is the OFDM modulator according to the first aspect, wherein the predistortion signal is estimated by receiving the signal restored by the quadrature demodulation means, and the estimated predistortion is estimated. The apparatus further includes a transmission signal estimating means for sending the signal to the modeling means, and the modeling means obtains a nonlinear characteristic of the nonlinear part using the estimated predistortion signal and the signal restored by the quadrature demodulation means.

According to a fourth aspect of the present invention, there is provided the OFDM modulator according to the third aspect, wherein the IDFT means receives a digital data signal to be transmitted and converts the digital data signal into a multilevel digital signal. The apparatus further includes a multi-value number selecting means for selecting a multi-value number representing the number of states of the I-axis and Q-axis data of the data. Is set by gradually changing from a small value to a large value.

According to a fifth aspect of the present invention, there is provided the OFDM modulator according to the first aspect, wherein the predistortion unit includes an amplitude adjusting unit for adjusting the amplitude of the transmission signal.

The OFDM modulator according to claim 6 is the OFDM modulator according to claim 1, wherein the modeling means comprises:
Among the nonlinear characteristics of the nonlinear part, the inverse characteristic relating to the amplitude distortion between the input and output signals is obtained first, and then the inverse characteristic corrected so that the slope of the linear part of the inverse characteristic relating to the amplitude distortion becomes 1 is obtained. Ask.

[0026]

FIG. 1 is a block diagram of an embodiment of the present invention.
1 is a conceptual diagram illustrating a configuration of a terrestrial television broadcasting system to which an FDM modulator is applied.

Referring to FIG. 1, the terrestrial television broadcasting system transmits a transmission signal corresponding to a program from a transmitter 2 of a broadcasting station 1 to a radio tower 4 via a cable 3,
The program is provided to the home 5 by radiating radio waves based on transmission signals from the home.

The transmitter 2 has an OFDM modulator 6. In the OFDM modulator 6, an OFDM modulation process based on digital data corresponding to the program is executed.
A modulated signal is created. The transmitter 2 transmits the created O
The FDM modulated signal is transmitted to the radio tower 4 via the cable 3 as a transmission signal.

At home 5, a receiving antenna 7, a tuner 8
And a television receiver 9. When a radio wave is received by the receiving antenna 7, the received signal based on the received radio wave is subjected to OFDM demodulation in the tuner 8, and the original digital data is restored. The restored digital data is provided to the television receiver 9.
Thereby, the program can be viewed on the television receiver 9.

FIG. 2 shows an OF according to the first embodiment of the present invention.
FIG. 2 is a schematic block diagram illustrating a configuration of a transmitter 2 including a DM modulator 100.

Referring to FIG. 2, a transmitter 2 receives a digital data sequence and outputs an OFDM signal x (t).
It includes an M modulator 6 and a non-linear unit 60 that receives the output of the OFDM modulator 6, performs high-output amplification with non-linear distortion, and sends out a transmission signal y (t).

The OFDM modulator 100 receives a data sequence to be transmitted, converts the data sequence into a complex data signal by, for example, 16QAM coding, and further performs an Inverse Discrete Fourier Transform (IDF).
T) and xi as the I-axis data and the Q-axis data of the baseband signal multiplexed by orthogonal division frequency division.
The IDFT means 10 for outputting (t) and xq (t) and the baseband signals xi (t) and xq (t) for compensating for the nonlinear distortion generated in the nonlinear unit 60, and these signals are converted in advance. A pre-distortion means 20 for performing a process of distorting and outputting pre-distortion signals Pi (t) and Pq (t); and applying a quadrature modulation process to the pre-distortion signal to convert the OFDM signal x (t) into a nonlinear unit 6
A quadrature modulation means 30 for outputting to 0,
Receive the transmission signal y (t) passing through the orthogonal demodulation means 30 and perform orthogonal demodulation processing which is the reverse processing of the orthogonal modulation means 30 to output demodulated signals yi (t) and yq (t).
And predistortion signals Pi (t) and Pq (t)
And a demodulation signal yi (t) and yq (t) to obtain a non-linear characteristic of the non-linear section 60, and a modeling means 50 for outputting the obtained characteristic to the pre-distortion means 20.

FIG. 3 is a schematic block diagram showing the configuration of the OFDM modulator 100 in more detail.

Referring to FIG. 3, IDFT means 10 receives a digital data string to be transmitted and, for example, 16QA
A multi-valued digital signal zn (= an + j) which is a complex data signal having I-axis and Q-axis data by M encoding
bn), receives the multi-valued digital signal zn, and receives an OFD signal.
S / to convert into N parallel data equal to the number of M carriers
A P conversion circuit 13, an IDFT circuit 14 for performing an N-point inverse discrete Fourier transform (IDFT) on the multi-valued digital data zn to perform frequency multiplexing with N carrier waves having mutually orthogonal frequencies, and an IDFT circuit And a guard interval adding circuit 15 that receives the output of 14 and adds a guard interval period to reduce the influence of multipath.

FIG. 4 is a conceptual diagram for explaining a guard interval. As shown in FIG. 4, the guard interval is a signal that is added so as not to cause inter-symbol interference, which is a major cause of decoding error, even when a signal is subjected to multipath interference and a delay time occurs. The guard interval is added by repeating a partial waveform of one end of the data in the effective symbol period corresponding to the transmission data. Thereby, adjacent symbols (in FIG. 4,
Even when an overlapping section occurs between (a) the main wave and (b) the multipath wave), if this section is within the guard interval, the demodulated signal can be obtained only by overlapping the data of the same effective symbol. Therefore, a decoding error due to intersymbol interference does not occur.

Referring again to FIG. 3, IDFT means 10
Further includes a P / S conversion circuit 16 that receives the parallel data string output from the guard interval addition circuit 15 and converts it into a serial data string.

The real part obtaining circuit 17a and the imaginary part obtaining circuit 17b receive the output of the P / S conversion circuit 16 and receive baseband signals xi (t), xq corresponding to the I-axis and Q-axis components of the complex data signal. (T) is generated. The baseband signals xi (t) and xq (t) are output to the pre-distortion circuit 21.

FIG. 5 is a block diagram for explaining the configuration of the pre-distortion circuit 21 in more detail. The baseband signals xi (t) and xq (t) undergo pre-distortion based on the instruction of the pre-distortion signal calculation circuit 24 via the address generation circuit 22, the I-axis RAM table 23a and the Q-axis RAM table 23b. And the predistortion signals Pi (t), P
It is converted to q (t).

Predistortion signal calculation circuit 24
Are the baseband signals xi (t), xq such that the nonlinear distortion disappears after the transmission signal passes through the nonlinear unit 60.
This is a circuit that achieves a function equivalent to Giard's patent described in the section of "Prior Art" in order to predistort (t). Predistortion signal calculation circuit 2
4 includes a model derivation circuit 5 by a method described later.
The non-linear characteristic of the non-linear section 60 obtained in step 2 is given. The pre-distortion signal calculation circuit 24 performs pre-distortion following the change with time of the nonlinear characteristic of the nonlinear unit 60 by reflecting the given nonlinear characteristic in the I-axis RAM table 23a and the Q-axis RAM table 23b.

Referring again to FIG. 3, the pre-distortion signal Pi, which is the output of the pre-distortion circuit 21,
(T) and Pq (t) are provided to multiplication circuits 31 and 32 included in the quadrature modulation means 30, respectively. The multiplication circuit 31 to which the pre-distortion signal Pi (t) is given
The multiplication circuit 32 modulates the amplitude of the carrier wave cos (ωct) with the predistortion signal Pi (t) and receives the predistortion signal Pq (t).
Is to modulate the amplitude of the carrier wave sin (ωct) by the predistortion signal Pq (t). As a result, the outputs x1 (t), x2 of the respective multiplication circuits 31, 32
(T) is as shown in the following equations (1) and (2). Note that ωc is the carrier frequency.

X1 (t) = Pi (t) cos (ωct) (1) x2 (t) = Pq (t) sin (ωct) (2) Each signal x1 (t), x2 (t) Is supplied to the addition circuit 33, and the addition is performed in the addition circuit 33. as a result,
As shown in the following equation (3), an OFDM signal x (t) is created.

X (t) = x1 (t) + x2 (t) = Pi (t) cos (ωct) + Pq (t) sin (ωct) (3) The nonlinear unit 60 calculates the OFDM signal x (t ) Includes an up-converter 61 that converts the frequency into an RF band, and a high-output amplifier 62 that amplifies the OFDM signal xrf (t) after the frequency conversion.

O after being amplified by the high power amplifier 62
The FDM signal yrf (t) is transmitted to the cable 3.
The high-output amplifier 62 has nonlinear characteristics, and the OFDM signal yrf (t) that has passed through the high-output amplifier is subjected to nonlinear distortion caused by the nonlinear characteristics.

The nonlinear unit 60 further includes a high-power amplifier 62
And a down-converter 63 for lowering the frequency of the OFDM signal yrf (t) after being amplified by IF to the IF band. The OFDM signal y (t) that has been frequency-converted by the down-converter 63 is provided to the quadrature demodulation means 40.

The quadrature demodulation means 40 comprises two multiplication circuits 4
1,42. The OFDM signal y (t) is divided into two and supplied to the multiplication circuits 41 and 42, respectively. Each of the multiplication circuits 41 and 42 applies a carrier c to the OFDM signal y (t).
os (ωct) and sin (ωct) are respectively multiplied. As a result, the OFDM signal y (t) is demodulated, and demodulated signals yi (t) and yq (t) are generated. Demodulated signals yi (t) and yq (t) are provided to modeling means 50.

The modeling means 50 outputs the demodulated signal yi
(T), yq (t) and predistortion signal P
A storage device (hereinafter referred to as a RAM) 51 that receives i (t) and Pq (t) and outputs these data to a model derivation circuit 52 each time the data is stored up to a predetermined number of data.
And the data sequence Pi (k), P output from the RAM 51
a model deriving circuit 52 that receives q (k), yi (k), and yq (k), appropriately models the nonlinear characteristics of the high-power amplifier 62, and provides the modeling result to the predistortion circuit 21.

Next, the model deriving process in the model deriving circuit 52 will be described. In the following, for the sake of convenience, after describing the nonlinear characteristic model of the nonlinear unit 60,
The modeling process will be described.

In the non-linear section 60, the input a (t) co
The non-linear characteristic with respect to s (ωct) is represented by an amplitude distortion F (a (t)) and a phase distortion G (a
(T)). Here, the amplitude distortion F (a (t))
Represents the degree of distortion relating to the amplitude between the input and output signals in the nonlinear unit 60, and represents the phase distortion characteristic G (a
(T)) indicates the degree of distortion related to the phase between input and output signals in the nonlinear unit 60.

F (a (t)) cos (ωct + G (a (t))) (4) In the orthogonal model, the input a (t) cos (ω
The output for ct) is expressed as in the following equation (5).

F (a (t)) cos (G (a (t))) cos (ωct) −F (a (t)) sin (G (a (t))) sin (ωct) 5) In the first embodiment, the OFDM signal x (t) which is the input signal of the nonlinear unit 60 is as shown in the above equation (3). Here, as shown in FIG. 6, θp (t) and rp
When (t) is defined, the expression (3) can be converted into the following expression (6).

X (t) = Pi (t) cos (ωct) + Pq (t) sin (ωct) = rp (t) cos (ωct−θp (t)) (6) Therefore, this input x When (t) is input to the nonlinear unit 60 according to the orthogonal model, the output O
The FDM signal y (t) is represented by the following equation (7) from the equation (5).

Y (t) = F (rp (t)) cos (G (rp (t))) cos ｛ωct−θp (t)｝ − F (rp (t)) sin (G (rp (t) ))) Sin {ωc t−θp (t)} (7) where Si (t) = F (rp (t)) cos (G (rp (t))) (8) Sq (t) = F (rp (t)) sin (G (rp (t))) (9) Equation (7) is converted into the following equation (10).

Y (t) = Si (t) cos ｛ωct−θp (t)｝ − Sq (t) sin ｛ωct−θp (t)｝ = ｛Si (t) cos (θp (t)) + Sq (t) sin (θp (t))｝ cos (ωct) + ｛Si (t) sin (θp (t)) − Sq (t) cos (θp (t))｝ sin (ωct) = F (Rp (t)) cos ｛θp (t) -G (rp (t))｝ cos (ωct) + F (rp (t)) sin ｛θp (t) -G (rp (t))｝ sin (Ωct) (10) Therefore, the OFDM signal y expressed by the equation (10)
(T) is replaced by two carrier waves cos (ωct) and sin (ωc
If demodulation is performed using t), the following equation (11) and (1)
As shown in equation (2), each demodulated signal yi (t), yq (t)
Is obtained.

Yi (t) = F (rp (t)) cos {θp (t) −G (rp (t))} (11) yq (t) = F (rp (t)) sin ｛θp ( t) −G (rp (t))｝ (12) Therefore, the pre-distortion signals Pi (t) and Pq
FIG. 7 shows the relationship between (t) and the demodulated signals yi (t) and yq (t). Therefore, the pre-distortion signals Pi (t), Pq (t) and the demodulated signal yi
If there is one set of (t) and yq (t), the amplitude distortion characteristic F (rp (t)) and the phase distortion for one amplitude value rp (t) as shown in the following equations (13) and (14). Characteristic G (rp
(T)).

F (rp (t)) = {yi (t) ² + yq (t) ² } (13) G (rp (t)) = θy (t) −θp (t) (14) The amplitude distortion characteristic F (rp (t)) described above is
It is obtained as point data (rp (t), F (rp (t))) when the horizontal axis and the vertical axis are rp (t) and F (rp (t)).

However, in the pre-distortion circuit 21, the characteristic relating to the amplitude necessary for distorting the baseband signals xi (t) and xq (t) is an amplitude distortion characteristic F (rp
This is the inverse characteristic of (t)). Therefore, the model derivation circuit 52 does not calculate the point data (rp (t), F (rp (t))), but sets the horizontal axis and the vertical axis to F (rp
(T)) and point data (F (rp (t)), rp (t)) for rp (t).

For the phase distortion characteristic G (rp (t)), which is a characteristic relating to the phase, the horizontal axis and the vertical axis respectively indicate
Point data (rp (t), G (rp (t))) for rp (t) and G (rp (t)) is obtained.

The model deriving circuit 52 calculates the point data (F (rp (t)), rp (t)) obtained as described above,
By accumulating (rp (t), G (rp (t))) by a predetermined number of samples and obtaining an approximate curve of each point sequence, the inverse characteristic of the amplitude distortion characteristic F (rp (t)) and the phase distortion are obtained. Characteristic G (rp
(T)).

Specifically, the RAM 51 stores the signals Pi
(T), Pq (t), yi (t), yq (t) are accumulated, and when a predetermined number of samples is reached, Pi (k), Pq
(K), yi (k), and yq (k) are output to the model derivation circuit 52. The model deriving circuit 52 calculates the Pi
From (k), Pq (k), yi (k), yq (k), point data (F (rp (k)), rp (k)), (rp (k), G
(Rp (k))) is obtained as in the examples shown in FIGS.

Then, the point data (F (rp (k)), rp
When (k)) and (rp (k), G (rp (k))) are accumulated for a predetermined number of samples, the model deriving circuit 52
While taking in each signal to M51 is prohibited, a plurality of point data (F (rp (k)), rp (k)), (rp (k),
G (rp (k))) is obtained.

The approximation curve may be determined by, for example, a single polynomial or a spline function connecting a plurality of polynomials. Regardless of the approximation method, it is necessary to determine the function by the least square method so that the approximate curve approaches the true nonlinear characteristic. Thus, the inverse characteristic of the amplitude distortion characteristic F (rp (k)) and the phase distortion characteristic G (rp (k)) are obtained.

The model deriving circuit 52 calculates the inverse characteristic of the obtained amplitude distortion characteristic F (rp (k)) and the phase distortion characteristic G (rp (rp (k))).
(K)) to the pre-distortion circuit 21. The pre-distortion circuit 21 generates a reverse characteristic and a phase distortion characteristic G (rp (k) of the given amplitude distortion characteristic F (rp (k)).
(K)), the baseband signals xi (t), x
Distorts q (t).

With the above operation, the pre-distortion circuit 21 can achieve pre-distortion according to the nonlinear characteristic of the nonlinear unit 60.

Each signal P of a predetermined number of samples is stored in the RAM 51.
Each time i (t), Pq (t), yi (t) and yq (t) are stored, the model derivation circuit 52 repeatedly executes the above-described processing. Therefore, the pre-distortion circuit 21 can perform the pre-distortion following the change with time of the nonlinear characteristic of the nonlinear unit 60.

The pre-distortion signals Pi (t) and Pq (t) necessary for performing the first modeling
While accumulating the demodulated signals yi (t) and yq (t), the predistortion circuit 21 outputs the baseband signals xi (t) and xq (t) as they are without distortion.

As described above, according to the first embodiment, the nonlinear characteristic is dynamically obtained by following the change with time, and the pre-distortion is performed based on the obtained nonlinear characteristic. Therefore, it is possible to cope with a change with time of the nonlinear characteristic, and it is possible to perform a good pre-distortion for a long period of time.

Therefore, by removing nonlinear distortion favorably from a transmission signal, high communication quality can be obtained for a long period of time.

FIG. 10 is a schematic block diagram showing a configuration of a transmitter to which OFDM modulator 200 according to the second embodiment of the present invention is applied. 10, parts having the same functions as those of the OFDM modulator 100 are denoted by the same reference numerals.

In the OFDM modulator 100, the predistortion signals Pi (t) and Pq (t) output from the predistortion circuit 21 to the modeling means 50.
Is given as it is, but this OF
In the DM modulator 200, the pre-distortion signals Pi (t) and Pq (t) are delayed by a delay circuit 65 and then applied to the modeling means 50.

Referring to FIG. 10, predistortion signal Pi output from predistortion circuit 21
(T) and Pq (t) are provided to the delay circuit 65. The delay circuit 65 is connected to the pre-distortion signal Pi
(T) and Pq (t) are given to the modeling means 50 with a delay of time Δt.

The time Δt corresponds to the pre-distortion signal P
i (t−Δt) and Pq (t−Δt) are output from the predistortion circuit 21 and the demodulated signals yi (t) respectively corresponding to the predistortion signals Pi (t−Δt) and Pq (t−Δt). , Yq (t) are generated.

Accordingly, the pre-distortion signals Pi (t-Δt), Pq which are not temporally shifted with respect to the demodulated signals yi (t), yq (t) are provided to the modeling means 50.
(T−Δt).

According to this configuration, the modeling means 50
Can determine the nonlinear characteristic in consideration of the signal delay, and thus can obtain a more accurate nonlinear characteristic than the OFDM modulator 100 of the first embodiment.

Therefore, compared with the OFDM modulator 100, the nonlinear distortion of the transmission signal can be more favorably removed.

FIG. 11 is a block diagram showing a configuration of a transmitter to which the OFDM modulator 300 according to the third embodiment of the present invention is applied. In FIG. 11, an OFDM modulator 1
Parts having the same functions as 00 are denoted by the same reference numerals.

The OFDM modulator 200 according to the second embodiment
In the OFDM modulator 300, a delay circuit 65 is provided to cope with the signal delay. On the other hand, in the OFDM modulator 300, the predistortion signals Pi (t-Δt), By estimating Pq (t−Δt), it is possible to cope with the signal delay without providing the delay circuit 65.

Referring to FIG. 11, OFDM modulator 300
Includes a transmission signal estimation circuit 70 in place of the delay circuit 65. The transmission signal estimating circuit 70 receives the demodulated signals yi (k) and yq (k) output from the RAM 51 as input signals, and estimates a predistortion signal based on the demodulated signals yi (k) and yq (k). The estimated pre-distortion signals (hereinafter referred to as estimated pre-distortion signals) Pi ′ (k) and Pq ′ (k) are provided to the model deriving circuit 52. The model deriving circuit 52 receives the given estimated pre-distortion signals Pi ′ (k), Pq ′
Non-linear characteristics are obtained based on (k) and demodulated signals yi (k) and yq (k).

FIG. 12 is a schematic block diagram showing a configuration of the transmission signal estimation circuit 70. Referring to FIG. 12, transmission signal estimating circuit 70 outputs demodulated signal yi from RAM 51.
(K), yq (k), a complex number conversion circuit 71 for converting yi (k) into a complex data signal having I-axis data and yq (k) as Q-axis data, and a parallel circuit receiving the complex data signal An S / P conversion circuit 72 for converting data into data, a guard interval removal circuit 73 for removing the guard interval given by the guard interval addition circuit 15 from the demodulated signal and extracting valid transmission data, and a guard interval removal circuit 73 Discrete Fourier Transform (Di
a DFT circuit 74 for performing a discrete Fourier Transform (hereinafter referred to as DFT); and an I / O of a multi-valued digital signal zn (= an + jbn) which is an output of the complex number conversion circuit 12 in the IDFT means 10 upon receiving the output of the DFT circuit 74
A determination circuit 75 for determining values corresponding to the axis data an and the Q axis data bn;

FIG. 13 is a conceptual diagram for explaining the operation of the determination circuit 75. For example, when 64-level QAM encoding is performed by the mapping circuit 11, the I-axis component an and the Q-axis component bn of the above-described multi-level digital data zn are each data of a multi-level number “8”. .

In this case, for example, digital data a
The eight possible values of n and bn are "-14, -10,
When “−6, −2, 2, 6, 10, 14” is set, the input / output characteristics of the determination circuit 75 for each of the I axis and the Q axis are as shown in FIG.

As an example, 0 <an <4, 4 <bn <8
Is given from the determination circuit, "2 + j
6 is output, and if data of 4 <an <8, 0 <bn <4 is given, a signal of “6 + j2” is output.

As described above, by associating one output value with an input value within a certain range, a deviation from the original multi-valued digital data zn is compensated.

Referring again to FIG. 12, transmission signal estimating circuit 70 is further obtained by receiving the output of determination circuit 75 and performing an inverse discrete Fourier transform (IDFT) with IDFT circuit 76 and inverse discrete Fourier transform. A guard interval addition circuit 77 for adding a guard interval again to the signal,
A P / S conversion circuit 78 which receives the output of the guard interval addition circuit 77 and converts it into serial data.

Based on the obtained serial data sequence, the estimated signals xi 'of the baseband signals xi (k) and xq (k) are output from the real part obtaining circuit 79a and the imaginary part obtaining circuit 79b as outputs.
(K) and xq '(k) are obtained.

Further, the obtained estimated signals xi '(k),
By providing xq ′ (k) to the pre-distortion circuit 25, the transmission signal estimation circuit 70 can obtain estimated pre-distortion signals Pi ′ (k) and Pq ′ (k) as the output of the pre-distortion circuit 25. Here, the configuration and function of the pre-distortion circuit 25 are the same as those of the pre-distortion circuit 21 in the pre-distortion means 20.

In this case, the estimated pre-distortion signals Pi ′ (k), Pq ′ (k) are converted into demodulated signals yi (k),
yq (k), the demodulated signal yi
There is no time lag between (k) and yq (k).

Therefore, the pre-distortion circuit 2
1, the predistortion signal Pi output from
As in the case where (t) and Pq (t) are delayed by the delay circuit 65, it is possible to cope with a signal delay. Therefore, more accurate nonlinear characteristics can be obtained.
Therefore, non-linear distortion of the transmission signal can be more favorably removed.

FIG. 14 is a schematic block diagram showing a configuration of a transmitter to which the OFDM modulator 400 according to the fourth embodiment of the present invention is applied. 14, parts having the same functions as those of the OFDM modulator 100 are denoted by the same reference numerals.

In the configuration of the OFDM modulator 300 according to the third embodiment, any demodulated signal yi
It is assumed that the predistortion signals Pi (t) and Pq (t) can be estimated from (t) and yq (t).

However, when the nonlinear distortion in the nonlinear unit 60 is large, the configuration of the OFDM modulator 300 allows accurate estimation predistortion Pi ′ (t), Pq ′.
It is difficult to obtain (t).

Specifically, actually, the baseband signal x
Even when i (t) = 2, if the nonlinear distortion is large, the demodulated signal yi (t) obtained by demodulation may be 6 in some cases. In this case, in the above-described determination circuit 75, xi '(t)
= 6, and an estimated pre-distortion signal Pi ′ (t) corresponding to the result of the erroneous estimation is output.

Referring to FIG. 14, OFDM modulator 400
Further includes a multi-value number selection circuit 80 in order to improve such a problem of erroneous estimation.

The operation of the multi-value number selection circuit 80 will be described below. In the determination circuit 75 described above, for example, when the original baseband signal xi (t) is “2”, the digital data xi (t) can be accurately estimated.
Only when 0 <yi (t) <4. That is, nonlinear distortion can be tolerated only up to the level of ± 2.

On the other hand, input digital data xi
(T) and xq (t) are, for example, "-12, -4, 4,
In the case of four values of “12”, the input / output characteristics of the determination circuit 75 can tolerate up to ± 4 levels of nonlinear distortion as shown in FIG. 15, for example.

That is, the digital data xi (t), x
If the number of multivalues of q (t) is small, the allowable range of the nonlinear distortion increases.

Therefore, the multi-value number selection circuit 80 converts the digital data sequence into the I-axis and Q-axis data values of the multi-valued digital signal in the mapping circuit 11, which is the multi-valued number of states of the I-axis and Q-axis data. The number is switched as needed.

Specifically, when the OFDM modulator 400 starts operating, the multi-level number selection circuit 80 gives a small multi-level number setting to the mapping circuit 11. This allows
The OFDM modulator 400 outputs an accurate estimated pre-distortion signal Pi ′ (t), Pq ′ even if the nonlinear distortion is large.
(T) can be obtained. Here, the multi-value number of the multi-value digital data set in the multi-value number selection circuit 80 is determined by the transmission signal estimation circuit 70.

More specifically, the transmission signal estimation circuit 70
Adopts a small value as the first multi-valued number, adopts a large value as the second and subsequent multi-valued numbers, and simultaneously changes the multi-valued number of the determination circuit 75 to the same value.

For example, the transmission signal estimating circuit 70 sets the multi-level number “2” to the multi-level number selection circuit 80 at the time of the first modeling, and at the time of the second and subsequent modelings, The multi-level number selection circuit 80 sets the multi-level number “8”.

As described above, by setting a small multilevel number first, the nonlinear distortion is large and the demodulated signal yi
Even if (t) and yq (t) are greatly distorted, the probability that the transmission signal estimation circuit 70 can obtain accurate estimated predistortion signals Pi '(t) and Pq' (t) increases.

Therefore, even if the multi-valued number is returned to a large value from the second modeling, the probability of obtaining accurate estimated predistortion signals Pi '(t) and Pq' (t) increases. Therefore, even if the influence of the nonlinear distortion in the nonlinear unit 60 is large, accurate estimated predistortion signals Pi ′ (t) and Pq ′ (t) can be obtained.

The method of setting the multi-valued number is changed every time modeling is performed, for example, as “2” → “4” → “8” → “8” → hereinafter “8”. Is also good.

Also, "2" → "2" → "4" → "4" →
It may be changed whenever modeling is performed, such as “8” → “8” → hereinafter “8”.

As described above, by changing the multilevel number stepwise over a plurality of times, more accurate estimated predistortion signals Pi '(t) and Pq' (t) can be obtained.

FIG. 16 is a schematic block diagram showing a configuration of a transmitter to which the OFDM modulator 500 according to the fifth embodiment of the present invention is applied. In FIG. 16, the same reference numerals are used for portions having the same functions as those of the OFDM modulator 100.

Referring to FIG. 16, OFDM modulator 500
Further includes amplitude adjustment circuits 91 and 92.

Up to now, the OFDM modulators 100 to 400
In, in determining the inverse characteristic of the amplitude distortion characteristic F (rp (t)), no consideration was given to its domain.

However, the amplitude distortion characteristic F (rp (t)) in which the amplitude of the output signal is saturated as the amplitude of the input signal increases.
When compensating for the non-linear part having, it is necessary to consider the domain.

In the OFDM modulator 500, the amplitude adjustment circuits 91 and 92 adjust the amplitude at the time of the first modeling in order to satisfy the conditions required for the definition area.

FIG. 17 is a conceptual diagram for explaining the functions of amplitude adjusting circuits 91 and 92. Referring to FIG. 17, the amplitude distortion characteristic indicates the amplitude of the output signal with respect to the amplitude of the signal input to the nonlinear unit.

Here, the maximum value of the absolute value rx (t) of the vector (xi (t), xq (t)) without amplitude adjustment is defined as r3, and the vector (yi (t), yq (t)) Ry (t) is the absolute value of (rx (t), ry
(T)) is distributed near the curve OB. The inverse function of this curve OB is curve OA.

For example, the vectors (xi (t), xq
If the absolute value of (t)) is r1, the predistortion circuit 21 outputs the vector (Pi) having an absolute value of r2.
(T), Pq (t)). Similarly, the pre-distortion circuit 21 has an absolute value of r3 (xi
When (t), xq (t)) is input, it is necessary to output (Pi (t), Pq (t)) whose absolute value is r4.

Therefore, the curve OC is required as the inverse characteristic instead of the curve OA. However, in order to obtain the curve OC, the curve OD needs to be obtained in advance. That is, it is necessary to adjust the amplitude so that the maximum value of rx (t) becomes r4 which is larger than r3.

The amplitude adjusting circuits 91 and 92 are circuits for adjusting the amplitude in response to the above-mentioned problems.

More specifically, amplitude adjusting circuits 91 and 92 each composed of an operational amplifier or an attenuator are arranged in a stage preceding the predistortion circuit 21 and (xi (t), xq (t)) used for the first modeling is stored in a memory. While storing in
The amplitude of the input signal is increased so that the maximum value of rx (t) becomes r4. After the above operation is completed, the increase in the amplitude is stopped.

Further, as the arrangement of the amplitude adjusting circuits 91 and 92, a stage following the predistortion circuit 21 or a stage following the quadrature modulation means 30 can be considered.

As described above, according to the OFDM modulator 500, even when the high-output amplifier 62 having the input / output characteristic in which the amplitude of the output signal is saturated is used, the domain required for the pre-distortion is reduced. Amplitude distortion characteristic F (rp
The inverse characteristic of (t)) can be obtained. As a result, good nonlinear distortion compensation can be stably performed.

FIG. 18 is a block diagram showing a configuration of a transmitter to which an OFDM modulator 600 according to the sixth embodiment of the present invention is applied. 18, portions having the same functions as those of the OFDM modulator 100 are denoted by the same reference numerals.

Referring to FIG. 18, OFDM modulator 600
Further includes a gain adjustment circuit 95 (hereinafter, referred to as an AGC circuit) between the nonlinear unit 60 and the quadrature demodulation means 40.

In the OFDM modulators 100 to 500, no particular consideration is given to gain adjustment.
The M modulator 600 models a nonlinear characteristic in consideration of gain adjustment.

More specifically, an AGC (Au) for gain adjustment is provided before the multipliers 41 and 42 of the quadrature demodulator 40.
to Gain Control) circuit 95 is provided.

The pre-distorted pre-distortion signals Pi (t) and Pq (t) pass through the non-linear section 60 where the amplification process is performed, and then the AGC circuit 9
5 given. The AGC circuit 95 amplifies the demodulated signal y (t), which is the output of the downconverter 63, and supplies the signal to the quadrature demodulation means 40 so that the total gain of the AGC circuit 95 and the non-linear section becomes “1”.

However, even if the AGC circuit 95 performs gain adjustment at this position, the demodulated signal y to be adjusted is adjusted.
Since the amplitude of (t) is distorted, gain adjustment cannot be performed accurately.

Therefore, OFDM modulator 600 uses AGC
In addition to the gain adjustment by the circuit 95, the model derivation circuit 52
Has a function of performing gain adjustment.

In OFDM modulator 600, model deriving circuit 52 outputs demodulated signal yi whose amplitude is not distorted.
(T) and yq (t) are extracted, and information necessary for amplitude adjustment is obtained from the extracted demodulated signals yi (t) and yq (t).

More specifically, the model derivation circuit 52
A portion that can be regarded as a straight line is extracted from the curve representing the inverse characteristic of the amplitude distortion characteristic F (rp (t)) already shown in FIG.

Thereafter, all the demodulated signals yi (t) and yq (t) are multiplied by a constant so that the slope of this range becomes “1”. As a result, the demodulated signal yi whose amplitude is not distorted
The gain for (t) and yq (t) is “1”. Thus, gain adjustment can be performed in consideration of the influence of amplitude distortion.

According to this configuration in which the model deriving circuit 52 is provided with the above-described function to perform amplitude adjustment, gain adjustment can be performed in consideration of the influence of amplitude distortion, so that more accurate nonlinear characteristics can be obtained. As a result, the effect of the nonlinear distortion can be more effectively removed.

Although the six embodiments of the present invention have been described above, it goes without saying that the present invention can take other embodiments.

For example, in the OFDM modulators of the first to sixth embodiments, the modulation method of each carrier is set to 16QAM.
64QAM, 256QAM or QPSK can be applied.

In the first to sixth embodiments, the case where the present invention is applied to a terrestrial television broadcasting system has been described as an example. However, the present invention can be easily applied to other systems such as a cable television. be able to.

That is, the present invention can be applied to any system that transmits a transmission signal subjected to OFDM modulation via a nonlinear unit.

The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[0134]

According to the OFDM modulator of the present invention, the nonlinear characteristic of the nonlinear portion is dynamically obtained by following a change with time, and the signal is distorted in advance by using the obtained nonlinear characteristic. Therefore, even if the nonlinear characteristic of the nonlinear part changes,
Following the change over time, nonlinear distortion can be satisfactorily removed from the transmission signal. Therefore, high communication quality can be maintained for a long time.

The OFDM modulator according to a second aspect of the present invention includes, in addition to the OFDM modulator according to the first aspect, a delay circuit that delays a predistortion signal and supplies the predistortion signal to a modeling circuit. Therefore, it is possible to eliminate a time difference between signals required for obtaining the non-linear characteristic, and to obtain the non-linear characteristic more accurately. Therefore, nonlinear distortion can be better removed from the transmission signal, and communication quality can be further improved.

The OFDM modulator according to claim 3 estimates a signal output from the predistortion means based on a signal obtained by demodulating a signal output from the non-linear section. Therefore, no time difference occurs between signals required for modeling, and the same effect as that of the OFDM modulator according to the second aspect can be obtained without using a delay circuit.

The OFDM modulator according to claim 4 further includes a multi-level number selection circuit for selecting a multi-level number corresponding to the number of states of the I-axis and Q-axis data of the multi-level digital data. Therefore, even if the signal restored by the demodulation means is distorted, the original signal can be satisfactorily estimated by the transmission signal estimation circuit. Therefore, the non-linear characteristic can be obtained more accurately, so that the non-linear distortion can be more properly removed from the transmission signal, and as a result, the communication quality can be further improved.

The OFDM modulator according to claim 5 further includes an amplitude adjustment circuit for adjusting the amplitude of a signal to be sent to the non-linear section.

Therefore, even when a high-output amplifier having an input / output characteristic in which the amplitude of an output signal is saturated is used, distortion compensation can be performed well, and communication quality can be further improved. .

In the OFDM modulator according to the sixth aspect, the model derivation circuit and the AGC circuit perform total gain adjustment in consideration of the influence of amplitude distortion so that the total gain of the non-linear section and the AGC circuit becomes one. Can do it. Therefore, it is possible to satisfactorily remove nonlinear distortion while performing gain adjustment in consideration of the influence of amplitude distortion and maintain communication quality.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a configuration of a terrestrial television broadcasting system to which an OFDM modulator of the present invention is applied.

FIG. 2 is a block diagram illustrating a configuration of a terrestrial television broadcasting system transmitter including the OFDM modulator 100 according to the first embodiment of the present invention.

FIG. 3 is a schematic block diagram illustrating a configuration of the OFDM modulator 100 according to the first embodiment of the present invention.

FIG. 4 is a conceptual diagram for explaining a guard interval.

FIG. 5 is a block diagram showing a configuration of a pre-distortion circuit 21 in more detail.

FIG. 6 is a diagram illustrating definitions of an amplitude rp (t) and a phase θp (t) of a predistortion signal.

FIG. 7 shows pre-distortion signals Pi (t) and Pq.
FIG. 3 is a conceptual diagram for explaining the relationship between (t) and the amplitude and phase of demodulated signals yi (t) and yq (t).

FIG. 8 is a conceptual diagram illustrating an example of an inverse characteristic of the amplitude distortion characteristic.

FIG. 9 is a conceptual diagram illustrating an example of a phase distortion characteristic.

FIG. 10 is a schematic block diagram illustrating a configuration of an OFDM modulator 200 according to a second embodiment of the present invention.

FIG. 11 is a schematic block diagram illustrating a configuration of an OFDM modulator 300 according to a third embodiment of the present invention.

FIG. 12 is a block diagram showing the configuration of a transmission signal estimation circuit 70 in more detail.

FIG. 13 is a conceptual diagram illustrating an example of a relationship between an input and an output of a determination circuit 75.

FIG. 14 is a schematic block diagram illustrating a configuration of an OFDM modulator 400 according to a fourth embodiment of the present invention.

FIG. 15 is a conceptual diagram for explaining the operation of the determination circuit 75 when the multi-value number is set to a small value.

FIG. 16 is a schematic block diagram illustrating a configuration of an OFDM modulator 500 according to a fifth embodiment of the present invention.

FIG. 17 is a conceptual diagram for explaining the operation of the amplitude adjustment circuits 91 and 92.

FIG. 18 is a schematic block diagram illustrating a configuration of an OFDM modulator 600 according to a sixth embodiment of the present invention.

FIG. 19 is a block diagram illustrating a configuration of a general OFDM modulation system.

FIG. 20 is a schematic block diagram for explaining a pre-distortion method according to Won's technique.

[Explanation of symbols]

2 transmitter 6,100,200,300,400,500,600
OFDM modulator 10 IDFT means 11 Mapping circuit 12, 71 Complex number conversion circuit 13, 72 S / P conversion circuit 14, 76 IDFT circuit 15, 77 Guard interval addition circuit 16, 78 P / S conversion circuit 17a, 79a Real part acquisition circuit 17b, 79b Imaginary part acquisition circuit 20 Predistortion means 21, 25 Predistortion circuit 22 Address generation circuit 23a I axis RAM table 23b Q axis RAM table 24 Predistortion signal calculation circuit 30 Quadrature modulation means 31, 32, 41, 42 Multiplication circuit 33 Addition circuit 40 Quadrature demodulation means 50 Modeling means 51 RAM 52 Model derivation circuit 60 Nonlinear unit 61 Upconverter 62 High power amplifier 63 Downconverter 65 Delay circuit 70 Transmission signal estimation circuit 73 Guard Ntabaru removing circuit 74 DFT circuit 75 determines the circuit 80 multilevel value selection circuit 91, 92 the amplitude adjustment circuit 95 AGC circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J090 AA01 AA24 AA26 AA41 AA66 CA01 CA22 CA25 FA08 FA17 GN03 GN05 GN06 GN11 HN04 HN06 KA25 KA53 KA55 MA11 MA20 NN16 SA14 TA01 TA02 5K022 DD01 DD23 DD24 5K060H DD03 FF06 H06

Claims (6)

[Claims] 1. A digital data stream to be transmitted is
An OFDM modulator that generates an FDM signal and outputs it to a non-linear section having non-linear characteristics. The OFDM modulator receives a digital data sequence to be transmitted and converts the digital data sequence into multi-valued digital data, which is a complex digital signal composed of I-axis and Q-axis data. IDFT means for further performing an inverse discrete Fourier transform to output a baseband signal; and receiving the baseband signal in order to compensate for non-linear distortion generated by the non-linear section, distorting the baseband signal in advance. A pre-distortion unit that outputs an obtained pre-distortion signal; an orthogonal modulation unit that performs an orthogonal modulation process on an output of the pre-distortion unit to generate an OFDM signal and sends the OFDM signal to the nonlinear unit; Receiving the OFDM signal that has passed through the section, Quadrature demodulation means for performing quadrature demodulation processing as processing, receives the output of the pre-distortion means and the signal restored by the quadrature demodulation means, obtains the non-linear characteristic of the non-linear part, and applies the non-linear characteristic to the pre-distortion means. OF with output modeling means
DM modulator. 2. Receiving the pre-distortion signal,
2. The apparatus according to claim 1, further comprising a delay unit that delays the predistortion signal and sends the delayed signal to the modeling unit.
An OFDM modulator as described. 3. The transmission signal estimating means for receiving the signal restored by the quadrature demodulation means, estimating the pre-distortion signal, and sending out the estimated estimated pre-distortion signal to the modeling means. The OFDM modulator according to claim 1, further comprising: the modeling unit obtains a nonlinear characteristic of the nonlinear unit using the estimated predistortion signal and a signal restored by the quadrature demodulation unit. 4. The multi-valued IDFT means receives a digital data signal to be transmitted and converts the digital data signal into a multi-valued digital signal. Further comprising a multi-valued number selecting means, wherein the multi-valued number selecting means changes the multi-valued number stepwise from a small value to a large value each time the predistortion signal estimation processing is performed. The OFDM modulator according to claim 3, wherein the setting is performed. 5. The OFDM modulator according to claim 1, wherein said predistortion means includes amplitude adjustment means for adjusting the amplitude of said transmission signal. 6. The modeling means first obtains, from among the nonlinear characteristics of the nonlinear portion, an inverse characteristic relating to amplitude distortion between input and output signals, and then calculates a slope of a linear portion of the inverse characteristic relating to the amplitude distortion. 2. The OFDM modulator according to claim 1, wherein an inverse characteristic corrected so that is equal to 1 is obtained.