MIMO (Multiple Input Multiple Output, multichannel input – multichannel output) – a method of coordinated use of multiple radio antennas in wireless network communications, common in modern home broadband routers and cellular networks LTE and WiMAX.
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How does it work?
Wi-Fi routers with MIMO technology use the same network protocols as conventional single-channel routers. They provide better performance by improving the efficiency of wireless data transmission and reception. In particular, the network traffic between the clients and the router is organized into separate streams transmitted in parallel, with their subsequent recovery by the receiving device.
MIMO technology can increase bandwidth, range, and transmission reliability with a high risk of interference from other wireless equipment.
MIMO in Wi-Fi
MIMO is included as standard with 802.11 n. It improves the performance and availability of network connections compared to conventional routers.
The number of antennas may vary. For example, MIMO 2x2 provides two antennas and two transmitters capable of receiving and transmitting on two channels.
To take advantage of this technology and realize its benefits, the client device and the router must establish a MIMO connection. The documentation for the hardware you are using should indicate whether it supports this feature. There is no other easy 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 802.11 n standard supported the single-user (SU) method. Compared to traditional solutions, where all the antennas of the router need to be coordinated to communicate with one client device, SU-MIMO allows you to distribute each of them among different equipment.
Multi-user (MU) MIMO technology was created for use in 802.11 ac Wi-Fi networks at 5 GHz. If the previous standard required routers to manage their client connections alternately (one at a time), MU-MIMO antennas can provide communication with multiple clients in parallel. The multi-user method improves the performance of connections. However, even if the 802.11 ac router has the necessary hardware support for MIMO, there are other limitations:
- a limited number of simultaneous client connections (2-4) are supported 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 the bandwidth is greater than in the bands licensed for 3G and 4G.
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 you're downloading a video, you can't stream an online video game to your console at the same time.
The user can run multiple devices on a Wi-Fi network, and the router very quickly takes turns forwarding bits of data to them. However, at the same time, it can only access one device, which is the main reason for reducing the quality of the connection if the Wi-Fi bandwidth is too low.
Since it works, the attention is paid little. However, you can improve the performance of a router that sends data to multiple devices at the same time. At the same time, it will work faster and provide more interesting network configurations. That's why there were developments like MU-MIMO, which were eventually incorporated into modern wireless standards. These developments allow the advanced router to communicate with several devices at once.
Brief history: SU-MIMO vs MU-MIMO
Single-and multi-user MIMO is different ways of communicating routers with multiple devices. The first one is older. The SU standard allowed the sending and receiving of 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 802.11 n update and began to be gradually introduced into new product lines.
However, SU-MIMO had limitations in addition to the antenna requirements. Although multiple devices can be connected, they still deal with a router that can only work with one at a time. Data rates have increased, interference has become less of a problem, but there is still much room for improvement.
MU-MIMO is a standard that has evolved from SU-MIMO and SDMA (multiple access with spatial channel separation). The technology allows the base station to communicate with multiple devices using a separate thread for each of them as if they all have their own router.
In the end, the MU support was added to upgrade to 802.11 ac in 2013 After several years of development, manufacturers began to include this feature in their products.
Advantages of MU-MIMO
It's fascinating technology because it has a significant impact on the everyday use of Wi-Fi without direct changes in throughput or other key parameters of the wireless connection. Networks are becoming much more efficient.
To ensure a stable connection to a laptop, phone, tablet, or computer, the standard does not require multiple 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 subjectively increases, 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.
Multi-user multi-access technology has a number of limitations, which are worth mentioning. The existing standards support 4 devices but allow you to add more and they will have to share the stream, which brings back to the problems of SU-MIMO. The technology is mainly used in downlink and is limited when it comes to outbound. In addition, the mu-MIMO router must have more information about the devices and the state of the channels than the previous standards required. This makes it difficult to manage and troubleshoot wireless networks.
MU-MIMO is also a directional technology. This means that 2 devices located side by side cannot use different channels at the same time. For example, if a husband is watching an online broadcast on TV and his wife is nearby streaming a PS4 game to their Vita via Remote Play, they'll still have to share bandwidth. The router can only provide discrete flows to devices that are located in different directions.
As we move towards fifth-generation (5G) wireless networks, the growth 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 a lot of attention in recent years, is designed to significantly increase the performance of telecommunication networks to unprecedented levels. With 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 explored in both academia and industry, where engineers seek to achieve theoretical results through 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 it is a key condition for increasing the speed of signal transmission. In cities, the interval between base stations is due to capacity 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 driven by coverage, the technology increases their range.
Massive MIMO radically changes the current practice, using a very large number of coherent and adaptively operating 4G service antennas (hundreds or thousands). This helps to focus the transmission and reception of signal energy in smaller areas of space, greatly improving performance and energy efficiency, especially when combined with the simultaneous scheduling of a large number of user terminals (tens or hundreds). The method was originally intended for time-division duplex transmission (TDD), but can potentially also be used in the mode of duplex (PDD) frequency division.
MIMO technology: advantages and disadvantages
The advantages of the method are the widespread use of low-cost low-power components, reducing latency, simplifying the level of access control (MAC), resistance to accidental and intentional interference. The expected throughput depends on the propagation medium providing asymptotically orthogonal channels to the terminals, and experiments have so far revealed no limitations in this regard.
However, along with the elimination of many problems, there are new ones that require urgent solutions. For example, in MIMO systems it is necessary to ensure effective collaboration of many inexpensive components of low accuracy, to collect data on the state of the channel and to allocate resources for the newly connected terminal. It is also necessary to use the additional degrees of freedom provided by the excess of service antennas, reduce internal power consumption to achieve overall energy efficiency and find new deployment scenarios.
The increase in the number of 4G antennas involved in the MIMO implementation usually requires a visit to each base station to change configuration and wiring. The initial deployment of LTE networks required the installation of new hardware. This made it possible to configure the MIMO 2x2 of the original LTE standard. 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 results in completely different behavior on the network than previous systems, which creates some scheduling uncertainty. Therefore, operators tend to use other developments first, especially if they can be deployed by updating the software.
Edited by DeZire