Mobile Phone Technology

MIMO - Multiple Input Multiple Output

Theoretical MIMO overview.

image ©unwiredinsight.com

MIMO stands for 'multiple input, multiple output', meaning that signals are transmitted at different frequencies to different antennas on the receiver, as can be seen in the diagram.

For example, if a phone has two antennas tuned to two different frequencies, it can receive half of the packets on one antenna and the other half on the other antenna. The packets are then put together to form the full transmission. In this example, the speed of receiving the data is twice that of a single input. However to speed up the transmission, the transmitter splits the data between two channels and uses multiplexing to send two signals to the receiver simultaneously. With two channels at each of the transmitter and receiver, the theoretical speed increase is therefore a factor of four. With four channels transmitted and two received by the mobile phone, there is the potential for an eight fold increase in network speed.

Real MIMO is much more complex than indicated here. It all depends on the strength of the signals and network conditions. The transmitter is much more dynamic than indicated above, picking the best transmission to the receiver dependent on these conditions.

One of the key improvements added to 5G is the ability to use massive MIMO (massive multiple input, multiple output). While a single device (phone or IoT) may only use one or two channels, the mast will have many channels, thus providing a large number of channels for sending or receiving data. The end device could potentially have many more channels. Automated cars and commercial drones are two potential applications where this could be advantageous.

In a traditional cell environment, the mast will distribute coverage over 360 degrees or, where three antennae are used, 120 degrees. However, within the area covered by such an antenna, some areas will have high demand and others will have low demand. Beamforming and beamsteering are techniques used with Massive-MIMO to direct coverage to areas in need. This can be done dynamically, reflecting the current need.

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