Baseband Hardware Designs in Modernised GNSS Receivers - Global Navigation Satellite Systems: Signal,Theory and Applications - page 41

operating frequency requirement of the correlator will go up and also an additional clock
divider circuit is required.
3.4 Multiple signal components
When a signal has more than one component (say pilot and data components), it is wise
to compute the correlation values independently for each signal component, thus allowing
the subsequent processing blocks to use efficient tracking techniques (Shivaramaiah, 2011).
One can optimise the correlation computation blocks by combining the logic for the signal
components but that would give a combined correlation value to the tracking loops. This
combined correlation value may suffer from loss due to the data and or code bit inversions
between the signal components. Therefore it is wise to isolate the different signal components
at the correlation computation stage (and combine in the succeeding stages if required).
3.5 Receiver bandwidth and the operating frequency
Receiver bandwidth has a direct impact on the sampling frequency and hence the operating
frequency of the circuit. While some baseband blocks can be fed a slower clock than the
sampling frequency (but still derived from the sampling frequency), some other blocks have
to operate at the sampling frequency itself. Any bandwidth reduction below the minimum
required (which is typically the bandwidth occupied by the main lobe(s)) done before the
correlation operation stage, will result in rounded auto-correlation peaks, which in turn result
in noisier range measurements. Therefore it is a good practice to keep the operating frequency
at least equal to the sampling frequency until the carrier mixing stage and at least equal to four
times the subcarrier frequency (or the twice the code frequency in the absence of subcarrier)
from the reference signal mixer stage onwards.
3.6 Complex modulation
In the case of AltBOC signals the lines generated within the core correlator portion in Fig. 2
carry complex signals. The local reference mixer LUT must cater for the complex correlation
operation.
There are basically two ways to realise this complex reference signal mixer: with the logic
or with LUTs. With the logic one would be using adders/subtracters and multipliers of
appropriate length to compute the reference signal mixer outputs. With the LUT, there are
many ways, each using different sizes of the LUT. In both the cases the resource requirement
would significantly increase compared to the GPS L1 C/A correlator (which requires no
reference signal mixer).
4. Core correlator architectural modifications for the new signals
4.1 New GNSS signals and general requirements
Table 1 revisits the centre frequency, typical receiver bandwidth and code lengths of some
of the new open service signals. These parameters largely determine the architecture and
complexity of the baseband signal processing stage in a GNSS receiver.
The following are the important points to note from the table.