High Sensitivity Techniques for GNSS Signal Acquisition - Global Navigation Satellite Systems: Signal,Theory and Applications - page 12

However, this technique might suffer from the combination loss due to the unknown data
transitions. Assuming that the chance of changing data bit sign after each data bit period is
50%, then if full code correlation (i.e. T int = 1 ms) is used, the average degradation due to

data overlay is 20 log

(

18/19

) ≈

0.47 dB. However, in the Galileo case, this loss is scaled to

( ) ≈

20 log 6 dB, because the data bit duration is equal to the code length of 4 ms. This fact

causes difficulties in applying the differential technique for Galileo E1 OS receivers.

1/2

As an expanded view of the conventional differential combination technique, generalized
differential combination techniques are introduced to further improve the sensitivity of the
acquisition process. These advanced differential techniques will be discussed in details in

Section 4.

3.3 External aiding approach for harsh environments

For this approach, basically, the availability of external aiding sources makes the value of
T int able to be larger than T b (i.e. T int >

T b ). Therefore, in this scenario, increasing T int
(or coherent combination of the correlator outputs) is the most suitable solution to give the
best sensitivity improvement to the acquisition stage operating in harsh environments. In

literature, this approach is also referred as assistance or assisted approach.

As pointed out in (Djuknic & Richton, 2001), the assisted technique enables HS acquisition,
since it provides the signal processing chain with preliminary (but approximate) code-phase /
Doppler frequency estimates along with fragments of the navigation message. This allows for
wiping off data-bit transitions and for extending the coherent integration time. The concept of
data-bit assistance has been also introduced by the 3rd Generation Partnership Project (3GPP)
in its technical specifications of the Assisted GNSS (A-GNSS) for UMTS (3GPP, 2008a) and
GSM/EDGE (3GPP, 2008b) networks.

In general, with all post correlation processing techniques presented in Section 3.2, sensitivity
losses are experienced due to

• the residual Doppler error (including the finite search resolution in frequency and the

contribution of the user dynamics)

• the uncertainty on the Local Oscillator (LO) frequency.

These effects impact the observed Radio Frequency (RF) carrier frequency and can be more
relevant with long coherent integrations (Chansarkar & Garin, 2000) as the case of the coherent
combination in this external aiding approach.

Finally, a trade-off between sensitivity and complexity is always necessary, particularly for
mass-market receivers (e.g. embedded in cellular phones) which require real-time processing
but low power consumption. Despite the recent improvements in chip-set sizes and speeds,
a real-time indoor-grade high-sensitivity receiver for cellular phones does not exist yet.
Reduced sampling rates are mandatory to minimize the computational load of the baseband
processing as well as the optimization of the assistance information exchange is fundamental
in order to minimize the communication load which is likely to be paid by the user, according
to the latest trends, such as the Secure User Plane for Location (SUPL) defined by Open Mobile
Alliance (OMA), (Mulassano & Dovis, 2010; OMA, 2007).