Baseband Hardware Designs in Modernised GNSS Receivers - Global Navigation Satellite Systems: Signal,Theory and Applications - page 36

and quadrature phase signals are generated with N car -bit quantization. The carrier mixer
output results in N 1 -bit values.
The local replica code+subcarrier signal, referred to here as the local "reference signal" is
N re f -bit wide. In the absence of the subcarrier, N re f =1 because the spreading code takes only
the values of 1 or 0. Since most of the signal tracking algorithms employed in a GNSS receiver
use the delay tracking principle, delayed versions of the local reference signal are generated
with the help of shift registers. R is the number of local reference signal “arms” (sometimes
referred to as "taps" or "fingers"), typically three: the Early, the Prompt and the Late).

The local reference mixer generates 2 R values each N 2 -bits wide as a result of combining
in phase and quadrature values with the local reference signal. These individual sample
correlation values are accumulated in a N acc -bit accumulator for a predefined “integration
duration”. The tracking loops act on these accumulator outputs and adjust the local carrier
frequency and the code delay so as to maintain lock (to be at the peak of the correlation
function). The tracking loops also produce the measurements and also demodulate the
navigation data bits present in the signal (Shivaramaiah, 2004).

2.3 Bit-width requirements of the correlator components

The parameters of interest for the complexity analysis of the core correlator are the number
of bits required to represent the intermediate signals, the bit-width of the accumulator and
other registers and the minimum frequency of operation required for a particular signal (or
any component of a signal thereof). The notations for the number of bits at different stages
are shown in Fig. 2, as N along with the thick lines. In the following paragraphs a brief

description of each of the underlying modules is given and the number of bits required for
the accumulator is derived.

•

2.3.1 ADC/IF ( N if )
The signal loss due to the quantisation beyond 2-bits is insignificant as long as the sampling
thresholds are sensibly set (Hegarty, 2009). However, 3-bits and more have been used to
alleviate the problems with the AGC in the presence of RF interference (Kaplan & Hegarty,
2006). Commercial mass-market receivers normally use 2-bit uniform sign-magnitude
quantisation with 4 levels

{±

1,

±

3

}

(Zarlink, 1999, 2001). Therefore for the examples in this

chapter it is safe to assume N if

=

2.

2.3.2 Local carrier generator ( N car )

The loss due to the local carrier quantisation is studied in Namgoong et al. (2000).
Typically, 3-bit uniform NCO phase quantisation and 2-bit amplitude quantisation with 4
levels

is used. More bits in the phase and amplitude quantisation increases the
Spurious-Free-Dynamic-Range (SFDR) and reduces the quantisation noise. However this has
a significant impact on the size of succeeding stages.

{±

1,

±

2

}