In communications, we often use the following modulation methods:
(1) Analog modulation: use a continuously changing signal to modulate a high frequency sine wave
Mainly include: 1. Amplitude modulation ( amplitude modulation AM, double sideband modulation DSBSC, single sideband amplitude modulation SBSSC, vestigial sideband modulation VSB and independent sideband ISB);
2. Angle modulation (frequency modulation FM, phase modulation PM) two kinds. Because the rate of change of the phase is the frequency, the phase modulation wave and the frequency modulation wave are closely related;
(B) digital modulation: a digital signal of a sine or cosine high frequency oscillations modulating
Mainly include: 1. Amplitude keying ASK ;
(3) Pulse modulation: use pulse sequence as carrier
Mainly include: 1. Pulse Amplitude Modulation (PAM: Pulse Amplitude Modulation);
2. Pulse width modulation (PDM: Pulse Duration Modulation);
3. The pulse position modulation (PPM: Pulse Position Modulation);
4. Pulse Code Modulation (PCM: Pulse Code Modulation);
With the increase of communication traffic, the spectrum resources are becoming more and more tense. In order to increase the capacity of the system, the channel spacing has been reduced from the initial 100kHz to 25kHz, and will be further reduced to 12.5kHz, or even smaller. Due to the flexible network construction of digital communications , It is easy to adopt digital error control technology and digital encryption, easy to integrate, and can enter the ISDN network, so the communication system is transitioning from the analog standard to the digital standard.
Therefore, digital modulation technology must be used in the system . However, general digital modulation technology, such as ASK, PSK and FSK, cannot meet the requirements of mobile communication due to low transmission efficiency. For this reason, it is necessary to specialize in some researches with strong anti-interference and good error performance. , Digital modulation technology with high spectrum utilization rate, as far as possible to increase the bit rate of the transmission data in the unit spectrum , in order to meet the requirements of mobile communication narrowband data transmission. Such as
Minimum Shift Keying (MSK-Minimum Shift Keying);
Gaussian Minimum Shift Keying (GMSK-Gaussian Filtered Minimum Shift Keying );
Quadrature Phase Shift Keying (QPSK-Quadrature Reference Phase Shift Keying);
Interleaved quadrature quadrature phase shift keying (OQPSK-Offset Quadrature Reference Phase Shift Keying);
Four-phase relative phase shift keying (DQPSK-Differential Quadrature Reference Phase Shift Keying);
π/4 quadrature phase shift keying (π/4－DQPSK－Differential Quadrature Reference Phase Shift Keying);
It has been widely used in digital cellular mobile communication systems .