1、Rec. ITU-R F.1337 1RECOMMENDATION ITU-R F.1337*FREQUENCY MANAGEMENT OF ADAPTIVEHF RADIO SYSTEMS AND NETWORKS USING FMCWOBLIQUE-INCIDENCE SOUNDING(Question ITU-R 205/9)(1997)Rec. ITU-R F.1337The ITU Radiocommunication Assembly,consideringa) the need for highly reliable HF communication services;b) th
2、e limited amount of HF spectrum available for transmission of voice and data;c) the constraints of the time-varying ionosphere, which limits the number of usable channels within the availablespectrum;d) that effective and efficient regulatory and frequency management techniques are essential to opti
3、mize use ofthe frequencies for fixed and some mobile services between 1.6 and 28 MHz;e) that experimental studies, as outlined in the Annex 1, have shown that substantial improvements in channelutilization can be achieved through the use of frequency and path diversity;f) that frequency modulated co
4、ntinuous wave (FMCW) oblique incident sounding schemes can provide anaccurate real-time assessment of the HF channels available to a network thereby forming the basis of dynamic frequencymanagement;g) that the FMCW “chirp” sounding method can be engineered to limit harmful interference, and can beim
5、plemented as an external process thereby eliminating reductions in system capacity,recommends1 that automatic and adaptive management schemes be considered for adaptive HF networks to include dynamicselection of optimum frequencies, the sharing of frequencies within a network, and adaptive selection
6、 of alternatenetwork paths;2 that FMCW “chirp” sounding be considered for use in dynamic frequency management schemes including: as a real-time input data source for updating resource management and propagation prediction programmes; as a means for updating the frequency scan lists of adaptive HF sy
7、stems; for modification and enhancement of the link quality analysis (LQA) matrices for adaptive HF systems; as a complement to the exclusive use of in-band channel sounding, thereby increasing network communicationcapacity and reducing interference introduced by channel sounding;3 that for adaptive
8、 HF networks in the fixed and mobile services, the information contained in the Annex 1 beconsidered for general spectral planning and in the design of network topologies;4 that further studies are required to evaluate the advantages of sounding technology for frequency managementof regional and glo
9、bal adaptive HF networks, emphasizing high latitude and equatorial regions during solar maximumconditions._*This Recommendation should be brought to the attention of Radiocommunication Study Groups 1 and 8.2 Rec. ITU-R F.1337ANNEX 1Long-term investigation of oblique incidence FMCW sounding as a meth
10、od fordynamic frequency management of HF communication networks1 IntroductionFMCW swept-frequency sounding has been used within centralized spectrum management networks to provide real-timechannel evaluation (RTCE) for multiple HF circuits. Often referred to as “chirp” sounders, these devices provid
11、e thebenefits associated with oblique incidence sounding, i.e. for use in the optimization of frequency utilization, improve-ment in circuit reliability, evaluation of signal-to-noise and interference ratios, and the measurement of ionosphericpropagation parameters used by some adaptive HF systems.
12、When linked to centralized spectrum management networks,these sounders can be operated in limited numbers on an intermittent, low-power basis, obtaining real-time spectrummanagement information for multiple users. This method decreases the need for sounding in general and also limits thetotal time t
13、hat a sounder will operate co-channel with other HF transmitters. The net result is improved information tomultiple users and a decrease in interference potential due to sounding.2 System conceptWithin centralized spectrum management networks, FMCW swept-frequency sounding information can be used to
14、produce presentations, called ionograms, of received signal time delay versus frequency of transmission. Because theionospheric channel may be comprised of multiple layers and scattering centres supporting HF communication, receivedsignals at a given frequency will experience varying amounts of time
15、 delay distortion. Traces on the ionogram record areindicative of various ionospheric propagation modes which will support communication.Ionograms are derived by processing a low power FMCW signal over a selectable frequency range, typically 2-30 MHz.The quality of these “chirp” ionograms is superio
16、r to those derived from pulse sounders for the same averagepower because the former employs a spread spectrum waveform characterized by processing gain which is appliedagainst narrow-band interference effects. The nominal average power (and peak envelope power (PEP) of these systemsis 10-100 W or po
17、ssibly less depending upon the circumstances.FMCW sounding also reduces the amount of time when any particular frequency is occupied. The nominal rate atwhich these sounders sweep through the 2-30 MHz band is 100 kHz/s. Consequently, the entire HF band is sampledin about 5 min. The temporal separati
18、on between successive sweeps for a given transmitter, called the scan interval,is typically 15 min, but may be set at 30 min or longer. Therefore, the FMCW swept frequency transmitter not only usesmodest power, but it has a momentary effect on other HF users within each scan, and the scan interval i
19、s relatively long.If a co-channel user is within the coverage pattern of the FMCW sounder, a typical 3 kHz channel may experiencea 30 ms blip no more than twice an hour if the scan interval is 30 min.When used within a centralized spectrum management network, the number of sounders can be limited wh
20、ile providingregional or worldwide coverage to a large number of users. This process eliminates the need for individual HF operatorsto perform their own sounding thereby decreasing the total amount of sounding needed to support a number of users.Channel sounding by individual transmitters, though pr
21、oviding useful information, can add to the congestion of the HFfrequency range. FMCW swept-frequency sounding used in conjunction with centralized spectrum managementnetworks can greatly decrease the potential for interference due to sounding.3 Experimental implications of relevance to the Recommend
22、ationMany experiments have been conducted over the years using FMCW swept-frequency oblique incidence sounders. Anextensive experimental program of the potential of frequency and path diversity was conducted between 1993 and 1996,a period of low solar activity. The program was conducted using ten si
23、tes in the Northern hemisphere. Figure 1 providesRec. ITU-R F.1337 3the geometry. The investigation was directed toward resolving problems associated with adaptive HF communicationsystems with special emphasis on automatic digital data communication. Other issues of concern included those of radiopr
24、opagation, frequency management, and system architecture. Paths that were examined have provided informationrelated to circuits within the polar cap, auroral zone, high-latitude trough, as well as mid-latitude channel environments.1337-0180 N70 N60 N50 N40 N30 N20 N10 N010 S20 S3 5704 6004 06024801
25、240146019503480133028803 02031505710230923001 6202 230850349026202 0002 9005040310627002 290176025001 200204822503 670SANFRANCISCOTx-RxMADRIDTxPUERTORICOTxFAIRBANKSRxVANCOUVERTxREYKJAVIKRx-TxST. JOHNSRx-TxJAN MAYENTxWINNIPEGRxWAINWRIGHTTxINVERNESSTxIQALUITRx-TxCHURCHILLRxFIGURE 1Geometry of the prop
26、agation experimentsDistances between the transmitter and receiversites are given in kilometres. Data were obtained between 1993 and 1996.N.CAROLINARxSTOCKHOLMRxFIGURE 11337-01 = 13.5 cmTransmitter powers used in the experiments ranged between 10 and 100 W, and the sounder scan interval was set at30
27、min. Over 40 path-years of data were collected and analysed, providing estimates of link and star-net communicationavailabilities under a variety of frequency and station diversity conditions.Not only was the information significant derived from the data taken during the experiments, but the extensi
28、ve long-termoperations provided evidence of the decreased potential for interference resulting from the used of FMCW sounders withcentralized spectrum management networks. During the entire period of operation of these FMCW sounders associatedwith the experimental activity conducted between 1993 and
29、 1996, there were no reports of interference.The following conclusions are based upon the analyses of the data collected. Some of the conclusions flow directly fromaxiomatic principles of HF propagation and the ionospheric interaction.4 Rec. ITU-R F.13374 Factors relating to FMCW sounding4.1 On spec
30、trumAdequate spectrum must be available to achieve optimum connectivity, whereas the conditions for adequacy willultimately depend upon factors such as traffic loading. Studies of star networks have indicated that long-termcommunication availabilities approaching 100% may be achieved provided there
31、is access to several widely-separatedground stations, and that dynamic selections based on real-time soundings can occur from among a pool of frequenciesresiding in a sufficient number of bands distributed across the HF spectrum. As a general rule, eight frequencies aregenerally adequate for good qu
32、ality fixed and mobile service applications. The condition for adequacy, however, mayrequire additional frequencies during disturbed conditions, when the number of diversity paths is reduced; or, wheneverionospheric correlation distances are increased. The converse is also true. That is, the frequen
33、cy requirements can bereduced under benign conditions or during periods of low traffic loading.In general the number of frequencies required to accommodate high levels of availability will be dependent upon thenumber of stations which are contributing to network capacity. Tests have examined this re
34、lationship and the followingrule is found to hold under most circumstances:Nf a + b/Ns(1)where Nfis the number of frequencies, Nsis the number of stations, and a and b are functions of network topology(i.e., node separations), the geographical area over which the communication service is to be provi
35、ded, the diurnal cycle,the season and the solar conditions. For the conditions tested, it has been found that a 4 and b 48 is fairly represen-tative for 3 Ns 5 except during disturbed conditions. There are also conditions during which sporadic E is acontrolling feature over one or more propagation p
36、aths. The condition of sporadic E, when available, may enable asingle station to host as many frequencies as necessary to accommodate all the traffic from a clusterhead to the groundstation and still achieve high availability. In this instance station diversity is unnecessary, and equation (1) does
37、not hold.A real-time frequency management system, based upon the FMCW technology outlined in this Annex will enable bothnormal and sporadic-E modes to be detected and evaluated.4.2 On network topologyStar net configurations consisting of a central node (i.e., clusterhead) and a surrounding cluster o
38、f external nodes exhibitenhanced connectivities if the cluster population is increased. Moreover, the connectivity characteristic approachessaturation for designs incorporating four or more widely spaced nodes. Thus the number of participating paths and thedegree of propagation independence of the s
39、pecified paths is quite important. Another condition for connectivity is thatthe participating paths are viable, since a null path cannot contribute to connectivity. These principles suggest thefollowing strategy for selection of cluster members: on one hand, association of paths having widely separ
40、ated controlpoints, on the other hand, association of paths residing in differing geophysical regimes, use of paths with largestpropagating bandwidth, including: longest possible paths in the equatorward direction, and paths possessing the largestsolar elevation angle. As a general rule, it is found
41、 that the region defined by great circle paths connecting nodes in thecluster should define the largest possible area containing the clusterhead thus enabling as much path diversity aspossible. However, given the greater possibility of limited viability for paths in the poleward direction, there sho
42、uld bean effort to specify equatorward circuits with as much diurnal separation as possible.An overall regional or global network may consist of an assembly of subnets or clusters where ionospheric properties atthe clusterheads and at other nodes in each cluster continue to evolve. For fixed service
43、s, where the clusterhead isstationary, the evolution results from ionospheric variability. For mobile services, since the clusterhead may be inmotion, the situation becomes more complicated. Frequencies which were once optimum will not remain so for verylong. It points to the importance of dynamic m
44、anagement of network resources, including the inter-exchange offrequencies between nodes in the cluster. Evidence for an excess of capacity from a given node to a generalizedcoverage area, owing to the presence of sporadic E is abundant. While sporadic E is recognized as a summer daytimephenomenon f
45、or mid-latitudes, it is seen that sporadic E bands due to the auroral zone may be used to circumvent F-region propagation disturbances.Rec. ITU-R F.1337 54.3 On the influence of magnetic activityMagnetic storms may have a significant impact upon the available HF spectrum, an effect which is generate
46、d bysignificant departures of the maximum observed frequency (MOF). Because magnetic storm effects may occur over avast area and can have such a long-lasting effect, it is evident that real-time ionospheric information over requisite linkswill be a decided improvement over long-term prediction metho
47、ds which are sometimes used to define adaptive HF scanlists. The largest diminutions in available bandwidth resulting from magnetic storms arise for mid-latitude circuits.While magnetic storms give rise to obvious MOF variations, there are also identifiable MOF fluctuations associated withelevated l
48、evels of magnetic activity. These may be associated with an increase in the number and magnitude of travellingionospheric disturbances (TIDs) which may originate in the neighbourhood of the auroral zone and propagateequatorward. While these lower level disturbances are not sufficiently well-organize
49、d to produce a magnetic storm, theydo introduce a variety of HF propagation effects, including: multipath, sidescatter, and spread-F. These effects areobserved directly by means of FMCW sounders.It is also well known that magnetic activity is correlated with the geographic position and strength of high latitudefeatures, including the mid-latitude trough and the auroral oval. For example, the oval thickens and moves equatorwardas magnetic activity increases, and these features contract as the activity becomes smaller. This is significant sincemagnetic a
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