What is GSM? How to understand it specifically?

1.GSM modulation mode is GMSK. GMSK modulation refers to gaussian minimum shift keying, which is a digital modulation. To some extent, it can be understood as frequency modulation, but the frequency change is carried out in a discrete (discontinuous) way, while frequency modulation is pure analog modulation, and the frequency change is continuous. GMSK adopts digital modulation, and one of its key technologies is time division multiple access (each user selects a carrier frequency in a certain time slot and can only receive information at a specific time). GSM system has several important features: good anti-theft and copying ability, large network capacity, rich number resources, clear call, strong stability, sensitive information, few dead calls and low power consumption of mobile phones. Therefore, its voice is clear and easy to keep secret, and it can provide many data transmission services.

CDMA uses HPSK modulation, which is also a digital modulation technology. My understanding of HPSK is very general, except that it uses complex spread spectrum to balance the amplitude of I/Q channel. Then, by delaying the Q branch of the PN sequence by 1 PN chip and extracting by a factor of 2, the zero-crossing rate of the I/Q constellation change of the signal is reduced. Specific examples can refer to the block diagram in WCDMA or cdma2000 protocol, and detailed explanations can also be found in monographs. In a word, the principle of CDMA technology is based on spread spectrum technology, that is, the information data with a certain signal bandwidth to be transmitted is modulated by a high-speed pseudo-random code with a bandwidth much larger than the signal bandwidth, so that the bandwidth of the original data signal is expanded, and then it is modulated by a carrier wave and sent out. The receiver uses the same pseudo-random code to correlate with the received bandwidth signal, and changes the broadband signal into the narrowband signal of the original information data, that is, despreading, thus realizing information communication.

In addition, GSM adopts GMSK modulation, and nonlinear power amplifier can be used to improve efficiency. When CDMA adopts HPSK modulation, linear power amplifier must be used to reduce distortion.