1、Rec. ITU-R F.1106 1RECOMMENDATION ITU-R F.1106EFFECTS OF PROPAGATION ON THE DESIGN AND OPERATIONOF TRANS-HORIZON RADIO-RELAY SYSTEMS(Question ITU-R 103/9)(1994)Rec. ITU-R F.1106The ITU Radiocommunication Assembly,consideringa) that the effects of propagation are crucial in the design and operation o
2、f trans-horizon radio-relay systems;b) that transmission characteristics can be determined in terms of the amplitude and phase properties of a receivedsignal;c) that amplitude variation as a function of time consists of a rapid variation superimposed on a slow change;d) that rapid variations, which
3、are basically caused by multipath phase interference phenomena, can be mitigatedin many cases by diversity techniques;e) that the effects of slow variations of received power may be minimized through the use of high-powertransmitting equipment, high-gain antennas, e.i.r.p. automatic level control, v
4、ery low-noise receiving systems, low-lossfeeders, adaptive message-loading techniques, and improved detection and other practices for system optimization,which are associated with the choice of radio-frequency carrier, bandwidth and modulation,recommends1. that the guidance contained in Annex 1 shou
5、ld be taken into consideration in the design and operation of trans-horizon radio-relay systems (see Notes 1 and 2);2. that, in particular, the following factors should be taken into consideration in the design and operation ofdigital trans-horizon radio-relay systems:2.1 the combined gain of the tr
6、ansmitting and receiving antennas may be less than the sum of their plane-wavegains as described in 2 of Annex 1; this apparent drop in gain is termed “gain degradation” or “antenna-to-mediumcoupling loss”;2.2 Recommendation ITU-R F.698 should be referred to for the preferred frequency bands for tra
7、ns-horizon radio-relay systems;2.3 diversity techniques, such as space diversity, frequency diversity and angle diversity are effective formitigating adverse effects caused by rapid variations of a received signal, as described in 4.1 of Annex 1;2.4 transmission bandwidth of a system utilizing diver
8、sity techniques can be estimated by the methods described in 4.2 of Annex 1;2.5 in order to mitigate the effects of multipath dispersion on digital trans-horizon radio-relay systems, order ofdiversity, adaptive equalization and selection of effective modems should be taken into consideration as desc
9、ribed in 5of Annex 1;2.6 in order to mitigate the effect of interference to other systems, caused by side lobe and overshoot signals underconditions of enhanced propagation, the use of automatic level control of the transmitter power, from the level receivedat associated distant receivers, should be
10、 considered.Note 1 This Recommendation applies to trans-horizon systems using tropospheric scatter but not necessarily to trans-horizon systems using other modes of propagation (diffraction, etc.).Note 2 Recommendation ITU-R PN.617 should also be referred to for the propagation data required for the
11、 design oftrans-horizon radio-relay systems.2 Rec. ITU-R F.1106ANNEX 1Factors concerning effects of propagation on the designand operation of trans-horizon radio-relay systems1. IntroductionThis Annex is based on consideration of systems carrying a relatively small number of voice channels or atelev
12、ision signal.Transmission characteristics can be defined in terms of the amplitude and phase properties of a received signal.Given a tropospheric-scatter path, both amplitude and phase transmission characteristics vary with frequency and time.Amplitude variation as a function of time consists of a r
13、apid variation superimposed on a slow change. Rapidvariations, which are basically caused by multipath phase interference phenomena, can be mitigated in many cases bydiversity techniques. The effects of slow variations of received power may be minimized through the use of high-powertransmitting equi
14、pment, high-gain antennas, very low-noise receiving systems, low-loss feeders, adaptive message-loading techniques, and improved detection and other practices for system optimization, which are associated with thechoice of radio-frequency carrier, bandwidth and modulation.2. Drop in antenna gainThe
15、combined gain of the transmitting and receiving antennas may be less than the sum of their plane-wavegains. This apparent drop in gain is termed “gain degradation” or “antenna-to-medium coupling loss”. Theoreticalanalyses state that the amount of loss is dependent on the antenna gain and scatter ang
16、le.The path antenna gain or total effective antenna gain over a trans-horizon system has been observedexperimentally to be practically independent of distance between about 150 and 500 km. According to theseexperiments, the total effective gain (see Fig. 1) may be assumed to depend only on the sum o
17、f the free-space antennagains, without large corrections provided that neither of the free-space antenna gains exceeds about 50 dB, and the gainsof the two antennas do not differ greatly.Further studies indicate that the drop in antenna gain is related to the slope of the refractive index gradient-a
18、ltitude curve within the common volume, consequently the variations of drop in antenna gain as a function of distancedepend slightly on the climate. These studies also show that, with increasing antenna gain, the slope of the curve inFig. 1 tends asymptotically to 1/4.3. Choice of frequency bandThe
19、choice of a frequency band for a trans-horizon radio-relay link depends on: propagation considerations, to obtain a sufficient received level and an adequate transmitting bandwidth; frequency sharing considerations, in relation with the usual high transmitting power trans-horizonsystems.These proble
20、ms are described in Recommendation ITU-R F.698.4. Diversity reception in trans-horizon radio-relay linksMost trans-horizon systems are designed to minimize the adverse effects of fluctuations due to propagation bymaking use of the partially correlated properties of the transmitted signal through div
21、ersity reception and/ortransmission. Diversity techniques are also very useful for trans-horizon digital radio-relay systems because they canlead to a reduction of short-term outages which are caused by multipath fading.Rec. ITU-R F.1106 340 50 60 70 80 90 100 110405060708090100110Total effectivegai
22、nGusing troposphericscatter(dB)pSum of the free-space antenna gain, G + G (dB)trFIGURE 1Relations between antenna gains (tropospheric scatter)D01FIGURE 1/F.1106.D01 = 12 CM4.1 Methods of obtaining diversity signals4.1.1 GeneralThe most commonly used methods are frequency and space diversity, i.e., s
23、imultaneous transmission of thesame signal over two or more channels (frequency diversity) and utilizing two (or more) antennas for reception and/ortransmission (space diversity). Some operational systems have used a combination of frequency and space diversity.Diversity reception, making use of rel
24、atively uncorrelated properties of the direction of arrival, has been alsodemonstrated to be beneficial in minimizing not only the rapid-fading problem, but also in lessening antenna-to-mediumcoupling loss.To realize substantial operational benefits from diversity reception, low orders of correlatio
25、n coefficient are notrequired.4 Rec. ITU-R F.11064.1.2 Space diversity in tropospheric-scatter systemsFor space diversity, the theoretical diversity distances have been examined, in terms of the horizontalcorrelation distance Dhnormal to the path, the horizontal correlation distance Daparallel to (a
26、long) the path and thevertical correlation distance Dv. Dhis the parameter most often utilized, expressed as:Dh= 3 a / 4d (1)where:d : path distancea : equivalent radius of the Earth : wavelength.Many tropospheric-scatter systems have been installed utilizing Dh= 100 .4.1.3 Frequency diversity in tr
27、opospheric-scatter systemsThe frequency correlation coefficient between the envelopes of two signals has been found theoretically to be: (2 1) = exp (2 pi )2(2 1)2(2)where: = 2l (sin ( / 2) / c)where: : angle of scatterc : velocity of lightl : standard deviation of each dimension of the scatter volu
28、me (assuming three dimensional rectangularcoordinates); l is a function of geometric and radiometeorological parameters.The subject of frequency correlation has received more experimental attention than space correlation.Frequency correlation is important both in assessing bandwidth capability and a
29、s a design parameter for frequency-diversity systems. The minimum necessary frequency separation depends on beamwidth of the antennas and the lengthof the path. A frequency separation of 3 MHz was adequate to produce a correlation coefficient of 0.6 or less on 226 kmand 345 km paths at either 600 or
30、 2 120 MHz, using a 10 m antenna for transmitting and a 3 m antenna for reception.4.1.4 Angle diversity in tropospheric-scatter systemsAngle diversity is a useful method which enables maximum use of a limited frequency band. Some UnitedKingdom test results between 1.7 and 2.7 GHz over paths of 250 t
31、o 350 km show that with antennas in the range 18 to25 m in diameter, correlation coefficients of 0.2 and 0.4 are obtained with two radio beams separated by approximatelyone beamwidth, with the lower figure resulting from vertical angle diversity. These results agree closely with theory. Asignificant
32、 medium-term decorrelation was apparent from the trials, as was a marked tendency for the short-term cross-correlation coefficient between beams to fall with decreasing signal levels. The former effect is unique to angle diversity.The overall performance of a correctly engineered angle diversity lin
33、k can be expected to approach that of frequencydiversity despite the small increase in transmission loss attributable to necessary engineering compromises.Theoretical considerations predict that angle diversity achieves most effective results at large scatter angles.Further confirmation of the effic
34、iency of angle diversity was obtained in trials in Japan at 1.8 GHz over a256 km path with 19 m diameter antennas (gain = 47.5 dB), where correlation coefficients of less than 0.4 were obtainedfor beam separations more than about 6 mrad.Rec. ITU-R F.1106 54.2 Considerations of transmission bandwidth
35、In estimating the effects of diversity on transmission bandwidth and quality, it is convenient to characterize thetransmission system by a network having amplitude/frequency and phase/frequency characteristics which vary in arandom manner with time.A theoretical evaluation of the linear amplitude di
36、spersion, z, can be made by assuming a Rayleigh distributionof the signal amplitude at the edge of the passband. The probability, z T) the intersymbol interference can only be combatted by usingspecial adaptive methods of reception.In practice, both short-term and long-term variations are observed i
37、n the multipath dispersion. Measurementsperformed in the United Kingdom indicate that for a median 2 value of 106 ns, the standard deviation of the long-termvariation in 2 taken from 92 s averages of 10 ms samples was 15 ns. The standard deviation of the short-term variationin 2 taken from the 10 ms
38、 samples was 50 ns. For the 124 km test link transmitting 2 048 kbit/s with 4-CPSK, themedian value of 2 / T was 0.1.5.3 Transmission methods for increasing the equivalent order of diversityWith a relatively low bit rate in the allocated frequency band, parallel or sequential multi-frequency signals
39、 canbe used for each transmitter.Rec. ITU-R F.1106 7Parallel multi-frequency signals are used to create a lattice of equidistant frequencies by means of additionalfrequency modulation, and on this basis it is possible to achieve additional reception diversity, for example, triplediversity reception.
40、Sequential multi-frequency (generally quadruple frequency) signals can be used for dual or quadruplefrequency-time diversity in combination with combined frequency and phase shift keying.A combination of known effective codes and bit interleaving for error decorrelation is considered as a variantof
41、the time diversity technique. However, this method of improving performance is accompanied by considerable delaysin the transmission of information.5.4 Adaptive equalizationThe use of adaptive equalization reduces intersymbol interferences and thereby increases the transmissioncapacity of digital tr
42、ans-horizon systems. Adaptive equalization permits the diversity inherent within multipathdispersion to be accommodated in receiver design thereby resulting in a predictable improvement in error performancefor a given value of .Ideally, adaptive equalization (linear equalization) in a multipath radi
43、o channel presupposes a cascadeconnection of the filter matched with the incoming signal and the transversal filter. However, in real conditions the inputfilter is matched with the transmitted signal so that, with the elimination of the influence of multi-symbol interference,the implicit diversity e
44、ffect cannot be adequately achieved.The use of decision-feedback is a non-linear method of signal processing and it can be used to compensate forthe intersymbol interference caused by precursor signal elements.Reception with evaluation of a discrete sequence by means of the Viterbi algorithm is cons
45、idered to be amethod for solving, with a maximum of a posteriori probability, the problem of evaluating the sequence of a timediscrete Markov process with a finite number of states. When account is taken of the matched filtering of the multipathsignal, the Viterbi algorithm is considered to be the m
46、ethod that offers optimum signal reception in a multipathcommunication channel.Spectral processing of a multipath signal involves extracting a number of signal frequency bands at thereceiving end when the signal spectrum is wider than the frequency correlation bandwidth of the communicationchannel.
47、Combining signal samples in the frequency range with specific weighting coefficients makes it possible notonly to eliminate intersymbol interference but also - with matched filtering of the output component signal - to achievean implicit diversity effect.Successful transmission of information rates
48、of up to 12.6 Mbit/s at 4.6 GHz using adaptive modems overtrans-horizon links has been reported.5.5 Comparative evaluation of effective modemsA comparative assessment of the basic effective modems used to implement the various transmission methodsdiscussed above is given in Table 1.To evaluate the n
49、oise immunity of the method, the ratio between the mean power of the received signal and thespectral noise density was determined for a given information bit rate with a bit error probability of 106. Quadruplespace-frequency diversity reception (two transmitters, two antennas) was considered in all cases. The signal-to-noiseratio corresponds to the equivalent diversity order. The parameter is determined, which characterizes the ratio betweenthe mean signal power and the spectral noise density per one bit of transmitted information. The parameter is
