Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-03-28
2003-07-22
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S344000
Reexamination Certificate
active
06597728
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a device and a method for correcting the local oscillation frequency of a local oscillator of a radio reception device according to pilot synchronous detection.
DESCRIPTION OF THE PRIOR ART
In mobile radio communication systems, higher usage efficiency of the radio wave frequency is desired for increasing the subscriber capacity of the mobile radio communication systems. For attaining the objects, mobile radio communication systems applying pilot synchronous detection to CDMA (Code Division Multiple Access) have been proposed. In the pilot synchronous detection, frequency difference (shift) between a received radio signal and a radio reception device (a local oscillator of the radio reception device) has to be corrected and eliminated. Especially, techniques capable of correcting the frequency shift in a wide frequency range (bandwidth) and with high accuracy are now being required in order to implement mobile radio communication devices capable of receiving radio signals from base stations correctly.
For the frequency correction in the pilot synchronous detection, pilot symbol blocks (each of which is composed of a predetermined number of pilot symbols) are inserted periodically in a transmitted radio signal. Generally, the frequency correction is executed by detecting phase shifts between adjacent pilot symbol blocks and taking the average of the phase shifts. The “phase shift between adjacent pilot symbol blocks” means a phase shift (relative to a local oscillation signal outputted by the local oscillator of the radio reception device) which occurs between the adjacent pilot symbol blocks. When there is no frequency difference between the transmitted (received) radio signal and the local oscillator of the radio reception device, each symbol of the received radio signal is received on each cycle of the local oscillation signal correctly, and thus the phase shift becomes 0. On the other hand, when a frequency difference exists between the transmitted (received) radio signal and the local oscillator, a certain amount of phase shift occurs. The averaged phase shift is used by a feedback circuit for correcting the frequency of the local oscillation signal (which is used for the detection of the received radio signal) which is outputted by the local oscillator of the radio reception device. In the frequency correction method using the phase shift between the pilot symbol blocks, the frequency correction can be executed correctly if the phase shift between adjacent pilot symbol blocks is within ±180°. In the case of the frequency correction method using the phase shift between the pilot symbol blocks, high accuracy frequency correction can be executed even though correctable frequency range is narrow.
Meanwhile, if another frequency correction method using the phase shift between adjacent pilot symbols in a pilot symbol block is employed, the frequency correction can be executed correctly if the phase shift between adjacent pilot symbols is within ±180°. In the case of the frequency correction method using the phase shift between the pilot symbols, frequency correction with a wide correctable frequency range can be implemented even though the accuracy of the frequency correction is lower.
Further, there has been proposed a frequency correction method for realizing high speed extraction and high trackability by combining the above two frequency correction methods together (T. Watanabe et al. “A performance of AFC for W-CDMA mobile station”, Proceedings of the 1998 Communications Society Conference of IEICE (the Institute of Electronics, Information and Communication Engineers (Japan)), B-5-146 (1998)).
FIG. 1
is a schematic block diagram showing a frequency correction device employing the conventional frequency correction method of the document.
In the conventional frequency correction method, pilot symbols are extracted from a despread signal (received signal after despreading (descrambling)) and thereafter two-mode frequency correction is executed. First, the frequency correction is executed in mode #1, in which the phase shifts between adjacent pilot symbols in a pilot symbol block (inter-symbol phase shifts) are detected, the average of the inter-symbol phase shifts is figured out, and frequency correction is executed by use of the averaged inter-symbol phase shift. After a predetermined time has passed, the frequency correction mode is switched into mode #2, in which the phase shifts between adjacent pilot symbol blocks (inter-pilot block phase shifts) are detected, the average of the inter-pilot block phase shifts is figured out, and frequency correction is executed by use of the averaged inter-pilot block phase shift.
However, in the above conventional frequency correction method successively employing the two frequency correction methods, the pilot synchronous detection becomes impossible if the phase shift between adjacent pilot symbol blocks becomes ±180° or more after the switching into the mode #2. In the above conventional frequency correction method, the accuracy of the frequency correction could be improved by first executing coarse frequency correction based on the detection of the inter-symbol phase shifts (mode #1) and thereafter executing fine frequency correction based on the detection of the inter-pilot block phase shifts (mode #2). However, the timing for switching the frequency correction mode from mode #1 to mode #2 has to be provided properly and correctly in the conventional frequency correction method, and the frequency correction becomes incorrect if the switching into the inter-pilot block phase shift detection (mode #2) is executed before the frequency correction (inter-pilot block phase shift) converges and decreases within ±180°. Further, if signal propagation status changed during the data communication based on the inter-pilot block phase shift and the frequency difference (inter-pilot block phase shift) increased ±180° or more, frequency correction thereafter can not be executed correctly.
SUMMARY OF THE INVENTION
It is therefore the primary object of the present invention to provide a frequency correction device and a frequency correction method for correcting the local oscillation frequency of a local oscillator of a radio reception device according to pilot synchronous detection, by which the frequency correction can be executed with high accuracy even if a large frequency difference (phase shift) occurred.
Another object of the present invention is to provide a frequency correction device and a frequency correction method for correcting the local oscillation frequency of a local oscillator of a radio reception device according to pilot synchronous detection, by which a large frequency difference can be corrected accurately regardless of whether it is at the beginning of communication or in the middle of communication, without the need of providing proper and correct timing for mode switching etc.
In accordance with a first aspect of the present invention, there is provided a frequency correction device for correcting the frequency of a local oscillator of a radio reception device according to pilot synchronous detection, comprising a pilot symbol detection means, an inter-symbol phase shift detection means, an inter-pilot block phase shift detection means, a frequency correction determination means and a frequency correction means. The pilot symbol detection means detects and extracts pilot symbols from a received radio signal after despreading. The inter-symbol phase shift detection means detects an inter-symbol phase shift between adjacent pilot symbols in a pilot symbol block. The inter-pilot block phase shift detection means detects an inter-pilot block phase shift between adjacent pilot symbol blocks. The frequency correction determination means determines a frequency correction based on the inter-symbol phase shift which has been detected by the inter-symbol phase shift detection means and the inter-pilot blo
Le Amanda T.
NEC Corporation
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