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1、Current Status and Developing Tendency for HighFrequency CommunicationsAbstract: High frequency (HF) communications play an important role in the field of their communications because of their skywave propagation characteristics. In this paper, two advanced HF radios are selected to introduce the cu
2、rrent status for HF communications. After analyzing the demand for future HF communications,the developing tendency for HF communications techniques is also presented.Key words: high frequency (HF) communications；adaptive；high speed modem；anti-jam；network1 IntroductionThe shortwave communication sai
3、d that the high frequency (HF) correspondence, the frequency of use scope is 330MHz, mainly uses the sky wave after the ionospheric reflection, does not need to establish relay station then to realize the remote communication. Simultaneously because the ionized layer cannot destroy the characteristi
4、c, the shortwave communication is one of throughout military command important means that but must obtain a more widespread application, the shortwave communication also waits for in the following several aspects enhancing:1) Correspondence reliability. Shortwave communication sky wave dissemination
5、 because of ionized layer change and multipath propagation influence, but is extremely unstable, signaling multi-diameter phenomenon is serious, retards in a big way, the Doppler shift is big, declines seriously. Simultaneously the shortwave channel is the band width the channel which limits, the ra
6、dio-frequency spectrum is been crowded, between the channel does to harass seriously, obtains the reliable correspondence quality has been the shortwave communication pursue goal.2) Data transfer rate. The tradition shortwave communication rises with difficulty a substantial clause is data transfer
7、rate is very low (does not surpass 600bit/s). In informationized war the military correspondence transmissions information, has developed from the simple direction order to such as the radar sounding data, the computed result, the high speed image facsimile and so on some requests the data to pass o
8、n the speed high information.3) Antijamming ability. Because the shortwave communication is under the war condition director the only reliable way, rate shortwave disturbance equipment develops along with the disturbance technologys development as well as some new great merit the successfully, short
9、wave communications antijamming way must diversify, the intellectualization, has in different electromagnetic environment survivability.4) Network. Along with the correspondence more and more to the network development, future shortwave communication should also more consider the network use and the
10、 network management. Uses net to wind the type correspondence, may cause the shortwave communication the information content and the information processing speed enhances greatly, but may also select the best link in the net, increases the information link survivable the nature and saves the nature
11、obstinately.2 shortwave communication development present situationBecause since the shortwave communication in the military correspondence the interchangeability, the shortwave communication has not started to receive in the 1980s takes seriously. Because many kinds of new technique the techniques
12、application, like the channel auto-adapted technology, the difference frequency-hopping spread spectrum, the wide band direct sequence spread spectrum technology, the channel coding technology, the channel balanced technology, short wave group the net technology, shortwave communication way existenc
13、es many questions are solved; And along with the microcomputer, the mobile communication and microelectronic technologys rapid development, person uses the microprocessor, the digital signal processing, enhanced the shortwave communication unceasingly the quality and the data transfer rate, the shor
14、twave communication and the equipment obtains the very big development, many companies has promoted some performance fine equipment and the system one after another, compared with famous included: USs shortwave communication broadcasting station modernization , Australias HF-90H, Swedens KV90, USs C
15、HESS, American HARRIS Corporations RF-5800 system , Collins companys SELSCAN system and so on.1) HF-90H Subminiature frequency-hopping shortwave broadcasting stationHF-90H promotes by Australian Q-MAC Corporation, the most prominent characteristic was picks to use the intelligent sideband frequency-
16、hopping spread spectrum.At present the world various broadcasting stations plant provides is the digital voice. The digital voice frequency-hoppings shortcoming is jumps the frequency frequency spectrum insufficient hiding, is distinguished easily, breaks the code and tracks, as shown in Figure 1 .
17、But HF-90H picks the intelligent sideband frequency-hopping pattern which uses is using SSB (Single Sideband Signal) the modulation system biography puts words in somebodys mouth the news number, the instantaneous frequency spectrum looks like the noise very much, in the frequency-hopping frequency
18、spectrum, because the frequency-hopping code concealment in the voice fluctuation, is unable to determine that the frequency-hopping frequency the establishment, as shown in Figure 2.Fig.1 Instantaneous and accumulated over time spectrum view of HF digital voice ho p perFig.2 Instantaneous and accum
19、ulated over time spectrum view of HF SSB voice hopperThe shortwave channel is often doping the intense noise and the unwanted signal, for example by the Broadcasting station either Fixed stations strong signal blocking or the clutter crosstalk, in addition fades to fall with the multi-diameter effec
20、t creates is unstable, causes the shortwave environment very severe. HF-90H uses the intelligence frequency-hopping spread spectrum to be possible to overcome these difficulties, the intelligence frequency-hopping is one kind has the greatly strengthened frequency band compatible technology, can per
21、iodically the automatic evaluation jump in the frequency band each channels signal strength, and instruction in the net the member abandons automatically with the noisy channel, through “the filtration” the frequency, causes the correspondence background obvious purification unceasingly, improves th
22、e correspondence quality.2) CHESS systemThe CHESS system Corporation promotes by American Lockheed Sanders, system structure as shown in Figure 3. The CHESS system take advanced digital signal processing the technology and high speed DSP (Data Signal Processor) the chip as the foundation, the freque
23、ncy-hopping band width is 2.56MHz, the frequency-hopping speed reaches as high as 5000Hops/s, data transmission rate highest may reach the 19.2kbit/s, CHESS system most prominent characteristic to use the difference frequency-hopping (Differential Frequency Hopping, DFH) technique the technique. The
24、 difference frequency-hopping materially is one kind the technology which unifies the frequency modulation and the code, through to the frequency coding, increased the frequency domain the redundance, enables the frequency-hopping frequency to have the specific relevance, the receiving end has used
25、this kind of relevance to carry on the frequency series decoding, the correction certain proportion frequency miscarriage of justice, plays by the frequency band receiving in exchange for signal-to-noise ratio or the SIR role, enhanced systems performance.Fig.3 CHESS system architecture3 shortwave c
26、ommunication technology trend of developmentThe shortwave communication technology developed in recent years is rapid, has made a series of breakthroughs and the progress, in the future must further study in the following several aspects3.1 shortwave auto-adapted digital communication technologyThe
27、shortwave channel multi-factor and so on diameter latency, amplitude fading, weather change influences is been unpredictable, must guarantee that the correspondence the reliability, needs the system according to short the wave channels change auto-adapted change system structure and the parameter. P
28、resents shortwave adaptive communication technology , mainly refers to the frequency auto-adapted technology, but future the shortwave adaptive communication technology should be omni-directional, including: Chooses the frequency and the channel establishment technology, the transmission speed auto-
29、adapted technology, auto-adapted channel auto-adapted weighs the technology, the auto-adapted antenna technology and so on.1) Chooses the frequency and the channel establishment technology auto-adapted. The present chooses the frequency and the channel establishment technology is auto-adapted with t
30、he correspondence unifies in together, will choose frequency quality to be lower than the frequency quality which like the special-purpose real-time frequency-selective system will provide. From now on the development direction should be unifies the special-purpose frequency-selective system and the
31、 adaptive communication system, enters one step to improve the shortwave communication quality.2) Transmission speed auto-adapted technology . Shortwave communication after designation operating frequency, must in obtain the biggest data volume of goods handled along with on the time variation chann
32、el, must use the transmission speed auto-adapted technology. Usually should choose as far as possible the high data transfer rate under the permission error rate condition, this need system uses the code and the modulator approach and the channel condition incident cross-correlation, when the channe
33、l dissemination characteristic is good with high transmission speed transmitter data, but when disseminates the characteristic variation cuts the transmission speed, enables the error rate throughout to satisfy the correspondence quality the request.3) Auto-adapted channel balanced technology. When
34、shortwave time-variable channel transmission signal, to eliminate the multi-diameter effect, the Doppler shift and so on serious code which brings to disturb, must use the auto-adapted channel balanced technology. The decision feedback equalizer (Decision Feedback Equalizers, DFE) is the present sho
35、rtwave communication is one kind of balanced technology which the series uses generally. In recent years proposed one kind of new balanced technical - Turbo balanced technology, it unifies the channel to arrange the decoding technology, full has used the channel information, enhanced 23dB after the
36、comparison in the shortwave communication system using the Turbo balanced technology compared with DFE.4) Auto-adapted antenna technology . The auto-adapted antenna technologys principle is the signal which receives through docking carries on the real-time processing, the control and the adjustment
37、antenna array element phase changes the antenna pattern characteristic, completes the auto-adapted beam forming, enables the antenna beam the zero position alignment disturbance direction, the signal direction gain to achieve most to be big, thus enhances the system anti-multi-diameter decline and a
38、ntijamming ability effectively.3.2 high speed modulation demodulation technologyAt present is widespread the application the narrow band shortwave broadcasting stations modem to have serial and the parallel two systems, the serial system uses the single carrier to modulate the transmitter data, befo
39、re item of , the maximum speed is 9.6kb/s, is very high to the balanced request; The parallel system is the data parallel allocation which transmits to many sub-carriers in transmits, the tradition and in the good system each sub-carrier does not overlap mutually in the frequency spectrum, separates
40、 each sub-channel in the receiving end with the filter group, between each sub-channel must leave leeway protects the frequency band, the frequency band use factor is low, moreover many filter realize also have the difficulty, at present the maximum speed is only 2.4kb/s. The Orthogonal Frequency Di
41、vision Multiplexing (Orthogonal Frequency Division Multiplexing, OFDM) the modulation system is quick by its transmission speed, frequency band use factor Gao Hekang multi-diameter ability strong and so on merits is valued peoples more and more, also starts to apply gradually in shortwave communicat
42、ion domain . Is opposite in the single carrier and non-orthogonal the frequency division multiplying way, OFDM applies in the shortwave communication has following superiority :1) Anti-frequency selectivity decline. The OFDM system and transforms the high speed data stream through the string, causes
43、 in each sub-carrier the data mark to continue length to increase, thus may reduce ISI which effectively wireless channels time dissemination brings (InterSymbol Interference), like this reduced has received the within the aircraft balanced order of complexity, sometimes even might not use the balan
44、cer, only through used the insertion prefix the method to eliminate ISI the adverse effect.2) The frequency spectrum use factor is high. Because the OFDM system between each sub-carrier has the orthogonality, allows the sub-channels frequency spectrum to overlap mutually, therefore compares with the
45、 conventional frequency division multiplying system, the OFDM system may maximum limit use the frequency spectrum resources.3) Realizes simply. Uses IDFT (Inverse Discrete Fourier Transform) /DFT (Discrete Fourier Transform) to realize OFDM. Namely causes regarding sub-carrier integer many systems,
46、is very easy to realize along with the large scale integrated circuit technology and the DSP technologys development.3.3 antijamming technologiesThe shortwave communication is under the war condition director the only reliable way, along with the disturbance method to the wide frequency range, the m
47、ulti-styles, the multi-level directions develops, antijamming the measure should also tend the integration, the intellectualization as well as multi-system coexisting, concrete sends the exhibition direction is:1) Signal processing . If auto-adapted frequency-hopping , its principle like chart 10 sh
48、ows. The auto-adapted frequency-hopping system has added on link in the convention frequency-hopping communications foundation the quality analysis, through the reliable information link quality analysis (Link Quality Analysis, LQA), determined that frequency spot which disturbs, gives may use jumps
49、 to lead the collection repeatedly, and transmits this frequency collection through the feedback channel for the launch side, causes the both sides automatic adaptation channel change situation, simultaneously deletes the complete frequency which disturbs, is not being had the disturbance or the disturbance very small frequency spot carries on the reliable communication.2) Spatial processing. If uses the auto-adapted antenna zero alignment technology, when receiving end receives the di
