Path Loss and Shadowing

Path Loss and Shadowing

The wireless radio channel poses a severe challenge as a medium for reliable high-speed communication.
It is not only susceptible to noise, interference, and other channel impediments, but these impediments
change over time in unpredictable ways due to user movement. In this chapter we will characterize the
variation in received signal power over distance due to path loss and shadowing. Path loss is caused by
dissipation of the power radiated by the transmitter as well as effects of the propagation channel. Path
loss models generally assume that path loss is the same at a given transmit-receive distance1. Shadowing
is caused by obstacles between the transmitter and receiver that absorb power. When the obstacle
absorbs all the power, the signal is blocked. Variation due to path loss occurs over very large distances
(100-1000 meters), whereas variation due to shadowing occurs over distances proportional to the length
of the obstructing object (10-100 meters in outdoor environments and less in indoor environments). Since
variations due to path loss and shadowing occur over relatively large distances, this variation is sometimes
refered to as large-scale propagation effects or local mean attenuation. Chapter 3 will deal with
variation due to the constructive and destructive addition of multipath signal components. Variation due
to multipath occurs over very short distances, on the order of the signal wavelength, so these variations are
sometimes refered to as small-scale propagation effects or multipath fading. Figure 2.1 illustrates
the ratio of the received-to-transmit power in dB versus log-distance for the combined effects of path loss,
shadowing, and multipath.
After a brief introduction and description of our signal model, we present the simplest model for
signal propagation: free space path loss. A signal propagating between two points with no attenuation or
reflection follows the free space propagation law. We then describe ray tracing propagation models. These
models are used to approximate wave propagation according to Maxwell’s equations, and are accurate
models when the number of multipath components is small and the physical environment is known. Ray
tracing models depend heavily on the geometry and dielectric properties of the region through which
the signal propagates. We therefore also present some simple generic models with a few parameters that
are commonly used in practice for system analysis and “back-of-the-envelope” system design. When the
number of multipath components is large, or the geometry and dielectric properties of the propagation
environment are unknown, statistical models must be used. These statistical multipath models will be
described later.
While this chapter gives a brief overview of channel models for path loss and shadowing, comprehen-
sive coverage of channel and propagation models at different frequencies of interest merits a book in its
own right, and in fact there are several excellent texts on this topic [3, 4]. Channel models for specialized
systems, e.g. multiple antenna (MIMO) and ultrawideband (UWB) systems.