common source of signal degradation, fading is another source of signal degradation that is
characterized as a non-additive signal disturbance in the wireless channel. Fading may either be
due to multipath propagation, referred to as multi-path (induced) fading, or to shadowing from
obstacles that affect the propagation of a radio wave, referred to as shadow fading.
The fading phenomenon in the wireless communication channel was initially modeled for
HF (High Frequency, 330 MHz), UHF (Ultra HF, 3003000 GHz), and SHF (Super HF,
330 GHz) bands in the 1950s and 1960s. Currently, the most popular wireless channel models
have been established for 800MHz to 2.5 GHz by extensive channel measurements in the field.
These include the ITU-R standard channel models specialized for a single-antenna communi-
cation system, typically referred to as a SISO (Single Input Single Output) communication,
over some frequency bands. Meanwhile, spatial channel models for a multi-antenna commu-
nication system, referred to as the MIMO (Multiple Input Mul tiple Output) system, have been
recently developed by the various research and standardiza tion activities such as IEEE 802,
METRA Project, 3GPP/3GPP2, and WINNER Projects, aiming at high-speed wireless
transmission and diversity gain.
The fading phenomenon can be broadly classified into two different types: large-scale fading
and small-scale fading. Large-scale fading occurs as the mobile moves through a large distance,
for example, a distance of the order of cell size [1]. It is caused by path loss of signal as a
function of distance and shadowing by large objects such as buildings, intervening terrains, and
vegetation. Shadowing is a slow fading process characterized by variation of median path loss
between the transmitter and receiver in fixed locations. In other words, large-scale fading is
characterized by average path loss and shadowing. On the other hand, small-scale fading refers
to rapid variation of signal levels due to the constructive and destructive interfer ence of multiple
signal paths (multi-paths) when the mobile station moves short distances. Depending on the
relative extent of a multipath, frequency selectivity of a channel is characterized (e.g., by
frequency-selective or frequency flat) for small-scaling fading. Meanwhile, depending on the
time variation in a channel due to mobile speed (characterized by the Doppler spread), short-
term fading can be classified as either fast fading or slow fading. Figur e 1.1 classifies the types
of fading channels.
Fading channel
Large-scale fading Small-scale fading
Path loss Shadowing Multi-path fading Time variance
Frequency-selective
fading
Flat fading Fast fading Slow fading
Figure 1.1 Classification of fading channels.
2 MIMO OFDM Wireless Communications with MATLAB