The list of articles
(theory and practice)
- Cluster Optimization vs. Auto Channel
- ACI: Adjacent-channel interference
- CCI: Co-channel interference
- 5GHz interference issues
- Wi-Fi: 2.4 GHz vs. 5 GHz
- The features of 802.11AC standard. Is this possible to reach the stated speed?
- Signal attenuations and quality of experience
- The impact of MIMO on multiple access
The features of 802.11AC standard. Is this possible to reach the stated speed?
In real life no one standard has been able to maximize its theoretical performance before, since many factors have the influence on the signal: electromagnetic interference from household appliances and electronics, obstacles in the signal path, signal reflections, and even magnetic storms. Because of this, the manufacturers continue to work under creation of even more efficient options for Wi-Fi standard. They will be more suitable not only for home, but also for active office use or for building extended networks. As a result of their commitment a new version of IEEE 802.11 - 802.11ac (or simply AC standard) was recently born.
There are not too many fundamental differences from 802.11n in this new standard, but they are all aimed at increasing the throughput of the wireless protocol. Basically, the developers chose the way to improve the advantages of the 802.11n standard. The most notable is the expansion of MIMO channels from a previously maximum three to eight. This means that soon we will be able to see wireless routers with eight antennas in stores. And eight antennas theoretically substantially increase the spatial streams.
The devices of 802.11abg standard operated on 20 MHz channels but 802.11n standard involved 40 MHz channels. The new standard has provided that AC routers have channels at 80 and 160 MHz, which means doubling and quadrupling of the double channel width.
Also, we should mention the improved implementation of MIMO technology provided in the standard that is known as MU-MIMO technology. The old 802.11n-compliant protocols supported half-duplex device-to-device packet transmission. It means that at the moment when a packet is transmitted by one device the other devices can only work for reception. Accordingly, if one of the devices connects to the router using the old standard, then the others will also work slower due to the increased packet transmission time to the device that uses the old standard. This may be the reason for slowdown the performance of the wireless network if many such devices are connected to it. MU-MIMO solves this problem by creating a multi-stream transmission channel that doesn’t wait for other devices to be used. At the same time, the AC router must be backward compatible with previous standards.
However, as it can be seen from our practical studies indicated in the picture below, despite the expansion of transmission channels, it is not possible to achieve the declared speed even under perfect conditions.
Credits: "Wi-Fi Capacity Analysis for 802.11ac and 802.11n: Theory & Practice By Timo Vanhatupa", Ph.D. Senior Research Scientist at Ekahau
Let's assume that we work with a good signal quality and therefore can use maximum modulation order (256-QAM) and maximum channel coding rate (⅚ LDPC). The 802.11ac OFDM frame has 4usec duration and contains 52 subcarriers. The resulting bit rate based on this PHY characteristics is 86,7 Mbps.
In spite of marketing the most common case is when UE (user equipment) has only 2 antennas. Therefore, we have usually no more than 2 parallel spatial streams.
Yes, we have the ability to use 160 MHz in 802.11ac and 802.11ax, however, this will mean more interference. Hence, 80 MHz channels are more preferable.
Finally, (assuming short guard (SG) interval and therefore SG factor = 1.11) we have 858,33 Mbps. And this is still a theoretical value! Practically, this value should be multiplied times a half at least due to interference and multipath.
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