The basic principle of frequency hopping systems
For example: radio broadcasts, a transmission frequency is typically a transmission program, different programs occupy different emission frequencies. Sometimes in order to let the audience can listen to a good show, radio program with the same set of simultaneous transmission of several transmission frequency. Thus, if been a serious interference on one frequency, listeners can also choose the clearest channel to listen to the program, which played a jamming effect. But the cost of doing so is the amount of spectrum required a lot of resources to transmit a program. If the transmission on several changing carrier frequency of a broadcast program, and also with radio audience tuned with the continuous reception on several frequencies, so that, even when subjected to interference on a certain frequency, also well listen to this program. This becomes a frequency-hopping system.
Also in the communication between the two sides in the fight against the enemy, the enemy attempted to find the frequency of our communications in order to intercept the content of information transmitted or found the orientation of our communication machine is located, in order to destroy to guide fire. Fixed-frequency communication system likely to expose and easy to be intercepted, this time, subtle frequency hopping communication is more difficult to be intercepted. Because the frequency hopping communication is "shot for a local" guerrilla communication strategy, so that the enemy is not easy to find a frequency used for communication, once the enemy is found, the frequency of communication have also been "transferred" to another on the frequency. When the enemy lost in the "transfer rule", it is difficult to intercept the communications of our content.
Therefore, the frequency hopping communication has the ability to jamming, anti-interception, and resource sharing of spectrum that could be done. So in the current modern electronic warfare frequency-hopping communications has shown great superiority. Further, frequency hopping communication is also applicable to civilian communications to fading, multipath, interference and anti gateway spectrum utilization is improved.
In order to prevent the enemy we know the frequency used for communication, the need for frequent changes the carrier frequency, that is, "shot for a place," adding that the carrier frequency hopping, frequency hopping communication carrier frequency change of the law, called frequency hopping pattern.
Usually we hope that the frequency hopping law change is not seen through the enemy, it is necessary to randomly change even no rules to follow to do. But if it is really no rule to follow, then, the two sides (or friendly) communications will also lose contact and communication can not be established. Thus, changing the often used pseudo-random hopping pattern.
Both sides know this hopping pattern only communication, but the enemy is absolutely confidential. The so-called "pseudo-random" is "false" random, in fact, there are laws to follow, but I do not know when the enemy hopping pattern, it is difficult to guess the law of its hopping.
Communication sending and receiving both the frequency hopping pattern is agreed in advance, or notified by the recipient of the sender. The hopping pattern is the enemy do not know. If you want to enemy interference frequency hopping communication, there are several strategies to choose from:
- Interference Embodiment 1, the long cast interference power at a certain frequency, i.e., single-frequency interference.
- 2 disturbances, long cast high-power interference on certain frequencies, i.e., multi-frequency interference.
- 3 disturbances; long cast interference power over several successive bands, frequency band part called scrambling.
- 4 disturbances, cast interference power at different frequencies at different times of the inner.
- Interference embodiment 5 in accordance with the law of frequency hopping pattern tracking cast high-power interference.
These interfere with the relationship between fashion and frequency hopping communication with each other just as "the child" as when two Duiyi, when a small cell in the same frequency domain when both sides of the board, "cloth child" falls, then the effective interference. Therefore, the hopping pattern tracking strategy is the best cast of interference interference frequency hopping communications strategy.
Consider a system of frequency hopping performance, should pay attention to the following indicators:
- Hopping bandwidth
- The number of frequency hopping
- Hopping rate
- The hopping code length (period)
- Synchronization time hopping system
Generally, it is desirable for a wide bandwidth, the frequency hopping, the frequency hopping to the number of multiple, faster rate of frequency hopping, the frequency hopping cycle longer codes, synchronization time is shorter FH system.
Hopping bandwidth size, interfering with the ability of anti-band concerned.
The wider the frequency hopping bandwidth, broadband anti-interference ability stronger. So we hope to full-band frequency hopping. For example, in the short wavelength, from 1.5MHz to 3MHz full-band hopping; at very high frequency, the whole band from 30MHz to 80MHz frequency hopping.
The number of hopping frequencies, capacity and anti-single-frequency interference and multi-frequency interference related.
The more the number of frequency hopping, the anti-single frequency, the stronger the multi-frequency comb and interference. In general frequency hopping radio, the number of frequency hopping is not more than 100.
The hopping rate, varying the number of times per second is frequency hopping, which is related to an anti-tracking ability of interference.
The faster hop rate, the stronger the resistance to tracking interference. In general HF frequency hopping, which is now no more than the hop rate of 100 hops / sec. VHF radio, a hopping rate of 500 hops general / sec. For some higher frequency hopping systems can work in tens of thousands of jumps per second level.
Length of the hopping code, it will determine the amount of hopping pattern continued length of time, the ability to index and anti-interception (decipher) concerned.
The longer the time hopping pattern continues, the more difficult to decipher enemy, anti-interception capability is also stronger. Cycle skipping code can be up to 10 years or even longer.
Synchronization time hopping system is a system that the sending and receiving sides of the hopping pattern and perfectly synchronized communication set-up time required. The length of time of the synchronization system will affect the degree of survivability of the system. Because the synchronization process once the enemy broken ring, can not be realized sending and receiving frequency hopping pattern is completely synchronized communication system, it will cause paralysis. Therefore, it is desirable to establish synchronization process as short as possible, the more subtle the better. Depending on the environment of use, the current synchronization time hopping radio may be in the order of seconds or a few hundred milliseconds.
Of course, the technical indicators a frequency hopping systems shall be in accordance with the terms of the purpose of use, and cost performance and other requirements into account in order to make the best choice. Features hopping system depends largely on its mechanism of spread spectrum. Frequency hopping spread spectrum on the mechanism of the direct sequence spread spectrum different.
The frequency separation between the carrier frequency is the number of carrier channel bandwidth, the frequency hopping channel bandwidth is multiplied by the frequency hopping bandwidth. Thus, frequency hopping system has the following characteristics:
Because it is the instantaneous narrowband system, it is easy to be compatible with the existing narrowband communication system.
Whether the current communication system is the analog modulation or digital modulation, it is usually narrow band communication systems. If the narrowband communication system to an existing installation so that it can be on a carrier frequency according to a certain hopping pattern and hopping means for synchronizing received can be realized, it can be transformed into a frequency hopping communication system.
Because it is a macro broadband system, which has a spread-spectrum anti-jamming capability.
Frequency hopping spread spectrum interference with anti-single-frequency, multi-frequency interference ability, but also has resistance to partial band and broadband interference.
In conventional fixed-frequency communication system, the oscillation frequency of the main oscillator of the transmitter is fixed, so it is a fixed carrier frequency. In order to obtain carrier frequency hopping signal hopping, requires the frequency of the master oscillator should be changed in accordance with the control command. This means generating a signal called the frequency hopping is frequency hopping. Frequency hopping frequency hopping sequence output, it is a frequency hopping pattern. Accordingly, what kind of frequency hopping command what the hopping pattern is generated? Typically, a pseudo-random generator to generate a hopping instruction or instructions to generate a frequency hopping by the software programming. Therefore, the key components of the system are hopping frequency hopping, and more particularly, is capable of producing good spectral purity of fast switching frequency synthesizer and hopping command generator pseudorandomness good.
Fixed frequency signal receiving apparatus, a generally used method superheterodyne receiver, i.e. the frequency difference between the receiver local oscillator an intermediate frequency than a carrier frequency of the received external signal, after mixing the intermediate frequency signal to produce a fixed and a mixer to produce a combination of frequency wave component. After filtering effect of the band-pass filters, filter combination wave frequency component enters the IF signal demodulator. Output of the demodulator is to be transmitted to the information receiving terminal.
Similar signals received frequency hopping, fixed-frequency with the process. In order to ensure that after the mixing to obtain an intermediate frequency signal, the frequency synthesizer of claim output frequency of an intermediate frequency higher than extraneous signals. Because the foreign signal carrier frequency hopping, requires local frequency synthesizer output frequency hop along with the law of varying the external signal transitions. Therefore, the receiver needs a controlled frequency hopping synchronization instruction is to determine from which frequency hopping, the stop time. As can be seen, frequency hopping is a key component of the frequency hopping system, the frequency hopping synchronization frequency hopping technique is the heart of the system.
To FH Frequency Hopping system, the condition signal is received correctly synchronized hopping frequency hopping system.
System synchronization includes the following items:
- Terminating and originating the same hopping pattern is generated, i.e. the same frequency hopping law.
- Received, the originator of the hopping frequencies should be guaranteed to produce a fixed intermediate frequency signal at the receiving end, i.e. hopping carrier frequency generated by the terminating local hopping frequencies differ by an intermediate frequency.
- Start and end times of the frequency hopping is synchronized in time, i.e. the synchronization jump, or phase coherence.
- When transmitting digital information, but also to achieve frame synchronization and bit synchronization.
At present the majority of manufacturers to provide radio stations in the world are ordinary digital frequency hopping. Disadvantage of digital frequency hopping is not enough spectrum frequency hopping subtle, easy to identify, decipher, tracking. The past two years there has been more advanced intelligent sideband frequency hopping mode, which is collectively referred to as sideband frequency hopping and intelligent frequency hopping. Sideband frequency hopping is more advanced technology in a digital frequency hopping based on the hopping code it implies sideband voice, the implied approximation signal sideband noise frequency hopping, frequency hopping than the average number is more difficult to identify, track and decipher. It is a kind of intelligent frequency hopping extremely band adaptation technology to automatically identify and deprecated congestion 256KHz frequency hopping channels within the frequency band. Purification clear background communication. For example, at night often occupied, various HF channel noisy signals, the use of intelligent frequency hopping, the entire communication network may be automatically adjusted to clean the channel region, the communication and quiet and clean background naturally more useful signal will change significantly clear