Digital Signal Processing Reference
In-Depth Information
taken. If, however, many thousands of subcarriers are used instead of one
carrier, the symbol rate is reduced by the factor of the number of subcarri-
ers and the symbols are correspondingly lengthened several thousand times
up to a millisecond. The fading problem is solved and, at the same time,
the problem of inter-symbol interference is also solved due to the longer
symbols and the appropriate pauses between them.
A multi-carrier method is born and is called Coded Orthogonal Fre-
quency Division Multiplex (COFDM). It is now only necessary to see that
the many adjacent carriers do not interfere with one another, i.e. are or-
thogonal to one another.
19.2 What is COFDM?
Orthogonal frequency division multiplex is a multi-carrier method with up
to thousands of subcarriers, none of which interfere with each other be-
cause they are orthogonal to one another. The information to be transmit-
ted is distributed interleaved to the many subcarriers, first having added
the appropriate error protection, resulting in coded orthogonal frequency
division multiplex (COFDM). Each of these subcarriers is vector modu-
lated, i.e. QPSK, 16QAM and often up to 64QAM modulated.
COFDM
Orthogonal
Frequency division multiplex
Coded
COFDM is a composite of orthogonal (at right angles to one another or,
in other words, not interfering with one another) and frequency division
multiplex (division of the information into many subcarriers in the fre-
quency domain).
In a transmission channel, information can be transmitted continuously
or in time slots. It is then possible to transport different messages in the
various time slots, e.g. data streams from different sources. This timeslot
method has long been applied, mainly in telephony for the transmission of
different calls on one line, one satellite channel or also one mobile radio
channel. The typical impulse-type interference caused by a mobile tele-
phone conforming to the GSM standard with irradiation into stereo sys-
tems and TV sets has its origin in this timeslot method, also called time di-
vision multiple access (TDMA) in this case. However, it is also possible to
subdivide a transmission channel of a certain bandwidth in the frequency
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