MIMO (communications engineering)

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The terms "SISO", "SIMO", "MISO" and "MIMO" refer to the transmission channel. Its "input" are the sending devices. Accordingly, the receivers are referred to as the "output" of the channel.

MIMO ( English Multiple Input Multiple Output ) denotes a number of areas in wireless transmission in the communications technology , a method and a transmission system for the use of multiple transmitting and receiving antennas for wireless communication.

This is the basis for special coding methods that use not only the temporal, but also the spatial dimension to transmit information ( space-time coding ). This can significantly improve the quality ( bit error frequency ) and data rate of a wireless connection. MIMO systems can transmit s per Hz bandwidth occupied substantially more bit / and thus have a higher spectral efficiency than conventional SISO systems ( English Single Input, Single Output ), each with an antenna on the sender and receiver side, or SIMO systems ( English Single Input, multiple output ) with one antenna on the transmitter side and several antennas on the receiver side.

MIMO technology has been and is constantly being developed. Most recently, in 2014, several router manufacturers presented multi-user MIMO (MU-MIMO). With this technology, an access point or router can send different data records to several clients at the same time. The radio channel becomes free again more quickly. This increases the efficiency of the system.

In addition to the presently described multi-variable system, there are MIMO SISO (SISO) in each of which make use of both sender and receiver, an antenna and "mixed" systems (SIMO and MISO ( English Multiple Input, Single Output )). At MISO, for example, a router uses three antennas and a smartphone only one antenna.

Working principle

Smart antennas / SIMO

The use of several antennas or receiving components at one end of the communication link has become widespread in recent decades. Intelligent (engl. Smart ) is connected to these antennas the downstream signal processing which assembles the received signals. In mobile radio systems such as GSM , in particular , the use of several receiving antennas on the base station ( BTS ) side is often found because this offers significant advantages: Several antennas can draw more energy from the electromagnetic field than a single one ( group gain ). Reflections on the propagation path cause multipath propagation , which can lead to signal fading due to destructive interference at the receiver . If several spatially separated receiving antennas are used in an environment with strong multipath propagation, the fading at the individual antennas is statistically independent and the probability that all antennas are affected by fading at the same time is very low. This effect is called spatial diversity (Engl. Spatial diversity ) and leads to a diversity gain , but does not grow linearly with the number of antennas, but quickly comes to saturation. Another approach is the beam steering (engl. Beamforming ), in which the main lobe of the antenna is directed specifically to the remote station. All of these methods can significantly increase the reliability of a connection, but not the average channel capacity .

Channel matrix

Illustration of the channel matrix

The advantages of MIMO go beyond those of the smart antennas. If one looks at a system with transmitting antennas and receiving antennas, then individual channels result. The resulting overall channel can be represented as a channel matrix with complex entries :

These different channels can be used at the same time with the same frequency, the transmission power is divided between the antennas. In a system with two participants, the different modes can be used to increase the data rate, but in a system with many users this can also be used as a multiple access method, e.g. B. to separate the signals of the individual users in a cellular network (as an alternative to the FDMA / TDMA in GSM or CDMA in UMTS used today ).

Simplifying example: In a system with four transmit and four receive antennas, a bit stream can be divided into four separate bit streams that are transmitted in parallel. On the receiving end, each antenna receives a composite signal from the transmitting antennas. In order to decode the bit stream and reassemble it, a system of equations with four equations for four unknowns has to be solved, which is only possible if the four equations are linearly independent, i.e. the channel matrix has full rank . In physical terms, this means that the individual channels must be very different, which is the case, for example, in environments with strong multipath propagation. If this condition is met, the system can transmit four times the amount of data in the same time without requiring additional bandwidth, which increases the spectral efficiency by a factor of four. A profit is thus achieved through spatial multiplexing .

Channel capacity

The channel capacity indicates the maximum number of bits / s / Hz that can be transmitted over a disturbed channel with an arbitrarily small error probability. For MIMO systems it is defined as


where the mean SNR at the receiver denotes the adjoint and the identity matrix . In a system with a large number of antennas, the average channel capacity is

Theoretically, it is possible here to increase the channel capacity over and over . The price for this, however, is the growing effort due to the number of antennas and the complexity of the RF receiver and signal processing. In addition, this information-theoretical variable is only an upper limit that is difficult to achieve in practice. In addition, the approximation formula only applies to uncorrelated, i.e. independent signal propagation paths (channels). In practice, however, the propagation paths of the signal are always correlated and the more so the more antennas are used.


MIMO technology is used in WLAN, WiMax and various cellular standards such as LTE.


The full MIMO support can only be used if both the sender and the recipient are proficient in the MIMO process. For example, if the access point uses MIMO with three antennas (3x3 MIMO), but only two antennas are available to the client (2x2 MIMO), the net throughput for 802.11ac components increases by approx. 20 with 3x2 MIMO compared to a 2x2 stream %.

First generation MIMO hardware

For the first MIMO devices based on the spring of 2005, their providers promised significantly higher radio coverage compared to the previous 802.11g standard. Examples of product names were or are at Netgear "RangeMax" or "SRX" at Linksys .

Second generation MIMO hardware

In December 2005 a new generation of routers (initially only from Netgear) with the new "Airgo" chipset came onto the market. This new chipset with MIMO technology made it possible for the first time to achieve net speeds similar to those in LAN via copper cables. The network components achieved a gross speed of up to 240 Mbit / s through the simultaneous use of two radio channels.

MIMO technology in the IEEE 802.11n WLAN standard

MIMO chip Atheros AR9220 in the Fritz! Box 7390

In the spring of 2006, WLAN components were presented for the first time at CeBit 2006, which can be operated with the WLAN standard 802.11n . Thanks to new chipsets and adapted technical specifications such as extended MIMO technology, these products had data throughput rates of up to 300 Mbit / s (gross). The technical specifications of these routers and WLAN adapters were initially only based on the preliminary version 802.11n draft. Many hardware components became fully compatible with the 802.11n standard adopted in 2009 with the help of firmware or software updates.

With the help of MIMO technology, data throughput rates of up to 600 Mbit / s (gross) are possible with the 802.11n WLAN standard as of 2012. The gross rate of 600 Mbit / s can only be achieved in the 5 GHz band with a channel bandwidth of 40 MHz and four antennas (4x4 MIMO) each on the transmitter and receiver side. The 11n standard recommends the MIMO OFDM method.

WiMax and cellular networks

MIMO technologies are included in the WiMax standard IEEE 802.16 adopted in 2009 . The 802.16e standard recommends the MIMO OFDMA method.

Example of an LTE MIMO antenna with 2 connections. The two antenna elements installed inside the antenna were offset by 90 ° to each other and thus use multiple input / output technology and antenna diversity .

Various cellular networks such as LTE also use MIMO processes. With MIMO it is possible for mobile network providers to offer high data speeds with a low error rate.

See also


Individual evidence

  1. a b c Ernst Ahlers: Funk overview. WLAN knowledge for device selection and troubleshooting . In: c't 15/2015, 178-181. ISSN  0724-8679
  2. What is LTE and is it worth it? Network coverage, costs and function . In: smartphone-mania.de . September 13, 2012. Retrieved October 26, 2012.