Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Reexamination Certificate
1999-09-27
2001-05-15
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
C327S047000, C375S326000, C375S340000
Reexamination Certificate
active
06232761
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a frequency estimating system for estimating a center frequency of the carrier wave of a received signal.
As the frequency estimating system of this kind, one as shown in
FIG. 6
is well known in the art. The illustrated frequency estimating system comprises a modulated component removing circuit
11
, a Fourier transforming circuit
12
, a power converting circuit
13
and a peak detecting circuit
14
. A received signal having an unknown carrier frequency, for instance, is coupled via an antenna (not shown) to the modulated component removing circuit
11
. The modulated component removing circuit
11
removes a modulated component from the received signal to obtain a non-modulated signal. The Fourier transform circuit
12
performs Fourier transform of the non-modulated signal and outputs the result as a frequency spectrum component. The power converting circuit
13
performs power conversion of the frequency spectrum component to feed a resultant power-frequency spectrum to the peak detecting circuit
14
. The peak detecting circuit
14
detects the peak power level in the power-frequency spectrum and outputs a frequency corresponding to the peak power level as an estimated center frequency fp.
In the frequency estimating system shown in
FIG. 6
, it is possible to accurately estimate the center frequency when the received signal is not subjected to fading or the like. However, when the received signals exposed to rice fading or the like so that its carrier wave component has a spread frequency range, right center frequency may not be estimated.
Taking the rice fading as an example, when the received signal is subjected to the rice fading during transmission, a fading component subjected to external disturbance by irregular reflection and a straight component not subject to any external disturbance are combined, and the resultant signal is received. In a mobile communication system or the like, in which a terminal (i.e., a mobile terminal) is moved at a high speed and receives great influence of the Doppler effect, the fading component and the center frequency of the straight component may fail to be coincident with each other.
For example, in a GSM system the received signal has a carrier wave spectrum as shown in FIG.
7
. As shown, the fading component and the center frequency of the straight component fail to be coincident. In the demodulation such that a received signal with a deviation between the fading component and the center frequency of the straight component, the best characteristic can be obtained when the received signal is inputted to a demodulator such that the center frequency of the fading component is zero. To make the fading component center frequency zero, estimation thereof is necessary. In the case of using the frequency estimating system as shown in
FIG. 6
, that is, in the case of detecting the peak of the carrier wave component, the frequency of the straight component is obtained as the estimated frequency (i.e., center frequency). In other words, when the received signal is subjected to fading during transmission, its carrier wave center frequency can not be accurately detected.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a frequency estimating system capable of accurate estimation of the received signal carrier wave center frequency even when the received signal is subjected to the influence of fading or the like in the transmission line.
According to an aspect of the present invention, there is provided a frequency estimating system for estimating a center frequency of carrier wave of a received signal subjected to the influence of fading in the transmission line, comprising a first means for obtaining power-frequency spectrum of the carrier wave of the received signal, a second means for obtaining a peak power level from power-frequency spectrum of the carrier wave and obtaining a peak power level frequency corresponding to the peak power level, and a third means for receiving the carrier wave power-frequency spectrum, the peak power level and the peak power level frequency, detecting a frequency, which is higher than the peak power level frequency and having a power level lower than the peak power level by a predetermined value, as a first lower power level frequency, detecting a frequency, which is lower than the peak power level frequency and having a power level lower than the peak power level by the predetermined value, as a second lower power level frequency, and a third means for obtaining the estimated center frequency by averaging the first and second lower power level frequencies.
The predetermined value may be 10 dB.
The third means includes a first detector for detecting the first lower power level frequency, a second detector for detecting the second lower power level frequency, and an averaging circuit for obtaining the center frequency by averaging the first and second lower power level frequencies.
The received signal contains a predetermined fixed signal series inserted therein, and the first means includes an inverse modulator for inversely modulating the received signal according to the fixed signal series to obtain a non-modulated signal free from any modulated component, a Fourier transform circuit for executing Fourier transform of the non-modulated signal to obtain a frequency spectrum, and a power converter for power converting the frequency spectrum to obtain the power-frequency spectrum.
The first means includes a demodulator for demodulating the received signal to obtain a demodulated signal, an inverse modulator for inversely modulating the received signal according to the demodulated signal to obtain a non-modulated signal free from any modulated component, a Fourier transform circuit for executing Fourier transform of the non-modulated signal to obtain a frequency spectrum, and a power converter for power converting the frequency spectrum to obtain the power-frequency spectrum.
The first means includes a frequency multiplier for frequency multiplying the received signal to obtain a non-modulated signal free from any modulated component, a Fourier transform circuit for executing Fourier transform of the non-modulated signal to obtain a frequency spectrum, and a power converter for power converting the frequency spectrum to obtain the power-frequency spectrum.
According to an aspect of the present invention, there is provided a frequency estimating method comprising steps of: obtaining a peak power level from power-frequency spectrum of carrier wave of received signal; obtaining a first and second frequencies having higher and lower than the peak power level frequency and having a power level lower than the peak power level by a predetermined value; and obtaining an estimated center frequency by averaging the first and second frequencies.
According to another aspect of the present invention, there is provided a frequency estimating method comprising steps of: obtaining a non-modulated signal free from any modulated component by inversely modulating a received signal on the basis of a predetermined fixed signal series inserted in the received signal; obtaining a peak power level from power-frequency spectrum of carrier wave of the received signal based on the non-modulated signal; obtaining a first and second frequencies having higher and lower than the peak power level frequency and having a power level lower than the peak power level by a predetermined value; and obtaining an estimated center frequency by averaging the first and second frequencies.
According to other aspect of the present invention, there is provided a frequency estimating method comprising steps of: demodulating a received signal to obtain a demodulated signal; inversely modulating the received signal according to the demodulated signal to obtain a non-modulated signal free from any modulated component; obtaining a peak power level from power-frequency spectrum of carrier wave of the received signal based on the non-modulated signal; obtaining a first and
Brown Glenn W.
Deb Anjan
McGuireWoods LLP
NEC Corporation
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