Galileo Signal - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 37

Chip length

Spreading Code

λ subcarrie r

Subcarrier

BOC Signal
No Carrier

λ ca r rier

Carrier

Final
Signal

FIGURE 3.3. Spreading code, subcarrier, carrier, and signal as result of the BOC modula-
tion principle. This figure does not show the navigation data.

These two parameters provide freedom to concentrate signal power within spe-
cific parts of the allocated band to reduce interference with the reception of other
signals. Furthermore, the redundancy in the upper and lower sidebands of BOC
modulations offers practical advantages in receiver processing for signal acquisi-
tion, code tracking, carrier tracking, and data demodulation; see Betz (2002).
Most Galileo signals come in pairs: a data signal and a data-free signal. They
are aligned in phase and consequently have the same Doppler frequency.
ABOC

signal is created by modulating a sine wave carrier with the prod-
uct of a PRN spreading code and a square wave subcarrier having each binary

( m , n )

1
values, see Figure 3.3. The parameter m stands for the ratio between the subcarrier
frequency and the reference frequency f 0 =

±

1 . 023 MHz, and n stands for the ratio

between the code rate and f 0 .Thus,BOC ( 10 , 5 ) means a 10 . 23 MHz subcarrier

frequency and a 5 115 MHz code rate.

The result of the subcarrier modulation is to split the classical BPSK spectrum
in two symmetrical components with no remaining power on carrier frequency;
see Martin et al. (2003).
The product is a symmetric split spectrum with two main lobes shifted from the
carrier frequency by the amount equal to the subcarrier frequency; see Figure 3.4.
We concentrate on BOC

.

( m , n ) = BOC ( 1 , 1 ) as this is likely to be used by the L1

signal transmitted by Galileo.
The ACF of BOC signals has a profile with more peaks that may be tracked.
For BOC signals it is important to make sure the channel is tracking the main peak
of the correlation pattern. So additional correlators are needed for measuring the
correlation profile at half a subcarrier phase from prompt correlator at either side.