外文翻译---短波通信的现状及发展趋势

Current Status and Developing Tendency for High
Frequency Communications
Abstract:
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 current 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；network
1 Introduction
The shortwave communication said that the high frequency (HF) correspondence, the frequency of use scope is 3~30MHz, 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 characteristic, 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
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 radio-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 rate is very low (does not surpass 600bit/s). In informationized war the military correspondence transmission's 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 on 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 technology's development as well as some new great merit the successfully, shortwave communication's 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 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 obstinately.
2 shortwave communication development present situation
Because 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 technique's 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 existence's many questions are solved; And along with the microcomputer, the mobile communication and microelectronic technology's rapid development, person uses the microprocessor, the digital signal processing, enhanced the shortwave communication unceasingly the quality and the data transfer rate, the shortwave 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:
US's shortwave communication broadcasting station modernization , Australia's HF-90H, Sweden's KV90, US's CHESS, American HARRIS Corporation's RF-5800 system , Collins company's SELSCAN system and so on.
1)HF-90H Subminiature frequency-hopping shortwave broadcasting station
HF-90H promotes by Australian Q-MAC Corporation, the most prominent characteristic was picks to use the intelligent sideband frequency-hopping spread spectrum.At present the world various broadcasting stations plant provides is the digital voice. The digital voice frequency-hopping's shortcoming is jumps the frequency frequency spectrum insufficient hiding, is distinguished easily, breaks the code and tracks, as shown in Figure 1 . 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 somebody's mouth the news number, the instantaneous frequency spectrum looks like the noise very much, in the frequency-hopping frequency 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 per
Fig.2 Instantaneous and accumulated over time spectrum view of HF SSB voice hopper
The shortwave channel is often doping the intense noise and the unwanted signal, for example by the Broadcasting station either Fixed station's strong signal blocking or the clutter crosstalk, in addition fades to fall with the multi-diameter effect 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 periodically the automatic evaluation jump in the frequency band each channel's signal strength, and instruction in the net t he member abandons automatically with the noisy channel, through “the filtration” the frequency, causes the correspondence background obvious purification unceasingly, improves the correspondence quality.
2)CHESS system
The 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 frequency-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 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 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 system's performance.
Fig.3 CHESS system architecture
3 shortwave communication technology trend of development
The 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 aspects
3.1 shortwave auto-adapted digital communication technology
The 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 channel's change auto-adapted change system structure and the parameter. Present's 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-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 the 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 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 channel, 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 channel 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 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 shortwave 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 2~3dB after the comparison in the shortwave communication system using the Turbo balanced technology compared with DFE.
4)Auto-adapted antenna technology . The auto-adapted antenna technology's
principle is the signal which receives through docking carries on the real-time processing, the control and the adjustment 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 antijamming ability effectively.
3.2 high speed modulation demodulation technology
At present is widespread the application the narrow band shortwave broadcasting station's modem to have serial and the parallel two systems, the serial system uses the single carrier to modulate the transmitter data, before 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 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 Division 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 people's more and more, also starts to apply gradually in shortwave communication 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 in each sub-carrier the data mark to continue length to increase, thus may reduce ISI which effectively wireless channel's time dissemination brings (InterSymbol Interference), like this reduced has received the within the aircraft balanced order of complexity, sometimes even might not use the balancer, 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-channel's frequency spectrum to overlap mutually, therefore compares with the 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, is very easy to realize along with the large scale integrated circuit technology and the DSP technology's development.
3.3 antijamming technologies
The shortwave communication is under the war condition director the only reliable way, along with the disturbance method to the wide frequency range, the multi-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
shows. 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 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 disturbance, causes its antenna pattern zero automatic direction disturbance direction, enhances communications receiver's SIR.
3)Time processing. If burst transmission technology and advanced error correction
coding technique. So-called burst transmission technology, first information storage, then when some flash by normalcy 10~100 time or higher speed burst. On the one hand may use the big pulse power to resist the intentional jamming, because on the other hand launches the time randomness and the temporary causes the intercept communications probability to reduce greatly; Uses close the shannon limit the Turbo code union to interweave the technology, the iterative technique as well as antijamming the technology may enhance system's resistance to interference to a certain extent.
4 conclusions
Because the shortwave communication holds in the military correspondence domain particularity, even if is the mobile communication and Internet so developed today, the world developed country the stop to the shortwave communication technology's research, still unceasingly had not made throughout the significant technological breakthrough in the shortwave communication domain, is promoting the shortwave communication technology development. Now, the shortwave communication already followed us to enter the information age, studies thorough along with the technical
progress and the people, once was considered that backwardness the shortwave communication must stride in the international advanced correspondence domain by the brand-new appearance the ranks.
短波通信的现状及发展趋势
摘要：
短波通信由于其天波传播特性，在通信领域具有其它通信手段无法替代的地位。

本文选择了两种先进的短波电台对短波通信的现状进行了介绍，分析了未来短波通信的需求以及短波通信技术的发展趋势。

关键词：短波通信；自适应；高速调制解调；抗干扰；组网
1 引言
短波通信又称高频(HF)通信，使用频率范围为3～30MHz，主要利用天波经电离层反射后，无需建立中继站即可实现远距离通信。

同时由于电离层的不可摧毁特性，短波通信始终是军事指挥的重要手段之一，但是要得到更广泛的应用，短波通信在以下几个方面还有待提高：
1) 通信的可靠性。

短波通信的天波传播因受电离层变化和多径传播的影响而极不稳定，信号传输多径现象严重，延迟大，多普勒频移大，衰落严重。

同时短波信道是带宽受限的信道，射频频谱非常拥挤，信道间相互干扰严重，获得可靠的通信质量一直是短波通信追求的目标。

2) 数据传输率。

传统短波通信难以崛起的一个重要原因就是数据传输率很低(不超过600bit/s)。

信息化战争中军事通信传递的信息，已从简单的指挥命令发展到诸如雷达探测的数据、计算结果、高速图像传真等一些要求数据传速率较高的信息。

3) 抗干扰能力。

由于短波通信是战事状态下指挥唯一可靠的途径，随着干扰技术的发展以及一些新型大功率短波干扰装备的研制成功，短波通信的抗干扰方式必须多样化，智能化，具有在不同电磁环境中的生存能力。

4) 网络化。

随着通信越来越向网络化发展，未来的短波通信也应更多地考虑组网使用和网络管理。

采用网络式通信，可以使短波通信的信息量和信息处理速度大大提高，还可在网内选用最佳链路，增加通信链路的抗毁性及顽存性。

2 短波通信的发展现状
由于短波通信在军事通信上的不可替代性，20 世纪80 年代以来，短波通信又开始受到重视。

由于多种新技术的应用，如信道自适应技术、差分跳频技术、宽带直接序列扩频技术、信道编码技术、信道均衡技术、短波组网技术，短波通
信方式存在的许多问题得到解决；并且随着微型计算机、移动通信和微电子技术的迅猛发展，人们利用微处理器、数字信号处理，不断提高短波通信的质量和数据传输速率，短波通信和装备得到了很大发展，许多公司陆续推出了一些性能优良的设备和系统，比较著名的有：美国的短波通信电台现代化、澳大利亚的
HF-90H、瑞典的KV90、美国的CHESS、美国HARRIS 公司的RF-5800 系统、柯林斯公司的SELSCAN系统等。

1) HF-90H 超小型跳频短波电台
HF-90H 由澳大利亚Q-MAC 公司推出，最突出的特点是采用了智能边带跳频技术。

目前世界各电台生产厂提供的都是数字话音。

数字话音跳频的缺点是跳频频谱不够隐蔽，容易被识别、破译和跟踪，如图1 所示。

而HF-90H 所采用的智能边带跳频模式是利用SSB(Single Sideband Signal)调制方式传送话音信号，瞬时频谱很象噪声，跳频频谱中由于跳频码隐含在话音的起伏中，无法确定跳频频率的设置，如图2所示。

Fig.1Instantaneous and accumulated over time spectrum view of HF digital voice ho p per
Fig.2 Instantaneous and accumulated over time spectrum view of HF SSB voice hopper 短波信道常掺杂着强烈的噪声和干扰信号，例如被广播电台或固定电台的强信号阻塞或杂波串扰，再加上衰落和多径效应造成的不稳定，使短波环境非常恶劣。

HF-90H 采用智能跳频技术可以克服这些困难，能跳频则是一种具有极强的频带适应性的技术，能够周期性地自动测评跳频段内每个信道的信号强度，并指令网内成员自动弃用嘈杂信道，通过不断地“过滤”频率，使通信背景明显净化，提高通信质量。

2) CHESS 系统
CHESS 系统由美国Lockheed Sanders 公司推出,系统结构如图3所示。

CHESS 系统以先进的数字信号处理技术和高速DSP(Data Signal Processor)芯片为基础，跳频带宽为2.56MHz，跳频速率高达5000Hops/s，数据传输率最高19.2kbit/s，CHESS 系统最突出的特点为采用了差分跳频（Differential Frequency Hopping，DFH）技术。

差分跳频实质上是一种将频率调制和编码相结合的技术，通过对频率编码，增加频率域的冗余度，使得跳频频率间具有了特定的相关性，接收端利用了这种相关性来进行频率序列译码，纠正一定比例频率误判，起到了以频带换取信噪比或信干比的作用，以提高系统的性能。

Fig.3 CHESS system architecture
3 短波通信技术的发展趋势
短波通信技术近年来发展非常迅速，已取得了一系列的突破和进展，未来在以下几个方面仍需进一步研究。

3.1 短波自适应数字通信技术
短波信道受多径时延、幅度衰落、天气变化等因素的影响变化莫测，要保证通信的可靠性，需要系统根据短波信道的变化自适应改变系统结构和参数。

现在的短波自适应通信技术，主要是指频率自适应技术，而未来的短波自适应通信技术应该是全方位的，包括：自适应选频与信道建立技术、传输速率自适应技术、自适应信道均衡技术、自适应天线技术等。

1) 自适应选频与信道建立技术。

现在的自适应选频与信道建立技术都是与通信结合在一起，这样选频质量会低于专用实时选频系统提供的频率质量。

今后发展方向应该是将专用选频系统和自适应通信系统结合起来，进一步提高短波通信质
量。

2) 传输速率自适应技术。

短波通信在选定工作频率后，要在随时间变化的信道上得到最大数据吞吐量，就必须采用传输速率自适应技术。

通常在允许的误码率条件下应选择尽可能高的数据传输率，这需要系统所采用的编码和调制方法与信道条件相互关联，当信道传播特性良好时用较高传输速率发送信息，而当传播特性变差时则降低传输速率，使误码率始终能满足通信质量的要求。

3) 自适应信道均衡技术。

在短波时变信道中传输信号时，为了消除多径效应、多普勒频移等带来的严重码间干扰，必须采用自适应信道均衡技术。

判决反馈均衡器(Decision Feedback Equalizers，DFE)是目前短波通信系统普遍采用的一种均衡技术。

近几年提出了一种新的均衡技术－Turbo 均衡技术，它结合信道编解码技术，充分利用了信道信息，经比较在短波通信系统中应用Turbo 均衡技术较之DFE 又提高了2～3dB。

4) 自适应天线技术。

自适应天线技术的原理是通过对接收到的信号进行实时处理，控制和调节天线阵元的相位来改变天线方向图特性，完成自适应波束形成，使天线波束的零位对准干扰方向，信号方向的增益达到最大，从而有效地提高系统抗多径衰落和抗干扰能力。

3.2 高速调制解调技术
目前广泛应用的窄带短波电台的调制解调器有串行和并行两种体制，串行体制使用单载波调制发送信息，目前最高速率为9.6kb/s，对均衡的要求很高；并行体制是将发送的数据并行分配到多个子载波上传输，传统的并行体制中各个子载波在频谱上互相不重叠，在接收端用滤波器组来分离各个子信道，各个子信道之间要留有保护频带，频带利用率低，而且多个滤波器的实现也有难度，目前最高速率仅为2.4kb/s。

正交频分复用(Orthogonal Frequency Division Multiplexing，OFDM)调制方式以其传输速率快、频带利用率高和抗多径能力强等优点越来越受到人们的重视，也开始逐步被应用于短波通信领域。

相对于单载波和非正交频分复用方式，OFDM 应用于短波通信具有以下优势：
1) 抗频率选择性衰落。

OFDM 系统把高速数据流通过串并转换，使得每个子载波上的数据符号持续长度相对增加，从而可以有效地减小无线信道的时间弥散所带来的ISI（InterSymbol Interference），这样就减小了接收机内均衡的复杂度，
有时甚至可以不采用均衡器，仅通过采用插入前缀的方法消除ISI 的不利影响。

2) 频谱利用率高。

OFDM 系统由于各个子载波之间存在正交性，允许子信道的频谱相互重叠，因此与常规的频分复用系统相比，OFDM 系统可以最大限度地利用频谱资源。

3) 实现简单。

采用IDFT(Inverse Discrete Fourier Transform)/DFT(Discrete Fourier Transform)实现OFDM。

即使对于子载波个数很多的系统，随着大规模集成电路技术与DSP 技术的发展都是很容易实现的。

3.3 抗干扰技术
短波通信是战事状态下指挥唯一可靠的途径，随着干扰手段向宽频域、多样式、多层次的方向发展，抗干扰措施也应趋于综合化、智能化以及多体制并存，具体的发展方向为：
1) 信号处理。

如自适应跳频。

自适应跳频系统在常规跳频通信的基础上加上了链路质量分析，通过可靠的通信链路质量分(Link Quality Analysis，LQA)，确定被干扰的频点，给出可以使用的跳频频率集，并把该频率集通过反馈信道传送给发射方，使双方自动适应信道变化情况，同时删除被干扰的全部频率，然后在无干扰或干扰很小的频点进行可靠通信。

2) 空间处理。

如采用自适应天线调零技术，当接收端受到干扰时，使其天线方向图零点自动指向干扰方向，以提高通信接收机的信干比。

3) 时间处理。

如猝发传输技术和先进的纠错编码技术。

所谓猝发传输技术，先将信息存储起来，然后在某一瞬间以正常时10～100倍或更高速率猝发。

一方面可使用较大的脉冲功率来抵御有意干扰，另一方面由于发射时间的随机性和短暂性使侦收概率大大降低；采用接近香农极限的Turbo码结合交织技术、迭代技术以及抗干扰技术在一定程度上可提高系统的抗干扰性能。

4 结论
由于短波通信在军事通信领域占有的特殊性，即便是移动通信和互联网如此发达的今天，世界发达国家始终没有停止对短波通信技术的研究，在短波通信领域仍不断取得重大技术突破，推动着短波通信技术的发展。

如今，短波通信已经伴随着我们进入了信息时代，随着技术的进步和人们研究的深入，曾一度被认为落后的短波通信必将以崭新的面貌跨入国际先进通信领域的行列。