Multiple-input multiple-output



Multiple-input and multiple-output, or MIMO, is the use of multiple antennas in order to improve the reliability and rate of communication. Typically, in wireless communications, this improved reliability and rate is achieved by means of utilizing the diverse set of spatial paths which undergo independent changes using multiple antennas at the transmitter, receiver or both. By suitable placement of antennas with appropriate distances of separation, independent statistics of channel variation is achieved, and statistical signal processing techniques can then utilize this to obtain reliability and rate improvements.

Introduction, notation and system model
The wireless MIMO channel is assumed to consist of a system with multiple transmit antennas and multiple receive antennas, which are connected by means of fading channels. Usually, by ensuring that antennas are placed in order to ensure that the channels across antennas are independent. Placement separation in units of $$\lambda/2$$ is one means to ensure independence.

Conventionally, for a MIMO system with $$M$$ transmit antennas and $$N$$ receive antennas, the number of channels, counting each link from a transmitter to a receiver separately, is $$MN$$. Much like in conventional wireless communication, these channels must be known at least at the receiver or transmitter or both, in order to communicate information successfully during the coherence interval. This is done by means of estimation of the channel coefficients using an appropriate technique.

In this page, we consider an $$M$$ transmitter, $$N$$ receiver MIMO system, represented as an $$N \times M$$ matrix denoted by $\mathbf{H}$. We assume a flat fading model for simplicity of analysis, since transform techniques such as OFDM can be used to covert frequency selective channels into flat fading ones. We also assume that the channel is known accurately to the receiver and does not change in the duration of this coherence interval.