What is quadrature amplitude modulation like?

Brief introduction of quadrature amplitude modulation

Quadrature amplitude modulation (QAM) is a kind of vector modulation, which maps the input bits onto a complex plane (constellation) to form complex modulation symbols. The quadrature amplitude modulation signal has two carriers with the same frequency, but the phase difference is 90 degrees (a quarter period, according to the integral term). One signal is called I signal and the other signal is called Q signal. Mathematically, one signal can be expressed as sine and the other as cosine. Two modulated carriers have been mixed at the time of transmission. After arriving at the destination, the carrier is separated, the data is extracted separately, and then added with the original modulation information. Compared with AM, the spectrum utilization ratio is twice as high.

QAM uses two independent baseband signals to suppress the amplitude modulation of two mutually orthogonal co-frequency carriers, and uses the orthogonality of the spectrum of this modulation signal in the same bandwidth to realize the transmission of two parallel digital information. This modulation method usually includes binary QAM(4QAM), quaternary QAM(l6QAM), octal QAM(64QAM), …, and the corresponding spatial signal vector endpoint distribution map is called constellation map, with 4, 16, 64, … vector endpoints respectively. At present, the highest QAM has reached 1024QAM. The more samples, the higher the transmission efficiency. However, the more samples, the better. With the increase of sample number, the bit error rate of QAM system will increase gradually. Therefore, in the environment with high reliability requirements, QAM at various points cannot be used. For 4QAM, when the amplitudes of the two signals are equal, its generation, demodulation, performance and phase vector are the same as those of 4PSK.

Second, the modulation and demodulation principle of QAM

The block diagram of MQAM modulation and demodulation is shown in Figure 2.2. 1. In the transmitter modulator, the serial/parallel conversion divides the input binary signal with the information rate of Rb into two binary signals with the rate of Rb/2, and the 2/L level conversion changes each binary signal with the rate of Rb/2 into a level signal with the rate of Rb/(2lbL), and then multiplies it with two orthogonal carriers respectively, and then adds it to get the MQAM signal. The orthogonal coherent demodulation method can be used in the receiver demodulator. The received signal enters the coherent demodulator of two orthogonal carriers in two paths, and then enters the decider to form L-ary signal and output binary signal respectively. Finally, the baseband signal is obtained after parallel/serial conversion.