Rain Attenuation Statistics over terrestrial Microwave Link operating at 28.75 GHz over Amritsar, INDIA( Tropical Region) Parshotam Sharma1, I.S. Hudiara2, M.L.Sigh3 1
K.C. College of Engineering & IT, Kariam Road, Nawanshar, Punjab , India
[email protected]
2
K.C. College of Engineering & IT, Kariam Road, Nawanshar, Punjab , India
[email protected] 3
Gurur Nanak Dev University, Amritsar, Punjab, India
[email protected]
Abstract Rain Induced attenuation over a terrestrial link operating at 28.75 GHz was measured during the period of Feb., 04 to Jan., 05. at Amritsar ( 31036’ N 740 52’ E) . The paper presents the results in the form of a proposed empirical formula. The experimental results have been measured with those predicted by ITU-R. It is observed that there is a significant difference between the attenuation levels measured and those predicted by using ITU-R model.
1. Introduction Much attention has recently been shown in the use of higher frequencies for terrestrial & satellite communication links for civil and military communication systems. The use of higher frequency bands for communication systems is necessary for higher channel capacities and also due to congestion in VHF and UHF bands. As the communication systems using higher frequencies are growing rapidly in tropical countries, there is an increasing need for the knowledge of propagation characteristics of microwave. The frequencies lying between 10 GHz and 30 GHz are now used for both terrestrial and satellite communication systems. However the frequencies above about 10 GHz are severely affected by the presence of rain over the link path, more so, in the tropical regions because of the high intensity of rain. Thus the knowledge of the fade margin to be provided, in the system design, to overcome the losses in the signal strength due to rain over the path is required for reliable communication. Though International Telecommunication Union (ITU-R) provides with the prediction of the attenuation that may be caused to the link of particular path length and at a particular frequency, the need for the locally experimentally determined parameters cannot be ignored. The present paper presents the results of the propagation experiment carried over Amritsar (INDIA) over the period of two years.
2. Experimental Setup The experimental site is at Amritsar (31036’ N 740 52’) about 229.4 mtrs above the sea level. The LOS link consists of a CW transmitter operating at 28.75 GHz and receiver at 28.75 GHz separated by a LOS distance of about 2.29 kms. The block diagram of the experimental arrangement is as shown in fig. 1.
LOS Link 2.29 km
Transmitter at 28.75 GHz
Strip Line Chart Recorder
Receiver at 28.75 GHz
Data Acquisition System
Fig. 1 Block Diagram of Experimental Setup
Tipping Bucket
Both transmitters and receivers have been housed in air conditioned labs to keep the temperature constant. Phase locked loop in the transmitter provides stable transmission. The systems have been designed to measure about 45 db of
attenuation. The rain rate measurements are made by using tipping bucket rain guage which is calibrated to give one tip for every 0.254 mm of rainfall. The time between the successive tips determine the rain rate in mm/hr. Receiver operating at 28.75 GHz was mounted on a horizontally and vertically steerable stands for aligning the transmitting antenna with receiving antenna in azimuth and elevation planes. The receiver was housed in the receiving hut at Guru Nanak Dev University, Amritsar.The specifications of the receiver and transmitter are as given in the table 1 and 2. Frequency (GHz)
28.75
DSB noise fig. of mixer (dB)
5.5
Mixer conversion loss (dB)
8 Dual
polarized,
front
Type of Antenna
fed Frequency (GHz)
28.75
Power Output (dBm)
23
Plane of Polarization
Hor/Ver
Type of Antenna
Dual
Antenna Size
front fed parabolic 0.61 m
parabolic Antenna Size
0.61 m
Antenna Gain (dB)
43
Antenna beam width
1.2 o
LO Frequency (GHz)
28.68
IF (MHz)
70
Antenna Gain (dB)
43
Dynamic Range (dB)
47
Antenna beam width
1.2 o
. Table 1: Specifications of the Receiver
polarized,
Table 2: Specifications of the Transmitter
3. Rain Attenuation of Microwave signal at 28. 75 GHz An LOS link at 28.75 GHz has been established between transmitter and receiver housed in the transmitter and receiver huts at Village Mallahan and the top of GNDU library (CRL) respectively. The data is monitored during periods of rain for deriving a relation between rain rate and excess attenuation due to rain. To correlate the attenuation with rain rate, we need to have (a) Rain Rate data and (b) Corresponding receiver data in the form of equivalent power in dbm. The data from the receiver was recorded in the form of DC voltage whereas the data from the tipping bucket was recorded in the form of AC voltage.The signal of the 28.75 GHz was recorded to be in volts in clear sky conditions while it dipped to the range of several milli-volts in the rainy event. The measurement of one such event is described Output of Tipping bucket below O/P of 28.75 in the form Of voltage
Tips of the rain guage
GHz Receiver
Fig 2. Snapshot of the Strip Chart graph Utility in Agilent Data Logger software showing a typical recoding of the data for rain induced attenuation
Concentrated Tips: More Rain rate: less signal: more attenuation
2.00E+01
60 Rain Rate
1.60E+01
Attenuation (dB)
Rain Rate mm/hr.
Attenuation dB
1.80E+01
50 40 30 20
1.40E+01 1.20E+01 1.00E+01 8.00E+00 6.00E+00 4.00E+00 2.00E+00
10
0.00E+00 0
0 0
20
40
10
20
30
40
50
60
Time (mins.)
60
Time in mins
Fig. 3 Variation of Rain rate with Time
Fig.4 Corresponding variation of path attenuation
Where ‘R’ is the rain rate in mm/hr and ‘t’ is the time calculated between two tips in hours. Again the corresponding voltage level of the receiver is converted back to the power level by calibration curve and the difference between this value and that of clearer sky level gives us the attenuation for that particular value of rain rate. A typical recording of the data is as shown in the fig 2. As it is illustrated, the strength of the signal is decreasing in direct proportion with the concentration of the rain gauge tips. After the raw data has been colleted then this data is processed to obtain the rain rates (at the tip point) and corresponding attenuation levels. The rain rate is calculated by the formula R=
0.254 mm / hr t
(1) LOS_2004 40
160 Feb Mar Apr Jun Jul Aug Oct Nov Dec Jan
Rain Rate (mm /hr.)
120
100
LOS_2004
35
30 Line of Sight Attenuation (dB)
140
80
60
25
20
15
10 40
5 20
0 0.001
0 0.0001 0.01
0.1
0.001
1
0.01
0.1
1
Percentage Time Exceeding
Percentage Time Exceeding
Fig. 5 CD’s of monthly rain rates
Fig. 6 CD of yearly attenuation levels
The rain induced attenuation data is presented in the form of time varying rain intensities and corresponding attenuation levels. The experimentally measured data has been compared with that predicted by using ITU-R [4]. The ITU-R prediction evaluated as below: A. ITU-R Prediction The rain induced attenuation can be determined as below A= α .Leff
(2)
Where ‘A’ is the total attenuation in dB ‘α’ is the specific attenuation (dB/Km) ‘Leff’’ is the Effective Path Length . The effective path length factor ‘Leff’ and specific attenuation’ α ‘ can be found by using equation 3 and 4 respectively 1 Leff = L (3) l 1+ 35.e − 0.15.R
α = a.Rb
(4)
Where ‘L’ is the LOS path length, ’R ‘ is the rain rate in mm/hr., ‘a’ and ‘b’ are frequency dependent temperature dependent coefficients which can be evaluated by using RDSD. The value of ‘a’ and ‘b’ has been chosen as .1667 and .9974 respectively as recommended by ITU-R Recommendation [4] P.838-2. B. Experimental Results The output of the receiver was recorded in the form of continuously varying DC voltage which was converted back to the power in dbm by the calibration equation. Rain rate was measured by using tipping bucket rain gauge which gives the output in the form of charging voltage of a capacitor. When the bucket gets filled, it tips and the switch connected to the capacitor gets closed, this momentarily closing of the switch is detected by the data acquisition system which also records the corresponding voltage of the receiver. The measurement procedure described above was employed for analyzing rainy events that occurred during 2004-05. The data was analyzed for forming CD of Attenuation for year 2004 -05. The CD are as shown in fig. 5 and 6 and are also compared with those predicted by using ITU-R model. Thus there is a need to revise the existing ITU-R models. This objective can be fulfilled by using CD of the full year to form the formulas for deriving the formula for attenuation prediction. The procedure used is given in ITU Recommendation and is as given below:
Ap A 0 . 01
Ap A0.01
= 0 . 12 p − ( 0 . 32 + . 04 log 10
= 0.12 p −(0.546+.043log10 p )
p)
for year 2004-05
ITU formula
(5)
(6)
4. Conclusions Rain attenuation was measured during rainy events that took place in the period of 2004-05. The CD of the attenuation level thus formed has been used for modeling rain attenuation which when compared with that given by ITU-R suggests revision of ITU-R model.
5. References 1] Albert Cohen, Comments on The Role of Rain in Satellite communications, IEEE Trans. Antenna and Propagation, 1976, 903-904, [2] Lousis J. Ippolito Jr., Radiowave Propagation in satellite communication, Van Nostrand Reinhold Company”, 1986,New York,. [3] Robert K. Crane, Electromagnetic Wave Propagation through Rain, 1996,John Willey & Sons, Inc., New York. [4] ITU-R Rec. ITU-R P.618-7, Propagation Data and Prediction Methods Required for the Design of Earth-Space Telecommunication Systems, 2001, ITU Radio communication Bureau, Geneva. [5] ITU-R Rec. ITU-R P.839-1, Rain Height Model for Prediction Methods, ITU Radiocommunication Bureau, Geneva, 1997. [6] Allinutt J.E., and Upton, S.A.J., Results of a 12 GHz radiometric experiment in Hong Kong, Electronics. Letter 25,( 1985), 1217-1219. [7] Yuei-An Liou , Radiometric Observation of Atmospheric Influence on Space to Earth Ka – Band Propagation in Taiwan , Proc. Nat. Sci. Comm. 24, (2000),238-247. [8] A. W. Dissanayake, Allinutt J.E., and Upton, S.A.J, D. K.Trimrthy, Radiometric Rain Attenuation Measurements at 11.6 GHz in Peru , Electronics. Letter,26 , (1997),169-170. [9] ITU-R Rec. ITU-R P.1322, Radiometric Estimation of Atmospheric attenuation. ITU Radio communication Bureau, Geneva, 1997. [10] R.L.Olsen, D.V.Rogers and D.B.Hodge, The aRb relation in the calculation of rain attenuation,"IEEE Trans. Antennas Propagation,AP-26, (1978), 318-329. [11] ITU-Rec. ITU-R P. 838-2, "Specific attenuation model for rain for use in prediction methods", ITU-R Recommendations, Geneva, 2003. [12] ITU-Rec. ITU-R P. 837-4, "Characteristics of precipitation for propagation modeling” ITU-R Recommendations, Geneva, 2003. [13] Parshotam Sharma, I S Hudiara, M L Singh, “ Estimation of effective rain height at 29 GHz at Amritsar ( India), IEEE Trans. Antenna and Wave Propagation ( In Press).
