Wi-Fi 6 vs. Wi-Fi 5 Key Changes to the RF Physical Layer
By Eve DanelMay 25, 2021
LitePoint’s Eve Danel is the author of this two-part blog series. Throughout these posts, you’ll learn about the key RF-PHY changes made in Wi-Fi 6/802.11ax compared to Wi-Fi 5 and older generations: the addition of a new frequency band, higher modulation rate, smaller subcarrier spacing, longer guard interval and the introduction of the OFDMA modulation technique providing capacity improvements and latency reduction.
Drivers for Wi-Fi 6 vs. Wi-Fi 5: Capacity Improvement
In previous generations, updates to the Wi-Fi standard were mainly focused on increasing the raw throughput. Wi-Fi 6 is faster as well, but different in that it also focuses on improving user experience, especially in dense, congested environments with a large number of users, such as airports, stadiums or educational settings. In these environments, the user experience suffers from inefficiencies when too many users are competing for bandwidth, and multiple overlapping networks interfere with each other.
Wi-Fi 6 adds features that improve average throughput per user by up to four times over Wi-Fi 5 in these highly congested environments. Wi-Fi 6 is also very well suited for use in home networks, where phones and tablets join televisions, home automation controls and other systems in competing for bandwidth. Wi-Fi 6 is particularly important in applications such as video conferencing and content streaming that require very low latency.
Wi-Fi 6 vs. Wi-Fi 5: Key Changes to the RF Physical Layer
Wi-Fi 6, also known as high efficiency (HE) Wi-Fi, is based on the 802.11ax standard and operates in the 2.4 GHz and 5 GHz frequency bands. Wi-Fi 6E, also based on the 802.11ax standard, operates in the 6 GHz frequency band.
Wi-Fi 6 vs. Wi-Fi 5 and Wi-Fi 4
The chart above summarizes the key RF-PHY changes made to increase capacity and efficiency in Wi-Fi 6 vs. Wi-Fi 5. Some new features are designed to improve Wi-Fi top speeds. Other new features are designed to improve the user experience in outdoor spaces or in high multipath fading environments. Other features are designed to improve capacity and reduce latency.
Other features not listed in the table, but that still have an impact on the network and devices include target wake time (TWT) for better battery savings and BSS coloring that is designed to improve spatial reuse in congested environments.
Wi-Fi 6 increases the modulation rates to improve peak data rates. The 802.11ax standard uses 1024 – quadrature amplitude modulation (1024QAM) with a peak data rate that is 25% higher than the peak data rate in the 802.11ac standard, which utilized 256QAM. 1024QAM used in Wi-Fi 6 encodes 10-bits of data per subcarrier, while 256QAM used in Wi-Fi 5 encodes 8-bits of data per subcarrier.
Wi-Fi QAM Rate Evolution
When encoding data at 1024QAM, the main challenge is ensuring high transmitter performance so that signals are correctly demodulated at the receiver. Error vector magnitude (EVM) is the metric that is used to ensure modulation accuracy. EVM measures the difference in dB between the ideal and the actual I/Q positions on a constellation diagram. The IEEE-defined EVM requirements for 1024QAM require a minimum EVM of -35dB, which is 3 dB better than 256QAM.
When measuring the EVM for a device with a tester, that tester’s EVM floor is critical. To ensure accuracy, the tester’s EVM floor should be 10 dB better than the device that is being measured. For 802.11ax, a tester that can achieve at least -45 dB to -48 dB residual EVM is needed.
OFDM Subcarrier Spacing and Symbol Duration
Another change to the PHY layer is that 802.11ax adds 4 times more subcarriers than 802.11ac, resulting in the subcarrier spacing becoming ¼ of what it was in 802.11ac. For 802.11ax, there is now a spectral spacing of 78.125 kHz between subcarriers. The symbol duration is inversely proportional to carrier spacing and increases four times over Wi-Fi 5, from 3.2 µs in 802.11ac to 12.8 µs in 802.11ax.
There are a few advantages from this change to the subcarrier spacing. The first is that with more subcarriers there can be finer resource unit (RU) granularity. The resource units are allocated in OFDMA to divide the channel bandwidth into smaller sub-channels assigned to different users. The smallest RU size contains 26 subcarriers (around 2 MHz), and allows up to 74 users in a 160 MHz channel (each user being assigned a 26 tones RU).
The second advantage is that 802.11ax is designed with a higher ratio of subcarriers that carry data vs. those that don’t carry data (null and pilot). The increase in the number of data subcarriers increases the efficiency of the transmission. This change results in about 10 percent improvement over 802.11ac.
It is important to keep in mind that because subcarrier spacing has decreased and the signal processing required is also more complex, Wi-Fi 6 devices need better frequency accuracy for proper demodulation.
Guard Interval / Cyclic Prefix
The standard also introduces new guard interval options. In Wi-Fi 6, longer guard intervals are designed to provide improved performance in environments with multi-path and delay spread. These longer guard intervals help to prevent inter-symbol interference in outdoor environments and therefore improve coverage and performance. 802.11ac had two Guard Interval (GI) options – long GI (0.8µs) and short GI (0.4µs). 802.11ax has three types of GI – the normal GI (0.8 µs), double (1.6 µs) GI and quadruple (3.2 µs) GI. Overall efficiency is preserved since in 802.11ax the symbol duration is four times longer than in 802.11ac, therefore the overhead generated by even the longest guard interval is the same percentage of the symbol time as in the previous generation, but with the added benefits of longer guard interval.
Wi-Fi 6 utilizes orthogonal frequency division multiple access (OFDMA). This is a fundamental change to the way that Wi-Fi operates. In previous OFDM-based Wi-Fi standards, bandwidth could be increased, but a user transmission would take the full channel for each transmission whether or not there was enough data to fill the bandwidth of the entire channel. Other senders would queue for access to the channel; creating a big problem in networks that supported a lot of simultaneous users.
By contrast, in OFDMA the channel is divided into subchannels called resource units (RUs). Each RU is made of a pre-defined number of subcarriers. The smallest RU can have 26 subcarriers, which is a little less than 2 MHz of spectrum and the largest RU can be as wide as 996 subcarriers, which is close to 80 MHz of spectrum. There is one user per RU for OFDMA;
Each RU is assigned to a different client station and the size and number of RUs allocated to each station is determined by the access point (AP), based on the data transmission requirements of each station.
For example, an application that requires a lot of data, such as content streaming, can be assigned a large RU. A device that requires less data, an IoT sensor for example, can be assigned a very small RU. Each RU can use a different modulation scheme, encoding rate and power level. RU assignments can vary on a frame-by-frame basis, based on what the AP determines is the most efficient use of the spectrum.
OFDMA does not directly impact Wi-Fi’s raw link speed, but instead, it increases its efficiency and will reduce delay for users that are sharing the spectrum. This provides benefits, especially in congested environments.
In my next blog post, I will expand on OFDMA and how it works, and will present important test considerations for Wi-Fi 6 vs. Wi-Fi 5. In the meantime, please visit LitePoint’s Wi-Fi 6 page to learn more about our test and measurement solutions for Wi-Fi 6 and click here for a replay of my webinar on this topic.
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