IQ quadrature modulation and constellation diagram

A signal has three characteristics that change over time: amplitude, phase, or frequency. However, phase and frequency are just different ways of looking at or measuring the same signal. One can modulate amplitude and phase simultaneously, or modulate them separately, but this is both difficult to produce and even more difficult to detect. But in a specially designed system, the signal can be decomposed into a set of relatively independent components: in-phase (I) and quadrature (Q) components. These two components are orthogonal and independent of each other.

Quadrature Amplitude Modulation (QAM) is a modulation method that performs amplitude modulation on two orthogonal carriers. The two carriers are usually sinusoidal waves with a phase difference of 90 degrees (π/2), so they are called orthogonal carriers. This modulation method gets its name from this.

In the QAM modulator in Figure 1, the I and Q signals come from the same signal source, with the same amplitude and frequency. The only difference is that the phase of the Q signal is 90 degrees different from that of the I signal. The specific relationship is shown in the figure below. When the amplitude of I is 1, the amplitude of Q is 0, and when the amplitude of I is 0, the amplitude of Q is 1. The two signals are independent of each other, with a phase difference of 90 degrees, and are orthogonal.

Phase modulation of analog signals and PSK of digital signals can be considered as special orthogonal amplitude modulation with constant amplitude and only phase change. Therefore, frequency modulation of analog signals and FSK of digital signals can also be considered as special cases of QAM, because they are essentially phase modulation.

The IQ modulation signal can be synthesized by adding the in-phase carrier and the 90-degree phase-shifted carrier. It directly involves the change of the carrier phase in the circuit, so it is easier to implement. Secondly, there are usually only a few fixed points on the IQ diagram, and a simple digital circuit is sufficient to do the encoding work. Moreover, the difference between different modulation technologies lies only in the different distribution of points on the IQ diagram, so as long as the IQ encoder is changed, different modulation results can be obtained using the same modulator.

The IQ demodulation conversion process is also very easy. As long as the same carrier signal as the transmitter is obtained, the demodulator block diagram is basically the reverse of the modulator. From the hardware point of view, there is no part in the modulator and demodulator block diagram that must be changed due to different IQ values ​​(different IQ modulation technologies), so these two block diagrams can be applied to all IQ modulation technologies.

Constellation diagram:

Polar coordinates are the best way to observe amplitude and phase. The carrier is the reference for frequency and phase. The signal is expressed as

The signal can be expressed in polar coordinates in terms of amplitude and phase. The phase is relative to the reference signal.

It is usually a carrier wave, and the amplitude is an absolute or relative value.

In digital communication, it is usually expressed as I and Q. In polar coordinates, the I axis is on the phase reference, while the Q axis is rotated 90 degrees. The projection of the vector signal on the I axis is the I component, and the projection on the Q axis is the Q component. The following figure shows the relationship between I and Q.

QAM modulation is actually a combination of amplitude modulation and phase modulation. The phase + amplitude states define a number or combination of numbers. The advantage of QAM is that it has a larger symbol rate, which can achieve higher system efficiency. The occupied bandwidth is usually determined by the symbol rate. Therefore, the more bits (basic information unit) per symbol, the higher the efficiency. For a given system, the number of symbols required is 2n, where n is the number of bits per symbol. For 16QAM, n = 4, so there are 16 symbols, each symbol represents 4 bits: 0000, 0001, 0010, etc. For 64QAM, n = 6, so there are 64 symbols, each symbol represents 6 bits: 000000, 000001, 000010, etc.

The above is the basic principle of QAM modulation. After channel coding, the binary MPEG-2 bit stream enters the QAM modulator, and the signal is divided into two paths, one for I and the other for Q. Each path gives 3 bits of data at a time. The 3-bit binary number has a total of 8 different states, corresponding to 8 different level amplitudes. In this way, I has 8 levels with different amplitudes, Q has 8 levels with different amplitudes, and the I and Q signals are orthogonal. In this way, any combination of the amplitude of I and any combination of the amplitude of Q will map a corresponding constellation point on the polar coordinate diagram, so that each constellation point represents a mapping composed of 6 bits of data. I and Q have a total of 8×8, a total of 64 combinations. The various possible data state combinations are finally mapped to the constellation diagram, which is the 64QAM constellation diagram shown in Figure 5.

Each constellation point corresponds to an analog signal of a certain amplitude and phase, which is then up-converted to a radio frequency signal and transmitted. Here, let me explain the difference between analog modulation and digital modulation: The difference between analog modulation and digital modulation lies in the modulation parameters. In both schemes, the amplitude, frequency or phase (or a combination of them) of the carrier signal is changed. In analog modulation, the carrier parameters are changed according to the continuous analog information signal, while in digital modulation, the parameters (amplitude, frequency or phase) are changed according to the discrete digital information.