Offset QPSK modulation analytic system

Details Offset QPSK modulation analytic system Offset QPSK modulation analytic system

1999-10-14

2002-10-22

Pham, Chi (Department: 2731)

Pulse or digital communications

Systems using alternating or pulsating current

Angle modulation

C375S226000, C375S261000, C375S316000, C375S323000, C375S326000

Reexamination Certificate

active

06470056

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an offset QPSK modulation analytic system for analyzing a transmitter waveform quality factor, etc. by receiving a signal modulated in an offset QPSK (OQPSK) system.
A digital cellular system in a CDMA system excels in communications quality and has been put to practical use also in Japan. In this digital cellular system, the QPSK modulation system is used in a forward link through which data is transmitted, for example, from a base station to a mobile station, and on the other hand, the OQPSK modulation system is used in an reverse link through which data is transmitted from the mobile station to the base station. In the digital cellular system, it is necessary to measure the waveform quality, etc. based on an actually received signal, and analyze it to maintain excellent communications quality.
FIG. 8
shows the outline of the coherent detection in a common transmission and reception system using an orthogonal modulation system. On a transmission side, an in-phase component I is multiplied by a local signal (carrier signal) at a predetermined frequency, an quadrature component Q is multiplied by a signal obtained by shifting the phase of the local signal by 90°, and the resultant signals obtained as the multiplication results are combined for transmission. On a receiving side, the frequency of a received signal is converted using a local signal having the same frequency as the signal on the transmission side and also using a signal obtained by shifting the phase of the local signal by 90°, and the in-phase component I and the quadrature component Q are separated from each other by passing the two types of frequency converted signals through a low-pass filter (LPF).
When a signal is transmitted and received by the above described orthogonal detection, and if there is a phase difference between the local signals on the transmission and receiving sides (the phase difference is referred to as an ‘initial phase’), then the initial phase is compensated on the receiving side so that the local signals on the transmission side and the receiving side should be synchronized with each other. For example, it is common that the phase of a local signal is synchronized on the receiving side using a well-known data pattern such as a synchronization word contained in communications data, etc.
When the QPSK modulation analysis is performed, any of the four variations of absolute phases, that is, +45°, +135°, −45°, and −135°, is applicable even when the initial phase is not known. If the symbol positions or points at the in-phase component I and the quadrature component Q can be obtained, the initial phase can be specified by forcibly assigning the leading symbol position to any of the four absolute phases.
Although the OQPSK modulation system is adopted, the symbol position of the in-phase component I can match the symbol position of the quadrature component Q by, for example, forcibly shifting the symbol position of the quadrature component Q by ½ symbol only if the initial phase and the frequency error can be specified by any means. As a result, a clock delay can be estimated and compensated in the same manner as in the QPSK modulation system, thereby successfully analyzing the waveform quality, etc. of an actual signal.
However, in the OQPSK modulation system, the initial phase cannot be specified by the method in the QPSK modulation system when there is a ½ symbol difference in time between the symbol position of the in-phase component I and the symbol position of the quadrature component Q. Therefore, the offset of the quadrature component Q cannot be eliminated. As a result, the conventional technology which estimates a clock delay and analyzes the waveform quality, etc. of an actual signal cannot be utilized.
The present invention has been developed to solve the above described problems, and aims at providing an offset QPSK modulation analytic system capable of specifying the initial phase contained in a received signal in the offset QPSK modulation system, and estimating a clock delay by regaining the QPSK modulation signal.
SUMMARY OF THE INVENTION
In a preferred embodiment of the present invention, in the offset QPSK modulation analytic system according to the present invention, the correlation coefficient is computed by a correlation coefficient computation unit between a signal (actual signal) obtained by actually performing a phase compensation using a predetermined candidate value of an initial phase and an ideal signal generated by repeating the offset QPSK demodulation and modulation on the obtained actual signal. Based on the computed correlation coefficient, the initial phase determination unit determines whether or not the candidate value of the initial phase is appropriate. The closer to the true value the candidate value of the initial phase is, the more alike the waveforms of the actual signal and the ideal signal become. Since the correlation coefficient of these two signals is a large value, the initial phase can be appropriately estimated using the correlation coefficient.
Especially, according to the present invention, the clock delay is estimated by generating a QPSK signal by eliminating the offset of the quadrature component of the signal after compensating the initial phase. Then, the correlation coefficient is calculated by a signal compensated corresponding to the estimated clock delay. Since a correct initial phase and a frequency error are estimated, and the offset of the quadrature component of a corrected signal is eliminated, a clock delay can be estimated. Using an actual signal corrected by compensating the clock delay and an ideal signal, the correlation coefficient between the two signals is obtained, thereby further improving the estimation precision of the initial phase.
Furthermore, it is desired that the above described initial phase determination unit retrieves the maximum value from among a plurality of correlation coefficients computed corresponding to each of a plurality of candidate values of the initial phase, and a corresponding candidate value is estimated as an initial phase. By using the above described correlation coefficient, the optimum value in the plurality of candidate values can be extracted, thereby easily estimating the initial phase.
In addition, it is desired that the estimation precision can be progressively enhanced by performing an estimating operation by the above described initial phase determination unit step by step and setting a plurality of candidate values used in each of the subsequent estimating operations based on the estimation result in the preceding step. When the estimating operation is performed step by step, an efficient initial phase estimating operation can be performed by first roughly estimating the initial phase, and then enhancing the estimation precision closer to the true value. Practically, to progressively improve the estimation precision, the interval among the plurality of candidate values may be set smaller, or the signal length whose correlation coefficient is to be computed may be set larger. Thus, an initial phase can be estimated with a smaller error.


REFERENCES:
patent: 5745525 (1998-04-01), Hunsinger et al.
patent: 5946359 (1999-08-01), Tajiri et al.
patent: 5999223 (1999-12-01), Patel et al.
patent: 6160838 (2000-12-01), Shinohara et al.

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