1. Main technical indicators
1.1 General characteristic indicators
The frequency bands commonly used by mobile phones in my country mainly include CDMA1X and 800MHz frequency bands occupied by CDMA mobile phones; 900/1800MHz frequency bands occupied by GSM mobile phones; 900/1800MHZ frequency band occupied by GSM1X dual mode; 1900/2000/2100MHz frequency band occupied by 3G.
Working frequency: GSMTx: 890～915MHz
Rx: 935～960MHz
DCSTx: 1710～1785MHz
Rx: 1805～ 1880MHz
PCSTx: 1850.2～1909.8MHz
Rx: 1930.2～1989.8MHz
Frequency error: ±0.1×f0×10
Working temperature: -10～+55°C
Reference frequency source: VCTCXO13MHz
1.1.1GSM dual frequency
The frequency bands occupied by GSM mobile phones in China are mainly 900MHZ and 1800MHZ. In fact, 1800MHZ is also due to the rapid increase in the number of mobile phone users, which has caused the mobile communication network system to be in an overloaded operation state, and finally caused the mobile phone to be prone to failures such as dropped calls, crosstalk, poor voice quality, and difficulty in accessing the Internet. Phenomenon. In order to solve these problems, more and more mobile phone operators and manufacturers are beginning to realize the urgency of solving this problem, and continue to take relevant measures to further expand the mobile phone network system, so GSM1800Mhz came into being, also known as As DCS1800 (Digital Cellular System), its appearance makes dual-frequency networks based on GSM900 and 1800 a reality.
Using a GSM900/GSM1800 dual-band mobile phone, users can freely switch between GSM900 and GSM1800, which can effectively avoid the problems of dropped calls, difficult calls and poor sound quality. Compared with the previous calls using only GSM900 network More convenient.
1.1.2GSM tri-band
The so-called "tri-band" includes three working frequencies, these three working frequencies are GSM900Mhz, DCS1800Mhz and PCS1900Mhz, and so on, the so-called "Tri-band mobile phone" means that the mobile phone can receive the signals of the three frequency bands of GSM900M, DCS1800Mhz and PCS1900Mhz at the same time, and make a choice among them. If the signal of that frequency band is strong, the signal of the base station is selected. If one of them cannot be connected, You can freely switch to the signal of another frequency band. It actually increases the connection rate of the mobile phone. In some areas where mobile phone users are relatively concentrated, it is especially suitable to use tri-band mobile phones, because tri-band mobile phones can flexibly switch between GSM900, DCS1800 and PCS1900, so as to always maintain continuous calls and call quality. PCS1900 mega network is a network segment commonly used in the field of communication networks in North America (the United States, Canada) and European countries.
Because tri-band mobile phones can work in three network segments with different frequencies at the same time, tri-band mobile phones undoubtedly have the characteristics of these three networks. From a technical point of view, because of the high frequency band, GSM1800 has strong signal penetration ability, so it can bring good call quality and communication coverage in the complex environment with tall buildings; while the PCS1900 channel is in North America (the United States, Canada) and The European region has good communication capabilities, which undoubtedly provides those who frequently travel between continents with the services they need.
For operators, the construction of a three-band network has completely alleviated the frequency band and capacity problems of GSM900, which has further optimized the network and effectively alleviated the peak traffic in hotspots. The connection rate is higher, which greatly increases the business volume.
1.2 Transmitter performance indicators
Maximum transmit power: GSM: 33±2dBm
DCS: 30±2dBm
PCS: 30 ±2dBm
Mean square phase error: <5°
Peak phase error: <20°
(3) Receiver performance index
Receiving reference sensitivity: -102dBm
Bit error rate (ClassⅡRBER): 2%
(4) Power supply
Nominal working voltage: 3.0 ～4.2V
Call current: average value﹤250mA
Waiting current: 3～10mA
Battery: Commonly used batteries include nickel-metal hydride batteries, lithium-ion batteries and lithium polymer battery.
1.3 System
The GSM system has several important features: good anti-theft copy capability, large network capacity, abundant mobile phone number resources, clear calls, strong stability, resistance to interference, and sensitive information , Less dead space for calls, low power consumption of mobile phones. The two main GSM systems in the world are GSM900 and GSM1800. Because of the different frequencies, the applicable mobile phones are not the same. The former developed earlier and used more countries, while the latter developed later and used fewer countries. In terms of physical characteristics, the former has a lower frequency spectrum, longer wavelength and poor penetration, but has a longer transmission distance, while mobile phones have stronger transmission power and higher power consumption, so the standby time is shorter; while the latter has a longer spectrum. High, short wavelength, good penetration, but the transmission distance is short, the transmission power of its mobile phone is small, and the standby time is correspondingly longer.
2. Basic features
① Frequency band: Dual-band mobile phone users can automatically switch between the GSM900MHz and DCS1800MHz frequency band network areas, and tri-band mobile phone users can also choose the PCS1900MHz frequency band in North America.
②Support GPRS.
③Display: single screen, dual screen.
④Chinese input method: T9, Zi, etc.
⑤Phone book and call records: phone numbers can be stored, and can be moved and copied between the phone and SIM card; can be searched by name and location; dialed, received, and missed calls can be displayed Number.
⑥Incoming call reminder: Different incoming calls use different tones, and different incoming calls use different colors of backlight flashing.
⑦Short message: Support SMS and enhanced short message (EMS), that is, support the input, storage, sending and receiving of short message, broadcast short message, short message service function setting and voice mail.
⑧Call management: call time reminder, emergency call, call billing, multi-party call, call hold, call waiting, set local number restriction, call and fax, call transfer, call restriction.
⑨Phone settings: ring size setting, SIM card PIN1, PIN2 management, phone lock (4-8 digits), setting time and date, network selection, ringing tone, language selection, vibration selection, Flip cover to answer, automatic answer, any key answer, low voltage alarm.
⑩Accessories: alarm, timer switch, world time, notepad, electronic dictionary, calculator, timer, stopwatch, game.
Infrared transmission: can transmit business cards or data to each other.
Go online.
STK: Support Chinese SIM card value-added services.
MP3 player, radio, physical camera.
Recording, voice dialing.
3. Mobile phone standards
Mobile phone standards mainly include GSM, CDMA, and 3G. Since the advent of mobile phones, it has experienced the first generation of analog mobile phones (1G) and the second generation. Digital mobile phones (2G) such as GSM and TDMA, 2.5-generation mobile communication technology CDMA, and third-generation mobile communication technology 3G.
Analog network: The signal of the analog network is modulated in an analog manner, and its analog series adopts frequency division multiple access. (The frequency band specified by mobile communication is 905-915MHZ, and every 25KHZ is a channel, which supports a pair of users to talk). China's analog network is divided into A network (Motorola equipment) and B network (Ericsson equipment), and the two networks have been interoperable.
Analog network signal distortion is small, so the sound quality can be comparable to wired phones. Moreover, due to the earlier construction and complete coverage, most county-level cities across the country have coverage. The disadvantage of the analog network is that the number of channels is relatively small and the confidentiality is poor.
GSM digital network: GSM: GSM (GlobalSystemForMobileCommunication) network is the global system for mobile communication, also known as "Global Communication", and many companies have participated in the formulation of standards. The GSM digital mobile communication system was designed by a standardization committee composed of major European telecommunication operators and manufacturers, and it was developed on the basis of a cellular system. Since the end of 1994, my country has been preparing to build a GSM cellular mobile communication network in more than ten provinces and cities. Its development momentum is sighed by the world. The GSM digital network has covered more than 30 provinces (regions, cities), more than 300 regions and more than 2,000. Counties and cities, and can roam with more than 40 countries.
GSM uses digital modulation technology. 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). The GSM system has Several important features: good anti-theft copy capability, large network capacity, rich number resources, clear calls, strong stability and resistance to interference, sensitive information, fewer dead spots for calls, and low power consumption of mobile phones. Therefore, the voice is clear, confidentiality is easy, and more data transmission services can be provided. The number of users supported by the GSM network is 1.8-2 times that of the analog network.
Because of the rapid development of GSM, after its 900MHZ frequency band was full, the GSM1800 frequency band was opened up, and the mobile phone worked in several frequency bands such as 900MHZ and 1.8GHZ frequency band and GSM1900MHz.
GPRS: GPRS is the English abbreviation of GeneralPacketRadioService, and the Chinese is General Packet Radio Service. It is a wireless packet switching technology based on the GSM system, providing end-to-end, wide-area wireless IP connection. Compared with the original GSM dial-up circuit-switched data transmission method, GPRS is a packet-switched technology with the advantages of "real-time online", "pay-as-you-go", "fast login", "high-speed transmission", and "free switching". In layman's terms, GPRS is a high-speed data processing technology. The method is to transmit data to users in the form of "packets". Although GPRS is a transitional technology for the transition from the existing GSM network to the third generation of mobile communications, it has significant advantages in many aspects.
Due to the use of "grouping" technology, users are relatively stable on the Internet, avoiding unnecessary disconnections. In addition, the method of using GPRS to surf the Internet is different from that of WAP. Using WAP to surf the Internet is like surfing the Internet at home. First, "dial-up connection", and then you cannot use the phone line at the same time. Simultaneously. Technically speaking, voice transmission (ie, call) continues to use GSM, while data transmission can use GPRS. In this way, the application of mobile phones will be upgraded to a higher level. Moreover, the development of GPRS technology is also very "economical", because it only needs to use the existing GSM network for development. GPRS has a wide range of uses, including sending and receiving e-mails via mobile phones, and browsing on the Internet.
TDMA: TDMA is the abbreviation of TimeDivisionMultipleAccess, which is a technology that uses Time-DivisionMultiplexing to provide wireless digital services. It represents a digital signal transmission technology for mobile phone systems . TDMA divides a radio frequency into multiple time slots, and then divides these time slots into multiple groups of calls. In this way, a radio can support multiple data channels at the same time, and this technology has become the basis of today's D-AMPS and SM systems.
4. Overall composition and basic working principle
4.1 Composition of GSM mobile phone
Figure 1 shows the overall block diagram of the mobile phone. The mobile phone is composed of radio frequency, baseband, It is composed of software and man-machine interface.
4.2 The basic working principle of GSM mobile phone
The following is described in five parts: radio frequency, baseband, man-machine interface, software, and SIM card.
4.2.1 Radio frequency unit
The composition of the radio frequency unit is shown in Figure 1.
The transmitter mixes the modulated baseband signal sent by the baseband unit with the local oscillator signal generated by the frequency synthesizer, transforms it into a radio frequency transmission frequency, and amplifies the modulated radio frequency signal to the required power by a power amplifier , And then fed to the mobile phone antenna through the duplexer for transmission.
The mobile phone receiver sends the weak modulated radio frequency signal received on the antenna from the base station transmitter through the duplexer to the low noise amplifier to amplify it to the required level, and the frequency synthesizer generates The local oscillator signal is mixed, transformed into a baseband signal and sent to the baseband unit.
The frequency synthesizer is based on a high-precision crystal oscillator, and can generate a series of high-precision frequency sources with a certain frequency interval through synthesis technology. There are two ways of synthesis: direct synthesis and phase-locked loop synthesis.
A duplexer is a device that allows simultaneous transmission and reception using the same antenna. In essence, it is a set of filters to avoid the strong signal from interfering with the weak signal received. Early duplexers It is a ceramic duplex filter with a larger volume. In order to reduce the size of the mobile phone, the usual approach is to use an electronic switch plus the necessary TX low-pass filter and RX-SAW filter, and integrate it into a module to realize the function of a duplex switch.
The radio frequency design introduced below is a set of radio frequency design schemes suitable for GSM900/DCS1800 dual-frequency mobile phones. It is different from the previous mobile phone solutions. It uses zero-IF technology and no longer needs intermediate frequency when receiving. Filters are very beneficial for reducing the size and cost of mobile phones. The working principle of the radio frequency part is briefly described below.
a. Receiver
The receiver principle is shown in Figure 2.
In this receiver, the main function is completed by the zero-IF transceiver (U4), which includes a GSM low-noise amplifier (LNA) and two quadrature mixers (GSM frequency band and DCS frequency band) , A local oscillator signal generator and two active filters.
When the receiver is working, the received signal enters the front-end module through the antenna (the front-end module includes the transceiver switch, low-pass filter and RX filter), and then the signal is sent to the DCS channel or the GSM channel respectively. For the GSM channel, only GSM_RF and GSM_RFB sent from the front-end module are sent to U4 for down-conversion processing; for the DCS frequency band, LNA (U2) and BALUN (U3) must be connected to convert the front-end filtered signal into DCS_RF and DCS_RFB The double-ended balanced signal is then sent to U4 for mixing. After the local oscillator signal generated by the system frequency synthesizer is divided in U4, it is mixed with the received signal to directly obtain the zero intermediate frequency I and Q signals. The I and Q signals are filtered through a low-pass filter to remove blocking interference and adjacent channel interference. , Is sent to the baseband circuit for demodulation processing.
External LNA (U2) voltage gain, U4 internal LNA, mixer and internal baseband amplifier gain, all can realize programmable gain control (AGC) through serial interface.
The AGC of the GSM mobile phone adjusts the gain of the receiver according to the received signal strength detected by the baseband, so that the peak-to-peak amplitude of the baseband signal output by the receiver is maintained at the required value to meet the dynamic range of the receiver Requirements. The AGC control signal is sent by the baseband unit.
b. Transmitter
The transmitter is mainly composed of a modulation loop, a power amplifier (PA) and a front-end module. The modulation loop is integrated in the zero-IF transceiver U4, which includes a quadrature modulator, a frequency divider, a high-speed phase-frequency detector and a down-conversion mixer, and it completes the transmission together with the external transmit voltage-controlled oscillator (TXVCO) modulation.
The principle of the transmitter is shown in Figure 3. The working process of the transmission path: the I and Q signals sent from the baseband circuit enter U4, U4 generates a quadrature modulated IF signal, and then uses the transfer loop technology to change the signal to the final TX frequency through TXVCO (GSM is 890 ~915MHz, DCS is 1710~1785MHz), and then the RF signal output by TXVCO is sent to the power amplifier (U7) for amplification, and then sent to the front-end module (U1) for filtering and then transmitted by the antenna. Because the TXVCO output spectrum is good, it only needs to integrate a low-pass filter in the front-end module to filter out the transmitted harmonics.
To put it simply, the transfer loop technology is a phase-locked loop with a down-converter in the feedback loop, which acts as a tracking band-pass filter, which not only emits low noise, but also Can eliminate parasitic modulation.
The power control of U7 is closed-loop control through a piece of IC (U6). On the one hand, it is necessary to keep the output power stable in each working time slot to meet the requirements of the GSM standard. On the other hand, use the power level control signal TX_RAMP from the baseband to control the output power of the mobile phone, which can be achieved without the maximum transmission power. In the case of better transmission quality, reduce the transmission power of the mobile phone, reduce interference to other communications, and extend the battery life of the mobile phone.
The process of power control is: the mobile phone reports the measured received signal strength and signal quality through the uplink, and the GSM system issues mobile phone power control commands through the downlink to determine whether to increase or decrease the mobile phone’s transmit power. After the mobile phone software selects according to the system instructions, it sends out the TX_RAMP signal to adjust the output power required by the power amplifier.
The principle block diagram of automatic power control (APC) is shown in Figure 4.
The implementation process of APC is as follows: U7 output couples a part of the signal through a directional coupler, and sends the voltage V1 of this part of the signal to an input of the comparator (U6), and the voltage of the control signal TX_RAMP from the baseband For comparison, the generated voltage difference is sent to the voltage control pin of U7 to automatically control the output power.
c. Frequency synthesizer
The program frequency synthesizer mainly includes 13MHz reference crystal oscillator (VCTCXO), fractional frequency phase locked loop (PLL) and radio frequency voltage controlled oscillator (RFVCO). The specific working process is shown in Figure 5. .
The automatic frequency control (AFC) signal controls the frequency of the VCTCXO and provides a reference frequency for the PLL. The frequency generated by the RFVCO is sent to the PLL, and after frequency division processing, it is compared with the 13MHz frequency, and the error voltage CP generated by the comparison is sent to the RFVCO to further control the frequency of the RFVCO until the frequency value reaches the requirement.
RFVCO is a wide-band, low-phase-noise oscillator, and its frequencies in different working modes are shown in Table 1.
Table 1 Frequency of RFVCO in different working modes
Transmit frequency p>(MHz) | Receiving frequency (MHz) | VCO frequency (MHz) | |
GSM mode | 890～915 | 915～960 | 1320～1440 |
DCS method | 1710～1785 | 1805～1880 | 1282.5～1440 p> |
RFVCO coverage frequency | 1282～1440 |
Radio frequency VCO covers GSM/DCS dual frequency It is not difficult, but because the mobile phone works at low voltage, it also requires fast PLL lock time (for GPRS<250μs) and low phase noise. Therefore, a charge pump is used in the loop to improve the control speed of the voltage-controlled oscillator. U1 is a fractional frequency PLL, and the loop phase frequency can be selected higher to speed up the lock time. U1 includes a sigma-delta modulator, an adder, a high-frequency prescaler, a low-noise phase detector, and a charge pump.
Under normal circumstances, the technical indicators of VCTCXO are: nominal center frequency f0=13MHz; frequency error under normal temperature conditions is ±5×f0×10; temperature stability is ±2.5×f0×10.
It can be seen from the technical parameters of the reference frequency oscillator: If AFC is not used, it obviously cannot meet the mobile phone frequency error of 0.1×f 0×10 requirements, therefore, AFC must be adopted.
In order to complete AFC, there must first be a frequency correction signal based on the base station frequency, which is sent by the BS on the downlink Slow Correlation Control Channel (SACCH). After the mobile phone receives the frequency correction data sent by the BS, it is transformed and filtered by the DAC to generate an AFC control signal, which is added to the AFC pin of the mobile reference frequency source U11 to adjust the reference frequency of the mobile phone, so as to fine-tune the transmission of the mobile phone. working frequency. After the mobile phone's transmitting frequency is received by the BS, it is judged by the BS. If the error exceeds the standard, the BS will re-adjust through the SACCH channel until the mobile phone's transmitting frequency error can meet the requirements under normal and extreme conditions.
d. Interface
There are many interfaces between the radio frequency circuit and the baseband circuit, including analog and digital. The main interfaces are as follows.
I, Q interface: The I and Q signals generated by the receiving path are sent to the baseband for demodulation, and finally become voice signals, while the I and Q signals required by the transmission path come from the baseband and pass through the radio frequency. The circuit is modulated and loaded with waves and then emitted.
SEN, SDATA, SCLK interfaces: these 3 interfaces are digital control interfaces between baseband circuit and radio frequency circuit, which can control many functions of radio frequency circuit, including receiver gain and frequency synthesizer control.
AFC interface: comes from the baseband circuit to realize the frequency control of VCTCXO.
RF_CLK interface: connects to the baseband circuit to provide an accurate reference clock for the baseband circuit.
TX_RAMP interface: from the baseband circuit, compare with the signal from the coupler to realize the power control of the power amplifier.
4.2.2 Baseband unit
In a wireless communication system, the baseband signal constitutes the modulated signal of the transmitter. What is transmitted in the GSM system is a binary digital signal. When transmitting, there are source coding, channel coding, interleaving, burst formatting, encryption and modulation. Through these processes, the analog source signal is converted into a digital baseband signal; Demodulation, decryption, burst formatting, de-interleaving, channel decoding and source decoding, the digital baseband signal is converted into an analog source signal through signal processing opposite to the transmission. These processes are shown in Figure 6.
The baseband part of the mobile phone is designed with a dedicated chip. The dedicated chip is a large-scale integrated circuit with a microprocessor, a microcontroller and a baseband interface chip as the core. Digital signal processor realizes mobile phone voice codec, adaptive equalization, encryption and decryption algorithms; microcontroller realizes control of mobile phone operation and communication protocol operation; baseband interface chip realizes baseband signal modulation/demodulation and A/D, D/ A conversion. The baseband also provides necessary supporting capabilities such as voice, data interface, and man-machine dialogue. The SIM card, which is a personal communication symbol, is also configured in the baseband. All system software and application software are stored in the baseband flash memory (FlashROM).
The following introduces a mobile phone baseband design scheme, which can support GPRS.
The work of the baseband unit of this scheme is carried out around two main chips: GSM processor U1 and baseband interface U2.
Figure 7 is a block diagram of the baseband part. There are two crystal oscillators in the block diagram. The 13MHz crystal oscillator is the reference frequency of the mobile phone, which requires high frequency accuracy. The 32kHz crystal oscillator mainly provides a reference frequency for the power-saving mode of each part.
a. Microprocessor U1 function introduction
U1 is mainly composed of three parts: 16bit digital signal processor (DSP), 32bit microprocessor (MCU) and peripheral interface. The functional block diagram is shown in Figure 8.
Ⅰ. Digital Signal Processor (DSP)
DSP specializes in functions such as voice encoding and decoding, channel equalization, channel encoding and decoding, and signal strength measurement. The code to realize these functions is usually stored in an external flash memory, and dynamically downloaded to the program RAM and cache of the DSP as needed.
DSP integrates two coprocessors and cache/program control system. The main task of the computing coprocessor is to perform encryption/decryption operations. The main task of the Viterbi coprocessor is to complete channel equalization and channel coding and decoding. Cache/program control serves as an intermediary and control system for communication between DSP and internal and external storage units, providing sufficient address space to complete the timing control of each part of the function.
DSP can access the code stored in flash or internal RAM through the cache system, and the cache system can automatically download the required code.
Ⅱ. FMCU
In the GSM system, the main function of the MCU subsystem is to execute GSM protocol layer software, man-machine interface software and other user application software. It is composed of ARM7 central processing unit, internal ROM, clock generator and access control module. The bus management module connected to the ARM controls the ARM to directly access one of the peripheral bus, the system RAM bus or the external bus.
Ⅲ. Peripheral interface
U1 peripheral interface includes keyboard, memory, display driver, SIM data interface and the communication interface required for various processing.
b. Audio interface chip U2 function introduction
The function of U2 is shown in Figure 9. It is mainly composed of three parts: baseband processing (signal modulation/demodulation), auxiliary processing and audio processing. The communication between each part and the microprocessor is carried out through the serial bus, among which: the baseband serial port processing and RF interface I and Q signals; the auxiliary serial port handles all control signals, ADC data and DAC data related to encoding and decoding; the audio serial port handles voice signals.
Ⅰ. The baseband processing part is always an analog signal from beginning to end, which directly provides drive interfaces for microphones and speakers; provides hands-free and external car equipment interfaces; and provides independent input and output channels. The input and output gains provide users with programmable features with maximum flexibility.
The transmit path sends the uplink I and Q signals received by the baseband serial port to the GMSK modulator, after modulation, it sends it to two high-speed DACs, and then sends it to the RF transmitter. The baseband processing modulator/demodulator is dual channel.
The balanced I and Q signals sent by the RF receiver are sampled in the receiving path, and then sent to two Σ-Δ modulators to reduce quantization noise. The I and Q signals after the ADC undergo high Performance digital filter to filter out adjacent channel noise and quantization noise.
Ⅱ. The auxiliary processing part mainly includes control registers, ADC'S, DAC'S.
Ⅲ. The audio processing part mainly deals with the transformation of audio signals.
c. Power management and charging
Mobile phone power system usually adopts the centralized control of power management module. The power management module of this solution provides 4 LDOs. These 4 LDOs have been optimized in performance according to circuit characteristics and actual needs. Each LDO has its own characteristics.
Digital LDO: Digital LDO always needs to be turned on after power-on, so LDO optimizes the quiescent current at low load.
Analog LDO: The analog LDO is also always on, so the quiescent current requirement is also very high. At the same time, since it needs to be connected to the radio frequency part, it is necessary to strengthen the low-frequency ripple filtering.
Crystal oscillator LDO: Crystal oscillator LDO requires good noise characteristics.
Real-time clock LDO: Real-time clock LDO charges the backup battery, and it will work even when shutting down.
Mobile phone charging can use a linear charging module, and a PMOS tube is used as a switch tube. A period of time before charging is constant current, when the battery voltage reaches 4.1V/4.2V, it becomes constant voltage charging. The charging circuit of this scheme is integrated in the power management module.
d. Display interface
There are two LCD interface modes: parallel and serial. In this solution, the interface between LCD and MCU is serial mode, and one bit of serial data is input on the rising edge of each clock. After all 8 bits of serial data are entered, the serial data is converted to 8 bits of parallel data for the next step in the drive module. The drive module has a built-in display RAM. One RAM bit corresponds to one LCD dot, so that the status of the LCD dot can be changed by changing the content of this RAM bit.
e. Radio frequency and system interface
Ⅰ. Baseband and RF part interface:
Baseband I/Q interface signals, such as IP, IN, QP and QN;
Serial data signals, such as SYNTHDATA, SYNTHEN and SYNTHCLK, are The baseband part provides a serial communication interface for RF control signals;
RF clock and control signals, such as RF_CLK, AFC, TX_RAMP;
Temperature detection signals, such as TEMP_SENSE.
Ⅱ. The system interface of the baseband part: ground (GND), digital power, analog voltage, universal system interface 0～6 (USC0～6), earphone jack, charger power interface.
4.2.3 Human-Machine Interface
The human-machine interface is the interface between the person performing mobile communication and the mobile phone that provides mobile communication services, as shown in Figure 10. It includes hardware and software: hardware includes keyboard, display, microphone, speaker and SIM card, etc.; software has menu and phone book functions, public mobile network functions, user SIM card functions, and basic man-machine interface functions.
4.2.4 Software
Refer to Figure 11, the GSM software includes the operation program of the internal function circuit of the baseband unit and the operation program of the first, second, and third layer of the communication protocol. The left side of the dotted line in the figure is the basic GSM software, and the right side is the part of the software that adds GPRS functions.
4.2.5SIM card
The SIM card is made of a large-scale integrated circuit chip. In the GSM digital mobile communication network, every user has a SIM card, which must be inserted into the mobile phone before the user can make a call. A mobile phone without a SIM card can only make emergency calls, and all other functions cannot be used. The SIM card technology is used in GSM mobile communication, which liberates radio communication from the unsecured situation.
There are two types of SIM cards: one is called a large card with a size of 85mm×54mm; the other is called a small card with a size of 25mm×15mm. Regardless of whether it is a large card or a small card, the integrated circuits installed are the same. Some large cards are embedded with small cards, which can be removed and used. The universal one is the small card. With the continuous communication of network value-added services, STK cards have also become popular. They can provide banking and other services. At the same time, the capacity of the card is larger than the general large and small cards. The STK card can store 100 phone numbers.
SIM card has the characteristics of separation of customer and mobile phone (separation of man and machine), safe and reliable communication, low cost and durability.
The contents stored in the SIM card include: a. User identification number, which represents the user's phone number. b. User key and secret algorithm. They can not only identify the user's identity, prevent illegal access to the network, but also prevent the user data transmitted on the wireless channel from being stolen, thereby eliminating the "double machine" phenomenon. c. Personal identification code (PIN code) and SIM card personal unlock code (PUK code). The PIN code is the personal password of the SIM card, which can prevent others from using the SIM card without authorization; when the PIN code is incorrectly pressed, the PUK code can be used to unlock the lock. d. Storage space used by users. Users can store some personal information such as fixed short messages and phone book in the SIM card.
5. Contrast
The voice of GSM digital mobile phone is transmitted on the wireless channel after being digitized. It is not as easily disturbed as an analog mobile phone, so the voice is clear during a call , Small interference. However, because the transmitted voice is digitized, there is also the disadvantage that the voice is somewhat distorted. The voice distortion of analog mobile phones is better than that of GSM digital phones. Relevant departments are researching and developing more advanced voice digital coding technology to reduce the voice distortion of GSM mobile phones.
CDMA digital network: CDMA is the English abbreviation of Code Division Multiple Access (CodeDivision Multiple Access), which is a new and mature wireless communication technology developed on the branch of digital technology-spread spectrum communication technology . It can meet the market's high requirements for mobile communication capacity and quality. It has the characteristics of high spectrum utilization, good voice quality, strong confidentiality, low call drop rate, low electromagnetic radiation, large capacity, and wide coverage. It can greatly reduce investment and Reduce operating costs.
Industry operators are working hard to increase the number of users in their systems, reduce the cost of each user, create greater profits and actively strengthen market penetration. Code division multiple access technology is one of the digital communication technologies to solve this problem.
The advantages of GSM mobile phones are:
Efficient frequency band utilization and larger network capacity
Simplify network planning
Improve Call quality
Enhance confidentiality
Improve coverage features
Extend user talk time
Soft volume and "soft" switching
Internet speed is faster