104 Keysight WLAN Measurement Guide
Concepts
Orthogonal Frequency Division Multiplexing (OFDM) Concepts
Orthogonal Frequency Division Multiplexing (OFDM) Concepts
OFDM (Orthogonal Frequency Division Multiplexing) - OFDM uses multiple
carriers, of which there are 52, spaced 312.5 kHz apart. Data is sent on 48
carriers simultaneously, with 4 used as pilots. The time to transmit each bit
increases in proportion to the number of carriers. This makes the system less
sensitive to multipath interference, a major source of distortion.
Figure 3-8
OFDM Carriers Separation
802.11a uses OFDM as its transmission scheme. For 802.11a there are 52
carriers in all, 48 of which are used to carry data and 4 which are used as pilots.
Orthogonal Frequency Division Multiplexing (OFDM) has been formally
accepted as the method to achieve high data rates, better than 20 Mbps in the
WLAN systems. OFDM provides multiple frequency channels at regular
spacing, each modulated by M-ary QAM. An OFDM signal consists of the sum
of a number of sub-carriers that are modulated using some form of PSK or
QAM. Data interleavers “spread” the information throughout the carriers.
As shown in
Figure 3-9, “OFDM training structure,”
an OFDM burst actually
has four distinct regions. The first is the
Short training sequence
, followed by a
Long training sequence
and finally by the
Signal
and
Data symbols
. From a RF
standpoint, the Signal symbol and the rest of the OFDM symbols are similar.
Figure 3-9
OFDM training structure