Galileo Signal - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 31

Additionally, the wealth of digital signal processing experience over that time
means many hands to the pump. The following considerations are based on Mattos
(2004).

The L1 OS signal is transmitted on the frequency f 1 =

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42 MHz. The signal
is composed of three channels , called A, B, and C. L1-A is identical to L1 PRS
(PRS for public regulated service), which is a restricted access signal. Its ranging
codes and navigation data are encrypted. The data signal is L1-B (meaning the
B channel within L1) and the data-free signal is L1-C (meaning the C channel
within L1). A data-free signal is also called a pilot signal. It is made of a ranging
code only, not modulated by a navigation data stream.
The L1 OS signal has a 4092 code length with a 1

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023 MHz chipping rate giving
it a repetition rate of 4 ms; on the pilot signal a secondary code of length 25 chips
extends the repetition interval to 100 ms.
Under some circumstances it may be difficult to separate the wanted signal
from the unwanted ones and the unwanted one is often a cross correlation from
another satellite as the inherent CDMA isolation of the codes is only around
21 dB. The cross-correlation problem is solved by using very long codes .How-
ever, longer codes also delay the acquisition process. In most cases the processor
must search at half-chip offsets; thus, 8184 possibilities for the L1 OS code. To
search the very long code lengths proposed for the new signals would be impracti-
cal, so the codes have been designed with escape routes. The most common one is
called a tiered code . This means it is built in layers so that when you have a strong
signal you can acquire on a simple layer, with less time-domain possibilities, only
switching to the full-length code when required.
The minimum bandwidth is generally twice the chipping rate for simple codes,
while for BOC codes it is twice the sum of chipping rate and offset code rate.
Thus, the minimum practical bandwidth for the Galileo L1 OS is 8 MHz. For pre-
cise tracking of the code a bandwidth wider than the minimum is generally used.
Within this 4 ms period the signal-to-noise ratio (SNR) prevents the download-
ing of data for signals weaker than 25 dB/Hz. The data-download situation is im-
proved by using forward error correction codes (FEC), and block interleave also
covers for burst errors. FEC convolutional codes spread the information from one
user data bit over many transmitted symbols. If some of these are lost, the data
bit can be recovered from the others. However, a burst error may destroy all the
relevant symbols. Interleaving, which transmits the symbols in a scrambled se-
quence, means that a single burst error cannot destroy all the symbols relevant to
a single user data bit. The downside is that it adds latency to the message, to allow
for the interleaving/de-interleaving process. However, on the Galileo signal with
1 s packets this is not a problem.
The 4 ms repetition rate is ideal because there is one symbol per code epoch.
When the code is synchronized, we know that we will not hit a data bit edge
because these always occur at the start of a code sequence.
The signal is the product of carrier, spreading code, BOC, and data. Tradition-
ally, the RF hardware removes the carrier, the correlators remove the BOC(1,1)

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