Multiplex communications – Generalized orthogonal or special mathematical techniques – Quadrature carriers
Patent
1995-11-22
1997-12-02
Hsu, Alpus H.
Multiplex communications
Generalized orthogonal or special mathematical techniques
Quadrature carriers
370335, 370342, 375206, 375207, 375210, H04B 7216, H04J 1302
Patent
active
056943881
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a wireless receiver used in digital mobile communications, and more particularly to a CDMA (Code Division Multiple Access) demodulator and demodulation method preferably employed in a spread spectrum CDMA receiver.
BACKGROUND ART
In a spread spectrum CDMA wireless system, a transmitter carries out a normal modulation of a transmitted signal followed by a second modulation using a spreading code, and sends out the wideband spread signal. A receiver, on the other hand, receiving a signal including a number of wideband spread signals, recovers a desired signal by converting one of the wideband signals into a narrowband signal through a process called despreading, and by carrying out a normal demodulation of the narrowband signal. The despreading process selectively produces only a desired received signal by detecting and utilizing a correlation between a spreading code in the received signal and the spreading code generated in the receiver. As a typical device for detecting the correlation, matched filters are well known. If no cross-correlations are present among the spreading codes, the matched filters will produce only autocorrelation of the desired signal. In general, however, since some cross-correlations are present among the spreading codes, the cross-correlation components are inevitably introduced into the despread output.
Besides, a signal of an intended channel can have cross-correlations between signals arriving through multiple transmission paths. FIG. 1 illustrates correlations in the case of three paths. The correlation of the first path signal is detected with a matched filter using a spreading code as tap coefficients. Since a second path signal and a third path signal differ in timing from the first path signal in this correlation detection, they are considered to have been despread with different spreading codes. As a result, the first signal will suffer interference caused by the cross-correlation between the first path signal and the second and third path signals. Incidentally, D(n) designates an n-th symbol in FIG. 1.
A method for minimizing such cross-correlations is disclosed in Yoshida, et al. "DS/CDMA adaptive interference canceller suitable for mobile communication environment, " Technical Report of the Institute of Electronics, Information and Communication Engineers of Japan, 93-76 (1993-11).
FIG. 2 shows a configuration for implementing the method. An orthogonal filter 3 has a tap length of a several symbol interval, and operates at a rate of m times the chip rate of the spreading code, where m is a positive integer. The orthogonal filter 3 is provided with a spread signal at an input terminal 1, extracts a signal for this station by despreading, and supplies it to a differential detector 7 as a narrowband despread signal. The output of the differential detector 7 is supplied to a decision block 11, so that decision data are produced from an output terminal 2 as a decoded output. The decision data are also fed to an error vector calculator 12 which calculates the differences between the decision data and the output of the differential detector 7. The differences are fed to a tap coefficient controller 14 after they are converted into linear values by an error vector/linear quantity converter 13. The tap coefficient controller 14 adaptively calculates tap coefficients that are orthogonal to the spreading codes of all the other stations, and feeds them back to the orthogonal filter 3. The adaptive control of the tap coefficient is carried out at a symbol interval, and the demodulation output is also obtained at the symbol interval. Thus, the interference components from the other stations are eliminated, and only the intended received signal is extracted.
The adaptive control of the orthogonal filter 3, however, cannot follow fast fluctuations in the transmission path due to Rayleigh fading when it is applied to mobile communications in a Rayleigh fading environment. Taking account of this, the system employs the differential det
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Adachi Fumiyuki
Andou Hidehiro
Sawahashi Mamoru
Hoare, Jr. George P.
Hsu Alpus H.
NTT Mobile Communications Network Inc.
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