From single-user OFDM to OFDMA
In a conventional OFDM system, all subcarriers can be assigned to one transmission during a given time interval. This works well for a single user sending a large block of data, but many users generating small, frequent packets must wait for successive transmission opportunities. With short packets, some assigned subcarriers remain unused for the rest of the interval even though the channel is reserved for one transmission. Channel access is then organized with time-division multiple access (TDMA).
OFDMA (Orthogonal Frequency-Division Multiple Access) extends OFDM with multiple access. It lets several users transmit in the same channel by assigning different subsets of subcarriers to them during the same time interval. The allocation preserves subcarrier orthogonality, so the assigned transmissions can coexist within one channel.
Analogy: Highway Traffic
A 12-lane highway can model the channel. OFDM would assign all lanes to one large truck for a brief moment, then to the next vehicle. A small car carrying a tiny package would occupy the whole highway. OFDMA divides the resources: two lanes go to the small car, six to a medium-sized van, and four to a bus. All vehicles move at the same time. The lanes represent simultaneous use of different subcarriers, while a new allocation represents the next time interval.
Resource Units in OFDMA
OFDMA divides the available subcarriers into defined groups called . A scheduling mechanism in a Wi-Fi access point (AP) dynamically assigns RUs to multiple users for uplink, from user to network, and downlink, from network to user, within the same time slot. In 5G NR, a gNB performs analogous scheduling with resource blocks and physical resource blocks under NR numerology and allocation rules. In Wi-Fi, the RU size determines how many subcarriers are available to a station and lets the allocation fit a small packet or a larger stream.
This time-frequency grid shows the difference in resource allocation. In an OFDM system scheduled with TDMA, users occupy the resources sequentially. In OFDMA, several users transmit in parallel, each in its assigned part of the spectrum. A shared time slot contains separate RUs, with each user receiving its own allocation.
Benefits of Subcarrier Allocation
Fine-grained subcarrier allocation matches resources to the number of users, packet sizes, and timing requirements. The scheduling mechanism can use otherwise unused subcarriers in the same interval instead of waiting for the entire transmission to finish.
- Efficiency for small packets: OFDMA fits environments where many users send small packets, such as VoIP calls, IoT sensor data, and instant messaging. Sending these packets together reduces access overhead and uses the available spectrum more effectively.
- Lower latency and contention: Simultaneous transmissions reduce the need to wait for a completely free channel and limit access-contention procedures. Shorter waiting times are useful for gaming, video conferencing, and other real-time applications.
- Quality-of-service support (QoS): The scheduling mechanism can assign more RUs to a user streaming 4K video and a small, low-latency RU to another user on a VoIP call. The allocation can reflect the requirements of both applications.
- Operation in dense environments: Coordinated transmissions and less contention improve network use in crowded places such as stadiums, airports, and lecture halls, where many devices compete for airtime.
Applications of OFDMA
OFDMA is used in several current wireless standards. In each case it combines OFDM multicarrier modulation with multiple access, while the allocation details come from the relevant standard:
- Wi-Fi 6 (IEEE 802.11ax): The standard defines OFDMA for downlink and uplink transmissions. The access point assigns Resource Units to stations, reducing contention overhead when many users transmit.
- 4G LTE (Long-Term Evolution): OFDMA is used for downlink transmission. For the uplink, a modified version called is used to conserve battery life on mobile devices.
- 5G NR (New Radio): 5G NR assigns OFDM resources in downlink and uplink transmissions. 3GPP specifies scalable subcarrier spacing and DFT-s-OFDM for selected uplink transmissions, supporting services from massive IoT to enhanced mobile broadband.