OFDMA

What Is OFDMA?

OFDMA (Orthogonal Frequency-Division Multiple Access) is a technology introduced in Wi-Fi 6 that enhances wireless network performance. It achieves this by dividing the available frequencies into independently modulated subcarriers. This orthogonal allocation of subcarriers enables simultaneous transmissions to and from multiple clients, optimizing spectrum utilization and increasing overall network capacity. By using OFDMA, Wi-Fi 6 can support more efficient and concurrent communication, resulting in improved network performance and enhanced user experience.

How Does OFDMA Work?

Division into RU (Resource Units):

OFDMA divides the Wi-Fi channel into smaller RUs, which are the frequency allocations used for communication. These RUs allow an access point to communicate with multiple clients simultaneously, assigning specific RUs to each client.

Orthogonal Spacing:

In Wi-Fi 5, all 64 312.5-kHz subcarriers were used to transmit data to a single client. With OFDMA, these subcarriers are spaced orthogonally, enabling Wi-Fi 6 to divide channels into smaller units without interference. This spatial separation enhances efficiency and reduces interference.

Assignment of Resource Units:

The number of RUs allocated to each client is determined by various factors, including device constraints, quality-of-service (QoS) requirements, and packet size. The access point schedules the RUs for each client, considering these factors. The flexibility in scheduling, along with the parallel nature of OFDMA, increases the productive airtime efficiency.

Advantages of OFDMA over OFDM

OFDM:

OFDM is an older technology used to increase wireless capacity and efficiency. It has been employed in various applications like cellular networking and broadcast media, as well as in previous versions of Wi-Fi. In OFDM, the available frequency spectrum is divided into multiple orthogonal subcarriers, each carrying a separate data stream. These subcarriers are allocated sequentially, one after another.

OFDMA:

OFDMA is a variation of OFDM designed specifically for supporting multiple users. It allocates resources in both the time domain and the frequency domain. This means that multiple users can be accommodated simultaneously, even if they have different usage patterns or data loads. With OFDMA, the available frequency spectrum is divided into subcarriers, similar to OFDM, but these subcarriers can be allocated to different users in both the time and frequency domains.

OFDMA vs. MU-MIMO

OFDMA and MU-MIMO are two key technologies that improve wireless connectivity in multi-user scenarios.

MU-MIMO:

MU-MIMO physically divides network resources to increase capacity and efficiency in high-bandwidth applications like video streaming and downloading. It enables multiple spatial streams to be transmitted simultaneously to different users, maximizing spatial stream utilization and effective bandwidth. However, MU-MIMO can be impacted by the stability of the client devices (STAs) being served.

OFDMA:

OFDMA supports multi-channel transmission in the frequency domain and is well-suited for low-bandwidth, small-packet applications such as web browsing and instant messaging. It enhances spatial stream utilization and transmission efficiency while reducing application latency and queuing time. OFDMA is generally more stable and less affected by the stability of STAs.

Compatibility and Joint Scheduling:

MU-MIMO and OFDMA can be used together without conflict. In fact, joint scheduling of MU-MIMO and OFDMA optimally allocates resources based on the specific services being used, such as web browsing, video streaming, downloading, and instant messaging. By properly designing the algorithms for MU-MIMO and OFDMA, Wi-Fi 6 access points can utilize this joint scheduling mode to reduce conflicts arising from random uplink and downlink access. This enhances the overall user experience, especially in high-density access scenarios.