What is the principle of sending signals and messages by mobile phones? Thank you, everyone.

The frequency of mobile phone signal is very high, generally around 900Mhz, and it propagates through ionospheric reflection. The signal of the calling mobile phone is transmitted to the nearest base station, that is, the mobile or connected signal tower, and then the frequency of the high-frequency signal is reduced by the base station, and the base station communicates with the base station. This signal travels in a straight line and will be blocked when it meets tall buildings, so those towers are very high and are transmitted to the base station near the mobile phone that answers the phone. Then it is converted into a high-frequency signal and sent to the GSM system of the mobile phone. The full name of GSM system is digital cellular mobile communication system. In which area the mobile phone is located, it will automatically track the nearest base station, so the above communication mode can be realized. Unicom's communication system is theoretically more advanced than mobile phones, because GSM is the first generation mobile communication system and CDMA is the second generation, which is the full name of narrowband code division multiple access digital access technology. However, due to the funds of China Unicom or other technical reasons, the signal in many areas is not as good as mobile. The landlord can refer to the books on modern communication principles and technologies, and if they are not very professional, they should be able to understand them.