CommScope's COVID-19 Customer & Partner Hub Visit
In part two of this series, we explored the basics of MU-MIMO, OFDMA, and 1024-QAM. In this blog post, we’ll take a closer look at Wi-Fi speeds, along with an in-depth look at OFDMA, MU-MIMO and BSS Coloring.
Theoretical peak speeds vs. network capacity and efficiency
As we’ve emphasized throughout this series, the 802.11 standard has rapidly and significantly evolved over the past two decades. For example, wireless LANs once focused on achieving theoretical peak speeds. With the advent of Wi-Fi 6 (802.11ax), the emphasis has shifted to overall network capacity and efficiency, in addition to throughput speeds. As the latest iteration of 802.11, Wi-Fi 6 (802.11ax) is expected to become prevalent in ultra-dense environments such as transport hubs, urban apartment complexes, college campuses, concert venues and sports stadiums. These are all locations where many clients routinely access the internet over Wi-Fi, as well as share UHD content and stream 4K video.
Currently, in advanced development, the IEEE 802.11ax standard is slated to be released in 2019. It is worth noting that the maximum theoretical speed of Wi-Fi 4 (802.11n) was 150 megabits per second, per stream. Wi-Fi 5 (802.11ac) increased this to a theoretical speed of 866 megabits per second, per stream, which is considered a six-fold jump. Wi-Fi 6 (802.11ax) supports maximum speeds of up to 1201 megabits per second. Although Wi-Fi 6 is certainly faster than its predecessor, it is not the six-fold increase seen with the release of Wi-Fi 5 (802.11ac).
Wi-Fi 6: 4x increase in throughput
More specifically, Wi-Fi 6 (802.11ax) is expected to boast a 4x increase in throughput for the average user. This is primarily due to more efficient spectrum utilization and various improvements for dense deployments. Clearly, speed is not the most important issue, as the maximum rates are notoriously inaccurate when it comes to real-world performance. These can vary widely based on a range of obstacles, other signals in the air, multipath reflections and the capabilities of both access points and client devices.
To address these issues, Wi-Fi 6 (802.11ax) aims to improve efficiency by delivering consistently higher real-world speeds than Wi-Fi 5 (802.11 ac). As we noted in part three of this series, W-Fi 6 introduces orthogonal frequency-division multiple access or OFDMA. The mechanism – which is (4G) LTE-proven – provides more efficient access for users. Essentially, OFDMA technology allows multiple users with varying bandwidth needs to be served simultaneously by dividing each wireless channel into multiple sub-channels. This allows multiple clients to talk to the AP – simultaneously – over a single-channel (depending on the channel size). More specifically: 9 clients over a 20 MHz channel, 18 over a 40 MHz channel, and 37 over an 80 MHz channel. With multiple smaller channels, the AP can offer flexible bandwidth allocation to each device based on specific data requirements, thereby increasing overall network performance. It should be noted that smaller sub-channels are known as Resource Units (RU) or RU tones. The minimum size of one RU is 26 tones or subcarriers, which equals approximately 2 MHz. In practical terms, this means a 20 MHz channel can serve up to 9 users.
Working in tandem: OFDMA & MU-MIMO
OFDMA works in tandem with MU-MIMO, the latter of which helps APs address multiple devices simultaneously, instead of one at a time. From a precise chronological perspective, MU-MIMO was introduced as part of Wi-Fi 5 (802.11n), but only supported the mechanism in downlink mode. In contrast, Wi-Fi 6 (802.11ax) supports up to 8x8 MU-MIMO in both downlink and uplink modes – allowing APs to serve up to 8 users simultaneously. It is important to understand that MU-MIMO also benefits the performance of legacy devices such as those designed to support Wi-Fi 5 (Wave 2) devices.
Another important Wi-Fi 6 (802.11ax) feature is Basic Service Set (BSS) Coloring, which can perhaps best be described as a six-bit identifier attached to each PHY header that indicates the origin of the wireless LAN. Since Wi-Fi is a half-duplex medium – meaning that only one radio can transmit on a frequency domain or channel at any given time – Wi-Fi 6 (802.11ax) will defer transmission if it ‘hears’ the PHY preamble transmission of any Wi-Fi 6 radio at a signal detect or SD threshold of four decibels or greater. This medium contention overhead is a major issue in high-density venues such as a stadium or large conference rooms due to the sheer number of APs and clients.
Unnecessary medium contention is referred to as overlapping basic service sets (OBSS), or co-channel interference (CCI). Wi-Fi 6 (802.11ax) addresses this challenge by improving spatial reuse, which is often referred to as BSS Coloring. This mechanism was initially introduced as part of 802.11ah to address medium contention overhead due to OBSS. It assigns a different color, a number between 0 and 63, which is added to the PHY header of the Wi-Fi 6 (802.11ax) frame to each BSS in an environment. With BSS coloring, an AP can identify which frames are coming from other networks – and ignore them if they are below a certain threshold of weakness to prevent interference. This helps avoid unnecessary wireless slowdowns.