MIMO

MIMO (Multiple Input Multiple Output) referred to in communications technology, the use of multiple transmit and receive antennas for wireless communication. This is the basis for specific coding methods that use not only the temporal but also the spatial dimension for information transmission ( Space-Time Coding). This quality ( bit error rate ) and data rate of a wireless link can be significantly increased. MIMO systems can transmit far more bit / s per Hz bandwidth occupied and thus have a higher spectral efficiency than conventional SISO or SIMO systems. Due to these properties, they are currently a central topic of international research.

• 2.1 Wi-Fi 2.1.1 MIMO hardware of the first generation
• 2.1.2 MIMO hardware of the second generation
• 2.1.3 MIMO technology in the IEEE 802.11n WLAN standard

Principle of operation

Smart Antennas / SIMO

The use of multiple antennas or receiving components at one end of the communication connection has been found in the last decades great distribution. Intelligent ( smart Sheet ) is connected to these antennas, the subsequent signal processing which assembles the received signals. In particular in mobile radio systems such as GSM, the use of multiple receive antennas at the side of the base station (BTS) is common, because it offers significant benefits: a plurality of antennas can be found more energy from the electromagnetic field, as a single ( group of gain). Reflections on the propagation path cause multipath propagation that can lead by destructive interference at the receiver to signal cancellation (English Fading). When a plurality of spatially separated receiver antennas are used in an environment subject to strong multipath, fading is statistically independent of the individual antennas and the probability that all antennas are simultaneously affected by fading is very low. This effect is called spatial diversity (English spatial diversity) and leads to a diversity gain, but not linearly increases with the number of antennas, but quickly comes to saturation. Another approach is the beam steering (English beamforming ), in which the main lobe of the antenna is directed specifically to the remote site. All of these processes can increase the reliability of a connection clear, but not the average channel capacity.

Channel matrix

The advantages of MIMO go beyond the smart antenna. If we consider a system with transmit antennas and receive antennas, the result is individual channels. 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 distributed to the antennas. In a system with two users, the different modes can be used to increase the data rate in a system with many users you can do this but also be used as multiple access method, to separate the signals of each user in a cellular network, for example ( as an alternative to today used FDMA / TDMA to GSM or CDMA in UMTS).

Simplistic EXAMPLE: In a system with four transmit and four receive antennas, a bit stream is divided into four separate bit streams are transmitted in parallel. On the receiver side, each antenna receives a sum signal of the transmit antennas. In order to decode the bit stream and reassemble a system of equations with four equations for four unknowns to be solved, which is only possible if the four equations are linearly independent, ie, the channel matrix has full rank. Physically, 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 may transmit within the same time four times the amount of data without requiring additional bandwidth, which increases the spectral efficiency by the factor of four. Thus generate a profit through spatial multiplexing (English spatial multiplexing).

Channel capacity

The channel capacity indicates how much bit / s / Hz can be transmitted over a maximum disturbed channel with arbitrarily small error probability. MIMO systems it is defined as

Wherein the average SNR referred to the receiver that adjoint and the identity matrix. In a system with a large number of antennas, an average channel capacity of

Theoretically, there is the possibility to increase the channel capacity over and any size. However, the price for this is the growing effort by the number of antennas and the complexity of the RF receiver and signal processing. In addition, this information-theoretic size is only an upper bound, which is difficult to achieve in practice. In addition, the approximate formula is only valid for uncorrelated, ie independent signal propagation paths (channels). In practice, the propagation paths of the signal are, however, always correlate specifically, the stronger will be the more antennas are used.

Applications

Wi-Fi

The MIMO support full will only be used if both the sender and receiver dominate the MIMO process. Uses only one side of MIMO technology, so the performance of normal 802.11g components increases by about 50 percent.

MIMO hardware of the first generation

For the first MIMO devices from the spring of 2005, their vendors promise much higher radio coverage compared to normal 802.11g standard, sometimes up to 1000%. The fact that the technology is still emerging, show the different product names (eg Netgear " RangeMax " or " SRX " for Linksys).

The MIMO technique was introduced in the spring of 2005 by various manufacturers. Whether these are really compatible with each other, is questionable, since the same time runs the standardization process for the future WLAN standard 802.11n and MIMO standard is expected to be adopted until after the official introduction of 802.11n by the normalization IEEE organization.

MIMO hardware of the second generation of

In December 2005, a new router generation came (at first only by the company Netgear ) with the new " Airgo " chipset on the market. This new chipset, which utilizes the MIMO technology, for the first time enables faster or the same speed as in the LAN via copper cables. The new network components reach a speed of up to 240 Mbit / s by using two radio channels. It also has Wi-Fi at the same time a full- duplex connection by using two radio channels.

MIMO technology in the IEEE 802.11n WLAN standard

In spring 2006 WLAN components were presented at CeBit 2006 for the first time, which can be operated with the wireless 802.11n standard. These new products are due to the new chipsets and technical specifications data throughput rates up to 300 Mbit / s have. This in turn is made ​​possible by the advanced MIMO technology. The technical specifications of this router and wireless adapter, however, based initially only on the 802.11n draft. According to the manufacturers ( Netgear or Linksys) this new hardware components are but be fully compatible with that with the help of firmware or software updates in adoption of the new 802.11n standard. With the help of MIMO technology in the 802.11n WLAN standard data rates, up to 600 Mbit / s are possible.

More

MIMO techniques will be included in future WiMax IEEE 802.16 standard and are already use in the mobile network, such as LTE. With MIMO technology, it is the mobile service providers are able to offer high data rates with low error rate.