Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 122

6.12 QUANTIZATION LEVELS ( 11-13 )
As discussed in Section 5.3, GPS is a CDMA signal. In order to receive the
maximum of signals, it is desirable to have comparable signal strength from
all visible satellites at the receiver. Under this condition, the dynamic range of
a GPS receiver need not be very high. An ADC with a few bits is relatively
easy to fabricate and may operate at high frequency. Another advantage of using
fewer bits is that it is easier to process the digitized data, especially when they
are processed through hardware. The disadvantage of using fewer bits is the
degradation of the signal-to-noise ratio. Spilker ( 11 ) indicated that a 1-bit ADC
degrades the signal-to-noise ratio by 1.96 dB and a 2-bit ADC degrades the
signal-to-noise by 0.55 dB. Many commercial GPS receivers use only 1- or 2-
bit ADCs.
Chang ( 12 ) claims that the degradation due to the number of bits of the ADC
is a function of input signal-to-noise ratio and sampling frequency. Low signal-
to-noise ratio signal sampled at a higher frequency causes less degradation in a
receiver. The GPS signal should belong to the low signal-to-noise ratio because
the signal is below the noise. At a Nyquist sampling rate, the minimum degrada-
tion is about 3.01 and 0.72 dB for 1- and 2-bit quantizers, respectively. At five
times the Nyquist sampling rate, the minimum degradation is 2.18 and .60 dB
for 1- and 2-bit quantizers, respectively. These values are slightly higher than the
results in reference 11.
The only time that a high number of bits in ADC is required in a GPS receiver
is to build a receiver with antijamming capability. Usually, the jamming signal
is much stronger than the desired GPS signals. An ADC with a small number of
bits will be easily saturated by the jamming signal. Under this condition, the GPS
signals might be masked by the jamming signal and the receiver cannot detect the
desired signals. If an ADC with a large number of bits is used, the dynamic range
of the receiver is high. Under this condition, the jamming signal can still disturb
the operation; however, the weak GPS signals are preserved in the digitized data.
If proper digital signal processing is applied, the GPS signals should be recovered.
This problem can be considered in the frequency domain. Assume that there are
two signals, a strong one and a weak one, and they are close in frequency.
In order to receive both signals, the receiver must have enough instantaneous
dynamic range, which is defined as the capability to receive strong and weak
signals simultaneously. If the ADC does not have enough dynamic range, the
weak signal may not be received. Reference 12 provides more information on
this subject. In Section 12.9, the number of ADC bit required under jamming is
discussed.
6.13 HILBERT TRANSFORM ( 10 )
In this topic a single channel is used to collect data and the software is written
to process real data. If a software receiver is designed to process complex data
and only real data are available, the real data can be changed to complex data