Modulation/demodulation method and system for realizing quadrature modulation/demodulation technique used in digital mobile radio system with complex signal processing

A complex signal demodulation method comprising the steps of:

• extracting from a received signal a quadrature-modulated signal in which a carrier having a frequency f.sub.C is quadrature-modulated by a modulation data signal having a signal band of 2f.sub.B, 2f.sub.B being smaller than f.sub.C ;
• converting the quadrature-modulated signal into a digital signal with an analog-to-digital converter;
• filtering said digital signal with a digital complex coefficient band pass filter which has a frequency characteristic including a pass band of 2f.sub.B with a center frequency f.sub.0 =f.sub.C and which operates at a sampling frequency f.sub.S =Nf.sub.0, N being an integer not smaller than 3;
• and
• decreasing a sampling frequency of a real part component or an imaginary part component output from said digital complex coefficient band pass filter to f.sub.C to quadrature-demodulate the quadrature-modulated signal.

A complex signal modulation system comprising:

• means for inputting a modulation data signal having a signal band of 2f.sub.B, a carrier frequency f.sub.C, and a sampling frequency f.sub.D, 2f.sub.B being smaller than f.sub.D ;
• means for filtering said modulation data signal, said filtering means including a digital complex coefficient band pass filter which has a frequency characteristic including a pass band of 2f.sub.B with a center frequency f.sub.0 =f.sub.C and which operates at a sampling frequency f.sub.S =Nf.sub.0, N being an integer not smaller than 3;
• digital-to-analog conversion means for converting a real part component and an imaginary part component output from said digital complex coefficient band pass filter into an analog signal;
• and
• means for extracting frequency components in a range of f.sub.C -f.sub.B to f.sub.C +f.sub.B from said analog signal to obtain a quadrature-modulated signal.

A complex signal demodulation system comprising:

• means for extracting from a received signal a quadrature-modulated signal in which a carrier having a frequency f.sub.C is quadrature-modulated by a modulation data signal having a signal band of 2f.sub.B, 2f.sub.B being smaller than f.sub.C ;
• analog-to-digital conversion means for converting the quadrature-modulated signal into a digital signal;
• means for filtering said digital signal, said filtering means including a digital complex coefficient band pass filter which has a frequency characteristic including a pass band of 2f.sub.B with a center frequency f.sub.0 =f.sub.C and which operates at a sampling frequency f.sub.S =Nf.sub.0, N being an integer not smaller than 3;
• and
• means for decreasing a sampling frequency of a real part component or an imaginary part component output from said digital complex coefficient band pass filter to f.sub.C to quadrature-demodulate the quadrature-modulated signal.