Global Positioning System Reference
In-Depth Information
Q
(
0
,
i
)
•
•
•
I
•
•
•
(
0
,
−
i
)
FIGURE 3.2.
IQ
plot of CASM output.
Table 3.1 shows the eight possible signal combinations of the three binary sig-
nals
e
A
,
e
B
,and
e
C
. Also shown are the real and imaginary parts of
S
(corre-
sponding to the in-phase and quadrature signal components) as well as the norm
of the
IQ
vector.
It is observed that the length of the
IQ
vector of
S
(
t
)
(
)
is always 1. This is also
seen from the
IQ
plot in Figure 3.2 where the unit circle is included for reference.
From Table 3.1 it also appears that the three binary signals are combined to a
representation with six possible phases, thus the name tricode hexaphase. It is seen
from the table that the transmitted PRS code is equal to the sign of the quadrature
component. It is also seen that the in-phase component can only assume three
different values, so unambiguous mapping from received in-phase signal to trans-
mitted signal is not possible. The table shows that the data channel is equal to the
sign of the in-phase component when the data and pilot channels are not equal,
and that the in-phase component is zero when the data channel is equal to the
pilot channel. This means that all the transmitted power is concentrated in the
quadrature channel whenever the data and pilot signals are equal.
t
3.2.3 Binary Offset Carrier Modulation
The Galileo signals and the planned modernized GPS signals inherit improved
performance compared to the existing GPS signals. One of the improvements is
the introduction of the binary offset carrier (BOC) modulation. BOC modulations
offer two independent design parameters
-
subcarrier frequency
f
s
in MHz, and
-
spreading code rate
f
c
in Mchip/s.
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