GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 115

sequence or PRN code . An excellent overview of PRN codes, including their genera-
tion, characteristics, and code families with good properties is provided in [2]. The
minimum interval of time between transitions in the PRN waveform is commonly
referred to as the chip period , T c ; the portion of the PRN waveform over one chip
period is known as a chip or spreading symbol ; and the reciprocal of the chip period
is known as the chipping rate , R c . The independent time parameter for the PRN
waveform is often expressed in units of chips and referred to as codephase .
The signal just described is called spread spectrum, because of the wider band-
width occupied by the signal after modulation by the high-rate PRN waveform. In
general, the bandwidth is proportional to the chipping rate.
There are three primary reasons why DSSS waveforms are employed for satel-
lite navigation. First and most importantly, the frequent phase inversions in the sig-
nal introduced by the PRN waveform enable precise ranging by the receiver.
Second, the use of different PRN sequences from a well-designed set enables multi-
ple satellites to transmit signals simultaneously and at the same frequency. A
receiver can distinguish among these signals, based on their different codes. For this
reason, the transmission of multiple DSSS signals having different spreading
sequences on a common carrier frequency is referred to as code division multiple
access (CDMA). Finally, as detailed in Chapter 6, DSSS provides significant rejec-
tion of narrowband interference.
It should be noted that the chip waveform in a DSSS signal does not need to be
rectangular (i.e., a constant amplitude over the chip period), as we have assumed
earlier. In principle, any shape could be used and different shapes can be used for
different chips. Henceforth, we will denote DSSS signals generated using BPSK sig-
naling with rectangular chips as BPSK-R signals. Several variations of the basic
DSSS signal that employ nonrectangular symbols have been investigated for satellite
navigation applications in recent years. Binary offset carrier (BOC) signals [3] are
generated using DSSS techniques but employ portions of a square wave for the
spreading symbols. A generalized treatment of the use of arbitrary binary patterns
to generate each spreading symbol is provided in [4]. Spreading symbol shapes, such
as raised cosines, whose amplitudes vary over a wide range of values, are used
extensively in digital communications. These shapes have also been considered for
satellite navigation but to date have not been used for practical reasons. For precise
ranging, it is necessary for the satellite and user equipment to be able to faithfully
reproduce the spreading waveform, which is facilitated through the use of signals
that can be generated using simple digital means. Furthermore, spectral efficiency,
which has motivated extensive studies in symbol shaping for communications appli-
cations, is generally not a concern for satellite navigation. Finally, DSSS signals with
constant envelope (e.g., those that employ binary-valued—one magnitude with two
possible polarities—spreading symbols) can be efficiently transmitted using

switching-class amplifiers.

4.2.2 Multiplexing Techniques
In navigation applications, it is frequently required to broadcast multiple signals
from a satellite constellation, from a single satellite, and even upon a single carrier
frequency. There are a number of techniques to facilitate this sharing of a common