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SCINTILLATION FADE AND ENHANCEMENT DURATIONSTATISTICS AT 20, 40 AND 50 GHz

Ali Savvaris, Constantinos N. Kassianides and Ifiok E. Otung

Radiowave Propagation and Systems Design Research UnitSchool of Electronics, University of GlamorganPontypridd CF37 1DL, United KingdomEmail: asavvari@glam.ac.uk

AbstractStatistics of observed durations of scintillation fadesand enhancements of the ITALSAT satellites at 20,40 and 50GHz are presented for various thresholdsignal levels, and their use in fade countermeasures isexamined. The analysis shows that most signalamplitude deviations from the mean level are of shortduration and do not depend on the threshold level.Distribution of fade durations at thresholds 0.1 and0.5dB were reasonably well approximated by thelognormal function.

Introduction

The saturation of C-band and the ever-increasingdemand for new services that require greaterbandwidth has led to the exploitation of higherfrequencies. The higher frequencies offer variousadvantages such as, increased bandwidth, smallerantennas, and smaller satellite footprint that givehigher EIRP and permit frequency reuse. The mainobstacle however is that they are subject to strongerpropagation degradation. The small size antennasemployed in VSAT and USAT systems significantlyreduce the cost of earth station terminals and alsoeliminate tracking requirements, but they lose themitigating effect of aperture averaging and henceexperience stronger scintillation [1].

Scintillations are rapid fluctuations in amplitude andphase of the received signal arising from fluctuationsin the atmospheric refractive index due to turbulence.Increasing the transmitted power or the receivingantenna diameter to provide an adequate fade marginis often not feasible in VSAT systems. An alternativescintillation countermeasure, which is beinginvestigated, is the use of Forward Error Correction(FEC) codes and adaptive modulation schemes.

Link budget engineers use annual and worst monthcumulative distribution functions to calculate the linkbudget for a new satellite communication system.The use of digital signal processing however requiresthe knowledge of the dynamics of troposphericscintillation, including the distribution of scintillationfade duration and inter-fade interval. Fade duration

statistics are particularly important for the design ofhigh frequency satellite systems. In such highfrequency systems, operating at a fixed small fademargin, the best way to mitigate propagationimpairments is by introducing adaptive techniques,adaptive fade countermeasure strategies. Fade andinterfade duration statistics provide the systemdesigner with useful information for evaluatingvarious mitigation techniques that will be employedto ensure a given system availability and quality ofservice. In this paper attention is focused on theanalysis of scintillation fades and enhancements at18.7, 39.6 and 49.5 GHz, which will be hereafterreferred to as the 20, 40 and 50GHz, respectively,using propagation data from the ITALSAT F1 andITALSAT F2 satellites.

Experiment and Analysis

ITALSAT was Italy's first operationalcommunication satellite launched on the 16th ofJanuary 1991 by an Ariane booster and stationed ingeostationary orbit at 13.2 degrees east. The designlife for the ITALSAT vehicle originally was only fiveyears, but ITALSAT F1 operated beyond its expectedlife, facilitated by the adoption of a propellant savingoption in which the North/South station keeping wasabandoned. As a consequence it became necessaryfor the beacon receivers to track the satellite position.Towards the end of 1997 the 50 GHz beacon receiverat Sparsholt was equipped with a tracking unit tocounter the problem. The same goal was attained forthe 40 GHz receiver in August 1998 [2].The data examined can be divided into two sets:

a) Data set 1: This contains data measurement fromthe ITALSAT F2 satellite, operating at 20 GHzand covering a 1-year period from September 99to August 2000.

b) Data set 2: This contains data measurement fromthe ITALSAT F1 satellite, operating at 40 and 50GHz and covering a 1-year period fromSeptember 96 to August 97.

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Both data sets were recorded at Sparsholt(51.0814°N, 1.3947°W), UK at a path elevation of30° and at a sampling rate of 1 Hz. Cassegrainantennas of diameters 1.22m for the 20 GHz and0.61m for both the 40 GHz and 50 GHz beaconswere employed. The propagation data together with arange of meteorological measurements recorded atthe same sampling rate were archived to a compactdisk once a month.

Pre-processing of the raw propagation data wascarried out. This involved visual inspection of graphsof the data to identify gaps and spurious samples, andhigh-pass filtering using a 6th order Butterworth filterwith a cut-off frequency of 0.04 Hz to extractscintillation [3].

Distribution of Scintillation Fade & Enhancement

In Figure 1, the observed annual cumulativedistributions of scintillation fades and enhancementsat 20, 40 and 50GHz are presented. It can be seenfrom this figure, that for 0.01% of the time the fadelevels at 20 and 50GHz are 0.8 dB and 1.5 dB,respectively. This represents an increase in thescintillation signal amplitude by a factor of 1.88.

Fade Duration

Fade duration D- at threshold level χT below themean level (= 0dB) of χ (scintillation amplitude) wastaken as the time interval over which the signal levelcontinuously fell below χT. Similarly, enhancementduration D+ at threshold level χT above 0dB wasobtained as the time interval over which the signallevel continuously exceeded χT.

Inter-fade duration or non-fade duration is thecomplement of the fade duration. It is defined as thecontinuous time over which the attenuation is lowerthan a given threshold value. Figure 2 gives thegraphical representation of this definition. A smallhysteresis of 0.005 dB was used in the detection ofthe signal levels in order to mask the effects ofsystem noise.

Results

The cumulative distribution function (cdf) at 20, 40and 50GHz of fades at selected threshold levels isshown Figure 3. Use has been made of a logarithmicscale for the durations, and for the probabilities, ascale such that a lognormal distribution is representedby a straight line. It was observed that that thedistribution of scintillation fade duration at thresholdlevels of 0.1dB and 0.5dB follows adequately alognormal distribution. However, at threshold levelsabove 0.8dB the small number of sample points did

not allow an accurate prediction, e.g. the number ofsample points corresponding to fades greater than1dB at 20GHz were only 389, during the entire 1-year period.

We note that the fade duration statistics from SIROmeasurements indicated a lognormal distribution [4]whereas the OLYMPUS campaign has suggested apower-law dependence for short fades (<20sec)which are generally the result of scintillation or noiseand a lognormal distribution for longer fades. Basedon the results obtained from Figure 3 it would appearthat short fades tend also to follow a lognormaldistribution. Similar conclusions were obtained fromthe analysis of enhancement duration statistics, whichwere on average of equal magnitude.

Furthermore, the measured annual mean fade andenhancement duration displays no dependence onthreshold level, see Figure 4. The mean duration ofboth fades and enhancement are on average of equalduration, lasting no longer than 1.4 seconds. Forexample, at 1dB threshold level at 40GHz the meanfade duration is ~1.125 seconds and at 2dB thresholdlevel the mean duration is ~1.05 seconds.

However, at higher frequencies the observedpercentage of long-fade duration exceeding a specificthreshold is greater than that observed at a lowerfrequency. For example at 20GHz the percentage of0.5dB fades lasting up to 8 seconds is ~ 0.01%,whereas at 50GHz the percentage is ~ 0.07%.

System design – fade countermeasures

The obvious need of fade duration statistics is todimension accurately the common pool for sharedresource fade countermeasures. Such systems allocateon demand the back-up resource to counteract deepfades encountered on any particular link. Toaccommodate any particular network, it is necessaryto know the expected duration over which theresource is likely to be engaged. Adaptive fadecountermeasure (FCM) systems will also rely on theknowledge of fade duration statistics to estimatebeforehand the length of time over which each levelof the FCM will be in use and to avoid an undesirablefrequent switching in levels of protection.

In order to overcome the problem of frequentswitching of modes in an adaptive system, a fixedfade margin can be introduced. For example, Figure 3shows that fades lasting one second are the mostcommon. Take for example the 50 GHz case. Forthresholds of 0.1, 0.5, 0.8 and 1.0 dB, the percentagesof fades lasting one second are respectively 25, 12,11 and 10 %. Fades that last one second above acertain threshold value are not likely to exit muchthat level. So by introducing a fixed margin slightly

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higher to that level these one-second fades willdisappear, and therefore the need for the adaptivesystem to adapt to these, so switching states regularlywill be avoided. For example, introducing a 0.5 dBmargin the one-second fades drop from 25% that athreshold of 0.1 dB gives, to 12%, meaning that thesystem has to deal with fewer one-second changes.

In a system that uses adaptive modulation as fadecountermeasure, changing modulation methoddepending on the fading present on the link, a generalidea of the total time that each modulation methodwill be used, say in a year can be determined. Thiswill give the throughput of the system in a year.Higher modulation techniques will transmit moreinformation for a particular time than lowermodulation methods. This is shown in Figure 5 for anadaptive system using three modulation methods. Fades below A1, are covered by a fixed fade marginand the system operates with the highest throughputusing the highest M-ary modulation scheme. A fadebelow A2 will cause the adaptive system to use alower modulation method and a fade above A3 willcause the adaptive system to use the lowestmodulation method available. By knowing the fadeduration above A3, the time that the lowestmodulation method will be in use will be determined.The time of the highest modulation method can bedetermined also by knowing the time that fades arebelow A2. The distance between the three levels canbe set to avoid frequent switching betweenmodulation levels while ensuring that the systemachieves the specified availability and quality ofservice.

Conclusion

This paper presented the statistics of scintillationfades and enhancements using propagation dataobtained from the ITALSAT satellites at 20, 40 and50GHz. The analysis showed that the signalamplitude deviations from the mean level (=0dB)were predominantly short lived with duration lastingless than 20second. The influence of these durationstatistics on the design of FCM measures based onadaptive modulation was also outlined.

References

[1] Otung I. E., 1998, “Accurate prediction ofscintillation degradation applicable tosatellite communication system design”,EPSRC research proposal.

[2] Report of the RCRU at RAL for the year1997, 1997 annual report, RCRU, RAL

[3] Savvaris A., Otung I.E., ‘PreliminaryPreprocessing of ITALSAT Data at 20, 40and 50 GHz’, URSI 2000 Symposium.

[4] Dintelmann F., “Analysis of 11GHz slantpath fade duration and fade slope”,Electronics letters, 1981, 17, (7), pp.267-268.

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0 0.5 1 1.5 2 2.510

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Figure 1. Yearly cumulative distribution function of fades and enhancements at 20, 40 and 50 GHz.

Time

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Episode Episode

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Figure 2. Definition of episode and inter-episode

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Figure 3. Starting from top to bottom the measured cumulative distribution of fade duration at various signal threshold levels at 20, 40 and 50GHz, respectively.

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0 0.5 1 1.5 2 2.5 3 3.5 41

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Figure 4. Measured annual mean fade and enhancement duration at various signal threshold levels.

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A3

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Attenuation A3 > A2 > A1

Figure 5. Adaptive modulation switching levels