Wireless HART Data Transmission Technology

Wireless HART uses a combination of frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) wireless transmission techniques to provide interference suppression (FHSS) and coding gain (DSSS), thus building a very powerful anti-interference handling mechanism. In the FHSS transmission system, the operating frequency band is split into a series of sub-channels and the carrier changes frequency according to a pseudo-random sequence. This pseudo-random series is a list of several frequencies, and the transmitter selects the transmitted sequence by applying a frequency hopping sequence. The transmitter frequency hops from one sub-channel to another. Bursts of short data are sent on each channel at a predefined time, and then a small number of time hops are applied to the next frequency (hopping time). When this list of frequencies is exhausted, the transmitter repeats this sequence. The frequency hopping sequence is synchronized between the receiver and the transmitter. Simply put, the transmitter and receiver continuously change the frequency of the radio transmission in a pre-agreed sequence known only to both parties. the FHSS transmission system avoids sources of interference by constantly changing the transmission frequency. For industrial process applications with strong interference, FHSS can transmit signals over long distances with low transmission rates, but is extremely resistant to interference and is a very reliable technology for transmitting data. 

Direct Sequence Spread Spectrum (DSSS) technology is another method of transmitting data used in the ISM band. DSSS is an extended spectrum technology that extends the signal with a high code rate spread spectrum code sequence at the transmitter side, and demodulates it with the same spread spectrum code sequence at the receiver side, reducing the extended spread spectrum signal to the original information. In other words, each information bit corresponds to the generation of a dummy arbitrary code, which will expand the useful narrow-frequency signal into a wide-frequency signal. The signal strength of a particular frequency is reduced because the spread causes the transmission energy to be dispersed. This broadband signal is sent through the antenna and received by the receiver using the same spread spectrum code generator, so that the useful signal can be received. When an interfering signal enters the wireless transmission, this narrow-band interfering signal is transmitted through the spread spectrum code generator to the receiver, where it has been converted into a wide-band signal (noise). DSSS is suitable for medium-distance signal transmission with high transmission rates, allowing the transmission of large amounts of data.