TOPIC 8: Terrain Models EE 542 Fall 2008 O. Kilic EE 542 Outline • Fixed Terrestrial Links – Terrain as sharp edges – Outdoor propagation models • Satellite Links References: Simon R. Saunders, “Antennas and Propagation for Wireless Communication Systems,” Wiley. O. Kilic EE 542 Fixed Terrestrial Links • Involve a pair of stations mounted on masts and separated by 10-100s of km. • Masts are typically many 10s of meters high. • Highly directional antennas are used to allow for a generous fade margin. O. Kilic EE 542 Free Space Path Loss Model From Frii’s equation: ⎛ λ ⎞ PR = PT GT GR ⎜ ⎟ R 4 π ⎝ ⎠ PR ⎛ λ ⎞ = GT GR ⎜ ⎟ PT ⎝ 4π R ⎠ 2 2 PT GT GR ⎛ 4π R ⎞ LFS =⎜ ⎟ PR ⎝ λ ⎠ LFS (dB) = 32.4 + 20log R + 20log f MHz 2 O. Kilic EE 542 Single Knife Edge Loss Model For (v>1) LKE (v) −20log O. Kilic EE 542 1 0.225 = −20log v πv 2 O. Kilic EE 542 . The TX antenna is mounted 20 m above the ground. while the height of the receiver antenna is TBD. b) Calculate the height of the RX antenna for the path loss to be just equal to the maximum acceptable value. a) Calculate the total path loss assuming the RX is mounted 20m above the ground. The ground is level except for a 80 m high hill located 10 km away from the TX. O. Kilic EE 542 .Single Obstruction Example A microwave link operating at 10GHz with a path length of 30 km has a maximum acceptable path loss of 169 dB. 4 + 20log30 + 20log10000 = 142 dB O. Kilic EE 542 .Solution a) Free space loss: LF (dB) = 32.4 + 20log R + 20log f MHz = 32. 5 dB ρo ro O.225 v . λ = 3 × 10−2 m ρo + ro vh 2 2 × 30 × 103 = 60 =6 R1 3 × 10−2 × 10 × 103 × 20 × 103 Since v>1. Kilic EE 542 0.Solution a) h = 60m ρo = 10 km ro = 20 km Single KE Loss: R1 = λ ρ o ro . use LKE (v) −20log = 28. 5 = 170.Solution a) Total path Loss: L = LFS + LKE (dB ) L = 142 + 28. Kilic EE 542 .5 (dB ) The total loss is in excess of the acceptable limit! O. we need to reduce the obstruction loss.225 v= =5 10 λρo ro h=v = v × 10 = 50 m 2( ρ o + ro ) −27 20 O.Solution b) We can’t do anything about free space loss unless we are allowed to change the geometry or frequency. Thus. Assuming the RX tower height is the only variable we can change.225 ⎞ LKE (dB) = 27 = −20log ⎜ ⎟ v ⎝ ⎠ 0. the RX antenna height can be determined as: ⎛ 0. Kilic EE 542 . The acceptable level for the obstruction loss is: 169-142 = 27 dB. O. Kilic EE 542 . Multiple Knife Edge Diffraction Models • • • • Bullington (1946) Epstein (1953) Deygout (1994) Giovanelli (modification to Deygout) O. Kilic EE 542 . Bullington Method Defines a new effective obstacle at the point where the LOS from the two antennas cross. Equivalent problem: hm RX TX O. Kilic EE 542 . Kilic EE 542 . • Not an accurate method in general as the same equivalent KE can be the solution to multiple scenarios. O.2 • Very simple method • Important obstacles can be ignored.Bullington Method . therefore losses can be underestimated • Reasonably accurate when two KEs re relatively close. Kilic EE 542 .Bullington Method -3 hm b b a a RX TX Cases a and b are treated identically. O. Kilic EE 542 d4 .Epstein-Peterson Method L = L1 + L2 L1: (TX-1-2) L1 = L(d1.h1) L2: (2-3-RX) L2 = L(d3.d4. O.d2.h3) h1 h3 2 1 3 TX RX d1 d2 d3 Draw lines-of-sight between relevant obstacles and add the diffraction losses at each obstacle. Kilic EE 542 .Epstein-Peterson . O. • Has large errors for two closely spaced obstructions.2 • Overcomes the primary limitation of Bullington – that important obstacles can be ignored. In this case Bullington method is better. i. O. the point with the highest value of v along the path.Deygout Method • Search the entire path for a main obstacle.e. Kilic EE 542 .. • Diffraction losses over "secondary" obstacles may be added to the diffraction loss over the main obstacle. • Diffraction for secondary obstacles is calculated wrt the main obstacle and the visible terminal. d3+d4) O.h2) h2 m 1 TX d1 2 d2 Main obstacle vmax d3 Lm: TX-m-RX Lm = L(d1+d2.Deygout Method .d2. Kilic EE 542 d4 RX .d4.h1) Main term h1 Secondary term L2: m-2-RX L2 = L(d3.hm.2 L = Lm + L1 + L2 Secondary term L1: TX-1-m L1 = L(d1. 3 • Typically agrees well with rigorous techniques • Overestimates loss. especially when there are multiple obstacles close together • The accuracy is higher when there is one dominant obstacle • Superior to Bullington and EpsteinPeterson methods for highly obstructed paths O.Deygout Method . Kilic EE 542 . • Find a reference point for diffraction calculations O.Giovanelli Method • Modification to Deygout Method • Identifies a main obstacle as in Deygout. Kilic EE 542 . d3.hm’) O.h1) Lm: TX-1-RX’ Lm = L(d1.d2+d3. Kilic EE 542 h1 RX .Giovanelli Method L = Lm + L1 Tangent line to secondary obstacle Main term L1 hm ’ RX’ m 1 TX d1 d3 d2 L1: m-1-RX L1 = L(d2. Kilic EE 542 .Other Methods • Many different approaches exist. • Examples: – Causebrook – Vogler (analytic approach) O. • Some are modifications to the methods mentioned. O. Kilic EE 542 . O. Kilic EE 542 . Vogler Good agreement for large h2 grazing O. Kilic EE 542 . Kilic EE 542 .O. Kilic EE 542 .Vogler O. Kilic EE 542 .O. Kilic EE 542 .O. buildings and other obstacles. O. • Numerous propagation models exist based on measurement data and statistical methods. Kilic EE 542 . • The terrain profile may vary from a simple curved earth profile to a highly mountainous profile with the presence of trees.Other Outdoor Propagation Models • The methods discussed so far all depend on reducing the terrain to sharp edges. • This results in deterministic + random components for the path loss. Kilic EE 542 .Outdoor Propagation Models • • • • • • Longley-Rice Durkin Okumura Hata Lee ….. So on… O. Okumura Model • One of the most widely used models for signal prediction in urban areas. based on extensive series of measurements made around Tokyo. • There is no attempt to base the prediction to a physical method. • Fully empirical method. • In general applicable to – f: [150 MHz – 1920 MHz] – D: [1km – 100 km] – H: [30 m – 1000 m] • Predictions are made via a series of graphs O. Kilic EE 542 . Okumura-Hata Model • Hata approximated Okumura’s measurements in a set of formulae. suburban and urban. • The urban values have been standardized by ITU for international use. • The method involves dividing the area into a series of categories: open. Kilic EE 542 . O. D O. Kilic EE 542 .Okumura-Hata Model • The median path loss are calculated using the following expressions: – URBAN: L(dB) = A + BlogR – E – SUBURBAN L(dB) = A + BlogR – C – OPEN L(dB) = A + BlogR . 97 for large cities.1log f c − 0.75hm ) ) − 4.Okumura-Hata Model A = 69.16log f c − 13.2 ( log(11.29 ( log(1.54hm ) ) − 1.82log hb B = 44.33log f c + 40.7 ) hm − (1.55log hb 2 f C = 2 ⎛⎜ log( c ) ⎞⎟ + 5.8 ) for medium to small cities O.4 28 ⎠ ⎝ D = 4.55 + 26.9 − 6.1 for large cities.78 ( log f c ) + 18. Kilic EE 542 . f c < 300 MHz 2 E = (1.94 2 E = 3.56log f c − 0. f c ≥ 300 MHz 2 E = 8. Fixed Satellite Links O. Kilic EE 542 . Kilic EE 542 .O. Kilic EE 542 .Specific Attenuation Through Trees O. Kilic EE 542 .O. O. Kilic EE 542 . O. Kilic EE 542 . Kilic EE 542 .O. O. Kilic EE 542 . Kilic EE 542 .O. O. Kilic EE 542 . O. Kilic EE 542 . O. Kilic EE 542 . Kilic EE 542 .O. Kilic EE 542 .O. Kilic EE 542 .O. O. Kilic EE 542 . Kilic EE 542 .Scintillation Event – Scintillation Only O. Kilic EE 542 .O. O. Kilic EE 542 . Kilic EE 542 .O.