In Galileo AltBOC has been
modified introducing two
different subcarriers and the
products of the components
in order to obtain a signal that
lies on an 8-PSK constellation
(constant envelope) which is
very important for satellites
high power amplifiers.
Figure 3 Construction and Power Spectral
Density of AltBOC Signal
Figure 5 Real and Imaginary part of E5 ideal signal,
E5 filtered signal and its samples (K=8)
Figure 5 Real and Imaginary part of E5 ideal signal,
E5 filtered signal and its samples (K=8)
Figure 6 Constellation of E5 signal
Figure 7 Shapes of autocorrelation of S(t) and
correlation between S(t) and Spilot(t)
In the following lines is
reported how, in the simulation,
the samples of E5 signal are
generated, for more details see
[11]. As known in AltBOC(n, m),
n represent the chip rate of the
signal and m the frequency of
the subcarriers. As one can see
the subcarrier period is Ts = (n/m)
Tc = (2/3)Tc. The two subcarriers
are defined every Ts/8, so after
the multiplexing scheme each
chip period is composed of N =
12 slots in which the values of
the signal can be different. The
components ex–
Y(nTc) are then
interpolated
with a “hold
interpolator” of
factor N = 12
and so directly
multiplexed
according to
the expression
of Eq. 2. The
output signal
is defined in
TAB = Tc/12. Now is shown the relationship between
the chip rate Rc = 1/Tc and the bandwidth
in which the signal should be transmitted.
The one side bandwidth is B = 25.575MHz
that B = 2.5Rc. Now is necessary to select
a correct sampling frequency: Fsamp =
KB with K > 2. So the ratio between the
sample time, Tsamp = 1/Fsamp, and TAB is
Tsamp/TAB = 24/5K. For example if K = 4
one have to upsample the signal by a factor
5, filter (In the simulation is introduced a
raised root cosine filter with roll-off factor
a =0.22) it in order to bandlimiting it, and
then sample it with a factor 6 obtaining
10 samples for chip. These equations
and values can be important in the
development of a software receiver for E5, in particular the value of K determine the
sampling frequency and so the resolution
of the receiver. The procedure illustrated
can be useful in a software receiver for
the efficient generation of the local signals
for the correlations with the input signal.
Real and imaginary part of
AltBOC(15,10) signal are in Fig. 5 and
its constellation is represented in Fig. 6.
Figure 8 Shapes of correlations r1(t) and r2(t)
Acquisition, Tracking
and Data Recovery
As previously said it has been developed
a receiver for the demodulation of the
overall E5 signal: the signal in
input at the receiver is directly
sampled (in a simulation the
signal is already sampled) and
the complex samples are then
processed. In the acquisition and
tracking process is performed a
correlation between the received
sampled signal and the samples
of a local AltBOC(15,10)
pilot signal defined as: Eq.4
This signal can been obtained
from Eq. 2 setting to zero the
data components ea–I and eb–I
and the different normalization
factor is introduced in order
to not reduce the peak of
correlation. In figure 7 one
can observe that there is no a
significant difference between
the autocorrelation of S(t) signal
and the correlation between
S(t) and Spilot(t). The acquisition
is performed using FFT based
technique [4] and the delay and
doppler frequency are estimated
with precision depending on
the sample time Tsamp and on the
length of correlation. Also for the tracking
of the signal the correlation is between
the received signal and pilot signal of Eq.
4. For the tracking it is implemented a
non-coherent Early minus Late DLL [6].
The data recovery operations use two local
sampled signals (Eq. 5 and 6) that are both
correlated with the incoming received
signal [9].
These signal
are obtained
setting to
zero the pilot
components
in S(t).
This choice
is made
because
using only
one signal
(Sdata–1(t))
shows a
problem
when the sign of the two data is opposite: there
is a zero in the middle of both real and
imaginary part of correlation
Making correlation with these two signals
for the decision for the transmitted data is
necessary to observe only real parts of the
two correlations. For example when the
data of component ea–I = –1 nd the data of
component eb–I = 1 the shapes of real and
imaginary parts of correlations r1(t) and
r2(t) between the received signal and Sdata–
1(t) and Sdata–2(t) respectively are in Fig. 8.
In the table 2 are summarized all the cases.
Conclusion
In this paper have been described the
main features of Galileo E5 signal and
some algorithms for signal generation
and software receivers. In particular have
been highlighted differences between
conventional AltBOC and that used in
Galileo: two subcarriers are used and the
signal has an 8-PSK constellation. Have
been given some useful equations for
the implementation of a signal generator
or the development of a simulation and
have been proposed some techniques
for the acquisition, tracking and data
recovery of the overall received signal.
References
[1] ESA,GJU ``Galileo Open Service: Signal In Space Interface
Control Document (OS SIS
ICD) Draft 0’’, 23-05-2006
[2] G. W. Hein, J. Godet, J.L. Issler, J.C.
Martin, P. Erhard, R. L.Rodriguez, T.
Pratt ``Status of Galileo Frequency
and Signal Design’’, Brussels , 2002
[3] L. Ries, L. Lestarquit, E.A. Miret, F.
Legrand, W. Vigneau, C. Bourga ``A
Software Simulation Tool for GNSS2
BOC Signals Analysis’’, 2002
[4] D.J.R. Van Nee, A.J.R.M.
Coenen ``New Fast GPS Code-
Acquisition Tecnique Using FFT’’,
ELECTRONICS LETTERS 17th
January 1991 Vol. 27 No. 2
[5] K. Krumvieda, P. Madhani, C. Cloman,
E. Olson, J. Thomas, P. Axelrad, W.
Kober ``A Complete IF Software GPS
Receiver: A Tutorial about the Details’’,
ION GPS 2001, Salt Lake City, 2001
[6] W. Zhuang, J. Tranquilla ``Modelling
and Analysis for the GPS Pseudo-
Range Observable’’, IEEE
TRANSACTIONS ON AEROSPACE
AND ELECTRONIC SYSTEMS
VOL. 31, NO. 2 APRIL 1995
[7] J.M. Sleewaegen, W. De
Wilde, M. Hollreiser ``Galileo
AltBOC Receiver’’, 2004
[8] N. Gerein, A. Manz, M. Clayton, M.
Olynik ``Galileo Sensor Station Ground
Reference Receiver Performance
Characteristics’’, ION GPS/GNSS
2003, 9-12 Sept 2003, Portland, OR
[9] N.Gerein ``Hardware Architecture
for Processing Galileo Alternate
Binary Offset Carrier (AltBOC)
signals’’, United States Patent,
No. 6,922,177 B2, July 2006
[10] M.C. Jeruchim, P. Balaban,
K.S. Shanmugan ``Simulation
of Communication Systems’’,
Plenum Publishing Corporation,
New York, 1992
[11] S. Fantinato, S. Pupolin, L.
Vangelista ``Analysis of Galileo
AltBOC(15,10) Signal for Simulations
and Software Receivers’’, Proceedings
of WPMC 2007, Jaipur (India)
[12] S. Fantinato, Thesis: ``Development
of a software receiver for Galileo
system’’, Padova, 2007
Fantinato Samuele
Fantinato Samuele:
he got a degree in
Telecommunications
Engineering in July 2007
at University of Padova,
Italy, with the top of the marks. He was at
``Department of Information Engineering’’.
He joined Qascom Srl (an Italian company
working in GPS security and authentication),
during the thesis period. He is now interested in
working in companies researching in navigation
and satellite communication systems.
samuele.fantinato@gmail.com
.gif)
