Signal Processing - September 2017 - 41

Alternatives to the
Code Tracking

Code Tracking Structures

Based on Subspace
Decomposition

Based on Multicorrelators

APME
Douple
Delta

BJ
DP

EML/DLL
MGD
NCORR
RML

CADLL

CELP

MEDLL

ELS
HRC

MET

Joint Implementation
with PVT
Computation

Quadratic
Optimization
Peak
Tracking

SAGE

S-Curve
Shaping

TLS-ESPRIT
ML

Teager
Kaiser

PAC

NELP
RSSML

MUSIC
POCS

E1/E2

FIMLA

Based on Nonlinear
Signal Processing

DPE
RDPE
VDLL

Wavelet

SBME
VE-VL

Strobe
Correlator

Vision
Correlator/
MMT

FIGURE 3. Our classification of the code trackers in generic DS-SS systems. PVT: position, velocity, time; MUSIC: multiple signal classification; SAGE:
space alternating generalized expectation maximization; VDLL: vector delay locked loop; MGD: multiple gate delay; FIMLA: fast iterative maximum
likelihood algorithm; RML: robust maximum likelihood; SBME: slope-based multipath envelope; VE-VL: very early-very late; MMT: multipath mitigation
technique; RSSML: reduced search space maximum likelihood; CELP: coherent early late processing; ELS: early late slope; POCS: projection onto convex
sets; TLS-ESPRIT: total least squares-estimation of signal parameters via rotational invariance techniques; NELP: noncoherent early late processing;
MEDLL: multipath estimating delay lock loop; CADLL: coupled amplitude delay lock loop; APME: a posteriori multipath envelope; EML/DLL: early minus
late/delay lock loop.

The motivation of having so many available code-tracking
structures (Figure 3) comes from the fact that there is not a
unique optimization criteria to be targeted, but instead, different trackers address different criteria, such as maximizing
accuracy, minimizing mean time to lose lock, minimizing
implementation complexity, etc. Furthermore, a systematic
approach for deriving these trackers is sometimes missing,
as they are often proposed in an ad-hoc manner based on
empirical evidences. In addition, finding nonpatented solutions may also be a target, as many of the structures enumerated in Figure 3 are already covered by patents. To the best
of our knowledge, there is no exhaustive comparison between
all of the available code-tracking structures in terms of their
performance and complexity, but partial results can be found,
e.g., in [12] for comparing several open-loop code trackers,
in [14] for a brief survey of tracking loop-based multipath
mitigation techniques, in [11] for another survey of multipath
mitigation techniques for GNSS, and, finally in [2] for a classification of code-tracking methods in GNSS. More advanced
tracking structures, such as those based on antenna arrays,
are discussed, for example, in [15] and are out the scope of
this article.

The code trackers enumerated in Figure 3 are generic (for
any DS-SS) and they do not treat explicitly the additional challenges created by BOC modulation, with the exception of the
bump jumping (BJ) technique (shown with different color),
which can be used with and without ambiguities. BJ will be
discussed in more detail in the section "Comparative Summary." However, as described next, such generic structures can
be combined with unambiguous stages.

BOC modulation and the challenges
of the ambiguities
BOC modulation
A BOC-modulated signal is a signal with a split spectrum,
when the baseband signal energy is moved away from the
zero frequency and there is a notch in the signal spectrum at
zero frequency. Such a frequency split is realized dividing
the signal into subchips or BOC units with alternating sign.
The number of BOC units per one chip is called the BOC
modulation factor, and it is denoted here via N B [16]. There
are two basic types of BOC modulation: a sine BOC modulation, referred to as BOC from now on (created by taking

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September 2017

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Table of Contents for the Digital Edition of Signal Processing - September 2017
