MIMO (Multiple Input Multiple Output, multi-channel input - multi-channel 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 it works?
Wi-Fi routers with MIMO technology use the same network protocols as conventional single-channel ones. They provide better performance by improving the efficiency of transmitting and receiving data over a wireless communication line. 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 the bandwidth, range and reliability of transmission with a high risk of interference from other wireless equipment.
Wi-Fi Application
MIMO technology is included in the standard from version 802.11n. Its use improves the performance and availability of network connections compared to conventional routers.
The number of antennas may vary. For example, MIMO 2x2 provides for two antennas and two transmitters capable of transmitting and receiving on two channels.
In order to take advantage of this technology and realize its advantages, the client device and the router must establish a MIMO connection between themselves. The documentation for the equipment used should indicate whether it supports this feature. There is no other simple way to check if this technology is used in a network connection.
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, when all the antennas of the router must be coordinated for communication with one client device, SU-MIMO allows you to distribute each of them between different equipment.
Multi-user (MU) MIMO technology was created for use in Wi-Fi 802.11ac networks at a frequency of 5 GHz. If the previous standard required routers to manage their client connections one at a time (one at a time), MU-MIMO antennas can communicate with multiple clients in parallel. The multi-user method improves connection performance. However, even if the 802.11ac router has the necessary hardware support for MIMO technology, there are other limitations:
- a limited number of simultaneous client connections are supported (2–4) depending on the antenna configuration;
- Antenna coordination is provided in only 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 waves - the use of microwave bands in which bandwidth is greater than in the ranges licensed for 3G and 4G.
Multi-user technology
To understand how MU-MIMO works, you should 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 uploaded, then you cannot simultaneously broadcast an online video game to the console.
A user can launch several devices on a Wi-Fi network, and the router very quickly in turn forwards data bits to them. However, at the same time, it can access only one device, which is the main reason for the decrease in connection quality if Wi-Fi bandwidth is too low.
Since this works, little attention is paid to itself. Nevertheless, the efficiency of a router that transfers data to several devices at the same time can be improved. At the same time, it will work faster and provide more interesting network configurations. That's why developments like MU-MIMO came about that were eventually incorporated into modern wireless standards. These developments allow advanced routers to communicate with multiple devices at once.
A Brief History: SU vs. MU
Single and multi-user MIMOs are different ways to communicate routers with multiple devices. The first one is older. The SU standard allowed sending and receiving data on several streams at once, depending on the available number of antennas, each of which could work with different devices. SU was included in the 2007 802.11n update and began to gradually be introduced into new product lines.
However, SU-MIMO had limitations in addition to the antenna requirements. Although several devices can be connected, they still deal with a router that can only work with one at a time. The data transfer speed has increased, interference has become a smaller problem, but there are many opportunities for improvement.
MU-MIMO is a standard that has evolved from SU-MIMO and SDMA (Space Division Multiple Access). The technology allows the base station to interact with several devices using a separate stream for each of them, as if they all have their own router.
Ultimately, MU support was added to the 802.11ac standard update in 2013. After several years of development, manufacturers began to incorporate this feature into their products.
Benefits of MU-MIMO
This is an exciting technology because it has a significant impact on the daily use of Wi-Fi without directly changing the bandwidth or other key parameters of the wireless connection. Networks are becoming much more efficient.
To ensure a stable connection with a laptop, phone, tablet or computer, the standard does not require the presence of several antennas on the router. Each such device may not share its MIMO channel with others. This is especially noticeable when streaming video or performing other complex tasks. The speed of the Internet is subjectively increased, and the connection is established more reliable, although in fact it becomes more reasonable to organize the network. The number of simultaneously serviced 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 you to add more, and they will have to share the stream, which returns to SU-MIMO problems. The technology is mainly used in downlink channels and is limited when it comes to outgoing channels. In addition, the MU-MIMO router should have more information about the devices and channel status than the previous standards required. This complicates the management and troubleshooting of wireless networks.
MU-MIMO is also a directional technology. This means that 2 devices located nearby cannot use different channels at the same time. For example, if a husband watches an online broadcast on TV, and next to his wife transmits a PS4 game to her Vita via Remote Play, they still have to share bandwidth. A router can provide discrete streams only to devices that are located in different directions.
Massive mimo
As we move towards fifth-generation (5G) wireless networks, the increase in the number 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 performance of telecommunication networks to unprecedented levels. Given the scarcity and high cost of available resources, operators are attracted by the opportunity to increase bandwidth in frequency bands below 6 GHz.
Despite significant progress, Massive MIMO is far from perfect. Technology is still being actively researched both in academia and in industry, where engineers seek to achieve theoretical results using commercially acceptable solutions.
Massive MIMO can help solve two key problems - bandwidth and coverage. For mobile operators, the frequency range remains a scarce and relatively expensive resource, but is a key condition for increasing the signal transmission speed. In cities, the interval between base stations is determined by bandwidth rather than coverage, which requires the deployment of a large number of them and leads to additional costs. Massive MIMO allows you to increase the capacity of an existing network. In areas where the deployment of base stations is due to coverage, the technology allows them to increase their range.
Concept
Massive MIMO is fundamentally changing current practices using a very large number of coherently and adaptively working 4G service antennas (hundreds or thousands). This helps to focus the transmission and reception of signal energy in smaller areas of space, significantly improving performance and energy efficiency, especially in combination with the simultaneous planning of a large number of user terminals (tens or hundreds). The method was originally intended for time division duplex (TDD) transmission, but could potentially also be used in full duplex (PDD) frequency division mode.
MIMO technology: advantages and disadvantages
The advantages of the method are the widespread use of low-cost low-power components, reduced latency, simplified access control (MAC), and resistance to random and intentional interference. The expected throughput depends on the propagation medium providing asymptotically orthogonal channels to the terminals, and experiments still have not revealed any limitations in this regard.
However, with the elimination of many problems, new ones appear that require an urgent solution. For example, in MIMO systems, it is necessary to ensure the effective collaboration of many low-cost low-precision components, collect channel status data, and allocate resources to newly connected terminals. It is also required to use additional degrees of freedom provided by the excess of service antennas, reduce internal energy consumption to achieve overall energy efficiency, and find new deployment scenarios.
The increase in the number of 4G antennas involved in MIMO implementation usually requires a visit to each base station for configuration and wiring. The initial deployment of LTE networks required the installation of new equipment. This made it possible to configure the MIMO 2x2 source standard LTE. Further changes to base stations are made only in extreme cases, and higher order implementations depend on the operating environment. Another problem is that the MIMO operation leads to completely different network behavior than previous systems, which creates some planning uncertainty. Therefore, operators tend to use other designs first, especially if they can be deployed by updating the software.