Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal
Frequency Division Multiple Access (OFDMA) are two different variants of
the same broadband wireless air interface that are often mistaken for
one another. OFDMA is a form of OFDM, which is the underlying
technology. Long Term Evolution (LTE) is an OFDMA-based technology standardized in 3GPP Release 8 and Release 9.
The interfaces of both OFDM and OFDMA work by separating a single
signal into subcarriers, or, in other words, by dividing one extremely
fast signal into numerous slow signals that optimize mobile access, as
the subchannels can then transmit data without being subject to the same
intensity of multipath distortion faced by single carrier transmission.
The numerous subcarriers are then collected at the receiver and
recombined to form one high speed transmission.
The difference between OFDM and OFDMA is that OFDMA has the ability to
dynamically assign a subset of those subcarriers to individual users,
making this the multi-user version of OFDM, using either Time Division
Multiple Access (TDMA) (separate time frames) or Frequency Division
Multiple Access (FDMA) (separate channels) for multiple users. OFDMA
simultaneously supports multiple users by assigning them specific
subchannels for intervals of time. Point-to-point systems are OFDM, and
do not support OFDMA. Point-to-multipoint fixed and mobile systems use
OFDMA.
OFDM technologies typically occupy nomadic, fixed and one-way transmission standards, ranging from TV transmission to Wi-Fi as well as fixed WiMAX and newer multicast wireless systems
like Qualcomm’s Forward Link Only (FLO). OFDMA, however, adds true
mobility to the mix, forming the backbone of many of the emerging
technologies including LTE and mobile WiMAX.
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