Abstract: Introduce the design and development of a high-speed GPS data transmission terminal for GPS vehicle monitoring and dispatching. The terminal is half-duplex communication, GMSK data modulation and demodulation, the transmission data rate is 9600bps, and it can also transmit voice. The vehicle monitoring and dispatching system using this digital transmission terminal can monitor more than 1200 vehicles / minute in real time.
In the GPS vehicle monitoring and dispatching system, the positioning data of the vehicle needs to be transmitted back to the monitoring and dispatching center through the wireless data communication platform. Commonly used wireless data communication platforms can be divided into two categories: public network and private network. The public network refers to GPRS, CDPD, GSM, etc. to use wireless data networks, and the private network refers to the wireless communication network specially established for dispatching systems. The GPS system using the public network has the advantages of small investment, large coverage, and small system maintenance, but its real-time performance is relatively poor, and GPS differential positioning cannot be performed. To use the GPS system of the private network to transmit data to the monitoring target by time-division multiplexing, make full use of wireless frequency resources, the transmission is fast and the real-time is good, GPS differential positioning can be performed, and the positioning accuracy is high. Therefore, the GPS vehicle monitoring and dispatching system of the private network is particularly suitable for applications requiring high real-time performance such as public security, firefighting, public transportation, and financial banknote transportation. The role of the GPS data transmission terminal in the private network in the system is mainly to realize GPS differential positioning and wireless communication. This article introduces the design method, performance and characteristics of the low-cost, high data rate, good real-time, reliable voice transmission GPS data transmission terminal for the private network.
1 Full utilization of frequency resources in the design of digital transmission terminals
In vehicle monitoring and dispatching systems, frequency resources are limited, 
It is not possible to allocate a frequency band to each terminal, usually all terminals share a data channel. Therefore, how to reuse this frequency resource to make full use of it and increase the data communication capacity of the system is a question worth discussing in the data transmission end and system design.
There are two common single-channel multiplexing methods: roll call method and time division multiplexing method. The roll call method is that in the whole system, the base station terminal first rolls the name and designates a specific mobile terminal to return data to it; in the subsequent period of time, the designated terminal returns data, other terminals remain silent, and the base station terminal Data; then the base station terminal continues to roll call. The time division multiplexing method is to allocate a time slot to each terminal in a time period, the terminal sends data in turn, and in the next time period, the terminal sends in turn, and so on. The shortcoming of the roll call method is that the base station terminal has to roll the roll call every time, so the communication efficiency is relatively low, and the data communication capacity is relatively small, which can only be applied to a relatively small system. The time division multiplexing method is more efficient than the roll call method and has a large data communication capacity, but all terminals need a common time reference. In mobile communications, this reference is usually provided by the base station through a separate channel, which requires a separate control channel, which has high requirements on the equipment. In vehicle monitoring and dispatching systems, this method cannot be used. Considering that when the GPS receiving module performs GPS positioning, it will also obtain a very accurate global synchronous clock. Using it as a time reference for time-division communication, time-division multiplexing can be achieved without increasing costs and equipment complexity.
In the GPS vehicle monitoring and dispatching system of time division communication, the mobile terminal does not have much time to send and receive data, and the terminal is often in an idle state. In vehicle monitoring and dispatching systems, the use of data to transmit positioning information and voice to achieve dispatching functions will greatly improve system performance. Therefore, if it is possible to transmit both data and voice without mutual interference on a half-duplex transmission platform, the performance of the entire system will be greatly improved without increasing costs. Consider the following two facts:
(1) In the time-division multiplexing monitoring and dispatching system, the time for each mobile terminal to transmit and receive data is very short. In each time-division multiplexing cycle, there are only one reception and one transmission, twice, each tens of milliseconds. The data transmission and reception time of the data transmission terminal of the base station is relatively long. 
(2) During voice communication, the voice is occasionally interrupted for less than 100 milliseconds, which basically does not affect the intelligibility of the voice, and the listener only feels a slight click or click.
The author adopts the following methods to achieve the simultaneous transmission of data and voice:
(1) Two channels with 25kHz bandwidth are used, one for voice communication and one for data communication;
(2) Most of the time, the mobile terminal is in the voice channel, and receives or sends voice. In the time slot for sending and receiving data, whether it is receiving or sending voice, it is forced to switch to the data channel to receive and send data. , Continue to send and receive voice. In this way, data transmission and reception will only cause the interruption of voice communication in less than 100 milliseconds, so the influence of voice communication can be ignored.
(3) Install two base station terminals in the monitoring and dispatching center, one dedicated to voice communication and one dedicated to data communication; each monitoring target is installed with a mobile terminal to send and receive data in a given time slot, and send and receive voice at other times; It is slightly different from the mobile terminal only in software. In this way, on a half-duplex platform, 
Simultaneously achieve half-duplex transmission of data and voice.
2 GPS digital transmission terminal hardware design
2.1 Selection of digital modulation
There are three main factors that determine system capacity in a time-division communication system: wireless data transmission rate, protection time for data transmission between different terminals, and the amount of data for each terminal. Increasing the data transmission rate can directly increase the capacity of the communication system. In vehicle monitoring and dispatching systems, bandwidth resources are very limited. To increase the communication data rate, a relatively efficient modulation method must be used.
ASK, PSK, FSK and other modulation methods are simple in modulation and demodulation, but have poor spectrum characteristics and low bandwidth utilization. Complex modulation methods such as QAM and TCM require more complicated modulation and demodulation methods, and the cost is relatively high. GMSK (Minimum Frequency Shift Keying with Gaussian Filtering) data modulation method is adopted here. GMSK is a constant-envelope modulation scheme that can be implemented with a simpler Class C amplifier, and it can maintain lower co-channel and adjacent channel interference while maintaining spectral efficiency. Realization of GMSK modulation can use orthogonal modulation or simple Gaussian low-pass filtering plus frequency modulation. The latter is used here, as shown in Figure 1.
During demodulation, a completely reverse process is used, first demodulating the frequency to obtain a Gaussian filtered baseband signal, then Gaussian inverse filtering to restore the signal before modulation.
2.2 Design of frequency modulation and demodulation
In order to ensure the stability and reliability of data transmission, the transmitting circuit uses two oscillators: an intermediate frequency oscillator and a local oscillator, and the data and voice respectively modulate these two oscillators. The advantage of separate modulation of digital speech is to avoid the mutual influence of the two channels, and the data signal directly modulates the intermediate frequency crystal oscillation circuit, which improves the stability of data modulation and is beneficial to the realization of MSK modulation and demodulation of the receiving circuit. The intermediate frequency oscillator uses an oscillator composed of a high-precision crystal; the local oscillator uses a programmable swallow PLL (phase-locked loop) frequency synthesizer, and the local oscillator VCO (voltage controlled oscillator) is locked to the high-precision crystal oscillator through the PLL The local oscillator has both high frequency stability and frequency can be changed by programming.
The block diagram of frequency modulation is shown in Figure 2.
The block diagram of the receiving demodulation circuit is shown in Figure 3.
After the RF signal received from the antenna is amplified, the baseband signal is obtained after two down-conversions and filtering. After the baseband signal is amplified, it can drive the speaker to sound or demodulate the digital signal to the Gaussian inverse filter.
Due to the relatively high cost of the PLL frequency synthesizer, considering the limitation of frequency resources in actual use, the digital transmission terminal adopts the half-duplex working mode, and the frequency modulation and demodulation share a PLL frequency synthesizer (local oscillator).
The conversion time of the PLL is an important indicator, and the size of the conversion time directly affects the performance of the terminal. The long conversion time makes the digital / voice communication channel conversion time of the terminal also long. The longer data protection time sent by different terminals will greatly reduce the digital communication capacity of the entire system and reduce the system performance; and the long PLL conversion time, data communication will make The interruption of voice communication for a long time seriously affects the quality of voice communication. Therefore, the design should reduce the PLL conversion time as much as possible to improve the PLL lock speed. Using the variable width method to accelerate PLL lock, the system performance has been greatly improved.
2.3 Gaussian low-pass filter and inverse filter circuit
Gaussian low-energy filter means that the frequency response of the filter is a Gaussian function:
The impulse response of the Gaussian filter is also a Gaussian function. It is impossible to realize this kind of filter by the analog method, and the Gaussian filter is usually implemented by the method of digital storage. A GMSK modem FX589 designed and produced by CML is used here.
FX589 is a low-voltage high-speed GMSK modem, it can realize Gaussian low-pass filtering and inverse filtering, the data rate is 4Kbps ~ 64Kbps.
In order to achieve the channel bandwidth required for wireless communication of 25kHz, out-of-band interference <-60dB, the selected data rate is 9600bps, BT = 0.5.
According to the working characteristics of FX589, the following measures have been taken to improve the performance of data communication:
(1) Carefully design the peripheral circuit of FX589 to work with FX589;
(2) Add / descramble the sent / received data to remove the DC and low frequency components in the signal to suit the requirements of FX589;
(3) Add the appropriate header code to the data, use FX589 to restore the receiving clock, and protect the integrity of the received data; 
(4) A data error detection and retransmission mechanism is adopted in the software to eliminate the impact of error codes on system performance.
2.4 The overall design of the digital transmission terminal
The design of the entire digital transmission terminal is centered on the MCU, and FPGA is used to integrate peripheral devices to improve the stability of the system and reduce the complexity of test maintenance. The overall block diagram of the digital transmission terminal is shown in Figure 4.
Serial EEPROM is used to store important information of the vehicle, such as serial number, license plate number, etc. FLASH is used to record vehicle operation information for query by the dispatch center. The SRAM memory is mainly used to store temporary data, such as GPS positioning information and differential positioning information. The GPS receiving module is used to receive GPS signals and realize GPS differential positioning. The display and control panel uses a backlit LCD display, controlled by the power volume knob, squelch adjustment knob and four touch buttons. RS-232 test setting port is used to communicate with PC or other devices. FPGA connects all devices into a whole, and the microcontroller controls the coordination of each module through the serial communication port, address data interface and general I / O port, and completes the functions of display, communication and data processing of the entire digital transmission terminal.
3 GPS digital transmission terminal control software design
The software design of the GPS data transmission terminal requires firstly to cooperate with the hardware to ensure the stability and reliability of the terminal's work; secondly, to reasonably control, give full play to the potential of the hardware and improve the performance of the terminal and the system; in addition, the system needs to be taken into account. 
Provide a good operation interface and certain additional functions and expansion capabilities.
The schematic diagram of the structure of the entire software is shown in Figure 5.
Because the terminal works in a time division communication system, each terminal can only send and receive data at a specified time, so in software design, real-time requirements are very high. If the real-time performance of the software control is not good, the data communication between different terminals will interfere with each other. In this case, to ensure reliable data transmission, only special system data communication capacity can be used to increase the protection time of data transmission between different terminals. The author uses the following methods on the software to improve the real-time control:
(1) The entire software is timed by a short-time (hundreds of microseconds) timer interrupt, combined with GPS high-precision time information, so that all terminals have synchronized and accurate time.
(2) The software adopts modular design. When designing the module, the work of each module is divided into multiple parts. When the module is running, only a part of it is run at a time, which reduces the execution time of the module and improves the real-time performance of software control.
(3) According to the module's real-time requirements, they are divided into different execution priorities. For example, the priority of the FM and demodulation circuit control module is set to the highest, and the priority of the EEPROM read-write module is set to the lowest.
In the software design, in order to reduce the protection time of data transmission between different terminals and increase the system capacity, according to the characteristics of the system's half-duplex data communication, a control method of early channel switching is adopted for the data transmission circuit. After adopting this method, the guard time does not include the PLL lock time, only the RF power settling time. Because the settling time of the RF power is very short and can be ignored, the time accuracy of the MCU control becomes the main factor that determines the protection time. As long as the real-time software control is good, the protection time can be reduced to within a few milliseconds. The schematic diagram is shown in Figure 6.
In addition, in the software design, the serial communication program adopts a layered design, which is divided into three layers: receiving, command analysis and command data processing, which is convenient for future expansion of commands to adapt to different vehicle monitoring and dispatching systems. 
The performance indicators of the prototype meet the design requirements, as follows:
(1) GPS positioning and GPS differential positioning functions;
(2) Half-duplex wireless communication, with a communication data rate of 9600bps in the frequency range of 430MHz to 450MHz, and can simultaneously transmit voice;
(3) Channel bandwidth 25kHz; frequency spurs <5kHz; transmitting adjacent frequency interference <-60dB;
(4) Receive sensitivity: 1.0μV signal input, demodulation output SNR> 30dB, BER <1.0e-5;
(5) Reception selectivity: ± 10kHz; -6dB; ± 25kHz: -50dB;
(6) Transmit power: 10W ～ 35W (adjustable);
(7) LCD backlight display; RS-232 serial data interface.
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