Pulse or digital communications – Repeaters – Testing
Patent
1995-06-06
1997-01-14
Chin, Stephen
Pulse or digital communications
Repeaters
Testing
375206, 375344, H04B 1500, H04K 100, H04L 2730
Patent
active
055947549
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a receiver of a code division multiple access (CDMA) communication system appropriate for mobile communications, and particularly to a spread spectrum communication receiver which corrects a frequency offset of a local signal with respect to a received signal in a baseband signal region.
The term "baseband signal" in this specification refers to a signal having no carrier signal components. More specifically, the baseband signal at a transmitter refers to a signal after the primary modulation or a signal after spreading, and the baseband signal at a receiver refers to a signal after quadrature detection and before despreading, and a signal after despreading.
BACKGROUND ART
Recently, tremendous research and development effort has been directed to CDMA systems. The CDMA system falls into a direct sequence (DC) system and a frequency hopping (FH) system. The FH system is seldom used at present. This is because the FH system resolves a symbol into elements called chips, and translates individual chips into carriers of different frequencies at a high rate, which is difficult to be implemented by a frequency synthesizer in the state of the art.
Thus, the DS system is commonly used. The DS system performs, at a transmitter, a primary modulation of an original signal use in QPSK or the like, spreading (secondary modulation) of the primary modulated signal into a wideband signal using a spreading code, and transmission thereof on a carrier, and at a receiver, removal of the carrier, despreading (secondary demodulation) of the wideband signal into the primary modulated signal using the spreading code identical to that of the transmitter, and recovery of the original signal by a primary demodulation.
Problems involved in applying the CDMA system to mobile communication systems such as portable telephony will be considered. Base station equipment of mobile communications today includes a highly stable reference oscillator, and the accuracy under the domestic standard for digital vehicle telephony in Japan is less than 0.05 ppm in absolute accuracy. On the other hand, mobile station equipment usually employs a temperature compensated crystal oscillator (TCXO) because of difficulty of using a highly stable reference oscillator with a thermostat. The frequency accuracy of the crystal oscillators is approximately 3 ppm in absolute accuracy in an 800 MHz band.
As a result, the frequency of the local signal of a mobile station will deviate from the center frequency of a transmitted signal of a base station (that is, a received signal of the mobile station). To compensate for the offset of the frequency to maintain stable receiving operation, an AFC (Automatic Frequency Control) circuit is required. In addition, since a mobile station is usually moving with respect to the base station, the center frequency of the received signal further deviates by an amount corresponding to the Doppler frequency.
FIG. 1 shows a major portion of a conventional CDMA receiver with an AFC circuit. An intermediate frequency (IF) received signal applied to an input terminal 10 is divided into two parts by a hybrid coil 10A, and is supplied to a quadrature detector 11. The quadrature detector 11 detects the IF received signal by a local signal from a voltage controlled oscillator (VCO) 12, and outputs an in-phase baseband signal I and a quadrature baseband signal Q. These baseband signals are baseband signals spread by a spreading code. The I baseband signal is supplied to an A/D converter 15 through an LPF (lowpass filter) 13, and is converted into a digital signal. Likewise, the Q baseband signal is supplied to an A/D converter 16 through an LPF 14, and is converted into a digital signal.
These digital signals are fed to a correlator 17 consisting of matched filters or a sliding correlator, which detects correlation between the received digital signals and the spreading code, thereby despreading the digital signals. In other words, the correlator 17 functions as a despreader, and outputs baseb
REFERENCES:
patent: 5150377 (1992-09-01), Vannucci
patent: 5329546 (1994-07-01), Lee
patent: 5467367 (1995-11-01), Izumi et al.
Adachi Fumiyuki
Dohi Tomohiro
Sawahashi Mamoru
Chin Stephen
NIT Mobile Communications Network Inc.
Phan Hai H.
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