GNSS Modulation Schemes

Fundamentals
Title	GNSS Modulation Schemes
Author(s)	J.A Ávila Rodríguez, University FAF Munich, Germany.
Level	Advanced
Year of Publication	2011

Galileo and GPS signals are generated using the well-known Direct Sequence Spread Spectrum (DSSS) technique. DSSS is a particular case of Spread Spectrum (SS) technique.

The SS principle seems simple and evident, but its implementation is indeed complex. In order to accomplish this objective, different SS techniques are available, but they all have one thing in common: they perform the spreading and dispreading operation by means of a pseudo random noise (PRN) code attached to the communication channel. The manner of inserting this code into the transmitting chain before the antenna is actually what defines the particular SS technique, as next figure shows:

Figure 1: SS techniques classification depending on the point in the system at which the PRN code is inserted in the communication channel.

According to this, a DSSS signal, s(t) can be represented as follows [C.J. Hegarty, 2003] and [C.J. Hegarty et al., 2005]:

where [math]\displaystyle{ \left \{ c_k \right \} }[/math] represents the pseudorandom code symbols (which may be periodic), p(t) is the chip waveform which is non-zero only over the interval support [math]\displaystyle{ [0,T_c) }[/math] refers to the chip period and N is the number of equal length divisions of one chip period. In the next chapters of this chapter we will study different waveforms and analyse them in terms of their spectral efficiency and characteristics.

Of all the possible chip waveforms, the binary solutions belong to the most interesting for satellite navigation. Nevertheless, technology progresses so fast that it is possible to imagine more complex non-binary alternative spreading waveforms in a near future. In this chapter we will discuss the possibilities and potential they could bring to the navigation world.