KR20070020470A - A method for signal processing and a signal processor in an ofdm system - Google Patents

Abstract

A signal processing method for processing an OFDM encoded digital signal and a signal processor for a receiver are disclosed. OFDM encoded digital signals are transmitted as data symbol sub-carriers on several frequency channels. The subset of sub-carriers is in the form of a pilot sub-carrier with a pilot value a _p known to the receiver. The method includes estimating the channel transfer function H and the derivative H 'of the channel transfer function from the received signal y by a channel estimation scheme. An estimate of the data a is then performed from the received signal y and the channel transfer function H. Finally, the estimation of the genuine received signal y ₂ takes into account at least one of previous and subsequent OFDM symbols, thereby eliminating inter-carrier interference so that the data a , the derivative of the channel transfer function ( H ') and the received signal y , followed by a repetition of the above-mentioned estimation.

Description

A method and signal processor for processing signals in an OPM system {A METHOD FOR SIGNAL PROCESSING AND A SIGNAL PROCESSOR IN AN OFDM SYSTEM}

The present invention relates to a signal processor corresponding to a method for processing an OFDM encoded digital signal in a communication system.

The invention also relates to a receiver arranged to receive an OFDM encoded signal and a mobile device arranged to receive an OFDM encoded signal. Finally, the invention relates to a communication system comprising such a mobile device. This method can be used to derive improved data estimation in systems using OFDM technology with pilot sub-carriers such as terrestrial video broadcast systems (DVB-T or DVB-H). The mobile device may be, for example, a portable T.V., a cell phone, a PDA or a portable PC (laptop), for example, or any combination thereof.

In wireless systems for the transmission of digital information such as voice and video signals, orthogonal frequency division multiplexing (OFDM) has been widely used. OFDM can be used to cope with frequency-selective fading of a wireless channel. Interleaving of data can be used for effective data recovery and data error correction schemes.

OFDM is used today, for example, in the digital audio broadcasting (DAB) system Eureka 147 and the terrestrial digital video broadcasting system (DVB-T). DVB-T supports 5-30 Mbps net bit rates, depending on modulation and coding mode, over 8 MHz bandwidth. For the 8K mode, 6817 sub-carriers (of 8192 total) are used with sub-carrier spacing of 1116 Hz. The OFDM symbol validity period is 896 ms, and the OFDM guard interval is 1/4, 1/8, 1/16 or 1/32 of the period.

However, in a mobile environment such as a car or train, the channel transfer function perceived by the receiver changes as a function of time. This variation in the transfer function within the OFDM symbol can cause inter-carrier interference (ICI) between OFDM sub-carriers, such as Doppler extension of the received signal. Inter-carrier interference increases with increasing vehicle speed, making it impossible to reliably detect without countermeasures above a threshold speed.

Signal processing methods were previously known from WO 02/067525, WO 02/067526 and WO 02/067527, where the signal a as well as the channel transfer function H of the OFDM symbol and the time derivative H 'of this function Is calculated for the particular OFDM symbol under consideration.

US 6,654,429 also discloses a method for pilot added channel estimation, where pilot symbols are inserted into each data packet at a known location, occupying a predetermined location in time-frequency space. The received signal is subjected to two-dimensional inverse Fourier transform, two-dimensional filtering and two-dimensional Fourier transform to recover the pilot symbols, resulting in estimating the channel transfer function.

It is an object of the present invention to provide a method for processing a signal less complicated.

It is another object of the present invention to provide a method for signal processing for the estimation of the channel transfer function, where the estimation is further improved by removing pilot induced interference.

This and other objects are achieved by a method of processing an OFDM encoded digital signal, wherein the OFDM encoded digital signal is transmitted as a data symbol sub-carrier in multiple frequency channels, the subset of sub-carriers being a known pilot It is a pilot sub-carrier with a value. The method comprises: estimating the derivative of the channel transfer function and the channel transfer function from the signal by a channel estimation scheme; Estimating data from the received signal and the channel transfer function; Estimating the data, the derivative of the channel transfer function and the signal by removing inter-carrier interference by considering at least one of a previous and subsequent OFDM symbol; And an iterative step of the above-mentioned estimation. In this way, an effective method of estimation of the data is obtained.

The estimation of the data may be performed by a set of M-tap equalizers. This equalizer can be recalculated for each iteration. The number of taps for the equalizer may be 1 or 3, the number of iterations may be 2 for 1-tap equalizers, and 1 for 3-tap equalizers.

In an embodiment of the present invention, pilot induced inter-carrier interference is eliminated using the derivative H 'of the channel transfer function and the known pilot value a _p .

에 의해 상기 수신된 신호(y)로부터 제거되고, p는 상기 파일롯 서브-캐리어의 인덱스이다. In another embodiment of the invention, the pilot value a _p is given by the following equation:

Is removed from the received signal y by p, where p is the index of the pilot sub-carrier.

In another embodiment of the invention, the method is formula:

 Removing the inter-carrier interference by

Ξ is an inter-carrier interference spreading matrix, which is

N is the number of sub-carriers and f _s is the sub-carrier spacing.

에 의해 정의되는 대역 매트릭스일 수 있다.To reduce the complexity of the calculation, the interference spreading matrix is given by the following formula:

It may be a band matrix defined by.

를 갖는 필터에 의해 필터링되고, 상기 필터의 합은 순정된 수신된 신호를 제공하기 위해 상기 수신된 신호로부터 차감된다.In another embodiment of the present invention, the product of the channel transfer function H and the data a is L taps and filter coefficients.

Filtered by a filter having a sum of the filters is subtracted from the received signal to provide a genuine received signal.

In another aspect of the invention, the invention comprises a signal processor for performing the steps of the above-mentioned method.

Other objects, features and advantages of the present invention will become apparent by reading the following description of exemplary embodiments of the invention with reference to the attached drawings.

1 is a schematic block diagram illustrating general signal processing of the present invention.

2 is a schematic block diagram of a complete channel estimation scheme in which the present invention may be used.

3 is a schematic block diagram illustrating a data estimation scheme.

4 is a schematic block diagram illustrating simplified cancellation of inter-carrier interference in accordance with the present invention.

In a mobile environment, due to the movement of the vehicle, the channel perceived by the receiver changes over time. In DVB-T systems using OFDM, this change causes the occurrence of inter-carrier interference (ICI). ICI levels increase with increasing speed of the vehicle. For reception in vehicles moving at high speeds, special countermeasures must be taken to achieve reliable detection.

A general framework for achieving reliable detection is shown in FIG. The data estimation scheme compensates for distortion in the received signal and estimates the transmitted symbol from the received signal. For this purpose, the data estimation scheme requires a channel parameter, which is estimated by the channel estimation scheme.

A complete scheme for channel estimation is shown in FIG.

The channel estimation method is based on the following channel modes. For all reasonable vehicle speeds, the signal received in the frequency domain is:

Can be approximated as

y : received signal

H : Complex channel transfer function vector for all subcarriers

H ': H derivative of H

Ξ: Fixed ICI Spreading Matrix

a : transmitted symbol vector

n : complex cyclic white Gaussian noise vector

N: number of sub-carriers

f _s : sub-carrier spacing

In the present invention, given the channel parameters H and H 'estimated from the received signal and the channel estimation scheme, the problem of how to estimate the transmitted data is solved. A possible solution is to use an N-tap equalizer for each data to get an estimate of the transmitted symbol. The equalizer is designed such that the estimated error is minimized in terms of mean-squares. However, in the present invention, a data estimation scheme with reduced complexity is disclosed.

)의 출력은 이 방식의 출력이고, 슬라이서로 또한 공급될 것이다. 만약 반복이 존재한다면,

는 보다 순정된 수신된 신호(y ₃)를 생성하기 위해

와 y ₁을 또한 취하는, ICI 제거 블록으로 공급된다. 그후, y ₃는 보다 양호한 데이터 추정치(

)를 생성하기 위해 데이터 추정기로 공급된다. 이러한 매커니즘은 강제된 반복의 횟수만큼 계속될 것이다.The proposed iterative data estimation scheme is shown in FIG. 3. This scheme consists of two blocks: a data estimator in the potential input path and an ICI cancellation block in the feedback path. In a first iteration, the data estimator is supplied with the output of pilot pre-removal from the channel estimator y ₁ . If no iteration is forced, the data estimator (

The output of) is the output of this way and will also be supplied to the slicer. If iterations exist,

To generate a more pure received signal y ₃

Taking and y ₁ also, it is supplied to the ICI removal block. Then y ₃ is a better data estimate (

Is fed to the data estimator to produce. This mechanism will continue the number of forced iterations.

The data estimator is a set of M-tap equalizers. In every iteration, the equalizer is recalculated because y ₃ has less ICI after every iteration. The number of tabs presented for the equalizer is 1 and 3. In the 1-tap case, the number of presented iterations is two, while for the 3-tap case, the number of presented iterations is one.

The calculation for obtaining the equalizer coefficients for the first iteration is described as follows. First, Equation 1:

Is rewritten as

to be.

The 1-tap equalizer applied to the sub-carrier (k) is calculated using the following non-energie principle:

to be.

The calculation of the power of the ICI in each sub-carrier requires a product of 3N (in addition to the square operation). This is for 8k DVB-T mode

Can be further simplified.

The sum is calculated in advance. The value shown in equation 9 is the average of the sum calculated in the middle of the frequency band. This calculation reduces the complexity to a multiplication of 2 per sub-carrier.

)은 재계산을 필요로 한다. 이 항은 다음과 같이 근사치가 추정된다:For the first iteration, the term (

) Requires recalculation. This term is approximated as follows:

Thus, the equalizer coefficient for the sub-carrier k is:

to be.

For the nth iteration, the coefficients and the MSE are

는 제2 H 비엔너 필터 The calculation is based on the assumption that H and H 'are completely known. For the estimated H and H 'two additional factors must be added to the denominators of equations 5, 11 and 13, i.e.

Is the second H binner filter

는 H' 비엔너 필터의 MSE이다.MSE of,

Is the MSE of the H 'non-ener filter.

A typical N-tap optimal non-ener equalizer is as follows:

W is an N × N matrix. Row k corresponds to the N-tap equalizer for sub-carrier k.

The calculation of W requires four matrix calculations and an NxN matrix inversion. This complexity goes beyond what can be handled normally in a real implementation. In the next part, complexity is reduced by using an M-tap equalizer instead of N, M << N, and reducing the number of multiplications.

For the sub-carrier k, the M-tap symmetric binner equalizer is calculated as follows.

The principle of orthogonality is:

here,

According to the same derivation,

Is obtained, where

이다.

to be.

To reduce the calculation, the following sum is approximated:

here,

here,

It should be noted that because of the Hermitian nature of the matrix Ξ _{k + lp, k + l} = (Ξ _{k + l, k + lp} ) ^* . In addition, for a particular p, the matrix ¨Teplitz matrix Ξ _{k + lp, k + l} = ΞNp _{, N} for all (k, l). Therefore sum

Can be calculated beforehand for all p.

The matrix under reversal is Hermitage,

It is therefore worth noting that only the upper or lower triangular part needs to be calculated. The rest is obtained by taking a conjugate of triangles.

과 알려진 파일롯 심볼(a_p)을 사용하고, 후속적으로 ICI를 y ₀로부터 상쇄한다.An additional operation may be performed prior to the first data estimation to ensure the removal of the white and noise processor of the residual ICI at the input of the second H filter, ie, the pilot derived ICI from the received signal. No. ID696812, hereby incorporated by reference, see patent applications filed in parallel here). This operation is used to regenerate the ICI caused by the pilot symbol on all subcarriers.

And known pilot symbol (a _p ) and subsequently cancel the ICI from y ₀ .

Since the pilot symbol is known, it can be removed from the received signal, i.e.

y _{l, p} = y _{0, p} -H _p a _p and p is the index of the pilot sub-carrier.

For M-tap equalizers, this operation is beneficial at the next sub-carrier of the pilot, i.e., indexes p + 1 and p-1, because interference from two sub-carriers is in the absence of the pilot. It will be the strongest, and therefore the equalizer on both sub-carriers will be able to get extra information from the residual signal in the pilot. Because of this operation, Equations 21 and 23 must be modified, i.e., for all pilot sub-carriers, the average power is zero.

The operations performed on ICI removal are as follows:

If this operation is performed in a conventional manner, this operation requires N (N + 1) multiplications or (N + 1) multiplications per sub-carrier.

인 L-탭 필터를 가지고

과

의 요소-곱의 필터링으로서 간주될 수 있다. 서브-캐리어 당 곱셈의 개수는 L+1이다. The presentation according to the invention is as follows. The meaningful value of Ξ is concentrated along the main diagonal for each sub-carrier instead of canceling the interference derived from all sub-carriers, so that only interference originating from the plurality of nearest sub-carriers can be canceled out. have. In addition, since Ξ is a depletion matrix, elements along each diagonal have the same value. This means that for all subcarriers, the elements involved in cancellation are identical. Therefore, the multiplication operation

Has an L-tap filter

and

It can be regarded as filtering of the element-product of. The number of multiplications per sub-carrier is L + 1.

4 shows a simplified operation.

The present invention is generally applicable to any OFDM system having a pilot structure and having the problem of ICI.

Other filters and operations may be performed by a dedicated digital signal processor (DSP) and in software. Alternatively, all or some of the steps of the method may be performed in hardware or a combination of hardware and software, such as an Application Specific Integrated Circuit (ASIC) and a Programmable Gate Array (PGA).

The expression “comprising” does not exclude other elements or steps, and it is mentioned that singular elements do not exclude a plurality of elements. In addition, reference numerals in the claims should not be construed as limiting the scope of the claims.

A number of embodiments of the invention have been described above with reference to the drawings. Those skilled in the art upon reading this specification will contemplate many other alternatives, which are intended to be within the scope of the present invention. Combinations of embodiments other than those specifically mentioned herein are intended to be within the scope of the present invention. The invention is only limited by the appended patent claims.

The present invention is applicable to a signal processor corresponding to a method for processing an OFDM encoded digital signal in a communication system.

Claims (15)

A method for processing an OFDM encoded digital signal, wherein the OFDM encoded digital signal is transmitted as a data symbol sub-carrier in several frequency channels, wherein the subset of sub-carriers has a known pilot value a _p . A method for processing an OFDM encoded digital signal that is a pilot sub-carrier, Estimating the channel transfer function H and the derivative H 'of the channel transfer function from the received signal y by a channel estimation scheme; Estimating data a from the received signal y and the channel transfer function H ; -By taking into account at least one of the preceding and subsequent OFDM symbols, the received signal y ₂ pure from the data a to remove inter-carrier interference, the derivative H 'of the channel transfer function and the Estimating the received signal y ; And Repeating the above-mentioned estimation, the method for processing an OFDM encoded digital signal. 2. The method of claim 1, wherein the estimation of data ( a ) is performed by a set of M-tap equalizers. 3. The method of claim 2, wherein the equalizer is recalculated for each iteration. 4. The OFDM encoded digital according to claim 2 or 3, wherein the number of taps for the equalizer is 1 or 3, the number of repetitions is 2 for 1-tap equalizer and 1 for 3-tap equalizer. Method for processing a signal. 5. The method of claim 1, further comprising removing pilot-induced inter-carrier interference using the derivative H ′ of the channel transfer function and the known pilot value a _p . And a method for processing an OFDM encoded digital signal. The method according to any one of claims 1 to 5, wherein the pilot value a _p is of the following formula:

Remove from the received signal ( y ) and p is the index of the pilot sub-carrier. The method according to any one of claims 1 to 6, - Formula:

Removing the inter-carrier interference by Ξ is an inter-carrier interference spreading matrix. The method of claim 7, wherein

N is the number of sub-carriers, and f _s is the sub-carrier spacing. 9. The method of claim 8 wherein the interference spreading matrix is

About

A method for processing an OFDM encoded digital signal, which is a band matrix defined by. 10. The method of claim 9, wherein the product of the channel transfer function H and the data a is L taps and filter coefficients.

A is filtered by the filter having the sum of the filter method for processing OFDM encoded digital signals, is subtracted from the signal (y) it received in order to provide the the pure received signal (y _2). A signal processor arranged to process an OFDM encoded digital signal, wherein the OFDM encoded digital signal is transmitted as a data symbol sub-carrier in multiple frequency channels, the subset of sub-carriers being a known pilot value (a _p). A signal processor arranged to process an OFDM encoded digital signal in the form of a pilot sub-carrier with A channel estimator arranged for estimating the channel transfer function H and the derivative H 'of the channel transfer function from the signal y by the channel estimation scheme; A first data estimator arranged to estimate data a from the received signal y and the channel transfer function H ; A pure reception from the data a , the derivative H 'of the channel transfer function and the received signal y by removing inter-carrier interference by taking into account at least one of previous and subsequent OFDM symbols A second data estimator arranged to estimate the received signal y ₂ , and A signal processor arranged for processing an OFDM encoded digital signal, comprising means for repeating the above-mentioned estimation. A receiver arranged to receive an OFDM encoded signal, wherein the OFDM encoded digital signal is transmitted as a data sub-carrier in several frequency channels, the subset of sub-carriers having a pilot sub-carrier having a known pilot value A receiver arranged to receive an OFDM encoded signal, which is in the form of: A channel estimator arranged for estimating the channel transfer function H and the derivative H 'of the channel transfer function from the signal y by the channel estimation scheme; A first data estimator arranged to estimate data a from the signal y and the channel transfer function H ; -Receiving at least one of previous and subsequent OFDM symbols, thereby eliminating inter-carrier interference, thereby purely receiving from the data a , the derivative H 'of the channel transfer function and the received signal y A second data estimator arranged to estimate the received signal y ₂ , and Means arranged for receiving an OFDM encoded signal, comprising means for repeating the above-mentioned estimation. A mobile device arranged to receive an OFDM encoded digital signal, the OFDM encoded digital signal being transmitted as a data sub-carrier in several frequency channels, the subset of sub-carriers being a pilot sub having a known pilot value. For a mobile device in carrier form, A channel estimator arranged for estimating the channel transfer function H and the derivative H 'of the channel transfer function from the signal y by the channel estimation scheme; A first data estimator arranged to estimate data a from the signal y and the channel transfer function H ; -Receiving at least one of previous and subsequent OFDM symbols, thereby eliminating inter-carrier interference, thereby purely receiving from the data a , the derivative H 'of the channel transfer function and the received signal y A second data estimator arranged to estimate the received signal y ₂ , and A mobile device arranged to receive an OFDM encoded digital signal, comprising means for repeating the above-mentioned estimation. A mobile device arranged to receive an OFDM encoded digital signal, the OFDM encoded digital signal being transmitted as a data sub-carrier in several frequency channels, the subset of sub-carriers being a pilot sub having a known pilot value. -Carrier type, said mobile device arranged to receive an OFDM encoded digital signal, arranged to perform the method of claim 1. Communication system comprising the mobile device according to claim 13.

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