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Sec. 8–6 Link Budget Analysis 613
Example 8–4 RECEIVED (E b /N O ) db FOR LINE-OF-SIGHT PROPAGATION
Using Eq. (8–46), let (P EIRP ) dBw = 30.5, f = 4 GHz, T syst = 154.5 K, R = 1.54 Mbs, and
d = 24,784 miles. The receiving antenna is a 1-meter diameter parabola. Calculate the (E b N o ) dB
at the receiver detector input. See Example8_04.m for the solution.
Path Loss for Urban Wireless Environments
Path loss for a non-free-space wireless environment, such as a path with obstructions consisting
of buildings and trees, is difficult to model. This is the case for terrestrial cellular and
wireless systems where multipath signals are received that consist of reflected path signals in
addition to the direct path signal, as well as additional signal attenuation as the signals pass
through foliage and building walls. Often, measurements of received signal strength along the
path are made to validate predicted results. For a free-space path, the exponent of the path loss
is n = 2; that is, the path loss varies as the square (n = 2) of the distance as shown by
Eq. (8–9). When obstructions are present, n is larger than 2. Usually it is in the range of 2 to 6
with a typical value being n = 3. When the path loss is expressed in dB between the transmit
and receive antennas, this gives a log-distance path loss model. Thus, the obstructed path loss
is [Rappaport, 2002, p. 138].
L dB (d) = L FSdB (d 0 ) + 10n log a d d 0
b + X dB
(8–47a)
where
L FSdB (d 0 ) = 20log [(4pd 0 )>l]
(8–47b)
L dB (d) is the path loss in dB for a distance of d between the antennas, L LFSdB (d 0 ) is the free
space loss for a distance d 0 that is close to the transmitter, but in the far field, and d 7 d 0 . n is
the path loss exponent, and X dB is a zero-mean Gaussian random variable representing the
variations in the path loss caused by multiple reflections. Typically, d 0 is taken to be 1 km for
large urban mobile systems, 100 m for microcell systems, and 1 m for indoor wireless
systems. An example of using Eq. (8–47) to evaluate the link budget and the BER for a
wireless personal communication device (PCD) is shown in SA8–3.
Example 8–5 LINK BUDGET EVALUATION FOR A TELEVISION RECEIVE-ONLY
TERMINAL FOR SATELLITE SIGNALS
The link budget for a television receive-only (TVRO) terminal used for receiving TV signals
from a satellite will be evaluated. It is assumed that this receiving terminal is located
at Washington. D.C., and is receiving signals from a satellite that is in a geostationary orbit at
134° W longitude above the equator. The specifications on the proposed receiving equipment
are given in Table 8–5 together with the receiving site coordinates and satellite parameters.