Introduction
This document describes the assumptions and methods used in SoftWright's implementation of the "Okumura" model for radio propagation, as described in "Field Strength and its Variability in VHF and UHF Land-Mobile Radio Service", Review of the Electrical Communication Laboratory, Vol 16, Numbers 9-10, Sep.-Oct, 1968, by Yoshihisa Okumura, et. al. Unless otherwise noted, page and figure numbers refer to this publication.
After the basic median field strength is computed, the user has the option of which other correction factors to include. Each factor can be individually user-selected as included or not included. The computed field strength generated by the program is usually the sum of the basic median field strength and all included correction factors. If this value exceeds the computed free-space field at that distance (for example, a high base antenna and a short receive distance), the free-space value is used (see p. 846).
The basic median field strength is computed from Figure 41 (a-d) as a function of frequency, base antenna height, and distance from the base station. The user-specified frequency is matched to the curve of the closest frequency (150, 450, 900, or 1500 MHz). No interpolation for frequencies between the curves is performed.
The effective base station antenna height is computed as specified by Okumura (p. 832) as the height of the base station antenna above the average terrain elevation from 3-15 km. For locations at a distance less than 15 km, the average is taken from 3 km to the location distance. For locations less than 3 km from the base, the effective base station antenna height is the difference between the base antenna and the receive antenna.
The basic median field strength obtained from Figure 41 (dBu for 1kW ERP) is adjusted for the actual base station Effective Radiated Power.
The basic median field strength curves of Figure 41 are referenced to a mobile receive antenna height of 1.5m AGL. Figure 27 provides a correction factor for other antenna heights used in urban areas. This figure is used to correct the field in urban areas only (specified as "Urban - large city" and "Urban - medium city" as described below). Although Figure 27 is referenced to 3m antenna heights and Figure 41 is referenced to 1.5m antenna heights, this negligible difference is ignored.
This factor corrects for the type of area being served. The basic Okumura curves yield basic median field strength for urban areas.
Okumura describes three fundamental area types (p. 834):
Urban "Built-up city or large town crowded with large buildings and two-or-more-storied houses, or in a larger village closely interspersed with houses and thickly-grown tall trees."
Suburban "Village or highway scattered with trees and houses - the area having some obstacles near the mobile radio car, but still not very congested."
Open "No obstacles like tall trees or buildings in the propagation path and a plot of land which is cleared of anything 300 to 400m ahead, as, for instance, farm-land, rice field, open fields, etc."
Two other distinctions are made elsewhere by Okumura. Urban areas are subclassified as "Large city" or "Medium city" for the purposes of correcting for mobile antenna heights (Figure 27). Open areas are subclassified as "Open" or "Quasi-open" (Figure 22). "Quasi-open" is defined as "midway between the open and suburban area" (p. 846).
One of five types of area can be specified for the program:
Urban - Large city
Urban - Medium city
Suburban
Quasi-open
Open
The basic median field strength will be increased from the Urban value from Figure 41 for Suburban (+6 to +14dB), Quasi-open (+18.5 to + 27.5dB), and Open areas (+23.5 to +32.5dB).
Care should be exercised in selecting the area type to use, keeping in mind the fact that these definitions are based on Japanese environs. Application to U.S. or other cities should make appropriate adjustments.
The field strength values can be adjusted for street orientation in the area being served. The computed field will be increased if streets are generally ALONG (i.e., roughly parallel to) the radial paths from the base transmitter site. The computed field will be decreased in streets are generally ACROSS (i.e., roughly perpendicular to) radial paths from the base transmitter site. The field will be adjusted from -8 to +8dB as a function of distance from the base station. At locations less than 5km from the base station, the maximum value from Figure 18 (+6.1dB or -4.7dB) is used.
The field strength values can be adjusted for the average slope of the path. The computed field will be increased for generally uphill paths, or reduced for generally downhill paths. This path slope and the corresponding field strength adjustment are computed only for paths greater than 5km. The average slope value is computed by averaging the slope from the base site elevation (not the antenna) to each point on the path. The computed slope is limited (or "clamped") to the limits of Figure 34. That is, slopes computed with a magnitude greater than ±20milliradians are evaluated at the limits of the figure (±20mr). (See Okumura Figure 7 and Figure 34.)
The stated frequency range for Figure 34 is 450 to 900MHz. No further adjustment is made when this correction factor is included for other frequencies.
The Okumura model includes procedures for adjusting the field strength values for mixed land-sea paths. The computed field will be increased for areas where a significant portions of the path between the base transmitter site and the area being served. (See Okumura Figure 35.)
TAP will distinguish land from sea using the interpolated elevation value at each point in your study. Unless otherwise indicated, a location with elevation at or below zero is considered sea. At your option, you may define sea level elevation to be a value other than zero.
The field strength values can be adjusted for the degree of irregularities in the terrain along the path between the base transmitter site and the area being served. The computed field will be reduced as a function of the computed roughness of the terrain, defined as the difference between the elevation value exceeding 10% of the elevations and the elevation value exceeding 90% of the terrain elevations, within a distance of 10km of the receiver location. This "terrain undulation height" (_h) is computed automatically from the radial elevation data in the specified elevation file. (See Okumura Figure 28.) For d H ranging from 10 to 500m, the field is adjusted from +1.5 to -25.5dB. When the calculation details are printed by the program, the terrain undulation value (in feet or meters) is printed, followed by the 10%, 50%, and 90% elevations in parentheses.
Note that the correction for this factor is specified for 453, 922, and 1430Mhz. Any frequency values between 453 and 1430 will be interpolated from these values. Frequencies outside of this range are evaluated with the 453 or 1430Mhz curves as appropriate.
Rolling Hilly Terrain (Fine Correction)
The field strength values can be adjusted for the location of the receiver site on hills in the area to be served. The computed field will be increased for receiver locations at or near the top of hills, and reduced for locations in valleys. (See Okumura Figure 29.)
For locations with elevations above the 90% elevation (or below the 10% elevation) the full correction factor value is added to (or subtracted from) the basic median field strength. For locations with intermediate elevations, the correction factor value is adjusted by linear interpolation based on the location elevation and the median (50%) elevation of the path.
The field strength values can be adjusted for isolated ridges along the path between the base transmitter site and the area to be served. If the SoftWright Path Geometry module (including the obstruction data base programs) is installed, this factor will be used to adjust the field strength for manmade obstructions added to the obstruction data base. The factor for "isolated ridges" in the topography will be computed automatically from the radial elevation data in the specified elevation file. (See Okumura Figure 31 and 32.)
For these purposes, an "isolated ridge" is defined as an obstruction that blocks line of sight to the receive location (see Okumura, p. 832, "in the way") and is above the average elevation on the path. The average elevation is computed from the base location to the obstruction, as implied by Figure 6a and Figure 31.
Only ridges within 10km of the receive location are considered. The correction factor is adjusted for the ridge height above the average path elevation using the formula specified with Figure 32 (.07_h).
When the calculation details are printed by the program, the distance to the ridge, the ridge height, and the distance from the ridge to the receive location are printed.
In the case where more than one ridge blocks line of sight to the receive location, only the single worst-case ridge is used to determine a correction factor.
The field strength values can be adjusted for the percent of locations receiving the computed field strength. The basic median field values from Figure 41 are for 50% of the locations in the coverage area. The calculations can be adjusted for any user-specified value above 50%, up to 99.9%. The standard deviation for the field at the specified frequency from Figure 39 is adjusted based on a standard normal curve to compute the correction factor. The correction factor (in dB) is subtracted from the field at each location.
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Copyright 1997 by SoftWright LLC