Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1998-06-03
2002-09-10
Legree, Tracy (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S550100, C455S110000, C331S016000, C332S123000, C375S375000, C375S346000
Reexamination Certificate
active
06449470
ABSTRACT:
The present invention relates to a device for estimating the frequency offset, in particular for a mobile-telephone demodulator.
Frequency modulation is the modulation technique most widely used in mobile radio systems.
In frequency modulation, the signals have all their information contained in the phase or the frequency of the carrier.
In order to recover the data, the demodulator which is used must estimate the phase or the frequency of the signal. If the carrier frequency is not exactly the expected frequency (because of a difference in frequency between the frequency of the oscillators of the transmitter and of the receiver), this may lead to an inaccuracy, and therefore increase the error rate of the demodulator.
It will be assumed below that the oscillator of the receiver is perfect but the oscillator of the transmitter is imperfect. In reality, both are imperfect.
One of the great problems with radio systems is that of reducing the impact of the frequency error of the received signal by compensating for it locally.
Indeed, in order to reduce the total system cost, manufacturers are reluctant to use oscillators which are accurate and therefore expensive.
There are usually two ways of compensating for the frequency error of the carrier:
a) the frequency of the oscillator of the receiver is changed in order to adapt it to the frequency of the carrier.
This involves external compensation using a VTCXO;
b) the frequency is corrected directly inside the demodulator. This therefore involves internal compensation.
In both cases, it is necessary to have a good estimate of the frequency error in a probably imperfect environment (low signal
oise ratio) in a fairly short time.
Furthermore, if the receiver successively receives signals from different transmitters, it must frequently estimate new frequency errors.
In this case, the frequency error estimator must give a reliable estimate on the basis of a very small number of samples.
An aspect of the invention is to create a very efficient frequency error estimator used with a differential phase demodulator.
A further aspect of the invention is to create a frequency error estimator which is particularly well-suited to a &pgr;/4 QPSK demodulator, having four-state phase modulation with a jump of &pgr;/4, dedicated to wireless application such as PHS and PDC mobile telephones.
The invention therefore relates to a method for estimating the frequency offset in a signal received by a demodulator of a mobile-telephone set, characterized in that it includes the steps consisting in:
calculating an error signal &egr; on the basis of the phase &phgr; of the output signal of the demodulator and of the ideal phase &phgr;
o
of this signal;
calculating estimates of the averages of the signal &egr;(FO-RAW) and of its absolute value (Q-RAW) over a certain number of samples, taking a certain number of bits (n
FO-RAW
, n
Q-RAW
) of these estimates and applying them to means for modeling a function &OHgr;(FO-RAW, Q-RAW) representing the frequency offset.
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Craver Charles
Kempler William B.
Legree Tracy
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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