What Is MIMO?

MIMO, short for multiple input, multiple output, is a way of using several antennas at both the transmitter and the receiver so that one radio channel carries more than one signal at once. “Input” and “output” describe the radio channel: several signals go into it and several come out.

A radio signal rarely travels by one path. It reflects off buildings, vehicles, machinery and the ground, so copies arrive at slightly different times and strengths. With one antenna at each end this multipath causes fading and drop-outs. With several antennas spaced apart, each receive antenna sees a slightly different mix of the transmitted signals, and the receiver’s signal processing can separate them again. MIMO turns multipath from a problem into extra capacity.

MIMO set-ups are written as transmit × receive antennas. A 2×2 link has two antennas at each end and can carry up to two independent data streams, and a 4×4 link up to four. The number of streams is limited by the smaller side, so a 4×4 base station talking to a 2×2 device still runs at most two streams to it.

The same antennas can be used in three ways, and radios switch between them as conditions change.

Single-user MIMO (SU-MIMO) sends all the streams to one device at a time. Multi-user MIMO (MU-MIMO) lets an access point or base station serve several devices at once by giving each one its own spatial streams, which helps when many devices share the network. Wi-Fi 5 (IEEE 802.11ac Wave 2) added downlink MU-MIMO, and Wi-Fi 6 (802.11ax) extended it to the uplink.

Massive MIMO is the base-station version, with 32, 64 or more antenna elements in one panel steering many beams at once. It is a core part of 5G mid-band networks and raises cell capacity, but nothing changes on the device side: a 5G module still uses its own small set of antennas.

Figures are drawn from the IEEE 802.11 and 3GPP specifications; the streams a device actually uses depend on its radio, its antennas and the network.

A MIMO antenna is a housing that holds two or more antenna elements for the same band, each with its own cable and connector. Because MIMO depends on each element seeing a different version of the signal, the design matters more than with a single antenna.

Elements should be spaced apart, ideally by at least half a wavelength (around 21 cm at 700 MHz, but only a few centimetres at 3.5 GHz), or use different polarisations so they stay independent in a small housing. Leads should be the same length and type, and each one must go to the matching port on the radio module. Combination antennas for vehicles often carry two or four cellular elements, two Wi-Fi elements and a GNSS element in one roof-mounted housing, which keeps the installation tidy but makes clear labelling essential.

On an industrial computer the radio is usually an M.2 or Mini PCIe module inside the case, and each of its antenna ports runs to a connector on the panel. A 4G module typically needs two cellular antennas for 2×2 MIMO and receive diversity, a 5G module four, and a Wi-Fi module two, with GNSS on its own port. That is why many of our in-vehicle computers and industrial IoT gateways list several antenna positions or cutouts rather than one.

A module will often connect with fewer antennas than it supports, which makes MIMO problems easy to miss. A 5G module fed by two antennas instead of four can lose a large share of its downlink speed and become less stable at the cell edge. Inside a metal cabinet or vehicle body, every antenna should be mounted outside the metal on short, low-loss cable. Our guide to cellular gateways and industrial cellular computers covers antenna placement for each type of installation, and our guide to private 5G networks at the edge explains why four antennas matter on a standalone network.

We fit the cellular and Wi-Fi modules, SIM and antenna leads in the computers we supply as standard, and we match the number of antennas to the module so MIMO works as designed.