Understanding of SISO, SIMO, MISO and MIMO

Understanding of SISO, SIMO, MISO and MIMO
In radio , multiple-input and multiple-output, or MIMO (commonly pronounced my-moh or me-moh), is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology.

MIMO technology has attracted attention in wireless  communications, because it offers significant increases in data throughput and link range without additional bandwidth or transmit power. It achieves this by higher spectral efficiency (more bits per second per hertz of bandwidth) and link reliability or diversity (reduced fading). Because of these properties, MIMO is an important part of modern wireless communication standards as IEEE 802.11n (Wifi), 3GPP Long Term Evolution  , WiMAX and HSPA+  . 

Functions of MIMO
MIMO can be sub-divided into three main categories, precoding  , spatial multiplexing or SM, and diversity  coding.

Precoding  is multi-stream beamforming , in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. In (single-layer) beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power
is maximized at the receiver input. The benefits of beamforming are to increase the received signal gain, by making signals emitted from different antennas add up constructively, and to reduce the multipath fading effect.
In the absence of scattering, beamforming results in a well defined directional pattern, but in typical cellular conventional beams are not a good analogy. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas, and precoding with multiple streams is used. Note that precoding requires knowledge of channel state information  (CSI) at the transmitter.

Spatial multiplexing  requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams into (almost) parallel channels. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios (SNR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers, known as space-division multiple access  . By scheduling receivers with different spatial signatures, good separability can be assured.

Diversity  Coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods, a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain  from diversity coding.
Spatial multiplexing can also be combined with precoding when the channel is known at the transmitter or combined with diversity coding when decoding reliability is in trade-off.

Forms of MIMO
MIMO communications
Up to now, multi-antenna MIMO (or Single user MIMO) technology has been mainly developed and is implemented in some standards, e.g. [802.11n] products.
 ●SISO /SIMO/MISO are degenerate cases of MIMO 
* Multiple-input and single-output (MISO) is a degenerate case when the receiver has a single antenna. 
* Single-input and multiple-output (SIMO) is a degenerate case when the transmitter has a single antenna. 
* single-input single-output (SISO) is a radio system where neither the transmitter nor receiver have multiple antenna. 
 ● Principal single-user MIMO techniques 
* Bell Laboratories Layered  Space-Time (BLAST), Gerard. J. Foschini (1996), see also V-blast 
* Per Antenna Rate Control (PARC), Varanasi, Guess (1998), Chung,Huang, Lozano (2001) 
* Selective Per Antenna Rate Control (SPARC), Ericsson (2004) 
 ● Some limitations 
* The physical antenna spacing are selected to be large; multiple wavelengths  at the base station. The antenna
   separation at the receiver is heavily space constrained in hand sets, though advanced antenna design and algorithm
   techniques are under discussion.