MIMO (Multiple Input Multiple Output, multichannel input - multichannel output) is a method of coordinated use of several radio antennas in wireless network communications, common in modern home broadband routers and in LTE and WiMAX cellular networks.
How does it work?
Wi-Fi routers with MIMO technology use the same network protocols as conventional single-link routers. They provide higher performance by improving the efficiency of transmitting and receiving data over a wireless link. In particular, network traffic between clients and the router is organized into separate streams transmitted in parallel, with their subsequent recovery by the receiving device.
MIMO technology can increase throughput, range and transmission reliability when there is a high risk of interference from other wireless equipment.
Wi-Fi Applications
MIMO technology has been included in the standard since version 802.11n. Its use improves the performance and availability of network connections compared to conventionalrouters.
The number of antennas may vary. For example, MIMO 2x2 has two antennas and two transmitters capable of transmitting and receiving on two channels.
To take advantage of this technology and realize its benefits, the client device and the router must establish a MIMO connection between them. The documentation for the equipment used should indicate whether it supports this feature. There is no other easy way to check if a network connection is using this technology.
SU-MIMO and MU-MIMO
The first generation of technology, introduced in the 802.11n standard, supported the Single User (SU) method. Compared to traditional solutions where all router antennas must be coordinated to communicate with a single client device, SU-MIMO allows each antenna to be distributed to different equipment.
Multi-user (MU) MIMO technology was created for use in 802.11ac Wi-Fi networks at 5 GHz. Whereas the previous standard required routers to manage their client connections in turn (one at a time), MU-MIMO antennas can communicate with multiple clients in parallel. The multiuser method improves the performance of connections. However, even if the 802.11ac router has the necessary hardware support for MIMO technology, there are other limitations:
- limited number of simultaneous client connections (2-4) supported depending on antenna configuration;
- Antenna coordination is provided only in one direction - from the router to the client.
MIMO and Cellular
The technology is used in different types of wireless networks. It is increasingly being used in cellular communications (4G and 5G) in several forms:
- Network MIMO - coordinated signal transmission between base stations;
- Massive MIMO - the use of a large number (hundreds) of antennas;
- millimeter wave - Enabling microwave bands, which have more bandwidth than the bands licensed for 3G and 4G.
Multiplayer technology
To understand how MU-MIMO works, consider how a traditional wireless router handles data packets. It does a good job of sending and receiving data, but only in one direction. In other words, it can only communicate with one device at a time. For example, if a video is being loaded, you cannot stream an online video game to the console at the same time.
A user can run multiple devices on a Wi-Fi network, and the router sends bits of data to them very quickly in turn. However, it can only access one device at a time, which is the main reason for poor connection quality if the Wi-Fi bandwidth is too low.
Because it works, little attention is paid to yourself. However, work efficiencya router that transmits data to multiple devices at the same time can be upgraded. At the same time, it will work faster and provide more interesting network configurations. This is why developments like MU-MIMO emerged and were eventually incorporated into today's wireless standards. These developments allow advanced routers to communicate with multiple devices at once.
Brief history: SU vs MU
Single and multi-user MIMO are different ways for routers to communicate with multiple devices. The first one is older. The SU standard allowed sending and receiving data on several streams at once, depending on the number of antennas available, each of which could work with different devices. SU was included in the 2007 802.11n update and has been gradually introduced into new product lines.
However, SU-MIMO had limitations in addition to antenna requirements. While multiple devices may be connected, they are still dealing with a router that can only handle one at a time. Data rates have increased and interference has become less of an issue, but there is still room for improvement.
MU-MIMO is a standard that evolved from SU-MIMO and SDMA (Space Division Multiple Access). The technology allows a base station to communicate with multiple devices using a separate stream for each of them, as if they all had their own router.
Ultimate supportMU was added to the 802.11ac update in 2013. After years of development, manufacturers have begun to include this feature in their products.
MU-MIMO Benefits
This is an exciting technology because it has a measurable impact on everyday Wi-Fi usage without directly changing bandwidth or other key wireless settings. Networks are getting much more efficient.
To ensure a stable connection with a laptop, phone, tablet or computer, the standard does not require the router to have multiple antennas. Each such device may not share its MIMO channel with others. This is especially noticeable when streaming video or performing other complex tasks. The Internet speed is subjectively increased and the connection is more reliable, although in fact the networking becomes more intelligent. The number of simultaneously served devices is also increasing.
MU-MIMO limitations
Multi-user multiple access technology has a number of limitations that are worth mentioning. Existing standards support 4 devices but allow more to be added and have to share the stream, which brings back SU-MIMO issues. The technology is mainly used in downlinks and is limited when it comes to outgoing. In addition, a MU-MIMO router must have more device and link state information than previous standards required. This complicates the management and troubleshooting ofwireless networks.
MU-MIMO is also a directional technology. This means that 2 devices side by side cannot use different channels at the same time. For example, if a husband is watching a live stream on TV and nearby his wife is streaming a PS4 game to her Vita via Remote Play, they still have to share the bandwidth. A router can only provide discrete streams to devices that are located in different directions.
Massive MIMO
As we move towards fifth-generation (5G) wireless networks, the growth of smartphones and new applications has led to a 100-fold increase in their required bandwidth compared to LTE. The new Massive MIMO technology, which has received much attention in recent years, is designed to significantly increase the efficiency of telecommunications networks to unprecedented levels. With available resources scarce and expensive, operators are attracted by the opportunity to increase bandwidth in the frequency bands below 6 GHz.
Despite significant progress, Massive MIMO is far from perfect. The technology continues to be actively explored in both academia and industry, where engineers strive to achieve theoretical results with commercially viable solutions.
Massive MIMO can help solve two key issues - throughput and coverage. For mobile operators, the frequency band remains a scarce and relatively expensive resource, but is a keycondition for increasing the speed of signal transmission. In cities, the spacing between base stations is driven by bandwidth rather than coverage, requiring large deployments and additional costs. Massive MIMO allows you to increase the capacity of an existing network. In areas where deployment of base stations is driven by coverage, the technology allows to increase the range of their coverage.
Concept
Massive MIMO revolutionizes current practice by using a very large number of coherently and adaptively operating 4G service antennas (hundreds or thousands). This helps to focus the transmission and reception of signal energy into smaller areas of space, greatly improving performance and power efficiency, especially when combined with scheduling a large number of user terminals (tens or hundreds) at the same time. The method was originally intended for time division duplex (TDD), but has the potential to be applied to frequency division duplex (PDD) as well.
MIMO technology: advantages and disadvantages
The advantages of the method are the widespread use of inexpensive low-power components, latency reduction, simplification of the access control level (MAC), resistance to random and deliberate interference. The expected throughput depends on the propagation medium providing asymptotically orthogonal channels to the terminals, and experiments so far have not revealed any limitations in this regard.
HoweverAlong with the elimination of many problems, new ones appear that require urgent solutions. For example, in MIMO systems, it is necessary to ensure that many low-cost, low-precision components work together efficiently, collect channel state data, and allocate resources to newly connected terminals. It also needs to take advantage of the additional degrees of freedom provided by excess service antennas, reduce internal power consumption to achieve overall energy efficiency, and find new deployment scenarios.
The growth in the number of 4G antennas involved in the implementation of MIMO usually requires a visit to each base station to change the configuration and wiring. The initial deployment of LTE networks required the installation of new equipment. This made it possible to produce the 2x2 MIMO configuration of the original LTE standard. Further changes to base stations are made only as a last resort, and higher-order implementations depend on the operating environment. Another problem is that the MIMO operation results in a completely different network behavior than previous systems, which introduces some scheduling uncertainty. Therefore, operators tend to use other designs first, especially if they can be deployed through software upgrades.
