Coded Orthogonal Frequency Division Multiplex (COFDM) - Digital Video and Audio Broadcasting Technology - page 311

19 Coded Orthogonal Frequency Division
Multiplex (COFDM)
Almost from the beginning of the electrical transmission of messages
about 100 years ago, single-carrier methods have been used for transmit-
ting information. The message to be transmitted is impressed on a sinusoi-
dal carrier by applying analog amplitude, frequency or phase modulation
techniques. Since the eighties, single-carrier transmission is more and
more by digital methods in the form of frequency shift keying (FSK) and
in many cases also by vector modulation (QPSK, QAM). The main appli-
cations for this are fax, modem, mobile radio, microwave links and satel-
lite transmission and the transmission of data over broadband cables.
However, the characteristics of many transmission paths are such that sin-
gle-carrier methods prove to be sensitive to interference, complex or in-
adequate. Since the days of Marconi and Hertz, however, it is precisely
these transmission links which are used most frequently. Today, every
child knows of transistor radios, television receivers and mobiles or the
simple walkie-talkies, all of which operate with a modulated carrier in a
terrestrial environment. And every car driver knows the effect of reception
of the radio program he is listening to suddenly ceasing when he stops at a
red light - he is in a 'dead spot'. Due to multi-path reception, fading occurs
which is frequency- and location-selective. In terrestrial radio transmis-
sion, narrowband or wideband sinusoidal or impulse-type interferers must
also be expected which can adversely affect reception. Location, type and
orientation and mobility, i.e. movement, all play a role. This applies both
to radio and TV reception and to reception via mobile radios. Terrestrial
conditions of reception are the most difficult types of reception of all. This
similarly applies to the old two-wire line in the telecommunications field.
There can be echoes, crosstalk from other pairs, impulse interferers and
amplitude and group delay response. However, the demand for data links
with higher bit rates from PC to Internet is increasing more and more. The
usual single-carrier methods and also data transmission systems such as
ISDN are already reaching their limits. For many people, 64 kbit/s or 128
kbit/s with grouped channels in ISDN is not enough. In the terrestrial radio
link it is now the broadcasting services, which have always had a wide