1、 Rec. ITU-R F.1110-3 1 RECOMMENDATION ITU-R F.1110-3 Adaptive radio systems for frequencies below about 30 MHz (Question ITU-R 147/9) (1994-1995-1997-2003) The ITU Radiocommunication Assembly, considering a) that HF sky-wave links allow communication over large distances; b) that, up to now, natural
2、 ionospheric variations and interference resulting from spectral congestion and propagation anomalies have made HF links difficult to operate and demanded highly-skilled operators; c) that technological progress in recent years has led to the development of adaptive systems which can be used to auto
3、mate HF links and enhance their quality; d) that adaptive systems make it possible: to achieve a higher quality of service by combining an ability to exploit modern radio-frequency technology with advanced real-time control software; the result is a system which is reliable, robust, cost-effective a
4、nd easy to use; to establish extensive HF networks with flexible, less hierarchical architecture; to reduce transmission times thereby: securing more efficient use of the spectrum, reducing the interference between users, providing the ability to increase traffic density, to reduce the reliance on s
5、killed operators; e) that adaptive automatic link establishment (ALE) systems are now in operation; f) that additional information on adaptive HF systems and networks can be found in the ITU-R Handbook on “Frequency-adaptive communication systems and networks in the MF/HF bands”, recommends 1 that a
6、daptive HF systems should have the general functions given in Annex 1. 2 Rec. ITU-R F.1110-3 Annex 1 General functions of HF adaptive systems 1 Introduction Frequency-agile, adaptive HF systems may be used for any type of fixed or mobile service, but have a greater applicability for digital technolo
7、gies where a high quality of service is required. An adaptive system automates the processes involved in establishing, maintaining and terminating HF links or networks. It dispenses with the need for skilled operators and improves the quality of service and the efficiency of the link. Basically, an
8、adaptive system has a triple function: automatic selection of the frequency and of other system parameters to be used; automatic operation as regards calling, establishing the communication (with possible switch-over to the peripheral equipment needed for the type of service to be provided), and dis
9、connecting; adaptivity during the communication so as to optimize at all times the quality of service according to the ionospheric conditions and spectrum congestion. 2 Automatic selection of the frequency to be used The selection of the frequency to be used should utilize some or all of the followi
10、ng information: list of assigned frequencies; a stored ionospheric prediction schedule for predicting link quality at different frequencies as a function of the time, the season and the year; data on the quality of previous links, which may reduce the ALE time if the ionospheric duct is sufficiently
11、 stationary (short-term) or sufficiently reproducible during the same time interval on successive days; passive real-time analysis of channels in order to sort out free channels from channels suffering interference (reducing spectrum congestion); possibly, information provided by another device, e.g
12、. ionospheric-sounding system. On the basis of all this information, a preferential list can be drawn up, when required, of the frequencies to be used for a given link. Rec. ITU-R F.1110-3 3 3 Automatic calling, link establishment and disconnection 3.1 Common calling/traffic channels The calling seq
13、uence should contain the following information provided by the user: identification of calling station; identification of called station; type of service; possibly, mode of operation for those cases where this is not imposed by the link and where there is not a one-to-one correspondence between the
14、mode of operation and the type of service. The frequency selection unit or function lists the frequencies in order of expected service quality. The calling sequence is carried out on the frequency classified as No. 1 by the frequency selection unit. This frequency is retained for the link if: a repl
15、y is received from the called station; this reply indicates that the measured quality of the link in the calling-called direction is sufficient to provide the required service (the quality might be sufficient for establishing the link, which is always done at a low bit rate, but it might be insuffic
16、ient for a service requiring a higher quality, for example analogue telephony or high bit rate transmission); the measured quality of the link in the called-calling direction is sufficient to provide the required service. A call on the frequency classified as No. 2 will be reinitiated if one of the
17、above three conditions is not met, etc. As soon as an adequate frequency has been found, the system switches over automatically to the peripheral equipment corresponding to the type of service to be provided. After disconnection, stations return to the watch-keeping mode. NOTE 1 By frequency classif
18、ied as No. 1, No. 2, , the following is meant: one single frequency for simplex operation; a pair of frequencies for half-duplex and duplex operation. It is highly advisable to employ procedures allowing independent selection of the frequencies for each direction of the link in the following cases:
19、presence of local interference; when the same frequencies are not available at the two ends of the link. 3.2 Separate calling and traffic channels For networks or systems where the traffic density or number of stations is large, the use of separate calling and traffic channels may be preferred. In s
20、uch cases the call establishment will generally follow the pattern of 3.1, except that the initial contact is made on one of a set of calling channels, which are monitored by all stations when watch-keeping. After this, a combination of passive channel assessment and active channel sounding is used
21、to determine the most suitable traffic frequency. 4 Rec. ITU-R F.1110-3 4 Adaptivity during a communication Due to its adaptivity, the system automatically maintains the quality of an HF transmission during a communication by varying the main transmission parameters in accordance with the changing s
22、tate of the channel. These parameters are, for example: radio equipment: frequency; transmitter power; choice of modulation; data terminal or telegraph peripheral equipment and their associated modems: bit rate; type of coding; shift excursion; sub-carrier frequency. In setting up an adaptive proces
23、s, it is necessary to: determine a measurable criterion representative of the quality of the link for a given type of service (for example, number of repetitions for an automatic repeat request (ARQ) telegraph link error rate for a digital transmission measurement of S/N ratio-jitter); decide on a v
24、alue for this criterion below which the quality is regarded as inadequate (threshold value); arrange for regular repeated measurements of the value of this criterion during the communication process; if this value drops below the threshold set for a specific time, vary one (or more) of the link para
25、meters so as to obtain the required quality again; regular monitoring of the selected channels for occupancy from both ends of each link, so as to avoid causing interference. This clearly presupposes that these parameters are programmable and that the various discrete values adopted may be modified
26、remotely. When the parameters are modified, the two terminals concerned have to be informed by a special signal that an adaptive process is in progress. NOTE 1 In analogue telephony, the criterion which is representative of the link quality can only be subjective; the user must therefore be able to take the decision to change the parameter. For example, a “restart” command activated by a user who is not receiving properly might tell the system that the adaptivity procedure needs to be triggered.
