Introduction to Multiple Antenna Systems: SIMO, MISO, MIMO

SISO

Radio transmissions traditionally use one antenna at the transmitter and one antenna at the receiver. This system is termed Single Input Single Output (SISO).

Picture. Single Input Single Output (SISO)
One antenna at both the transmitter and the receiver.
 Employs no diversity technique.

Both the transmitter and the receiver have one RF chain (that's coder and modulator). SISO is relatively simple and cheap to implement and it has been used age long since the birth of radio technology. It is used in radio and TV broadcast and our personal wireless technologies (e.g. Wi-Fi and Bluetooth).

SIMO

Picture. Single Input Multiple Output (SIMO), 1x2
One antenna at the transmitter, two antennas the receiver.
 Employs a receive diversity technique.

To improve performance, a multiple antenna technique has been developed. A system which uses a single antenna at the transmitter and multiple antennas at the receiver is named Single Input Multiple Output (SIMO). The receiver can either choose the best antenna to receive a stronger signal or combine signals from all antennas in such a way that maximizes SNR (Signal to Noise Ratio). The first technique is known as switched diversity or selection diversity. The latter is known as maximal ratio combining (MRC).