Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1998-12-22
2001-10-02
Trost, William (Department: 2683)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S226100
Reexamination Certificate
active
06298227
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of wireless communication systems, and more particularly, to a signal down-converter that can compensate for a limited-accuracy oscillator.
BACKGROUND OF THE INVENTION
Almost all radio systems receive a signal at one frequency and then convert the signal to an intermediate or baseband frequency. The radio system then demodulates the signal on the intermediate (baseband) frequency. Such radio systems require this step so that they can receive signals on a range of frequencies, but only need to demodulate the signal on one (intermediate) frequency, rather than having to demodulate the signal on each of the frequencies in the range. To convert the signal, a locally-generated intermediate frequency signal is mixed with the incoming radio frequency signal and then the resultant signal is filtered to remove all frequencies except for the signal on the intermediate frequency. This signal is then demodulated. A crystal oscillator in the receiver generates this intermediate frequency. When a complex signal that requires precise demodulation is received (such as those in wireless transceivers), the frequency reference must be very accurate so that the demodulator can recover the content cleanly.
A transceiver in current mobile stations requires a very precise intermediate frequency oscillator in order to provide a frequency reference that does not add distortion to the signal. Hence, mobile stations employ expensive, temperature-compensated crystal oscillators. There is a trade-off between the expense of precise oscillators and the degree of distortion that may be acceptable.
The present invention is directed toward overcoming one or more of the problems set forth above.
SUMMARY OF THE INVENTION
According to one aspect of this invention, a frequency converter is disclosed that compensates for a limited-accuracy local oscillator that generates an intermediate (baseband) frequency. The frequency correction device includes a frequency converter configured to receive an analog signal at a first frequency and convert the analog signal to an intermediate frequency using the intermediate frequency. An analog-to-digital converter converts the intermediate frequency signal to a digital representation of the signal. The frequency correction device also includes a plurality of digital filters, each configured to filter the digital representation of the signal at a predefined frequency offset, each digital filter filtering at a different frequency offset. There are also a plurality of correlators (advantageously equal in number to the plurality of digital filters), each configured to generate a correlation of the filtered digital representation of the signal to known synchronization words, each word having been previously shifted by the frequency offset of the correlator's respective digital filter. A selector derives a final frequency offset for use in correcting the intermediate frequency of the signal to remove any distortion introduced by the limited-accuracy oscillator.
In accordance with another aspect of this invention, the selector is configured to derive the final frequency offset by finding the highest correlation, by averaging the correlation results, by taking a weighted average of the correlation results or by averaging all of the correlation results with previous correlation results.
In accordance with another aspect of this invention, the number of filters and correlators vary with the desired accuracy of the frequency control device. Further, the filters and/or correlators may operate in parallel or in series.
In accordance with another aspect of this invention, a frequency converter is described that generates a frequency offset to compensate for a limited-accuracy oscillator. The frequency correction device comprises a limited-accuracy local oscillator generating an intermediate frequency, a summer for summing the frequency offset with the intermediate frequency from the local oscillator and a frequency converter configured to receive an analog signal at a first frequency and convert the signal to an intermediate frequency using the summed intermediate frequency. An analog-to-digital converter converts the signal to a digital representation of the signal. A plurality of digital filters are configured to filter the digital representation of the signal at assigned frequency offsets. There are also a plurality of correlators that are configured to receive filtered digital representation of the signal from the plurality of filters and configured to generate a correlation of the filtered digital representation of the signal to known synchronization words shifted by an assigned frequency offset. A selector derives a final frequency offset from a plurality of correlation results and delivers the final frequency offset to the summer.
In accordance with yet another aspect of this invention, the summer is configured to sum the oscillator's reference frequency with a portion of the final frequency offset.
In accordance with yet another embodiment of this invention, a method is described for deriving a frequency offset to correct an intermediate frequency signal for frequency errors introduced in the down-conversion by a limited-accuracy oscillator. The method comprises the steps of converting a signal received at a first frequency to a second frequency using the limited accuracy oscillator, filtering adjacent and other signals from the signal and converting the filtered signal to digital symbols. The method further includes the steps of filtering the digital symbols by plurality of frequency offsets, correlating the filtered digital symbols with a plurality of digital representations of synchronization words to generate a correlation and selecting one of the correlations as the frequency offset for decoding the signal.
In accordance with another aspect of this invention, the step of filtering occurs simultaneously for all filters or the step could occur for each filter serially. Finally, the step of synchronizing could occur simultaneously for all correlators or for each correlator serially.
REFERENCES:
patent: 5796777 (1998-08-01), Terlep et al.
patent: 5859842 (1999-01-01), Scott
patent: 6128469 (2000-10-01), Zenick, Jr. et al.
J. Ahmad et al., “DSP Based Carrier Recovery Techniques for OQPSK Mobile Satellite Communications,”Signal Processing VI: Theories and Applications, 1992 Elsevier Science Publishers B.V., pp. 1677-1680.
J. Ahmad et al., “Digital Frequency Error Detectors for OQPSK Satellite Modems,”Electronics Letters, Sep. 26, 1991, vol. 27, No. 20, pp. 1821-1823.
S. Holm, “Optimum FFT-Based Frequency Acquisition with Application to COSPAS-SARSAT,”IEEE Transactions on Aerospace and Electronic Systems, vol. 29, No. 2, Apr. 1993, pp. 464-475.
Coats & Bennett
Ericsson Inc.
Perez-Gutierrez Rafael
Trost William
LandOfFree
Method and apparatus for frequency conversion for a receiver... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for frequency conversion for a receiver..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for frequency conversion for a receiver... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2611704