Invited Paper Experimental study of rain induced effects on microwave propagation at 2 and 3 GHz LS Hudiara Department of Electronics Technology, Guru Nanak Dev University, Amritsar, India hudiarais@yahoo.com ABSTRACT Rain induced attenuation degrades the performance of communication systems. Thus the knowledge of rain-induced attenuation at the frequency of operation is necessary to design a reliable communication system at a particular location. Attenuation of radio waves by rain depends on the shape, size and distribution of raindrops and rate of rainfall (mmlhr). Hence this research work has been undertaken to study rain-induced effects on microwave propagation at 2 and 3 GHz at Amritsar for planning future earth-space communication links at these frequencies. Present research work envisaged collection of propagation data over line of sight links, radiometric data and meteorological data in the form of rainfall and raindrop size distribution. The data thus collected, using proposed experimental setup, have been analyzed for developing empirical models which are required to develop a statistical model for the prediction of rain induced slant path attenuation for our location which will help us to design an efficient and reliable terrestrial and satellite communication systems which will work for 99.99 % of time. Keywords: Radiometer, Rain attenuation, microwave propagation, Slant path attenuation, reduction factor, rain fall rate and microwave communication link. 1. INTRODUCTION Microwave links operating at frequencies above about 1 GHz are severely affected by the presence of rain over the link path, more so, in the tropical region because of the high intensity of rain and large raindrops. The efficient design of reliable communication link requires the knowledge of rain fade margin to be provided in the system design to overcome the losses in the signal strength due to rain over the path [1]. Hence there is a need for the development of reliable and accurate propagation model for attenuation prediction at the operating frequency of the system. Currently existing prediction methods based on experimental observation are reasonably accurate for temperate latitudes [2]. When these models are applied to tropical, high rainfall zones, they give unsatisfactory results. Therefore to develop accurate rain attenuation prediction models at 2 and 3GHz an experimental study was undertaken at Guru Nanak Dev University, Amritsar, (India). A line of sight link at 19.4 and 28.75 GHz was established at, Amritsar (31 36'E 74 54'N), which is 229.4 m above sea level. The length of LOS link was 2.29Km. A zenith looking radiometer-operating atl9.9ghz was also installed for radiometric measurements. All the measuring instruments were housed in air-conditioned huts. Tipping bucket rain gauges have been used for measuring rainfall and distrometer has been used for measuring the size and number of raindrops falling over a particular time of interval. Following data were collected over a period of two years for developing various empirical models. 1. Rain attenuation data at 19.4 and 28.75 GHz over LOS link. 2. Radiometric data using zenith looking radiometer. 3. Meterological data in the form of rainfall rate and rain drop size distribution. Empirical models based on experimental observation have been compared with the existing models highlighting point of agreement and disagreement. These models will be extremely useful in predicting various rain induced effects on the propagation of microwaves at 2 and 3 GHz, which can be taken care off while designing a microwave communication link for terrestrial or space to earth- path at these frequencies. 35 Microwave and Optical Technology 23, edited by Jaromír Pištora, Kamil Postava, Miroslav Hrabovský, Banmali S. Rawat, Proceedings of SPIE Vol. 5445 (SPIE, Bellingham, WA, 24) 277-786X/4/$15 doi: 1.1117/12.56777
2. ANALYSIS OF EXPERIMENTAL DATA The experimental data were collected for a period of two years, during 2 and 21. Data were collected during winter and monsoon rains on round-the-clock basis. Data on rainfall rate and raindrop size collected using co-located rain gauges and Distrometer have been analyzed for annual rain rate (R) statistics and rain drop size distribution (RDSD) respectively. Raindrop size data have also been analyzed for specific attenuation (a db/km). Rainfall rate and raindrop sizes distribution are the two important parameters required for rain attenuation data analysis. Rain attenuation data collected over line of sight links at 2 and 3 GHz have been analyzed for cumulative attenuation statistics and path length reduction factor (r). Radiometric data, using zenith looking radiometer were also collected to predict zenith attenuation statistics and effective rain height (HR). Finally an algorithm has been developed for predicting slant path attenuation at 2 & 3 GHz. 2.1 Rain rate statistics The point rainfall intensity was measured with tipping bucket rain gauge, which usually provides good approximation to the instantaneous rain rates. The rain gauge uses a 12 inches diameter orifice and a tipping bucket mechanism coupled to a mercury switch. The buckets are calibrated to make one tip for each.254mm of rainfall. As one-bucket fills and tips, the second bucket starts collecting water. At the time oftip a magnet moves and momentarily closes the mercury switch, which is electrically connected to event marker of the stripline chart recorder where each event is recorded. The time between the two tips gives rate of rainfall. The rain rate (mm/hr) at a particular instant is calculated, by measuring the distance between the two tips, as follow: R (mm/hr) = (.254 x)/ d (1) where d is the distance in mm, between the two tips and x is the speed ofthe chart in mm/hr. The point rainfall data for the winter and monsoon seasons for the year 2 & 21 have been analyzed for the cumulative rain statistics for the whole year. Table 1 summarizes the data on rate of rainfall exceeding various percentage of time for rain zone L (which includes Amritsar) as given by ITU-R [3] with measured values ofrain rate exceeding same percentage of time. Table 1 Rain Climate Zone L and measured Rain Intensity exceeded (mm/hr) % time rain rate exceeded Measured Rain rate (mm /hr) 2 21 ITLJ-R (L).1 168 11.5 15.3 146 88.5 15.1 89.5 62 6.3 41 39.7 33.5 29 29.5 24.8 19 16.3 17.1 15.5 12.6 15.3 7.5 2 7.5 4.75 4 Comparison of table 1 shows year-to-year variability of the rain rate statistics. It has been observed from table 1 that there is a wide difference between the measured values of rate of rainfall exceeding small percentages of time as compared with ITU-R values. Therefore, the applications of ITU-R values, for rain rate, are likely to give erroneous results when applied for this location. Proc. of SPIE Vol. 5445 351
2.2 Rain-induced specific attenuation Measurements on rain attenuation on terrestrial and space to earth path and the rate of rain fall by earlier observations [4] led to the following relation between specific attenuation a (db/km) and rate ofrain fall R (mmlhr) as a=arb (2) where a & b are functions of frequency, rain temperature, refractive index ofraindrops and RDSD. In the present study Medhurst technique has been adopted for the evaluation of specific attenuation based on rain drop size data collected by distrometer. The method is based on the proportion of the total volume of water reaching the ground, which consists of drops of different diameter interval D1, and the attenuation caused by one drop per cubic cm with uniform drop diameter D for each rain rate. The specific attenuation [5] canbe written as a = P ( dd1 11.885 x 1 6 V D3 ) pd1 db/km (3) Di where P is the rain rate (mmlhr), ddi is the loss/km for a rain concentration of one drop per cubic cm with a uniform drop diameter of D (cm). pd1 is the proportion of the total volume of water reaching the ground consisting of drops whose diameters fall in the interval centered on D cm. Specific attenuation at 2 and 3 GHz have been evaluated at different rain rates by using above equation. It is worth noting here that the rain rate is based on 1-minute integration time data collected by distrometer. An empirical relation obtained from the regression analysis over specific attenuation values calculated at various rain rates has resulted in the following models 2O O.71R''154 (4) a3 O.186R' 478 (5) 3. ANALYSIS OF RADIOMETRIC DATA The sky noise being received by the radiometer and recorded on strip line chart in terms of voltage was converted to antenna temperature, Ta using calibration curves obtained for the radiometer using hot and cold loads. The antenna temperature is further converted in to excess path attenuation values. The path attenuation, Ar, can be found using [6] Ar =1 log (Tm Tcs) 1 (Tm Ta) db (6) where Tm is the effective medium temperature, Tcs is the cosmic noise temperature (3 K) and Ta is the radiometric antenna temperature. The excess path attenuation due to rain over the attenuation value under the clear sky condition is found by measuring the sky noise temperature (Antenna temperature), Ta' and Ta" under the clear sky and rainy conditions respectively. Therefore the excess path attenuation due to rain is given by Ar =1 log (Tm Ta')I ( Tm Ta") db (7) In the present work the medium temperature, Tm, has been taken as 295 k for 1% of time, a reasonable average of measured surface temperature for rainy period ofobservation [8]. For percentage times above 1%, 285 k has been taken to calculate the zenith path attenuation during fogy days and in clear sky. Since scattering losses are assumed negligible, the attenuation that has been calculated from the zenith sky noise temperature measurement only takes into account nonscattering losses that are mainly due to absorption of energy by raindrops. Using equations (7), the zenith attenuation values have been calculated from measured sky noise temperature at various rain rates. Seventy-five rainy days have been taken from the two-year data to determine the relationship between attenuation and rain rate as shown in fig. 1. A regression analysis of the measured data yields the following best-fit curve equation. Ar =.4873 R 798 (8) 352 Proc. of SPIE Vol. 5445
On the basis of results shown in fig. 1, it is concluded that attenuation up to approximately 1 8 db can be calculated accurately from sky noise measured by radiometer at rain up to rain rates of 11 mm/hr. Zenith attenuation is well correlated with rain rate and can be estimated from regression analysis which has been obtained from the scattered values of measured zenith attenuation. 25 Rain Rain Rain height intensity attenuation km mm/hr db 11.5 62 39.7 29.5 16.3 2.123 12.741 8.956 7.816 4.43 1.7114 2.256 2.383 2.5199 2.9844 12.6 3.6138 3.2262 2.843 5.526.75.3881 7.3511 Average 3.4559 :. 2 C C.C a- C w N 1 2 3 4 5 6 7 8 9 1 11 Rain rate (mm/hr) Table 2- Values ofattenuation and rain height over Amritsar for the year 21 at 19.9 GHz Fig. 1 Scatter plot ofzenith Path Attenuation verses rain rate 3.1 Effective rain height Effective rain height is one of the parameters for the prediction of slant-path attenuation. The simple vertical structure assumes that rainfall is uniform from the ground to the "rain height" HR. The effective rain height at various rain rates R (mm/hr) has been calculated using the experimentally measured Zenith path attenuation and specific attenuation at a given rain rate using the relation given below Hr = Ar/ ar (9) where Ar and a are the total zenith attenuation and specific attenuation due to rain at rain rate R (mm/hr). The dependence of effective rain height on rain rate is also illustrated in Table 2. It has been found that the average value of the effective rain height estimated from the measured data is found to be 3 km during year 2 & 3.45 km during year 21 at Amritsar, which is in good agreement with the mean rain height given by ITU -R [7]. 4. ANALYSIS OF LOS DATA The rain attenuation data collected over line of sight (LOS) link have been analyzed for cumulative attenuation statistics. Attenuation data have been collected for 2632 minute in the year 21 to determine relationship between rain induced attenuation and rainfall rate. Fig. 2 & 3 shows the plot ofrain attenuation versus rate ofrainfall at 19.4 GHz and 28.75 GHz respectively. Finding the mean attenuation for specified rain rate and rejecting attenuation data falling outside the 99% confidence refine the data plotted in fig. 2 & 3. Regression analysis over the measured values of rain attenuation gives best fit curve equations and are compared with that obtained by using MP model [8] and ITU-R model [3] for rain attenuation at the same rain rates. It is observed from figure 2 that the attenuation values at 1 9.4 GHz over the path measured for all rain rates, over estimates the ITU-R model. The predicted model is in good agreement with that of MP model up to the rain rate of 5-6 mm/hr. The measured values of attenuation corresponding to the rain rates more than 5 mm/hr underestimate MP model. It is also observed from fig. 3 that the attenuation values at 28.75 GHz corresponding to the rain rates more than 5 mm/hr underestimate MP model. This finding is due to the fact that the rainfall rate up to 5 mm/hr can be considered uniform along the entire path and the instantaneous attenuation values measured at the receiving end can be related to the Proc. of SPIE Vol. 5445 353
point rainfall rate measured at that instant (it can again be justified by taking measured specific attenuation value at these rain rates). As the rain rate increases the non-uniformity of rain increases. Due to non-uniformity of rain over the path, the instantaneous attenuation value measured at the receiving end cannot be related to the average point rain intensity measured at that instant. Thus the non-uniformity of rain along the horizontal path is accounted for, by using reduction factor to convert the physical path length to an effective path length. 9 45 8 4 a E7 D,)n C 6 V.2 : 25 5 C 4 2 Cu.c 15 2 1 1 5 5 1 15 2 5 1 15 Rain Rain rate (mm/hr) rate (mmlhr) 2 Fig. 2 Rain attenuation at 19.4 GHz as a function ofrain rate Rain rate (mm/hr) 1 1.69 2 1.35 3 1.243 4 1.11 5 1.1 6.947 7.891 8.847 9.89 1.777 11.749 12.725 13.718 14.683 Fig. 3 Rain attenuation at 28.75 GHz as a function ofrain rate 4.1 Path length reduction factor Path length reduction factor (r) has been derived at various rain rates from the experimental data on rain attenuation over LOS path and experimental specific attenuation values at same rain rates, using the relation r=a/al (1) where A is the observed attenuation over LOS path, a is the specific attenuation (db/km) as derived earlier from experimental data on drop size distribution. L is the actual path length of LOS link (2.29 km in this case). By substituting the measured values of A and a, in equation 1, the values of r at various rain rates has been calculated for vertical polarization at two frequencies and are given in table 3. It is observed that the value of r reduces with increase in rain rate. Table 3- Average Value ofpath Length Reduction Factor Reduction (r) Average value factor 354 Proc. of SPIE Vol. 5445
5. SLANT-PATH ATTENUATION PREDICTION Slant-path attenuation at an elevation angle consists of determining the effective path length through the rain and multiplying by the specific attenuation of the rain appropriate for the region under consideration. A model is presented that describes the relevant statistics of rain attenuation on satellite link (at 2 & 3 GHz) for this location. This model can also be applicable for any location for which there is a long-term data record of rainfall statistics is available. The following location dependent parameters are required for determining the statistics of slant path rain attenuation. 1. Location dependent rain rate statistics R (P%), from table 1. 2. The specific attenuation, a at rain rate R, from equations 4 & 5. 3. The effective rain height, HR, from table 2. The implementation of the model is straightforward and can be used on any small desktop computer with the ability to read the database disk. 6. CONCLUSION Rain-induced effects on microwave propagation at 2 & 3 GHz at Amritsar have been experimentally studied. Rain induced attenuation degrades the performance of communication systems. Therefore its quantitative measurements are required for planning and engineering of future communication systems at these frequencies. This paper summaries the results obtained from experimentally measured data, collected over LOS links at 19.4 & 28.75 GHz, Zenith looking radiometer at 19.9 GHz and data on rain rate and rain drop size distribution. Empirical models based on experimental data have been developed and compared with the existing models highlighting points of agreement and disagreement and a suitable explanation has also been given. REFERENCES [1] F.C. Medeirosfilho, R.S.Cole and A.D.Sarma: "Millimeter-wave rain induced attenuation: theory and experiments" TEE Proceedings Microwave Antennas Propagation, vol.133, pt.h,pp.38-314,1986. [2] ITU-R recommendation P.618-7 "propagation data and prediction methods required for Earth-space telecommunication systems" 21 [3] ITU-R recommendation P.837-3 "Characteristics ofprecipitation for propagation modelling" 21. [4] R.L. Olsen, D.V Rogers and D.B. Hodge. "The arb relation in the calculation of rain attenuation" IEEE Trans. of Antenna Propagation., vol. AP-26, pp. 3 18-329,1978. [5] R.G. Medhurst, 'Rainfall attenuation of centimeter waves: Comparison of theory and measurement', IEEE Transaction on Antenna & Propagation, vol. AP-13, 1965, pp.55-564. [6] The Tnt. J. of Satellite Communication, vol.8, No. 3, pp. 239-249, 199. [7] ITU-R recommendation P.839-3 "Rain height model for prediction methods" 21. [8] R. L. Olsen, D. V. Rogers and D. B. Hodge, "The ar relation in the calculation of rain attenuation," IEEE Trans. Antennas Propagation, Vol. 26, 1978, pp.318-329. Proc. of SPIE Vol. 5445 355