Multiple-input and multiple-output (MIMO) technology is the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO technology offers significant increases in data throughput and link range without additional bandwidth or transmit power. It achieves this by higher spectral efficiency (more bits per second per hertz of bandwidth) and link reliability or diversity (reduced fading). Because of these properties, MIMO is an important part of modern wireless communication standards such as WiMAX, HSPA+, 3GPP Long Term Evolution, 4G, and IEEE 802.11n (Wifi).

Figure 1. Conventional SISO antenna system.

Figure 2. MIMO antenna system.

The MIMO antenna configuration can be used for spatial multiplexing. In this case a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams into (almost) parallel channels.

Accordingly the spatial multiplexing by using MIMO antennas is a powerful technique for increasing channel capacity at higher signal-to-noise-and-interference ratios (SNIR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver.

Typical MIMO schemes are MIMO 2x2 (that is two antennas each at both transmitter and receiver), and MIMO 4x4. In case of spatial multiplexing each MIMO antenna element transmits a separate MIMO data stream. The MIMO scheme 4x2 transmits the MIMO stream 1 from two antenna elements and the MIMO stream 2 from two other antenna elements, combining MIMO with a distributed antenna system (DAS). The receiver includes also two antenna elements (for separating the two different MIMO streams).