Galileo OS SIS ICD.indd - GSA - Europa

3.3. Primary Codes Generation
The primary spreading codes can be either
● Linear feedback shift register-based pseudo-noise sequences, or
● Optimized pseudo-noise sequences
Optimized codes need to be stored in memory and therefore are often called ‘memory
codes’. Register based codes used in Galileo are generated as combinations of two
M-sequences, being truncated to the appropriate length. These codes can be generated
either with pairs of LFSR or might be also stored in memory.
Figure 10 shows an example standard implementation of the LFSR method for the
generation of truncated and combined M sequences. Two parallel shift registers are
used: base register 1 and base register 2. The primary code output sequence is the
exclusive OR of base register 1 and 2 output sequences, the shift between these two
sequences is zero. Each shift register i (i=1 for base register 1 and i = 2 base register
2) of length R is fed back with a particular set of feedback taps {a i,j} j=1…R = [a i,1,a i,2,…,a i,R]
and its content is represented by a vector {c i,j} j=1…R = [c i,1,c i,2,…,c i,R]. For truncation to
primary code length N, the content of the two shift registers is reinitialised (reset) after
N cycles with the so-called start-values {s i,j} j=1…R = [s i,1,s i,2,…,s i,R].
Figure 10. LFSR Based Code Generator for Truncated and Combined M-sequences
3.4. Primary Codes Defi nition
3.4.1. E5 Primary Codes
In case of memory codes, the E5a-I, E5a-Q, E5b-I and E5b-Q primary codes are
according to the representation provided in Annex C. In case of generation via LFSR,
those primary codes use the principle defi ned in paragraph 3.3, using the parameters
defi ned in Table 14. Note that each set of codes for each signal component comprises
50 members.
© European Union 2010
Document subject to terms of use and disclaimers p. ii-iii
OD SIS ICD, Issue 1, February 2010
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