Antenna Design Considerations for MIMO and Diversity Systems

Because of the signal fading induced by multipath propagation, it can be difficult to offer reliable wireless communication in many practical environments. However, because multipath propagation is characterized by electromagnetic waves that depart at a variety of different transmit angles and arrive at different angles at the receiver, it is theoretically possible to achieve high capacity in these scenarios by exploiting these spatial propagation characteristics. For example, if the transmit and receive antennas were to provide infinite spatial selectivity (antenna beams that could excite a single plane wave), unique data streams could be transmitted on each multipath component. While practical considerations clearly make this infeasible, this simple conceptual illustration reveals the high potential capacity available in a rich scattering environment. While infinite antenna resolution is infeasible, some degree of spatial selectivity can be obtained using multiple antenna elements at the transmit and receive ends of a link. Such multiple-input multiple-output (MIMO) wireless communications exploit the multipath channel characteristics to provide a new resource, namely, spatial processing, that allows improvement of the system performance. This new resource can be used to increase data throughput, improve signal reliability, or reduce transmitted power (leading to extended battery life in mobile devices), all without requiring an increase in the spectrum used for communication. Given these benefits, MIMO has received considerable attention in the research community and is now integrated into emerging communications standards. This chapter focuses on antenna issues related to MIMO systems [1]. Specifically, we explore the interaction of the antenna elements with the electromagnetic propagation, a study that reveals principles that aid in the design of antennas suitable for MIMO systems. Antenna radiation characteristics such as pattern shape and polarization as well as array configuration naturally represent an important part of the discussion. We also focus on mutual coupling and array supergain, which represent significant issues when implementing MIMO technology on mobile devices where the array must be compact. Finally, the discussion turns to the synthesis of optimal antennas for MIMO communication and provides a way to compare practical designs to this optimal benchmark.

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