Signal Processing - September 2017 - 70

integration length to 2T1. In such a case, a receiver employing
the SDHT technique utilizes the next N 1 signal samples to perform an additional coherent search with Tco = T1 for the same
tested Doppler frequencies and coherently adds the results to
the previous correlation results to obtain the coherent correlation output for N i = 2 at the same tested Doppler-frequency
hypotheses. When no signal is found yet, the receiver can
find coherent correlation output for Doppler frequencies in
between the above-tested Doppler frequencies (i.e., -4,750 Hz,
-4,250 Hz,..., 4,750 Hz). As a result, the receiver obtains coherent correlation output for Tco = 2T1 at all Doppler-frequency
hypotheses at every 250 Hz within [-5, 5] kHz. This process
can be repeated for N i = 4, 8,..., until the receiver detects the
signal. In [34], a sample averaging technique is also utilized in
the SDHT technique and achieves a lower computational cost
than the 2-D-FFT technique. It should be noted that since the
SDHT technique does not assume data-free (wiped-off) signal
samples and finds consecutive correlation outputs R 6h, : , :@ for
h = 1, 2, f, it is possible to apply the data bit search process to
the result of the SDHT technique.

Assisted acquisition technique
To enhance the acquisition sensitivity and to reduce the timeto-first-fix (time to make a first position fix in a cold start)
of GNSS receivers connected to the cellular networks through
mobile phones, the GNSS receivers can make use of GNSS
signal observations made by the network. This is possible in
the A-GNSS technique that is a high sensitivity and fast acquisition technique different from those techniques for standalone
GNSS receivers introduced in the sections "Sample-Domain
Techniques: Averaging Techniques" and "Frequency-Domain
Techniques: Fast Computation Algorithms." There is various
assistance in the A-GNSS technique, including acquisition
assistance to help the mobile GNSS receiver (i.e., A-GNSS
receiver) narrow down the search area much smaller than the
whole 2-D hypothesis plane, and the conventional correlation
techniques and search strategies in the sections "Fundamentals
of GNSS Signal Acquisition" and "Channel Combining Techniques for New GNSS Signals" can be applied. This section
introduces the assisted acquisition in the A-GNSS technique
and some technical issues with the recent cellular networks
such as long-term evolution (LTE).
In the A-GNSS technique [35], the assistance generated
by the assistance server equipped with a reference GNSS
station is to help the A-GNSS receivers for acquisition and
positioning. The acquisition assistance sent to the A-GNSS
receiver includes the satellite ID, expected Doppler frequency and Doppler rate, Doppler uncertainty, code phase,
integer code phase (the number of code periods from the last
bit boundary), code-phase search window, GPS bit number,
azimuth, and elevation [36]. These are the essential information elements (IEs) needed to indicate (with uncertainties) the prompt Doppler frequency and the prompt code
phase of the GNSS signal being observed at the remote
A-GNSS receiver. These IEs make it possible for the acquisition function in the A-GNSS receiver to narrow the search
70

space down to a small, local search space composed of a
small number of hypotheses. As a result, the technique
improves the acquisition performance significantly in terms
of speed and complexity. Other assistance for positioning
includes the reference time, reference location (usually, the
base station location), differential corrections, navigation
model (i.e., satellite ephemeris and clock corrections), ionospheric model, coordinated universal time model, and realtime integrity [36].
In practice, there are two operational modes of the A-GNSS
technique: the mobile station (MS) assisted and the MS based. In
the MS-assisted mode, an A-GNSS receiver acquires and measures the incoming GNSS signals and the measurements are sent
to the network for position calculations, whereas in the MS-based
mode, an A-GNSS receiver not only acquires and measures the
incoming GNSS signals, but also calculates its own position
[37]. However, the acquisition assistance is only delivered in the
MS-assisted mode. In the MS-based mode, a mobile phone can
download the other assistance data from the assistance server in
advance, so that the A-GNSS receiver can exploit the data to narrow down the search space [3].
While the principle of A-GNSS technique has been standardized accordingly through the generations of cellular networks, it
should be noted that the performance benefit from the acquisition
assistance strongly relies on the clock and frequency accuracies of
the downlink pilot signal. In the cellular networks synchronized
to GPS, such as second-generation code-division multiple access
(2G CDMA) and LTE in time-division duplexing mode, the
(downlink) pilot channel and the cell-specific reference signal can
be used as a fine frequency reference to stabilize the mobile phone
oscillator and to narrow down the Doppler-frequency search
range. The pilot channel and cell-specific reference are also used
as an accurate time reference to narrow the code-phase search
range. In this case, an A-GNSS receiver, determining a smaller
search space using the assistance, may only need to consider the
Doppler-frequency uncertainties due to the user motion. Note
that the code-phase uncertainty due to the downlink propagation
is included in the assistance information. On the other hand, in
the asynchronous cellular networks (to GNSS) such as widebandCDMA and LTE in the frequency-division duplexing mode [38]-
[40], the Doppler-frequency uncertainty to define the search space
needs to be large enough, and the prompt code phase information
in the acquisition assistance may be not accurate.

Conclusions and future directions
To meet the growing demands for GNSS positioning in GNSSchallenged environments, high sensitivity and fast acquisition
have become the two most desired features for GNSS receivers.
First introduced were the fundamentals of the GNSS acquisition function and some of the new acquisition techniques for new
GNSS signals composed of pilot and data channels in QPSK or
in the same phase. In the second part, we have investigated fast
GNSS acquisition techniques, which achieve both low computational cost and high sensitivity, for standalone GNSS and
A-GNSS receivers. It has been found that the fast GNSS acquisition techniques use specialized search strategies according to their

IEEE SIGNAL PROCESSING MAGAZINE

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September 2017

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

Signal Processing - September 2017 - Cover1
Signal Processing - September 2017 - Cover2
Signal Processing - September 2017 - 1
Signal Processing - September 2017 - 2
Signal Processing - September 2017 - 3
Signal Processing - September 2017 - 4
Signal Processing - September 2017 - 5
Signal Processing - September 2017 - 6
Signal Processing - September 2017 - 7
Signal Processing - September 2017 - 8
Signal Processing - September 2017 - 9
Signal Processing - September 2017 - 10
Signal Processing - September 2017 - 11
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Signal Processing - September 2017 - 13
Signal Processing - September 2017 - 14
Signal Processing - September 2017 - 15
Signal Processing - September 2017 - 16
Signal Processing - September 2017 - 17
Signal Processing - September 2017 - 18
Signal Processing - September 2017 - 19
Signal Processing - September 2017 - 20
Signal Processing - September 2017 - 21
Signal Processing - September 2017 - 22
Signal Processing - September 2017 - 23
Signal Processing - September 2017 - 24
Signal Processing - September 2017 - 25
Signal Processing - September 2017 - 26
Signal Processing - September 2017 - 27
Signal Processing - September 2017 - 28
Signal Processing - September 2017 - 29
Signal Processing - September 2017 - 30
Signal Processing - September 2017 - 31
Signal Processing - September 2017 - 32
Signal Processing - September 2017 - 33
Signal Processing - September 2017 - 34
Signal Processing - September 2017 - 35
Signal Processing - September 2017 - 36
Signal Processing - September 2017 - 37
Signal Processing - September 2017 - 38
Signal Processing - September 2017 - 39
Signal Processing - September 2017 - 40
Signal Processing - September 2017 - 41
Signal Processing - September 2017 - 42
Signal Processing - September 2017 - 43
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Signal Processing - September 2017 - 45
Signal Processing - September 2017 - 46
Signal Processing - September 2017 - 47
Signal Processing - September 2017 - 48
Signal Processing - September 2017 - 49
Signal Processing - September 2017 - 50
Signal Processing - September 2017 - 51
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Signal Processing - September 2017 - 53
Signal Processing - September 2017 - 54
Signal Processing - September 2017 - 55
Signal Processing - September 2017 - 56
Signal Processing - September 2017 - 57
Signal Processing - September 2017 - 58
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Signal Processing - September 2017 - 60
Signal Processing - September 2017 - 61
Signal Processing - September 2017 - 62
Signal Processing - September 2017 - 63
Signal Processing - September 2017 - 64
Signal Processing - September 2017 - 65
Signal Processing - September 2017 - 66
Signal Processing - September 2017 - 67
Signal Processing - September 2017 - 68
Signal Processing - September 2017 - 69
Signal Processing - September 2017 - 70
Signal Processing - September 2017 - 71
Signal Processing - September 2017 - 72
Signal Processing - September 2017 - 73
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Signal Processing - September 2017 - 85
Signal Processing - September 2017 - 86
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Signal Processing - September 2017 - 88
Signal Processing - September 2017 - 89
Signal Processing - September 2017 - 90
Signal Processing - September 2017 - 91
Signal Processing - September 2017 - 92
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Signal Processing - September 2017 - 94
Signal Processing - September 2017 - 95
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Signal Processing - September 2017 - 97
Signal Processing - September 2017 - 98
Signal Processing - September 2017 - 99
Signal Processing - September 2017 - 100
Signal Processing - September 2017 - 101
Signal Processing - September 2017 - 102
Signal Processing - September 2017 - 103
Signal Processing - September 2017 - 104
Signal Processing - September 2017 - 105
Signal Processing - September 2017 - 106
Signal Processing - September 2017 - 107
Signal Processing - September 2017 - 108
Signal Processing - September 2017 - 109
Signal Processing - September 2017 - 110
Signal Processing - September 2017 - 111
Signal Processing - September 2017 - 112
Signal Processing - September 2017 - 113
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Signal Processing - September 2017 - 125
Signal Processing - September 2017 - 126
Signal Processing - September 2017 - 127
Signal Processing - September 2017 - 128
Signal Processing - September 2017 - 129
Signal Processing - September 2017 - 130
Signal Processing - September 2017 - 131
Signal Processing - September 2017 - 132
Signal Processing - September 2017 - 133
Signal Processing - September 2017 - 134
Signal Processing - September 2017 - 135
Signal Processing - September 2017 - 136
Signal Processing - September 2017 - 137
Signal Processing - September 2017 - 138
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Signal Processing - September 2017 - 140
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Signal Processing - September 2017 - 143
Signal Processing - September 2017 - 144
Signal Processing - September 2017 - 145
Signal Processing - September 2017 - 146
Signal Processing - September 2017 - 147
Signal Processing - September 2017 - 148
Signal Processing - September 2017 - 149
Signal Processing - September 2017 - 150
Signal Processing - September 2017 - 151
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Signal Processing - September 2017 - 153
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Signal Processing - September 2017 - 196
Signal Processing - September 2017 - Cover3
Signal Processing - September 2017 - Cover4
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