Orthogonal Frequency Division Multiplexing (OFDM) and IEEE 802.11a

Orthogonal Frequency Division Multiplexing (OFDM) and IEEE 802.11a
IEEE 802.11a (5 GHz) uses OFDM as its frequency management technique and adds several versions of
quadrature amplitude modulation (QAM) in support of data rates up to 54 Mbps. In 1970, Bell Labs
patented OFDM, which is based on a mathematical process called Fast Fourier Transform (FFT). FFT
enables 52 channels to overlap without losing their individuality or orthoganality. Overlapping channels is
a more efficient use of the spectrum and enables them to be processed at the receiver more efficiently.
IEEE 802.11a OFDM divides the carrier frequency into 52 low-speed subcarriers. Forty-eight of these
carriers are used for data and four are used as pilot carriers. The pilot subcarriers allow frequency
alignment at the receiver.
One of the biggest advantages of OFDM is its resistance to multipath interference and delay spread.
Multipath is caused when radio waves reflect and pass through objects in the environment. Radio waves
are attenuated or weakened in a wide range depending on the object's materials. Some materials (such
as metal) are opaque to radio transmissions. As you can see, a cluttered environment would be very
different from an open warehouse environment for radio wave transmission and reception. This
environmental variability is why it is so hard to estimate the range and data rate of an IEEE 802.11
system. Because of reflections and attenuation, a single transmission can be at different signal strengths
and from different directions depending on the types of materials it encounters. This is multipath. IEEE
802.11a supports data rates from 6 to 54 Mbps. It utilizes BPSK, QPSK, and QAM to achieve the various
data rates.
Delay spread is associated with multipath. Because the signal is traveling over different paths to the
receiver, the signal arrives at different times. This is delay spread. As the transmission rate increases, the
likelihood of interference from previously transmitted signals increases. Multipath and delay spread are
not much of an issue at data rates less than 3 or 4 Mbps, but some sort of mechanism is required as
rates increase to mitigate the effect of multipath and delay spread. In IEEE 802.11b, it is CCK
modulation. In 802.11a, it is OFDM. The IEEE 802.11g specification also uses OFDM as its frequency
management mechanism.[6]
The adoption and refinement of advanced semiconductor materials and radio transmission technologies
for IEEE 802.11 provides a solid basis for the implementation of higher-level functions. The next step up
the protocol ladder is the definition of access functionality. Without structured access, the physical
medium would be unusable.[