some form of inherent delay in propagation between transmitter and receiver.
This is true for wire-line systems, optical systems and wireless radio systems,
where propagation velocity is related
to the dielectric constant of the medium through which the signal passes.
Propagation velocity is expressed as a
percentage of the speed of light, and in
vacuums (dielectric constant = 1) and
in air (dielectric constant = 1.00054),
propagation velocity can be considered
to be 100 percent of the speed of light
for most practical purposes.
Therefore, in most wireless communication systems the propagation delay
between a transmitter and receiver can
be very closely approximated by divid-ing the straight line distance between
the transmitter and the receiver, by the
speed of light.
For the specific flight path dis-
cussed earlier, the range delay profile
of Figure 7 can be expected. Model-
ing different types of moving platforms
and flight paths will produce dramati-
cally different results. When perform-
ing one-way tests, where a receiver or
transmitter is being tested, the channel
simulator must be capable of signal de-
lay ranges dictated by both the closest
and farthest expected separation be-
tween transmitter and receiver, plus
margin. Complete simulations of re-
lay communications scenarios require
that the channel simulator be capable
of delaying for the full communication
Note that these
setups allow test-
ing of all receive-
such as antennas,
tors, encryptors, de-
ers, decryptors, bit
syncs, etc., because
the input signal is
into and out of channel simulators can be over cables, near-field RF, or long distance RF.
As Equation 1 describes, the Doppler shift is frequency dependent.
Since data signals have non-zero bandwidth, various portions of the signal
are actually at different frequencies
as can be observed with the 120 kbit/
sec QPSK signal shown in Figure 6.
For precise simulation, the Doppler
shift capacity of the channel simulator
must apply appropriate and different
Doppler shifts across its bandwidth. In
Figure 6, for example, the left side of
the waveform would receive a slightly
lower Doppler shift than the right side,
since the left side is at a lower frequency than the right side. This is especially
important at high data rates that result
in wide bandwidth data signals.
Referring back to Figure 3, it also
shows that the Doppler shift rate
changes throughout the flight. The
flatter, more horizontal areas of the
plot are where the Doppler shift remains relatively constant due to comparatively small changes in the closing
velocity between the aircraft and the
ground station. The steeper portion of
the curve is where the aircraft’s range
from the ground station is changing
more rapidly; as the plot crosses the
X axis, the velocity changes sign from
positive values (aircraft approaching
receiver) to negative values (aircraft
moving away from receiver). Channel
simulators, configured as those shown
previously, must apply Doppler shift
rates both within
and beyond the anticipated ranges for
verification of appropriate receiver
s Fig. 4 Channel simulator configuration for receiver system testing.
s Fig. 5 Channel simulator configuration for transmitter testing.
S YS TEM
UNDER TES T
All communication systems have s Fig. 6 Typical 60 kHz bandwidth of a 120 kbit/sec ( 60 kSymbols/
sec) QPSK signal.