Baseband Hardware Designs in Modernised GNSS Receivers - Global Navigation Satellite Systems: Signal,Theory and Applications - page 35

1 1

Local
Reference
Mixers

Decision and
Feedback
Control

IF sam ples

Carrier
Mixer

N acc - bit
Accumulators

Measurements

N if

N 1 2R 2R

N car N 2 N acc

Navigation
data

...

1 R

Convolution
Decoder

Shift
Register

N ref Correlation computation

Timing
Control (to
all
sequential
blocks)

Subcarrier
Modulator

C L K

N ref

Carrier
Generator

Code
Generator

Sub-carrier
Generator

Carrier
NCO

Subcarrier
NCO

Code NCO

N nco1

N nco2 N nco3

Local signal generation and control

Fig. 2. A functional diagram of the baseband hardware (thick lines carry N • bits, dashed
boxes are optional )

2. Correlator Controller
The controller processes the correlation values produced by the core hardware and makes
decisions based on a set of feedback control algorithms. The threshold detection during the
acquisition, the carrier and code locked loops during the tracking, the process of dictating
the parameters for the local replica carrier and replica code generation, are all included in
this part.

The core correlator hardware functionality can again be divided into two parts.

1. Correlation Computation
2. Local Signal Generation

The necessity of this second level functionality division is due to the new signals that will
be dealt with in future sections. This type of segregation of the core correlator hardware
functionality helps accommodate new signals both at the same frequency that may belong to
a different constellation and the signals at different frequency bands of the same constellation.

The input to the baseband hardware is the sampled and digitized IF signal with N if -bit
quantization at a sampling frequency of f s Hz. In order to demodulate the navigation data
bits, the basebandmodule must first remove the carrier and the spreading code from the signal
(Braasch & van Dierendonck, 1999).

The IF samples are mixed with the locally generated carrier in the carrier mixer. The local
carrier frequency generator aims to match the frequency of the input signal. Both in phase