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MIMO Antenna Design

What is MIMO? MIMO stands for multiple-input, multiple-output and is an emerging wireless communications approach, involving N transmitting and M receiving antennas, where both N and M are greater than one and are usually equal. Rather than sending information with a single antenna and have the signal received by one antenna, the MIMO communications system breaks up the information it wishes to transmit into smaller blocks and distributes the smaller blocks of information to each antenna in an appropriate manner, with the receiving antennas receiving information from all antennas. The result is a communications link that has a drastically increased spectral capacity and can therefore transmit more information over the same bandwidth as its single antenna counterparts. The dramatic increase in capacity is achieved in part due to fading. Contrary to intuition, if the fading is uncorrelated on each of the communications channels setup by the MIMO system, the capacity benefits ­ even though fading is almost always a bad thing, the fact that the fading is unique on each channel means the extraction of data (and suppression of noise) turns out to be much easier than in the single antenna case. Currently, the radio spectrum is fast becoming congested, and a spectrally efficient communications approach is required ­ MIMO is such a technology. The focus in this summary is to outline the electromagnetically-based design requirements for antenna design, rather than the mathematical principles behind MIMO theory. MIMO antenna design has four fundamental principles to adhere to · Low envelope correlation between antennas o Low envelope correlation can result from having sets of antenna patterns that are diverse in the spatial, phase and polarization senses. In other words, if all antenna patterns are "looking" in different directions or in different ways (phase / polarization) one has low correlation. · Large individual antenna mean effective gain (MEG) o MEG refers to the gain of an antenna in a particular environment relative to some reference antenna (e.g. isotropic radiator). In other words, it is a measure of how well an antenna receives power for a particular scattering environment. · All antennas with roughly the same mean effective gain o The MEG of each antenna needs to be roughly the same for a balanced system · Low total array reflection coefficient (TARC) o Antennas radiate simultaneously and must have minimal crosstalk pairs. The biggest challenges in MIMO antenna design are as follows · There are simple design rules to follow for an ideal scattering environment, but deviation from the ideal case leads to antenna design with no design guidelines. · Low envelope correlation is at odds with large mean effective gain. If the region we wish to have large gain constitutes a small window of the far-field, it means we must have diversity in this window while simultaneously having large gain in said window ­ both of which are clearly at odds. · There is a strong push towards mobile communications and MIMO on handhelds, which leads to closely spaced radiating elements, thereby conflicting with all requirements of MIMO antenna design!

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