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precise than at a high flight level as shown in Fig. 6.7.3.1, but still is in rather good
agreement with the measured results. Also, a greater azimuthal asymmetry between
the left side and the right side of the measured curve was observed.

6.7.3.2.2 NLOS Scenario In the case of the less frequent NLOS scenario, the spherical
diffraction waves occur when the signal is in the NLOS and propagate mainly in the
diffraction region. In this region, signal strength decreases rapidly as distance increases.
1
A deterministic approach has been done5 to compute large-scale fading with the uni­
form theory of diffraction, and the experiment/theoretical model comparison with the
measured data showed errors between 5 and 15 dB. Air-to-ground radio communica­
tion usually avoids this condition.

6.7.3.2.3 A LOS Channel-Satellite Aircraft Channel The satellite aircraft channel is
always in a LOS condition. The atmosphere condition causes additional losses from free
2
.
space path loss, a described in Sec. 6.7 1 . 1 . 1 . Most of the published literature,45•5 has
focused on the satellite aircraft channel.
6. . 4 Prediction of Bullet Train Communication Signal
7
The bullet train communication medium is an unchanged medium. The train is always
running on a fixed route. Therefore, the path loss of this medium can be calculated
straightforward without difficulty. The path losses of their signals follow the free space
path loss.
However, because of the fast speed of the train, the received signal will be con­
sidered differently than the signal received from land mobiles. The bullet train
systems in different countries are different. China and Germany53•54 use remote radio
heads, as in the cloud-based high-speed train communication system architecture, as
shown in Fig. 6.7.4.1, or radio over fiber, which consists of radio access units along
the rail for wideband communications system.
In Japan, 55 most bullet train communications use coaxial cables. Outer conductors
of the coaxial cables have slots radiating a portion of the transmitted energy. There are
two cables along the railroad-one for inbound and one for outbound. Before the
signal goes through the cable and gets weaker, a relay is installed along the railroad
every given distance apart to boast up the signal level. The two slot antennas are
mounted on the train, one on each side. Therefore, the link budget of the signal is
constant.
Because of the fast speed of the train, the received signal suffers the frequency
dependence (time-delay spread) and time variance (Doppler spread) of the channel.
The Doppler spread directly influences the inter-subcarrier interference of an OFDM
system. One approach to reduce Doppler spread and time-delay spread is to use the
tracking directional antenna aiming at the base station.56

7
6. . 5 Millimeter Wave Signal
The millimeter wave signal can be only used for indoor communications. In 1973, Lee
had installed a 3-mm link between the Empire State and Pan Am Building, using a
15-mW IMPATT diode as a source, two 30-in parabola dish antennas for a link gain of
106 dB, and a Schottky diode for detection. The link was clearly established, but the
rainfall attenuation during the heavy rainfall caused the 3-mm wave signal faded.57
Therefore, we concluded at that time that an outdoor millimeter wave communication