MIMO - Multiple Input Multiple Output

MIMOⁱ, Multiple Input Multiple Output is another of the LTEⁱ major technology innovations used to improve the performance of the system. This technology provides LTE with the ability to further improve its data throughput and spectral efficiency above that obtained by the use of OFDMⁱ.

Although MIMO adds complexity to the system in terms of processing and the number of antennas required, it enables far high data rates to be achieved along with much improved spectral efficiency. As a result, MIMO has been included as an integral part of LTE.

LTE MIMO basics

The basic concept of MIMO utilises the multipath signal propagation that is present in all terrestrial communications. Rather than providing interference, these paths can be used to advantage.

MIMO is being used increasingly in many high data rate technologies including Wi-Fi and other wireless and cellular technologies to provide improved levels of efficiency. Essentially MIMO employs multiple antennas on the receiver and transmitter to utilise the multi-path effects that always exist to transmit additional data, rather than causing interference.

The schemes employed in LTE again vary slightly between the uplink and downlink. The reason for this is to keep the terminal cost low as there are far more terminals than base stations and as a result terminal works cost price is far more sensitive.

For the downlink, a configuration of two transmit antennas at the base station and two receive antennas on the mobile terminal is used as baseline, although configurations with four antennas are also being considered.

For the uplink from the mobile terminal to the base station, a scheme called MU-MIMO (Multi-User MIMO) is to be employed. Using this, even though the base station requires multiple antennas, the mobiles only have one transmit antenna and this considerably reduces the cost of the mobile. In operation, multiple mobile terminals may transmit simultaneously on the same channel or channels, but they do not cause interference to each other because mutually orthogonal pilot patterns are used. This techniques is also referred to as spatial domain multiple access (SDMA).