Constellation in Digital Modulation

Constellation is a basic concept in digital modulation. When we send digital signals, we usually do not send 0 or 1 directly, but first form a group of 0 and 1 signals (bits) according to one or several. For example, every two bits form a group, that is, 00, 01, 10, and 11. There are four states in total (if there are no three bits, there are eight states, and so on). At this time, we can choose QPSK (four-phase modulation, corresponding to the preceding four states of 00, 01, 10, and 11), The four QPSK points form a QPSK constellation. Each point is 90 degrees away from the adjacent points (the amplitude is the same). A constellation point corresponds to a modulation symbol. In this way, every modulation symbol sent is twice as much information as a bit sent. 

QPSK modulation constellation 

The constellation diagram of the received signal

When receiving and demodulating the QPSK signal, judge which signal is sent according to the distance between the received signal and the four points in the constellation (generally referred to as the European distance), and determine which point is closest to which point for demodulation. 
So, the constellation diagram is mostly used for mapping during modulation (like QPSK, 16QAM, 64QAM, etc.) and for figuring out which point is sent during reception so that data can be demodulated correctly. 
The above is the knowledge explanation of the Constellation Chart brought by Shenzhen Haidiwei Optoelectronics Technology Co., Ltd., which is an optical communication manufacturer and produces communication products.