JP2001211135A - Ofdm transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an output amplification part usable even in a nonlinear area. SOLUTION: A pilot symbol of a base-band signal before orthogonal modulation and a pilot symbol of a base-band signal obtained by orthogonally demodulating the signal outputted from the output amplification part 15 are inputted to an impulse response estimation part 21 in the same timing to perform estimation and reverse convolutional operation parts 8 and 9 add inverse distortion canceling distortion generated at the output amplification part 15 to the base-band signal before the orthogonal modulation according to the estimation result.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to OFDM (Orthog
The present invention relates to a technique for compensating for distortion generated in an output amplification unit in an onal Frequency Division Multiplex (orthogonal frequency division multiplex) transmission device.

[0002]

2. Description of the Related Art OFDM modulation is a system which has recently been studied or adopted in digital audio broadcasting in Europe, terrestrial digital television broadcasting in Japan and Europe, and wireless LAN. This OFDM modulation method uses, for example, QP for several to several thousand subcarriers that are orthogonal to each other.
The data to be transmitted is mapped to the real part and the imaginary part according to a predetermined modulation scheme such as SK or 16QAM,
This is converted from data on the frequency axis to data on the time axis by discrete inverse Fourier transform, and a guard interval is added to this to make one symbol.

For example, as shown in FIG. 4, there are 52 subcarriers which are orthogonal to each other (having an integral multiple of the fundamental frequency) (numbers are -26, -25,...,-).
, +25, +26), and four of the pilot carriers p (numbers are −21, −7,
+7, +21) (for correction at the time of demodulation on the receiving side) is included in each of 52-4 = 48 subcarriers.
For example, in the case of QPSK, data of 2 bits (1 bit for the real part and 1 bit for the imaginary part) can be mapped, so that 96-bit data can be mapped each time. Note that the number 0
Are set to the same frequency as the carrier when transmitting to the transmission line, and no data mapping is performed here.

[0004] The subcarriers mapped in this way are subjected to discrete inverse Fourier transform to form time-axis data, and the latter half of the time-axis data, for example, 1/4 of the data is copied to the beginning and added as a guard interval, and this is added to 1 This is a symbol signal. This is repeated for a predetermined number of symbols, and two or three symbols for synchronization and service identification are added to the head to form one frame. After D / A conversion of the baseband OFDM signal created in this way, the signal is orthogonally modulated, mounted on a carrier wave, and transmitted to a transmission path.

[0005] On the receiving side, the received signal is subjected to, for example, quadrature demodulation to remove a carrier wave, to extract a baseband OFDM signal, A / D-converted, and further converted to a signal on the frequency axis by discrete Fourier transform. Data is extracted from the subcarrier by demapping the signal on the frequency axis.

[0006] In such an OFDM modulation system, if the modulated signal is subjected to nonlinear distortion, the orthogonal relationship between the subcarriers is broken, and inter-subcarrier interference occurs.
There is a problem that transmission characteristics deteriorate.

[0007] Therefore, in general, a sufficient back-off is taken, and the above problem is avoided by performing linear amplification in the output amplifying section of the OFDM transmitting apparatus.

[0008]

However, when the back-off is increased, another problem occurs in that the amplification efficiency in the output amplifying unit is deteriorated.

The present invention has been made in view of such a point, and an object of the present invention is to provide an OFDM transmission apparatus in which characteristics are not degraded even when a nonlinear region in a range not clipped by an output amplifier is used. is there.

[0010]

According to a first aspect of the present invention, a signal on a frequency axis, on which data is mapped to a plurality of subcarriers, is subjected to discrete inverse Fourier transform, and a baseband signal on a time axis is obtained. A guard interval is added to the baseband signal, a plurality of symbols are created as one symbol, a symbol for synchronization or the like is added to the beginning of the plurality of symbols to form one frame, and after D / A conversion, the carrier A pilot symbol adding means for inserting a pilot symbol and finally including the pilot symbol in the frame, wherein
Delay means for taking in and delaying the pilot symbol after A / D conversion and before input to the output amplifying section; and the same signal form extracted from the output side of the output symbol and the pilot symbol delayed by the delay means Impulse response estimation means for inputting the pilot symbols at the same timing and estimating the impulse response thereof, and applying an inverse distortion to the signal before input to the output amplifying section according to the estimation result obtained by the impulse response estimation means Reverse distortion adding means.

In a second aspect based on the first aspect, the pilot symbol is a code having a large autocorrelation when there is no delay, and having a small autocorrelation otherwise.

[0012] In a third aspect based on the first or second aspect, the same symbol as a symbol for synchronization or the like added to the head of the frame is used as the pilot symbol.

In a fourth aspect based on any one of the first to third aspects, the delay means takes in the pilot symbol before modulation using the carrier and delays the pilot symbol, and the impulse response estimator has The pilot symbol obtained from the delay unit and the pilot symbol extracted and demodulated from the output side of the output amplifying unit are input, and the inverse distortion adding unit receives the base signal before the modulation based on the estimation result of the impulse response estimation unit. The band signal is subjected to deconvolution operation, and the operation unit adds an inverse distortion to the band signal and sends it to the modulation unit.

In a fifth aspect based on the fourth aspect, the inverse distortion adding means includes a discrete Fourier transform of an estimation result of the impulse response estimation means, performs a reciprocal operation on the result, and assigns the result to each symbol on the frequency axis. And a means for applying inverse distortion by multiplying by, and performing the discrete inverse Fourier transform on the result of the multiplying means.

[0015]

FIG. 1 is a block diagram showing the configuration of an OFDM transmitting apparatus according to one embodiment of the present invention. 1 is a scramble section for scrambling input data, 2
Is a mapping unit that sequentially maps a plurality of data after scrambling to a plurality of subcarriers by QPSK and inserts a pilot carrier, and 3 performs discrete inverse Fourier transform on the plurality of mapped subcarriers to convert data on the frequency axis into time. IFFT to convert to on-axis data
Part 4, the latter half of one unit of the time axis data, for example, 1/4
Is inserted as a guard interval at the beginning of the data and a guard interval inserting unit for using this as one symbol, 5 is a pilot symbol adding unit (pilot symbol adding means) for adding a pilot symbol described later, and 6 and 7 are D / A converting units. Thus, an in-phase component (I) and a quadrature component (Q) of the OFDM baseband signal are obtained.

As a baseband signal of OFDM, a plurality of the symbols to which data is mapped and a guard interval is added are repeated, and a bilot symbol (arbitrary addition of a guard interval) is included in the middle of the repetition. And one or more symbols for service addition are added to form one frame. Note that the pilot symbols may be arranged at the beginning rather than inside the frame.

Reference numerals 8 and 9 denote inverse convolution operation units (inverse distortion adding means) for in-phase components and quadrature components for adding inverse distortion, which will be described later. 10, 11 are multipliers, 12 is both multipliers 10, 11
Is an adder for adding the result of multiplication. Reference numeral 13 denotes a local oscillator, and the generated carrier is input to the multiplier 10 and multiplied by the OFDM baseband signal having the in-phase component.
The carrier obtained by shifting the phase of the carrier by the 90-degree phase shifter 14 is input to the multiplier 11 and multiplied by the OFDM baseband signal of the quadrature component, and quadrature modulated by these. Reference numeral 15 denotes an output amplifying unit that amplifies the output signal of the adder 12 by using the output signal up to the non-linear range without clipping. The output signal of the output amplifier 15 is transmitted to the transmission path.

Reference numeral 16 denotes an attenuator which takes in a part of the output signal of the output amplifying unit 15 and attenuates it to a predetermined level (such as the level on the input side of the output amplifying unit 15), and 17 and 18 denote multipliers. A multiplier 17 multiplies the output signal of the attenuator 16 by the carrier, and demodulates a signal corresponding to the in-phase component of the OFDM baseband signal. The multiplier 18 multiplies the output signal of the attenuator 16 by a carrier obtained by shifting the carrier by 90 degrees, and demodulates a signal corresponding to the orthogonal component of the OFDM baseband signal.

Reference numerals 19 and 20 denote delayers (delay means) for delaying the in-phase component and the quadrature component of the OFDM baseband signal. Reference numeral 21 fetches the in-phase component and the quadrature component of the pilot symbol of the baseband signal delayed by the delay units 19 and 20, and the same in-phase component and the quadrature component of the pilot symbol of the demodulated baseband signal, and estimates an impulse response. It is an impulse response estimation unit (impulse response estimation means). The deconvolution operation units 8 and 9 are:
A deconvolution operation is performed using a coefficient corresponding to the impulse response estimation result obtained by the impulse response estimation unit 21 to apply in advance non-linear distortion (inverse distortion) to the in-phase component and the quadrature component of the OFDM baseband signal in advance.

In the above description, the pilot symbol added by pilot symbol adding section 5 has a code that shows a sufficiently large autocorrelation when the delay time is 0, and shows a sufficiently small autocorrelation otherwise, for example, a code having a correlation characteristic. A code having a property like noise (for example, a synchronization code added to the head of the frame as shown in FIG. 2) is used. The delay amounts of the delay units 19 and 20 are set such that the in-phase component and the quadrature component of the pilot symbol captured by the impulse response estimation unit 21 and the in-phase component and the quadrature component of the demodulated pilot symbol indicate signals at the same time. D / A
The transit time from converters 6 and 7 to multipliers 17 and 18 is assumed.

Now, assuming that the baseband signal of the pilot symbol before orthogonal modulation is s (t), the baseband signal of the orthogonally demodulated pilot symbol is S (t), and Ts is one symbol period, the impulse response estimator At 21, p (t) = Σ (τ = 0 to Ts) s (t + τ + Δ) · S (t + τ) = Σ (τ = 0 to Ts) [s _x (t + τ + Δ) + J · s _y (t + τ + Δ)] ×
An impulse response p (t) of [S _x (t + τ) + j · S _y (t + τ)] is obtained. Δ is delay unit 1
The delay time at 9,20, τ is a parameter representing time, s _x
The real part of s (t), s _y is the imaginary part, the S _x is the real part, S _y is the imaginary part of S (t). Note that Σ (τ = 0 to Ts) represents a process of adding the results for the time of the period Ts.

At this time, when there is no distortion in the demodulated signal S (t), p (t) = δ (to), and an impulse waveform is obtained. When there is distortion, this value has a certain width. δ is δ
(t) = 1 (δ (t = 0)) and δ (t) = 0 (δ (t ≠ 0)).

Therefore, if the subsequent baseband signal is subjected to deconvolution integration in advance by the deconvolution operation units 8 and 9 using the coefficient based on the estimation result of the impulse response, the output amplification unit 15 applies the baseband signal to the output amplification unit 15. Since the inverse distortion for canceling the generated distortion is added, the signal output from the output amplifier 15 is a signal in which the nonlinear distortion in the output amplifier 15 is compensated and the distortion is removed. As a result, the output amplifying unit 15 uses not only the linear region but also the non-linear region, so that efficient amplification processing can be performed.

Such a distortion compensation may be performed periodically in a system in which data is constantly transmitted, such as a broadcast. For example, a pilot symbol may be replaced by one of several packets of one packet. What is necessary is just to insert it into a frame and to periodically update the deconvolution coefficients of the deconvolution operation units 8 and 9. Also, in burst communication, distortion compensation can be performed on a packet by inserting a bi-rot symbol in the first frame of the packet.

The delay units 19 and 20 may have at least a function of delaying pilot symbols.

FIG. 3 is a block diagram showing a configuration of an OFDM transmitting apparatus according to another embodiment in which the inverse distortion adding means is realized by another configuration. Here, the deconvolution operation units 8 and 9 in FIG. 1 are deleted, and the in-phase component data line between the mapping unit 2 and the discrete inverse Fourier transform unit 3 is added to the multiplier 2
5, a multiplier 26 is arranged on the quadrature component data line, and the estimation result of the in-phase component and the quadrature component obtained by the impulse response estimator 21 is converted by the discrete Fourier transformer 22 into a data signal on the frequency axis. The in-phase component and the quadrature component are reciprocally operated by the reciprocal calculators 23 and 24, respectively, and the results are input to the multipliers 25 and 26.

In the present embodiment, a process equivalent to deconvolution on the time axis is performed on the frequency axis, and a component for canceling the distortion component in the output amplifier 15 is added to the data on the frequency axis in advance. Become.

[0028]

As described above, according to the present invention, an efficient amplification process can be performed by using not only the linear region but also the nonlinear region in the output amplifier.

[Brief description of the drawings]

FIG. 1 is a block diagram of an OFDM receiver according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of an example of a pilot symbol.

FIG. 3 is a block diagram of an OFDM receiver according to another embodiment.

FIG. 4 is an explanatory diagram of a subcarrier.

[Explanation of symbols]

1: scrambling section, 2: mapping section, 3: discrete inverse Fourier transform section, 4: guard interval insertion section, 5: pilot symbol addition section, 6, 7: D / A conversion section, 8,
9: deconvolution unit, 10, 11: multiplier, 12: adder, 13: local oscillator, 14: 90-degree phase shifter, 1
5: output amplifying unit, 16: attenuator, 17, 18: multiplier,
19, 20: delay unit, 21: impulse response estimation unit, 2
2: discrete Fourier transform unit, 24, 25: reciprocal operation unit, 2
5, 26: multiplier.

Claims (5)

[Claims] 1. A signal on a frequency axis obtained by mapping data on a plurality of subcarriers is subjected to a discrete inverse Fourier transform to obtain a baseband signal on a time axis, a guard interval is added to the baseband signal, and this is represented by one symbol. , A symbol for synchronization or the like is added to the beginning of the plurality of symbols to form one frame, and after D / A conversion, modulated using a carrier, amplified by an output amplifier and output to a transmission line. A pilot symbol inserting means for inserting a pilot symbol and finally including the pilot symbol in the frame; and taking in the pilot symbol after A / D conversion and before input to the output amplification unit. Delay means for delaying, the pilot symbol delayed by the delay means and an output side of the output amplifying unit. Impulse response estimating means for inputting the extracted pilot symbols of the same signal form at the same timing and estimating the impulse response thereof; and, based on the estimation result obtained by the impulse response estimating means, And an inverse distortion adding means for applying an inverse distortion to the signal. 2. The OFDM transmission apparatus according to claim 1, wherein the pilot symbol is a code having a large autocorrelation when there is no delay, and having a small autocorrelation otherwise. 3. The OFDM transmission apparatus according to claim 1, wherein the same symbol as a symbol for synchronization added to the head of the frame is used as the pilot symbol. 4. The delay unit according to claim 1, wherein the delay unit fetches and delays the pilot symbol before modulation using the carrier, and the impulse response estimation unit is obtained from the delay unit. Receiving the demodulated pilot symbol and demodulated pilot symbol from the output side of the output amplifying unit, and the inverse distortion adding means deconvolution operation of the baseband signal before modulation based on the estimation result of the impulse response estimating means. And an inverse distortion added by the arithmetic means and sent to the modulation means.
Transmission device. 5. The inverse distortion adding means according to claim 4, wherein the inverse distortion adding means performs a discrete Fourier transform on the estimation result of the impulse response estimation means, performs a reciprocal operation on the result, and multiplies each symbol on the frequency axis. An OFDM transmission apparatus characterized in that a means for applying a distortion is replaced and the result of the multiplication means is subjected to the discrete inverse Fourier transform.