What is an IQ signal?

I- Q modulation signal can be synthesized by adding in-phase carrier and 90-degree phase shift carrier, so it is easy to realize. Secondly, there are usually only a few fixed points on the I-Q diagram, and a simple digital circuit is enough for coding. Moreover, the difference between different modulation techniques is only the distribution of points on the I-Q diagram, so different modulation results can be obtained by changing the I-Q encoder and using the same modulator.

The process of I-Q demodulation is also very easy. As long as the same carrier signal as the transmitter is obtained, the block diagram of the demodulator is basically the inverse process of the modulator. As far as the hardware is concerned, there is no part in the block diagram of modulator and demodulator that must be changed because of different I-Q values (different I-Q modulation technologies), so these two block diagrams can be applied to all I-Q modulation technologies.

BPSK (Double Phase Shift Keying) is the simplest method to modulate digital elements, as shown in Figure 6-5. When the fundamental frequency data is 1, the amplitude and phase of the carrier remain unchanged. When the fundamental frequency data is 0, the amplitude of the carrier remains unchanged, but the phase changes by 180 degrees. If data 1 and 0 are transmitted alternately, the carrier phase will change greatly by 180 degrees, resulting in signal discontinuity. Therefore, the signal bandwidth after BPSK modulation is larger. There are only two points on the I-Q diagram or constellation diagram of BPSK, one on each side of the origin. The distance between the two points and the origin is the same, but the phase difference is 180 degrees.

QPSK (Quadrature Phase Keying) has four points on the constellation diagram, forming a square with the origin as the center, as shown in Figure 6-6. The distance between the four points on the constellation is the same as the origin, so the amplitude of the carrier has not changed, only the phase has changed. Because there are only four points on the constellation diagram, there are four possible modulation situations, and each situation can be represented by two data bits.

Every point on the I-Q graph is defined as a symbol, and the frequency at which a point appears on the graph is the symbol rote or boud rate, that is, the rate at which the actual carrier changes. In QPSK, each symbol represents two data bits, so the data transmission rate is twice that of the symbol. On the contrary, the symbol transmission rate is half of the data transmission rate. If it is known that a symbol represents several data bits, then the symbol transmission rate is the data transmission rate divided by the number of data bits represented by a symbol.

The bandwidth of the modulated signal is directly proportional to the symbol transmission rate, while the probability that QPSK directly changes the carrier by 180 degrees is relatively less than that of BPSK. At the same symbol transmission rate, QPSK will occupy less bandwidth, but in fact the data transmission rate is twice that of BPSK.

FSK (frequency shift keying) is a digital element of frequency modulation, as shown in Figure 6-9. The transmitted data are 1 and 0, each representing a carrier frequency, and the carrier amplitude remains unchanged. Using the existing FM demodulation technology, the fundamental frequency data can be easily recovered.

MSK(Minimum Shift Keying) is a special FSK, in which the difference △f between data 1 and 0 is 1/2 of the data transmission rate. MSK can also be regarded as a variant of QPSK. Like QPSK, MSK has four points on the constellation, but the points can only move forward or backward by 90 degrees at a time, and cannot move diagonally, that is to say, the phase of the carrier will not change by 180 degrees, so the frequency spectrum of the modulated signal will not be scattered, and the frequency utilization will be more efficient.

Modulation means changing a known signal carrier in some way at the signal sending end, while demodulation means finding out how the known carrier signal changes at the receiving end and the information to be conveyed behind this change. In an analog communication system, analog baseband signals (such as voice) are information to be transmitted by modulation and demodulation. In digital communication systems, the transmitted information is usually data processed by fundamental frequency signals. That is to say, digital modulation is the process of loading data onto the RF carrier, while demodulation is the process of taking data out of the RF signal. Only three parameters of radio frequency carrier signal Aces(2πfct+θ) can be changed: amplitude, frequency and phase. The modulation method of changing the carrier amplitude is called amplitude modulation. The way to change the carrier frequency is called frequency modulation (FM); The modulation method for changing the carrier phase is called phase modulation (PM). Because the differential of phase is frequency, the amplitude and phase of carrier signal can be said to be two main modulation variables.