Revised PRN Code Structures for Galileo E1 OS (2008)

Pseudo Random Noise (PRN) Codes are a fundamental
element in any Code Division Multiple Access (CDMA)
system such as GPS, Galileo, QZSS and Compass. In fact,
these codes are the tool that enables a GNSS receiver to
distinguish one satellite from another.
The current codes of Galileo for E5a/b, E6 and E1 OS
were defined and selected in 2004. At that time, different
approaches for code generation and optimization were
considered. After the evaluation of all code sets, the Random
Codes (available in the SIS-ICD [2]) were shown to
provide the best performance for Galileo E1 OS according
to the set of criteria defined at that time [6].
As shown in those papers published by the people involved
in the Galileo code design at that time, the code
families of each particular band were optimized taking
into account only code properties. This means that the real
modulation characteristics of the signal, that is their par-ticular spreading waveform and multiplex, were not considered
in the code design. In fact, the derived codes of E1
OS would only be optimal in the wide sense if the data
and pilot signals were transmitted in quadrature. However,
as we know from the Galileo SIS ICD [2], the data
and pilot components constituting the Galileo E1 OS signal
will be transmitted in phase using a modified Interplex
modulation. In other words, a typical receiver will see
either the sum or the difference of data and pilot code.
In this paper a new set of random codes is presented.
These are optimized for Interplex modulation and offer
thus an interesting improvement in performance regarding
the main figures of merit for PRN codes, namely the autoand
crosscorrelation.
Compared to other PRN code families which are derived
following a certain generation algorithm and can be generated
in the receiver at run time, Random Codes like
those of Galileo E1 OS need to be stored in memory at
receiver level as they are not based on any generation
scheme in general. This introduces additional complexity
and costs especially for low-end mass-market receivers.
This paper will introduce an interesting alternative family
based on Sidelnikov/Lempel-Cohn-Eastman (SLCE) sequences
that fit exactly the desired code length of 4092
chips. These sequences do not require any truncatation or
addition of any chip. Furthermore, this new code family
offers extremely good autocorrelation properties.
Especially for low cost receivers, a cheap and thus memory
saving code generation unit as well as s rapid acquisition
are of great importance and very desirable. An application
of this concept to the Galileo E1 OS codes would
however not be optimum since shortening the 4092 chips
long Galileo E1 OS codes would imply a considerable
loss of sensitivity as non-desired correlation side peaks
would appear in the partial correlations. An alternative
solution could be to introduce shorter code structures
within the long code of 4092 chips, such that the receiver
would only concentrate on the long or short parts of it
depending on its particular requirements. In order to introduce
this substructure into the 4092 chip code, with
which the correlation for the low end receiver could be
accomplished, the concept of majority voting will be applied
under the constrain of guaranteeing the correlation
properties of the long 4092 chip codes.