Baseband Hardware Designs in Modernised GNSS Receivers - Global Navigation Satellite Systems: Signal,Theory and Applications - page 49

Carrier Mixer
Output

+

32x5
LUT
s I r I

32x5
LUT
s Q r Q

s I 6

3

Sample
Correlation

s Q

3

32x5
LUT
s I r Q

32x5
LUT
s Q r I

+

6

Reference
Signal

r I

2

r Q

2

Fig. 8. Local reference mixer for the complex modulation signals

It should be noted that the reference signal mixer example for the complex signal is chosen
and dealt in more detail here because it is the correlator block which has a major impact due
to the signal structure and is drastically different to the implementation of the reference signal
mixer for the GPS L1 C/A signal. It is not difficult to identify other such resource hungry and
power hungry blocks and is essentially a part of the baseband hardware design process.

5.4.2 Operating frequency considerations

One of the major contributors for the higher power consumption of the correlators that
process new signals is the correlator operating frequency. The operating frequency of the
correlator is typically the sampling frequency at which the IF signal samples are received.
However in most of the situations, once the signal is brought to the baseband after the carrier
mix operation (the signal at this point may still contain residual Doppler) the result can be
resampled to a lower sampling frequency. The minimum operating frequency for the stages
after the carrier mix operation can then be reduced to twice the spreading code chipping rate
(Namgoong & Meng, 2001a,b; Namgoong et al., 2000). Reduction below twice the spreading
code chipping rate is possible but care should be taken to trade-off wisely the signal loss
vs. correlator power consumption advantage. The carrier mixer output should undergo
proper filtering before the sampling frequency reduction which will increase the resource
requirement (by the amount of resource consumed by the filter). Initial implementation
results show that the resource requirements of the filter are not significant and hence it is
not a significant overhead.

5.4.3 Processing signal components separately vs. processing together

The accumulator at the end of the correlator computation chain is a power hungry block.
Typically six accumulators are required for the correlator that implements three delayed (early
prompt and late) reference signals for the reference signal correlation. The requirement of
separate correlation values for the individual signal components increases the requirement of
the number of accumulators. For example, the GPS L5 signal requires 12 sets of accumulators