GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 334

to reduce the sampling frequency. The next section will cover the number of bits
required under strong interference.

12.9 NUMBER OF ADC BITS REQUIRED UNDER JAMMING ( 7 )

In commercial GPS receivers, the ADC used is only one or two bits because all
the signals have comparable strength under normal environment. Under inten-
tional jamming, however, it is interesting to consider how many bits are needed
to process the GPS signals. This problem can be very complicated because there
are different types of jamming signals and many different approaches to mitigate
the jamming signal. If the jamming signal cannot be completely removed, the
GPS signal may not be found. In order to simplify this study, simulated data with
a very simple jamming signal will be evaluated.
Only one jamming and one GPS signal will be used to study this problem.
In order to further simplify the problem, the GPS signal strength considered
will be at nominal power level, such that 1 ms of data can be used for acquisi-
tion. The correlation output then has 5000 points instead of 2500 points in the
time domain to achieve the maximum correlation output. The following signal
conditions are used:

1. The signals are sampled at 5 MHz; thus the equivalent bandwidth

is 2.5 MHz.
2. The input GPS signal S/N

=− 18 dB. The noise bandwidth is 2.5 MHz
because of the sampling rate used in this simulation. After 1-ms acquisition
through coherent integration, the output S/N

≈ 14 . 4dB.
3. The jamming signal is a cw with a fixed frequency. The frequency matches
one of the frequency bins after the FFT operation. Thus the jamming signal
can be easily removed in the frequency domain.

The jamming-to-signal ratio ( J / S ) is the jamming power to signal power ratio,
usually expressed in dB. The J / N is the jamming power to noise power ratio
expressed in dB. The J / N is used as an input parameter in the simulation. Because
the signal is 18 dB below the noise, the relation between J / S and J / N is

J/S = J/N − 18 dB ( 12 . 18 )

For example, if the J/S = 120 dB, the J / N is 102 dB.

The input data are digitized by an ADC with a different number of bits to
generate 5000 points. Before performing acquisition on the input data, an FFT
operation is performed. Since the input data are real values, and the jamming
signal is on a frequency bin, two spectral lines are produced that are symmetrical
with respect to the 2500th frequency bin. This jamming signal is removed by
making the two spectral lines equal to zero. The remaining results are converted
back to time domain through an inverse fast Fourier transform (IFFT). A regular