DE10325835A1 - Modeling optimized model-filter in crest-factor reduction circuit of multi-carrier data transmission system, using approximation to determine filter coefficients using window in which impulse response has highest energy - Google Patents

Abstract

The method involves providing a non-recursive model-filter with M coefficients, representing a filter chain connected to the output of the crest-factor reduction circuit. The impulse response of the filter chain is determined. Filter coefficients are determined for an optimized model filter by approximation using a window of defined length M, in which the impulse response has the highest energy content. An independent claim is included for a circuit for crest-factor reduction.

Description

The The invention relates to a method for modeling an optimized Model filter, which is in a model path of a circuit for crest factor reduction a multi-carrier data transmission system is arranged. The invention further relates to a circuit for Crest factor reduction.

In modern telecommunication plays the hochbitratige data transmission an increasingly important role on a subscriber line, especially because you get a bigger usable from her Bandwidth of the to be transmitted Data combined with bidirectional data communication promises.

A Technique, the most recent Time is gaining in importance, is the so-called multi-carrier data transmission, also called "multi-carrier" transmission, as "discrete Multitone (DMT) "transmission or as "Orthogonal Frequency Division Multiplexing (OFDM) "Transmission is known. Such a data transfer is used, for example, in wire-bound systems, but also in the Radio range, for Broadcast systems and for used the access to data networks. Such systems for transmission of data with multicarrier transmission a variety of carrier frequencies, being for the data transmission the data stream to be transmitted into many parallel partial streams which are transmitted independently in frequency multiplexing become. These partial flows are also considered a single carrier designated.

One Representative of the multi-carrier data transmission is the ADSL technology, being ADSL for "Asymmetric Digital Subscriber Ruler stands. ADSL is a technique called the the transfer a high bit rate bitstream from a central office to the subscriber and a low bit rate, leading from the participant to a central office Bitstream allowed. In this technique, the telecommunications line in at least one channel for conventional Telephone services (ie voice transmission) and subdivided at least one further channel for data transmission.

Although already many problems with such multi-carrier data transmission systems as ADSL solved are still some problems remain unresolved.

One associated with this multi-carrier data transmission going problem arises from the fact that due to the overlay very many individual carriers These briefly too high peak values in the transmission signal can add up. The relationship from peak to rms is called crest factor, its square referred to as PAR (Peak to Average Ratio). Especially for multi-carrier systems Like ADSL, the crest factor can be very large - for example, larger than 6 - become. Even though these peaks in the resulting amplitude very rare and typically only for very short periods of time, make a big one Disadvantage of multi-carrier data transmission represents.

A large crest factor causes various problems in the overall system of data transmission:
The maximum possible modulation of the digital / analog converters and of the analog circuit components, for example filters and line drivers, must be designed in their modulation range and their dynamics or resolution for the maximum occurring peak values. This means that these circuit parts must be dimensioned much larger than the effective modulation. This is accompanied by a correspondingly high operating voltage, which also leads directly to a high power loss. Especially with line drivers, which generally have a non-negligible non-linearity, this leads to a distortion of the signal to be transmitted.

One Another problem of data transmission High crest factors is that a very high peak in the transmission signal the maximum possible Exceed modulation can. In this case, a limitation of the transmission signal begins - one speaks here from a clipping. In these cases, however, the transmission signal represents no longer the original one Transmission signal sequence, causing it to transmission errors comes.

For this reason, multi-carrier data transmission systems have the need to suppress or avoid such peak values as much as possible. This problem is known in the literature under the term crest factor reduction or PAR reduction. There are several approaches to reduce the crest factor here:
In a known method, some carriers or carrier frequencies are reserved from the multicarrier data transmission system (typically about 5% of the spectrum), which are then no longer available for data transmission. This means that these carriers are initially set to zero. From these reserved carriers, a function in the time domain with the highest possible time-limited peak value is generated, which forms the correction signal or the so-called kernel. Iteratively, this kernel, which occupies only the reserved carriers, is weighted with an amplitude factor that is proportional to the difference of maximum peak and desired maximum value and subtracted from the transmit signal in the time domain. It is the Kernel in place of the corresponding peak value of the transmission signal, which is responsible for the excessive crest factor, cyclically shifted. The shift rate of the DFT transformation ensures that even after the shift, only the reserved carriers are occupied.

The just described method works only in the time domain and is therefore characterized by its speed and low complexity. However, a major disadvantage of this method is that that subsequent circuit parts and filters for the determination of the kernel no longer considered become. These filters, which are always present in practice, in turn increase the crest factor of the signal superimposed with the kernel and make thus negating some of the crest factor reduction. The The method described above is suitable from this alone Not in practice.

In International Patent Application WO 03/026240 A2 is a described method based on the method described above. through of the correction signal, the high peaks in the to be sent Time signal for too high a crest factor, iteratively reduced. The advantage of the method described in WO 03/026240 A2 is that for the generation of the correction signal and thus for the crest factor reduction also subsequent filter functions are taken into account, thus even after the overlay with the correction signal no longer to an excessive crest factor in the to be sent Lead time signal can. For this purpose, in the model path of the crest factor reduction circuit a model filter is provided, which is the circuit of the crest factor reduction downstream Imitates filter chain.

Around now that of the circuit for crest factor reduction downstream Filter chain as possible optimally replicate, typically a model filter is possible high order used. Such a model filter has a variety of filter coefficients. As a filter here recursive filters, for Example, an IIR filter (IIR = infinite impulse response), or non-recursive filters, for example a finite impulse response (FIR) filter, be used. Such filters can be formed in terms of hardware or implement in a digital signal processor, since here the arithmetic units of the processors mostly multipliers with downstream ones Provide accumulators with which the immediate execution of the discrete folding possible is. In the case of a higher order FIR filter is for calculation the correction signal, however, requires a computational effort, which, in the case of m coefficients, essentially consists of a to be performed m times Multiplication results. However, multiplications require over others Arithmetic operations an increased Computational effort. A high amount of computation reduces the performance on the one hand the entire system and on the other hand, this is a correspondingly complex Hardware, which directly to an increase in the chip area leads.

outgoing thereof is the object of the present invention, the Computing effort in the model path, in particular in the model filter, a circuit arrangement for crest factor reduction too reduce.

According to the invention this Task by a method having the features of the claim 1 and a circuit with the features of claim 10 solved.

The The idea underlying the present invention is that that for modeling the circuit for crest factor reduction downstream filter chain this not necessarily identical must be modeled. For a reduction in the computational effort has proven to be advantageous the filter chain in the signal path of the circuit arrangement for crest factor reduction by a suitable replacement filter if possible imitate low order. Very common is this filter chain in the signal path up to a much higher, than the model path used oversampling Are defined. It is therefore recommended as an intermediate for a modeling This filter chain subsampling the impulse response of the filter chain make and a replica with a filter model sufficient high order, but with a lower order than for exact modeling to use the filter chain in the signal path.

Of the The present invention is based on the finding that it is in Case of crest factor reduction crucial for the quality of the model filter is one as possible faithful reproduction of the time domain properties, ie the impulse response the filter chain, and not the frequency response of the filter chain itself make. It has also been shown that the immediate band limiting components in the filter chain on a dominant influence the crest factor of the to be transferred Have time signal. A reduction of the model filter to just this Components, such as the high-pass components and at least the first stage of the low pass, has almost no influence on the Shape of the impulse response of the filter and thus the quality of the crest factor reduction.

In the method according to the invention, therefore, initially from the subsampled Im pulse response of the filter chain a window of defined length with the highest energy content is used as an approximation of appropriate length for the model filter. In a very advantageous embodiment, coefficients that are not taken into account, for example the first coefficients of the impulse response of the filter chain, are replaced by delay elements or even completely eliminated. The resulting time offset between model path and signal path does not or only insignificantly influences the performance of the method. In addition, the coefficients of the model filter thus obtained are quantized to a limited number of possible amplitude values, so that in the subsequent convolution operation, several samples can be added before multiplication by the filter coefficient. In this way, the number of multiplication per sampling step is significantly reduced. At the same computational effort, a much longer impulse response can be approximated.

advantageously, is a sequential, non-recursive model filter, in particular a FIR filter is provided, which sequentially transmits the signal to be transmitted executing.

In According to an advantageous embodiment, the arithmetic unit is program-controlled Unit, in particular as a microprocessor or microcontroller formed.

advantageous Embodiments and further developments are the dependent claims and the description with reference to the drawings.

The Invention will be described below with reference to the schematic figures The drawings specified embodiments explained in more detail. It shows attended:

1 a block diagram of a circuit for crest factor reduction of a multi-carrier transmission system with inventive FIR model filter;

2 a canonical representation of an example of an adaptive quantization of an FIR model filter with 8 coefficients, which is reduced in its complexity by the method according to the invention to 3 multiplication steps;

3 a canonical representation of an example of an adaptive quantization of a realization of a polyphase FIR model filter with 2-fold oversampling of the input signal;

4 a comparison of the impulse responses in the case of a FIR filter with 50 coefficients and in the case of a reduction according to the invention to 5 multiplications per polyphase with 2-fold oversampling of the input signal;

5 the quantization steps for the first polyphase component (a) and for the second polyphase component (b).

In all figures of the drawing are identical or functionally identical elements, Signals and functions - provided nothing else is indicated - the same Service.

1 shows a block diagram of a circuit for crest factor reduction - hereinafter referred to as CF circuit - a multi-carrier transmission system with inventive FIR model filter.

In 1 is with reference numerals 1 a section of a multi-carrier data transmission system called. With regard to the structure and operation of such a multi-carrier data transmission system 1 and in particular a CF circuit reference is made to the already mentioned above WO 03/026240 A2, which is fully incorporated with respect to these objects in the present patent application.

In 1 is merely a section of the transmitter-side transmission path 5 shown, which is arranged between a transmitter, not shown, and a likewise not shown hybrid circuit which is connected to the corresponding telephone line.

In the transmission path 5 of the multicarrier data transmission system are successively an IFFT module 2 , A circuit for crest factor reduction 3 and an output filter 4 arranged. The IFFT module 2 An input signal X0 supplied by the transmitter is supplied by the IFFT module 2 is modulated by inverse Fourier transform. The thus modulated input signal X, which has a sampling frequency of 2.208 MHz in the case of ADSL data transmission and a sampling frequency of 4.416 MHz in the case of ADSL + data transmission, becomes the downstream CF circuit 3 fed. The CF circuit 3 generates a crest factor-reduced output signal Z, which is the downstream output filter 4 is supplied. After filtering the crest factor reduced output signal Z, the output filter is output 4 a signal Z ', which is still Crest-factor reduced, although through this output filter 4 the output signal Z has been changed and thus the possibility of a renewed generation of an excessive crest factor exists. This is only possible by a special design of the CF circuit 3 that just the action of this filter 4 taken into account by the output signal Z by the output filter 4 modeled as well as possible.

The CF circuit 3 has a model path for this purpose 6 on, which is parallel to a part 5 ' of the transmission path 5 is arranged.

The model path 6 branches at the input of the CF circuit 3 from the transmission path 5 ' off, leaving the model path 6 also the modulated input signal X is supplied. At the beginning of the model path 6 is an oversampling block 11 provided that the original, formed by the user data time signal X L-fold, for example, 2-fold or 4-fold oversampled. You may be able to access this oversampling block 11 also be waived.

The oversampled signal X 'becomes a downstream model filter 12 fed. In the model filter 12 it is a modeling of the output filter 4 or all, the CF circuit 3 downstream filter 4 or filter chains. The procedure for modeling this model filter 12 is described below on the basis of 2 to 5 described in detail.

The model filter 12 is an arithmetic unit 13 downstream, which searches from the oversampled and filtered signal the corresponding maximum point. In this case, a method is described which essentially corresponds to that in WO 03/026240 A2 and the method described above. Starting therefrom, correction signals Y _{CF are} generated, for example dirac-like pattern functions, which are stored, for example, in a memory. In the adding device 10 These correction signals Y _CF in the transmission path part 5 ' superimposed with the temporally suitably delayed transmission signal X with appropriate weighting. In this case, the correction signal Y _{CF is} shifted to the corresponding point of the time signal X, at which the maximum is currently located. The superimposition is then carried out, for example, by subtracting the correction signal Y _CF from the corresponding part of the time signal X in the adder 10 , The oversampled correction signal Y * _CF becomes in the adder 16 in the model path 6 superimposed with the temporally suitably delayed signal Y.

The feedback paths 17 . 18 in the send path part 5 ' and model path 6 as well as the appropriate switches 7 . 8th . 14 serve the iterative treatment of the respective signals. For storage or for buffering are the buffer devices 9 . 15 intended. The send path part shows 5 ' the iterative change of the non-oversampled time function X during the model path 6 performs the associated modification in the oversampled time grid X '.

For the iterative treatment of the input signals X contains the transmission path part 5 ' a first switch 7 (Start), a second switch 8th (Stop), and one between these switches 7 . 8th arranged buffers 9 and adder 10 , The buffer device 9 serves the buffering, ie the delay of the input side supplied time signal X by a time delay in the model path 6 Account for the storage of the respective intermediate values from the iteration.

For the iterative treatment of the oversampled signals X 'is in the model path 6 a first switch 14 (Start), a buffer device 15 and an adder 16 intended. The computing and storage unit 13 leads just the correction signal and the adder 16 in which the correction signal Y * _{CF is} subtracted from the time signal Y in the model path. This results in a second iteration loop.

According to the invention, the model filter forms 12 the output filter 4 not exactly after, but provides a simplified model of the output filter 4 dar. The modeling of this model filter 12 is described below.

The following assumes that the model filter 12 a non-recursive filter, especially an FIR filter.

In a first approximation section, the actual impulse response of the output filter is determined 4 , which may well have recursive character, as already described above, cut out a window of length M, which has the greater energy content.

an die Koeffizienten des Ausgangsfilters 4 angepasst.Aim of further simplification of the model filter 12 is it, in a realization of a model filter 12 with M coefficients g = [g 0 , g 1 , ... g M-1 ] T significantly reduce the number of multiplications required per filter step. For this, a set of K << M adaptive quantization levels is used L = {l 0 , l 1 , ..., l K-1 } defined, which make up a proximity filter (model filter 12 ) with the filter coefficients ḡ = [ḡ 0 , ḡ 1 , ..., ḡ M-1 ] T with | ḡ _i | ∈ L, which thus requires only k multiplications per filter step. The individual quantization stages I _k are in the following optimization step

to the coefficients of the output filter 4 customized.

The principle of this simplification is in the canonical representation in 2 graphed, where 2 (a) represents the filter in the initial situation (not optimized) and 2 B) denotes the optimized filter reduced in number of multiplications. As can be seen, the number of necessary additions does not decrease, or only when some of the coefficients are quantized to zero.

In the case of an oversampled input signal X is an efficient polyphase realization of the model filter 12 as a practical implementation desirable. Depending on the type of implementation, it may be necessary to apply the adaptive quantization separately to the individual polyphase filters g _m and to keep the number of quantization steps for the respective sets L _m constant in order to ensure a uniform utilization of the hardware. This is exemplary in 3 shown for a double oversampling.

The form of the simplified FIR realization of the model filter in 3 has proven to be very efficient for crest factor reduction. First of all, the time domain properties of the model filter 12 and in particular the positions of the peak values are of particular importance, the coefficients can be very roughly quantized without significant performance loss. Advantageously, several coefficients can also be quantized to zero, which additionally reduces the computational effort.

In 4 is an example of the impulse response IA of a model filter 12 with 50 coefficients (n), in which, assuming a 2-times oversampled input signal X, the number of multiplications per polyphase is reduced from the original 25 to 5 (B). The 5 shows the assignment of the individual coefficients of the polyphase components to the respective quantization levels. 5 (a) shows the quantization _steps for the first polyphase _component I _0i (even coefficients) and 5 (b) shows the quantization _steps for the second polyphase _component l _1i (odd coefficients; 5 (b) ).

Even though the present invention above based on preferred embodiments It is not limited to this, but in many ways modifiable.

Especially the invention is not limited to the above data transmission systems and methods limited but lets for the purpose of crest factor reduction to all, on multi-carrier data transmission Expand based systems and procedures. In particular, be the Invention is not limited to ADSL data transmission, but let yourself on all xDSL data transmissions expand. Also conceivable are mobile applications such as DAB (= Digital Audio Broadcasting), DVB-T (= Digital Video Broadcasting-Terrestrial) or OFDM-based wireless local area network (WLAN) applications.

Although the invention has been described above with reference to FIR filters, do not be limited to but is on all possible non-recursives Expandable filter or transversal filter. Also, the specified Numerical examples for the Number of filter coefficients or the number of reduced multiplication steps only to be understood as an example and can, of course, in Be varied as desired within the scope of the invention. It is only essential in the case of the modeling method according to the invention the number of multiplication steps performed in the model filter opposite the number of coefficients or the multiplication steps originally required significantly reduced.

Also the invention is not limited to the specified high-pass filters, Bandpass filter or low pass filter limited, but can be on however, expand any trained filter types.

Especially the invention is not limited to the above figures, but can be in the context of the invention and the expert knowledge in arbitrary Modify way.

It It is understood that the elements of the crest factor reduction circuit as well as the specified IFFT modules and filters conventional hardware components are, but also implemented software can be.

Also The invention is not necessarily a 2-fold or 4-fold oversampling limited to the data signal to be sent. Rather, it can also be provided be that there is no oversampling, even an undersampling or an arbitrarily high oversampling takes place.

1

Multi-carrier data transmission system

2

IFFT module

3

circuit for crest factor reduction, CF circuit

4

Output filter, downstream filter chain

5

transmission path

5 '

part of the transmission path

6

model path

7

switch

8th

switch

9

Buffer means, Storage

10

adder

11

block for oversampling

12

model filter

13

Rake- and storage unit

14

switch

15

Buffer means, Storage

16

adder

17

Iterationspfad

18

Iterationspfad

X0

input

X

(IFFT modulated) input signal

X '

(Oversampled, modulated) input signal

Y

filtered signal

Z

output

Z '

filtered output

Y _CF

correction signal for the transmission path

Y * _CF

oversampled Correction signal for the model path

Claims (15)

Method for modeling an optimized model filter ( 12 ), which in a model path ( 6 ) a circuit for crest factor reduction ( 3 ) of a multi-carrier data transmission system ( 1 ), with the method steps: (a) a non-recursive model filter ( 12 ) having a first number (M) of coefficients which one of the crest factor reduction circuits ( 3 ) downstream filter chain ( 4 ) is provided; (b) The impulse response of the filter chain ( 4 ) is determined; (c) From the impulse response determined in method step (b), filter coefficients for an optimized model filter are determined by approximation of a window of defined length M in which the impulse response has the highest energy content. 12 ) certainly. Method according to Claim 1, characterized in that, after method step (c), the filter coefficients thus approximated are quantized to a second number (K) of possible amplitude values of the filter coefficients, the second number (K) being smaller than the first number (M) and the number of optimized model filters ( 12 ) multiplications per filter step. Method according to one of the preceding claims, characterized marked that for the determination of the impulse response in method step (b) only the Bandbegrenzenden elements of the filter chain, in particular the High-pass component and / or the first stage of the low-pass, taken into account become. Method according to one of the preceding claims, characterized in that for the quantization of the filter coefficients adaptive quantization levels are defined, from which the optimized model filter ( 12 ), which approximates the non-recursive model filter ( 12 ) and which has a second number (K) of multiplications per filtering step. Method according to one of the preceding claims, characterized in that in order to optimize the coefficients of the optimized model filter ( 12 ) each have a quantization step corresponding to the coefficients of the non-recursive model filter ( 12 ) is adjusted. Method according to one of the preceding claims, characterized in that filter coefficients of the filter chain ( 4 ), which are outside the window, are replaced by delays and / or set to zero. Method according to one of the preceding claims, characterized in that when an oversampling of the transmission signal (X) the optimized model filter ( 12 ) is implemented as a block of polyphase filters. Method according to one of the preceding claims, characterized characterized in that the adaptive quantization is separate to the single polyphase filter and the number of quantization levels per polyphase filter compared to a filter without oversampling is held equal. Method according to one of the preceding claims, characterized characterized in that the data symbol to be sent is a function of a Variety of provided within a given data frame signals and each of these signals is assigned to a carrier, each one carrier each occupied at least one frequency from a transmission data spectrum, being at least one carrier reserved, not for the data transmission is provided. Circuit for crest factor reduction ( 3 ), in particular for carrying out a method according to one of the preceding claims, the sen detached in a multi-carrier data transmission system ( 1 ) is arranged, (A) with a transmission path ( 5 ) with a signal (X) to be transmitted, (B) with a signal parallel to a section of the transmission path ( 5 ' ) model path ( 6 ), in which a non-recursive model filter ( 12 ), to which the signal (X) to be transmitted can be fed, which determines the characteristic of one of the circuits for crest factor reduction ( 3 ) downstream filter chain ( 4 ), which has a first number (M) of filter coefficients, the first number being less than the number of filter coefficients for an exact replica of the filter chain ( 4 ), with a model filter ( 12 ) downstream processing unit ( 13 ), one of the model filter ( 12 ) filtered signal (Y), which generates a correction signal (Y _CF ) dependent thereon, (C) with a subtracting device ( 10 ), the output side with the model path ( 6 ) and the transmission path ( 5 ) and which subtracts the correction signal (Y _CF ) therefrom to reduce the crest factor in the signal (X) to be transmitted. Circuit according to Claim 10, characterized in that the signal (Y _CF ) for the transmission path ( 5 ) and the correction signal (Y * _CF ) for the model path ( 6 ) exhibit. Circuit according to one of Claims 10 or 11, characterized in that in the model path ( 6 ) one unity ( 11 ) is provided for oversampling, which before the model filter ( 12 ) is arranged and which performs an oversampling of the signal to be transmitted (X). Circuit according to one of claims 10-12, characterized in that delay elements are provided which are not in the model filter ( 12 ) replace filter coefficients that are outside the window and before it. Circuit according to one of Claims 10-13, characterized in that a sequentially operating, non-recursive model filter ( 12 ), in particular a FIR filter, is provided which processes the signal (X) to be transmitted sequentially. Circuit according to one of claims 10-14, characterized in that the arithmetic unit ( 13 ) is designed as a program-controlled unit, in particular as a microprocessor or microcontroller.