
The OFDM Interface Frame
The OFDM (orthogonal frequency division multiplexing) interface is the access
technique used by PLC. This access technique is also used by Wi-Fi in the IEEE
802.11a and 802.11g standards and by the ADSL and terrestrial TV broadcasting
technologies.
This technique is highly robust with regard to communication media interfer-
ence. The OFDM technique principle is to separate the frequency band into narrow
sub-bands, with each sub-band transporting part of the binary information. The fre-
quency responses of each sub-band are orthogonal and slightly overlap to obtain
good spectral efficiency.
OFDM Symbols
As explained above, the HomePlug 1.0, Turbo, and AV frames consist of OFDM
symbols of binary data combined into blocks.
Figure 5.5 gives a temporal and frequencies representation of the OFDM fre-
quency bands used by PLC technologies. The frequency band is divided into 84
sub-bands; only 78 of these sub-bands are used in order to comply with frequency
regulations concerning radio amateur networks (compliance with 40m, 30m, 20m,
and 17m amateur bands).
The OFDM Interface Frame 91
Figure 5.5 Temporal and frequential representation of OFDM frequency bands
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