Duct-induced terrestrial microwave link degradation in Nigeria: Minimization factors

Transcription

1 Indian Journal of Radio & Space Physics Vol 41, June 2012, pp Duct-induced terrestrial microwave link degradation in Nigeria: Minimization factors O D Oyedum Department of Physics, Federal University of Technology, Minna, Nigeria oyedumod@yahoo.com Received 8 March 2011; revised 14 January 2012; accepted 10 May 2012 Radio propagation at VHF and higher frequency bands are characterized by fading and anomalous enhancements associated with meteorological conditions. In particular, link disruptions on line-of-sight terrestrial microwave paths are often associated with fading losses that often depend on the height and position of the receiver as well as season of the year. Such propagation problems may be minimized by appropriate choice of the height and relative position of the transmitting and receiving antenna. Using results based on analysis of radiosonde data obtained during for surface air temperature, pressure and relative humidity from the Nigerian meteorological stations of Lagos (06 28 N, E), on the Atlantic coast of West Africa; and an inland city of Kano (12 02 N, E), height-distance curves have been derived for estimating suitable heights and relative positions of antennas, to minimize fade losses on line-of-sight terrestrial paths during surface duct events. A detailed account of theoretical applications of such curves in both cities is also attempted. Keywords: Surface duct, Shadow zone, Antenna height, Signal degradation, Link degradation, Link disruption, Terrestrial microwave path fading loss PACS Nos: 41.20jb; Ta 1 Introduction Terrestrial microwave line-of-sight (LOS) paths involve propagation within the radio horizon, normally characterized by high and reliable signal strength. However, disruptions on such links sometimes occur due to a number of factors and may result in deep, prolonged space-wave fadeouts associated with super refraction and formation of surface ducts between the transmitter and receiver. Such effects can be significant for wideband propagation in microwave ranges 1 and may lead to erroneous radar estimates of target distance 2. Formation of surface duct such as evaporation duct may occur over water surface due to strong vertical humidity gradient, while surface-based duct may occur over land when a low-level temperature inversion traps sufficient water vapour below it. Surface ducts and surface-based ducts show considerable diurnal and seasonal characteristics 3. Drop in power (or field strength value) of free air radio wave may occur in LOS paths within the horizon due to discontinuities in refractivity gradient, dn/dz, which may be expressed in terms of modified refractivity gradient, dm/dz. For a surface duct, dn/dz < -157 N units per km or dm/dz is negative at the surface and returns to normal positive values at some height above the surface, while for surfacebased duct, negative dm/dz begins at some height above the surface and reaches a minimum value that is less than the surface value before returning to normal positive values 4. Fading within the horizon is also influenced by passage cold fronts. Propagation conditions and the associated meteorological factors have been highlighted by a number of researchers 5-7, while an appraisal of potential for duct-induced degradation on microwave links in Nigeria has also been attempted 8. High incidence of super refraction and ducting in Nigeria has been reported 9-10, while recent studies highlighted the prevalence and relative strength of surface ducts in the two Nigerian cities under study. Super refraction occurs if dn/dz < -40 N units per km or dm/dz < 118 M units per km and may lead to ducting when dm/dz < 0. Field strength fading within the radio horizon of a transmitter may result from defocusing of the lobe pattern of the transmitter, and may simultaneously give rise to enhanced field strengths beyond the horizon, where it has potential for adverse co-channel interference. Fadeouts of this nature can cause serious disruptions on microwave links; and since they depend on the location and height of the receiver relative to the transmitter, proper choice of these

2 340 INDIAN J RADIO & SPACE PHYS, JUNE 2012 parameters is important to reduce such link disruptions 13. In order to enhance optimal planning of terrestrial radio networks, possible methods of choosing the height and distance of the receiver antenna (relative to the height and location of the transmitter) so as to ensure that the receiver is not sited in a loss region or shadow zone of the transmitter 14 should be explored in the environment concerned. This is the focus of this effort with regard to the two Nigerian cities. It may be stated that signal fading within the radio horizon due to dn/dz discontinuities, which may result in surface ducts, is the focus here, and other causes of fading and signal degradation such as diffraction from nearby objects that may sometimes prevail as well, or adverse interference from transhorizon circuits, are not considered. Theoretical considerations 13 show that during a surface duct event, loss regions or shadow zones exist at certain distances from a given transmitter located above the duct depending on the duct height and the receiver antenna height. A ray leaving a transmitter at a launch angle θ 0 makes a grazing incidence on a surface duct of height h D at point A and splits into rays AB which penetrates the duct, and AB which continues above the duct to define onset of the first shadow zone (Fig. 1). The ray AB may be reflected by the ground at B, and further split at C, defining a second shadow zone. Thus, ground distances d 0 (from the transmitter to first shadow zone), d (length of first shadow zone) and corresponding half-length distances d hl =d 1 =d 2 are as shown. Theoretically, performance of a receiver placed within distance d 0 from the transmitter may not be affected by the presence of the duct; but beyond d 0 performance of a receiver may be affected by losses in the first or subsequent shadow zones of the transmitter, depending on the height h R of the receiver and/or its distance d R from the transmitter. By careful choice of h R and d R, the circuit engineer can enhance performance of a receiver in a duct-prone area. This can be achieved from values of d 0 and d hl obtained from suitable curves derived from meteorological data in the region. Shadow zone half-lengths d hl (Fig. 2) may be computed for both maximum and minimum ducting conditions for a given surface duct of known height h D. The half length of a given shadow zone d sz may be obtained from the relation 9 : 6 2 N 10 dhl = a θ p θ p where, 6 hd θ p 2 N 10 a (1) (2) where, a = earth s radius (m); N = difference in refractivity N between ground and top of surface duct; θ P = penetration angle defined as the minimum elevation angle for a ray from a transmitter above the duct that can penetrate the duct. Also, the distance (measured on the earth s surface), which a radio ray from an antenna of height h travels in an exponential reference atmosphere before grazing the surface at a point A (or vice versa) may be computed from the relation: 6 2 N 10 dh = a ( θ p θh ) θ p + θh and (3) 2 2h θh θ p + 2 N (4) a where θ h = grazing angle at height h; 0 θ h θ p ; N = difference between N at ground level and the reference height. Fig. 1 Surface duct, first and second shadow zones (Source: Bean & Dutton, Radio Meteorology, 1968) Fig. 2 Antenna positions and heights for shadow zone clearance (Source: Bean & Dutton, Radio Meteorology, 1968)

3 OYEDUM: DUCT-INDUCED TERRESTRIAL MICROWAVE LINK DEGRADATION IN NIGERIA 341 Optimal height and position of a receiving antenna may be derived from the resulting d hl and heightdistance curves, depending on whether the receiving antenna is above or below the top of the surface duct. For a given surface duct of height h D and transmitter of height h t > h D, the ground distance d o from the transmitter to point A (representing onset of first shadow zone) may be obtained from Fig. 2 as d = d( h h ) (5) 0 t D And the maximum distance of the first shadow zone d sz from the transmitter is d = d + 2d (6) sz(max) o hl The maximum receiving antenna height h max (for antenna within the surface duct) or minimum height h min (for antenna higher than the duct) required to be free from the shadow loss region of the transmitter depends on which side of the ground reflection point B it is located, the season of the year, and the distance d GR from B, provided that (7) 0 dgr dhl and hmax hlr hmin (8) h lr is any height within the loss region (Fig. 2). For a receiving antenna at a desired distance d from the transmitter and above the surface duct, the minimum height h min required to clear from the loss region may be deduced from appropriate distanceheight curves using corresponding d GR given by: dgr = do + n dhl d, if d is on LHS of B (9) or dgr = d ( do + n d hl ),if d is on RHS of B (10) where, n = number of shadow zone half-lengths between the transmitter and the receiver; d o = distance from transmitter to onset of first shadow zone, point A; and d hl = shadow zone half-length. 2 Computational methods Shadow zone half-lengths, d hl, were computed for maximum and minimum ducting conditions in two Nigerian cities, Lagos (06 28 N, E) on the Atlantic coast of West Africa and Kano (12 02 N, E) in sub-saharan Africa. Daily values of air temperature, air pressure and water vapour pressure from radiosonde data, obtained at hrs GMT in these stations during the period , are analyzed to determine relative prevalence of surface ducts in the two cities. The half-length, d hl, of a shadow zone of length, d sz, is obtained from Eqs (1-2). Lagos and Kano, respectively are at about 19 m and 480 m above mean sea level. Typical meteorological characteristics and relative prevalence of ducting in the period of study are shown in Table 1. Computed values of d hl are plotted against corresponding duct heights, as shown in Fig. 3. In addition, the distance d h (measured on the earth's surface) which a ray from a transmitting antenna of height, h, travels in the exponential reference atmosphere before grazing the surface duct at a point A (or vice versa) is computed for duct heights between 50 and 400 m using Eqs (5-6). These computations are made for the months of January (representing dry season in Nigeria) and July (representing the wet season), for the corresponding exponential reference atmospheres earlier derived for Nigeria 15. Computed values of d h for heights in the range 0-20,000 m are shown in the curves of Fig. 4(a) for Lagos, and Fig. 4(b) for Kano. Table 1 Monthly values of surface temperature, relative humidity and occurrence of ducting in Lagos and Kano Month Maximum temp, C Relative humidity, % % Occurrence of ducting Lagos Kano Lagos Kano Lagos Kano January February March April May June July August September October November December

4 342 INDIAN J RADIO & SPACE PHYS, JUNE 2012 Fig. 3 Shadow zone half-lengths for maximum and minimum ducting conditions in Lagos and Kano 3 Results and Discussion From Figs (3 and 4), it is clear that for surface ducts of width of about m, d hl in Lagos varies km under maximum ducting conditions in January (dry season), or km under minimum ducting conditions; however, in July (wet season) the range of d hl variation increases to about km under maximum ducting conditions, but decreases to about km under minimum ducting conditions. In Kano, d hl variation for the same range of duct widths in April (when maximum dry season ducting is available) is considerably increased over that of Lagos to about km; but in July (wet season), only little corresponding increase in d hl variation in the range km is observed. Depending on the ducting condition, variation of d hl for a typical surface duct width (~100 m in Lagos) is between km in January (dry season) and km in July (wet season) or km in the year. For a typical duct width of 300 m in Kano, the variation is ~450 km in dry season (April) and ~550 km in wet season (July). Thus, shadow zone distances from given transmitters are long, and subject to considerable seasonal variations, which must be adequately taken care of in link design to ensure year-round reliability. The ground distance (such as d o in Fig. 2), which a ray from an elevated transmitting height travels in an exponential atmosphere before grazing the surface, as computed for various transmitting heights, is shown in Fig. 4 (a and b) for Lagos and Kano, respectively for both seasons of the year. Exponential atmosphere specified by surface refractivity N s =366 N units in dry season and N s =376 N units in wet season for Nigeria 15 are used. Figure 4(a) shows that for both seasons in Lagos, the first shadow zone begins at a relatively short distance, d o of ~5 km from a transmitter of height ~40 m above a surface duct; or ~ km for a transmitter on a higher altitude such as a Low Earth Orbit (LEO) satellite. However, in Kano [Fig. 4(b)] the corresponding figures for such a transmitter or LEO satellite are respectively ~4 km and ~470 km in wet season (July). These observations imply that in Lagos, where width of surface duct is typically ~100 m, transmitting antennas should be substantially higher than 100 m above the surface, ~500 m (or ~400 m above a surface duct) so as to ensure that d o extends to a reasonable distance (~40 km) away from the transmitter. However, in Kano where surface ducts have widths of ~300 km, the effort should be to keep both transmitter and receiver below ~300 m (i.e. within the duct),

5 OYEDUM: DUCT-INDUCED TERRESTRIAL MICROWAVE LINK DEGRADATION IN NIGERIA 343 Fig. 4(a) Ground distance (d h ) and antenna height (h) above surface duct for average Lagos atmosphere in January and July Fig. 4(b) Ground distance vs height above surface duct for average Kano atmosphere in July as cost of transmitting antenna above such ducts by ~400 m (i.e. ~700 m above surface) is prohibitive. 3.1 Using d hl vs duct height and distance vs height curves For a given surface duct of height, h D, and transmitter of height, h t > h D, the ground distance d o from the transmitter to point A (representing onset of first shadow zone) may be obtained from Fig. 4 using Eq. (5) while the total shadow zone distance, d sz, of the transmitter is computed from Eq. (6). From Fig. 3, it is seen that in Lagos, for a surface duct of width 200 m, d hl is 40 km during maximum ducting conditions and ~370 km during minimum ducting conditions in January. In July, the corresponding

6 344 INDIAN J RADIO & SPACE PHYS, JUNE 2012 figures are about 60 km and 200 km, respectively. Thus, shadow zone half-length for a transmitter above a 200 m surface duct may vary by about 330 km in January or 140 km in July. In Kano, d hl values for a transmitter above a 200 m surface duct are about 350 km and 450 km, respectively for the few ducting events observed in April and July. No ducting events were observed in January as the air is generally dry during this period. From Fig. 4, it is also deduced that for a transmitting antenna of height 200 m and a surface duct of height 150 m, Eq. (5) gives distance from the transmitter to onset of first shadow zone as: d 0 = d ( ) = d 50 This gives d 0 = 6 km from both Fig. 4(a) (Lagos) and Fig. 4(b) (Kano). Duct widths are typically more than 300 m in Kano but in Lagos, for typical duct height of ~100 m, d o is ~12 km in January and ~11 km in July. Observed values of d hl and d 0 show that shadow zones are relatively close to the transmitter in both Lagos and Kano, but extend considerably. Whereas the former allows for more flexibility with height and location of a receiving antenna within the surface duct, the latter could present significant propagation problems during surface duct events. Thus, the circuit engineer must contend with trade-off between use of taller transmitters for maximum unimpeded area coverage (with possible disruptions at distances beyond d o during surface duct events), and use of shorter transmitters that remain within a surface duct, hence, free from duct-related disruptions but with reduced coverage area. 3.2 Estimating optimal height and location of receiver within a surface duct For a receiver antenna within a surface duct, the maximum height h max required to be free from shadow zone loss of the transmitter is used. This is obtained for various distances, d GR, from the ground reflection point B (or similar points), using Eqs (9-10), depending on: (i) receiver distance from the transmitter and the distance d GR from the point B, (ii) whether the receiver is on left-hand-side (LHS) or right-hand-side (RHS) of point B, and (iii) season of the year. These distances are such that Eqs (7 and 8) are satisfied. The computations mentioned above are carried out for maximum ducting conditions for the months of January and July, which respectively represent the dry and wet seasons in Nigeria. Ducting condition prevails when the surface refractivity gradient dn/dz < -157 N units per km, and maximum ducting conditions are associated with least dn/dz values. Curves of maximum receiver heights are not to be exceeded in order to be free from the shadow zone, and the corresponding distances d GR from the ground reflection point (Fig. 2), are presented for duct heights of m (in January) and for duct heights of m (July) for Lagos [Fig. 5(a)] while similar curves for Kano are presented in Fig. 5(b). Comparison of Fig. 5(a) with Fig. 5(b) shows that for any given antenna height, the d GR distance is higher in Kano than in Lagos. Lagos also shows only slight differences between the two seasons, reflecting the fact that ducting prevails throughout the year in Lagos but in Kano d GR values are considerably higher in wet season than in dry season when ducting sparingly prevails. Values of d GR for given antenna heights and given duct heights also reflect the same trend. Signal reception in secondary shadow zones will naturally be progressively weaker; and improved reception may be achieved in such situations if the receiving antenna is kept within the surface duct. Charts of distance (d GR ) and height of receiver given in Fig. 5 can also be quite handy in such cases for obtaining suitable estimates of height and location of the receiving antenna. Thus, Fig. 5 enables the radio circuit engineer to determine optimal height and position of a receiver antenna (relative to the transmitter) so as to be free from shadow loss region of the transmitter during surface duct events in either Lagos or Kano. 3.3 Estimating optimal height and location of receiver above a surface duct For a receiver antenna above a surface duct, the minimum height, h min, required for clearance from the first shadow zone, depending on the distance from the transmitter and season of the year, may be determined from the curves of Figs (3 and 4) plotted for receiver heights in the range 0-20,000 km. Depending on the side of the ground reflection point the receiver antenna is located, the distance, d GR, that is used to determine the corresponding h min from these curves are derived from results of computations based on Eq. (9) or Eq. (10). Using these curves for Lagos, the following results are obtained: (i) For a surface duct of height h D = 150 m under maximum ducting conditions, shadow zone half-

7 OYEDUM: DUCT-INDUCED TERRESTRIAL MICROWAVE LINK DEGRADATION IN NIGERIA 345 Fig. 5(a) Maximum receiver height (h max ) vs distance (d GR ) for different surface ducts heights (d h ) during maximum ducting conditions in Lagos Fig. 5(b) Maximum receiver height (h max ) vs distance (d GR ) for different surface ducts heights (d h ) during maximum ducting conditions in Kano length obtained from Fig. 3(a) is 50 km in July and 32 km in January. Under minimum ducting conditions, d hl equals 180 km in July or 300 km in January [Fig. 3(b)]. (ii) For a transmitter of height h t = 210 m, d o is obtained from Fig. 4(a) [B] using d o =d ( ) = d 60, which is equal to 6.5 km in July or 7.0 km in January. (iii) For a transmitter and surface duct specified above, the minimum height (h min > h D ) of a receiver at a desired distance (d>d o ) from the transmitter, required to be free from the shadow zone may be obtained from d-d o = d (hmin hd). Hence, for a receiver located 10 km from the transmitter, the minimum height of the receiving antenna in July is obtained from = d (hmin-150) for which

8 346 INDIAN J RADIO & SPACE PHYS, JUNE 2012 Fig. 4(a) [B] gives d (hmin-150) = d (30), so that h min = 30, giving h min = 180 m in July. Using d o = 7.0 km in January and the same procedure, h min = 178. Since the minimum height specification for July (180 m) also satisfies that of January (178 m), it should be adopted for both seasons of the year in Lagos. (iv) For a receiver of desired height h>h D, the maximum distance d, from the transmitter required for clearance from the loss region is given by d o + d (h-hd). Thus, if desired receiver height h in wet season (July) is 160 m, then d = d o + d ( ) = d 10. From Fig. 4(a), d 10 = 95 m, which gives d = m in July. The corresponding distance in January (dry season) using d o =7.0 is ~ 92 m, which also meets the wet season requirement for transmitter-receiver distance and should be adopted for both seasons. (v) For a receiver of desired height h>h D and desired position d from the transmitter, the minimum height h min is obtained from Fig. 4 using d=d (hmin-hd). (vi) For a receiver of height h<h D and transmitterreceiver distance, d > d o + d hl (i.e receiver located on RHS of ground reflection point, B), d= d o +d hl +d GR, so that d GR = d-d o -d hl ; the corresponding receiver height h max is obtained from Fig. 5. If d is on LHS of B (i.e. d < d o +d hl ), then d GR = d o +d hl -d, and the corresponding receiver height h max is determined from Fig. 5. Thus, for any desired transmitter-receiver distance d, d GR may be obtained for dry season and wet season; and the smaller of the two values of h max adopted for the whole year in Lagos. Values of h max in dry season are generally smaller and used since greater heights associated with enlarged duct widths during the wet season are likely to project into the shadow zone in dry season when surface ducts become shallower in Lagos. In Kano, the same procedure is adopted for using the curves of Figs (3-5). However, since surface ducts are not observed during dry season in Kano, comparison can be made between observed individual events of maximum ducting in April and minimum ducting in July, to estimate year-round limits of relative receiver-transmitter distances and heights for reliable communications links. 4 Conclusions Depending on the relative positions of the transmitting and receiving antennas within and outside a radio duct, presence of the duct can cause considerable degradation of the signal on line-of-sight paths due to defocusing and shadow zones; signal enhancement and anomalous propagation to great distances beyond the horizon can also result with potential for interference on co-channel links. High occurrence of super refraction and ducting is observed in Nigeria, especially in the southern city of Lagos 16, on the Atlantic coast of West Africa. Consequently, disruptions experienced on line-ofsight microwave circuits in the region may be partly attributed to prevalence of surface ducts and the associated shadow zones. Suitable height-distance curves based on proper characterization of the surface ducts and their seasonality enables the circuit engineer to minimize fading losses during such duct events. In particular, such height-distance curves enhance good choice of transmitter and receiver heights, as well as their relative positions. The height-distance curves presented in this work will enhance terrestrial microwave link designs in Nigeria, especially for the rapidly increasing number of GSM circuits. There is need to complement this effort with empirical results based on field strength measurements to corroborate the theoretical deductions. Acknowledgements The author is indebted to the Nigerian Meteorological Agency for providing the radiosonde data used for this study. Appreciation is also hereby expressed to Prof D O Adefolalu for his assistance in the data acquisition and analysis; Dover Publications Inc for the copyright permission to use some relevant materials; and the reviewers for their contributions which have greatly enhanced this effort. References 1. Sirkova I, A clear-air propagation system: evaporation duct application, Bulgarian J of Phys, 25 (3/4) (1998), Moszkowicz S, Ciach G J & Krajewski W F, Statistical detection of anomalous propagation in radar reflectivity patterns, J Atmos Ocean Technol (USA), 11 (1994) Babin S M, Surface duct height distributions for Wallops Island, Virginia, J Appl Meteorol (USA), 35 (1996) Sizun H, Radio wave applications for telecommunications applications (Springer, Berlin), 2005, Babin S M, A case study of sub-refractive conditions at Wallops Island, Virginia, J Appl Meteorol (USA), 34 (1995) 1028.

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