Page 272 - Electrical Engineering Dictionary

fading channel signal fluctuation caused
by multiple propagation paths over a radio
F channel is called fading. May be categorized
as fast fading or slow fading, depending on
the rate of fading with respect to the informa-
tion symbol rate. May also be categorized as
frequency selective or frequency nonselec-
f H common notation for higher band edge tive, depending on the transfer function of
frequency in hertz. the radio channel. See also fading margin,
fading Rayleigh, fading Rician.
f L common notation for lower band edge
frequency in hertz. fading margin the margin by which the
average signal-to-noise ratio (SNR) in a ra-
Fabry–Perot etalon interferometer con- dio communications link is over-designed,
sisting of two highly reflecting flat or spher- in order to compensate for variations in the
ical mirrors; only resonant frequencies are short-term SNR that occur due to fading of
transmitted. the signal. The signal fading is typically due
to multipath propagation, which arises due
Fabry–Perot laser a laser source where to the presence of multiple reflectors in the
the gain medium is placed within a Fabry– radio link. Utilizing diversity schemes and
Perot cavity, which provides feedback into transmitter power control are typically em-
the laser medium. Several simultaneous las-
ployed in order to reduce the fading margin
ing modes are supported in such cavities.
required in a radio system.
Fabry–Perot resonator any open (as op-
fading rate the rate at which the received
posed to a cavity) resonator, usually the as-
signal level crosses the median signal level
sembly of two parallel plates resembling the
in a downward direction (i.e., with a negative
optical Fabry–Perot interferometer. See also
slope). It is usually expressed in fades per
standing-wave resonator.
second or fades per minute, depending on the
actual rate of fading.
Fabry–Perot structure Fabry–Perot
etalon or interferometer that has an optically
nonlinear medium in its cavity. fading Rayleigh in mobile wireless com-
munications, wide fluctuations in received
facsimile the process of making an ex- signal strength (e.g., swings of 30 – 40 dB)
act copy of a document through scanning of and phase caused by scattering of the trans-
the subject copy, electronic transmission of mitted signal off of surrounding objects. The
the resultant signals modulated by the subject scattering induces a Gaussian distribution on
copy, and making a record copy at a remote the in-phase and quadrature signal compo-
location. nents, so that the received signal envelope has
a Rayleigh distribution. See also Rayleigh
facsimile encoding a bilevel coding meth- distribution.
od applied to the encoding and transmission
of documents. Facsimile systems may in- fading Rician similar to Rayleigh fading,
clude supportfor grayscale imagecodingtoo, the only difference being that a direct line-
which is described under still image coding. of-sight component is present in the received
signal in addition to the scattered signal. The
FACTS See flexible AC transmission received signal envelope has a Rician distri-
system. bution. See also Rice distribution.

c
2000 by CRC Press LLC