A base band signal transmitting device in a base station of a mobile communications system is disclosed including a CDMA modem unit constructed in a manner that a plurality of CDMA modems are detachably mounted on one board, each of the CDMA modems converting a digital voice signal into direct band spread signal by sectors, to generate a digital base band signal; a digital signal summing unit constructed in a manner that a digital summing device is mounted on another board, the digital summing device for digital-summing the direct band spread digital signals received from the CDMA modem unit by sectors and transmitting them in parallel; and a signal processing unit including a FIR low pass filter for converting the digital signals received from the digital signal summing unit into parallel signals and removing high frequency component from them, a digital/analog converter for converting the output signals of the FIR low pass filter into analog signals, and a base/intermediate frequency band upward conversion circuit for upward-converting the base band analog signals converted by the converter into analog signals of intermediate frequency band and high frequency band, the FIR low pass filter, digital/analog converter and base/intermediate frequency band upward conversion circuit being mounted on another separate board.

The input data to a CDMA transmitter is used to introduce a slight frequency offset to the clock that is used to generate the CDMA code. At the receiver, the frequency-offset CDMA signal proportionately affects the magnitude of output signals from one or more CDMA correlators. A composite signal that is based on the magnitudes of the output signals is compared to a set of predefined threshold levels to provide the demodulated output data.

For a CDMA communication system using an interference canceller, a communication system which enables the interference canceller to easily confirm pulling-in of its own signal is disclosed. Input transmission signals are stored through a serial-parallel converter 101 in storage circuits 301 and 302, each of which holds one frame of transmission signals for I and Q axes. An area for holding one cycle of spreading codes for the I and Q axes output from a spreading code generator 105 is formed in the rearmost part of each of the transmission data strings of the storage circuits 301 and 302. One frame of transmission signals prepared by a pilot inserting device 102 and frame synchronizing signals are transferred to a transmitted circuit 103 in sequence. These signals are then transmitted as data for the I and Q axes by symbol rate clocks. The data for the I and Q axes are multiplied by chip rate spreading codes output from a spreading code generator 105, a spreading code for the I axis and a spreading code for the Q axis in a spreader 104 and then output as spread spectrum signals.

A radio frequency transmitter, of the type supplied with two signals in baseband and in quadrature, I(nT) and q(nT), which are images from two binary streams representing information to be transmitted, 1) provides a first transposition into the digital domain, at an intermediate frequency .omega..sub.0, for the baseband signals and generates, by combination, two signals of intermediate frequency in quadrature, 2) provides a second transposition into the analog domain, after multiplication by a frequency .omega..sub.1, followed by a summation of the two signals at intermediate frequency and in quadrature, in such a way that a resultant signal is generated which is found finally around a frequency .omega..sub.2, where .omega..sub.2 =.omega..sub.0 +.omega..sub.1. In an advantageous variant, the radio frequency transmitter additionally digitally compensates gain and phase imperfections of the direct conversion.

A CDMA synchronous acquisition circuit for receiving a CDMA signal and performing synchronous acquisition on the CDMA signal, comprises a receiving unit for receiving a signal and decoding the received signal to a reception base band signal which is spread-modulated in a CDMA mode, a correlation circuit for calculating a correlation value for the received base band signal every timing within one chip of a spread code by a correlator, and means for detecting the maximum value of the correlation value to estimate the spread code generation timing of a spread-modulated transmission base band signal within a precision of one chip of the spread code, wherein the correlator includes offset removing means for removing a DC offset component due to unbalance of the code balance of the spread code.