E5 band (1164 -1215 MHz), composed
of E5a and E5b bands, is part of the
spectrum allocated by ITU for new Radio
Navigation Satellite Services in 2000. E5
signal has the wider bandwidth (51.150
MHz) never used in satellite navigation.
Galileo E5 signal is composed by two data
components and two pilot components
broadcasted together by means of the
multiplexing scheme AltBOC(15,10). E5a
band will be used for Freely/NAV message
(Open Service) and the codes of data and
pilot components are uncripted, E5b band
will be used for Integrity/NAV message
for Safety of Life and Open Service.
Integrity of signal is probably the most
advanced service introduced by Galileo
[1]. One of the main feature of E5 band
is that the signal can be received in two
ways: the first is to filter and demodulate
only one of side bands E5a or E5b (see
Fig. 1) the second is to process the overall
received signal containing the components
of both bands [2],[3]. In this sections is
considered the second way because it will
be adopted by professional receivers and
this will avoid to obtain all the advantages
of Galileo signal. E5 is the most promising
signal in terms of performance in
multipath environment and positioning
for critical applications but also the
most challenging for a receiver or a
simulation. In this paper will be presented
the simulation of the transmission and
reception of E5 signal. It will be first
described the generation of codes, then it
will be illustrated Galileo AltBOC(15,10) signal structure and its differences with
a conventional AltBOC, it will be shown
a way for the generation of that signal
and some basic characteristics for the
development of a software receiver for E5.
Table 1 Feature of E5 components
Simulation
The simulation is developed using
Matlab. Four satellites are transmitting
Galileo AltBOC(15,10) signal
in steps of one millisecond.
For each transmitted signal some effects
are introduced like doppler shift due to
relative motion between satellite and
receiver, delay, multipath channel and
gaussian noise and then the overall
received signal is processed with some
algorithms typical of software receivers.
All the blocks are simulated at baseband.
Signal Generation
The first step in the simulation is the
generation of Galileo E5 codes. In E5
signal there are four components or
channels: , , and . Each
component has its own code which is the
product of the repetition of a primary code
of 10230 chips, corresponding to 1 ms of
signal, and a secondary code of different
length, see Tab. 1. “Galileo Signal In
Space Interface Control Document” (SIS
ICD) gives the basis for generation of
codes: the primary codes are generated
by means of two Linear Feedback Shift
Registers. For each satellite SIS ICD
gives the “start values” and the values
of feedback taps as polynomials in octal
notation and some instructions to build
correctly the LFSR. The secondary
codes are only the conversion in binary
representation of a hexadecimal number.
Through some Matlab functions primary
and secondary codes are generated for
all satellites involved in simulation. As
known in a Matlab simulation is necessary to define the reference time domain of
each signal. The bits of the codes are then
mapped in +1 BPSK values producing
the discrete signals ex–y(nTc) with Tc
97.75ns that is the chip period. and
are data channels with data rate
reported in Tab. 1, and so the ranging
codes are multiplied by two different data
streams, and are pilot channels
containing only the ranging codes.
In conventional AltBOC the complex
signal eb (t) = eb–I (t) + jeb–Q (t) is multiplied
by a complex squared subcarrier er
(t) = cr (t) + jsr (t) where cr (t) is the
sign of a cosine function and Sr (t)
is the sign of a sine function and the
complex signal ea (t) = ea–I (t) + jea–Q (t)
is multiplied by the complex conjugate
of er (t), i.e. er
*(t) obtaining the signal:
In Figure 3 is represented how an
AltBOC signal is built and its power
spectral density in which one can
recognize the two main lobes that
represent the bands E5a and E5b.
The Galileo AltBOC(15,10) signal
structure is more complex than
conventional AltBOC and has expression:
where:
where SCS(t) and SCp(t) are two subcarriers
quantized in four values (Fig. 4).
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