Wireless digital communication: Working principle of IQ modulation
IQ modulation is the foundation of various high-order modulation methods widely used in the LTE and WiFi fields, such as BPSK, QPSK, QAM16, QAM64, QAM256, etc. Understanding the working principle of IQ modulation is essential for a better understanding of the underlying OFDM implementation mechanism in LTE and WiFi. Because the modulation methods used on each subcarrier of OFDM are still various QAM modulation techniques based on IQ modulation, and essentially, the implementation of OFDM is the superposition of IQ modulation on multiple orthogonal subcarriers.
The concepts of I and Q in IQ modulation correspond to the abbreviations of two English words: In Phase and Quadrature, respectively. Simply speaking from the literal meaning, the names of I and Q signals have no meaning. What is in-phase? What is orthogonality? What signal is used as a reference to determine in-phase and quadrature? In practical implementation, a set of cosine and sine waves of the same frequency are generally used as the I and Q signals. Therefore, the cosine signal used as the I signal is actually the reference signal, which is of course an In Phase signal. The cosine signal that is 90 degrees out of phase with the I signal is naturally its Quadrature signal. So the simplest understanding of IQ modulation signals is to consider the I signal as a cosine carrier signal, and the Q signal as a sine carrier signal with a frequency difference of 90 degrees from the I signal.
The so-called IQ modulation is to use a set of I signals, namely cosine signals, and Q signals, namely sine signals (with a phase difference of 90 degrees and orthogonal to each other) of the same frequency as carriers, to simultaneously modulate two baseband signals to the two carriers, and then superimpose the two modulated signals together and transmit them through an RF circuit. After receiving the radio frequency signal, the receiving end performs IQ demodulation. During IQ demodulation, because the two carriers are orthogonal to each other, the two baseband signals can be separated from the mixed signal based on the I and Q carrier signals used in the previous modulation process and demodulated separately. For both the I and Q signals modulated by IQ, increasing the relative amplitude of one side not only increases the amplitude of the modulated signal, but also causes the phase of the modulated signal to shift towards the side with increased amplitude. So essentially, IQ modulation is amplitude modulation performed on the I signal and Q signal, but the modulation result not only affects the amplitude of the modulated signal, but also its phase change. This is the basic working principle of IQ modulation and demodulation.
Post time: Mar-05-2025