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Orthogonal frequency-division multiple access

lastmod 2026-03-31created 2024-11-072 edits

Orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the popular OFDM (orthogonal frequency-division multiplexing) digital modulation scheme. In OFDMA, multiple access is achieved by assigning subsets of subcarriers to individual users. This allows simultaneous low-data-rate transmission from several users.

About

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OFDMA (Orthogonal frequency-division multiple access) technology, applied starting with Wi-Fi 6 (IEEE 802.11ax), is one of the key technologies that improve connection density, spectral efficiency, latency, and traffic capacity.

The main idea of the technology is to split the bandwidth into more resource units (Reource Unit) so that more user data and traffic can be transmitted simultaneously between clients and the router.

Through OFDMA, Wi-Fi 6 supports more clients on the same channel than Wi-Fi5 and improves efficiency with higher throughput and lower latency, and it increases the number of channels by subdividing the spectrum into resource units. As a result, it can provide an ideal environment for high-density user environments such as airports or public transportation, for high-capacity gaming and 4K media use, for connecting multiple devices such as IoT devices, and for businesses or industrial sites that need many client connections.

Benefits of AX routers with OFDMA:

  • Lower overhead and better performance – combining short packets into one yields significant savings.
  • Low-latency transmission - parallel communication is more efficient with long frames and low-speed transmission.
  • Improves overall QoS and uses bandwidth effectively in high-density environments.
  • More clients - using OFDMA per packet lets the router support more clients.
  • Reduces device battery usage after data transmission.

Comparison

OFDMA is often compared with the combination of OFDM and statistical time-division multiplexing. See also the page listing the key features of OFDM.

Pros

  • Allows simultaneous low-data-rate transmission from several users.
  • Pulsed carriers can be avoided.
  • Lower maximum transmission power for low-data-rate users
  • Shorter and more consistent delay
  • Contention-based multiple access (collision avoidance) is simplified.
  • Further improves OFDM robustness to fading and interference.
  • Combats narrowband interference.
  • Flexibility to be deployed across various frequency bands with little modification to the air interface
  • Averaging of interference from neighboring cells by using different basic carrier permutations between users in different cells
  • Interference within the cell is averaged by using allocation with cyclic permutations.
  • Enables single-frequency network coverage where coverage is a problem, and provides excellent coverage.
  • Provides frequency diversity by spreading the carriers all over the used spectrum.
  • Allows per-channel or per-subchannel power.

Cons

  • Higher sensitivity to frequency offsets and phase noise
  • Asynchronous data communication services such as web access are characterized by short communication bursts at high data rates. A few users in a base station cell are transmitting data simultaneously at a low, constant data rate.
  • The complex OFDM electronics, including the FFT algorithm and forward error correction, are constantly active and consume power regardless of the data rate, whereas with OFDM combined with data packet scheduling the FFT algorithm can sometimes be put into hibernation.
  • The OFDM diversity gain and resistance to frequency-selective fading may be partly lost if very few subcarriers are assigned to each user and the same carriers are used in every OFDM symbol. Therefore, adaptive subcarrier assignment based on fast feedback information about the channel, or subcarrier frequency hopping, is desirable.
  • Dealing with co-channel interference from nearby cells is more complex in OFDM than in CDMA. It requires dynamic channel allocation with advanced coordination among adjacent base stations.
  • Fast channel feedback information and adaptive subcarrier assignment are more complex than CDMA fast power control.

See also

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