Page 87 - Integrated Wireless Propagation Models

M a c r o c e l l P r e d i c t i o n M o d e l s - P a r t 1 : A r e a - t o - A r e a M o d e l s 65

2.8 Carey Model
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The Carey propagation modeP is based on Part 22 of the US FCC Rules and Regulations.
The model is essentially a simplified statistical method of estimating field strength and
coverage based only on the effective radiated power (ERP) and height above average ter­
rain (HAAT) at the base station. Since the terrain information is averaged, the model
takes into account specific individual localized obstructions or shadowing. The range of
the terrain for the model to use is between 3 and 16 km from the transmitter site. The ter­
rain obstructions outside of this range are ignored. Any terrain obstructions within 3 km
or beyond 16 km that block the line of sight over the radio path would get the same cal­
culation as the LOS condition. The main use for this model is for license applications such
as cellular radio licensing applications or other submissions to the FCC that specifically
require the use of the methods described in Part 22 of the FCC Rules or other administra­
tive requirements, such as certain frequency coordination procedures.
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Carey curves give the F(L, T) field strength versus distance noted that F(L, T )
o
o
a
denotes field strength exceed t L percent f locations during T percent f the time.
F(50, 50) and F(50, 10) field strength versus distance for propagation under average
terrain conditions with: (1) mobile antenna height of 1.8 m, (2) base station antenna
heights ranging from 30 to 1500 m above average terrain, and (3) distances up to 130 km
for the F(50, 50) curves and up to 240 km for the F(50, 10) curves.U The Carey curves for
450 to 1000 MHz were based on a CCIR recommendation covering the entire band.
Median transmission loss for this model can be expressed as
.
8
L = l l 0.7- 19 1 l ogh+55logD (dB), for ::; D < 48 (2.8.1)
L = 9 1 . 8 - 1 8 l ogh + 66logD (dB), for 48 ::; D < 9 6 (2.8.2)
The curves are derived from CCIR curves for television broadcasting. The latter were
adjusted downward by 9 dB to account for the . 8-m height of mobile station antennas.
1
The Carey formula is used to calculate the FCC Cellular Geographic Service Area
formula. This formula approximates this distance to the 32-dBj..t contour predicted by
Carey:
o
1
d = . 05 * Ifl·34 * p .J7
where d is the distance in miles from the cell site antenna to the reliable service area
boundary, i s the antenna height in feet above average terrain, and i s the ERP in watts.
H
P
The service area usually is defined by connecting the specific coverage distance on
the 32-dBj..tV /m point along different angle of radius. It is called Carey curve or Carey
1
contour. Conversion between dBj..t and dBm in power delivery 3 is calculated as
dBm = dBj..tV /m - 20 log( ) - 77.21 (2.8.3)
j
where f is the frequency in MHz. Equation (2.8.3) is based on the antenna gain of 3 dBd
(above dipole). The 32-dBm VIm contour was an eased version from that of the 39-dBj..t VIm
contour for cellular license application.
The Carey contour was based on the two-way system (for link balance) or paging/
broadcast (forward link only) in early measurements by Carey. Actually, Carey mea­
sured only a number of frequencies, and others (nonspecific to his actual measurement)
were interpolated. In general, the higher the value in dBj..tV /m, the stronger the signal
received, as shown in Eq. (2.8.3).