Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1997-12-29
2001-01-30
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Receivers
Angle modulation
C329S307000, C329S304000
Reexamination Certificate
active
06181750
ABSTRACT:
The invention relates to digital signal transmission, and more precisely to apparatus for estimating the frequency difference existing between the carrier frequency of a digital signal received by a receiver, and the frequency of the output signal of a local oscillator contained in the receiver and serving to demodulate the received signal or at least to transpose it into intermediate frequency.
BACKGROUND OF THE INVENTION
The invention is particularly applicable to satellite signal transmission in which, in particular, the Doppler effect, satellite drift, and the instability of the frequency transposition equipment impart frequency shifts to the received signal. At a ground station receiving the signal from the satellite, a rotation of the constellation is observed after the received signal has been transposed into baseband, and this does not enable the received signal to be sampled at the instants when the eye diagram is at its most open.
FIG. 1
is a block diagram of a digital signal receiver including an estimator for estimating the frequency difference existing between the carrier frequency of a received digital signal and the frequency of a receive local oscillator.
The receiver shown in
FIG. 1
includes a direct-conversion quadrature demodulator to which the received signal, referenced SR, is applied. More precisely, the signal SR is applied to two mixers
10
,
11
also receiving respective demodulation signals from a local oscillator
12
, one of the mixers (
10
) receiving the demodulation signal via a 90° phase-shifter referenced
13
. The mixers
10
and
11
deliver baseband signals to respective analog-to-digital converters (ADC)
14
,
15
followed by receive filters
16
,
17
that are conventionally Nyquist root band-pass pass filters (complex Nyquist filters). The filters
16
and
17
reject adjacent channels outside the band of the modulated signal SR and deliver respective trains I and Q respectively representing the in-phase component and the quadrature component of the modulated signal SR. Each received symbol is encoded over K samples (for example, K is equal to 4). The trains I and Q are applied to an estimator
18
serving to determine the frequency difference between the signal SR and the output signal of the local oscillator
12
. Said frequency difference is referenced &egr; and is applied to the local oscillator
12
so as to perform automatic frequency control.
The estimator
18
operates conventionally using the Alberty algorithm. The drawback with that algorithm is that it does not work when interference is present for frequency shifts greater than Ds/4, where Ds is the symbol rate. In satellite transmission, such interference is constituted by the channels adjacent to the demodulated channel. Whenever one of the adjacent channels enters the band of a receive filter at ±Ds/2, the estimator
18
locks onto the adjacent channel and the frequency difference can no longer be measured. As a result the expected signal is lost.
SUMMARY OF THE INVENTION
A particular object of the invention is to remedy these drawbacks.
More precisely, one of the objects of the invention is to provide apparatus for estimating the frequency difference existing between the carrier frequency of a received digital signal and the frequency of a local oscillator contained in a receiver of the received signal, the frequency estimation apparatus working in a wider band than the Alberty algorithm, and being less sensitive than that algorithm to the presence of interference.
This object, as well as others that appear below, is achieved by means of apparatus for estimating the frequency difference existing between the carrier frequency of a received digital signal and the frequency of a signal from a local oscillator contained in a receiver of the received signal, the receiver performing quadrature demodulation on the received signal, the apparatus comprising:
detector means for detecting the noise level received in the receive filters bands;
computing means for computing discrete Fourier transforms on the received signal transposed into baseband;
summing means for summing the energy levels detected on either side of the receive filter center frequencies;
subtractor means for subtracting the noise level from the energy bands; and
comparator means for comparing the resulting energy levels, the comparator means delivering an estimate of the frequency difference.
Such an estimator offers the advantage of working for frequency shifts less than Ds/2 in the absence of interference.
Advantageously, the comparator means are means for computing the ratio of the resulting energy levels.
The apparatus of the invention preferably includes means for detecting a frequency shift greater than Ds/4, where Ds is the symbol rate.
These means for detecting a frequency shift greater than Ds/4 advantageously compare the energy level of the midpoint (pm) of the spectrum obtained by means of the computing means with the energy levels of two points situated immediately below the −3 dB energy levels of the spectrum, and they cause the frequency of the local oscillator to be shifted by ±Ds/4 when one of the two energy levels is greater than the energy level of the midpoint.
The invention also provides a digital signal receiver including such estimation apparatus.
REFERENCES:
patent: 5077531 (1991-12-01), Takeuchi et al.
patent: 5103431 (1992-04-01), Freeman et al.
patent: 5272446 (1993-12-01), Chalmers et al.
patent: 5995483 (1999-11-01), Marchok et al.
Ferrad Abdelkrim
Guillemain Christian
Alcatel
Pham Chi H.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Tran Khai
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