BPSK-like Methods for Hybrid-Search Acquisition
of Galileo Signals
Adina Burian, Elena Simona Lohan, Markku Renfors
Institute of Communications Engineering, Tampere University of Technology
P.O.Box 553, FIN-33101, Finland
adina.burian@tut.fi, elena-simona.lohan@tut.fi, markku.renfors@tut.fi
Abstract— The Binary Offset Carrier (BOC) modulation which
has been proposed for future Galileo and GPS M-code signals,
provides a higher spectral separation from BPSK-modulated
signals, such as GPS C/A code. The absolute value of the auto-
correlation function of a BOC signal has a narrower main lobe,
which may increase the resolution of delay estimates, but also
presents deep fades, which may lead to a higher number of timing
hypotheses to acquire the signal. In order to get rid of these
ambiguities, several approaches have been proposed in literature,
which provide an unambiguous BPSK-like shape of correlation
function. In this paper we analyze, compare and develop further
two BPSK-like methods which allow to acquire a BOC-signal
unambiguously. The focus is on hybrid search, where several
time-frequency bins are searched in parallel.
We introduce here a modified version of a BPSK-like method
which decreases the receiver complexity and is valid for both
odd and even BOC orders. We analyze both single-side band
(SSB) processing (i.e., only one band is used) and dual-side band
(DSB) processing (i.e., upper and lower bands are combined
non-coherently). While eliminating the ambiguities in auto-
correlation function, both SSB and DSB processing present some
performance degradation, induced by the band selection and
non-coherent processing. The analysis is done in the presence
of multipath fading channels. As a benchmark, we keep also the
ambiguous BOC processing. We consider parameters specified in
the proposals for Galileo system Open Service (OS), respectively
Publicly Regulated Service (PRS).
I. INTRODUCTION
The BOC modulation has been selected for both Galileo
and modernized GPS signals [1]. The main target of BOC
modulation has been to provide a better spectral separation
with existing BPSK-modulated GPS signals, while allowing
optimal usage of the available bandwidth for different GNSS
signals [1], [2]. Also, BOC signals present a narrower main
lobe of the absolute value of their autocorrelation function
(ACF), which enhances the tracking accuracy. On the other
hand, the additional fades which appear in ACF within the
two-chip intervals may induce a missed detection due to a
zero (or very low) sampling point and may lead to a longer
acquisition time. In consequence, the necessary step to search
a given time-uncertainty window, ∆t
bin
, should be small
enough in order to find the main lobe of ACF. Therefore, the
computational complexity in acquisition process is increased,
the computational load being inversely proportional to the
square of step time bin ∆t
bin
, as reported in [3].
ABOC(m, n) signal (m and n are not necessarily integers)
is created by a square sub-carrier modulation, where the signal
is multiplied by a rectangular sub-carrier (e.g., with +1 and -1
values) at sub-carrier frequency. The modulation parameters
are equal to m = f
sc
/f
ref
and n = f
c
/f
ref
where f
sc
is
the sub-carrier frequency, f
c
is the code rate and f
ref
=1.023
MHz is the reference frequency. Thus, the power spectrum
is split into two symmetrical components around the carrier
frequency. The common baseline for OS structure (agreed by
US and European negotiation in June 2004) employs the sine
BOC(1,1) modulation, which uses a 1.023 MHz square-wave
sub-carrier modulated by spreading code chips at a chip rate
f
c
=1.023 MHz [8]. For PRS services, both sine and cosine
BOC(15,2.5) have been proposed [8]. Other BOC modulations
have also been considered, such as BOC(15,10), BOC(10,5)
or BOC(10,4) [5], [6].
In order to eliminate the ambiguities of the ACF envelope,
different methods have been approached in literature [3], [4],
[5], [6], [7]. The main idea behind these ”BPSK-like” methods
(generic name proposed in [5]) is that the BOC-modulated
signal can be obtained as the sum of two BPSK-modulated
signals, located at positive and negative sub-carrier frequen-
cies. The effect of sub-carrier modulation can be removed by
using a pair of single-sideband correlators. We may have a
single-side band (SSB) receiver, where either the negative or
the positive of the sidebands correlators is used, or a dual-side
band (DSB) r eceiver, when both bands are combined non-
coherently. Due to filtering and correlation losses, the BPSK-
like methods bring some degradation in the signal level.
In [3], [5] it is asserted that a SSB BPSK-like method
induces at least 3 dB degradation in SNR. If DSB processing
is used, this loss can be partially compensated, excepting
anyway the squaring losses in non-coherent integration [5].
Compared to coherent processing of both sidebands, non-
coherent processing looses about 0.5 dB of SNR.
The ’BPSK-like’ methods proposed so far in the literature
fall in of the following two categories: either the main lobes
of the signal and of the reference BOC-modulated PRN code
are selected (via filtering) and then correlated [3], [4], [7],
or both the main lobes and secondary lobes between of the
received signal are kept and the reference code is based on
the BPSK-modulated PRN code [5], [6]. We will refer to
the former method as Fishman & al. method [3], [4], [7],
and to the later method as the Martin & al. method [5], [6]
(from the authors’ names). The Martin & al. method, if the
reference code is properly selected, has the advantage of lower
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2006 proceedings.
1-4244-0355-3/06/$20.00 (c) 2006 IEEE
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