In a wireless network, two wireless nodes communicate by transmitting and receiving data on a radio channel. This channel uses a certain amount of wireless spectrum – and in many areas, suitable spectrum is scarce, making using more of it impractical.
So, how can a wireless network multiply its capacity without multiplying the spectrum it uses?
Multiple-input multiple-output, or MIMO, is a range of technologies used to multiply the capacity of a wireless connection without requiring more spectrum. Many modern wireless technologies, from WiFi to LTE, use MIMO techniques to achieve more capacity without more spectrum.
There is an important distinction between MIMO – technologies designed to allow transmitting and receiving more than one data signal simultaneously over the same radio channel, and beamforming, which is designed to enhance a single or small number of data signals.
A fundamental MIMO technique is to transmit and receive data not just on a single polarity of the radio wave used to communicate between wireless nodes, but on two at once – this is known as a dually-polarized system. A network using this technique can utilize both vertical and horizontal polarities to transmit and receive data, doubling the network capacity compared to a singly-polarized system.
More advanced MIMO techniques use multiple antennas to transmit and receive data. This requires more advanced processing capability in the wireless nodes and introduces many technical challenges; for one, each node must be able to determine the data transmitted from one antenna to that transmitted from another, otherwise network performance will be severely limited. This requires sophisticated channel estimation and sounding techniques.
As the number of antennas and the corresponding number of data streams in a MIMO system increase, the complexity required in the wireless nodes rapidly increases. It requires highly advanced signal processing and system design for the network to function well as the number of antennas increases. Systems incorporating a high number of antennas such as this are known as Massive MIMO systems.
There is currently no set definition of how many antennas a system must have to be considered Massive MIMO, but a system with greater than 8×8 antennas is generally considered a Massive MIMO system. The 8×8 figure refers to 8 transmit and 8 receive antennas.
Massive MIMO systems, although very difficult to practically implement, offer significant advantages over non-Massive MIMO systems. By building upon MIMO principles, they allow network capacity to be significantly multiplied without requiring more wireless spectrum.