What is MIMO? First a definition and then we’ll discuss the guts of how MIMO makes 802.11n even better.
MIMO stands for Multiple-Input / Multiple-Output. Here’s a definition of Mimo —
A radio system (transceiver) with multiple inputs into the receiver and multiple outputs from the transmitter capable of sending or receiving multiple spatial streams of data.
In indoor environments, RF signals typically encounter several barriers (walls, doors, partitions, etc.) in their communication path towards the receiver. These barriers either reflect or absorb the original RF signal, creating multiple reflected or “secondary” waveforms.
Multipath results when multiple copies of the original RF signal travel different paths to arrive at the receiver. Multipath traditionally has been considered the enemy of RF communication. The higher the multipath in an environment, the more the likelihood of poor RF performance resulting from weaker received signal strength (RSSI), increased retries, and dead spots. Conventional RF system design has addressed the problem of multipath through the use of antenna diversity using two antennas for each radio in an access point.
Antennas in a diversity configuration function almost like redundant antennas. A signal from only one antenna is used at any time. Diversity switching logic implemented on the access point decides when to switch between the primary and the secondary (diversity) antenna for receiving the best signal.
MIMO introduces a new paradigm in RF systems design. MIMO-capable radios actually perform better within a multipath-rich environment. A MIMO system has multiple radio chains each of which is a transceiver with its own antenna. A radio chain refers to the hardware necessary for transmit/receive signal processing. A MIMO radio can then apply several techniques to enhance signal quality and deliver more throughput. It is this ability to add signal components from multiple antennas that differentiates MIMO access points from traditional access points that use antennas in a diversity configuration. An access point with antenna diversity selects signal components from the antenna that provides the best signal performance and ignores the other antenna.
A MIMO system has multiple Radio Frequency (RF) chains implemented in the radio allowing the processing of multiple RF signals from multiple antennas. Depending on the number of transmit/receive antennas and the number of spatial streams, a MIMO system is often classified as a TxR:S system. Under this nomenclature T refers to the number of transmit antennas, R refers to the number of receive antennas and S refers to the number of spatial streams (transmitted data streams) the system can process. For example a 3×3:2 system can transmit and receive 2 data streams on its 3 antennas. When evaluating wireless infrastructure, be sure to compare apples-to-apples. Some manufacturers sell very inexpensive MIMO access points that are only capable of 1×1.