Orthogonal frequency-division multiple access
Multi-user OFDM variant assigning subcarriers to individual users.
Orthogonal frequency-division multiple access (OFDMA) is a multi-user adaptation of the orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. It works by giving different users their own sets of subcarriers, which lets several users send data at low rates at the same time.
Compared to combining OFDM with statistical time-division multiplexing, OFDMA has several benefits. It avoids the need for pulsed carriers, lowers the maximum transmission power for users with low data rates, and provides shorter, more consistent delays. It also simplifies contention-based access, improves resistance to fading and interference, and handles narrow-band interference well. The system is flexible across different frequency bands with minimal air interface changes, and it averages interference from neighboring cells by using different carrier permutations. Within a cell, cyclic permutations further average out interference. OFDMA supports single-frequency network coverage, offers frequency diversity by spreading carriers across the spectrum, and allows per-channel or per-subchannel power control.
On the downside, OFDMA is more sensitive to frequency offsets and phase noise. Its complex electronics, including the FFT algorithm and forward error correction, consume power constantly regardless of data rate, whereas OFDM with packet scheduling could let the FFT hibernate. If very few subcarriers are assigned per user or the same carrier is used in every symbol, some diversity gain and resistance to frequency-selective fading may be lost, making adaptive subcarrier assignment or frequency hopping desirable. Handling co-channel interference from nearby cells is more complex than in CDMA, requiring dynamic channel allocation with advanced coordination. Fast channel feedback and adaptive subcarrier assignment are also more complex than CDMA’s fast power control.
In operation, OFDMA can assign different numbers of subcarriers to different users based on feedback about channel conditions, supporting differentiated quality of service by controlling data rate and error probability per user. It serves as an alternative to combining OFDM with time-division multiple access or statistical multiplexing, allowing low-data-rate users to transmit continuously at low power instead of using pulsed high-power carriers.
- Used in
- IEEE 802.16 WiMAX, IEEE 802.11ax (Wi-Fi 6/6E), IEEE 802.20 MBWA, MoCA 2.0, 3GPP LTE downlink, 3GPP 5G NR downlink and uplink, Qualcomm Flarion Mobile Flash-OFDM, Qualcomm/3GPP2 UMB
- Also candidate for
- IEEE 802.22 WRAN, DECT-5G
- Proposed by
- Timo A. Weiss and Friedrich K. Jondral of the University of Karlsruhe (spectrum pooling system)
Lore & Background
OFDMA is a multi-user version of the popular orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved by assigning subsets of subcarriers to individual users, allowing simultaneous low-data-rate transmission from several users. It can be described as a combination of frequency-domain and time-domain multiple access, where resources are partitioned in the time–frequency space, and slots are assigned along the OFDM symbol index as well as the OFDM subcarrier index. Based on feedback information about channel conditions, adaptive user-to-subcarrier assignment can be achieved, and different numbers of subcarriers can be assigned to different users to support differentiated quality of service. In spectrum sensing cognitive radio, Timo A. Weiss and Friedrich K. Jondral of the University of Karlsruhe proposed a spectrum pooling system in which free bands sensed by nodes were immediately filled by OFDMA subbands.
Reader's Guide
OFDMA allows simultaneous low-data-rate transmission from several users, avoiding pulsed carriers and enabling lower maximal transmission power for low-data-rate users. It offers shorter and constant delay, simplifies contention-based multiple access, and further improves OFDM robustness to fading and interference. It combats narrow-band interference, provides flexibility of deployment across various frequency bands with little modification to the air interface, and averages interferences from neighboring cells using different basic carrier permutations. Single-frequency network coverage is enabled, and frequency diversity is offered by spreading carriers over the used spectrum. Per-channel or per-subchannel power control is possible. Disadvantages include higher sensitivity to frequency offsets and phase noise; the complex OFDM electronics, including the FFT algorithm and forward error correction, are constantly active, consuming power independent of data rate. The OFDM diversity gain and resistance to frequency-selective fading may be partly lost if very few subcarriers are assigned to each user and if the same carrier is used in every OFDM symbol. Dealing with co-channel interference from nearby cells is more complex in OFDM than in CDMA, requiring dynamic channel allocation with advanced coordination among adjacent base stations. Fast channel feedback information and adaptive subcarrier assignment is more complex than CDMA fast power control.
Did You Know?
- OFDMA is used in the downlink of 3GPP LTE and in both downlink and uplink of 3GPP 5G NR.
- Low-data-rate users can send continuously with low transmission power instead of using a pulsed high-power carrier.
- Timo A. Weiss and Friedrich K. Jondral proposed a spectrum pooling system using OFDMA subbands for cognitive radio.
More in Radio Modulation Modes, Part 2 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced