Data transmission can be simply divided into wired (including erection of cable, telecom cable or leased line) and wireless (divided into establishment of a dedicated wireless data transmission system or borrow CDPD, GSM, CDMA network and other public information platform) two major ways. Compared with the establishment of a dedicated wireless data radio transmission with less investment than the other way, opening fast and simple operation and maintenance advantages, disadvantages is "not so reliable."
Wireless transmission of data is really unstable reliable? Clearly not. Can not live our lives more and more television, satellite, mobile phones and other communication tools have a "wireless" approach, it is quite reliable. Whether wired or wireless communications, computing can be a reliable and unreliable measure of the amount that the bit error rate (BER). General requirements for communications systems in the 10-6 bit error rate, the combined transmission error control protocol, such as feedback retransmission ARQ, KERMIT, XMODEM, ZMODEM, etc., can guarantee error-free, but the system needs a large error rate multiple feedback retransmission, transmission efficiency is reduced.
Wireless transmission of data is really unstable reliable? Clearly not. Can not live our lives more and more television, satellite, mobile phones and other communication tools have a "wireless" approach, it is quite reliable. Whether wired or wireless communications, computing can be a reliable and unreliable measure of the amount that the bit error rate (BER). General requirements for communications systems in the 10-6 bit error rate, the combined transmission error control protocol, such as feedback retransmission ARQ, KERMIT, XMODEM, ZMODEM, etc., can guarantee error-free, but the system needs a large error rate multiple feedback retransmission, transmission efficiency is reduced.
So why do people feel the wireless transmission of data reliable? Or we are using the wireless transmission does not reliable? The reason is simple, choose one of the radio is not specific data products. Now most of the country is remote telemetry stations plus FM MODEM chip analog car radio restructuring, it is not a professional data products, but work on the 230MHz band radio data transmission. In this radio station does not transmit data reliably, because there are two fundamental issues can not solve: PTT launch control delay (100ms or more) to bring the leading characters of data loss and the end of FM static noise interference caused by extra characters, especially in high-speed data transfer or conversion transceiver applications such as fast access PLC. In the low transmission rate, proximity, reliability, less demanding, and if we can capture so many times, because of its low price, with a lot. But in the required high-speed, long-range, highly reliable transmission, we recommend users to use direct digital modem, a forward error correction, using DSP technology products, such as Shenzhen Huaxia Sheng WDS, DATALINC other manufacturers equipment. The second is according to the master and sub-stations and geographic distribution of the specific circumstances of the networking environment, the design and field testing.
1、theoretical calculations
Far distance between two points can not pass? Through after the error rate? How strong anti-interference stability? In theory depends on the following two factors.
A. line of sight transmission
Radio signal transmission according to the operating wavelength (frequency) of different lengths with a ground-wave transmission (long wave), ionospheric transmission (short wave), air (VHF, microwave) in three ways, our radio management departments will be dedicated wireless data transmission mainly assigned to the 220 ~ 240MHz frequency band (in addition to 800MHz, 2.4GHz and other bands), this band of wave propagation is carried out through the air. As the curvature of the Earth, two points (respectively, H m antenna height and h m) maximum between the visual distance D km is assumed office master antenna mounted on the roof (height of about 100m), the remote station antenna mounted on the cottage top (height of about 4m), then
D = 4.12 (√ 100 + √ 4) = 49.44 kilometer
D=4.12（√H+√h）
Taking into account a certain radio frequency bands 230 megabytes of diffraction capabilities of the kinds of assumptions set antenna, in theory, as far as you can pass 50 km.
Assuming the same master and remote station antenna height is 1 meter (such as hand-held), the theoretical maximum transmission distance of 6 km.
Therefore, the relative height of the antenna set up is to determine the communication distance of the first factor.
B. to receive field strength
Issued from the radio waves, through the feeder and antenna, the distance through the air to spread the signal by the attenuation, to a remote receiver, the field strength level for the
Pr = Pt + Gt + Gr-Lt-Lr-Lo
Pr: normal receive level (dBm)
Pt: writing power (dBm)
Gt, Gr: receive antenna gain (dBm)
Lt, Lr: send and receive feeder loss (dBm)
Lo: free space loss (dBm), Lo = 32.45 +20 logf (MHz) +20 logD (Km)
Radio signal reaches the receiver's field strength is different demodulated output signal to noise ratio will be different, thus affecting the system's decision caused error. If the field is too small, then even if the distance has not received nearly the receiver. Therefore, the decision to receive communications from the field is the second factor.
Radio signal reaches the receiver's field strength is different demodulated output signal to noise ratio will be different, thus affecting the system's decision caused error. If the field is too small, then even if the distance has not received nearly the receiver. Therefore, the decision to receive communications from the field is the second factor.
Pr received field strength and receiver threshold level (ie, at a BER of less than 10-6, the receiver requires the value of field strength level, a different radio station, the different indicators, generally-110dBm) gap is the decline of the reserves . Pr and receiver threshold level the greater the gap, the more reserves decline, the stronger anti-interference ability, fewer errors, the general decline of reserve requirements in more than 20dB.
2、the actual test
As the site of the actual environment can not be totally flat, if the hills, buildings, trees, different levels and way affect the wave transmission. Therefore, the implementation of the project, it must be on-site radio propagation and reception field strength tested (accurate to the number of dBm), to the scene environment and job requirements, identify the main station and the remote station radio power, antenna type, height and other parameters set so up and down the line to achieve sufficient signal anti-interference ability, to achieve effective and reliable wireless data transmission.
In short, as long as the selection of high specification, quality, reliable radio station, and then field-tested to determine the antenna type, height and radio power set up your wireless data transmission system must be stable and reliable operation.
