Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1999-12-03
2001-07-31
Pham, Chi (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C375S326000
Reexamination Certificate
active
06269118
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a signal carrier recovery process.
The invention applies more particularly to the case where the signal whose carrier is recovered is transmitted in a medium disturbed by echoes of large amplitude.
The invention also finds an advantageous application in the context of on-carrier digital quadrature transmission using a large number of states forming a constellation.
BACKGROUND OF THE INVENTION
Generally, with the aim of transmitting digital data through a transmission channel, these data are modulated using for example a modulation of the pulse amplitude type (“Pulse Amplitude Modulation” or “PAM”). Quadrature amplitude modulations or QAM are used to increase the sum of the data which can be transmitted within a bandwidth of an available channel. QAM modulation is a form of PAM modulation in which a plurality of information bits are transmitted together in an arrangement subsequently referred to as a constellation.
With a view to synchronizing with the signal received, the digital receiver must be provided with a device for generating a reference signal in phase with the signal received. Having been synchronized, the demodulator allows the demodulation of signals containing information in their phase. For example, in QAM modulation, the modulation of “0” and “1” bits corresponds to phases, in the modulated signal, which are determined according to rules which are known per se. Thus, the demodulator must generate a reference signal which must be synchronized in phase with the data carrier. This process is known by the name of carrier phase recovery.
In PAM modulation, each signal is a pulse whose amplitude level is determined by a transmitted symbol. In QAM modulation, for example in 16-QAM modulation, the amplitudes of the symbols −3, −1, 1 and 3 in each quadrature channel are used. It happens that the effect of each symbol transmitted through a channel extends beyond the time interval used to represent this symbol. The distortion caused by the resulting overspill of the symbols received is termed intersymbol interference (or ISI). This distortion has been one of the principal obstacles for data transmissions at high bit rate on limited bandwidth noisy channels. A device known as an “equalizer” is then used to remedy this intersymbol interference problem.
With the aim of reducing the intersymbol interference introduced by the transmission channel, accurate equalization is required. Since the characteristics of the channel are not known in advance, a statistical equalizer is thus used which carries out a mean compensation of the domain of the channels required in terms of amplitude and delay characteristics. The mean square error stochastic gradient algorithm, also known as the LMS algorithm (standing for Least Mean Squares) is generally used as adaptive equalization algorithm.
Thus, one of the essential functions of the receiver in digital transmission systems is therefore the extraction of a carrier synchronized in phase and in frequency with the carrier at the transmission end. A poor phase or a poor frequency at demodulation level reduces the power of the useful signal and creates interference between the quadrature components I and Q of the demodulated QAM signal, thus explaining the importance of the recovered phase.
Another essential function is also, as seen above, the elimination of the distortions of the signal received. Moreover, the response of the channel generally being unknown and, furthermore, susceptible of variation over time, its equalization then requires an adaptive equalizer capable of adapting itself to the channel and of tracking its temporal variations.
Now, in conventional receivers, the adaptive equalizer and the carrier recovery device, comprising a frequency estimator and a phase estimator which are switched as a function of particular criteria, follow one another in the reception chain. In this context, it is not possible to effect carrier recovery in the presence of echoes of large amplitude or of too considerable a phase shift.
SUMMARY OF THE INVENTION
The aim of the present invention is to recover considerable discrepancies in carrier frequency and, simultaneously to compensate for the large amplitude echoes.
To this end, the subject of the invention is a carrier recovery process for a received signal comprising a step of estimating the carrier frequency of this signal and a step of estimating the phase associated with this signal subsequent to the frequency estimation step, characterized in that the following steps are carried out:
during the frequency estimation step, a signal equalization step is implemented during which an equalizer comprising a direct adaptive filter and a recursive adaptive filter adapts only the coefficients of the recursive adaptive filter,
during the phase estimation step, the equalizer continues adapting for a predetermined time only the coefficients of the recursive adaptive filter.
Thus, the invention makes it possible to effect carrier recovery in the presence of echoes of large amplitude or of too considerable a frequency phase shift. The adaptation, firstly, of the coefficients of the recursive filter makes it possible to correct long echoes first before refining the equalization. Advantageously, the invention makes it possible to compensate for the powerful echoes in respect of constellations of high order, such as, for example, the constellation implementing 256-state quadrature amplitude modulation.
According to one embodiment, following the equalization by the equalizer of the coefficients of the recursive adaptive filter, a signal equalization step is implemented during which the said equalizer adapts the coefficients of the direct adaptive filter and also the coefficients of the recursive adaptive filter.
According to one embodiment, the frequency estimation step is triggered when the points received by the frequency estimator are superimposed on the patterns of the constellation representing the modulation scheme used, the 90° phase ambiguity being easily resolved in practice by differential coding, and when the points received are akin to predetermined ideal points of the constellation, these received points being situated in an acquisition zone characterized by a considerable distance to the origin and traversed by a diagonal connecting the origin of the constellation to the said point which the zone surrounds.
According to one embodiment, the frequency estimation is carried out by a measurement of phase difference corresponding to the angular distance between the point representing the signal received in the said constellation and the said diagonal.
According to one embodiment, during the frequency estimation step, an updating of an accumulator delivering the carrier frequency error to a demodulator is carried out following the measurement of the phase difference for N consecutive points received and the said ideal points corresponding thereto, N being an integer.
According to one embodiment, the equalization step during the frequency estimation step and/or the signal equalization step during the phase estimation step are carried out according to a blind or auto-recovery algorithm of the Constant Modulus or CMA type known per se, reducing the acquisition time of the system. The blind algorithm, being independent of the phase, allows the equalizer to remain stable even when the carrier is out of lock.
According to one embodiment, the convergence of the algorithm during the frequency estimation step is accelerated by increasing the number N of measurements.
According to one embodiment, the said equalization step carried out during the phase estimation step does not begin until after the estimation of the phase error relating to M points received, M being a predetermined integer.
According to one embodiment, when the convergence of the algorithm used for the phase estimation is achieved, the direct adaptive filter and the recursive adaptive filter are adapted by calculation of the error measured according to a decision feedback algorithm known per
Chevreau Sylvain
Hanna Charaf
Lopez Patrick
Kurdyla Ronald H.
Laks Joseph S.
Pham Chi
Phu Phuong
Thomson Licensing S.A.
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