fade slope on 10 to 30 ghz earth-space communication links-measurements and modelling

Fade slope on 10 to 30GHz earth-space communication links - measurements and modelling

B. Nelson W.L. Stutzman

Indexing terms: Ruin uttenuution, Sutellite eomrnunicution systems

Abstract: One year of atmospheric propagation data were collected and analysed at Blacksburg, Virginia, USA, using the Olympus satellite 12.5, 20 and 30GHz beacons. The results are in the form of statistics which communication system designers can use to quantify precipitation effects on Ka-band links. The paper presents the results for rain fade dynamics including fade slope statistics as well as a simple model for estimating fade slope statistics for any frequency in the range 10 to 30GHz.

1 Introduction

The Satellite Communications Group at Virginia Poly- technic Institute and State University (Virginia Tech) under NASAIJPL sponsorship conducted a measure- ment program to investigate the atmospheric effects of radiowave propagation in the Ku- and Ka-bands. The 12.5, 19.77 and 29.66 GHz beacons from the European Space Agency's OLYMPUS satellite were received in Blacksburg, Virginia, at an elevation angle of 14". Bea- con signal strengths and sky noise were measured at each frequency. In addition to Virginia Tech's partici- pation in North America, there were many European experiment sites [l]. Data were collected in Blacksburg from August 1990 to August 1992. One complete year of attenuation statistics, as well as a general description of the experiment, was reported by Stutzman et al. [2]. This paper presents results of fade slope measurements and modelling.

Atmospheric effects on earth-space microwaveimilli- metre-wave signals include scintillation, depolarisation and attenuation. For communication links operating above about 15GHz and above about 10" elevation angle, system performance will be limited by fading. Such fading is due to precipitation, primarily in the form of rain. Not only are the statistics of rain fading 0 IEE, 1996 IEE Proceedings online no. 19960587 Paper first received 31st August 1995 and in final revised form 15th May 1996 B. Nelson is with Hughes Network Systems, 11717 Exploration Lane, Germantown, MD 20876, USA and formerly with the Virginia Polytech- nic lnstitute and State University, Bradley Department of Electrical Engi- neering, Blacksburg, VA 2406141 1 1, USA W. L. Stutnnan is with the Virginia Polvtechnic Institute and State Uni- versity, Bradley Department of Electrical Enyneenng, Blacksburg, VA 240614111, USA

of interest to the system designer for link power budget evaluation, but rain fade dynamics must be understood for signal performance estimation. In fact, adaptive rain fade compensation techniques will be incorporated into future satellite communication systems [3]. Rain compensation algorithms for operational systems are based on fade slope data. In the typical scenario, as rain intensity increases along the uplink path, earth sta- tion transmitter power can be increased to overcome fading. This requires an understanding of the frequency of occurrence for given fade levels of the time rate of change of fading (fade slope), as well as the relationship between fade depth and fade slope. Secondary statistics such as fade slope are not derivable from primary rain fade statistics and must be extracted from the time-sequence data.

Rucker [4] reported on the attenuation dependence of fade slope, the frequency dependence of fade slope, and statistical rade slope ratios using fade slope results from an experiment that used 12, 20 and 30GHz beacons from the Olympus satellite. Howell et al. [5] conducted experiments using the 12, 20 and 30GHz beacons from the Olympus satellite and the 11.6 and 11.8GHz beacons from the OTS satellite, and reported fade slope occurrence distributions. Howell et ul. used a technique to calculate fade slope that is sensitive to large signal fluctuations and they calculated fade slope at only three attenuation levels. Dintelman [6] performed experiments with the OTS satellite and reported a cumulative fade slope for 107 fades at the 5dB attenuation level. Mattricciani [7] performed an experiment using 11.6GHz data from the Sirio satellite and reported on the similarity of log-nolrmal distributions between positive and negative fade slopes and on the attenuation dependence of fade slope. Webber [SI collected radiometer data at 13 GHz and reported similar log-normal distributions for both poisitive and negative fade slopes.

This paper reports fade slope results from the Olym- pus satellite beacon data at 12, 20 and 30GHz. Results similar to those in [448] were found in this research, in addition to other results. All effects except those due to rain were removed from the data set used for analysis. Fade slope calcula.tions were performed for all attenuation levels measured in rain at 12. 20 and 30GHz. Instantaneous fade slope ratio results are reported in this paper. A fade slope occurrence prediction model is also presented.

2

Separate terminals were used to receive the 12.5, 19.77

Experiment description and data processing

353 IEE Proc.-Microw. Antennus Propug., Vol. 143, No. 4, August 1996

and 29.66GHz beacons, referred to as 12, 20 and 30GHz. Each terminal had a beacon receiver and a total power radiometer. The Olympus beacons were coherent and all the Virginia Tech receivers were frequency locked to the 12GHz receiver because it experi- enced much less fading than that at 20 or 30GHz. Each receiver had a low noise amplifier followed by a mixer- preamplifier with an output frequency at 1120MHz and successive downconversions to 70 MHz and 10kHz. The l0kHz carrier was sampled at a 1 kHz rate by a 12 bit A/D converter and filtered with a 3Hz bandwidth FIR filter which had 1116 coefficients stored in ROM. The filter output was recorded at a 10Hz rate [9]. 111 each 0.1 s sample period 100 new samples of the 1 kHz stream entered the filter and the 100 oldest samples were dumped. The 12.5GHz system included an analogue receiver section to track the carrier frequency and maintain the signal within a 3Hz window.

The radiometer output was converted to sky temper- ature and used to set zero reference levels for the beacon signals [lo]. Unusable data and diurnal beacon level variations were removed from the data in a preprocessing stage. Preprocessing was followed by an analysis stage to produce several quantities. Attenua- tion with respect to free space (AFS) is the received signal relative to that if it had propagated in a vacuum. AFS includes effects of hydrometeors, clouds and gases, as well as scintillation. Attenuation with respect to clear air (ACA) is the received signal relative to that if it had propagated in clear air. Effects included in ACA are those from hydrometeors and clouds, as well as scintillation. Gaseous absorption is excluded.

3 Fadeslope

Fade slope is the rate of change of attenuation in dB/s. Since fade slope is a derivative, the absolute attenuation level is unimportant and either ACA or AFS can be used. We used AFS to determine all fade slope statistics. Attenuation data are first block averaged to remove short term signal fluctuations, primarily due to scintillation activity. Block average attenuation is calculated from 0. l s attenuation samples; the ith sample from the beginning of the day (0O:OO:OO Universal Time, UT) is

where AFSj is the instantaneous value of attenuation at each 0.1 s interval.

Fade slope is calculated at each 0.1s data sample. Fade slope is defined as the difference between attenuation values in dB associated with two data samples divided by the time between the two samples. The attenuation values are block averaged values calculated using eqn. 1. The first block average attenuation value is calculated using the 0.1s samples of attenuation within the 10s immediately before the 0.1s sample at which the fade slope is calculated. The second block average attenuation value is calculated using the 0.1 s samples of attenuation within the 10s immediately after the 0.1s sample at which the fade slope is calculated. The fade slope is calculated at an attenuation level that is defined by the 10s block average attenuation centred at the 0.1 s sample at which the fade slope is calculated.

The result is the block average fade slope (FSB):

FSB,(AFS, ) = - (AFSz+50 - X F S - s o ) [dB/s]

(2) (:o) -

Block averages have been used by others [4, 111 to reduce scintillation effects. Fade slope was computed for attenuation (AFS) thresholds ranging from -8 to 39dB in 1dB bins. Fade slopes were sorted into bins of width 0.05dBls ranging from -1.25 to +1.25dB/s.

The representative rain event from 14 May 1991 in Fig. 1 shows measured attenuation and fade slope at 30GHz. This 45 min data set was smoothed using 3 min block averages. Of course, positive (negative) fade slope values indicate increasing (decreasing) attenuation.

3 0 , ,006

-0 06 i 0 4 50 59 6a 77 a6 95

minutes past 17:OO UT Fig. 1 the representative rain event of14 May, 1991 FSB and AFS were smoothed using 3 min block averages

~ attenuation (AFS), ~ ~ ~ fade slope (FSB)

Fade slope and attenuation at 30 GHz us a function of time fou

4 Results

The Olympus satellite was out of its assigned 19"W orbit location and no beacon data were collected during June, July and August of 1991. The summer months of 1992 were substituted for the missing months. Our analysis year, then, consists of January- May 1991, September-December 1991, and June- August 1992.

1 -0.3 -0.2 -0.1 0 0.1 0.2 0.3

fade slope bins, d B l s Fig. 2 Cumulative fade slope statistics of attenwltion with respect to free space (AFS) for the analysis year of Jun-May, SepDec 1991 and Jun- Aug 1992 ut I2GHz AFS = (i) ldB, (ii) 3dB, (iii) 5dB, (iv) lOdB

Figs. 2 4 show the fade slope statistics for various attenuation levels for all three beacon frequencies expressed as a percentage of system uptime (the total time of accurate data). From Fig. 2 one could read, for example, that for a frequency of 12GHz and an attenuation level of ldB, the fade slope lies between -0.20

354 IEE Proc -Microw Antennas Propag, Vol 143, No. 4, Auxust 1996

and -O.lOdB/s for 0.07 % of the time. It is obvious from Figs. 2 4 that positive and negative fade slopes are nearly equally distributed, dispelling the myth that positive fade slopes occur more frequently than negative fade slopes because the leading edge of a convec- tive rain cell is sharper than the trailing edge. This nearly equal frequency of occurrence of positive and negative fade slopes has been noted in other experiments [4, 6-81.

100 j I a .__^___

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 fade slope bins,dBls

Fig. 3 Cumulative fade slope statistics of attenuation with respect to free space (AFS) for the analysis year of Jan-May, SepDec 1991 and Jun- Aug 1992 at 2OGHz AFS = (i) IdB, (ii) 3dB, (iii) 5dB, (iv) IOdB, (v) 15dB, (vi) 20dB

..........

. . . . . . . . ........ ........

0.3 0.2 0.1 0 0.1 0.2 0.3

Fig. 4 Cumulative fade slope statistics of attenuation with respect to free space (AFS) f o r the analysis year of Jan-May, SepDec I991 and Jun- Aug 1992 at 3OGHz AFS = (i) ldB, (ii) 3dB, (iii) 5dB, (iv) lOdB

fade slope bins,dBls

1 .21 I

U d

Y-

0.01 0.1 1 10 100 percentage of up-time in rain

Fig.5 SepDec I991 and Jun-Aug, I992 at 12, 20 and 30 GHz

Cumulative fade slope statistics for the analysis year of Jan-May,

Note from Figs. 2 4 that at a given fade level the occurrence of a fade slope value increases with frequency. This effect is revealed more clearly by examin-

IEE Proc.-Microw. Antennas Propag., Vol. 143, No. 4, August 1996

ing data during rain. The presence of a rain fade is taken to be when AFS at 12GHz is 2 1dB and when AFS at 20 and 30GHz is 2 3dB. Attenuation rarely exceeds this level when rain is not present along the path. The fade slope statistics for the analysis year rain time in Fig. 5 clearly shows the increase in fade slope with frequency. If time dynamic considerations are ignored, it is easy to show that in theory fade slope should scale with frequency in the same way as attenuation. First, the ratio of attenuations is primarily dependent only on the ratios of frequencies as follows:

(3)

where A , and A, are the attenuations at the upper and lower frequencies f, and fL. In this frequency range n - 1.9. It also turns out that attenuation based on the statistics of attenuation also applies to the instantaneous attenuation ratio [ 121. The instantaneous attenuation ratio can then be used to establish the ratio of fade slopes:

(4)

FSdt) and FSL(t) are the instantaneous values of fade slope at the upper and lower frequencies, respectively. Fade slope values are found at discrete time samples using finite time differences:

The result in eqn. 5 shows that the fade slope should scale with frequency in the same way as fades. This would be a valuable result, because frequency scaling of attenuation is fairly well understood. However, if this scaling law is tested with the fade slope data of Fig. 5, it is found to underpredicf the fade slope at a higher frequency. We conclude that the fade slope cannot be scaled in the same manner as attenuation and the complex behaviour of a rain fade requires direct measurements to determine fade dynamics. That fade slope and fade do not scale with frequency in the same way is demonstrated in Fig. 6, which shows data from a May 1991 rain event. 'The instantaneous values of 3 min block averaged fade slope ratio RFSB and attenuation ratio RA are not close.

-8-1 0 5 10 15 20 2 5 30 35 40 45

minutes past 17:OO (UT) Fig.6 2OGHz for a rain event on 14 May, 1991 __ RFSB(t), - - - RA(t)

Fade slope ratio (RFSB) and attenuation ratio (RA) for 30 to

Fade slope is not only frequency dependent but is also fade level dependent. That is, fade slope increases with fade depth. Figs. 7-9 show the occurrence of the

355

absolute values of fade slope for the analysis year with the data pooled into three fade depth groups. AFS at 12GHz was between 1 and 5dB for 62 898 min, between S and lOdB for SO2 min of the data set and between 10 and 15dB for 77 min of the data set in Fig. 7. Similar results for the 20 and 30GHz data are displayed in Figs. 8 and 9. The increase of fade slope

fade level is apparent.

1 0 *r----- 0 . 4 ........................................................................

0 2 ..........................................................................................................

0 0 01 01 1 10 100

percentage of time in coincident AFS range Fig. 7 using individual time buses for dqferent atrenuation levels ut I2 GHr (I) l0dB < AFS < 15dB; 77 min (ii) 5dB < AFS < 10dB; 502 min (iii) IdB < AFS e 5dB; 62 898 min

Attenuation dependence cdfade slope displuyed in percentage time

fade level. Thus, the deeper a fade is the more rapidly is the fade changing.

n 7, " ' I I

...................................................

Q 0

a U

10-6 10-5 10-4 10-3 10-2 10-1 I io 100 percentage of common time base

Attenuation dependence of ade slope displayed in percentage Fig. 10 time using U common time base for d d r e n t attenuation levels at I2GHz (I) lOdB < AFS < l5dB (11) 5dB c AFS < IOdB (m) IdB < AFS < 5dB

1 0 ,

10-5 10-4 10-3 10-2 10-1 1 IO 100 percentage of common time base

Fig. 11 time using a common time base for d d e n t attenuation levels at 20GHz (i) 20dB < AFS e 30dB (ii) lOdB < AFS < 20dB (iii) 3dB < AFS < lOdB

Attenuation dependence of ade slope displayed in percentage

0.01 01 1 10 100 percentage of time in coincident AFS range

Attenuation dependence of jude slope displayed in percentage time Fig. 8 using individual time bases jb r dgferent attenuation levels at 20GHz (i) 20dB < AFS < 30dB; 151 min (ii) l0dB < AFS < 20dB; 973 min (iii) 3dB c AFS e 10dB; 27 513 rnin

1 .21 I

m7 IO-^ IO-& 10-3 IO-* 10-1 I IO 100 percentage of common time base

Fig. 12 rime using ii common time base for d d e n t attenuation levels ut 30GHz (I) 20dB < AFS < 30dB (11) lOdB i AFS < 20dB (111) 3dB < AFS < lOdB

Attenuation dependence of ade slope displayed in percentage

6.01 01 1 10 100 percentage of time in coincident AFS range

Fig. 9 using individual time bases for different attenuation levels at 30GHz

Attenuation dependence of fade slope displayed in percentage time

(i) 28dB < AFS < 30dB; 565 min"" (ii) IOdB < AFS < 20dB; 3320 rnin (ui) 3dB < AFS < IOdB; 32 387 min

The same results are displayed in Figs. 10-12 for a common time base. These results show that for fade slopes above about 0.2dBls fade slope increases with

356

5 Fade slope model

For communication system applications it is often important to quantify the statistics of fade slope. A model was developed through empirical fitting to the data of Figs. 2 4 . The model is of the following log- normal form:

P ( F S B ) = a. eb l F S B l (6) where P is the percentage of the time rain occurs along

JEE Proc -Microw Antennas Propag , Vol 143 No 4, August 1996

the path that the fade slope is in the bin centred on block-averaged fade slope value FSB. a and b are empirical parameters that depend on attenuation level and frequency, and are given by

AT, f ) = __ a ( A T , ”1 - ( f - 12) + a(AT , 12) 20 - 12

a(&, 30) - AT, 20) for AT > 3,12 5 f 5 20 (7a)

(7b)

(7c)

( 7 4

(f - 20) + a(&, 20) 30 - 20 a(AT , f ) =

for AT > 3,20 5 f 5 30 AT, 20) - A AT, 12)

20 - 12

A AT, 30) - AT, 20)

(f - 12) + AT, 12) b ( A T , f ) =

for AT > 3,12 5 f 5 20

(f - 20) + b(AT , 20) b ( A T > f ) = SO - 2o

for AT > 3,20 5 f 5 30 where A , is the threshold attenuation (AFS) level in dB and f is any frequency in the range 12-30GHz. The constants in eqn. 7 are a , ( ~ T , 12) = 52.93~(-1.45”~+0.07AZT -0.0013.4%)

a ( ~ T , 20) = 717,71e(-1.07A~+0.038A$-0.00056A$)

a(AT, 30) = 4 0 4 . 2 2 F ( - 1 . 0 5 A 1 + 0 ~ 0 6 3 A ~ - 0 . 0 0 1 8 A ~ )

UP AT, 12) = -0.0315Ag + 1.168A;- - 14.94A~ + 72.72

(gal

(8b)

(8c)

(84 b(Ar, 20) = 0.0202AF - 0.3149Ag - 3 . 1 0 5 A ~ + 61.62

AT, 30) = 0.0134A$ - 0.2647A$ - 1 . 1 7 8 A ~ + 47.82 ( 8 f

The model in eqn. 8 uses 3rd-order equations and predicts fade slope occurrence during rain within 5% of measured values at low attenuation levels. The prediction accuracy is even better at higher attenuation levels. The 7th-order equations in eqn. 8 yield fade slope occurrence predictions during rain within 1% of measured values.

6 Conclusions

The measured fade slope statistics of Figs. 2-4 suggest a symmetry between the rate of fade increase and the sate of fade decrease. Other experimenters have also noted the symmetry between positive and negative fade slopes for similar experiments conducted in the Ku and Ka frequency bands [4, 6-81. Although it appears to be universally true that the statistics of positive and negative fade slopes are symmetric, it cannot be inferred that the same is true instantaneously. That is, individual rain storms may demonstrate unequal occurrences of positive and negative fade slopes, primarily due to differences at the start and finish of a storm. However, when many events are considered, symmetry exists.

Our measured results demonstrated that fade slope increases with frequency for a fixed occurrence level, as

did Ruckes [4]. Simple tlheory suggests that fade slope should scale with frequency in the same fashion as attenuation; see eqns. 7-9. However, we conclude from the experiment that there is no relationship between fade slope ratio and attenuation ratio in real time.

Our measured results also suggest that, as the attenuation level increases, the occurrence of large fade slope magnitudes increases. Rucker [4], Matricciani [7], and Webber [SI also reported a dependence of fade slope on attenuation. If fade slope occurrence increases with increasing attenuation, one could possibly infer that individual rain fades have higher fa de slope magnitudes with deeper fading. However, this depends on many parameters, such as geographic location of the earth terminal, propagation path orientation, weather pat- terns and atmospheric conditions. Fade slope statistics, in general, cannot be used to determine the real time behaviour of fades. We examined instantaneous fade events (Fig. 1 is an example) and we could not extract a correlation between fade slope and fade depth on an instantaneous basis.

The log-normal empirical model of eqns. 6-8 is pro- posed for estimating the percentage time occurrence of a fade slope level during rain. Inputs to the model are fade slope bin, attenuation level between 3 and 15dB, and frequency between 12 and 30GHz.

7 Acknowledgments

The support of the Jet Propulsion Laboratory and NASA is gratefully acknowledged.

8

1

2

3

4

5

6

7

8

9

References

ARBESSER-RASTBURG, B R , dnd BRUSSAARD, G ‘Prop- agation iescarch in Europe using the OLYMPUS satellite’, IEEE Proc , 1993, 10, pp 865-875 STUTZMAN, W L , PRATT, T , SAFAAI-JAZI, A , REMAK- LUS, P W , LASTER, J , NELSON, B , and AJAZ, H ‘Results from the Vireinia Tech orooaeation exrienment usine the Olvm- pus satellite r2, 20, and 30 b& beacod.,’, IEEE Trans Anteinus Propug, 1995, 43, pp 5 4 6 2 BRUSSAARD, G . and ROGERS, D V ‘Prowzation considerations in satellite communication systems’, I E E E h c . , 1990, 78,

RUCKER, F.: ‘Frequency and attenuat~on dependent fade slope statistics’, Electron. Lett., 1993, 29, pp. 744-746 HOWELL, R.G., HARRIS, J.W., and MEHLER, M.: ‘Satellite co-polar measurements at BT Laboratories’, BT Technol. J. , 1992, 10, pp. 34-51 DINTELMAN, F.: ‘Analysis of 1 1 GHz slant path fade duration and fade slope’, Electron. Lett.. 1987, 17, pp. 267-268 MATTRICCIANI, E.: ‘Rate of change omf signal attenualion from SIRIO at 11.6 GHz’, Electvon. Lett., 1981, 17, pp. 139-141 WEBBER. R.V.. and SCHLESAK. J.J.: ‘Fade rates a1 13 GHz

pp. 1275-1281

on Earth-space paths’, Ann Telecomniun , 1986, 41, pp 562-567 STUTZMAN, W.L., and REMAKLUS, P W ‘The effect of sample rate on propagation measurements’, Microw. Opt. Tech- nol. Lett., 1994, 7, pp. 404406

10 STUTZMAN, W.L., HAIDARA, F., and REMAKLUS, P.W.: ‘Correction of satellite beacon propagation data using radiometer measurements’, IEE Proc., Microw. Antcnnas Propag., 1994, 141, pp. 62-64

11 UPTON, S., and LARSEN, J.R.: ‘Analysis of fading charactcris- tics for communications applications’. Proceedings of O l y n p s utilisation conference, Vienna, April 1989, pp. 247-250

12 LASTER, J.D., and STUTZMAN, W.L..: ‘Frequency scaling of rain attenuation for satellite communication links’, IEEE Trans. Antennas Propug., 1995, 43, pp. 1207-12’16

IEE Proc -Microw. Antennus Propag.. Vol. 143, No. 4, August 1996 357

fade slope on 10 to 30 ghz earth-space communication links-measurements and modelling

Documents