GNSS Signal Simulation - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 159

new code generators (BOC, PRN, and secondary code) and longer mini-
mum dwell times. A generic L1 OS tracking architecture is shown in Fig-
ure B.7.
The downconversion from IF to baseband is done with the usual carrier
NCO and mixer. A separate PRN code generator (or look-up table depend-
ing on the final implementation) is used for the local data and pilot channel
PRN code replicas. Both local codes are combined with the BOC subcar-
rier. The pilot channel code is combined with the slow varying secondary
code. All code generators are controlled by the strobe signal from the code
NCO, although the secondary code generator uses a divided version of this
timing signal. Suitable discriminators and loop filters provide the control
signals for both NCOs as usual.
Both the data and pilot signals must be tracked together (combined track-
ing) to maximize receiver performance.
Data Demodulation and Decoding The demodulation will need an updated pre-
amble search function. The code will have to employ a deinterleaver to put
the received symbols (FEC encoded bits) in the right order. The MATLAB
communication toolbox has built-in functions for interleaving and deinter-
leaving.
A Viterbi decoder must be used to decode the symbol stream encoded by
the FEC algorithm. The MATLAB communication toolbox has a built-in
function vitdec to decode the FEC stream. The subframes have 6 tail bits set
to zero to “initialize” the Viterbi decoder at the start of the next subframe.
Therefore, the decoding can commence at the start of any subframe without
prior knowledge of the navigation data stream.
The CRC check must be computed to detect any errors. The algorithm is
very similar but not identical with that of GPS.
The actual locations of all transmitted data parameters in the data messages
will be different. Also, some data will be transmitted in the Galileo sys-
tem that is not transmitted in GPS. Therefore, new code must be written to
decode navigation messages.
Decoding of ephemerides and almanac data is very similar to that of GPS
once the data fields are read from the data messages. Decoding of time-
related fields will need minor changes due to new width of the data fields
and scale factors.
Position Computation The position computation is exactly the same as in GPS
once the time-related data and ephemerides are decoded. There is a small
difference in the coordinate systems (few centimeters) between GPS and
Galileo. Care must be taken to handle this properly in case of very precise
measurements.