Method and device for improving the estimation of a propagation channel of a multicarrier signal

Translation of description of equivalent WO 2005011144 A2

Method and device for estimating a propagation channel of a multicarrier signal. 1. Field of the Invention The field of the invention is that the applied digital communications, digital terrestrial broadcasting systems, the type belonging to the group including DAB, DVB-T, DRM, but also telecommunications (ADSL, Hyperlan2, etc.). More specifically, the invention relates to receivers such DAB, DVB-T, DRM using the OFDM demodulation ( "Orthogonal Frequency Division Multiplex" in English, "Multiplexing orthogonal frequency division"), which is increasingly used in the above areas. 2. The prior art 2.1. Reminder of the principle of OFDM modulation OFDM is to divide in the time / frequency space of Tu-term data (called useful symbol time) on a plurality of carrier frequencies modulated independently, such as QPSK or QAM. OFDM and subdivides the channel into cells along the axes 11 and time frequencies 12, as shown in Figure 1. Each of the carriers is orthogonal to the previous one. The predetermined length of channel 13 then consists of a series of frequency sub-bands 14 and a sequence of time segments 15. Each cell frequency / time is assigned a dedicated carrier. So we will distribute information to carry out all these carriers, each modulated at low speed, for example, by modulation of QPSK or QAM. An OFDM symbol includes all the information carried by all carriers at time t. This modulation technique is particularly effective in situations where you meet multipath. As illustrated in Figure 2 which presents a set of 21 OFDM symbols, one sequence of symbols arriving at a receiver via two different paths are present as the same information arriving at two different instants and which are added together. These echoes cause two types of defects: intra symbol interference: addition of a symbol with himself slightly out of phase; - Interference inter symbol: addition of a symbol with the following over the previous shifted slightly. Between each transmitted symbol, is inserted a zone "dead" called guard interval 22, the length 23 is chosen to be sufficiently large compared to the spreading of the echoes. These precautions will limit the inter symbol interference (the latter being absorbed by the guard interval). At the reception, the carrier underwent further an attenuation (destructive echoes) or amplified (constructive echoes) and / or a phase rotation. To calculate the transfer function of the channel and thus performing an equalization signal before demodulation, inserting synchronization pilot carriers (often of greater amplitude to carrier payload). The value and location of these drivers in the time / frequency space are predefined and known receptors. After interpolation in time and frequency, yields an estimate of the response of the relevant channel more or less depending on the number of reference pilots and their distribution throughout the time / frequency domain. 2.2. Application in the AM bands (DRM) OFDM modulation is increasingly used in digital broadcasting as it adapted very well to changes in the radio channel: the echoes, Doppler. Engineers, choose the OFDM structure best suited, first examine the characteristics of the radio channel that vary in function of the transmit frequency, the signal bandwidth but also for digital radio in the AM bands ( DRM), the propagation conditions between day and night and solar cycles. Receivers used for OFDM demodulation essentially exploit the response of the channel calculated from the reference drivers. The accuracy of this estimate depends on the proportion of reference pilots inserted in OFDM symbols. 2.3. Technical drawbacks of the prior art A correction of the common phase error algorithm is known, but the error treated corresponds to the relative error between two successive OFDM symbols, then the objective of correcting phase errors due to defects of the oscillators used in receivers. However, especially for DRM, we see rapid changes in channel, especially when traveling by car, which can lead to temporary loss of services (partial or total). In particular, to correct the phase variations between two successive OFDM symbols, known techniques are based on the calculation of the common phase error by differentiating between two successive symbols. This correction is performed before the channel estimation. This is for example the case of the solution proposed by France Telecom and Broadcasting of France (TDF) in their French Patent No. FR 2,768,278. However, this may not be enough, especially in the case of DRM. 3. Objectives of the invention The invention particularly aims to overcome these major drawbacks of the prior art. More specifically, an aim of the invention is to provide a method and a device for optimizing the estimation of a propagation channel, in particular an OFDM channel, for example for DRM applications. In particular, a further object of the invention is to fight against the strong fainting that can be encountered, for example in the case of DRM. Another object of the invention is to provide a method and a device for correcting the reference pilot in accordance with a corresponding propagation channel. A further object of the invention is to provide a method and a device for estimating to refine the synchronization of the receivers. Yet another object of the invention is to provide a method and apparatus that are easy to implement, while remaining at a reasonable cost. A final objective of the invention is to provide a method and a device which also corrects the common phase error problem on an OFDM signal in reception, for example induced by the oscillator phase noise, in addition to the common mistake OFDM symbols on the amplitude. 4. Main features of the invention These objects, and others which will become apparent later are achieved using a method of estimating a propagation channel formed by successive symbols of a multicarrier signal comprising each at least at least a reference pilot, and a plurality of frequencies carrying data. Such a method advantageously comprises at least one correction stage of the reference pilot, according to a first estimation of a propagation channel, so as to provide a second channel estimation more accurate. This approach is based in particular on the observation made by the inventors, that the algorithms currently known from the solutions of the prior art does not use any information it is possible to extract reference pilots. The article "The effect of Phase noise in COFDM" or "the effect of phase noise in COFDM" published in 1998 in the journal "EBU Technical Review" by J. Scott identifies the problem of the common phase error on an OFDM signal in reception, for example induced by the oscillator phase noise. This article of the prior art, however offers no correction to this problem. Advantageously, this first step of estimating a propagation channel takes into account the received pilots before they are corrected. Preferably, the correction step includes a step of calculating an error vector of amplitude and / or phase, for each of the reference pilots. Preferably, the step of calculating an error vector implements an averaging of a set of error vectors obtained on at least one symbol. This averaging may for example correspond to an integration on the error vectors obtained for each symbol, so as to eliminate any risk of introducing noise that could lead to a consideration of atypical drivers. Advantageously, the averaging is calculated over each symbol. Advantageously, the set of error vectors includes only the error vectors corresponding to at least a predetermined quality criterion. Preferably, the step of calculating an amplitude error vector and / or phase comprises a preliminary step of release, for this calculation, pilot whose amplitude is less than a first predetermined minimum threshold means and / or greater than a second predetermined maximum threshold means, so as to avoid any risk of bias introduced into the calculation of the error vector of amplitude and / or phase. Indeed, as a simple illustrative example, it is common in the classical estimation of a channel to obtain an error in amplitude and phase errors that are significant enough percentage to make it as unworkable.

On the contrary, the method according to the invention allows to demodulate all pilots to extract a common estimate applicable to the channel, applying the channel estimation on the data itself.

Thus, the number of drivers is important for a symbol and the better the estimation errors and the additive white Gaussian noise will be minimal. Drivers whose amplitude is very low compared to the average symbol or too high (which can be considered a parasite) are not included in the transaction. Advantageously, the second estimate comprises an equalization step, depending on the first channel estimation. Advantageously, the smoothing step is performed on all the carrier frequencies of each of the symbols. Da Also advantageously, the equalizing step is followed by a step of calculating an impulse response of a propagation channel, based on reference pilot equalized for refining time synchronization of the receivers. Preferably, the reference pilot correction step includes a division of drivers by the first estimate of the propagation channel. Preferably, the reference drivers correction step further comprises a final step of correcting all the useful carriers equalized taking into account the average value obtained by averaging the results. The method according to the invention is advantageously used for correction of at least one phase error and / or common amplitude to two cells of the same symbol of the OFDM ( "Orthogonal Frequency Division

Multiplex "in English," Multiplexing orthogonal frequency division "). The invention also relates to a device for estimating a propagation channel formed of successive symbols of a multicarrier signal each comprising at least at least a reference pilot, and a plurality of frequencies carrying data. Such a device thus comprises, preferably, correction means of the reference pilots, according to a first estimation of a propagation channel, so as to provide a second channel estimation more accurate. 5. LIST OF FIGURES Other features and advantages of the invention will become more apparent from reading the following description of a preferred embodiment given as a simple illustrative and not restrictive, and the appended drawings, which: - Figure 1 previously described in the introduction, is a time / frequency representation of an OFDM channel divided into cells, the channel then being constituted by a sequence of sub-frequency bands and a sequence of time segments; - Figure 2, also described above, presents a set of OFDM symbols; - Figure 3 illustrates an exemplary OFDM structure in A mode of a set of DRM symbols; - Figure 4 illustrates a second exemplary OFDM structure, but for a set of DVB-T symbols; - Figure 5 is a schematic description of the functional algorithm of the method according to the invention. It is detailed below. - Figure 6 shows an example of the determination of a global error vector from September error vectors respectively obtained for N = 7 drivers. It is detailed below. - Figure 7 shows an example of results obtained without the application and implementation of the algorithm according to the invention on an experimental broadcast of a DRM signal from a tower to 26 MHz with the mode A. It sets obviously the residual error of the channel estimation before and after application of the common error corrections. It is detailed in paragraph 7.2 of description of a preferred embodiment of the invention. 7. Reminder general principle of the invention therefore is to allow the correction of one or more errors of phase and / or amplitude of the common cells included in the same OFDM symbol, to optimize the estimation of an OFDM channel. In more detail, in the OFDM signal, reference of said drivers are integrated effect to estimate the propagation channel. This estimate allows one hand to correct such data drivers vis-à-vis the propagation channel, and secondly to obtain the channel impulse response, the latter then being used to refine the time synchronization receivers . For information, reference drivers use in the digital sound broadcasting systems, including COFDM type ( 'Coding

Orthogonal Frequency Division Multiplex "in English," Orthogonal Frequency Multiplexing Coded "in French) is described in French Patent No. FR 2658016 of 6 February 1990 on a" scattering method digital data, especially for broadcasting high-speed to mobile, in time-frequency interleaving and coherent demodulation, and corresponding receiver. Such a method includes determining, by interpolation, a phase and amplitude reference for each of the digital elements, and so realize a coherent demodulation. 7. Description of a preferred embodiment of the invention It is therefore possible with the method according to the invention to determine the phase rotation of an OFDM symbol after channel correction. Such a technique allows to compensate the phase changes from the oscillators of the receivers, but it is also possible to determine the common amplitude variations in a symbol. Both information used to track changes in the channel at the rate of the symbols in the time domain. In a preferred embodiment of the invention, it is a correct phase and amplitude error common to the cells included in the same OFDM symbol. This technique allows to track changes in channel at the rate of the OFDM symbols, which is above that allows the vector channel estimate due to the distribution in time and frequency reference pilots are often limited to not to decrease throughput. Once the time and frequency synchronization "coarse" established by correlation of the guard intervals, for example, it is possible to obtain the channel response by interpolation on the reference drivers. Once this estimate obtained, equalization is applied to all reference drivers or data OFDM symbols. At the reference drivers, after equalization, it becomes possible to determine a residual error between the transmitted and received pilot pilots? This usually comes from an additive white Gaussian noise or, an error related to the limits of time and frequency interpolation, that is to say, the channel estimation. By integrating all the vectors on a symbol, which is to average the yields an overall error vector. Confidence vectors elementary is obtained from the channel estimation, this operation allowing the rejection of weak signals (buried in noise) or spurious peaks. The phase of this vector is the common phase error "ECP" and the magnitude of this vector to the common amplitude error 'ECA', both expressed through the following mathematical formulas and illustrated in Figure 6 . This 6 shows an illustrative example of determining an overall error vector 61 from September error vectors (62, 63, 64, 65, 66, 67, 68) respectively obtained for N = 7 drivers. The common phase error "ECP" 69 is expressed:

<img id="imgf000011_0001" he="16" wi="42" file="imgf000011_0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" /> and the common amplitude error "ECA" 61:

<img id="imgf000011_0002" he="23" wi="39" file="imgf000011_0002.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" /> or o P EJTN: is the complex residual error on the reference drivers i, where i is the index (0 to N-1) of N references considered pilots; o N: is the variable defining the number of drivers in the calculation. Complementarily, if H is the first estimate of the channel response before correction, the weight of this first channel estimate with the global error vector provides a new more accurate estimate denoted channel H₂ which takes into account the phase errors of the oscillators of the receivers. This second more accurate estimate also takes account of channel variations which could normally not be measured with the techniques of the prior art and which are partly responsible for the improvement of the channel estimation applied to the demodulation OFDM. This second more accurate channel estimation is expressed by the following mathematical formula: ^hi^not&gt;⁼ ECA where h (n) is the channel response before correction. It then becomes possible to evaluate the refined estimation of value assigned to all cells equalized by the estimate by the following mathematical formula:<img id="imgf000012_0001" he="6" wi="32" file="imgf000012_0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" /> where: oy (n): is the set of cells equalized by the estimate; Oh₂(N) is the channel response after correction by the PCE and ACE; ox (n): is the set of cells associated with an OFDM symbol. 5 is a schematic description of the functional algorithm described above and for which we recall the main steps: - Step 1: a Fast Fourier Transform ( "Fast Fourier Transformation" in English) 510 is applied to the received signal reduced baseband to identify the set x (n) 511 cells associated with each of the OFDM symbols; - Step 2: a demultiplexing 512 cells 511 then enables to separate on the one hand the reference pilots 513, and secondly the 514 data carriers; - Step 3: an interpolation time / frequency 515 is performed on all drivers reference 513, to obtain a first estimate h (n) of the channel response 516 before correction; - Step 4: 517 calculations phase common mistakes (ECP) and joint amplitude errors (ECA) are then made from the reference drivers 513, and the first estimate h (n) of the response channel 516 before correction; - Step 5: calculation of a second more accurate estimate h₂(N) of the channel response 518 after correction; - Step 6: EQ 519 all 512 cells from the set of 514 data carriers and results of the second estimate 518, so as to define the set of 520 cells equalized y (n) by estimate; Step 7: application of data processing and calculation 521 522 of the complex residual error on i reference drivers corrected, so as to perform one last time adjustment 523 of the corrected drivers. 7 illustrates an example of results obtained without application and with application of the algorithm according to the invention, on an experimental broadcasting of a DRM signal from a tower to the 26 MHz with A (which has a driver reference every twenty cells in frequency and all five cells in time). When one moves near the tower to 50km / h the 71 audio service is very disturbed: the uncorrected channel 72 includes bellies and knots 73 whose recurrence is greater than what allows the OFDM method requirements. With the technique of the invention, after correction, the audio service 71 works perfectly, as illustrated by reference 74. Figure 7 also illustrates the signals obtained for: uncorrected unit 75; a fixed 76 module; uncorrected stage 77; a phase corrected 8. 78. Advantages of the solution according to the invention The method and device for estimating a propagation channel formed of successive symbols of a multicarrier signal each comprising at least at least a reference pilot, and a plurality of frequencies carrying data, as proposed by the invention have a number of advantages, including a non-exhaustive list is given below: optimizing channel estimation in OFDM; Correction pilot data vis-a-vis the propagation channel; - Possibility of obtaining the channel impulse response, the latter can be used to refine the time synchronization receivers application to the reception OFDM mobility of the type comprising: DRM,

DVB-T, etc .; Implementation simplicity; quality and relevance of the results in terms of accuracy; efficiency of the solution and of the algorithm.