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PROPAGATION OFSIGNALS 125Table 1Various Frequency Bands Used in Wireless Systems, Their Identification, and
Their CharacteristicsFREQUENCY BAND FREQUENCY RANGE ANTENNA SIZE DATA RATE
VLF (very low frequency) 3–30 kHz Large Low
Low frequency (LF) 30–300 kHz ↑
Medium frequency (MF) 300 kHz–30 MHz
High frequency (HF) 3–30 MHz
Very high frequency (VHF) 30–300 MHz
Ultra high frequency (UHF) 300 MHz–3 GHz
Super high frequency (SHF) 3–30 GHz ↓
Extra high frequency (EHF) 30–300 GHz Small HighHaving narrowed the operational frequency bands in
practical wireless systems, we now look at two key factors
that have a significant impact on the performance of wire-
less communication systems: the signal strength received
and any variability of the signal strength. These factors
play a major role in determining how far the signal will
travel (i.e., separation between transmitter and receiver)
and how much data can be transmitted usefully. The for-
mer is dictated by the level of loss suffered by the signal
as it reaches the receiver, and the latter is dictated by the
level variations. Both are uniquely determined by the ter-
rain between the transmitter and receiver.PROPAGATION OF SIGNALS
The following sections consider the attenuation and the
signal variability experienced by the wireless signals.Transmission Loss
As the electromagnetic waves travel from the transmit-
ter to the receiver, they encounter various objects in their
path. In typical urban environments, it may not be pos-
sible to have an LOS path between the transmitter and
the receiver. The signal leaving the transmitter reaches
the receiver through a number of mechanisms, such as
reflection, diffraction, and scattering (Feher, 1995; IEEE,Transmitter ReceiverLine of sightFigure 1: A line of sight propagation is shown.1988; Jakes, 1974; Rappaport, 2002; Shankar, 2001). These
are depicted in Figure 2. Reflection occurs when the signal
encounters objects that are much larger than the wave-
length. This is shown in Figure 2 (left). While reflection is
taking place, refraction of the wave may also take place,
in which case the signal will penetrate the object, which
may be a wall or a partition. The signal may also undergo
diffraction (i.e., bending over obstacles) when the signal
encounters sharp boundaries as seen in Figure 2 (center).
Scattering occurs when the surface of the object is rough
and is on the order of the wavelength. The signal is scat-
tered in all directions as seen in Figure 2 (right) as opposed
to getting reflected in a specific direction in the case of re-
flection where the angle of incidence is equal to the angle
of reflection. Thus, in outdoor areas, a signal reaches the
receiver through reflection, scattering, and diffraction, as
it encounters building, trees, and other artificial or nat-
ural objects. In indoor areas, such as in malls, factories,
and office buildings, the signal will reach the receiver pen-
etrating floors, walls, ceilings, and so on while undergoing
effects of reflection, scattering, and diffraction.
A typical signal received in urban wireless systems is
shown in Figure 3a. It is seen that the received power de-
creases as the distance increases. The power is plotted in
dBm. (The relationship between power in watts and dBm
is given in the Appendix.) In a short segment of this curve,
as shown in Figure 3b, the power loss is not a straight
line. The power fluctuates as it decreases. These fluctu-
ations are referred to as long-term fading or shadowing
(Pahlavan & Levesque, 1993; Parsons, 1996). Zooming fur-
ther into the power versus distance curve, the power fluc-
tuates around a mean value (Fig. 3c). These fluctuations
are of short duration compared with those seen in Fig-
ure 3b and are referred to as short-term fading. They
are also known as Rayleigh fading, based on the statis-
tical fluctuations in the received envelope of the signal
(Rappaport, 2002; Shankar, 2001; Steele & Hanzo, 1999;
Stein, 1987). Nakagami fading and Rician fading are also
used to describe short-term fading.
It is thus clear that the signal undergoes attenuation
and fluctuations as it reaches the receiver. The fluctuations
introduce random variations in the received power, mak-
ing it necessary to take additional steps to design the link.
The next section explores ways of modeling the power loss
and the fluctuations in power.