802.11ac technology

The new rate standard for Wi-Fi, IEEE 802.11ac, was launched in June (2012) with great fanfare, initially only by laptops and wireless routers, and then by television (such as LG), and then by top-level intelligence. Mobile phones (HTC new One, Apple iPhone 5, etc.) are also adopted. After all types of products are adopted, it is a new technology that cannot be ignored.

Although 11ac has been gradually developed, but strictly speaking, the 11ac standard has not yet been formally finalized, and the standard that started in June 2012 was the second draft of the technical proposal (Draft 2.0), which was followed one year later in June 2013. The Wi-Fi Alliance formally put forward 11ac's test verification program, but it is still not the official version. It is only the third draft version (Draft 3.0). The official version of 11ac is based on the progress made, and the progress is delayed. It will not be finalized until 2014.

Wide range of technologies

The 11ac standard is the same as 11n, the rate standard before 11ac. Except for the commercialization of the draft proposal, 11ac is also broad in technology and contains many optional features. Not every industry can fully implement all technical features.

First, with the number of antennas (strictly speaking, academically speaking, it will be referred to as spatial stream), the 11n standard allows the transmission and reception of up to four antennas, but in practice, most operators only implement three groups, and only two groups. . Similarly, the 11ac standard can have up to 8 antennas, but 11ac products have been implemented for a year, and 3 antennas are still used.

In addition, 11n each channel has a bandwidth of 20MHz and is compatible with 11g, but it can also use 40MHz to double the transmission rate. Similarly, 11ac is compatible with 20MHz of 11G and 11MHz of 40MHz, but 80MHz and 160MHz can be further used to increase the transmission rate by 2 times and 4 times. Not only this, 11n up to 64QAM modulation transmission, 11ac can further 256QAM modulation.

In addition, 11ac also has a number of optional features, such as Low-Density Parity Check Code (LDPC), Space-Time Block Coding (STBC), Transmit Beamforming (TxBF), and shorter frequency guard interval (Short). Guard Interval (SGI), which is shortened from 800nS (nanoseconds) to 400nS.

Reality and ideal

From the above description, it can be seen that the ultimate extremely powerful 11ac is used by 8 groups of antennas at the same time, the channel bandwidth is 160MHz, and 256QAM modulation is used, the code rate is 5/6, the guard interval is 400nS, etc., but in fact it can be done. ? The current answer is no.

Due to the wide range of 11ac technology, the industry will implement it in two waves and two phases. The first phase (Wave 1) will still be dominated by 80MHz, and the second phase will adopt 160MHz. At the same time, the first phase will be based on three groups of antennas. Only four groups of antennas will be introduced in the second stage.

Therefore, the current 11ac products only advertise up to Gbps transmission rate, generally about 1.3Gbps, this data is the use of 3 groups of antennas, each group is configured with 400nS interval, 80MHz channel bandwidth, so each group of antennas 433.3 Mbps, which is 3 times that of about 1.3Gbps.

MU-MIMO is the main difference

It can also be seen from the above that the acceleration method of 11ac is roughly the same as that of 11n, but it is more enhanced and refined, but one thing is that 11ac is obviously different from 11n, that is, 11n is only MIMO, but 11ac has MU-MIMO (MU is Multi- User multi-user means that an 11ac router can serve 2 terminals at the same time, and no longer can serve only one at a time.

Therefore, the so-called 11ac highest eight groups of antennas cannot serve eight units all serving a single device. A single device can obtain up to four groups of antenna services. If the 160MHz bandwidth is used again, each group of antennas is 866.7Mbps, and four groups of antennas are about 3.47Gbps. This is also the 11ac transmission limit.

In other words, if there are 4 groups of 11ac antennas in the laptop and TV, the highest transmission rate is 3.47Gbps. However, if the number of antennas is reduced, the transmission rate will be reduced. If only one set of Wi-Fi antennas is allowed in the mobile phone, then The transmission rate will be limited to 866.7Mbps (160MHz, 400nS). At this stage, it is mostly 433.3Mbps (80MHz, 400nS).

Does the new Apple AirPort Extreme have 6 antennas?

Finally, recently (June 2013) Apple announced that the new AirPort Extreme claimed to have 6 antennas, indicating that it has already supported the MU-MIMO function of 11ac? The answer is no, the so-called 6 groups of antennas, only 11n use 3 groups, 11ac use 3 groups, rather than 11n or 11ac has reached the level of 4 groups of antennas, but not even the ultimate state of MU-MIMO.

The official version still needs to wait

Looking to the future, if you look at the 11n model, the official version of 11ac may also be a good thing. 11n is also a commercial sneak peek starting from Draft 2.0, starting from the beginning of 2007 and finally completing the official version in October 2009. After 11 sketches. Similarly, 11ac began with draft version 2.0 in June 2012, and later announced the delay in the official version, currently Draft 3.0, it seems that the official version still needs to wait for some time.