Digital Transmission Techniques III: Modulation - Signals, Processes, and Systems - page 402

• Is it preferable to work with just one carrier frequency and many different discrete
states, or with many different carrier frequencies and only a few discrete states?

• But there are even more situations imaginable. For instance a mobile telephony
network where all participants use the same frequency range at the same time with
channels being separated by means of a special coding. In such a case all other
channels would make themselves felt as additional interference of the channel used.

This is an interrelationship of entirely different discrete states (amplitude, phase,
frequency) of a signal with regard to the optimum utilization of the limited bandwidth of
a transmission medium. Coding is added as the fourth "dimension". This means that
Claude SHANNON's information theory needs to be taken into account with regard to
this problem and there remain only two ways to gain a deeper insight:

• Well-aimed experiments with DASY Lab , always bearing in mind that in signal
physics as well as in other fields of science the motto always is: the results of proven
experiments that can be explained by physics are scientific truth. In technology
nothing is possible which would contradict the laws of nature.

• Interpretation of the fundamental statements of SHANNON's information theory.
Which of the results of this theory are of relevance for our questions? To what extent
do they contribute to solving our problem?

Quadrature Phase Shift Keying - QPSK
QPSK is the first step into a two-dimensional signal space. The aim is to use a trick to
transmit twice the mount of data (per time unit) using the same bandwidth.

For this purpose the input bit sequence is converted into two bit sequences of half the
pulse frequency (see Illustration 263, top right). Two successive serial bits of the
frequency f Bit are transformed into a "parallel dibit" with the frequency f Dibit . In this case
f Dibit is only half as large as f Bit .

This task is assumed by the so-called demultiplexer (multiplexers combine several
channels to form one channel, demultiplexers reverse this process, i.e. they split one
receiver channel into several output channels).

One component of this dibit is to denote the horizontal, the other one the vertical part in
the state of the signal space. This means that 2 2 = 4 different states are possible.

For a successful outcome (see Illustration 97 ff) a real part - which is related to cosine-
shaped oscillations - and an imaginary part which is related to sinusoidal oscillations -

are needed. Both parts are phase-shifted by just 90 degrees.

−