802.11ax fundamentals: Orthogonal Frequency-Division Multiple Access
Orthogonal Frequency-Division Multiple Access (OFDMA) enables an 802.11ax access point to simultaneously communicate with multiple devices by dividing each Wi-Fi channel into smaller sub-channels and choosing how best to allocate these resources. This technology ensures consistent service in high-density environments.
The base version of the IEEE 802.11 standard debuted in 1997 with specifications for data rates of 1 and 2 megabits per second (Mbit/s). Since then, the Institute of Electrical and Electronics Engineers (IEEE) has ratified 5 major iterations of the 802.11 Wi-Fi protocol, culminating with 802.11ac in 2013. However, despite a significant increase in speed, many organizations are finding themselves limited by the 802.11ac standard, particularly in high-density venues such as stadiums, convention centers, transportation challenges of high-density deployments, the IEEE recently introduced the 802.11ax standard, which is backward compatible with previous generations of Wi-Fi, to deliver faster network performance and connect more devices simultaneously. With an expected four-fold capacity increase over its 802.11ac Wave 2 predecessor, 802.11ax is successfully transitioning Wi-Fi from a ‘best-effort’ endeavor to a deterministic wireless technology that is fast becoming the de-facto medium for internet connectivity. Indeed, 802.11ax access points (APs) deployed in dense device environments will support higher service-level agreements (SLAs) to more concurrently connected users and devices – with more diverse usage profiles. This is made possible by a range of technologies that optimize spectral efficiency, increase throughput and reduce power consumption. These include Orthogonal Frequency-Division Multiple Access (OFDMA), Target Wake Time (TWT), BSS Coloring, 1024-QAM and MU-MIMO. In this article, we’ll be taking a closer at OFDMA and how 802.11ax wireless APs can utilize this mechanism to help achieve a 4X increase in network throughput.
How OFDMA Increases Throughput
Current and older iterations of Wi-Fi are client-centric with a randomized, contention-based approach that relegates Wi-Fi to a “free-to-send” first come, first served paradigm which is untenable for modern high-density deployments. In contrast, OFDMA is AP-centric and enables an 802.11ax access point to simultaneously communicate with multiple devices by dividing each Wi-Fi channel into smaller sub-channels known as Resource Units (RU). The access point determines how to allocate the sub-channels, as each individual RU (or sub-channel) can be utilized for different clients that are serviced simultaneously. In other words, an AP can choose to allocate the whole channel (all sub-channels within a channel) to a single user in a given time frame – or it may partition the whole channel to serve multiple devices simultaneously. This technique improves the usable throughput for all devices connected to an AP. The allocation and scheduling capability supported by OFDMA allows multiple devices to receive more consistent attention. This reduces the use of the contention methodology that preceded 802.11ax and moves Wi-Fi from a contention-based to a scheduled-based service, which is analogous to an unmanaged four-way street intersection finally getting a traffic light. This helps stabilize Wi-Fi performance, especially in higher density environments such as stadiums, convention centers, transportation hubs, and auditoriums. Indeed, OFDMA is most useful when multiple connections transmit limited amounts of data – which allows the protocol to squeeze smaller data packets through multiple sub-channels. Think of a delivery truck that can only send a package to one house at a time (prior to 802.11ax), versus that same truck able to carry multiple packages to multiple houses (802.11ax). Moreover, OFDMA is effective at all ranges (close, medium and far) and can help mitigate Overlapping Basic Service Set (OBSS) interference issues.
Evolving Wi-Fi Technology
According to analysts at IDC, 802.11ax deployment will ramp significantly in 2019 and become the dominant enterprise Wi-Fi standard by 2021. Designed for high-density connectivity, the new IEEE 802.11ax standard offers up to a four-fold capacity increase over its 802.11ac Wave 2 predecessor. With 802.11ax, multiple APs deployed in dense device environments can collectively deliver required quality-of-service (QoS) to more clients with more diverse usage profiles. This is made possible by a range of technologies – such as OFDMA – which enables 802.11ax access points to mitigate the effects of contention by scheduling simultaneous communication with multiple devices. 802.11ax will play a critical role in helping Wi-Fi evolve into a collision-free, deterministic wireless technology that dramatically increases aggregate network throughput to address high-density venues and beyond. Last, but certainly not least, 802.11ax access points will help improve the Wi-Fi performance for legacy wireless devices, although this is a topic for another blog.
How many antennas can the 802.11 ax standard support?
This means it has 8 transmit antennas, 8 receive antennas, and 8 spatial streams. Client devices do not have to be 802.11ax-capable to take advantage of APs with 8x8:8 antennas.