GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 116

transmission channel without the broadcast signals interfering with each other. The
use of different carrier frequencies to transmit multiple signals is referred to as fre-
quency division multiple access (FDMA) or frequency division multiplexing (FDM).
Sharing a transmitter over time among two or more signals is referred to as time
division multiple access (TDMA) or time division multiplexing (TDM). CDMA, or
the use of different spreading codes to allow the sharing of a common carrier
frequency, was introduced previously in Section 4.2.1.
When a common transmitter is used to broadcast multiple signals on a single
carrier, it is desirable to combine these signals in a manner that forms a composite
signal with a constant envelope for the reason discussed in Section 4.2.1. Two
binary DSSS signals may be combined using quadrature phase shift keying (QPSK).
In QPSK, the two signals are generated using RF carriers that are in phase quadra-
ture (i.e., they have a relative phase difference of 90º, such as cosine and sine func-
tions of the same time parameter) and are simply added together. The two
constituents of a QPSK signal are referred to as the in-phase and quadraphase
components.
When it is desired to combine more than two signals on a common carrier, more
complicated multiplexing techniques are required. Interplexing combines three
binary DSSS signals on a common carrier while retaining constant envelope [5]. To
accomplish this feat, a fourth signal that is completely determined by the three
desired signals, is also transmitted. The overall transmitted signal may be expressed
in the form of a QPSK signal:

() () ( ) () ( )

st = s t cos 2 π ft − s t sin 2 π ft (4.1)

I c Q c

with in-phase and quadra-phase components, s I ( t ) and s Q ( t ), respectively, as:

() () ( ) () ( )

s t = 2 P s t cos mP − 2 s t sin m

I I 1 Q 2

(4.2)

() () ( ) () () () ( )

s t = 2 P s t cos mP s + 2 t s t s t sin m

Q Q 3 I 1 2 3

where s 1 ( t ), s 2 ( t ), and s 3 ( t ) are the three desired signals, f c is the carrier frequency, and
m is an index that is set in conjunction with the power parameters P I and P Q to
achieve the desired power levels for the four multiplexed (three desired plus one
additional) signals.
Other techniques for multiplexing more than two binary DSSS signals while
retaining constant envelope include majority vote [6] and intervoting [7]. In major-
ity vote, an odd number of DSSS signals are combined by taking the majority of their
underlying PRN sequence values at every instant in time to generate a composite
DSSS signal. Intervoting consists of the simultaneous application of interplexing and
majority vote.

4.2.3 Signal Models and Characteristics
In addition to the general quadrature signal representation in (4.1) for GNSS signals,
we will find it occasionally convenient to use the complex-envelope or lowpass rep-
resentation, s l ( t ), defined by the relation: