Friday, 20 December 2013

OFDM/OFDMA: Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access

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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