Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 209

13 Basic Principles of Digital Modulation
To begin with, this chapter quite generally creates a basis for an approach
to the digital modulation methods. Following this chapter, it would also be
possible to continue e.g. in the field of mobile radio technology (GSM,
IS95 or UMTS) as the basic knowledge discussed here applies to the field
of communication technology and its applications overall. However, its
prime intent is to create the foundation for the subsequent chapters on
DVB-S, DVB-C, OFDM/COFDM, DVB-T, ATSC and ISDB-T. Experts,
of course can simply skip this chapter.
13.1 Introduction
Analog transmission of information has long been effected by means of
amplitude modulation (AM) and frequency modulation (FM). The infor-
mation to be transmitted is impressed on the carrier by varying either its
amplitude or frequency or phase, this process being referred to as modula-
tion.
To transmit data signals, i.e. digital signals, amplitude or frequency shift
keying was used in the early times of data transmission. To transmit a data
stream of e.g. 10 Mbit/s by means of simple amplitude shift keying (ASK),
a bandwidth of at least 10 MHz is required if a non-return-to-zero code
(NRZ) is used. According to the Nyquist theorem, a bandwidth corre-
sponding to at least half the data rate is required for the NRZ baseband
signal. Using ASK produces two sidebands and that gives a RF signal with
a bandwidth which is equal to the data rate of the baseband signal. The
bandwidth actually required is even larger because of the signal filtering
necessary to suppress adjacent-channel interference.
An analog telephone channel is about 3 kHz wide. Initially, a data rate
of 1200 bit/s could be achieved for this channel. Today, 56 kbit/s is no
problem any more. We are used to our fax and modem links operating at
such data rates. This quantum leap ahead was possible only through the
use of modern digital modulation methods known as IQ modulation. IQ
modulation is basically a form of amplitude modulation.