Also known as: OFDMA, orthogonal frequency-division multiple access
OFDMA (orthogonal frequency-division multiple access) is OFDM used as a multiple-access method: instead of one transmitter filling every subcarrier, the pool of subcarriers is partitioned and different groups of subcarriers are assigned to different users in each time slot.1 It combines the multipath robustness of OFDM with the flexible, fine-grained sharing of a scheduler — a user can be given many subcarriers for a short time, or a few for longer, matched to its data need and channel quality.
How it works
The base station (or Wi-Fi access point) runs the same IFFT/FFT OFDM engine, but a scheduler decides, slot by slot, which subcarriers belong to which user. The smallest schedulable unit is a resource block in LTE/5G (12 subcarriers over one slot) or a resource unit in Wi-Fi 6. Because subcarriers stay orthogonal, users’ allocations do not interfere even though they transmit simultaneously in the shared band.
OFDMA is best understood as a hybrid of the classic access schemes: it divides users in both frequency (like FDMA) and time (like TDMA) on one grid, giving the scheduler two degrees of freedom. This lets it exploit frequency- selective scheduling — assigning each user the subcarriers where its channel happens to be strong, since a deep fade rarely hits the whole band at once — for a multi-user diversity gain that single-carrier schemes cannot match.
On the uplink, pure OFDMA’s high peak-to-average power ratio is hard on battery-powered handset amplifiers. LTE therefore uses a precoded variant, SC-FDMA (single-carrier FDMA), which lowers PAPR while keeping the same resource-block structure; 5G NR uses it as an option.
Relevance to SDR
OFDMA is the air-interface backbone of 4G LTE and 5G NR and was added to Wi-Fi in 802.11ax (Wi-Fi 6) to let one access point serve many devices efficiently in the same transmission. It is the natural evolution of OFDM from a point-to-point waveform into a cellular multiple-access layer, and a peer of CDMA as a way for many users to share spectrum.
None of GopherTrunk’s target land-mobile trunking protocols use OFDMA — they rely on FDMA and TDMA over narrowband single carriers — so the scanner does not decode OFDMA. It is documented here to complete the multiple-access picture (FDMA, TDMA, CDMA, OFDMA) and to explain how modern broadband cellular schedules its shared spectrum.
Sources
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Orthogonal frequency-division multiple access — Wikipedia, for resource-block assignment, the FDMA/TDMA hybrid view, and SC-FDMA. ↩