Acquisition - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 72

I

∫

( ) 2

Incoming
signal Σ

Output

Q

∫

( ) 2

90°

Local
oscillator

PRN code
generator

FIGURE 6.1. Block diagram of the serial search algorithm.

In an ordinary receiver, the acquisition is usually performed in an application-
specific integrated circuit (ASIC). In a software receiver, it is implemented in soft-
ware. The following sections describe the theory behind three standard methods
of acquisition to demonstrate the possibility of implementing an efficient method
in a software receiver.

6.2 Serial Search Acquisition

Serial search acquisition is an often-used method for acquisition in code-division
multiple access systems (CDMA). GPS is a CDMA system. Figure 6.1 is a block
diagram of the serial search algorithm.
As seen in Figure 6.1, the algorithm is based on multiplication of locally gener-
ated PRN code sequences and locally generated carrier signals. The PRN genera-
tor generates a PRN sequence corresponding to a specific satellite. The generated
sequence has a certain code phase, from 0 to 1022 chips. The incoming signal
is initially multiplied by this locally generated PRN sequence. After multiplica-
tion with the PRN sequence, the signal is multiplied by a locally generated carrier
signal. Multiplication with the locally generated carrier signal generates the in-
phase signal I , and multiplication with a 90 ◦ phase-shifted version of the locally
generated carrier signal generates the quadrature signal Q .
The I and Q signals are integrated over 1 ms, corresponding to the length of
one C/A code, and finally squared and added. Ideally, the signal power should be
located in the I part of the signal, as the C/A code is only modulated onto that.
However, in this case the I signal generated at the satellite does not necessarily
correspond to the demodulated I . This is because the phase of the received signal
is unknown. So to be certain that the signal is detected, it is necessary to investi-
gate both the I and the Q signal. The output is a value of correlation between the
incoming signal and the locally generated signal. If a predefined threshold is ex-
ceeded, the frequency and code phase parameters are correct, and the parameters
can be passed on to the tracking algorithms.