Application design of wireless data transmission radio in tap water industry

0 Introduction
In industrial automatic control systems, data transmission is the basis for achieving automatic control. Data transmission can be simply divided into two methods: wired (including laying optical cables or renting telephone lines) and wireless (divided into establishing a dedicated wireless data transmission system or borrowing public network information platforms such as CDPD, GSM, CDMA, etc.). In general, there is a certain distance between the water pump station and the water treatment plant in the tap water industry (generally a few kilometers to tens of kilometers), and the central control room is generally located in the water treatment plant. Therefore, the remote substation of the water pump station is required to collect its signals and transmit them back to the central control room in time, and at the same time, quickly transmit the instructions of the central control room to the remote substation to realize the automatic control of the water pump station. This requires the wireless communication system to have the characteristics of high transmission rate, short polling time, and fast response time. Therefore, a reliable wireless data transmission system must be established to realize the automatic control of the water pump station.

1 Overall structure of dedicated wireless data transmission system
Figure 1 shows the overall structure of a wireless data transmission radio. At present, most of the remote control and telemetry radios in China are modified from analog FM intercom transmitters (i.e., car radios) plus MODEM chips. It is not a professional digital data transmission device, but a walkie-talkie that works in the 230 MHz/800 MHz data transmission business frequency band and the 350 MHz/800 MHz trunking business frequency band. This method has long been eliminated abroad. Using this radio to transmit data has the disadvantages of poor reliability, low performance indicators, high bit error rate, low sensitivity, and no network management. If the data cannot be transmitted back or the control cannot be issued, it will not be able to determine whether it is a power outage, a bad power supply, a poor signal, an antenna feeder mismatch, a high temperature, or no data from the PLC/RTU. Therefore, in transmissions that require high speed, long distance, and high reliability, the digital transmission radio based on DSP (Digital Signal Processing) and other technologies given in this article should be used. The equipment of manufacturers such as MDS and DATAIANC in the United States also has these functions.

2 Working principle of wireless data transmission
The communication between the host computer and the module in the wireless data transmission system is completed through an asynchronous serial port. Before communication, the data frame format (8 data bits or 9 data bits) and rate of the serial port should be consistent with the module. There are two types of communication content between the host computer and the module, one is data and the other is command. Data or command can be distinguished by DTR or DSR signal. Whether the data is transmitted from the host computer to the module or from the module to the host computer, it is transmitted in an unformatted manner (transparent transmission). In use, general users will not involve data transmission control and format between modules. However, as an understanding of the basic working principle of the module, and in applications with strict timing requirements, there should be a certain understanding of the transmission format between modules and the time occupied by each part of the transmission.

2.1 Module sending process
When the module receives data from the host computer, the module first determines whether the received data is a command or data through the DTR line. If it is a command, the corresponding command is executed; if it is sending data, the data to be sent is first sent to the sending buffer, and the module state is converted from the receiving state to the transmitting state at the same time. This conversion process takes 100 ms. After the state conversion is completed, the sending package program is started. The function of the sending package program is to package the data in the buffer into a data packet suitable for wireless transmission, and dynamically insert some control signals into the data packet, and then send the data of this data packet to the data modulation port in the module, and transmit it in FSK modulation according to the V23 protocol. In this process, all processes are completed in parallel.
2.2 Module receiving process
In the receiving state, the receiver always receives the synchronization information in the code stream. Once the synchronization information is received, it will immediately perform bit synchronization. After obtaining bit synchronization, it will perform code synchronization. After code synchronization is completed, data and control information can be received. After receiving the data, it is transmitted to the host computer according to the specified serial port frame format.
2.3 Basic data transmission functions
The functions of common data transmission modules are generally divided into three types: basic data transmission functions, extended functions, and advanced extended functions. Here we mainly introduce the hardware connection method for realizing basic data transmission functions. If TX is used to represent the data sending terminal (data leaving the functional block) on the device port, and RX is used to represent the data receiving terminal (data entering the functional block) on the device port, then the serial port connection method between device A and device B is shown in Figure 2.

If the serial port connection is also regarded as a two-port device, and the terminals of the two ports are marked, the connection circuit for serial communication between the two devices is shown in Figure 3, where the serial port connection in the middle box of Figure 3 can be regarded as a two-port device. If a wireless data transmission module is used to replace the serial port connection line, the communication connection circuit between device A and device B is shown in Figure 4.
By comparing Figures 3 and 4, it can be seen that if the wireless channel composed of two wireless modules is also regarded as a two-port device, then for device A and device B, the corresponding relationship between the wired connection and wireless connection terminals during serial communication is the same.

3 Network design and field test
3.1 Theoretical calculation
The communication distance of wireless data transmission stations, the bit error rate after connection, and the system anti-interference and stability indicators are theoretically determined by two factors: transmission line of sight and receiving object strength.
For line-of-sight transmission, the transmission of radio wave signals varies according to the length of the working wavelength (frequency band), including ground wave transmission (long wave), ionospheric reflection transmission (short wave) and air transmission (ultra-short wave, microwave 1). China's radio management department allocates dedicated wireless data transmission services mainly to the 200-240 MHz frequency band (there are also 800 MHz and other frequency bands). The radio wave propagation in this frequency band is carried out through the air. Due to the influence of the earth's curvature, the maximum visible distance D between two points (the main station antenna height is H, and the remote station antenna height is h) is:

Assuming that the main station antenna is erected on the roof of an office building (about 100m high) and the remote station antenna is erected on the roof of a bungalow (about 4m high), then:

Considering that 230 MHz radio waves have a certain diffraction ability. And assume that the antenna can theoretically pass up to about 50 km. Therefore, the relative height of the antenna is the first factor determining the communication distance.

When the receiving field strength radio wave is sent from the radio station, it will pass through the feeder and antenna and propagate far away through the air. When the signal is attenuated and reaches the remote receiver, its field strength level is:
Pr=P1+G1+Gr-Lt-Lr-Lo
, where Pr is the normal receiving level (dBm), Gt and Gr are the transmitting and receiving antenna gains (dBm), Lt and Lr are the transmitting and receiving feeder losses (dBm), and Lo is the free space loss (dBm). The free space loss is Lo=32.45+201gf(MHz)+201gD(km). The unit of f is MHz and the unit of D is km. The
signal-to-noise ratio of the demodulated output signal will be different when the field strength of the radio wave signal reaches the receiver, which will affect the judgment of the system and cause bit errors. If the field strength is too small, the receiver will not be able to receive it even if the distance is close. Therefore, the receiving field strength is the second factor that determines the communication distance.
The difference between the receiving field strength Pr and the threshold level of the receiver (i.e., the field strength level value required by the receiver when the BER is less than 10-6. This indicator is different for different radio stations, generally -110 dBm) is the fading reserve. The larger the difference between Pr and the receiving threshold level, the more fading reserve should be, the stronger the anti-interference ability is, and the fewer bit errors are. Generally, the fading reserve is required to be above 20 dBm.
3.2 Actual side test
Due to the requirements of the tap water system, there is generally a certain distance and height difference between the water intake pump station and the water treatment plant in order to fully utilize the potential energy to transport water. Therefore, the actual environment of the remote substation and the main station often has hills, buildings, trees, etc. This situation will affect the transmission of signals to varying degrees and in different ways. Therefore, before engineering design, it is necessary to conduct on-site signal transmission and receiving field strength tests (should be accurate to dBm) so as to determine the functions, antenna types, installation heights and other parameters of the main station and each remote substation radio station according to the on-site environment and work requirements. Only when the upper and lower signals have sufficient anti-interference ability can effective and reliable data acquisition and control be achieved.

4 Conclusion
This article provides a new way to achieve efficient and reliable wireless data transmission. In fact, as long as you choose wireless data transmission products with high technical indicators and stable and reliable quality, and then determine the antenna type, installation height and radio power through field testing, then you can use wireless data transmission radio to achieve high-speed, high-stability and high-reliability wireless data transmission.