Multiple Input Multiple Output communications, abbreviated MIMO, refers to a collection of signal processing techniques that have been developed to enhance the performance of wireless communication systems using multiple antennas at the transmitter, receiver, or both. MIMO techniques improve
communications performance by either combating or exploiting multipath scattering in
the communications channel between a transmitter and receiver.

MIMO techniques in categorized into two:

1. Spatial Diversity: The focus of MIMO processing is on creating spatial diversity, which improves reliability by combating fading. The information bits are normally encoded and modulated using conventional error correction coding and modulation techniques prior to undergoing some form of space-time coding (STC). At the receiver, space-time decoding is performed followed by demodulation and error decoding.
   1. Transmit Diversity
   2. Receive Diversity
2. Spatial multiplexing: It is a technique that exploits multipath fading by performing spatial multiplexing and the purpose is to maximize throughput. The information error encoded bits are passed through a serial-to-parallel converter and the individual output streams are modulated before being transmitted over separate antennas. At the receiver, each antenna receives a signal that consists of the sum of the signals from all of the transmit antennas; therefore, it is necessary to strip off each of the transmitted streams {si} before demodulating them.
   1. Single-user MIMO (SU-MIMO) refers to conventional MIMO where there is a one transmitting device and one receiving device at a time in a given wireless channel. For example, a single WiFi router can transmit or receive data from only a single WiFi device at a time, even though multiple WiFi devices have been linked to the WiFi network hosted by the Router.
   2. In multi-user MIMO (MU-MIMO), a single transmitter device can transmit data to multiple receiving devices at a time in a given wireless channel. For example, a single WiFi router can transmit or receive from multiple WiFi devices simultaneously.

MIMO systems with N_t transmit antennas and N_r receive antennas are referred to as N_t × N_r MIMO systems. Thus, for example, a 2 × 4 MIMO system implies that there are two transmit antennas and four receive antennas.

The communications channel is the reference point, and the term Multiple Input referred to the signals from multiple transmit antennas that were“entering” or “being input” to the communications channel. Similarly, the term Multiple Output referred to signals arriving at multiple receiver antennas, which were viewed as “exiting” or “being output” from the channel.

Diversity

In most environments where wireless communication systems operate, the strength of the received signal varies with time, which is called fading. Unfortunately, fading significantly degrades communications performance by causing the probability of bit error to increase compared to what it would be if only white noise were present.

In order to reduce the impact of fading, the concept of diversity is often employed. Diversity refers to transmitting replicas of the same signal over a fading channel in such a way that each replica fades independently of the others. When this happens, each replica tends to fade at a different time, so the probability that all the replicas fade simultaneously decreases as the number of replicas gets larger. By combining the replicas, however, the depths of the fades, and, so too, their adverse effects, can be
significantly reduced because the fades do not tend to occur at the same time.

There are various ways to generate replicas of a signal for diversity purposes.

1. Frequency diversity : Transmit the signal on different RF frequencies that are spaced far enough apart that the fading occurs independently on each carrier. This is called frequency diversity.
2. Time diversity: It involves transmitting the same signal at different times. In a multipath environment, this occurs naturally because the same signal arrives at the receiver by traveling over multiple physical paths, which tend to experience independent fading.
3. Polarization diversity: To transmit the same information on signals having different polarizations.
4. Spatial diversity: To transmit the information over different physical paths between the transmitter and receiver.

Spatial Diversity

1. Transmit Diversity
2. Receive diversity
   1. Selective combining: It involves comparing the replicas at each sample time and choosing the largest value for the output of the combiner.
   2. Equal gain combining: It involves adding the replicas together.
   3. Maximal Ratio Combining (MRC): The replicas are added together in the same way as they are in equal gain combining, but prior to being added they are first scaled in proportion to the signal-to-noise ratio of each replica.

Receive Diversity

The transmitted signal is denoted by s, and the communications channel has the effect of multiplying the transmitted signal by a complex value, which we call the channel response, and denote by h_i, i = 1, . . . , N_r, where N_r represents the number of receive antennas. The inputs to the combiner, therefore, consist of the set of signals {r^_i = h_is}. If the receive antennas are spaced far enough apart, the random variables {h_i} are independent, so the receiver is able to reduce the effect of fading by combining multiple independently fading signals.

For example, if the base station is assumed to have multiple antennas, and if a signal, s, is simply transmitted from each of