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Introduction
The Multiple-input multiple-output (MIMO) systems are today regarded as one of the most promising research areas of wireless communications. In 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.It transmits two or more data bits in same channel at same time using multiantennas at transmitter and receiver. It enables the increase of data rates by transmission of several independent multiplexed data streams on the different transmit antennas. It can enable robust communications, especially in challenging environments for radio propagation, by sending instead redundant information over the multiple antennas. Multiple data streams enable higher data speeds, while with redundancy under less radio-friendly conditions, if one signal is disrupted by interference, the receiver can recover all data from the other. It achieves this by higher spectral efficiency (more bits per second per hertz of bandwidth) and link reliability or diversity (reduced fading).It enables us to make use of variety of signal paths. The increase in spectral efficiency offered by MIMO systems is based on the utilization of space (or antenna) diversity at both the transmitter and the receiver. Space divercity increases robustness of system as it eliminates fades and increases average received signal to noise ratio. According to Shannon’s formula, C = B ln(1 + SNR) as SNR of system increases linearly with space divercity, growth of capacity of channel is logarithmic.
MIMO is also used in Mobile Radio Telephone standards such as recent 3GPP and 3GPP2 standards. In 3GPP, HSPA+(High Speed Packet Acess Plus) and LTE(Long Term Evolution) standards take MIMO into account. As spectral bandwidth is becoming an ever more valuable commodity for radio communications systems, techniques are needed to use the available bandwidth more effectively. MIMO wireless technology is one of these techniques. This makes MIMO wireless technology one of the most important wireless techniques to be employed in recent years.
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Functions of MIMO:
MIMO can be sub-divided into three main categories.
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) weighing 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.
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.
Diversity coding techniques are used when there is no channel state information 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.
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Forms of MIMO:
The terminology and methods used in MIMO can differ from system to system, however most fall into one of two categories:
SU-MIMO (Single User - MIMO) - this utilizes MIMO technology to improve the performance towards a single user.
MU-MIMO (Multi User - MIMO) - this enables multiple users to be served through the use of spatial multiplexing techniques.
The STC (Space Time Coding) method is typically used in SU-MIMO. This utilizes multiple antennas to transmit the same information, however the data on each antenna is diversity coded, i.e. intelligently reordered in time. The following diagram illustrates this concept. In a standard SISO (Single Input Single Output) or SIMO (Single Input Multiple Output) system, only a single transmit stream is used (this is illustrated as the blue line). It can be seen that the additional diversity stream (green line) is time coded and transmitted from a second antenna, i.e. spatial diversity. This provides a combination of space and time coding. If interference / errors occur on one of the streams, the other is still able to carry the data successfully. In addition, if there was interference in the time domain, which affected both streams (denoted by the “X” in the diagram) the bits lost are duplicated at different times on the different streams. This permits the system to handle more interference and thus enables the use of HOM (Higher Order Modulation) schemes and / or reduction in FEC (Forward Error Correction) overhead. As such, STC can either provide users with a more efficient connection, or it could be used to extend the range of a cell.