What Is the Difference Between UHF and VHF?

Frequency hopping is one of the most commonly used methods of spread spectrum. Its working principle refers to a communication method in which the carrier frequency of the transmission signal of the sending and receiving sides changes discretely according to a predetermined rule. That is, the carrier frequency used in communication is subject to a pseudo-random change code. Control and randomly jump. In terms of the implementation of communication technology, "frequency hopping" is a multi-frequency frequency shift keying communication method using code sequences, and it is also a communication system with code-controlled carrier frequency hopping.

Chinese name: Frequency hopping communication

Foreign name: frequency hopping communication
Function: Ensure the confidentiality and immunity of communications

Why frequency hopping communication uses frequency hopping technology

The frequency hopping technology is adopted to ensure the confidentiality and anti-interference of communication. Compared with fixed frequency communication, frequency hopping communication is more hidden and difficult to be intercepted. As long as the other party does not know the law of carrier frequency hopping, it is 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 performed at other uninterrupted frequency points. Because the frequency hopping communication system is an instantaneous narrowband system, it is easy to be compatible with other narrowband communication systems, that is, frequency hopping radios can communicate with conventional narrowband radios, which is conducive to the update of equipment.

Frequency hopping communication

The frequency hopping function is mainly:

(1) Improve fading.

(2) A mobile station that moves at a high speed in a multipath environment adopts frequency hopping technology, which greatly improves the communication quality of the mobile station, which is equivalent to frequency diversity.

Basic concepts of frequency hopping communication

In the use of short-wave communication in a wide area, we hope that the communication channel is smooth and confidential. However, they often encounter problems such as eavesdropping, electronic countermeasures, and channel congestion. Conventional shortwave radios transmit and receive at a fixed frequency. Therefore, eavesdropping, human interference, and channel congestion cannot be avoided. These problems must be completely overcome using frequency hopping technology.

"Frequency hopping communication" is to address the disadvantages of the above-mentioned traditional radio communication, so that the original fixed radio signal frequency jumps back and forth according to a certain rule and speed, and let the other party agree to track and receive simultaneously. Because the enemy does not understand the transition law of our radio signal, it is difficult to intercept the information. Although it can also use the "tracking interference" method to interfere with our radio station, because the frequency hopping spectrum is erratic, it is often that the enemy has just searched for a certain transmission frequency, and it immediately changes again, making it difficult to keep up. . Some people may come up with a method to fully implement interference (ie, "broadband blocking interference"), but this will not only consume huge power, but may also expose themselves and cause serious interference to their own communications.

Frequency hopping communication is a type of digital communication and a type of spread spectrum communication. In this communication method, the radio frequency bandwidth used for signal transmission is tens, hundreds, or even thousands of times the original signal bandwidth. But only for a moment, it only works at a certain frequency.

Frequency Hopping Communication

The principle of frequency hopping is: according to the preset procedure of the entire network, all stations in the network are automatically controlled to change the frequency multiple times in one second and stay on each frequency hopping channel for a short time. Periodic synchronization signaling is sent from the master station, instructing all slave stations to jump to change the operating frequency at the same time.

The principle of frequency hopping communication is not complicated. It is to add a "code-controlled frequency hopper" on the basis of ordinary radio shortwave communication. Its main function is to make the carrier transmitted by frequency hopping communication change according to a certain random hopping sequence. The key to achieving frequency hopping communication is that the local oscillator frequency controlled by the pseudo random code and used to change the carrier frequency must be strictly synchronized. The change of the frequency hopping pseudo-random code can be controlled by a microcomputer; changing the computer program can change the frequency hopping law. There are more than one frequency-hopping solution, and it is often changed. It is very difficult to spot it like a needle in a haystack.

In terms of communication security, frequency hopping shortwave communication is more reliable than satellite communication. This is because the agency providing satellite services has assumed strategic responsibility for its country and must be controlled by the government of that country. Frequency-hopping short-wave communication is completely autonomous and therefore the most reliable. Occasionally, the status of frequency hopping shortwave communication is irreplaceable.

At present, most of the world's radio manufacturers provide ordinary digital frequency hopping. The disadvantage of digital frequency hopping is that the frequency hopping spectrum is not concealed enough to be easily identified, decoded, and tracked. In the past two years, more advanced intelligent sideband frequency hopping modes have emerged. This is a collective term for sideband frequency hopping and intelligent frequency hopping. Sideband frequency hopping is a more advanced technology developed on the basis of digital frequency hopping. It hides the frequency hopping code in the sideband voice. The implicit frequency hopping signal is similar to the sideband noise, which is more difficult than ordinary digital frequency hopping. Identify, decipher and track. Intelligent frequency hopping is a very strong frequency band adaptation technology that can automatically identify and abandon congested channels in the 256KHz frequency hopping frequency band. Clearly purifies communication background. For example, at night, short-wave channels are often occupied by various noisy signals. With intelligent frequency hopping, the entire communication network can be automatically adjusted to a clean channel area. The communication background will naturally be much cleaner and quieter, and the useful signals will change significantly. Clear.

Frequency hopping in the GSM system is divided into two types: baseband frequency hopping (BBH) and radio frequency hopping (SFH). The principle of radio frequency hopping is that a voice signal is fixedly transmitted on a transmitter, but the transmission frequency of the transmitter is constantly changing, and the specific change process is controlled by a frequency hopping sequence. RF frequency hopping has higher performance and ability to resist co-channel interference than baseband frequency hopping. Current GSM actual networks generally use radio frequency hopping.

Frequency hopping communication functions and features

Fast and slow frequency hopping

Frequency hopping communication is generally divided into two types: when the frequency hopping frequency is higher than the cell code rate, it is called fast frequency hopping. When the frequency hopping frequency is lower than the cell code rate, it is called slow frequency hopping.

The fast anti-interference ability of fast frequency hopping is basically considered as unbreakable. However, the cost of the system is high, and it is currently only used in the field of military communications.

Slow FM is characterized by changing the frequency used by a channel at fixed intervals. GSM in 2G uses slow frequency modulation. According to the GSM recommendation, the frequency hopping of the base station's wireless channel is based on each physical channel, so for a mobile station, it only needs to hop once in the corresponding time slot of each frame, that is, 217 times per second . It sends and receives with a fixed frequency in a time slot, and then it needs to jump to the next TDMA frame after this time slot. Since it takes time to monitor other base stations, the time allowed for frequency hopping is about lms, and the transmission and reception frequency is duplex. frequency. Relatively simple to implement.

Frequency hopping communication conventional and adaptive frequency hopping

The frequency hopping patterns of both the communication sender and the receiver are predetermined, and hopping is performed synchronously according to the frequency hopping pattern. This frequency hopping method is called Normal FH. As electronic countermeasures in modern warfare have intensified, adaptive frequency hopping has been proposed on the basis of conventional frequency hopping. It adds two aspects of frequency adaptive control and power adaptive control. In frequency-hopping communication, the frequency-hopping pattern reflects the law of the carrier frequency of the two parties of the communication, which ensures that the transmitting party's sending frequency can be followed regularly, but it is not easy to be found by the other party. Commonly used frequency hopping code sequences are pseudo-random sequences designed based on m-sequences, M-sequences, RS codes, and the like. These pseudo-random code sequences are realized by a shift register and a feedback structure. The structure is simple, the performance is stable, and synchronization can be achieved relatively quickly. They can achieve a longer period, and the Hamming correlation characteristics are also better, but when there is artificial intentional interference (such as tracking interference after the prediction code sequence), the anti-interference ability of these sequences is poor ^[1] .

Like general digital communication systems, frequency hopping systems require carrier synchronization, bit synchronization, and frame synchronization. In addition, because the carrier frequency of the frequency hopping system changes according to a pseudo-random sequence, in order to achieve normal communication between stations, the transceiver must hop to the same frequency at the same time, so the frequency hopping system also requires frequency hopping pattern synchronization. Frequency hopping system has two basic requirements for synchronization: one is fast synchronization, and the other is strong synchronization capability. At present, the frequency synchronization methods of frequency hopping stations include precise clock method, synchronization header method, self-synchronization method, FFT acquisition method, autoregressive spectrum estimation method and so on. In practical applications, synchronization schemes often use a combination of synchronization methods. For example, the scanning and staying synchronization method commonly used in tactical frequency hopping systems uses a combination of three synchronization methods: precision clock method, synchronization header method, and self-synchronization method, which are divided into two phases: scanning and staying. The scanning phase completes the capture of the synchronization header frequency, and the resident phase extracts the synchronization information from the synchronization header to complete the synchronization between the sender and receiver.

In adaptive frequency hopping, synchronization also includes the synchronization of the frequency set updates of the sending and receiving parties, ensuring that the two parties can achieve the replacement of bad frequency points synchronously, otherwise the frequency tables of the sending and receiving parties will be inconsistent, causing communication failure. The frequency synthesizer is a key part of the frequency-hopping communication system. Currently, the frequency synthesizer used in most frequency-hopping radios uses phase-locked loop (PLL) frequency synthesis technology, but the frequency conversion speed of this technology is close to its limit. The technical difficulty to further improve is getting more and more difficult, and the resolution is lower. In order to further increase the frequency hopping rate, a direct digital frequency combiner (DDS) is proposed. It uses all-digital technology, which has the advantages of high frequency resolution, fast frequency conversion time, high output frequency and good stability, and low phase noise. It can meet the requirements of frequency synthesizers for fast frequency hopping radios. For example, in JTIDS in the United States, the jump speed reaches 35,800 beats per second, which can only be achieved by using a direct digital frequency combiner. However, DDS is expensive and complicated, and it is difficult to directly use it for tactical frequency hopping radio. If the DDS + PLL method is used, combining the advantages of the two, you can obtain the effects that are difficult to achieve with a single technology. In a frequency hopping system, even under good channel conditions, errors may still occur in a few hops, so it is necessary to perform error control. The error control methods are mainly divided into two categories: one is the automatic request retransmission error correction (ARQ) technology; the other is the forward error correction (FEC) technology. ARQ technology can deal with random errors and burst errors very well. It requires a feedback circuit. When the channel conditions are not good, frequent retransmissions are required, which may eventually lead to communication failure. FEC technology does not require a feedback circuit, but requires a large amount of signal redundancy to achieve excellent error correction, which will reduce channel efficiency. Because the error correction code has a poor error correction capability for burst errors, and the errors in the channel can be randomized by interleaving technology, a combination of coding and interleaving technology is often used to obtain good error correction performance. The error correction coding techniques commonly used in frequency hopping systems are Hamming code, BCH code, trellis code, RS code, Golay code, convolutional code, hard decision decoding, soft decision decoding, etc. The TURBO code was proposed in 1993, and its signal-to-noise ratio was close to the Shannon limit, which aroused great interest. Compared with commonly used frequency hopping codes such as RS codes, TURBO codes show great application potential in frequency hopping systems. In addition, different coding methods can be combined together to complement each other and perform joint coding. In the fast frequency hopping mode, the retransmission of large numbers can also be used to overcome the fast fading in the frequency hopping frequency band.

Frequency Hopping Communication Network

Frequency-hopping radios usually form a frequency-hopping communication network in practical applications, so that any two communication terminals in the network can achieve point-to-point normal communication. In addition to avoiding interference from near-end to far-end, inter-symbol interference, electromagnetic interference, and other noise interference caused by the system, the network must also avoid co-channel interference, adjacent-channel interference, Intermodulation interference, blocking interference, etc. The frequency-hopping multiple-access communication network has many advantages: strong anti-interference ability, low interception probability, low detection probability, good suppression effect on frequency selective fading and so on. However, compared with commonly used DS / CDMA systems, the maximum number of users in a frequency hopping network is relatively small. ^[2]

Frequency-hopping communication networks can be divided into synchronous communication networks and asynchronous communication networks. Frequency-hopping communication networks have multiple networking methods, such as sub-band frequency hopping networking, full-band orthogonal frequency hopping networking, and so on. In sub-band frequency hopping networking, the system divides the entire frequency band into several sub-bands, and different communication links use different sub-bands for communication, thereby effectively preventing interference between the same communication network. The full-band orthogonal frequency-hopping networking method is only used in synchronous frequency-hopping communication networks, that is, there is only one reference clock in the entire communication network. Networking is performed, so that although the communication between the terminals uses the same frequency band, because the instantaneous frequency hopping frequency points are different, it can be ensured that no co-channel interference occurs between them. In the adaptive frequency hopping communication system, the channels with bad communication conditions will be removed during the communication process, so co-channel interference may occur after the frequency update. Therefore, a good frequency update algorithm must be designed to ensure the updated Frequency hopping sequences are still orthogonal, otherwise frequency collisions between communication nodes may occur frequently, resulting in failure to communicate properly. In practical applications, the above two networking methods can also be combined. For example, Jaguar series radios of British Recal-Tacticom Company have adopted these two types of networking at the same time, and the number of networks can reach 200-300.

Frequency-hopping communication modulation and demodulation method

In addition to the above key technologies, modulation and demodulation methods are also very important in frequency hopping systems. Various modulation methods such as FSK, QAM, QPSK, QASK, DPSK, QPR, and digital chirp modulation can be used. The adaptive frequency hopping system is based on the conventional frequency hopping system and removes fixed or semi-fixed interference in real time, so as to adaptively automatically select a good channel set and perform frequency hopping communication to maintain a good communication state of the communication system. In other words, in addition to the functions of the conventional frequency hopping system, it also needs to realize the real-time adaptive frequency control and adaptive power control functions, so it needs a reverse channel to transmit frequency control and power control information. After a reliable channel quality evaluation algorithm is found, the interference frequency points should be deleted from the frequency table of the sender and receiver and replaced with good frequency points to maintain the fixed size of the frequency table. This detection and replacement occurs in real time. In order to increase the concealment and anti-deciphering ability of the frequency hopping signal, in addition to having a good pseudo-randomness and long period, the frequency hopping pattern should have good uniformity over a long period of time. After introducing the adaptive frequency replacement algorithm to update the frequency table in real time, in order to ensure the system performance, the frequency hopping pattern is still required to have good uniformity, so the different interfered ones should be replaced with different frequency points with better quality in turn. Frequency. The update of the receiving frequency table will cause inconsistencies in the sending and receiving frequency table. In order to synchronize the sending and receiving frequency table, the sender must be notified of the frequency update information of the receiving end through the feedback channel. This mutual exchange of information is a closed-loop control process, and a corresponding information exchange protocol needs to be formulated to ensure reliable and synchronous update of the frequency table. Another important indicator to measure the effectiveness of the protocol is the speed of frequency removal. After the interference frequency is detected, the faster the interference frequency is removed, the smaller the impact on the communication. Another role of channel quality assessment is to perform adaptive power control. Power control is to allocate the limited transmission power to each frequency hopping channel best, so that each channel can achieve normal communication with the minimum transmitter power, thereby improving the concealment and anti-interception ability of the frequency hopping signal. In the adaptive frequency hopping system, the system detects the communication status of each channel, and performs power control on each frequency hopping channel individually through the power control algorithm in the channel quality evaluation unit.

The power control algorithm can be based on two principles: one is the principle of minimum bit error rate, the algorithm selects the appropriate power for each frequency hopping channel, so that the bit error rate of the data received by the receiver reaches a predetermined error threshold; the second is to wait Signal-to-interference ratio principle. This algorithm adjusts the average power of each frequency hopping channel so that the signal-to-interference ratio is the same on each frequency-hopping channel. Here, the signal-to-interference ratio refers to the signal power on each frequency-hopping channel / (corresponding to interference on the channel Power + transmission loss power). The performance of these two algorithms is similar.

Development of Frequency Hopping Technology

With the continuous development of frequency hopping technology, its application is becoming more and more extensive. The main purpose of using frequency hopping technology in tactical radio is to improve the anti-jamming ability of communication. As early as the 1970s, research on frequency hopping systems began, and low-frequency 30-88MHz, UHF (above 300MHz), and HF (1.5-30MHz) frequency hopping have been developed in VHF (30-300MHz). Applications. With the continuous deepening of research, the frequency hopping rate and data rate are getting higher and higher. Now the CHESS high-speed short-wave frequency hopping radio station of Sanders Corporation in the United States has achieved a frequency hopping rate of 5000 hops / second, and the highest data rate can reach 19200bps . In addition, the CHESS frequency hopping radio is different from the general frequency hopping radio. It is based on DSP and uses differential frequency hopping (DFH) technology. Through modern digital processing technology, the CHESS frequency hopping radio station can better solve the problems of limited bandwidth of the short-wave system (cause of low data rate), mutual interference between signals, and multipath fading. At the same time, its instantaneous signal bandwidth is very narrow, and it has little effect on other signals. It can be seen that frequency hopping radios that achieve higher speeds and higher data rates are the future direction of frequency hopping communication systems, and the concept of software radio has gradually been applied to new types of frequency hopping radios. Short-wave adaptive frequency hopping radios have already occupied a very important part of current military communications. Unlike the VHF / UHF frequency band, short-wave channels have many inherent characteristics. For example, due to the effects of multipath delay, amplitude fading, and weather changes, channel conditions are unpredictable. But with the emergence of various new technologies, the reliability of short-wave communication has been technically guaranteed, and adaptive frequency hopping technology is one of these new technologies. It analyzes the frequency occupancy on the band and automatically searches for interference-free or unoccupied frequency-hopping channels for frequency hopping, which not only avoids natural interference, but also is not affected by the large-scale occupation of the short-wave spectrum. It will automatically change the frequency hopping sequence as needed, effectively adapting to harsh environments. Its superiority in the Gulf War aroused great attention from all countries.

In the existing DS / CDMA system, the near-far effect is a big problem. Because high-power signals only produce near-far effects on a certain frequency, they are not affected when the carrier frequency hops to another frequency, so the frequency-hopping system has no obvious near-far effects, which makes it easy to be applied and used in mobile communications. development of. In a digital cellular mobile communication system, if mutually orthogonal frequency hopping patterns are used for synchronous frequency hopping between links, or low frequency cross-correlation frequency hopping patterns are used for asynchronous frequency hopping, interference between links can be completely or substantially eliminated. It is of great significance to increase the capacity of the system. In addition, frequency hopping is an instantaneous narrowband system, and its frequency allocation has great flexibility. This is of great significance under the condition that the existing frequency resources are very crowded.

Frequency-hopping multiple-access performance is of great significance for networking. Laval University of Canada has proposed the application of fast frequency hopping technology in fiber optic networks. This system uses Bragg grating to replace the frequency synthesizer in the traditional frequency hopping system, and the hop speed reaches 10G order of magnitude. The system has a data rate of 500Mb / s under the condition of 30 users and a bit error rate of 10-9. Compared with non-coherent DS / CDMA optical fiber network, when the same number of users are used at the same time, the bit error rate of FFH / CDMA system is significantly better than DS / CDMA system.

In addition, frequency hopping technology has also been widely used in many fields such as GSM, wireless local area network, indoor wireless communication, satellite communication, underwater communication, radar, and microwave.

Because the frequency hopping system itself also has some shortcomings and limitations, such as poor signal concealment, resistance to multi-frequency interference, and limited tracking interference, etc., another method of spread spectrum direct sequence spread spectrum has better concealment and Ability to resist multi-frequency interference. Combining these two types of spread spectrum technology constitutes a direct sequence / frequency hopping spread spectrum technology. It adds the function of carrier frequency hopping on the basis of the direct sequence spread spectrum system. The pseudo random sequence used in the direct spread system and the pseudo random frequency hopping pattern used in the frequency hopping system are generated by the same pseudo random code generator. Time is interrelated and uses the same clock for timing control. Telettra's Hydra V radio in Italy is the first generation of tactical radios using direct sequence / frequency hopping hybrid spread spectrum technology. Due to the direct sequence spread spectrum DBPSK modulation method, 9dB more processing gain is achieved than using frequency hopping technology alone, thereby improving the anti-interference performance of the radio station.