In conventional wireless communications, a single antenna is installed at the source (transmitter), and another single antenna is installed at the destination (receiver). In some cases, this gives rise to problems with multipath effects. When an electromagnetic field is met with obstructions such as hills, buildings, and utility wires, the wavefronts are scattered, and thus they take many paths to reach the destination. The late arrival of scattered portions of the signal causes problems such as fading, cut-out, and intermittent reception.
What is MIMO?
MIMO stands for Multiple-Input Multiple-Output. MIMO is one of a forms of smart antenna technology by installing multiple antennas on the receiver and/or the transmitter in order to improve transmission performance. MIMO technology using multiple antennas makes use of reflected signals (multipaths - see Note 1 below) to provide gains in data throughput and increase in link range without additional bandwidth or increased transmit power. MIMO helps to combat destructive effects of signal fading, as well as improving link reliability and signal quality of a communication system.
Note 1: Multipath is a condition that arises when a transmitted signal undergoes reflection from various obstacles in the environment. This gives rise to multiple signals arriving from different directions causing interference at the receiver. By using MIMO technology, these additional paths can be used to increase the capacity of a link, thereby providing increased link capacity and spectral efficiency using what were previously seen as interference paths.
MIMO systems provide several major advantages over single antenna communications: 1) spatial multiplexing; 2) diversity gain; and 3) array gain.
- Spatial Multiplexing. It provide additional data capacity by ultilising the multipaths and effectively using them as additional channels to carry data.
- Diversity Gain. Improvement in link reliability is obtained by sending the same data on independently fading branches.
- Array Gain. This is the increase in signal power at the receiver due to the coherent combing of signals from multiple antennas at the receiver and/or the transmitter.