Design of wireless data transmission system based on EM200 module

Introduction
With the comprehensive advancement of China's informatization process, various industries have also put forward higher requirements for informatization. The understanding of transmission informatization has also been upgraded from simple digitization to the unity of digitization, networking and wireless. In view of the current informatization requirements, although the original wired transmission system can be digitized and networked, the complex wiring and high maintenance costs have greatly limited the distribution range of network nodes, which has largely hindered the in-depth and popularization of data transmission informatization. Mobile monitoring system can solve the above problems very well. Mobile monitoring system is generally composed of data acquisition equipment, terminal management computer and monitoring center. It can install data acquisition equipment on a removable carrier, so that the data collected on site is processed by the terminal management computer and transmitted to the monitoring center through the wireless data transmission channel, so that the monitoring center can understand the situation on site at any time, thereby realizing remote wireless mobile monitoring. Considering the needs of practical applications, this paper designs a network data transmission system based on CDMA or GPRS. The system can replace the corresponding modules according to different transmission requirements to complete more functions.

1 Wireless data transmission methods
At present, the methods of mobile wireless data transmission mainly include GSM short message, CDMA, GPRS and other methods (such as Bluetooth, ZigBee, etc.), each of which has different characteristics.
GSM short message transmission uses signaling channel, adopts the storage and forwarding method, and the transmission delay is not fixed, which depends on the current short message service usage. The disadvantages of this method are low data transmission rate, delay and uncertain time.
CDMA (Code Division Multiple Access), also known as code division multiple access, is a wireless communication technology. The system can assign each user a specific address code. The address codes are mutually quasi-orthogonal, so they can overlap in time, space and frequency. The theoretical transmission rate of CDMA 1X can reach 300 kbps, and the current actual transmission rate is about 100 kbps. It can support mobile IP services and can be used for Internet connection, data transmission, etc.
GPRS (General Packet Radio Service) is the abbreviation of general packet radio service. It is often described as "2.5G", that is, this technology is between the second generation (2G) and the third generation (3G) mobile communication technology. GPRS uses packet switching technology. It can efficiently transmit high-speed or low-speed data and signaling. The theoretical transmission rate of GPRS can reach 171.2 kbps, and the actual transmission rate is about 40 kbps.
Bluetooth technology is actually a short-range wireless communication technology and an open global specification for wireless data and voice communication. The Bluetooth system generally consists of four functional units: wireless unit, link control (firmware) unit, link management (software) unit and Bluetooth software (protocol stack) unit. The transmission distance of Bluetooth is 10 cm to 10 m. If the power is increased or some peripherals are added, the transmission distance can reach 100 m. It uses the 2.4 GHz ISM band and frequency modulation and frequency hopping technology, and uses right-directed error correction coding, ARQ, IDD and baseband protocol.
ZigBee is a set of technical standards for networking, security and application software developed based on the IEEE-approved 802.15.4 wireless standard. Its effective coverage range is 10 to 75 m. When the network rate is reduced to 28 kbps, the transmission range can even be expanded to 134 m, thus having higher reliability.
This article uses a high-performance microprocessor LPC2210, which can be widely used in the field of industrial and civil wireless data transmission, especially in some areas where wiring is difficult and remote areas. The CDMA wireless data transmission module used in the design adopts Huawei EM200 and is connected to the processor through a standard RS232 interface. This article introduces how to transmit user data information. In fact, user data information is only an application of the entire information transmission management system. By replacing different data source modules, different information transmission can be achieved, thus forming different information transmission management systems, such as remote meter reading systems, hydrological data collection systems, etc.

2 Networking solutions
M2M (machine to machine) data transmission methods are currently widely used in many industries. It can collect data distributed in various places through data collection terminals, and link the collected data to the CDMA wireless transmission module through a universal interface, and then transparently transmit the data through it, and send the corresponding data to the corresponding data center. Figure 1 shows the structural principle diagram of a mobile wireless data transmission system based on CDMA. The wireless data transmission system and the user's data center can generally establish a data connection through TCP/UDP. The user's data center, as the communication server, has a public fixed IP and a designated listening port. The data transmission module in the wireless data transmission system can be set to automatically dial when powered on. If the wireless data transmission system and the user's data center use the TCP protocol to transmit data, a TCP connection is automatically established with the server.

3 Hardware composition of wireless data transmission terminal
The hardware part of the wireless data transmission terminal mainly includes three parts: ARM CPU control module, CDMA Modem module and power supply. Its hardware connection relationship and structural block diagram are shown in Figure 2.

LPC2210 is a 16/32-bit ARM7TDMI-S CPU microcontroller from Philips. It is a RISC microprocessor based on the ARM general architecture, with the advantages of low cost, low power consumption, and high performance. Usually, after receiving user data through the RS232/485 port and converting the data into IP packets, the EM200 module can be used to access the CDMA network, and then the data can be sent to the external network or data processing center through various gateways and routes. The embedded CPU chip is the core of the entire data acquisition terminal. It must not only support the embedded operating system, but also provide a Modem interface for controlling CDMA; the EM200 module mainly completes the wireless Internet access function. It can be connected to the Modem interface of the ARM CPU through the RS232 interface to complete the control of the module (such as dialing and switching modes, etc.). Once the module is connected to the Internet, the collected data can be sent to any host with a public IP address using TCP/IP transmission, thereby realizing wireless data transmission.
3.1 UIM card interface design
The EM200 CDMA wireless data transmission module integrates a UIM card interface that complies with the IS07816-3 standard. It can be connected to the module B2B connector through PCB routing. This provides UIM card interface signals for the external UIM card holder. The EM200 CDMA wireless data transmission module can support and automatically detect 3.0V and 1.8V UIM cards. This design uses a 3.0V UIM card. Table 1 lists the interface definitions of the UIM card pins on the system connector.

Considering the requirements of EMC (Electromagnetic Compatibility), the UIM card holder can be designed as close to the interface of the EM200 wireless data transmission module as possible in the design, so as to avoid waveform deformation caused by long wiring and reduce the impact on signal communication and data transmission. In the actual design, a 0.1μF capacitor can be connected in parallel between UIM-VCC and UIM-GND, and a 33p capacitor can be connected in parallel between UIM-CLK, UIM-IO, UIM-RST and UIM-GND, so as to filter out the interference of RF signals. At the same time, a 33Ω resistor should be connected in series on the UIMCL-K, UIM-IO, and UIM-RST wiring. The interface circuit connection diagram of UIM and EM200 module is shown in Figure 3.

3.2 System Audio Design
The EM200 module provides a complete audio interface, and only a small number of peripheral auxiliary components need to be added during design. The audio is divided into two parts: the main channel and the auxiliary channel. In addition, the audio design should be kept as far away from the RF part of the module as possible to reduce the interference of RF on the audio. Since the first audio input and output channels (MICI-P, MICI-N; EARl-P, EARl-N) are all differential signals, the requirements for differential signal routing must be met during the specific PCB wiring, and the routing should be as short as possible to avoid interference of RF signals on the audio. If only one audio channel is used, it is best to use the first main audio channel. The design circuit of the main audio input and output channel is shown in Figure 4.

3.3 System status indicator light design
In order to more intuitively understand the working status of the wireless data transmission system. This system designs an LED light controlled by the LPG pin of the EM200 CDMA wireless data transmission module as a sign to indicate the network status. And the different flashing modes of the turntable indicator light indicate different network states. In the actual design, the LPG pin cannot directly drive the LED, but needs to be used with a triode. The specific status indicator light circuit is shown in Figure 5.

4 Mobile terminal software design
The software of this system consists of three parts: the host computer software, the slave computer software, the server application interface and the database. The slave computer software is responsible for the serial port communication between the ARM microcontroller and the CDMA module. Its flow chart is shown in Figure 6.

In order for the wireless data transmission system to transmit data correctly, it must be connected to the network smoothly. The wireless data transmission system needs to log in to the CDMA network and negotiate with the PDSN gateway through the LCP, PA-P, and IPCP protocols. The frame structures of the LCP, PAP, and IPCP protocols are similar, and the most commonly used frames are request REQ, response ACK, and rejection NAK. The LPC2210 microcontroller and the PDSN negotiate for each party. Either party can send a REQ frame to request a certain configuration. If the other party thinks that the configuration is unacceptable, it will respond with a NAK frame; if it is acceptable, it will respond with an ACK frame.

5 Conclusion
According to the development trend of monitoring systems and CDMA data communication technology in recent years, this article introduces a method for implementing wireless data transmission between computers by mobile terminals. On this basis, if data receiving software and database are established on the monitoring end to receive and store data from the CDMA network for query, data transmission and historical query can be easily realized. If the ARM controller is used to directly read the data of the monitor or sensor to complete data collection, data collection and wireless transmission control can be realized. At present, the system has been widely used in taxi, meteorology, public security, transportation, water conservancy and other fields.