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OVERVIEW OFRF WIRELESSCOMMUNICATION 179then to convert the signal back to its original form. This
is achieved through the process of modulation (or en-
coding) at the Tx side and demodulation (or decoding)
at the Rx side. The channel is the medium by which the
signal propagates, such as free space, unshielded twisted
pair, coaxial cable, or fiber-optic cable. In wireless com-
munication the channel is free space. Noise and interfer-
ence is added to the signal in the channel, which causes
attenuation, distortion, and eventually error in the re-
ceived signal.
The transmitter and receiver are very complex systems
consisting of many internal components. A block diagram
representation of some of the components is shown in
Figure 1. Components are denoted as transmitter pro-
cesses, receiver processes, amplifiers, mixers, local oscilla-
tors (LO), filters, and antennas. The transmitter processes
represents functions of the transmitter such as modula-
tion, encoding, analog-to-digital conversion, multiplex-
ing, addressing, and routing information. The receiver
processes, on the other hand, denote inverse functions
such as demodulation, decoding, digital-to-analog conver-
sion, and demultiplexing, as well as addressing and rout-
ing information. Effective transmission and reception of
radio waves involves processes such as amplification and
filtering of the signal at various internal stages, mixing of
the desired signal with a local oscillator signal, translating
the signal from one frequency to another, and transmis-
sion or reception of the RF energy through the antenna.
The amplifier is characterized by its gain, noise figure (or
output power), and linearity (Weisman, 2003). The gain
(in dB) of the amplifier is a measure of how much big-
ger the output signal is than the input signal. The noise
figure (or noise ratio) is a measure of the quality of the re-
ceiver system. Mixers are commonly found in the Tx and
Rx subsystems and are used to create new frequencies or
translate existing frequencies to new ones. They are some-
times called up or down converters. The most common
translation of frequency is from intermediate frequency
(IF) to RF and vice versa. The mixer performs this func-
tion by effectively multiplying two signals at two different
frequencies. A signal source that provides one of the in-
puts to the mixer is the LO. A common type of LO is a
voltage-controlled oscillator. The function of the filter is
frequency selectivity. Filters select signals based on their
frequency components. Regardless of the construction, all
filters can be classified as lowpass, highpass, bandpass, or
bandstop. These names are descriptive of the function of
the filter. For example, a lowpass filter will select signals
with low frequency and reject signals with high frequency.
A special type of filter commonly used in RF systems is
the duplexer. It is used to combine the functions of two
filters into one. The duplexer facilitates the use of one an-
tenna for both transmission and reception. The sink or
destination can vary as much as the source and type of
information.
In the channel, external noise in the form of manmade
noise (generated by electrical manmade objects), atmo-
spheric noise, and extraterrestrial noise is introduced.
Atmospheric noise is produced by electrical activities of
the atmosphere. This type of noise is predominant in the
range 0–30 MHz and is inversely proportional to its fre-
quency. Extraterrestrial noise is produced by activities of
the cosmos, including the sun.Radio Spectrum Classification
Radio frequencies or radio waves constitute the portion of
the electromagnetic spectrum extending from 30 kHz to
300 GHz. The entire RF spectrum is classified into differ-
ent bands and ranges, based on propagation properties.
Baseband signals or source signals (e.g., audio signals)
are in the low-frequency range below 30 kHz. This range
of frequencies is classified as very low frequency (VLF),
which must be translated into RF before transmission.
Radio waves are also described by their wavelength,
λ, as belonging to a particular wavelength range such as
shortwave, medium-wave, or millimeter-wave. The higher
the frequency, the lower the wavelength, becauseλ=c/fc,
wherec=3.9× 108 m/s is the speed of light, and fcis
the carrier frequency. The wavelength is related to the
realizable antenna length,L, system bandwidth,B, and
other practical system parameters. For example, higher
frequency radio waves produce smallerλ, require shorter
L, have higher bandwidth efficiency,ρ, are more suscepti-
ble to fading, and suffer from atmospheric distortion. The
characteristics and applications of radio frequencies are
summarized in Table 1.
Within each frequency range, several bands of frequen-
cies can be designated for communication. These bands
are commonly identified by eitherfcor a letter symbol,
as illustrated in Figure 2 (Acosta, 1999; Federal Commu-
nications Commission, 1997). For example, in practical
applications one could describe an RF system as operat-
ing in the C, X, K, or KAband instead of using the actual
frequency numbers. A complete list of the radio-frequency
allocation can be found inSelected U.S. Radio Frequency
Allocations and Applications(2002).
Because of the congestion or unavailability of usable
spectrum at the lower frequency bands (below 20 GHz)
and the recent demand for multimedia communication
at high data-rate capabilities, system designers have di-
rected their attention toward the use of SHF and EHF for
communication (Acosta, 1999). Currently, there is a great
deal of research on developing RF systems operating at
frequencies above 20 GHz (KAband and above) (National
Aeronautics and Space Administration, 1998).
This interest in the EHF band is justified due to its
potential benefits, such as the availability of usable spec-
trum, high data-rate capability, reduced interference, and
high achievable gain with narrow beam widths of small
antennas (Ippolito, 1989). The drawback, though, is that
at these frequencies atmospheric distortion, especially
rain attenuation, is very severe (Acosta & Horton, 1998;
Xu, Rappaport, Boyle, & Schaffner, 2000). The severity
of the meteorological effects increases with increasing
frequency. At some frequency bands, the meteorological
effects can cause a reduction in signal amplitude, depolar-
ization of the radio wave, and increase in thermal noise
(Ippolito, 1989).Radio Wave Characteristics
When electrical energy in the form of high-frequency volt-
age or current is applied to an antenna, it is converted to
electromagnetic (EM) waves or radio-frequency energy.
At the Tx, the antenna converts a time-varying voltage or
current into a time-varying propagating EM wave. The
resulting EM wave propagates in space away from the