The present invention relates to a digital signal receiver and, more particularly, to a method of compensating the Doppler effect, and to the associated device for processing a receive signal affected by a distortion due to the Doppler effect. It falls in particular within the scope of the implementation of the emergent DVB-H (Digital Video Broadcast Handheld) standard, the object of which is to enable digital television programmes to be received from mobile terminals.
The reception of digital television programmes in moving vehicles in an urban, suburban or rural environment is technically possible but requires powerful receivers in order to minimize the effects of propagation and, more particularly, the Doppler effect due to the speed which causes, among other things, a frequency shift on the channel.
The two-channel or four-channel antenna diversity solutions remain an effective means of combating these phenomena which cause unwanted breaks in reception. They require the implementation on the one hand of multiple antennas on a vehicle and a receiver provided with a number of complete receive subsystems, associated with the antennas, and on the other hand, a relatively costly processing system. It would also not be appropriate for battery-powered applications.
The DVB-H system, through an intermediate choice of the number of carriers, in the event of the “4K carriers and COFDM (Combining Orthogonal Frequency Division Multiplex)” type, offers an acceptable trade-off in terms of channel bit rate and tolerance to Doppler effect.
Currently, digital processing functions incorporated in COFDM demodulator circuits make it possible to significantly improve the tolerance to Doppler effect, but this phenomenon nonetheless still remains present and detrimental to mobile reception.
The invention proposes to remedy this problem.
The invention consists of a method of compensating the Doppler effect for a mobile receiver on a signal affected by a distortion due to the Doppler effect. It consists in implementing, based on the signal to be demodulated, a phase-locked loop acting on the frequency control of the tuner element for selecting channels.
The step for implementing the phase-locked loop comprises:
The method proposed by the invention thus makes it possible to very significantly improve the tolerance to Doppler spreading since it makes it possible to deliver a signal partly purged of the distortion inherent to this phenomenon. It provides a way of reinforcing the effectiveness of the demodulator and therefore, consequently, of the receiver.
The invention also relates to a digital data receiver comprising, upstream of the demodulator, a means of processing the intermediate-frequency signal to be demodulated to determine the image of the spurious modulation, a means for defining the sign of the Doppler shift, and a means for creating a tuner element frequency control indication, from the preceding data, for correcting, before modulation, the frequency drift generated by the Doppler effect.
The compensation device thus makes it possible to very significantly improve the tolerance to Doppler spreading since it delivers to the demodulator a signal already partially purged of the distortion inherent to this phenomenon. This demodulator, which itself comprises an integrated Doppler compensation digital processing circuit, thus makes it possible to largely eliminate the disturbances due to this Doppler effect.
The receiver according to the invention can be integrated in a macro-module, which offers the advantage of enabling it to be inserted in mobile terrestrial digital receivers.
The characteristics and advantages of the invention mentioned above, and others, will become more clearly apparent from reading the following description, given in conjunction with the appended drawings, in which:
The principle of the device or the method according to the invention is based on the analysis of the Doppler phenomenon explained below:
If the signal received by the antenna 1 of the receiver is affected by an echo but without Doppler shift, then the echo of this OFDM-modulated signal t of frequency ωm and of amplitude a, has an amplitude b and delay
R=a sin(ωmt)+b sin ωm(t+.
The amplitude of such a signal takes the form:
A#√{square root over ( )}(a2+b2)+b cos (ω.m
The amplitude is a function of the frequency of the carrier with maxima for ωm
If the receiver is mobile, the echo b is then affected by the Doppler effect and subjected to a frequency shift (Δfm), the resultant signal seen by the receiver at the instant t then becomes:
R=a sin(ωmt)+b sin(ωm+Δωm)(t+
The OFDM signal then has an amplitude of the form:
A#√{square root over ( )}(a2+b2)+2b cos(Δωm
The maxima correspond to the Doppler frequency (Δωm=2
The result is that the carriers of the OFDM signal for a given received channel are affected by a spurious modulation in pace with the Doppler frequency.
The method according to the invention then consists in determining the image of the spurious modulation applied by the Doppler effect and the sign of the Doppler shift Δωm, then in processing these two data items, so as to form a control signal to adjust the frequency of the tuner.
An exemplary embodiment of the device according to the invention, associated with the method, is represented by
The signal received by the antenna 1 is applied to the tuner 2 which selects a receive channel. The intermediate-frequency signal (for example 36 MHz) is then applied to the amplifier 3 and then to the demodulator 4 thus delivering the demodulated signal. In the present case, it is a COFDM demodulator, but any other demodulator corresponding to the modulation of the received signal can be envisaged. This demodulator incorporates a Doppler correction circuit enabling a partial compensation of the Doppler effect.
According to the invention, a compensation of the Doppler effect is provided by a circuit sampling the intermediate-frequency IF signal upstream of the demodulator 4 and thus makes it possible to apply, by a phase-locked loop, on the tuner element, an adjustment of the channel selection frequency. This IF signal input to this compensation circuit is sampled by a coupler, not shown in
The input signal of the compensation circuit is an intermediate-frequency IF signal which is applied to a frequency discriminator formed by a mixer 11 and a delay circuit 12, then filtered by a low-pass filter 13. The intermediate-frequency signal is applied to the RF input of the mixer and this same signal, affected by the delay T, introduced by the delay circuit 12, is applied to the LO input. This circuit therefore constitutes a delay-line demodulator known in the state of the art. It can be demonstrated that, for example, for a delay of
By applying this OFDM signal at intermediate frequency affected by the distortion added by the Doppler effect on the frequency discriminator formed by the mixer 11 and the delay element 12, the result is a signal for which the frequency modulation indication (Doppler fault) is contained in the frequency of the signal, after filtering. This signal therefore represents the image of the spurious modulation caused by the Doppler effect.
A second block 14 is responsible for determining the sign of the Doppler shift from the intermediate-frequency signal.
The two Doppler shift and sign indications are then sent to a processing block 15 managed by microcontroller charged with creating a new control indication to correct the adjustment of the tuner channel selection frequency.
Since the Doppler shift at its maximum is of the order of a few hundred Hz, the frequency synthesizer 5 of the tuner element 2 will in this case be implemented by a “Fractional N” type phase-locked loop providing an accuracy of about 1 Hz.
The possibility of integrating all these circuits within a macro-module thus allows this data processing circuit to be inserted into mobile terrestrial digital receivers.
This block comprises, as main element, a demodulator 20. A reference signal created by a digital synthesizer 21 is applied to the LO input of the demodulator 20. A circuit 23, phase-shifter and coupler, will be used to send the signal phase-shifted by 90° on one of its outputs, whereas it will be sent without phase shift on the other output. These two signals are respectively applied to the LO inputs of two mixers 25 and 26. The intermediate-frequency IF signal, sampled upstream of the demodulator (see
Other variants of the invention are possible. The examples described previously show a reception of the COFDM-modulated signal. Other modulations can be envisaged.
The examples described above show a circuit receiving the intermediate-frequency signal upstream of the demodulator. Any signal, affected by the disturbances of the Doppler effect, can be sampled upstream of the demodulator, at the amplifier or at other levels to control this phase-locked loop.
Number | Date | Country | Kind |
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0553160 | Oct 2005 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2006/067435 | 10/16/2006 | WO | 00 | 4/9/2008 |