Orthogonal Frequency Division Multiplexing
OFDM is not a form of spread spectrum. Instead, OFDM divides a data signal across 48 separate
subcarriers within a 20-MHz channel to provide transmissions of 6, 9, 12, 18, 24, 36, 48, or
54 Mbps. Data rates of 6 Mbps, 12 Mbps, and 24 Mbps are mandatory for all 802.11-compliant
products. OFDM is extremely efficient, which enables it to provide the higher data rates. In
addition, OFDM is highly immune to multipath propagation problems that cause significant
performance issues with spread-spectrum techniques.
An 802.11a modulator converts the binary signal into an analog OFDM waveform through the use
of different modulation types, depending on which data rate is chosen. For example, with 6-Mbps
operation, the PMD uses binary phase shift keying (BPSK), which shifts the phase of the transmit
center frequency to represent different data bit patterns. The higher data rates, such as 54 Mbps,
employ quadrature amplitude modulation (QAM) to represent data bits by varying the transmit
center frequency with different amplitude levels in addition to phase shifts.
Table G-6 summarizes the primary attributes of the various wireless LAN technologies.
Table G-6 Wireless LAN Technology Comparison
Spread Spectrum? Max Data Rate Standards
FHSS Yes 2 Mbps 802.11 FHSS
DSSS Yes 11 Mbps 802.11b
OFDM No 54 Mbps 802.11a and 802.11g