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What is Frequency Hopping Spread Spectrum (FHSS)

Sep 21, 2018

What is Frequency Hopping Spread Spectrum (FHSS)

Frequency Hopping Spread Spectrum; In the case of synchronization, and at the same time, the sending and receiving ends transmit the signal with a certain type of narrow frequency carrier. For a non-specific receiver, the pulsation signal generated by FHSS is only impulse noise.

The signals deployed by FHSS can be specially designed to avoid noise or one-to-many non-duplicated channels, and these fh signals must comply with the requirements of the FCC, using more than 75 fh signals and the maximum Time interval for fh to the next frequency is 400ms.

Principle Description

Frequency hopping is one of the most commonly used spread mode. Its working principle refers to the communication mode in which the carrier frequency of the transmission signal of both transmitting and receiving sides is discretely changed according to the predetermined law. In other words, the carrier frequency used in communication is randomly changed by pseudo-random variation code. In terms of the realization of communication technology, "frequency hopping" is a communication mode of multi-frequency shift keying with code sequence, and also a communication system of code control carrier frequency hopping.

In time domain, frequency hopping signal is a frequency shift keying signal with multiple frequencies. In the frequency domain, the spectrum of a frequency hopping signal is a random jump at unequal intervals over a very broad band. Where: frequency hopping controller is the core component, including frequency hopping pattern generation, synchronization, adaptive control and other functions; Frequency synthesizer under the control of frequency hopping controller; Data terminals include error control of data.

Compared with fixed-frequency communication, fh communication is more hidden and difficult to be intercepted. As long as the other side is not clear carrier frequency hopping law, it will be difficult to intercept our communication content.

At the same time, frequency hopping communication also has good anti-interference ability. Even if some frequency points are interfered, normal communication can still be conducted at other uninterfered frequency points. As the fh communication system is instantaneous narrowband system, it is easy to be compatible with other narrowband communication systems. That is to say, fh radio can be interconnected with conventional narrowband radio, which is conducive to equipment update.

The Conventional Frequency Hopping

The frequency hopping pattern of the two communication transceivers is prearranged and changes synchronously according to the frequency hopping pattern. This frequency hopping method is called Normal FH. With the development of electronic warfare, adaptive frequency hopping is proposed based on conventional frequency hopping. It adds two aspects: frequency adaptive control and power adaptive control. In frequency hopping communication, frequency hopping pattern reflects the law of carrier frequency of signals of both sides of communication, which ensures that the sending frequency of the communication party is regular, but it is not easy to be found by the other party. The commonly used frequency hopping code sequence is a pseudo-random sequence based on m sequence, m sequence, RS code, etc. These pseudo-random code sequences are realized by the shift register and feedback structure. They can achieve a longer period and have better hamming correlation characteristics, but when there is artificial and intentional interference (such as tracking interference after the prediction code sequence), the anti-interference ability of these sequences is poor.

Important Significance

In the existing DS/CDMA system, the near and far effect is a big problem. Since the high-power signal only produces near distance effect at one frequency, it is not affected when the carrier frequency jumps to another frequency, so the frequency hopping system has no obvious near distance effect, which makes it easy to be applied and developed in mobile communication. In the digital cellular mobile communication system, if links adopt mutually orthogonal frequency hopping patterns synchronous frequency hopping, or use low correlation frequency hopping patterns asynchronous frequency hopping, the interference between links can be completely eliminated or basically eliminated, which is of great significance to improve the capacity of the system. In addition, frequency hopping is a instantaneous narrowband system with great flexibility in frequency allocation.

The multi-access performance of frequency hopping is very important for networking. Laval university of Canada has proposed the application of fast frequency hopping technology in optical fiber networks. The system USES Bragg grating to replace the frequency synthesizer in the traditional frequency hopping system. The system has a data rate of 500Mb/s for 30 users with a bit error rate of 10-9. FFH/CDMA system has a significantly better bit error rate than DS/CDMA system when the same number of users are using it, compared with fiber optic networks using incoherent DS/CDMA technology.

In addition, frequency hopping technology has been widely used in GSM, wireless LAN, indoor wireless communication, satellite communication, underwater communication, radar, microwave and other fields.

Through the efforts of technicians day and night, we have developed a two-way, frequency hopping burglar alarm with strong anti-interference ability, which greatly improves the safety performance of the alarm.

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