986 IEEE COMMUNICATIONS LETTERS, VOL. 16, NO. 7, JULY 2012
Unambiguous BOC Modulated Signals Synchronization Technique
Jiamin Qi, Jiapin Chen, Zhenbo Li, and Dawei Zhang
Abstract—In this letter, a new unambiguous tracking technique
is proposed for binary offset carrier (BOC) modulated signals,
which will be utilized in global navigation satellite systems
(GNSS). The proposed technique is a synthesized correlation
function obtained by subtracting the cross-correlation between
the received B OC signal and the local auxiliary signal from the
cross-correlation of BOC signal between another local auxiliary
BOC signals. The theoretical standard tracking error formulas
are given. The results show that this technique can be a solution
to remove the ambiguity completely with slightly degradation in
acquisition and tracking for low modulation order BOC signals.
Index Terms—BOC modulation, ambiguity, synchronization,
standard deviation error.
I. INTRODUCTION
I
N order to enhance the code tracking performance and
provides the advantages of multipath effect reduction and
narrow band interference r ejection between the current GNSS
systems, a new type of modulated signals named BOC mod-
ulation have been adopted in modernized GPS [1], European
Galileo project [2] and Chinese Compass/BeiDou II [3]. The
BOC signal uses a square wave sub-carrier to modulate the
pseudo random noise (PRN) sequence.The subcarrier has
two types, sine phased or cosine phased. The sine phased
BOC signals are expressed as sBOC(m,n), where m and
n represents the ratio to 1.023MHz. However, there is an
ambiguity threat in acquisition and tracking processes, because
the auto-correlation fun c tion has more than one peak. With
the awkward ratio between subcarrier and local PN (Pseudo-
Noise) sequence, the difference b etween the maximum energy
of side peak and the m a in peak decreases. I n multipath a nd
interference e nvironments, it has the potential to lock on the
false zero point. If false acquisition occurs, the code tracking
loop will initially lock on the side peaks. In order to tackle
this problem, some unambiguous acquisition and tracking
approaches were proposed. These technique which can be
summarized into the following categories: the Bump Jumping
technique [4], the BPSK-like technique [5], The Sub Carrier
Phase Cancelation (SCPC) technique [6], and the Side-Peak
Cancelation Technique (SPCT) [7-10]. The essence of SPCT is
to minimize th e side-peaks of a uto-corre lation function since
they are the origin of the false lock tracking. The method
presented in [7, 8] have some drawbacks in the tracking stage,
because the effect of the side-peak is not removed completely.
So it can po tentially lock o n the false original peak. The
Manuscript received December 13, 2011. The associate editor coordinating
the revie w of this letter and approving it for publication was Y.-C. Wu.
The authors are with the Department of Information Process and System
Control, MNRI, JiaoTong University, ShangHai, China (e-mail: {chenjp, jmqi,
zbli, dawei}@sjtu.edu.cn).
Digital Object Identifier 10.1109/LCOMM.2012.050112.112521
proposed method in [8], is only dedicated to sinBOC(n,n)
signals is another flaw. For cosine BOC signals, it is useless.
In addition, bump-jumping (BJ) technique is also based upon
magnitude comparison, so it still has a high probability of
failure when signal-to-noise ratio is low. In this letter, a
Symmetrical Pulse Ambiguity Removing technique which is
suitable for all type BOC signals is proposed and its implement
structure is also simple and convenient to implement.
II. P
ROPOSED SYNCHRONIZATION TECHNIQUE
Inspired by autocorrelation correlated with its local replica
and cross-correlation function correlated with PRN sequence,
we can see that we can acquire a correlation function with only
one peak by construct several local auxiliary signals, the cross-
correlation of which with the received BOC signal can be
used to combine together to remove the undesired side peaks.
Taking the complexity into consideration, we choose the least
two correlations to combined the proposed correlation. So
the main idea of the proposed technique is to construct two
local auxiliary signals, the cross-correlation of w hich with
the received BOC signal can be used to combine together
to remove the undesired side peaks. We choose the combined
criterion as follows
R
comb
= |R
B/S1
|−|R
B/S2
| (1)
where R
B/S1
and R
B/S2
are the cross correlation between
the received BOC signal and two the local BOC-like signals,
respectively. The receiver locally generated reference BOC-
like waveform are two modified PRN defined as (x =1, 2):
S
x
= P
rn
S
c
⎧
⎨
⎩
1/2w, (k −1)T
c
<t<(k − 1+w)T
c
0, (k −1+w)T
c
<t<(k − w)T
c
(−1)
x
/2w, (k − w)T
c
<t<kT
c
(2)
where, S
c
is the subcarrier. P
rn
is the PRN spreading sequence
and k is the chip number within the sequence respectively.
And w is the pulse width of the local BOC-like code. It is
well known that the cross-corr elation is a linear superposition
of several triangular functions (in Fig.1). So by choosing the
local auxiliary BOC-like waveform, one can change the shape
of the combined DLL discriminator. In order to completely
remove the effect of side-peak, the pulse width of the local
generated signal must be smaller than 3n/8m chips. Julien’s
method [8] can be taken as an example with the pulse width
0.5 chips for the proposed tracking method (in Fig.2). And it
is out of the local pulse width range. So it can not eliminate
the effect of the side-peaks completely. We can understand this
phenomenon clearly in Fig.2, which shows the envelope of the
proposed composed correlation function with different pulse
width. In Fig.2, we can clearly see that the side peaks could
be removed completely b y choosing correct pulse width of
1089-7798/12$31.00
c
2012 IEEE
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