Introduction to MIMO Antenna Design
Multiple-input multiple-output (MIMO) wireless technology uses multiple antennas at the transmitter and receiver to produce significant capacity gains over single-input single-output (SISO) systems using the same bandwidth and transmit power. It has been shown that the capacity of a MIMO system increases linearly with the number of antennas in the presence of a scattering-rich environment. This will ensure that the signals at the antennas in the array are sufficiently uncorrelated with each other. This is where antenna design comes in for MIMO systems.
The primary aim of MIMO antenna design is to reduce correlation between received signals by exploiting various forms of diversity that arise due to the presence of multiple antennas, like space diversity (spacing antennas far apart), pattern diversity (using antennas with different or orthogonal radiation patterns), polarization diversity (using antennas with different polarizations) etc. These 3 forms of diversity are pictorially represented as shown.
Previous Research
Previous research at WSIL showed the benefits of pattern diversity over space diversity. While the latter is viable only when sufficient real estate is available, pattern diversity can be utilized even in the case where space is limited – like in cellular phones or handsets. Research carried out in WSIL established that circular patch array (CPA) yields significant capacity gains over the conventional uniform linear array with the advantage of reducing the physical size of the array significantly. It was shown that collocated CPAs produce orthogonal radiation patterns, which gives excellent pattern diversity. The work also involved optimizing overall systems performance, using CEM and EM software tools, to maximize antenna theory and communication theoretic metrics for given size constraints. The output of the optimization problem was used to tune antenna array parameters (i.e., size, antenna material, feed points).
Current Research in WSIL
We are currently working towards MIMO antenna design for base stations using the IEEE 802.16e-2005 and 3GPP-LTE channel models. We aim to develop channel simulators for these standards with transmission modes like Tx-AA and D-TxAA, to analyze the performance of different antenna designs with capacity as the metric. The challenge here is to employ MIMO antenna design techniques to a cellular system, thereby extending ‘theoretical’ analysis to workable solutions.
We are also analyzing the performance of reconfigurable antenna arrays for MIMO systems, and demonstrating the impact of antenna geometry and configurations on the capacity of different transmission schemes for MIMO systems. We hope that this would lead to the development of algorithms for reconfigurable antenna arrays in MIMO systems that would switch their configuration in accordance with the channel statistics to maximize capacity.
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