The wireless data transmission system consists of 2 parties: the transmitter and the receiver, whereby data is transmitted into the air from the antenna of the transmitter and received by the antenna of the receiver.
In a Single Input Single Output (SISO) system, both the transmitter and the receiver have one antenna, and data transmission over the air is through a single radio frequency (RF) signal chain.
An example of SISO type of wireless is Bluetooth.
Multiple antenna technique has been developed to improve wireless performance.
In a Single Input Multiple Output (SIMO) system, there is one antenna at the transmitter side and multiple antennas (each with an RF chain respectively) at the receiver side.
In a switched diversity or selection diversity implementation, the receiver chooses the best antenna to receive a stronger signal from the transmitter. In a maximal ratio combining (MRC) implementation, the receiver combines signals from all its antennas so that to maximize the Signal to Noise Ratio (SNR).
The Multiple Input Single Output (MISO) system is the other way round, with multiple antennas (each with an RF chain respectively) at the transmitter and a single antenna at the receiver.
A technique known as Alamouti Space Time Coding (STC) is employed at the transmitter with 2 antennas, allowing the transmitter to transmit signals both in time and space. This means data is transmitted by the 2 antennas at 2 different times consecutively.
Multiple Input Multiple Output (MIMO) system is commonly used in today's wireless technology, including 802.11n WiFi, WiMAX, LTE, etc. Multiple antennas (and therefore multiple RF chains) are put at both the transmitter and the receiver.
A MIMO system with same amount of antennas at both the transmitter and the receiver in a point-to-point (PTP) link is able to multiply the system throughput linearly with every additional antenna. For example, a 2x2 MIMO will double up the throughput.
Spatial Multiplexing (SM) technique is used in MIMO to enable signal to be transmitted across different spatial domains. This is used to provide additional data capacity.
Therefore, when buying a Wireless-N access point or router, to get a stabler WiFi with higher bandwidth, look for those that support MIMO and with more antennas.
Acknowledgement: the above diagrams are taken from the Radio-Electronics.com website.