1、 Rec. ITU-R F.1611 1 RECOMMENDATION ITU-R F.1611*Prediction methods for adaptive HF system planning and operation (Questions ITU-R 205/9 and ITU-R 147/9) (2003) The ITU Radiocommunication Assembly, considering a) that the number of adaptive HF systems in operational use is growing, specifically auto
2、matic link establishment (ALE) systems; b) that Recommendation ITU-R F.1110 specifies the general characteristics of adaptive HF systems, and specifically recognizes that adaptive HF systems make it possible to achieve the following: a higher quality of service by combining the ability to exploit mo
3、dern radio frequency technology with advanced real-time control software; to reduce transmission times, thereby securing most efficient use of the spectrum, reduced interference between users, and the ability to increase traffic density; c) that ITU has developed an adaptive HF Handbook which descri
4、bes the nature of adaptive HF systems and their use; d) that frequency-adaptive HF systems are constrained to use the minimum number of active frequency channels in order to limit the potential for interference with other users; e) that Recommendation ITU-R F.1337 recommends that automatic and adapt
5、ive management schemes be utilized for adaptive HF networks; f) that frequency planning with an accurate HF performance prediction model will reduce the margins over which adaptive HF systems must be designed to adapt, with the result that operational procedures can reduce the potential for interfer
6、ence, and overall cost may be reduced; g) that Recommendation ITU-R P.533 (and its software program REC533) is the established ITU-R method for HF performance predictions, and is well-suited to be an umbrella for additional (related) prediction methods, some of which are currently used in the contro
7、l of various quasi-adaptive HF systems and in the design of systems employing forms of ALE; h) that other related prediction methods, such as the IONCAP family of programs, are maintained on the same publicly-available Institute for Telecommunication Sciences (ITS) website as is Recommendation ITU-R
8、 P.533; *This Recommendation should be brought to the attention of Radiocommunication Study Group 3. 2 Rec. ITU-R F.1611 j) that all methods downloaded from the ITS website (including Recommendation ITU-R P.533, VOACAP and ICEPAC) have input/output methods that can be reconciled with some additional
9、 effort, recommends 1 that administrations which intend to procure and deploy adaptive HF and ALE systems, based upon the information in Annex 1, should explore the use of HF performance prediction models such as those contained in Recommendation ITU-R P.533 and related models in advance of deployme
10、nt to establish adaptivity bounds; 2 that the models contained in Recommendation ITU-R P.533, augmented by material such as that contained in the (optional) IONCAP family of programs, are preferred methods for the design of adaptive HF systems and for possible incorporation within software modules f
11、or real-time adaptation using recognized real-time channel evaluation (RTCE) technologies (i.e. in-band sounders, advanced channel probes, and out-of-band frequency modulated-continuous wave (FM-CW) sounding). Annex 1 Adaptive HF system planning and operation using prediction methods 1 Introduction
12、The ionospheric channel provides the connectivity for the links in an adaptive HF radio circuit or network. To provide proper utilization of this resource the radio system must operate on the ideal frequency or as close to it as practical. This may be the same frequency for a simplex circuit or two
13、different but closely related frequencies for a full duplex circuit. Frequency changes will be dictated by the natural cycles of ionospheric propagation; diurnal, seasonal and sunspot variations. Natural or man-made radio interference may dictate unpredicted changes in the operating frequency. Also
14、solar flares and geomagnetic storms may cause communications disruptions which will also require changes in the operating frequency. For the most part, the frequency adaptive HF radio system will detect link failure, find another usable frequency, bring the link up on the new frequency and re-establ
15、ish communications without operator intervention. Several things can create intolerable outages even with the most sophisticated adaptive HF radio system. Equipment failure is one that is unavoidable over the life cycle of the system. It can be mitigated by fault detection, backup power supplies and
16、 equipment redundancy in the system design. However, most disruptions are attributed incorrectly to propagation failure. Only Rec. ITU-R F.1611 3 under the most rare of ionospheric conditions should propagation failure occur. Most cases, where the signal power on any frequency across the band is unu
17、sable, can be directly traced to improper system design. The areas, which must be considered in the system design, will be discussed in the following paragraphs. Let it suffice to say at this point that the most common areas are: inadequate number of frequencies in the authorization list; underestim
18、ation of radio noise environment at the receive sites; antenna radiation patterns which do not match the takeoff and arrival angles of the ionospheric channel; and excessive losses in the transmission lines between the transmitter and the transmit antenna or the receiver and its antenna. 2 Frequency
19、 planning Frequency planning begins early in the design phase of the HF radio system. For frequency adaptive HF radio systems, it is essential that the ionospheric model used for making the frequency predictions include as much knowledge as possible concerning the expected variation of the hourly ma
20、ximum usable frequency (MUF) about the monthly median value (i.e. the MUF) for each of the probable modes on each link in the proposed radio system. The needed accuracy in the prediction model is directly related to the very low S/Ns required for the sophisticated adaptivity techniques employed in t
21、he modern radio equipment and their associated modems. We will now outline a sample procedure by which prediction methods can be used to design and operate adaptive HF systems. The discussion in this Annex is based upon the VOACAP procedure that is used within the United States of America administra
22、tion, having some experience in its application for adaptive HF systems. It is likely that similar procedures could be developed for other methods such as the preferred ITU method, Recommendation ITU-R P.533. At this time the preferred programs in the IONCAP family are VOACAP and ICEPAC, which, alon
23、g with Recommendation ITU-R 533, are maintained and available at no cost via the Internet from the US Department of Commerce (i.e. http:/elbert.its.bldrdoc.gov/pc-hf/hfwin32.html). The Recommen-dation ITU-R P.533 computer program predicts the monthly median MUF. It currently does not explicitly give
24、, as an output, the expected distribution of daily MUF values over the days of the month at that hour for the possible modes. In evaluating an adaptive HF system, each link in the radio net needs to be evaluated using a recommended prediction program. (In this example, we specify VOACAP, which belon
25、gs to the IONCAP family of programs.) These prediction methods were originally developed for non-adaptive systems. However they can be utilized for adaptive system planning. For conventional HF radio links, the optimum working frequency (OWF) would be determined by finding the most reliable frequenc
26、y at each hour, season and for high and low sunspot activity. However, for frequency adaptive HF radio systems, the highest probable frequency (HPF) needs to be found. This is the frequency which would not be exceeded more than 10% of the days in the month at that hour and sunspot number; whereas th
27、e OWF is the frequency which is exceeded on 4 Rec. ITU-R F.1611 approximately 90% of the days per month. The frequency adaptive system using ALE should have the capability of operating on frequencies above the monthly median MUF on half of the days per month. The frequency authorization list for suc
28、h a system should include frequencies above the MUF but not to exceed the HPF. This feature of frequency adaptivity demands the use of accurate prediction programs in the systems design of the radio circuit or network. It is essential in the planning for an adaptive HF radio system that the equipmen
29、t be designed to work over the full range of usable frequencies including those below the MUF but above the lowest usable frequency (LUF). When possible, the system should operate at the highest usable frequency which permits error-free connectivity. The higher frequencies will experience less atmos
30、pheric and man-made radio noise and provide higher S/Ns for less transmitter power and antenna gain. By designing the adaptive system to take advantage of the highest possible frequency usage, cost savings can be made in the power capacity of the transmitter and the size of the transmit and receive
31、antennas. At the same time, it should be recognized that practical circumstances may require operation of individual links at less than optimal frequencies to avoid self-interference or to avoid harmful interference to other users in a shared environment. Frequency predictions provide an a priori ca
32、pability for assessing groups of sub-optimal channels to provide the best possible system reliability under specified conditions. Also care must be taken in the system design to assure that the signal-plus-noise power delivered to the receiver is meets the sensitivity requirements of the proposed re
33、ceiver. Another frequency consideration which will be important in the operation of the frequency adaptive system is that the frequency assignment should be several times per day and for seasonal and sunspot number changes. Ideally the frequency sampling should not exceed more than half a dozen freq
34、uencies at a given time epoch. This allows the adaptive system to perform link establishment more rapidly and reduces the needless interference caused by sounding on useless frequencies. Also a priority of frequency sampling should be established so that during a given time epoch the most reliable f
35、requency is sampled first in the link establishment process. If there is a frequency which is 90% reliable, then on 27 days out of the month at that hour the system should lock up on that frequency and on the first try. Short term forecasting or now-casting techniques can further assure the success
36、of finding the best operating frequency in the shortest time possible. The advantage of using predicted frequencies for first priority is that they are more likely to remain stable over the entire hour than a frequency of opportunity found by random sampling of the spectrum. In short, there are two
37、levels of frequency steering in a frequency adaptive HF system. At the first level, frequency predictions can be used to reduce the range of frequencies to be sampled or sounded in a real-time scenario. The second level of steering is defined by channel properties best derived from sounding or equiv
38、alent RTCE schemes, supplemented by a knowledge of the expected trends in frequency availability obtained from an appropriate prediction procedure. In-band channel soundings (e.g. ALE sounding), full-band FM-CW soundings (i.e. chirp sounding), or parameters derived from the modulation in use may be
39、used. (See Chapter 6 of the Handbook on Frequency Rec. ITU-R F.1611 5 adaptive communication systems and networks in the MF/HF bands, on sounding and RTCE technologies.) Still, the use of frequency predictions is essential for the development of requisite lists of the most likely frequencies to be u
40、sed during the day and night path conditions by an adaptive HF radio system. 3 Power budget The next phase in the design for a frequency adaptive HF radio system is to establish the power budget for each of the ionospheric links in the circuit or network. The power budget may be considered as the me
41、ans of finding the most cost-efficient design which will permit for acceptable service over the life cycle of the system. Again for adaptive HF system design, performance prediction models with the highest degree of accuracy in modelling the signal and noise variations should be used. Programs shoul
42、d employ the full statistics of the expected variation in transmission loss by mode and the time variation of the atmospheric and man-made radio noise. The reason for needing this sophistication in the prediction model is that adaptive HF radio systems operate at very low S/Ns. In order to be certai
43、n that the selected transmitter power and antennas for each of the links are adequate, great care must be taken in the calculation of the time- and frequency-variant signal and the noise power delivered to the receiver. Errors in this design step will result in needless cost in over-design or commun
44、ications failure when the signal power is below the detection threshold of the receiver. 4 Design process The first step in the design process is to evaluate the predicted required power gain (RPWRG) on each link of the radio circuit or network. It is defined as the additional or excess power, expre
45、ssed in dB, required to achieve the required S/N at a specified reliability. A value of 0 dB at 90% reliability would mean that the required S/N would be achieved or exceeded 90% of the days of the month at that circuit hour. Generally, the prediction program is set up with an assumed transmitter po
46、wer of 1 kW and isotropic (0 dBi) antennas. To prevent modes from being considered at unreasonably low take-off or arrival angles, the minimum take-off angle to be considered is set at 3 above the horizon. At this time a revised frequency list (up to 11 frequency bands between 2 and 30 MHz can be sp
47、ecified in one run of the Recommendation ITU-R P.533 and VOACAP-family of programs) needs to be established for this phase of the system design. The goal is to select frequencies which fall between the lowest usable frequency and the highest probable frequency needed over the hours, seasons and suns
48、pot numbers representing the expect life cycle of the system. In other words the performance prediction should be made for only those frequencies for which we expect ionospheric support. The LUF is not predicted as such in the IONCAP-family or the Recommendation ITU-R P.533 programs. Instead, the re
49、liability expected at various frequencies is predicted, which may be compared with the service requirement. (For design purposes the LUF is defined as the lowest frequency which will provide 90% reliability of meeting the required S/N or as approximately 90% of the MUF.) The next step is to decide upon the minimum required S/N density 6 Rec. ITU-R F.1611 ratio needed to allow the adaptive system to provide a minimum acceptable grade of service. The value of the required S/N is not well established this time for adaptive HF radio equipment. However, there are several guidelines w
