EP2100421A1

EP2100421A1 - Apparatus and method for producing a signal to reduce the par in a multicarrier system

Info

Publication number: EP2100421A1
Application number: EP07821800A
Authority: EP
Inventors: Bernd Hassler; Björn JELONNEK; Gunter Wolff
Original assignee: Nokia Siemens Networks GmbH and Co KG
Current assignee: Nokia Solutions and Networks GmbH and Co KG
Priority date: 2006-11-16
Filing date: 2007-10-24
Publication date: 2009-09-16
Also published as: US20100128811A1; CN101601247B; CN101601247A; EP1924044A1; WO2008058840A1; US8467463B2

Abstract

The invention relates to an apparatus (400) for producing a reduction signal for a signal having a plurality of subcarriers, comprising - a limiter (402) which is designed to produce a limited signal(S) from the signal having a plurality of subcarriers; - a coefficient ascertainment unit (404) which is designed to determine coefficients (CK) for unused subcarriers of the signal having a plurality of subcarriers from the limited signal; and - a signal generator (406) which is designed to generate the reduction signal (Xn) from the coefficients, said reduction signal being able to be combined with the signal in order to form a reduced signal. The coefficient ascertainment unit is designed to project the limited signals onto a K-th base formula (1).

Description

description

DEVICE AND METHOD FOR PRODUCING A SIGNAL TO THE PAR IN ONE

REDUCE MULTI-CARRIER SYSTEM

The invention relates to a device for generating a reduction signal for a signal having a plurality of subcarriers according to claim 1 and a corresponding method according to claim 14.

Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) based cellular standards are multi-carrier systems. The combination of the individual carriers creates a signal whose maximum instantaneous power significantly exceeds the average power. In addition, even higher order modulation techniques such as Quadrature Phase Shift Keying (QPSK), Phase Shift Keying (8PSK) or Quadrature Amplitude Modulation (16QAM) with more than two possible information states per transmit symbol can be used to modulate the different carriers used , In the case of these modulation methods, a constant of the instantaneous transmission power in favor of higher data transmission rates is dispensed with in the individual carrier.

Depending on the modulation method and combination of the individual carriers, the maximum instantaneous power of a transmission signal can therefore be significantly higher than the mean value of the transmission power in the case of multi-carrier systems. The ratio of maximum to average transmission power is referred to as Peak to Average Ratio (PAR> 1) or Peak to Average Power Ratio (PAPR> 1). A problem now is that at higher PAR of the transmission amplifier of a transmission system must hold considerable headroom reserves to nonlinear distortions of the transmission signal, for example, due to limiting or

To avoid clipping effects. This increases the complexity and power consumption of the amplifier and reduces the efficiency of the overall system. DE10219318A1, for example, deals with a method for forming a carrier-frequency output signal from a plurality of sub-signals of a base frequency band. Each sub-signal is amplitude-limited in a main branch. In addition, for each sub-signal in a parallel sub-branch, estimated values are determined with which the amplitude limitation in the main branch is controlled.

DE19838295A1 describes a multi-carrier transmission method for the parallel transmission of information in the form of a signal on a plurality of subchannels. By means of a threshold decision, subchannels which are excluded from an information transmission are each given a correction function such that a ratio between a peak value of the signal and an effective value of the signal is minimized or at least reduced.

EP1248429A2 describes a DMT (Discrete Multitone) modulation method with which the ratio of maximum to average transmission power of a signal to be transmitted can be reduced. For this purpose, an experimental value of a limit prevention signal is initially determined and it is determined whether the limit-prevention signal is suitable for limiting the signal to be transmitted. If this is not the case, it will

Limiting prevention signal changed until it is suitable for limiting the signal to be transmitted. The finally suitable limit prevention signal is transmitted in idle subcarriers of the signal to be transmitted in order to lower the power peaks of the signal to be transmitted. An iteration loop is used to determine the appropriate limit prevention signal. However, this can lead to different signal delays because it is not known from the outset how often the iteration loop will be traversed.

An object of the present invention is now to provide an apparatus and a method for generating a reduction nals for a signal having a plurality of subcarriers, wherein the reduction signal may be for forming a reduced signal from the signal having a plurality of subcarriers.

This object is achieved by a device for generating a reduction signal for a signal having a plurality of subcarriers according to claim 1 and a corresponding method according to claim 14. Further embodiments of the invention düng result from the dependent claims.

An essential idea of the invention is to limit a signal to be reduced with a plurality of subcarriers, in particular the amplitude of the signal, to calculate coefficients for unused subcarriers of the signal from the limited signal, and to generate a reduction signal with the aid of these coefficients the original signal, ie the signal to be reduced, can be combined in order to reduce the peak values and thus the PAR of the signal to be reduced. As a result, in particular a frequency-selective limitation of a signal in the baseband can be achieved, which is much more efficient in comparison to conventional limiting methods, in particular for multi-carrier systems such as OFDM and OFDMA transmission systems. Above all, with the invention transmission signals in the digital complex baseband can be limited in amplitude and the maximum values of the transmission power can be reduced more than is possible with the conventional methods explained in the introduction, in particular without disturbing losses in the signal quality.

Clipping a signal in the sense of the present invention means that peak values of the signal and thus the ratio of maximum to average transmission power of the signal are reduced. The reduced signal thus corresponds to the original signal, but may be reduced in amplitude and in transmission power. The signal may in particular be a transmission signal of a multiple carrier system in which higher Current modulation methods, such as QPSK, 8PSK or 16QAM are used with more than two possible information states per transmit symbol. It may, for example, be a signal in the complex baseband of a transmission system. The signal has a plurality of subcarriers. Some of the subcarriers are used for information transmission, while the other subcarriers are unused, that is, not used for information transmission.

The present invention allows generation of a reduction signal by projecting a clipped signal onto the unused subcarriers. Alternatively, other educational regulations may be used that result in a greater reduction in peak values. It is not necessary to use an iteration loop for determining the reduction signal, which is executed until a reduced signal no longer undergoes any change by trimming and thus no longer exceeds a threshold value specified when limiting. Instead, a fixed number of reduction stages with constant signal delay can be used. According to the present invention, a different threshold is allowed in each reduction stage. Also, the educational requirements for the reduction signal or the coefficients of reduction level to reduction stage may differ.

The invention, according to one embodiment, now relates to an apparatus for generating a reduction signal for a signal having a plurality of subcarriers, comprising: a limiter adapted to generate a limited signal from the signal having a plurality of subcarriers;

a coefficient determiner configured to determine from the limited signal coefficients for unused subcarriers of the signal having a plurality of subcarriers; and

a signal generator configured to generate from the coefficients the reduction signal associated with the Signal can be combined to form a reduced signal.

In one embodiment, the apparatus may further include a combiner configured to combine the signal with the reduction signal to form the reduced signal. The combiner can be designed, for example, as an adder.

The coefficient determiner may be configured to weight the coefficients with weighting factors. By means of the weighting factors, the accuracy of the reduction can be adjusted.

The signal may be a modulated signal having more than two possible states of information per symbol. Furthermore, the coefficient determiner can be designed to determine the coefficients by a symbol-by-symbol projection of the limited signal. In this case, the coefficient determiner can determine the coefficients and the signal generator can determine the reduction signal in each case based on predetermined algorithms or formulas. In particular, according to an embodiment of the invention, the coefficient determiner may be designed to convert the coefficients by a symbol-by-symbol projection of the limited signal, comprising N samples for a symbol S _n ; n = 1 ... N, to the kth basis function

which is assigned to the kth unused subcarrier.

According to another embodiment of the invention, the coefficient determiner may be configured to provide a coefficient for the k-th unused subcarrier according to the formula: N

C * = - Σ _Sπ -e ² ^ '^ ¹ ! ^ ¹ , 1 ≤ k ≤ N

N n = 1

to calculate.

Furthermore, according to an embodiment of the invention, the signal generator may be configured to generate the reduction signal according to the formula:

X _f , "L we N, π-1 ... N k-uftusect

to calculate.

According to one embodiment, the coefficient determiner may be configured to extract the coefficients from the limited one

Signal and to determine the signal. In particular, the coefficient determiner may be configured to determine the coefficients by means of a matrix operation of the signal and the limited signal.

Furthermore, the limiter may have an adjustable limiting threshold, with which the limit of the signal is adjustable. The limiting threshold thus defines how much the signal can be limited.

The device according to the invention is particularly suitable for use in OFDM and OFDMA-based communication systems and can bring about a substantial reduction of the PAR and PAPR, whereby the requirements for transmission amplifiers with regard to complexity and power consumption are lower than in the case of conventional systems with high PAR and PAPR. Another embodiment of the invention relates to a system for generating a time domain signal from complex input symbols, comprising:

a converter configured to transform the complex input symbols according to a predetermined transformation into a signal having a plurality of subcarriers; and

- A device for generating a reduction signal for a signal having a plurality of subcarriers according to the invention, which is supplied with the signal having a plurality of subcarriers for forming a reduced signal, wherein the reduced signal corresponds to the time domain signal.

The system may further comprise another device according to the invention for generating a reduction signal, wherein the devices are connected in series. By cascading several PAPR reduction blocks, in the form of the devices according to the invention, a further improvement of the PAPR can be achieved.

The predetermined transformation can be an inverse Fourier transformation or an inverse fast Fourier transformation IFFT (Inverse Fast Fourier Transformation). The system may be an OFDM or OFDMA based transmission system.

A further embodiment of the invention relates to a method for generating a reduction signal for a signal having a plurality of subcarriers, comprising the following steps:

Generating a limited signal from the signal having a plurality of subcarriers;

Determining coefficients for unused subcarriers of the signal having a plurality of subcarriers from the limited signal; and Generating a reduction signal from the coefficients that can be combined with the signal having a plurality of subcarriers to obtain a reduced signal.

The approach according to the invention makes it possible, in a particularly advantageous manner and without disrupting the signal quality, to limit the amplitude of transmission signals in the digital complex baseband and to reduce the maximum values of the transmission power more than is possible with conventional methods.

The inventive method also offers the advantage that no additional FFT and IFFT transformations are required.

By exploiting a greater latitude for signal deviations, it is possible to achieve a reduction of the PAR with the same error vector magnitude EVM (Error-Vector Magnitude). This scope is especially increased in mobile radio systems with higher-order modulation methods.

The inventive approach allows the use of low-power and thus cost-effective amplifier to achieve a predetermined average output power without disturbing loss of signal quality.

Further advantages and possible applications of the present invention will become apparent from the following description in conjunction with the Ausfüh- tion examples shown in the drawings.

In the description, in the claims, in the abstract and in the drawings, the terms and associated reference numerals used in the list of reference numerals recited below are used. The drawings show in:

Fig. 1 is a block diagram of a UMTS transmission system;

FIG. 2 shows a block diagram of another OFDM-based sender system; FIG.

3 is a block diagram of another transmission system;

4 is a block diagram of a system including a device according to an embodiment of the invention;

5 shows a block diagram of a system having a device according to a further exemplary embodiment of the invention; and

Fig. 6 is a block diagram of a system according to another embodiment of the invention.

Hereinafter, the same and / or functionally identical elements may be provided with the same reference numerals.

FIG. 1 illustrates, very much simplified, a transmission system based on the UMTS (Universal Mobile Telecommunications System) example. In particular, a simplified representation of a generation of a transmission signal is also shown.

A signal to be transmitted is present in the complex baseband as an input signal. A baseband clipping 134 limits the maximum power of the signal to be transmitted. In a subsequent block 136, the bandwidth of the signal is limited by a RRC (Root Raised Cosine) filter and brought by further interpolation filter to a higher sampling rate of a digital intermediate frequency IF (Intermediate Frequency).

Through a mixer, the signal undergoes a frequency offset. Several complex baseband signals can be superimposed on the intermediate frequency, as indicated in FIG. 1 by a plurality of digital frequency converters. In a further block 138, the signal undergoes a further interpolation or a further limitation in the intermediate frequency, and possibly another interpolation in a further block 150, Digital / analog conversion and modulation to the actual high frequency RF (radio frequency). Thus, two different placement possibilities of limiting the transmission signal are shown in FIG. They differ essentially in their type of distortion of the RF output signal.

While the baseband limiter 134 can greatly distort the useful signal in the band and thus leads to an increase of the EVM, due to the subsequent RRC filters it influences the neighboring bands only insignificantly. However, the RRC filter forms linear combinations of the limited payload, thereby increasing the maximum signal amplitude that occurs. In contrast, the intermediate frequency limit 138 guarantees the maximum signal amplitude, but produces strong interference in the adjacent bands, also called ACLR (Adjacent Channel Leakage Ratio).

To limit the signal in OFDM-based systems, the rectangular limiting as well as the circular limiting can be used. In rectangular clamping, the in-phase and quadrature components of the complex signal are independently limited. In circular limiting, the magnitude of the complex baseband signal does not exceed a maximum value.

2 shows a simplified representation of the generation of the transmission signal in OFDM-based systems. In OFDM-based systems, the limiting algorithms described with reference to FIG. 1 are used in the same way. However, the complex IQ baseband signal received by baseband limiter 134 is from IFFT-210 transformed complex input symbols. In addition, in many transmission methods, a temporal guard interval is inserted between the symbols in a block 232.

A disadvantage of the baseband limiting, namely the increase of the maximum signal amplitude by the following RRC filter and the further interpolation, can be achieved by a preliminary Calculation of the expected signal overshoot and a corresponding limitation of the baseband signal can be avoided. This achieves a reduction of the PAR with the same EVM.

Statistically, the error is distributed uniformly over the entire baseband and spectrally shaped by the subsequent transmission filters. Especially in the case of higher-valued modulation methods, the error tolerance in the baseband is lower and the effectiveness of this method diminishes because it narrows the scope for signal deviations which are intended to bring about a reduction in the maximum instantaneous power.

In the frequency band of the useful signal, however, there are areas that tolerate larger signal deviations. These are characterized in that they are located at the edge of the baseband, where the signal due to the receive filter in the associated receiver is attenuated more, or in the unused frequency spectrum of the useful signal and are not occupied with information. This fact is used in the system shown in FIG. This can be achieved with the same EVM a greater reduction of the PAR.

FIG. 3 shows a simplified representation of the principle of interpolating limiting, ie the generation of a limited transmission signal with subcarrier-specific error weighting.

First, a time signal is generated from the input symbols by means of IFFT. This is subjected to a baseband boundary 134. The modified time signal is transformed back with an FFT. By clipping the original symbols were falsified.

The output signal is composed of the input signal multiplied by wk and the errored signal multiplied by (1-wk). Wn (0 ≤ wk ≤ 1, k = 1 ... n) can be calculated for each unbalance Carrier k the accuracy can be adjusted individually. Wk = 1 means highest accuracy, ie no error and wk = 0 maximum fault tolerance. In between are values with which the fault tolerance can be restricted.

On the output side, a complete FFT is required to obtain the subcarrier symbols. In addition, another IFFT is needed to generate the transmit signal.

4 shows a system with a device 400 for reducing a signal having a plurality of subcarriers. The device 400 is configured to receive the signal to be reduced and to output a reduced signal. Compared to the signal to be reduced, the reduced signal may be limited in amplitude. Preferably, peak values of the signal to be reduced are reduced in the reduced signal. The device 400 has a limiter 402, a coefficient determiner 404 and a signal generator 406. In addition, the device 400 may include a combiner 408.

The limiter 402 is configured to receive the signal to be reduced with a plurality of subcarriers, to generate from the received signal a limited signal (S), a so-called clipped signal, and to provide it to the coefficient determiner 404. The limiter 402 may be a known clipping block that is configured to limit the maximum power of the signal.

The coefficient determiner 404 is designed to receive the limited signal S and to determine from the limited signal coefficients C _K for unused subcarriers of the signal. According to this embodiment, a separate coefficient C _{κ is} determined for each unused subcarrier of the signal. For subcarriers used no coefficient is determined. Alternatively, coefficients can only be determined for selected unused subcarriers. The coefficient Tenermittler 404 is further configured to provide the coefficients C _R to the signal generator 406.

The signal generator 406 is designed to receive the coefficients C _κ ZU and to generate and provide a reduction signal X _n from the received coefficients.

By combining the reduction signal X _n and the signal to be reduced, the signal with a plurality of subcarriers can be reduced, ie limited. In particular, a reduction of the peak values of the signal can be achieved. According to this exemplary embodiment, the device 400 has a combiner 408 in the form of an addition circuit, which is designed to add the reduction signal X _n to the signal and in this way to generate a reduced signal from the signal provided by the device 400 or can be issued.

The device 400 is part of a system configured to receive complex input symbols and output a time domain signal. For this purpose, the system has a converter 410, which is designed to receive the complex input symbols and to generate the time domain signal from the input symbols by means of a suitable transformation. The converter 410 is further configured to provide the time domain signal as a signal to the device 400.

The system is a transmission system according to an OFDM or OFDMA based mobile standard. The converter 410 is designed to perform an inverse fast Fourier transformation IFFT. This means that the signal to be reduced can be a complex IQ baseband signal, which is determined from IFFT-transformed complex symbols. In each case a temporal guard interval (not shown in FIG. 4) can be inserted between the symbols. The limiter 402 is configured to perform baseband limiting. The calculation of each of the coefficients C _k is performed by a symbol-wise projection of the limited signal S, umfas ^¬ transmitting N samples of a symbol S _n; n = 1 ... N, to the k

^' Jk ¹ UJJJI te basic function

which is assigned to the respective, unused subcarrier. Thus, only those are utilized k, whose base radio ^¬ functions are assigned to unused subcarriers. This exploits the fact that the basic functions of all subcarriers are mutually orthogonal. The calculation of the co-efficient ^¬ C _k is performed according to the formula:

The signal generator 406 is configured to reduce the signal _k from these coef ^¬ coefficient C X _n

k-unused

to create.

A threshold may be varied in the limiter 402 to achieve optimal reduction. Further, the coefficient coef ^¬ can be provided with respective weighting factors. Depending on the circumstances, both the

Threshold and the weighting factors to be fixed or variable, for example, to be adjusted during operation.

Alternatively, the coefficient C can _κ using other formulas, algorithms or other methods, for example, calculated matrix operations or determined ^¬ the. Fig. 5 shows a further embodiment of the system shown in Fig.l. According to this embodiment, the coefficient determiner 404 is configured to receive both the limited signal S and the signal to be limited. Thus, in the determination of the coefficients C _κ and the unlimited signal flows. The reduction of the peak values of the signal thus takes place with additional consideration of the unlimited signal.

Fig. 6 shows another embodiment of the system according to the present invention. According to this embodiment, the system is configured to generate and output from the complex input symbols a time domain signal that has been reduced in two or more stages. The system includes a transducer 410 and two or more devices 400 connected in series.

The converter 410 is configured to perform an IFFT over the complex input symbols to generate and provide the signal to a first device 400. The first device 400, which may be any of the devices 400 described above, is configured to receive the signal and provide it as a reduced signal to the second device 400.

Again, the second device 400 may be any of the devices 400 described above. That is, the cascaded devices 400 may be identical or different. The second device 400 is configured to receive and again reduce the reduced signal from the first device 400.

In each device 400, each corresponding to a PAPR reduction block, a limiting threshold and a weighting of the coefficients may be selected individually. A number of the cascaded devices 400 are not limited and can be chosen depending on the situation. The present invention has been described with reference to mobile radio transmission systems, but is not limited to such systems. Rather, the inventive approach can be used for all systems in which a limitation of a signal is desired.

reference numeral

134 baseband limit

136 Interpolation

138 intermediate frequency limit

150 modulation

210 Transformation 232 Guard interval inserter

400 device

402 limiters

404 coefficient determinants

406 signal generator 408 combiner

410 transducers

Claims

claims

An apparatus (400) for generating a reduction signal for a signal having a plurality of subcarriers, comprising - a limiter (402) arranged to generate a limited signal (S) from the signal having a plurality of subcarriers;

- a coefficient determiner (404) arranged to determine from the limited signal coefficients (C _κ ) for unused subcarriers of the signal having a plurality of subcarriers; and

a signal generator (406) configured to generate from the coefficients the reduction signal (X _n ) that can be combined with the signal to form a reduced signal.

2. Apparatus according to claim 1, characterized in that the apparatus further comprises a combiner (408) arranged to combine the signal with the reduction signal (X _n ) to supply the reduced signal form .

3. Device according to one of the preceding claims, characterized in that the coefficient determiner (404) is designed to determine the coefficients (C _K ) by taking into account weighting factors.

4. Device according to one of the preceding claims, characterized in that the signal is a modulated signal with more than two possible information states per symbol and wherein the coefficient determiner (404) is formed, around the coefficients (C _K ) by a symbol-by-frame projection of the limited signal (S), comprising N samples for a symbol S _n ; n = 1 ... N, to the kth basis function

, fk-men-f) ..

which is assigned to the kth unused subcarrier.

5. The apparatus of claim 4, characterized in that the coefficient determiner (404) is adapted to obtain a coefficient (C _K ) for the k th unused sub-carrier according to the formula n ~ 1

to calculate.

6. Apparatus according to claim 5, characterized in that the signal generator (406) is adapted to the reduction signal (X _n ) according to the formula

k-unused

to calculate.

7. Device according to one of the preceding claims, characterized in that the coefficient determiner (404) is designed to determine the coefficients (C _K ) from the limited signal (S) and the signal.

8. Device according to claim 7, characterized in that the coefficient determiner (404) is designed to determine the coefficients by means of a matrix operation from the signal and the limited signal (S).

9. Device according to one of the preceding claims, characterized in that the limiter (402) has an adjustable limiting device.

Threshold, which defines how much the signal can be limited.

10. System for generating a time domain signal from complex input symbols, comprising

- A converter (410) which is adapted to transform the complex input symbols according to a predetermined transformation into a signal having a plurality of subcarriers; and - a device (400) for generating a reduction signal for a signal having a plurality of subcarriers according to one of claims 1 to 9, to which the signal having a plurality of subcarriers is supplied for forming a reduced signal, the reduced signal corresponding to the time domain signal ,

The system of claim 10, wherein the system further comprises another device (400) according to any one of claims 1 to 8, wherein the device and the further device are connected in series.

12. The system according to claim 10, wherein the predetermined transformation is an inverse Fourier transformation.

13. A system according to any one of claims 10 to 12, wherein said system is an OFDM- or OFDMA-based transmission sys- tem.

14. A method of generating a reduction signal for a signal having a plurality of subcarriers, comprising the steps of:

- generating a limited signal (S) from the signal having a plurality of subcarriers;

- determining coefficients (C _K ) for unused subcarriers of the signal having a plurality of subcarriers from the limited signal; and

Generating a reduction signal (X _n ) from the coefficients that can be combined with the signal having a plurality of subcarriers to obtain a reduced signal.