A smart antenna is a digital wireless communications antenna system that takes advantage of the diversity effect at the transmitter, the receiver, or both. The diversity effect involves the transmission and/or reception of multiple radio frequency (RF) waves to increase data speed and reduce the error rate.

In conventional wireless communications, a single antenna is used at the source, and another single antenna is used at the destination. This is called SISO (single input, single output). Such systems are vulnerable to problems caused by multipath effects. The use of smart antennas can reduce or eliminate the trouble caused by multipath wave propagation.

Smart antennas fall into three major categories:

1. SIMO (single input, multiple output): One antenna is used at the source, and two or more antennas are used at the destination.
2. MISO (multiple input, single output): Two or more antennas are used at the source, and one antenna is used at the destination.
3. MIMO (multiple input, multiple output): Multiple antennas are employed at both the source and the destination.

MIMO has attracted the most attention recently because it cannot only eliminate the adverse effects of multipath propagation, but in some cases can turn it into an advantage. Smart antenna techniques are used notably in cellular systems like W-CDMA and UMTS.