Introduction to GSM mobile phone principles

GSM is a communication technology that combines FDMA (Frequency Division) with TDMA (Time Division). All users in the network use different frequencies for communication. CDMA is a communication system that uses code division multiple access technology. In this system, all users use the same frequency. Comparison of FDMA, TDMA, and CDMA is as follows:

The GSM system is the second generation of digital cellular mobile communication system. It uses MHz band and adds MHz band and MHz band in the later stage. For convenience, the GSM system is called, DCS1800, and pcs1900.
GPRS (General Packet Radio Service, Universal Wireless grouping Service), as the second generation of mobile communication technology, the transition from GSM to the third generation mobile communication (3G) technology, it was proposed by Bt cellnet as early as 1993. It is a GSM-based mobile grouping data service that provides users with IP addresses or X.25 connections for mobile groups; now GPRS has developed an enhanced type of GPRS, also known as edge. The transmission speed is faster than that of the tutorial and generally becomes the 2.75 generation network. Is a GSM source code and channel code.

The principle diagram of the GSM mobile phone is as follows:

The circuit structure of a mobile phone (hereinafter referred to as a mobile phone) can be divided into four parts: Wireless, transmission and processing, interface, and power supply. The circuit principle can be summarized into two parts: RF circuit and Baseband circuit.
1. Wireless part
It includes antenna circuit, transmission, receiving, modulation and demodulation, oscillator, and other high-frequency systems. The sending part consists of a RF power amplifier and a band-pass filter. The receiving part consists of high-frequency filtering, high-frequency amplification, frequency conversion, and medium-frequency filter. The Modem uses GMSK.

2. transmission processing
2.1 sending channel processing includes voice encoding, channel encoding, encryption, and TDMA frame formation.
1) voice encoding: the user's voice is converted into an electrical signal through mic. This electrical signal is converted into a digital 13 kbitps stream representing the voice through the ADC.
2) channel coding: In order to detect or even correct errors during transmission, the attention code is introduced into the data stream to increase the rate of the information calculated from the source data. The channel encoding result is a code stream.
3) intertwined: mixing the bits of several codewords, so that the bits close to each other in the modulated signals can be extended to several codewords. Because the possibility of continuous errors in the modulation stream is closely related, and when the error is irrelevant, the channel encoding performance will be improved, and the information flow will become the sequence of information blocks after interweaving.
4) burst pulse formatting: To facilitate the synchronization and balancing of received signals, add some binary information to the encrypted information block to make it a binary information block.
5) encryption: you can modify the content of the information block only through the encryption method known to the mobile station and the receiving and receiving station of the base station.
6) modulation: GMSK modulation technology is used to convert digital signals into analog signals with appropriate frequencies at appropriate times, and then transmit signals in the form of radio waves through RF circuit processing.
2.2 The processing of the receiving channel includes balancing, Channel Separation, decryption, Channel Decoding, and speech decoding.
1) demodulation: after the radio wave is received by the antenna, the receiver receives the corresponding information according to the multi-access rules. This part of the signal is demodulated with the help of additional information introduced during burst pulse formatting, and the result is a sequence of binary information blocks.
2) balance: the purpose of balanced demodulation is to correct radio signal distortion caused by complex terrain.
3) Decryption: modify these bits in the opposite way to encryption.
4) deinterlace: in order to reconstruct the code word, put the bits of different burst pulses back into the original position.
5) Channel Decoding: uses the append verification code to detect or correct possible errors in the output of the modem and recover the source information from the output of the modem.
6) speech decoding: digital speech information is restored to analog speech signals by using the DAC decoder.
The control part controls and manages mobile phones, including timing control, digital system control, antenna system control, and man-machine interface control. If frequency hopping is adopted, the frequency hopping control should also be included. The controller uses a microprocessor.
3. Interface Section
It consists of three parts: analog voice interface, digital interface, and man-machine interface. Analog voice interfaces include A/D, D/A conversion, microphone, and speaker. The digital interface is primarily a digital terminal adapter, and the human-machine interface mainly includes a display and a keyboard.
4. Power supply
The power supply includes the circuit directly powered by the battery and the circuit that is powered by the battery by a dedicated integrated power IC into various DC voltages.

5. Related Concepts

1) MS --- Mobile Station, relative to the base station, can be said to be the whole mobile phone. BS-base stations, compared with mobile stations, each base station transmits and receives GSM wireless signals and covers a certain range. Ms and Bs are a pair of relative concepts, as shown below: BS --- radio channel --- MS
2) me --- mobile device, which is relative to Sim after GSM achieves device/card separation. Me runs the GSM protocol stack and MMI applications. SIM is the SIM card, which is relative to me after the GSM card is separated. the SIM card stores user information and authentication password. Me and SIM are a pair of relative concepts, as shown below.

Me --- R/W interface --- Sim

3) Te --- terminal device. Compared with Ta, in the dual-CPU solution, it refers to a system consisting of the primary CPU to run MMI applications and other applications. Ta --- terminal adapter, relative to te, refers to a system in the dual-CPU solution that consists of a CPU running protocol stack (PS. At --- commands for communication between TE and TA, attention.
MMI-human-computer interaction interface. At commands are difficult to use and unfriendly, so a friendly user interface is required. Most mobile phone software designers now call all upper-layer applications MMI. My Understanding of MMI refers to applications related to the GSM protocol. TE and TA are a pair of relative concepts, as shown below.
Te --- at command --- Ta
MMI PS

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