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| Analog modulation |
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Orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the popularorthogonal frequency-division multiplexing (OFDM)digital modulation scheme.Multiple access is achieved in OFDMA by assigning subsets ofsubcarriers to individual users. This allows simultaneous low-data-rate transmission from several users.
OFDMA is often compared to the combination of OFDM withstatistical time-division multiplexing. The advantages and disadvantages summarized below are further discussed in theCharacteristics and principles of operation section. See also the list ofOFDM key features.
Based on feedback information about the channel conditions, adaptive user-to-subcarrier assignment can be achieved.[2] If the assignment is done sufficiently fast, this further improves the OFDM robustness to fastfading and narrow-band cochannel interference, and makes it possible to achieve even bettersystem spectral efficiency.
Different numbers of sub-carriers can be assigned to different users, in view to support differentiatedquality of service (QoS), i.e. to control the data rate and error probability individually for each user.
OFDMA can be seen as an alternative to combining OFDM withtime-division multiple access (TDMA) or time-domainstatistical multiplexing communication. Low-data-rate users can send continuously with low transmission power instead of using a "pulsed" high-power carrier. Constant delay, and shorter delay, can be achieved.
OFDMA can also be described as a combination of frequency-domain and time-domain multiple access, where the resources are partitioned in the time–frequency space, and slots are assigned along the OFDM symbol index, as well as OFDM sub-carrier index.
OFDMA is considered as highly suitable for broadband wireless networks, due to advantages including scalability and use of multiple antennas (MIMO-friendliness), and ability to take advantage of channel frequency selectivity.[1]
In spectrum sensingcognitive radio, OFDMA is a possible approach to filling freeradio frequency bands adaptively. Timo A. Weiss and Friedrich K. Jondral of the University of Karlsruhe proposed aspectrum pooling system in which free bands sensed by nodes were immediately filled by OFDMA subbands.[relevant?][citation needed]
OFDMA is used in:
OFDMA is also a candidate access method for theIEEE 802.22Wireless Regional Area Networks (WRAN), acognitive radio technology which useswhite spaces intelevision spectrum, and the proposed access method forDECT-5G specification which aims to fulfillIMT-2020 requirements for high-throughput mobile broadband (eMMB) and ultra-reliable low-latency (URLLC) applications.
