Abstract:
A COFDM demodulator including a fast Fourier transform circuit receiving a signal on an information channel, the received signal corresponding to a sequence of symbols, each conveying several carriers, some of which are pilots, each carrier of a symbol being modulated in plase and/or in amplitude by a current complex coefficient, the fast Fourier transform circuit providing for each carrier the associated current complex coefficient; a circuit for determining an estimate of the frequency response of the information channel providing, for each pilot, a complex coefficient estimated based on the current complex coefficient associated with the pilot, and a circuit for determining the time variation of the frequency response of the information channel based on the estimated and current complex numbers associated with at least one pilot.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a Coded Orthogonal Frequency Division Multiplex (COFDM) demodulator. 
   2. Discussion of the Related Art 
     FIG. 1  illustrates the principle of a COFDM modulation. Data packets to be transmitted are put in the form of N complex coefficients associated with N respective frequencies (or carriers). Number N of the frequencies is equal, for example, to 1,705 for the so-called “2K” mode and to 6,817 for the so-called “8K” mode, in digital television radio transmission. Each complex coefficient corresponds to a vector which is illustrated in  FIG. 1  as starting from a frequency axis at a point indicating the frequency associated with the coefficient. 
   The set of these N coefficients is processed by inverse fast Fourier transform (IFFT), which generates a “symbol” formed of a sum of modulated carriers, each carrier having an amplitude and a phase determined by the associated complex coefficient. The symbol thus generated is transmitted. 
   Conventionally, in radio transmission, the width of the information channel is 6, 7, or 8 MHz and each carrier is separated from the next one by a frequency difference Δf=1/Tu. Tu is the transmission time of a symbol and is called the operating lifetime. The operating lifetime is on the order of 224 μs in 2K mode and 896 μs in 8K mode, for a 8-MHz passband. 
   Upon reception, a receiver submits the symbol to the inverse processing, that is, mainly, a fast Fourier transform (FFT) to restore the initial complex coefficients. 
   The receiver actually receives the signal transmitted by the transmitter as well as a multitude of attenuated and delayed signals originating from different echoes. The information channel, taken by the signal to be demodulated, is then said to be a multiple-path channel. Such a channel has a frequency response which is not flat, but comprises holes and bumps due to the echoes and reflections between the transmitter and the receiver. The channel is said to be of fixed type when it has a frequency response which is substantially constant along time. Conversely, the channel is said to be of time-variable type when it has a frequency response which varies along time. 
   To determine an estimate of the frequency response of the information channel, regularly distributed vectors P 1 , P 2 , P 3  . . . having a known constant value are used. These vectors, or the corresponding carriers, are said to be pilots. They are used to reflect the distortions undergone by the transmitted channel and by the information that they provide on the channel response, they enable correcting the unknown vectors located between pilots. 
     FIG. 2  schematically shows the place of pilots in the symbols. The symbols are gathered in frames of 68 symbols, conventionally in digital television radio transmission (standard ETSI EN 300 744, V1.4.1). 
   In  FIG. 2 , each line represents a symbol and each box represents the position of a carrier. The carriers are defined as going from a position  0  to a position Kmax, Kmax being equal to 1,704 in 2K mode and 6,816 in 8K mode. Indeed, a portion only of the possible frequencies is used, especially due to risks of losses at the channel border. The pilots are of two types. 
   On the one hand, there are, in each symbol, continuous pilots Pc. The continuous pilots correspond to specific frequencies distributed in the channel. In the above-mentioned ETSI standard, there are 45 of these in 2K mode and 177 in 8K mode. Continuous pilots are present in all the symbols and always occupy the same frequency position. In  FIG. 2 , only the continuous pilots corresponding to positions  0  and Kmax have been shown. 
   On the other hand, there are, in each symbol, so-called “scattered pilots” Pr, which are arranged every 12 carriers, and shifted by three positions between two successive symbols. Thus, every four symbols, the same arrangement of scattered pilots Pr can be found. 
   The continuous and scattered pilots, of constant amplitude on transmission, are used to know the frequency and pulse response of the channel. For this purpose, at the receiver, the complex time received signal, after having been put in baseband, is provided to a fast Fourier transform unit providing the symbol in the frequency field. The pilots are sampled from this symbol. An estimate of the frequency response of the channel is determined based on the continuous and scattered pilots sampled from four successive symbols. The estimate of the frequency response is especially used to correct the vectors associated with the carriers located between pilots. The pulse response of the channel, obtained from the inverse Fourier transform of the frequency response estimate, is especially used to finely position a window on which the fast Fourier transform is performed. 
   Many operating parameters of the COFDM demodulator are generally optimized on operation of the COFDM to improve the demodulator performance. 
   A method to adjust the operating parameters of the demodulator consists of analyzing the data received after complete decoding and determining bit error rate BER. The previously-described operating parameters of the demodulator can then be modified by trial and error until the bit error rate is acceptable. 
   Another criterion of the measurement of the demodulation quality corresponds to the signal-to-noise ratio SNR which can be determined on extraction of the continuous or scattered pilots. 
   Among the operating parameters of the demodulator that can be optimized, some depend on the fixed or time-variable type of the channel. For example, according to the nature of the channel, a specific method can be privileged to determine the channel frequency response, thus enabling more efficiently obtaining a precise estimate of the frequency response of the channel. Further, according to the channel type, the amplifier gains provided at the demodulator input may be set in adapted fashion to amplify the signal received by the demodulator. Further, before performing the fast Fourier transform, the complex signal is generally corrected in frequency and in time by algorithms implementing time constants that can be adjusted according to the type of channel. 
   However, the method for optimizing the operating parameters of the demodulator using bit error rate BER is relatively inaccurate and slow as concerns the operating parameters that can be optimized according to the type of information channel. Indeed, the complete data decoding must be awaited, which can be very long. A modification of the operating parameters of the demodulator based on such a criterion is thus performed long after a channel variation, and thus generally too late to avoid a data loss. Further, bit error rate BER does not indicate how the operating parameters of the demodulator are to be modified to improve its performances. Indeed, a proper adjustment of the operating parameters of the demodulator will only be acknowledged by a subsequent decrease in the bit error rate. 
   Like for bit error rate BER, it is not possible to deduce from signal-to-noise ratio SNR the reasons of the degradation of the demodulation. A parameter optimization method implementing signal-to-noise ratio SNR is thus slow and inaccurate since the operating parameters of the demodulator must thus be modified by trial and error until the signal-to-noise ratio is acceptable. 
   SUMMARY OF THE INVENTION 
   The present invention aims at a method and circuit, for a COFDM demodulator, providing an indicator enabling adjusting, in a fast and accurate manner, the operating parameters of the demodulator that can be optimized according to the fixed or time-variable type of the information channel. 
   According to another object, the present invention aims at a method and a circuit providing an indicator representative of the way in which the operating parameters of the demodulator, which can be optimized according to the fixed or time-variable type in the information channel, are adjusted. 
   To achieve this and other objects, the present invention provides a COFDM demodulator comprising a fast Fourier transform circuit receiving a signal on an information channel, the received signal corresponding to a sequence of symbols, each conveying several carriers, some of which are pilots, each carrier of a symbol being modulated in phase and/or in amplitude by a current complex coefficient, said fast Fourier transform circuit providing for each carrier the associated current complex coefficient; a circuit for determining an estimate of the frequency response of the information channel providing, for each pilot, a complex coefficient estimated based on the current complex coefficient associated with the pilot; and a circuit for determining the time variation of the frequency response of the information channel based on the estimated and current complex numbers associated with at least one pilot. 
   According to an embodiment of the present invention, the circuit for determining the frequency response estimate comprises a circuit for providing, for each pilot of a symbol, a differential complex coefficient obtained based on the difference between the current and estimated coefficients associated with the pilot. 
   According to an embodiment of the present invention, the circuit for determining the time variation of the frequency response of the information channel comprises a circuit for providing a first value representative of the modulus of the estimated complex coefficient associated with the pilot; a circuit for providing a second value representative of the modulus of the differential complex coefficient associated with the pilot; and a circuit for providing an instantaneous indicator of the time variation of the frequency response of the information channel for the pilot, corresponding to the ratio between the second value and the first value. 
   According to an embodiment of the present invention, the circuit for determining the time variation of the frequency response of the information channel comprises a circuit for determining the sum of the instantaneous indicators for all the pilots of a symbol. 
   According to an embodiment of the present invention, the circuit for determining the time variation of the frequency response of the information channel comprises a circuit for determining a mean indicator equal to the time average of the sum of the instantaneous indicators. 
   According to an embodiment of the present invention, the demodulator comprises a correction circuit receiving the mean indicator and modifying an operating parameter of the demodulator according to the mean indicator. 
   The present invention also provides a COFDM-type demodulation method comprising a step of fast Fourier transformation of a signal received from an information channel, the received signal corresponding to a sequence of symbols each carrying several carriers, some of which are pilots, each carrier of a symbol being modulated in phase and/or in amplitude by a current complex coefficient; a step for determining an estimate of the frequency response of the information channel based on the current complex coefficients associated with the pilots; and a step for determining the time variation of the frequency response of the information channel, comprising providing, for each pilot, a complex coefficient associated with the pilot based on the estimated complex coefficient associated with at least one pilot and on the current complex coefficient associated with the pilot. 
   According to an embodiment of the present invention, the step of determining the time variation of the frequency response of the information channel comprises the steps of providing a first value representative of the modulus of the estimated complex coefficient associated with the pilot; providing a second value representative of the modulus of a differential complex coefficient obtained from the difference between the current and estimated complex coefficients associated with the pilot; and providing an instantaneous indicator of the time variation of the frequency response of the information channel for the pilot, corresponding to the ratio between the second value and the first value. 
   According to an embodiment of the present invention, the step of determining the time variation of the frequency response of the information channel comprises the determination of the sum of the instantaneous indicators for all the pilots of a symbol. 
   According to an embodiment of the present invention, the step of determining the time variation of the frequency response of the information channel comprises the determination of the time average of the sum of the instantaneous indicators. 
   The foregoing object, features, and advantages, as well as others, of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , previously described, shows phase- and amplitude-modulated carriers in a COFDM transmission system; 
       FIG. 2 , previously described, schematically shows the position of pilots in symbols; 
       FIG. 3  shows an example of a demodulator according to the present invention; 
       FIG. 4  shows an example of the forming of a portion of a unit for estimating the frequency response of an information channel according to the present invention; and 
       FIG. 5  shows an example of the forming of a unit for determining the type, fixed or variable along time, of the channel. 
   

   DETAILED DESCRIPTION 
   The present invention aims at providing, in quasi-instantaneous fashion, an indicator representative of the fixed or time-variable type of the information channel. This enables obtaining a fine demodulation since the adjustments of the operating parameters of the demodulator which depend on the information channel type can then be performed vary fast. 
     FIG. 3  shows an example of a demodulator according to the present invention. The received signal especially comprises continuous pilots, scattered pilots, and data carriers. 
   In  FIG. 3 , an input E of the demodulator receives a signal IF of intermediary frequency enabling a sampling, for example, at 36 MHz. Signal IF corresponds to the signal received after various frequency changes or transpositions. 
   Input E is coupled to an analog-to-digital converter  10  (ADC) which digitizes input signal IF. Analog-to-digital converter  10  drives a unit  11  for suppressing pulse interferences. Unit  11  provides a signal corresponding to the signal provided by analog-to-digital converter  10  in which pulse interferences have been suppressed. Unit  11  drives a frequency change unit  12 . Unit  12  provides a signal substantially in baseband, the signal spectrum at the output of unit  12  being centered on a frequency substantially equal to zero. Unit  12  is coupled to a unit  14 , enabling on the one hand fine setting of the central frequency of the signal spectrum and, on the other hand, providing time samples at times appropriate to the subsequent processing. At the output of unit  14 , the signal spectrum is centered on a frequency equal to 0 and the number and the time position of the samples are adapted to the transformation by Fourier transform which is performed in the next unit. Unit  14  is controlled by connections  15  and  15 ′ connecting unit  14  to a unit  16  for processing the continuous and scattered pilots. 
   The output of unit  14  drives a fast Fourier transform unit  20  (FFT) which provides the frequencies corresponding to a symbol. Unit  20  is driven by a unit  22  which provides, via a connection  24 , a signal for setting the analysis window of the Fourier transform. 
   The output of unit  20  is coupled to unit  16  which extracts and processes the continuous and scattered pilots. Unit  16  provides on connections  15  and  15 ′ the signals intended to correct the central frequency of the spectrum and the sampling frequency of the signal. 
   The output of unit  20  drives a unit  30  in which the signal is connected by means of an estimate of the frequency response of the channel. The estimate of the channel frequency response is obtained in unit  16  by means of the pilots. This estimate is provided by unit  16  on a connection  55 , having a branch  55   a  coupled to unit  30 . At the output of unit  30 , the signal especially comprises the corrected carriers conveying the data. 
   The estimate of the channel frequency response, provided by unit  16 , supplies, via connection  55  and a branch  55   b  of connection  55 , an inverse fast Fourier transform unit  26  (IFFT), to determine the channel pulse response. Unit  26  provides the channel pulse response to unit  22 , to dynamically adjust the positioning of the FFT analysis window. 
   The processing of the carriers conveying the data is ensured in a data processing and provision circuit  40 . Circuit  40  has a conventional structure and may comprise, as shown in  FIG. 3 , a symbol disinterlacing unit  42 , a so-called “demapping” unit  44 , a bit disinterlacing unit  46 , and an error correction unit  48  (FEC). The output of unit  48  forms output S of circuit  40  and of the demodulator and provides data corresponding to the transmitted data. 
     FIG. 4  shows a more detailed example of the forming of a circuit  50  which corresponds to the portion of unit  16  relative to the determination of the estimate of the channel frequency response. Circuit  50  is used in integrated circuit STV0360E, available from STMicroelectronics. Circuit  50  is duplicated for each continuous and scattered pilot contained in a symbol and, for a considered pilot, circuit  50  is duplicated for the real part and for the imaginary part of the complex coefficient associated with the pilot. The operation of the different components of circuit  50  is synchronized by a clock signal, not shown. 
   Circuit  50  receives as an input a signal PILOT corresponding to the real part or to the imaginary part of the complex coefficient associated with a pilot extracted by another portion of unit  16 , not shown. Circuit  50  comprises a circuit  60 , also called a static estimator, which for example determines the time average of signal PILOT. As an example, signal PILOT is a digital signal coded over eight signed bits. Circuit  60  comprises a multiplication unit  62  receiving signal PILOT and the content of a memory  64  in which is stored a gain MP. Multiplication unit  62  provides signal PILOT multiplied by gain MP to a first input of an adder  66 . The output signal of adder  66  is stored in a latch (L)  68 . A multiplication unit  70  multiplies the content of latch  68  by a gain 1-MP stored in a memory  72 . Multiplication unit  70  drives a second input of adder  66 . Latch  68  provides a signal Emean corresponding to the time average of signal PILOT. Gains MP and 1-MP act as time constants for the determination of signal Emean. As an example, MP is equal to 4/16. Circuit  60  can be deactivated by setting MP to 1. 
   Circuit  50  comprises a subtractor  74  receiving signal PILOT and signal Emean and providing a signal Eprediction corresponding to the difference between signal PILOT and signal Emean, and thus to the difference between the last value of signal PILOT provided to circuit  50  and the mean value of signal PILOT. 
   Circuit  50  comprises a conventional circuit  76  for correcting signal Eprediction, also called a dynamic estimator. Circuit  76  for example is of predictor or interpolator type. Circuit  76  receives signal Emean and signal Eprediction and provides a signal Eestimate which corresponds to an estimate of signal Eprediction and a signal Echannel-estimate which corresponds to the sum of signal Emean and of signal Eestimate. Signal Echannel-estimate thus corresponds to an estimate of signal PILOT. 
     FIG. 5  shows an example of the forming of unit  110  for determining the fixed or time-variable type of the information channel. For this purpose, unit  110  determines an indicator representative of the fixed or time-variable type of the channel. Any variation of the indicator translates a time variation of the channel characteristics. For each successive symbol, and within a symbol, for each continuous and scattered pilot, unit  110  receives signals Emean R  and Emean I  which correspond to signal Emean respectively obtained from the real part and from the imaginary part of the complex coefficient associated with the considered pilot, and signals Eprediction R  and Eprediction I , which correspond to signal Eprediction respectively obtained from the real part and the imaginary part of the complex coefficient associated with the considered pilot. Signals Emean R  and Eprediction R  are provided by a same circuit  50  and signals Emean I  and Eprediction I  are provided by a same circuit  50 . Signals Emean R  and Emean I  drive a circuit  114  for determining the modulus of the complex number Em having Emean R  as a real part and Emean I  as an imaginary part. Similarly, signals Eprediction R  and Eprediction I  drive a circuit  114  for determining the modulus of complex number Ep having Eprediction R  as a real part and Eprediction I  as an imaginary part. Since the determination of a modulus requires performing multiplication operations, it may be desirable to use an approximate value of the modulus of a complex number z, of real part a and of imaginary part b, given by the following relation:
 modulus( z )=( d max+3/8* d min), with   d min=min(abs( a ),abs( b )) and  d max=max(abs( a ),abs( b )) 
   where abs corresponds to the absolute value function, min corresponds to the minimum function, and max corresponds to the maximum function. It should be clear that the modulus of complex number Ep may be determined according to any technique known by those skilled in the art. 
   Unit  110  comprises a division unit  116  connected to circuits  112 ,  114  and providing a signal DYNinst corresponding to the ratio of the moduli of Ep and of Em. Signal DYNinst corresponds to an instantaneous indicator representative of a time variation of the frequency response of the channel for the considered pilot. Given that a division is performed at the level of unit  116 , it is desirable to provide for circuit  112  to set the modulus of complex number Em at a minimum non-zero constant value, when the modulus is smaller than a determined threshold. 
   The output of unit  116  drives a first input of an adder  118  providing a signal stored in a latch  120 . A second input of adder  118  is connected to the output of latch  120 . When signals Emean R , Emean I , Eprediction R , Eprediction I  associated with the continuous and scattered pilots of a symbol have successively been transmitted to unit  116 , the signal provided by latch  120  is equal to the sum of the instantaneous indicators associated with the continuous and scattered pilots of a symbol. Latch  120  drives a multiplication unit  122  capable of multiplying the signal provided by latch  120  by a gain CST stored in a memory  124 . An adder  126  receives at a first input the signal provided by multiplication unit  122  and provides a signal stored in a latch  128 . Latch  128  provides a signal DYNmean, corresponding to an averaged indicator, which is transmitted to a multiplication unit  130  multiplying signal DYNmean by a gain 1-CST stored in a memory  132 , the result of the multiplication being provided to a second input of adder  126 . Signal DYNmean thus corresponds to the time average of the sum of the instantaneous indicators. Gains CST and 1-CST correspond to the time constants used to determine the time average of the sum of the instantaneous indicators. As an example, gain CST is equal to 1/16. 
   Signal DYNmean being coded over a limited number of bits, it may be necessary to multiply signal DYNmean by a scale factor, which may depend on the type of demodulation used (“2K” or “8K” demodulation), especially to obtain an easily-interpretable numerical value. 
   The higher the instantaneous indicator DYNinst associated with a pilot, the more the channel frequency response varies along time with respect to the considered pilot. Conversely, the smaller the instantaneous indicator DYNinst associated with a pilot, the more constant the frequency response of the channel is along time with respect to the considered pilot. 
   The higher averaged indicator DYNmean, the more globally variable the channel frequency response is along time. Conversely, the smaller averaged indicator DYNmean, the more constant the channel frequency response is along time. 
   The values of the instantaneous and averaged indicators may be provided to various components of the demodulator to modify operating parameters of the demodulator. For example, according to the values of the instantaneous indicators and/or of the averaged indicator, the values of gains MP and of the coefficients used in the circuit  76  can be modified to privilege an estimate of the frequency response of the channel for the continuous or scattered pilots via average detection circuit  60  or circuit  76  of correction of unit  16 . According to another example, the gains of the amplifiers provided upstream of analog-to-digital converter  10  may be modified according to the values of the instantaneous indicators and/or to the averaged indicator. According to another example, the time constants of the algorithms implemented by unit  14  may be adjusted according to the values of the instantaneous dynamism indicator and/or of the averaged indicator. 
   According to an alternative of the present invention, unit  110  for determining the fixed or time-variable type of the information channel receives signal Eestimate instead of signal Eprediction. Signal Eestimate is then used instead of signal Eprediction in the rest of unit  110  which is further identical to what has been described previously. Indeed, signal Eestimate, which corresponds to an estimate of signal Eprediction, has the same information content as signal Eprediction. 
   Of course, the present invention is likely to have various, alterations, improvements, and modifications which will readily occur to those skilled in the art. In particular, the detail of the processing of the continuous and scattered pilots is a non-limiting example only, and it is within the abilities of those skilled in the art to appropriately modify this processing. 
   Also, in the example of a demodulator of  FIG. 5 , all units may be modified or replaced with appropriate elements. For example, input E of the circuit may directly receive a signal centered on approximately 4.5 MHz. The analog-to-digital converter may be external to the demodulator. 
   The present invention has mainly been described in the context of the digital television radio transmission, defined by standard ETSI EN 300 744, V1.4.1. However, the present invention is neither limited to this standard, nor to this field, and may be applied in and to any device comprising a COFDM demodulator, be it a television receiver or not. For example, the demodulator according to the present invention may be used in a portable phone. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.